Effects of Running Diesel at Low RPM

[sgharford]

With all the talk of running your boat at hull speed to save gas I thought I would post a question to see what specific engine problems one potentially faces by not running at ‘cruising rpm’ for extended periods. I have a 43 DC with Cummins VT 903s (turbos).

[MikeP]

It's temperature related. If the engine cooling system will allow the engine to reach proper operating temp at low RPM, then it's not an issue. Car engines can do this from idle on up; boat engines' cooling systems - at least older ones - are usually unable to achieve proper operating temp at low RPM (and often can't hold it down to proper operating temp high RPM either!)

Consequently, lots of folks run the engines up to fairly high RPM periodically to "blow" everything out. Other folks question whether that really does anything useful but it often looks impressive and the smoke cloud may kill a lot of mosquitos. That's not a bad thing! ;)

[REBrueckner]

See Sam's for prior posts on this subject.

I just tested my own 1972 8V71TI's at 1500 RPM and found turbo exhaust temps about 475 to almost 500 degrees...perhaps a bit cool for extended period operation, but not bad. Mike had previously suggested 525 degrees or higher might be acceptable...

My engines are slightly overpropped and turn up about 2150 rather than 2300 at WOT. When I speed up to 1800 or 1900 RPM I get no smoke after perhaps 5 to 10 hours of 1500 RPM operation, slight smoke at around 15 to 20 hours operation at 1500 RPM which clears quickly...maybe 30 seconds maximum...I use 170 degree thermostats.

Four cycle turbos, like Cummins, may have somewhat different operating characteristics and requirements.

[sgharford]

I tried a few searches related to this but could not find anything in depth. Seen various information pertaining to this in other post but nothing solely revolving around the issue. I always thought the biggest deal with not running hard was clogging the exhaust elbow (this came from my sailboat days after I installed an 18 HP Yanmar in my 30’ sailboat. Think someone else was calling these elbows "surge tubes" the other day).

What happens to a turbo if exhaust gas not hot enough? Is it just lose of efficiency and/or do you hurt them mechanically as well?

[dbltime]

It's temperature related. If the engine cooling system will allow the engine to reach proper operating temp at low RPM, then it's not an issue. Car engines can do this from idle on up; boat engines' cooling systems - at least older ones - are usually unable to achieve proper operating temp at low RPM (and often can't hold it down to proper operating temp high RPM either!)

Consequently, lots of folks run the engines up to fairly high RPM periodically to "blow" everything out. Other folks question whether that really does anything useful but it often looks impressive and the smoke cloud may kill a lot of mosquitos. That's not a bad thing! ;)


Jim Stitz the long time J & T service manager said the same thing. Engine runs at design temp you are good to go. Too cool and raw fuel leaves the exhaust after not being combusted in the chamber.

Older Burgers would swap injectors to make the long run to Puerto Rico or Mexico. remove the 105mm and use 90mm for long range. Change the jumper lines.

[DCMY #92]

It is my understanding that low load operation, at least in DDs, does not generate sufficient combustion chamber pressure to force the rings into the cylinder walls with sufficient force. This causes glazing of the cylinder walls and increased blow-by of combustion gasses past the rings into the crankcase. Eventually blow-by deposits form behind the rings and they become stuck. At some point the a ring breaks and wrecks the cylinder.

If the engine is not brought to normal operating temperature (oil temperature) for a sufficiently long period there is not enough heat to evaporate moisture in the oil that comes from the blow-by gasses. The carbon in the fuel burns to form CO2 (and CO if combustion is incomplete). The hydrogen burns to form water. CO2 and water forms carbonic acid. The sulfur in the fuel forms (after a couple of chemical reactions) sulphuric acid.

Increased blow-by of the nasties above from low load operation adds significantly to the normal rate of oil contamination. The additive package in the oil will give out much sooner and allow the acids to start working on the rod, main and cam bearings.

At least on a normally aspirated engine, it is not the RPM that matters, but the load. If you want to cruise for long periods at low RPM, prop up so that the engine is normally loaded and you will avoid all of this trouble. Of course you have to be sure your gear can take the loads imposed at the lower speed (oil pump capacity and hydraulic pressure on the discs) and that the shaft can take the increased torque. Most of the gears and shafts in our boats probably can.

[Brian Degulis]

The water jacket temperature can and should be at operating range at low RPM but your still doing long term harm because the combustion temp is to low. There is no direct link to jacket temp and combustion temp you could be 180 at idle no load with no combustion temp at all. Or you could be 150 at 2300 with high combustion temps. On DDs around 400 is the absolute minimum 475 to 700 is perfect. On 4 stroke engines the ranges are much higher.

Brian

[Nonchalant1]

Detroit Diesel specifically recommends running 8V71TIs at least 900 RPM wherever possible to limit carbon formation in the rings, but of course, it's not always possible.

We put 6,000 miles on our 8V71TIs last year and a lot of it had to be at lower speeds, like 900 to 1,200 RPMs. Based on this forum, I ran them up on plane at about 1950 RPMs at least once per day for at least 45 minutes to get them to operating temps. Of course, there were lots of days when I couldn't, like in the Trent Severn canal, but I did heat them up when I could. It's fun too!

We plan to cruise the Tennessee river system this Fall and then go to Florida in January to April. I'll probably go at hull speed a lot to save fuel, so I'll do the same to heat them up daily.

Doug

[sgharford]

Do I understand this correctly - the reason for running under “load” is important as it corresponds to combustion chamber temperature? Since diesels use a constant volume of air in compression and only vary the amount of fuel sprayed in, I assume the additional fuel needed to run at the same RPM under load is what is causing the increased combustion temperatures. Additionally, more fuel leads to bigger bang which is higher pressure (higher pressure also corresponds to higher temp in fixed volume – whole working principal behind diesels) which leads to more fuel burned completely in combustion, pushing the rings out, and clearing out deposits.

So in order to really pull this off correctly one would need to mount temperature sensors somewhere in the exhaust line, correct? Would you place temp sensors before the turbos, which would seem to make sense to get most accurate reading.

[SKYCHENEY]

Pyrometers are mounted in the exhaust manifold before the turbos. They can give you some great info on your engine. On a vee, you would have one on each bank.

[sgharford]

So their called Pyrometers – thanks for the info. Could anybody recommend a good manufacturer of these for marine use? Sounds like a spot for 4 more gauges would also be necessary. I would be a little worried about drilling and tapping the exhaust manifolds for senders in fear they may crack with thermal expansion/contraction.

I would hate to ruin my 1976 VT903s over trying to save a couple grand on gas.

[Avenger]

Your manifolds may already have a port or ports machined and plugged for pyrometers. I'm not familiar enough with Cumaparts to know for sure, but many engines do have them.

Also, you may not need two. It depends on your exhaust configuration. If you have a single turbo, and a single collection point you can do with one pyrometer.

Another trick: if you do need two thermocouples (the sensor) you can wire them through a selector switch to a single gauge. Less holes in the dashboard.

One manufacturer you can look at is Isspro. www.isspro.com

[MikeP]

Frankly, I think the level of concern is unnecessary. When you consider DDs recommendations (above 900 RPM) it is clear that they were/are concerned about operating temps (water temp), not combustion chamber temp or turbo temp or any other temp.

Certainly, it's not a bad idea to monitor all this if you are inclined to do so but IMHO, such monitoring will have a better chance of making you crazy/worried than anything else.

The cooling system's job (though badly done on marine DDs) is to maintain the overall temp of the engine at an optimum level. If you run at higher loads, the cooling system is supposed to pull whatever heat away it takes to keep the engine at the proper temp. Same is true on the other end - it (is supposed to) restricts the amount of heat it pulls away to maintain the temp.

Very few car/SUV engine ever operate at much more that a fraction of their load capability for their entire lives. Yet there is no issue with combustion chamber temps, turbo temps, etc. The only thing that is important is that the coolant temp - and therefore the engine operating temp - is within the design parameters for the engine.

[REBrueckner]

The purpose in having high enough internal combustion temperatures, as monitored by the turbo exhaust temps, is to insure reasonably complete combustion to avoid carbon build up...and subsequent wear on rings/liners etc.
Pyrometers are very nice monitoring devices to have to continually test operating conditions, but an ocassional check of turbo exhaust temperatures via infra red thermometer is a reasonable alternative.

DD may recommend 900 RPM on 8V71TI's, but when I run my 8V71TI's at 900 or 1,000 RPM for a few hours, when I speed up I notice more temporary smoke than when running in the 1300 to 1500 RPM range...And I get really bored at that slow cruise speed!!!! I don't know that its harmful, but I don't like the smoke feedback when I speed up so I don't run at those low RPMs much.

[sgharford]

Great trick to use switch so you can get away with one gauge. I do have one turbo per engine, I will look for thermo coupler port when I am on boat tonight. With the name “pyrometer” was able to find so additional good info in other post on this site and also google. Apparently pyrometers are a very good thing for truckers pulling big loads to monitor engine/fuel performance. From an older post I see they require no electricity other then the illuminating light for night time use and you can even get the one gauge to give you two simultaneous thermo coupler readings. Looks fairly inexpensive too – think $200 will get you a dual reading gauge and two thermocouples.

I hear what Mike P is saying too – The cooling system, if properly designed, should keep the combustion chamber in a happy place. I am extra worried about my engines as I just bought her, plan on using her (did 20 hours over the weekend), and cannot really afford new engines for her. Unless I hooked alarm up to them as well, sure I will not be looking at them when they are reading a prelude to disaster. This is all good education, knowing factors that can mess up my engines (low engine coolant temp and unloaded running) and what I can possibly do to avoid it outside of running normal cruising speed. Atleast oil came down $4/barrel today (a trend that will hopefully continue with stronger dollar) making the latter more feasible and enjoyable.

[SKYCHENEY]

They make a pyro with two needles in one gauge. You would only need one guage for each engine, but the gauge will show the temps for both banks on it. It is nice to see the temps on both side of a vee. If you see variations in temp from one side to the other it could be time to investigate to see if you have a bad injector, etc. None of this monitoring is necessary, but it sure is nice to have.

[REBrueckner]

sgharford....
a great diagnostic/preventive engine monitor device (to prevent engine failure) is a salt water flow meter...and helm alarm display. It's a pipe fitting insert with a flap valve and a magnetic switch put in the engine salt water flow line....when raw water is present, the flap opens...no alarm; if you lose salt water flow (bad impeller, clogged intake,etc) the flap closes and you get an instantaneous alarm...no waiting for the egine temp to climb before an alarm sounds...I got mine via West Marine, don't remember the brand name....

Nice insurance when you a running in windy/rough weather or anytime you can't hear your exhaust sound.....

[Avenger]

It is nice to see the temps on both side of a vee. If you see variations in temp from one side to the other it could be time to investigate to see if you have a bad injector, etc. None of this monitoring is necessary, but it sure is nice to have.Real engines (i.e. tugboat / ship propulsion, electrical powerplants etc.) have a thermocouple in each exhaust port and one at the turbo, all on a selector switch. Loads of great information. It just depends on how involved you want to get. :)

You may get forced into it too. My 3208s have a dual plenum exhaust collector with one pyrometer port in each chamber. There is no single connection point. :(

[Boss Lady]

The issue to be concerned with on low rpm/low load operation on 2-stroke DDs is a condition called "wet stacking". Actually this condition can affect any diesel. I have only seen it personally occur on generators, where it is very common. In the army we used load banks to keep the generators loaded to at least 50% load at all times and never had a problem, until the army discontinued using load banks in the late 1980's. When we lost our load banks we had large generators that may only have 10-30% loading and we started having wet stack problems and cylinder wall glazing.

Low combustion chamber temps results in incomplete combustion products building up and creating a sludge that increases in viscosity over time, it also builds up in the exhaust system. This sludge can cause head gasket failures, and in extreme cases broken rings.

If you pay attention you should be able to avoid having problems, while it is not something to be alarmed about, you should still pay attention to the fact that it can occur under the right (wrong) conditions.

My opinion on this topic from experience is that most of the real problems I have seen are from light loads at high RPM as found in generator applications. I don't really see this being a serious problem in a boat being operated at low rpm, because there is some load there, but I would stress that you should operate at an rpm to get to at least the low side of the temperature range.

Maybe someone has actually experienced this in a similiar loading/rpm situation and can shed more light on this.

[Brian Degulis]

So their called Pyrometers – thanks for the info. Could anybody recommend a good manufacturer of these for marine use? Sounds like a spot for 4 more gauges would also be necessary. I would be a little worried about drilling and tapping the exhaust manifolds for senders in fear they may crack with thermal expansion/contraction.

I would hate to ruin my 1976 VT903s over trying to save a couple grand on gas.


Here you go
http://www.hewittindustries.com/instruments.htm

Brian

[Brian Degulis]

Frankly, I think the level of concern is unnecessary. When you consider DDs recommendations (above 900 RPM) it is clear that they were/are concerned about operating temps (water temp), not combustion chamber temp or turbo temp or any other temp.

Certainly, it's not a bad idea to monitor all this if you are inclined to do so but IMHO, such monitoring will have a better chance of making you crazy/worried than anything else.

The cooling system's job (though badly done on marine DDs) is to maintain the overall temp of the engine at an optimum level. If you run at higher loads, the cooling system is supposed to pull whatever heat away it takes to keep the engine at the proper temp. Same is true on the other end - it (is supposed to) restricts the amount of heat it pulls away to maintain the temp.

Very few car/SUV engine ever operate at much more that a fraction of their load capability for their entire lives. Yet there is no issue with combustion chamber temps, turbo temps, etc. The only thing that is important is that the coolant temp - and therefore the engine operating temp - is within the design parameters for the engine.

The reason you don't see low RPM problems on cars or trucks is because they have transmissions that change drive ratios to properly load the engine at lower RPMs. Our boats don't in addition to that there is the inherent diffrence in the way a prop absorbs power and the way the engine produces it. A engine operating at 900 RPM turning a fixed pitch prop is only producing around 20 percent of the power available at 900 RPM so it's very lightly loaded and without a proper load combustion is in complete at any RPM. The problem is not RPM it's load but in boats low RPM translates to low load. Generators operating at a fixed RPM properly loaded have long lives lightly loaded have very short lives. This is true even though in both cases water jacket temps are within range.

If you planing to run slow for long periods get a pyrometer ( or check it with an infra red gun)so you know what's going on. Use a Cetane booster in your fuel. Consider larger props but understand if you go that route you will no longer be able to use the full HP of the engine.

Brian

[Boss Lady]

I have to agree with Brian. Trucks do not run continuously at light loads, they start and stop, shift through constantly varying loads as terrain changes. I would not use light trucks used as transportation as a measuring stick either. These modern engines run very hot coolant temperatures compared to our DDs.

I would not make any assumptions about low rpm operation based upon anacedotal evidence. Especially since DD warns against low rpm operation and always has. There are people who operate at low rpm without apparent problems. This however is not conclusive evidence, it is their experience doing so, and I would recommend searching out as much information on the subject as possible, and then decide if this is a risk that makes sense for you.

Repropping to place more load on the engines at lower rpm makes the most sense and will ensure that the engines at least reach proper operating temps and a higher percentage of load. I think this is a step in the right direction if one intends to operate at low rpm for long periods of time. My humble opinion.

[Genesis]

1000-1100 RPM all day long for extended periods of time is fine.

Idle is NOT.

If the temperatures stay in range (>=170) you're ok. The cooling system is regulating the temperature as it is designed to. When you get into a situation where the thermostats are fully closed and the water temperature continues to fall (e.g. below 170F or so on the coolant) then you're in trouble because the engine is producing insufficient heat to require the cooling system to draw any of it off.

Insufficient heat for proper combustion is a serious issue, but we're not talking about operating there. If you do this run it up to cruise (1800 RPM) once every 3-4 hours and I bet you see anything you burn off clear within 30 seconds or so - if so, there's no issue.

Engines need to have enough load put on them to reach proper internal operating temperatures. If the cooling system is being forced to work that is taking place.

BTW aftercooled (as opposed to intercooled) engines do a better job of regulating internal temperatures due to the fact that the aftercooler is in the fresh water system.

For those who say that car diesels, for example, don't run for long periods of time at very light power settings - they're simply wrong. My Jetta, on a flat road (Midwest, etc) tooling along at 60mph is running at less than 20% of its rated output, measured by the fuel consumption. When I take long trips I will get to that speed and set the cruise, running there in some cases for hours at a time. Its fine.

[Brian Degulis]

For those who say that car diesels, for example, don't run for long periods of time at very light power settings - they're simply wrong. My Jetta, on a flat road (Midwest, etc) tooling along at 60mph is running at less than 20% of its rated output, measured by the fuel consumption. When I take long trips I will get to that speed and set the cruise, running there in some cases for hours at a time. Its fine. (quote)

Your mis understanding Your Jetta probably is running around 20% of the rated output but it's running around 80% of the rated output FOR THAT RPM. That is the diffrence between a car and a boat without the ability to change drive ratios a boat @ 1000 RPM is only using around 15% of the power available @ 1000 RPM.

Here are some #s from the DD power/ propeller curve for a 1271TI 650 HP @ 2300 RPM

At 1200 RPM the engine is capable of producing 375HP The propeller load with OEM props @ 1200 RPM is 60 HP. So only 16% of the HP available is being used and only around 9% of the total rated output is being used. That's just way to low for a complete burn even if the jacket temp is OK.

Brian

[Nonchalant1]

DDs 8V71 manuals indicate that for some uses, such as large generators, 160 degree thermostats are to be used, so I'd say DD designed the engines to be run at 160 degrees for long periods of time. Generators can run 24/7 and only have large loads for a few hours of those 24, while the RPMs will stay constant. I'd have no problems running 900 RPMs and 160 degrees for long periods. I ran at 900 or so for days with no ability to run them up in the Trent Severn canal.

Doug

[REBrueckner]

Brians comment about the 650 HP is right on...

OEM for my 8V71TI at 1500 RPM is about 125HP, capability (rated maximum SHP) is about 325HP at 1500 RPM. I'm overpropped by about 150 RPM, so my actual HP might be closer to 150 at 1500 RPM...yet my turbo exhaust temp is only 475 to 500 degrees...ok, but no way I could get enough exhaust temp at the low outputs Brian references....say 1,000 RPM on my engines, even though my coolant temperature is at 170 degrees....and at these lower RPM's, as I posted here already , I get extra initial smoke when speeding up from lower RPM's, proof that combustion is not as complete as desired....

[GJH]

When I had my 8V92TIs surveyed last year by reputedly one of the best DD guys in Maryland (Tom Hug of MR Power), I asked him during the sea trial about engine performance at various RPM as we ran up and down the scale from idle to WOT. He had all his own gauges hooked up directly to the engines. So at one point we were running at 1000 RPM, and I went down and asked him about running at that speed, or 900 his response was "do that and they'll out live all of us!" So my next question was "OK how long do I have to live?" When I asked about running them up and blowing them out every so often, he said merely that "it helped". His strong opinion was that the very worst thing you could do was idle them with no load.

My mechanic in North Carolina, also of good repute (Jim Oberci of Oberci Diesel), basically says the same thing, but is a bigger advocate of running them up to 1800 for a little while towards the end of the day.

So, for our trip from Baltimore to Marathon and back we usually run somewhere between 1000 and 1200 (gives us 8-9 knots depending on current and conditions) and do the blow out thing at end of day. The starboard engine, which now has 660 hours SMOH, doesn't smoke at all. Port, with about 2000 hours, smokes for about 5 seconds then clears up. On days where we have had to run at 1400 to 1600 mostly (ouch!), there is no smoke at the 1800 to 2000 mark.

As an aside, if I use the block heaters over night neither engine smokes at start up. Without the BHs, starboard usually nothing, and port some for about the same 5 seconds.

The temperature specs for my engines are 165 -185 degrees; mine are usually in the 170 to 180 range.

George

[Genesis]

What I can tell 'ya is that of people who have run these engines in the 1000-1200 RPM range as their "usual operating practice", and I've had a chance to peek inside, there ain't a thing I'd complain about.

The "gotta let the dog eat" crowd, on the other hand, well, if you like overhauls on 1200 hour intervals then that's just fine.

[bobk]

I bought my 1981 48MY with about 400 Hrs SMOH, and have since added a good seven hundred hours at 1000-1100 rpm with periods at 1200-1300 to make bridges etc. After one 10-12 hour day at 1000 rpm, if I crank to 1900, the smoke will clear in less than a minute (probably 30 seconds), clearly before the engine reaches 180F. If I can't, or forget to crank them up, after two days at low RPM it might take up to three minutes to clear the smoke.

QUESTION: How long should I continue to run after the smoke clears to be sure rings etc will be OK?

Bob
Chateau de Mer
1981 Series I MY

PS, agree Tom Hugg is one of the best DD guys in the Baltimore area.

[Genesis]

A few minutes, 5-10 maybe.

[Brian Degulis]

When I had my 8V92TIs surveyed last year by reputedly one of the best DD guys in Maryland (Tom Hug of MR Power), I asked him during the sea trial about engine performance at various RPM as we ran up and down the scale from idle to WOT. He had all his own gauges hooked up directly to the engines. So at one point we were running at 1000 RPM, and I went down and asked him about running at that speed, or 900 his response was "do that and they'll out live all of us!" So my next question was "OK how long do I have to live?" When I asked about running them up and blowing them out every so often, he said merely that "it helped". His strong opinion was that the very worst thing you could do was idle them with no load.

My mechanic in North Carolina, also of good repute (Jim Oberci of Oberci Diesel), basically says the same thing, but is a bigger advocate of running them up to 1800 for a little while towards the end of the day.

So, for our trip from Baltimore to Marathon and back we usually run somewhere between 1000 and 1200 (gives us 8-9 knots depending on current and conditions) and do the blow out thing at end of day. The starboard engine, which now has 660 hours SMOH, doesn't smoke at all. Port, with about 2000 hours, smokes for about 5 seconds then clears up. On days where we have had to run at 1400 to 1600 mostly (ouch!), there is no smoke at the 1800 to 2000 mark.

As an aside, if I use the block heaters over night neither engine smokes at start up. Without the BHs, starboard usually nothing, and port some for about the same 5 seconds.

The temperature specs for my engines are 165 -185 degrees; mine are usually in the 170 to 180 range.

George

At 1000 RPM you will have 0 turbo boost and your stack temperature will be somewhere between 200 and 300 degrees. To make matters worse your TI engines have 17:1 kits in them so your compression is around 90lbs less than a natural. So your running like a natural that's beat to death and your pushing un burned fuel thru the engine that's not my opinon that's fact. If your mechanic thinks that's just dandy I'd find another mechanic or better yet talk to an aplication engineer they understand this stuff mechanics don't.

I'm not saying your going to destroy an engine in a week or that you shouldn't run this way that's up to you. But ignoring obvious facts and figuring I do it and nothings gone wrong or this guy said this or saw that just is not the way to make an informed decision.

In my opinoin if your going to run slow most of the time then go to larger props or at a minimum add some pitch. Your engine will be running in a much better state and you will save fuel. (larger props on my boat improved fuel consumption by around 30% at 10 kts) use a cetane booster or an additive that contains one (Diesel Kleen). And run them up once a day enough to bring the exhaust temp to at least 700 degrees. If you do regular oil analysis you will see the diffrence that increasing the load will make inside of 50 hours.

Brian

[MikeP]

Again - I think we are severely over-worrying about this issue. To paraphrase Krush (as I recall), when did an engine ever fail from running slow? Never heard of that happening. We can talk about all the "bad" things that occur when motors are run too slowly. And the truth is that those bad things (carbon, etc) DO happen but the reality is, the engines just keep running.

Surely no one would bet that an 8V71TI, for example, run at 1800 RPM with all the good temps/load, etc, would actually outlast an 8V71TI run at 1100 RPM? If you'd bet on that, your just going to lose money! ;)

[Brian Degulis]

Again - I think we are severely over-worrying about this issue. To paraphrase Krush (as I recall), when did an engine ever fail from running slow? Never heard of that happening. We can talk about all the "bad" things that occur when motors are run too slowly. And the truth is that those bad things (carbon, etc) DO happen but the reality is, the engines just keep running.

Surely no one would bet that an 8V71TI, for example, run at 1800 RPM with all the good temps/load, etc, would actually outlast an 8V71TI run at 1100 RPM? If you'd bet on that, your just going to lose money! ;)

If that engine is in a typical boat I'd take that bet in a heartbeat. If it was a natural or if you brought it up to 2000 I'd start thinking about it but 1800 on a TI that's going to be about 70% of rated load perfect.

Brian

[bobk]

A few minutes, 5-10 maybe.

Thanks Karl. that's about what I have been doing.

Bob

[GJH]

Well I just had the engines supposedly completely checked out, after over 300 hours running this way, and the turbos were just fine. The injectors were fine. Oil analysis just fine. They don't smoke (of any color) or show any other signs of distress. Fuel consumption is in line with published figures. Mechanical judgement rendered by people who know I won't be an ongoing customer, so no motivation to BS me to get rebuild/repair biz later. Not only from people that have actually opened up the engines, but informal polls of numerous mechanics at various DD parts departments or around the docks working on other boats. So...

I'm still trying to find a mechanic that has had to deal with engines ruined by running as "slow" and in the manner as I am. Can anyone here find me one to talk to? And when I find this mechanic what exactly is he going to look for and find that several different guys (not just those mentioned) who have done this for 30 years are over looking? I am not asking that cynically. I can't find anything in DD's documentation either that says anything I am doing is wrong or outside of their specs.

I understand (but don't know) that 1600 is perhaps "better", but the issue is that 1000-1200 is not ruinous, and that in this day and age, the fuel savings will more than cover the theoretical (but to me unproven) cost in engine life. I am on track to buy about 6000 gallons of diesel in this operating year, so I could just about pay for a MOH running at 10 gph instead of 20. But there is no indication that the engines are on the road to needing one.

In the meantime, I am going to have to go with the pros I have dealt with so far and the actual physical evidence at hand.

George

[Brian Degulis]

I'm still trying to find a mechanic that has had to deal with engines ruined by running as "slow" and in the manner as I am. Can anyone here find me one to talk to? And when I find this mechanic what exactly is he going to look for and find that several different guys (not just those mentioned) who have done this for 30 years are over looking? I am not asking that cynically. I can't find anything in DD's documentation either that says anything I am doing is wrong or outside of their specs.

Your turbos or injectors will suffer no harm from running cold and slow you will see a negative diffrence in oil analysis and over time a bore scope inspection will clearly show the efects. If you guage the engines out and bring the readings to an aplication engineer at DD they will tell you it's not a good state to run an engine the information is there but you have to look in the right places. I don't think anyone on this forum has suggested it's going to wreck your engine in 300 hours only that it will do long term harm. What surprizes me is that a guy like you who obviously wants to take care of his boat would choose to ingnore a problem that is so easily corrected.

You know I'm sure that when you change your oil you make sure that your using the right stuff. But the simple truth is that if you didn't if you used a multi weight your engines would still check out just fine for a long time. So why take the time and trouble to get the right stuff when your not seeing any harm?

Brian

[Brian Degulis]

I'm still trying to find a mechanic that has had to deal with engines ruined by running as "slow" and in the manner as I am. Can anyone here find me one to talk to? And when I find this mechanic what exactly is he going to look for and find that several different guys (not just those mentioned) who have done this for 30 years are over looking? I am not asking that cynically. I can't find anything in DD's documentation either that says anything I am doing is wrong or outside of their specs. (QUOTE)

Your turbos or injectors will suffer no harm from running cold and slow you will see a negative diffrence in oil analysis and over time a bore scope inspection will clearly show the efects. If you guage the engines out and bring the readings to an aplication engineer at DD they will tell you it's not a good state to run an engine the information is there but you have to look in the right places. I don't think anyone on this forum has suggested it's going to wreck your engine in 300 hours only that it will do long term harm. What surprizes me is that a guy like you who obviously wants to take care of his boat would choose to ingnore a problem that is so easily corrected.

You know I'm sure that when you change your oil you make sure that your using the right stuff. But the simple truth is that if you didn't if you used a multi weight your engines would still check out just fine for a long time. So why take the time and trouble to get the right stuff when your not seeing any harm?

Brian

[REBrueckner]

One of the difficulties is these discussions is that we are NOT talking industrial/commercial annual use here. For the one hundred to five hundred or so hours many of us run our engines annually, anybody would be hard pressed to wear an engine out unless it was really run WOT all the time or idled all the time. An mechanics, as noted, are not engineers and only see what they are told by owners..and most owners, present group excepted, have little idea what they are doing.

Let's face it, most of our engines suffer more from a lack of use, sitting unused in the damp humid marine environment for weeks and months, sometimes for six months in freezing conditions, than from actual running time.

[sgharford]

This may be material for new thread (I will start one if no one answers this here) - Brian made mention that using a multi-weight oil will not give you accurate oil analysis.

Local Cummins (truck) shop where I bought oil told me to use 15w-40 even though specs call for 30w (I have 1976 Cummins VT903's). He said multi-weights were not even invented in 1976 and that I should unquestionably use multi-weight (which I currently am - Shell Rotella brand).

[Genesis]

For 903s that may be true but DO NOT run multigrades in a Detroit 2-stroke.

The issue is film strength. If you get unlucky (the highest risk of damage lies with the camshaft) its a bitch as the camshaft is long enough that getting it out with the engine in the boat is usually impossible.

Detroit 2-strokes also get very pissy about suflated ash, and will build valve deposits with sulfated ash figures over 1.0%. This takes hundreds or even thousands of hours to cause trouble but it DOES happen; I've seen the results. The problem is that TBN is basically impossible to jack up without also jacking sulfated ash, and most 4-stroke oils have a lot of TBN boosters in them (they counteract acidity from the sulfur in fuel, which has now been mostly eliminated - but used to be a very big deal.) This is a trade-off that you can't make in a Detroit.

There IS a difference between a Cx-II and Cx-4 rating on engine oil.

[GJH]

Brian, the oil analysis showed no adverse result from the one taken 300 some hours ago; it almost looked like the same report (used DD's Power Trac service) Summary was "no corrective action required." I plan on having analysis done with every change from now on. I am not trying to argue with you, I am looking for definitive facts to steer me the right way.

Perhaps our actual mix of running RPMs is higher than I am characterizing. Allowing for the generator burn, we have averaged about .85 statute mile (.75 nautical) per gallon out of the two engines combined. I have got 1mpg when running more frequently in the 1000
Q1I890;LK-1200 range.

I just had the props redone; three out four blades on each need some reshaping. I look forward to how that may affect performance. I have kept the bottom and the running gear clean. Other than one 100 hour cycle on Kendall Super D30, based on a recommendation here, the boat has been on 40W CF2 Rotella or Detroit Power Guard. Filters either NAPA, Detroit or Baldwin.

So I am trying to do everything I can and then some to take care of these babies religiously. It just seems we all read a slightly different version of the Bible sometimes.

George

[Brian Degulis]

George if you want definitive facts you have to look in the right places it's not a different interpretation of the bible the problem is that you have not yet read the bible. Matching engines to transmissions and props is a large part of what I do for a living. It's not overly complicated but the information is not readily available either mechanics know almost nothing about it there job is to diagnose and repair not design.

Lets try a different approach. When Hatteras decided to build your model of boat and I'm not sure of what boat you have I'm going to assume MY. At some point in the design phase they sat down with application engineers for Detroit Diesel. They said we are going to build this boat it's a planing hull and we are figuring a design speed of 19 kts. We estimate that will require around 1200 HP and we want to use 892TIs the engineers felt it was a good match and approved it I've over simplified and the #s are a guess but you get the idea.

Now if they had come to the engineers with the same boat with a design speed of 9 kts they never would have approved those engines cause they are way to big. They would have recommended a pair of 6 or even 471 naturals cause you only need around 200 hp or less to make displacement speed. An assumption was made by all that a planeing hull would be run at least most of the time on plane. Those engineers would have said that those 892TIs will be to lightly loaded for proper operation. The simple fact is that your engines are much to big for the way you want to use your boat.

Now you can ignore all this and pretend that everything is fine based on the opinions of those that are out of the realm of they're expertise. Or you can gather good information and bring it to the right people and get those definitive facts you say your looking for.

Don't get me wrong I'm all for going slow I might be the biggest advocate for going slow on this forum. But I believe in doing correctly 1000 RPM is very slow if we were talking 1200 to 1400 I think you could get away with that. But at 1000 you really should see what your exhaust temp is running I suspect it will be under 300 degrees. If that's the case then you should consider re engineering specifically larger props or more pitch to get a little more load you might be pleasantly surprised at the results as far as efficiency and performance are concerned.

Brian

[garyd]

Interesting discussion. The DDEC system runs only 3 of the six cylinders. I'm guessing they alternate the three. This happens all the way up to almost 1500 RPM. So it looks like they were or are trying to accomplish the small diesel correctly loaded at low RPM and the big Diesel correctly loaded at the high RPM.

Since there is a cycle where there is no fuel injected any unburned residual fuel from the previous up stroke gets scavanged on the next up stroke. Maybe DDEC's are really a GOD send in this go slow society.

Garyd

[bobk]

This discussion brings up the question of fuel additives. If without an additive you get some transom soot at low rpm, I'd interpret that to mean combustion was incomplete and you'd have a greater potential to foul rings etc. If you use an additive that eliminates the soot, would you not think there was better combustion and as a result a reduced potential to foul the innards at the low rpm we are talking about? Regardless of exhaust temperature? Comments?

Bob

[Genesis]

DDEC does that to try to improve the scavenging at lower speeds (no boost); its a fuel and emissions game, not a combustion completeness one.

It was one of the many things that Detroit tried to meet the emissions regs with the 2-stroke engines, but they were never able to succeed.

[MikeP]

I completely agree with Brian and his speculation of how the engines for these (or any) boat are specced. Certainly, if these boats had been designed to run at the speeds that most of us are now running them, they would have had small engines and Jack Hargrave would have designed a different hull shape. Like he did when they made the LRCs.

Most people I have met with 53/58MYs say they used to run them almost exclusively on plane and that's what they were designed to do. So I agree that running them slow and off plane is a major shift out of the design envelope.

The part where I take some issue is the actual effect of running them slow. And I guess "Slow" needs to be clarified. I think of slow as a 9-10k speed which, on our 8v71TIs is around 1400-1500RPM. I don't believe that there is the slightest negative effect on engine life by running at these speeds for as long as you care to do so.

On our boat, if we run at 1400 RPM for several hours and then go to WOT, there is no cloud of smoke of any kind as the RPM comes up to WOT. However, when we motored out of our Marina in Northport, it was about 30 minutes of no wake speed - maybe 700 RPM. If, when we cleared the no wake area, we went to WOT, there was a HUGE cloud of blue/white smoke for maybe 10 seconds.

So I agree that very slow running is a bad thing - which DD says in the service and operators manual. But cruising at hull speed or thereabouts? With proper maintanance I'm not sure you could wear out an 8V71TI in your cruising life! ;)

[Brian Degulis]

This discussion brings up the question of fuel additives. If without an additive you get some transom soot at low rpm, I'd interpret that to mean combustion was incomplete and you'd have a greater potential to foul rings etc. If you use an additive that eliminates the soot, would you not think there was better combustion and as a result a reduced potential to foul the innards at the low rpm we are talking about? Regardless of exhaust temperature? Comments?

Bob

An aditive with a cetane improver helps a lot

Brian

[Brian Degulis]

The part where I take some issue is the actual effect of running them slow. And I guess "Slow" needs to be clarified. I think of slow as a 9-10k speed which, on our 8v71TIs is around 1400-1500RPM. I don't believe that there is the slightest negative effect on engine life by running at these speeds for as long as you care to do so. (quote)


Mike the exhaust temp tells the story but I'm just about certain that at 14 RPM your OK. The discussion was based on 900 to 1000 RPM.

Brian

[Genesis]

Just make sure you're looking at the exhaust temp AT THE PORT.

Oh wait, you're not - and the manifolds are water-cooled, plus you're producing a huge amount of over-scavenging due to the blower at low power output.

Uh yah.

200-300F at the port eh? Uh uh. Not at 1100RPM you're not.

A couple of knots over hull speed on my 45C (~10-11kts) was right around 1100 RPM. I bet you wear out before the engines do at that power setting.

Let's not forget the original design output for these engines either. It ain't 500HP.

[Brian Degulis]

Well if you believe that then you would have to believe that a 20KW gen set operating continously with a 3 KW load is just going to out last us all? Cause the percentages are about the same that 20KW gen set running at 3KW percentage wise is the same as your 8V92TI driving a prop at 1100 RPM. Now if you call up Northern lights or onan or any of them and tell them you want to do that they're going to tell you don't. They're going to tell you that that unit will be to lightly loaded it's going to carbon up and wet stack and have a very short life. And with a typical gen set your talking about a naturaly aspirated 4 cycle engine which is going to tolerate light loading a whole lot better than a turbo DD.

Brian

[Genesis]

Well if its carboning up and wetstacking when run that way how come I'm not seeing it when I look?

I recently had a buddy's boat that I swapped injectors in - he's run in this operating environment now for several years, and basically NEVER kicks it up other than to clean things up at the end of the day. Right around 1100 is where he runs - all the time.

The cylinders looked like they were brand new, rings and grooves great, etc.

Where's the problem?

[Brian Degulis]

Karl I can't explain one engine run by one guy and inspected by another. But if you have the experience working on these engines that you say you do then you know as well as I do that lots of things can be wrong and the effects may not be obvious. Iv' e removed cylinder heads to correct burnt valves and found cracks. Should I just do the valve job and slap them back on cause the cracks weren't causing problems? I've seen engines run for years on the wrong oil and they look fine should the owner just keep using the wrong oil cause he's not seeing any problems? Or would recommend that he go by the specification and the known facts?

If your smart your decision as to how your going to run your engines should be based on fact not this guy said this or that. If your running out of design parameters you gauge it out see what it's doing look at other applications check with the manufacturer and make an informed decision.

Brian

[Genesis]

Absolutely Brian.

My problem with the pontification that running at 1100 RPM is "damaging" is that when I HAVE gauged it out I don't see a problem, and I also don't see a problem when I look into an engine that has some 1500 hours on it in precisely this operating regime, with an unknown number before that (who knows whether the hobbs are accurate or if the claimed history before the guy who owns it now bought it is real)

This is a boat and owner I know very well. He's not bullshitting me. There's absolutely nothing wrong with these engines in terms of cylinder condition.

Based on the fuel consumption (12gph @ 11kts) we're pulling ~100 HP out of each of these engines. The original design horsepower for the 6V92TTA is in the 300-350HP range @ 2100 RPM. They were "cranked up" for recreational marine use; their marine rating is not a valid place to look at what's up with them in terms of percentage of design load, as there were no changes made to the internals or turbocharging - only the injectors were changed. This, in fact, is one of the reasons that these engines have a habit of going "boom" due to overheats - the cooling systems were NOT enhanced materially when the fuel flow was increased, significantly reducing the safety margins. The same thing applies to the 6-71s and the rest of the DD line.

Prop demand curves are not dispositive - fuel consumption is the best and final arbitrer of the actual power produced. Diesels make about 10 hp for every 0.6gph of fuel consumed. If we're burning 6gph per engine we're pulling about 100HP.

So based on this we're pulling 30% of original design load and likely 50-60% of possible horsepower at that RPM. After-turbo pyrometers are not dispositive of stack temperatures in this operating mode, but they show temperatures in the 500F range - if we do the "by feel" assumption you're seeing 700F at the ports, cleanly into the "normal" range. Water jacket temperatures are right up the middle - 170F, which on 160 stats means they're partially open and regulating the engine temperature.

What's the problem? The engines run cleanly, there is no carbon buildup in the cylinders or ring groves, the crowns look ok, no airbox deposits of materiality, the cylinder condition is fine and the gauge numbers that we can obtain display no cause for alarm.

[GJH]

Thanks Karl for your insights. The point about the injectors is a good one. Certainly we see many more instances of short engine life spans due to "over horse powering " as well as over loading and running at the pins.

So it seems we are playing a little game of Goldilocks here to figure out what's just right. My confusion lays solely with the fact that it is harder for me to find cases of engine failure due to running in that 1100ish range, but I may just not be hanging around the right crowd.

Looks like I picked up about half a knot average from the prop tune up. She made WOT of 2300 rpm, but without a lot of fussing with the tabs, only does about 14.5 knots through the water at WOT. Admittedly, I did not spend much time playing around at WOT. Regardless, this is not a go-fast boat.

Brian the boat is a 1981 56MY, which has the 18'2" beam hull form. She just weighed in at 90,000 lbs with full fuel and water tanks.

I don't think a generator is a good analogy at all. A generator has to run at a constant RPM, so under loading it is something like running your boat in neutral at 1100 rpm, something everyone agrees is a bad thing. We have a (too big, in my opinion) 20 kw Onan that we do load up gratuitously when it has to run.

Now I am going to ask something that even I suspect might be dumb, but let's throw it out there anyway. Aren't the engines the HP they are so the boat can go fast? Wouldn't the designers take into account that the boat would be spending many hours in no wake zones, waiting for bridges, or in conditions where running on plane was ill advised?
What's the "average" RPM assumed in the design?

It may well be that in actual overall use, Karl's buddy and I are operating within that spec. Time will tell. Assuming Brian is completely right (despite the evidence so far), at the savings in fuel cost, I can afford the occasional premature MOH, but given the attention and testing the engines are getting, we can hopefully head that off at the pass.

George

[REBrueckner]

Is there any advantage to using smaller injectors when planning to run for the forseeable future at significantly lower speed to save fuel.
Smaller injectors would require some increased RPM to reach the same reduced HP as larger injectors...would that help alleviate concerns such as lower turbo boost,etc....or would smaller injectors cause other problems???

[Genesis]

Rob, no.

Actually, its exactly the opposite, believe it or not.

The IDEAL circumstance for a mechanical injection engine (where you can't really have multiple injection events as you can with an electronic pizeo injector) is that you want ALL the fuel in the cylinder instantaneously with just enough "lead" to TDC that the pressure rise occurs exactly as the piston begins its travel downward.

In the real world injection does not happen instantaneously, ignition does occur instantaneously and with a mechanical engine you can only be exactly correct on your injection timing with an exactly-known load and RPM.

A very significant percentage (in fact, most!) of the gains in fuel efficiency that are had by electronic control are found in the ability to map injection timing more exactly to load at any given RPM, thereby optimizing exactly when ignition occurs. This results in greater power delivered to the crankshaft for a given amount of fuel consumed, more efficient combustion, and less wasted energy going out the exhaust.

As a consequence there is no real advantage to a smaller nozzle if you never intend to use all of the capability, and in fact depending on the particular injector and its fuel delivery curve it may be the exact opposite, as you may get closer to "ideal" with the bigger squirter.

Going to a deeper prop to place more load on the motor is an alternative BUT if you're going to do that I would STRONGLY recommend cutting down the maximum governed RPM at the same time to prevent trying to load the engine beyond where it can safely operate. This, of course, removes the flexability to use all of the horsepower that used to be available.

IN GENERAL I see no benefit to tampering with the prop or injectors for this sort of use (e.g. 1100-1200 RPMish) at all for most of these boats. By today's standards these vessels are underpowered and you're going to be in the sort of range I was talking about before.

The trick here is to figure out the engine's output as a function of fuel consumption, and instrument to the best of your ability. If you are within normal operating parameters given that, and for most of these boats with OEM power you will be in the 1100-1200 RPM area, then I doubt very much you're going to see any sort of difficulty. As a protective mechanism against possible trouble running up to 1800 for 10-15 minutes at the end of each day of 1100 RPM operation is cheap insurance, but its also likely unnecessary insurance.

[REBrueckner]

Running main engines at low RPM is an effort to save some money on fuel...and maybe one side benefit is more time cruising in a relaxing and ...is one tool to try to offset higher fuel costs. It works mostly because the boat (hull) is more efficient at slower speeds.

Does anybody know if fuel consumption per HP varies much over a reasonable operating (RPM) range?? I would suspect not, but it could. But in general HP/fuel consumed is not magic: want more HP? You have to burn more hourly fuel to obtain the power...

Another tool is a prop scan (is that a brand name?) or any other computer aided tool for tuning up props...that should aid efficiency a bit but isn't a big deal for only 100 hours or running annually. (But if you can gain say 2% efficiency, 2% at 500 hours and say 20 gals/hour saves 200 gal/yr, maybe $1,000 bucks and that's good payback!!)

A third tool is increasing the diameter of props, maybe increased pitch helps a bit as well: Did we ever reach a consensus on increasing pitch resulting in a MORE efficient prop...I don't remember...
Another trick, which I posted separately, is to reduce your genny run time by matching charging capacity to battery bank size...and battery type...And of course a related approach is to charge batteries and heat water, for example, when you cook rather than run the genny separately for a longer period.

Above Karl says smaller injectors offer no real fuel efficiency advantage....I don't understand the pros and cons, but I do know you can put in larger (oversize) injectors in my 8V71TI's and boost HP...maybe a few other changes are required as well...My mechanic did that, and I posted several years ago about it, in a 1970's vintage 46 Bertram SF...The owner told me he got another three knots...roughly 24 to 27 knots WOT, or therabouts...My mechanic recommended against it for me, saying it would offer speed but reduced engine lifetime, which makes sense.

I'd be really, really cautious, as Brian suggests, about a few years observation, anechdotal stories, local mechanics intuition or a few hundred hours of tests...that's NOT the way to make longer term engine operating decisions. For example, in Nova Scotia, all the commercial boats with two cycle Detroit's use 15W-30 lube oil...I spoke with at least a half dozen crews/captains/mechanics and they said it works fine...but Detroit Diesel says don't do it, so I'm not switching from straight 40 weight...

Also, what about running with slightly warmer thermostats for lower RPM cruising....Apparently some of you have 160 degree thermostats, I use 170 degree...because my mechanic installed them during major work....One was a 180 in error, which I changed out two years ago...Is 180 a better choice?

Any other fuel saving tips??????????????????????

[MikeP]

Per DD for the serial numbers of my 8V71TIs, they were delivered with 160 Tstats and that's what's in there now though when we bought the boat, one engine had 180's and the other had 170's. Go figure...

If this was a gas engine, with which I have a lot of experience on every level of engine building from stock to mild to competition, I would have immediately put 180's in both engines which would make for better eng efficiency than 160's. But based on input here, I went with the oem 160's. However, when the port eng had 180's, the temp at cruise (mechanical gauge) held at 195, which is within the specs of a 180 Tstat.

I know that a lot of folks say 195 is too hot for these engines. Although my experience with gas engines would lead me to disagree with that, my lack of equal experience with DDs makes me reluctant to argue the point.

However, I have been saying for months that I am going to put 180's in; I have just never got around to doing it. I am confident that it would improve efficiency in the RPM range we currently use. Whether it would be satisfactory at planing speeds would be a function of how clean the RW system is. It is obvious to me that, unlike most automotive engines and as Karl has often pointed out, these marine DD cooling systems have nearly no excess cooling capacity to handle warm water temps, etc at WOT.

My gut feeling is that engines are engines and that the DDs would operate best at around 195 but obviously, 195 is not too far from a dangerous temp and any shortcoming of the cooling system could put you up there quite rapidly...

[Brian Degulis]

Absolutely Brian.

My problem with the pontification that running at 1100 RPM is "damaging" is that when I HAVE gauged it out I don't see a problem, and I also don't see a problem when I look into an engine that has some 1500 hours on it in precisely this operating regime, with an unknown number before that (who knows whether the hobbs are accurate or if the claimed history before the guy who owns it now bought it is real)

This is a boat and owner I know very well. He's not bullshitting me. There's absolutely nothing wrong with these engines in terms of cylinder condition.

Based on the fuel consumption (12gph @ 11kts) we're pulling ~100 HP out of each of these engines. The original design horsepower for the 6V92TTA is in the 300-350HP range @ 2100 RPM. They were "cranked up" for recreational marine use; their marine rating is not a valid place to look at what's up with them in terms of percentage of design load, as there were no changes made to the internals or turbocharging - only the injectors were changed. This, in fact, is one of the reasons that these engines have a habit of going "boom" due to overheats - the cooling systems were NOT enhanced materially when the fuel flow was increased, significantly reducing the safety margins. The same thing applies to the 6-71s and the rest of the DD line.

Prop demand curves are not dispositive - fuel consumption is the best and final arbitrer of the actual power produced. Diesels make about 10 hp for every 0.6gph of fuel consumed. If we're burning 6gph per engine we're pulling about 100HP.

So based on this we're pulling 30% of original design load and likely 50-60% of possible horsepower at that RPM. After-turbo pyrometers are not dispositive of stack temperatures in this operating mode, but they show temperatures in the 500F range - if we do the "by feel" assumption you're seeing 700F at the ports, cleanly into the "normal" range. Water jacket temperatures are right up the middle - 170F, which on 160 stats means they're partially open and regulating the engine temperature.

What's the problem? The engines run cleanly, there is no carbon buildup in the cylinders or ring groves, the crowns look ok, no airbox deposits of materiality, the cylinder condition is fine and the gauge numbers that we can obtain display no cause for alarm.

Karl

If you had the readings all along why did you wait so long into the discussion to produce them?

Your thinking is flawed you can't take some original lower design HP and use it here because your prop was sized to the actual rating of the engine in the boat not some lower rating on some other engine. If the prop was sized for 2100 RPM it would have been larger than the prop called for for 2300 RPM. This is more about the prop than the engine.

If you want to use fuel consumption to determine HP produced you better make sure that the engine is at least close to operating near it's design parameters. Engines running very lightly loaded are incredibly inefficient the un burned fuel going out the exhaust and the cold burn all contribute to fuel consumtion but don't make HP. This is why the most efficent displacment speed boats use engines that are sized to operate around 80% load at cruising RPM.

No one is measuring exhaust temp after the turbo and it's not measured at the ports either (you need 6 or 8 pyrometers to measure at the ports?) DD and all other manufactures want the thermo couple between the maifold discharge and the turbo. Our discussion was about a set of 892TIs not 6s low RPM is not as much of a problem with smaller engines. Now if your readings are true and acurate and you have 500 degrees of exhaust temperature after the turbo or even in between the turbo and the manifold at 1100 RPM then your friends boat is over propped and not capable of reaching 2300 RPM. That would explain the excellent condition of the engines and the good fuel economy and if that's the case I agree with you 100 %. Our discussion was with boats with OEM (reach rated RPM props).

Don't bother trying to tell me that he's OEM proped and reaches rated RPM with 500 degrees of exhaust temp @ 1100 RPM cause it's just not posible. If your producing that kind of heat @1100 then at 2300 you would be so hot the valves wouldn't last a week.

Guys this is not rocket science or vo doo and it sure as hell isn't my opinon based on one engine. All this info loads heat running parameters is old well proven science and the info is available you just have to look in the right places. How much harm over how much time I think is impossible to say. Most of us have seen high hour engines from the late 70s early 80s some still run clean and strong others dirty and week. Of course care and maitenance play a part but so do operating parameters and subtle changes show themselves over time not right away. The problem is that when you start to see it the harm has already been done. So I believe that if you change the operating parameters you might be wise to re engineer a little and make some adjustments to protect your equiptment now that's just my opinion. But like it or not at 1000-1100 RPM your to lightly loaded that's a fact.



Brian

[Brian Degulis]

Does anybody know if fuel consumption per HP varies much over a reasonable operating (RPM) range?? I would suspect not, but it could. But in general HP/fuel consumed is not magic: want more HP? You have to burn more hourly fuel to obtain the power...(quote Rob Brueckner)

There is an article in Power and Motor Yacht came out about 2 months ago. The article discusses diesel electric drive but it talks about efficiency losses when running large engines lightly loaded. The author says that you can see a 40% loss running a high HP engine at lightly loaded low RPM. I think he's right on the money because when I switched to larger props and lowered RPM for the same near displacement cruising speed. With the engine loaded more at a lower RPM I gained over 30% in efficiency which today translates to around a $20 per hour savings @ 10.5 kts and much happier engines.

Brian

[Boss Lady]

thermodynamic efficiency is part of the equation. This is why combustion chamber temps are important.

When it comes to thermostats, a 180 puts normal temps up around 195 degrees which is max safe temp. If you exceed the cooling capacity you will not get any indication of a problem before hand. If you stick to the 160s, and operate at 180, then if you are running out of cooling capacity you will see it from the temperature climb and possibly head off a catastrophic overheat. If you live in a warm climate you really need that 15 degree buffer. Especially on the 92 series. Otherwise you are rolling the dice to the tune of about $40K-$80K depending upon model.

[Genesis]

Whatever.

You folks do what you want.

I'm tell you what I see. You can claim I'm not seeing it if you want. As soon as someone calls me a liar I'm done with the conversation.

This is now a circle jerk and I don't have time for it. Sayonara; talk to Brian about future engine matters, since he has proclaimed himself your expert.

[captddis]

Don't like it when the shoe is on the other foot? You like to be critical of everyone else. Waaaaa.

[Genesis]

Whatever.

Again, fuel consumption doesn't lie and neither do stack temperatures and a clean exhaust.

As soon as someone says that I'm lying about instrumented results there's nothing further to talk about. Either you believe that my floscans showed what they did or you do not. You either believe that if my floscan said I burned 200 gallons going out to fish and when I came in and filled that tank at 201 I needed a 1 gallon jug on the vent to catch the overflow or you don't.

That's all.

Have fun.

[Brian Degulis]

Look Karl Speak for yourself not me I haven't proclaimed myself anything. Like anyone else there are things I know something about and things I don't. I'm not arrogant enough to believe that the people on this forum are going to blindly follow one persons advise and ignore all other information only a fool does that and there aren't to many fools on this forum.

Boy for a guy that's awfully sarcastic and critical of others you sure do bruise easily

Brian

[Roscoe]

Brian, you mentioned replacing your OEM props with a larger size but not the optimal props you wanted due to cost. After analysing the changes you made to cruise at displacement speed, what comments can you provide regarding optimal prop sizes for economy and how those props are affected by Hatteras hull design?

[REBrueckner]

My question if using 180 degree thermostats was a good idea was focused on lower RPM and lower speed cruising for efficiency. Not normal 1800 or 1900 RPM cruising...

Posts above suggest 180 thermostats may run about 195 degrees....isn't that at normal cruise RPM, say 1800 or 1900 RPM? That's what my stb engine always did at those RPM's with a 180 degree thermostat...My port 8V71TI always ran about 175 degrees at 1500 and about 180 degrees at 1800 or 1900 RPM, and now that I have 170 stats in both engines, they both run at those temps....I agree 195 temps don't leave enough margin.

My mechanic said NOT to run Detroits above 195 but I don't recall just why..whether he thought there was not enough margin (likely) or they just weren't supposed to be that hot...I run in NE waters where summertime water temps are not over 80 degrees, more like 70 to 75 more typical....I would NOT like to run Detroits anywhere near 195 degrees here let alone tropical waters...

When originally trying to figure out why my stb engine was running warmer than my port, I took infrared thermometer readings at different spots...As I recall, those readings varied four maybe five degrees in different spots on each individual engine, but were darn close to the temperature gauges at my helms...And most of the readings between engines at the same relative locations were different by about six to eight degrees or so, which is what ultimately made me think "maybe it IS a 180 thermostat" in the warmer engine...it was...

[Genesis]

The reason not to run over 195F is that this is the temperature at the probe (in the elbow); the localized temperature at hot spots in the water jacket (specifically, in the cylinder head) is a good bit higher.

With a 7psi cap there is a risk of localized boiloff in the cylinder head water passages. If that happens the head will crack.

In addition you have the same risk in the block with the seals in a 92-series engine; if you compromise a seal then you get water/oil contamination and its "game over."

Modern automotive engines tend to run 15psi caps which give you more margin. I have been told by multiple people who are smarter than I am that Detroits were not engineered internally in their seals to handle a 15psi cap, so you can't run one to gain more margin.

There is an additional consideration if you're running inhibited water instead of coolant in that you have reduced boiling margin. This is counteracted by the greater heat-carrying capacity of inhibited water .vs. coolant (about 15% more BTU transport capability)

The underlying issue here is that Detroit cooling systems were not designed to handle the horsepower they are set up for in recreational service. As such you're playing with very thin margins in these high output engines; unless you're prepared to spend a buttload of money installing much higher-capacity heat exchangers you don't want to do things that further erode your safety margins.

[Brian Degulis]

Brian, you mentioned replacing your OEM props with a larger size but not the optimal props you wanted due to cost. After analysing the changes you made to cruise at displacement speed, what comments can you provide regarding optimal prop sizes for economy and how those props are affected by Hatteras hull design?

You want to use the largest diameter you can swing the limitation is clearance 10% of diameter between the hull and prop is ideal. A bare minimum of 2" usually works the pitch should be sized acording to RPM and HP. Of course it's all comprimises and everything needs to be factored in but more diameter is better.

The big gain in fuel consumption is still the loading of the engines at a lower RPM that probably acounts for 90% of the improvment. So if it's not feasable to go to max diameter you can go as big as possible and increase pitch to load at a lower RPM.

Be sure you consider everything first determine how your going to set the governers what will be the new lower max RPM and what HP can be taken at that RPM. Then size the prop consider clearance shaft loads tramsmission loads how much thrust there will be at idle. If you go to far you might end up with to much thrust at idle and or a cruising RPM so low that things like oil pressure and raw water cooling capacity need to be considered.

I didn't run into any of these problems and I did make a large change my max RPM went from 2300 down to 1600 big diffrence. I do have a little more thrust at idle than I would like but no diffrent than many high HP boats it's managable. So I don't think anything mentioned above should be a problem but it all needs to be looked at.

Brian

[BUSTER]

The reason not to run over 195F is that this is the temperature at the probe (in the elbow); the localized temperature at hot spots in the water jacket (specifically, in the cylinder head) is a good bit higher.

With a 7psi cap there is a risk of localized boiloff in the cylinder head water passages. If that happens the head will crack.

In addition you have the same risk in the block with the seals in a 92-series engine; if you compromise a seal then you get water/oil contamination and its "game over."

Modern automotive engines tend to run 15psi caps which give you more margin. I have been told by multiple people who are smarter than I am that Detroits were not engineered internally in their seals to handle a 15psi cap, so you can't run one to gain more margin.

There is an additional consideration if you're running inhibited water instead of coolant in that you have reduced boiling margin. This is counteracted by the greater heat-carrying capacity of inhibited water .vs. coolant (about 15% more BTU transport capability)

The underlying issue here is that Detroit cooling systems were not designed to handle the horsepower they are set up for in recreational service. As such you're playing with very thin margins in these high output engines; unless you're prepared to spend a buttload of money installing much higher-capacity heat exchangers you don't want to do things that further erode your safety margins.


All absolutely true,especially the marginal high hp,Detroit cooling systems.

I agree with your previous posts concerning running at 1100 rpm's.I don't know of a Lobsterman around here that hasn't gotten at least 10,000 or many more hrs on Detroits "even 92 series" idling from trawl to trawl and pulling them up.The key to it is to make sure to pick up the RPM's from time to time to both dry up the exhaust and prevent low rpm pattern wear.The same goes for Cummins,Volvo, JD or CAT. One of my Lobsterman customers has over 22,000hrs on an 8.3 Cummins and the thing runs as new.
Tony

[REBrueckner]

Buster's posts again illustrates that we recreational (low annual hour users) Detroit owners likely obsess unnecessarily, myself included, about engine life...turbo versus natural, WOT RPM, and on and on.

For the most part if you run reasonable RPMS, not WOT, not idle, at cruise and take normal precautions on operating temp, oil fill, etc, our old engines would benefit from a lot more run time than we give them rather than sitting around unused for weeks and months at a time.

I was aboard a wooden Ferry in Maine a few years ago during haul out; talked with the owner/operator of the vessel...Volvo diesel....He told me the engine had over 20,000 hours and except for things like injectors,impellers, and oil, she was original!!!!

[Walter P]

My nephew is the skipper of a fairly good size MY on the West Coast. The boat was built by a company called Knight and Carver in Calif. It is powered by a pair of 8/92s Naturals and has almost 20,000 (twenty thousand) hours on them. The engines still run strong. It is a fairly heavy boat about 80 feet OAL and obviously runs at displacement speeds.

The boat rarely sits for more than a couple of days and thats probably a major factor with it's long life. They never run on the pins or idle for long periods either. My point is that moderation seems to be the key to long life - - - not only for people but for our boats.

Walt

[chris]

My nephew is the skipper of a fairly good size MY on the West Coast. The boat was built by a company called Knight and Carver in Calif. It is powered by a pair of 8/92s Naturals and has almost 20,000 (twenty thousand) hours on them. The engines still run strong. It is a fairly heavy boat about 80 feet OAL and obviously runs at displacement speeds.

The boat rarely sits for more than a couple of days and thats probably a major factor with it's long life. They never run on the pins or idle for long periods either. My point is that moderation seems to be the key to long life - - - not only for people but for our boats.

Walt

Walt
Our 48MY's were designed by Hatteras to run at displacement speeds with 6v92TA's, displacement hulls, and 26x22 4 blade props. They did advise an occasional run up in rpm when running long periods at 1000 rpm.
That is pretty much how I run mine, thinking the original design must be ok.

Chris

[mstailey]

This may be material for new thread (I will start one if no one answers this here) - Brian made mention that using a multi-weight oil will not give you accurate oil analysis.

Local Cummins (truck) shop where I bought oil told me to use 15w-40 even though specs call for 30w (I have 1976 Cummins VT903's). He said multi-weights were not even invented in 1976 and that I should unquestionably use multi-weight (which I currently am - Shell Rotella brand).


sgharford,
Per Cummins you need to use Valvoline Premium Blue 15w-40. It is the oil recomended by Cummins. I've had good performance with this lube in my VT903 M1's meaning very, very low oil consumption between oil changes (less than 1 qt at 100 hrs between changes). I also run the engines @ 2400 rpm. Per the M1 specs (you did not say which marine model you have) max rpm is 2800 on the M1 series. Acording to the local factory Cummins mechanic here in MI you do not want to run these engines for an extended period of time at low rpm (< 1500) as it will have issues with combustion temp's not being optimum.

[Brian Degulis]

You shouldn't run multi weight oil in the 2 stroke detroits for many other engines it's fine.

Brian

[Pete]

Chris,

Having looked at a few 48 MY, I thought the story from Hatteras was a dual mode hull design, with an option of 671N's for those that wanted a displacement speed vessel and 6V92TA's for those who wanted a planning vessel. I am not aware of anything done to the 6V92TA's that make them any more suitable for low RPM running than any other 6V92TA of similar horsepower rating. I could be wrong but that is what I recall from 7 or 8 years ago.

Pete

[chris]

Chris,

Having looked at a few 48 MY, I thought the story from Hatteras was a dual mode hull design, with an option of 671N's for those that wanted a displacement speed vessel and 6V92TA's for those who wanted a planning vessel. I am not aware of anything done to the 6V92TA's that make them any more suitable for low RPM running than any other 6V92TA of similar horsepower rating. I could be wrong but that is what I recall from 7 or 8 years ago.

Pete
They were advertised as a dual mode hull.
In their advertising they say with the 6v92's you had the option the run the boat on plane. They also mention if running long periods at low rpm's to run the rpm's up occasionally.

[Walter P]

Usually when you try to offer too many benefits from the same design, something is not optimal. I'm sure most of you have heard the old saw about motor sailing yachts (the 1983 65' Hatteras included) that they function fairly well in each mode but not great. They were not really sail boats or power yachts, sort of somewhere in between. In my opinion as with most things with boats, they are compromises. Boating conditions are very fluid (no pun intended) and thereby constantly changing from calm seas to horendous nastyness. The designers are constantly torn with how to best give them fair performance in between calm and rough.

The Series I 48 MY (1981 - 1984) was designed with a dual mode hull by Jack Hargraves design team. (I understand the running surfaces were actually designed by Dudley Dawson when he worked for J. Hargrave). It is obviously a compromise design, not as good at planing as a hard chine semi-disp hull as well as not as efficient as the 48 Hatteras LRC for displacement cruising. They have come pretty close to being damn good at doing both planing and disp. cruising.

While a few were made with 6/71Ns, most had the optional 6/92 TAs rated at about 435 hp. A couple were made with somewhat higher HP 6/92s which I won't go into at this time.

I frankly do not have the extensive time on my 48 (1982 model) with the higher HP 92s to offer very much valuable information, but at least one sistership owned by another well known HOF member, Bob Kassal does extensive cruising at 1000 rpm back and forth to Fla from the Chesapeake each year with no apparant adverse effect and fantastic mileage. Chris is also very happy with the dual performance capabilities of his 48. I'm not sure how he normally runs his however and therefore I can't comment.

While I still believe that just about any Hatteras with Detroits can perform very well for long periods at displacement speeds, I believe that the few of us who have one of the 30 or so 48 MY models manufactured have a pretty good compromise boat, with the option to run ahead of a storm if necessary. I will not worry about wrecking my engines by not running hard enough, just use common sense and enjoy the ride for many years.

Walt

[krush]

To paraphrase Krush (as I recall), when did an engine ever fail from running slow? Never heard of that happening.

I leave the country for a month and holy heck all hell breaks loose! And to be coorect in the quote, my orinignal point was really how can somebody really say "running on plane for 30 min a day prevents damage" is ok. How does one know that 1100 all the time is any different?

I can tell you one thing though. Running at 1100rpm without properly loading the engine is REALLY killing your fuel economy.

Brian has proven this with his data. Brian also specs and sizes power plants for boats for a living (and spends times talking to naval architects), so he does have some experience in this area that most people are clueless about. In fact, I've met very few people that have enough knowledge to discuss the fundamental performance (in the engineering sense) aspect of engines.

I've stayed on and worked on the boat Genesis is talking about. It is in great shape and runs nicely, but it also has relatively fresh engines. I think it has around 1500 on them motors, which in my opinion is hardly enoguh to draw concrete conclusions from.

I will say this and am not going to argue it because I don't need to prove anything (LOL I finally got the piece of paper on the wall for that): The brake specific fuel consumption of an engine WILL vary greatly throughout the operating envelope. This is why propping can create significant fuel savings like Brian experienced.

Using GPH measurements to directly calculate HP output is misinformed at best.

[fucifino]

Hello all,I am a newbie and liveaboard on a 1985 connie w/ 6v92ta's 550hp.I am not a 1600-1800 rpm operator,I like 12-1400.I understand 170 degrees is the sweet spot to make the engines happy.Is there a way to get 170 degrees at 1400 rpm ?I don't want to reprop ,that would just up the rpms at a lower speed.I am on disabilty and can't afford 1800rpms and certainly an engine rebuild.Please help!!!Thanks Someone said removing the turbo's would lower the horsepower,but I don't think thats my answer.

[Fanfare]

I have your engines. Lately we run 1950 rpm to go on plane for deep water runs. But in shallow or sheltered water we run about 1300 which gives us 8.8 kts, just below our hull speed of 9.1. I have 170 degree thermostats. At this cruise speed my engine temperatures run 165 degrees. We have traveled hundreds of hours at this speed and have had no problems in about 25 years. I do try to blow the engines out for a few minutes each day if I don't have to watch my wake. Coming back from the Bahamas I always have at least 55 nmi back to Florida which at 1950 rpm seems to purge anything in the oil. In the Intracoastal I have to run about 750 rpm for the wake. This may not be the best but has never hurt my engines.

[bobk]

I have a 48 MY with stabilizers, 6V92 425 HP. I have about 4000 hours on them at mostly 1000-1100 rpm with no issues. I do use Howes MPK for cetane booster and 170 t-stats.

Bobk

[Seapig100]

I have been thinking about this thread and thought I would add my 2 cents. IMO we are doing zero damage to our engines running them at 1200 or 1300 rpm. It takes quite a bit of power to move these boats at 8 or 9 knots. I would actually argue that this is the ideal rpm to run at. In a over the road truck it's not uncommon to go 6 or 7 hundred thousand miles even a million miles is not that rare. Let's assume an average speed of 50 mph that would be about 20,000 hours before major overhaul. How many boats get 20,000 hours between major overhauls? A car or truck uses a fraction of the available power to cruise at 70mph a car uses about 30 hp to cruise that fast. My car has 300 hp so it's using 10 percent of its available power. How much is a truck using 125/150 hp? And today's truck have upwards of 600 hp available. My Detroits have 325 hp and I read that maybe 60 or 70 hp is used to move at 8 knots. This is plenty to keep the engine healthy if it's run up occasionally.

[Glory]

DDs 8V71 manuals indicate that for some uses, such as large generators, 160 degree thermostats are to be used, so I'd say DD designed the engines to be run at 160 degrees for long periods of time. Generators can run 24/7 and only have large loads for a few hours of those 24, while the RPMs will stay constant. I'd have no problems running 900 RPMs and 160 degrees for long periods. I ran at 900 or so for days with no ability to run them up in the Trent Severn canal.

Doug

8V71Generator RPM is not going to be 900. Generator manufacturers normally suggest loading the unit.

[Pete]

In most all discussion on this subject, the scavenger air volume does not get included in the consideration. On DD 2 cycles the blower is gear attached to the RPM of the engine. Thus when you slow the engine down you also diminish the cooling air going through the engine. This makes the comparison to over the road usage invalid in my opinion. In over the road applications, the engine runs in the 1400 to 1600 range, producing on average very little horsepower. Lots of air going through the engine with very little fuel, and stress.

By comparison, many boaters will over prop to save a little fuel, utilizing more fuel with lesser volumes of scavenger/cooling air, increasing the stress at low RPM. IMO if you want the engine to go further between rebuilds, you underprop it and let it breathe/cool better as it produces the same horsepower at a higher rpm. And this needs to be done where the engine temperature is managed in the proper operating range. Over the road applications manage proper engine temperature at light loads, so we know it can be done.

And for the overprop/lower rpm/save fuel crowd, I suggest they complete the thought to overprop/lower rpm/save fuel/rebuild sooner.

Pete

[Seapig100]

Up until about 15 years ago every truck in our fleet were powered with Detroits. Many of the trucks had over 150,000 miles. These are fire trucks they spend the majority of their life idling. I know of one major overhaul that took place and it's because the oil pan got ripped off. This is not high miles but the hours are through the roof. Turbo rebuilds absolutely like clock work at 100k miles. This however is because they were shut down hot. So the life of a fire truck is run like hell for 3 min then idle for 20 minutes to a half hour then drive back like a normal vehicle and shut her down. They just kept going. Running at 1400 rpm puts me at 10 knots just a bit to fast. I guess I can probably do 1300 instead of 1200 but not sure it will make a measurable difference. I know for sure running 1800 to 2000 will just rack them out so slow it is for us.

[krush]

By comparison, many boaters will over prop to save a little fuel, utilizing more fuel with lesser volumes of scavenger/cooling air, increasing the stress at low RPM. IMO if you want the engine to go further between rebuilds, you underprop it and let it breathe/cool better as it produces the same horsepower at a higher rpm. And this needs to be done where the engine temperature is managed in the proper operating range. Over the road applications manage proper engine temperature at light loads, so we know it can be done.

And for the overprop/lower rpm/save fuel crowd, I suggest they complete the thought to overprop/lower rpm/save fuel/rebuild sooner.

Pete

"Overprop" debate at low RPM is irrelevant as the engine is still very lightly loaded.

[Pete]

Disagree. The engine will benefit from the additional scavenger air flow for whatever power is produced. Also, the overproping folk always recommend an occasional run up in the rpm to "clean out the carbon" and the engine will be well under the designed scavenger air flow during this operation.

Pete


"Overprop" debate at low RPM is irrelevant as the engine is still very lightly loaded.

[Top Hatt Craig]

Here's some verbiage from the Wartsila 9L26 (inline 9!) heavy marine engine. I wish DD could be as specific. . "Operation at low load and idlingThe engine can be started, stopped and operated on heavy fuel under all operating conditions. Continuous operation on heavy fuel is preferred rather than changing over to diesel fuel at low load operation and manoeuvring. The following recommendations apply:Absolute idling (declutched main engine, disconnected generator)● Maximum 10 minutes if the engine is to be stopped after the idling. 3-5 minutes idling before stop is recommended.● Maximum 6 hours if the engine is to be loaded after the idling.Operation below 20 % load● Maximum 100 hours continuous operation. At intervals of 100 operating hours the engine must be loaded to minimum 70 % of the rated output.Operation above 20 % load● No restrictions." kinda apples and oranges but what the heck.

[krush]

Disagree. The engine will benefit from the additional scavenger air flow for whatever power is produced. Also, the overproping folk always recommend an occasional run up in the rpm to "clean out the carbon" and the engine will be well under the designed scavenger air flow during this operation.

Pete

Even when "overpropped", at 1200 RPM, the engine will still be able to make much more HP than the prop is absorbing. And, when "overpropping", one should also derate the engine by lowering the MAX RPM operated at OR (more better) lower the governor high-idle setting.

Also, most of our boats have turbo chargers. Loading the engine makes the turbo pump more air.

[rsmith]

Those of us that fish spend up to 12 hours a day at less than 1000 rpm trolling.at the end of the day when you push the throttles up all that smoke is from unburned crap that's coated the exhaust valves and port area. These areas were never designed for combustion. If you crank it up and burn it off before the buildup gets too bad you may be ok. I wasn't so lucky. After years of offshore canyon fishing in the North Atlantic with 4 hour runs to the fishing grounds I decided to leave the boat in Fla. With manatee areas between my dock and the inlet and only 11nm till the fishing grounds it hardly makes any sense pushing the throttles up. It's gentlemen fishing at its best. After a few of these trips we got caught in a typical Florida afternoon white out squall. Cranked it up to go home and one engine started pouring out white smoke. When we pulled the head one of the valves was torched actually melted down to the stem. The rest were coked up with the residue from not running the engine hard enough.
Thermostat may keep up the coolant temp to prevent acids and condensation in the oil but the only answer for low combustion temp it to run em hard

[krush]

Thermostat may keep up the coolant temp to prevent acids and condensation in the oil but the only answer for low combustion temp it to run em hard

Or put bigger props on to load the engines more at lower RPM.

My pyrometers are mounted after the turbo and the reading doesn't budge at idle in neutral. But when I put in gear, the needle climbs up a bit...something I did not expect. And when I'm running at 1800RPM cruise, the needle isn't much higher than 1000rpm in gear. Now, I'm assuming that the values are correct....and I need to verify the instrumentation calibration.

[Pete]

On DD engines, the volume of blower scavenger air is directly proportional to the RPM of the engine, as it is gear driven. The scavenger air volume is the primary contributor to the air volume exhausted through the turbo to increase the spool rpm. Thus, requiring more power at a lower engine RPM will have little influence on the turbo output to the blower and thus the scavenger air volume. Granted the purpose of the turbo is to raise the input air pressure on the blower, but at low RPM that is being discussed this is minor. It would be very interesting to see a RPM versus turbo output pressure versus scavenger air pressure graph to better understand the precise relationship.

But what we do know empirically is that low rpm operation, especially in an over propped setup, does carbon up the combustion chamber and exhaust, as everyone sees the increased black smoke when the air flow and temperature are increased by higher RPM operation. That black smoke is the carbon deposits of unburnt fuel, some in the combustion chamber, other on the pistons, rings, exhaust valves, exhaust manifolds, where they do no good.

Circulating back to those million mile over the road engines, they do not lug along at 1000 to 1200 RPM for hours, then blow them out for 30 minutes. Simple logic will tell us, if you want similar results from a marine engine, best idea is probably to emulate the operating environment that produces those results.

Pete

[Avenger]

Wow!!!

[Top Hatt Craig]

"The scavenger air volume is the primary contributor to the air volume exhausted through the turbo to increase the spool rpm" are you sure? Isn't the expanding combustion gas the source of power to drive the hot side turbine against the load of the cold side compressor/turbine? The scavenger air is not much over atmospheric, there's very little energy there. Throttling up against a load produces (as you know) more expanding gases that accelerates turbines, providing boost. I'll betcha' if we could rig a test where a very large electric motor were to spin the crankshaft of a DD at 2100 rpm with no fuel injected, we wouldn't see near as much, if any manifold boost compared to our boat at 12 knots or a truck climbing The Grape Vine. Regards, Craig

[Avenger]

You don't need an electric motor. Spool one up to the governor in neutral and you will find zero manifold pressure from the turbo. No load = no boost.

[kelpy]

You are correct, Running the engine up to high idle, say 2350 RPM, will show no boost. The burning fuel provides the energy to spool up the turbos. Later engines had bypass blowers to eliminate them at high boost.

[Seapig100]

Over the road trucks burn about 8 gallons an hour. So to emulate this I need to be right about 1500 rpm as a guess. This is by far the worst place to be on my boat. So I have a choice burn 4 gallons an hour per engine or run them up to 1900 and burn 17 gallons an hour per engine. I would bet that I get many more miles from the engines if I burn 4 gallons an hour.

[Pete]

Run the engine up in neutral to no load limit and take a look at the exhaust outlet. Where is that air volume coming from that shoots the exhaust water 50 feet or more across the water. Also, what is manifold pressure at WOT under load?

Seapig, the solution to your dilemma is to take pitch out of your props such that 1500 RPM produces the cruise speed you desire. You will give up WOT speed in doing so, but that is the trade off.

Pete

[krush]

The scavenger air volume is the primary contributor to the air volume exhausted through the turbo to increase the spool rpm. Thus, requiring more power at a lower engine RPM will have little influence on the turbo output to the blower and thus the scavenger air volume. Granted the purpose of the turbo is to raise the input air pressure on the blower, but at low RPM that is being discussed this is minor. It would be very interesting to see a RPM versus turbo output pressure versus scavenger air pressure graph to better understand the precise relationship.


Volume and mass flow are different things. Volume per rotation through the positive displacement blower on a detroit is practically the same at all loads and RPM. However, mass per rotation varies greatly.

You don't need a graph to figure this stuff out. A boost gauge will tell the whole story.

I'm not here to argue on the interwebs LOL do what makes you feel good.

[krush]

Run the engine up in neutral to no load limit and take a look at the exhaust outlet. Where is that air volume coming from that shoots the exhaust water 50 feet or more across the water. Also, what is manifold pressure at WOT under load?



They both said there is little to no boost at high idle in neutral. Nowhere did they use the term "volume".

[Pete]

If you prefer substitute the word "pressure" for "volume". Was not aware that the exact same terms must be used in replies. BTW, pressure difference are required to move volumes of air.


Pete

[Seapig100]

A lot of this talk is tied to turbo's, what about naturals.

[Seapig100]

The 71 series Detroits were used in generators the designed rpm's for these generators is 1200 rpm's were these engines built different then the 71s in my boat?

[kelpy]

A lot of this talk is tied to turbo's, what about naturals.

What about them? The blowers supply excess air for scavenging the cylinders. There is very little boost, if any, from the time the valves close to the time the ports are covered. They are fueled accordingly.

[GJH]

My boat came with a set of spare props with 2" more of pitch to them. I put them on once when I had the usual props out for repair and tuning. We almost always ran the boat slow in the 1000-1350 range (56MY with 8v92tii) so I figured why not?
Where there was once a very clean transom was now covered in soot. Put the old ones back on as soon as they came out of the shop.

[Pete]

Let me address the easy one for me. DD 1200 RPM generator engines. I do not know anything about the build of these engines, beyond the obvious that the variable speed governors we have on boat main engines would be replaced with constant speed governors. Keep in mind that DD two cycles are basically kit engines with many, many parts available and designed to handle specific application needs. I would be very surprised to learn that the generators did not have internal parts different from those for marine applications.

As for the other base discussion, operational considerations apply to NA engines equally, except the specific operation and influence of the turbo. NA engines have scavenger blowers and the principles of operation are the same NA or turbo. Turbo DD present more questions than NA. One is that turbo engines have lower compression ratios than their cousin NA engines. Thus they will be less efficient until the turbo makes up the difference with higher air box pressure, which only occurs when the turbo spools up. To get deeper into this requires turbo spool up data, I do not have. Suffice to say, turbo engines are designed to run with turbo air pressure present. Running a turbo engine without turbo pressure is the same as running a worn out NA engine.

Pete

[kelpy]

No it isn't the same but I'm not going to bother trying to explain why though. You will just have to figure it out yourself.

[Seapig100]

[QUOTE=kelpy;318400]No it isn't the same but I'm not going to bother trying to explain why though. You will just have to figure it out yourself.[/QUOTEYou shouldn't keep secrets.

[Pete]

This has to be the best well articulated nothing I have seen in a long time. Perhaps just a disguise for another problem.

Pete


No it isn't the same but I'm not going to bother trying to explain why though. You will just have to figure it out yourself.

[kelpy]

The statement actually refers to your apparent lack of understanding of how Detroit Diesels work.
I'm done with this thread. I wish everyone a great Labor Day and enjoy yourselves!!

[Pete]

I really appreciate your sensitivity in not exposing my lack of understanding on how Detroit Diesels operate. And there is no doubt in my mind that you could thoroughly embarrass me with you vast and absolutely comprehensive mastery of the subject. Too bad you appear to be a guy who tosses one in the punch bowl, and runs out the back door, leaving us all to wonder what was that all about.

BTW, you are current with the meds?????

Pete


The statement actually refers to your lack of understanding of how Detroit Diesels work.

[Seapig100]

I know I am just an uneducated idiot, however one thing I have learned is not to take advice without first researching said advice. In the process of doing this research some very interesting points have been learned. First we get somewhere between 18 and 20 hp per gallon of fuel burned second Detroit 8-71's were put in generators governed at 1200 rpm's. There have been a few points made as to why I should not run my engines at 1200 rpm's. I listened and researched and asked questions. The questions are based on these two facts. If a person has no ability to answer my questions without taking their ball and going home then I have to wave the bs Flagg on their whole argument. My research tells me I am producing about 80 hp per motor (hardly lugging a 325 hp engine) and I am running the motors at almost peak torque. I know and it's not disputable that the ideal place to operate an engine is the rpm that makes peak torque. So there is zero doubt to whether this is doing damage to the engines it is not. Now are the blowers operating with enough cfm to scavenge the spent exhaust? I would be very surprised if Detroit has multiple blowers for the engines. I looked and find no such evidence. What I did find was that the generators were considered marine engines. In other words the generators governed at 1200 rpm are the exact same engines as I have in my boat. So this blows up the scavenging air problem. We can then look at all the boats and what boats are getting the most hours from their engines. Up here in Boston the lobster guys take the prize. They have little problem getting 20,000 hours from their Detroits. These guys run their motors at or just above idle for hours and hours at a time. Show me a boat that's been run at 1800 to 2000 rpm's with 20,000 hours on them. So I think I am in my right to question anyone who makes claim that running these engines at 1200 rpm's is causing excessive wear.

[Pete]

Let me add one more research point. As I already noted DD engines are kit engines with many interchangeable parts that look identical and but are speced differentially, including scavenger blower volume ratings. The 71's you mention are basically the same engines but I do know the parts hung off the blocks vary with the application. To make this short, 8V71's came in recreation/pleasure boat setups and on the other end continuous duty/commercial duty setups. I would think the constant RPM setup for generators is also different but have no facts.

As an example 671s came rated at about 180 HP continuous duty NA. Recreational duty got up to 390 HP still NA. Add a turbo and they got up to 485 HP. Same basic block, same 671 designation, big differences.

Those 20,000 hours engines I would wager are continuous duty ones. Not what you have in your Hatteras. Check me out on this.

Pete

[Seapig100]

Thanks Pete I will try to find out if the blower is different then maybe the best solution is to change the blower. Our engines are most certainly 100% duty cycle engines. For those not understanding what a duty cycle is, it is how long a piece of equipment can run before being shutdown down for a designated shutdown period. Our diesels can no doubt run indefinitely without being shutdown making the 100% rated. Now is the scavenger pump different? I will attempt to get the answer to the question.

[kelpy]

Well Pete, you lured me back with your post. I think you are doing this on purpose.

A 390 HP 6-71 NA? Good luck showing us one of those!
And I don't believe that there are different displacement blowers either. Why would there be? At any given RPM, there is always excess air for scavanging. 426 CI displacement (6-71) is the same regardless of power output. The blower neither knows nor cares how much power is being produced. But a turbo does.
And "kit" engines? All manufacturers supply engines that are used in various applications. They simply are engineered and built to perform their designed task. Nothing "kit" about them.
In our applications, the Detroits are refined (for their time), precisly engineered marine propulsion engines. If you leave them as designed, prop them correctly to reach full RPM at WOT, and tune them up once a year or so, they are able to chug along for thousands of hours with proper care.

Here is a link that will help define & describe marine Diesel engine duty ratings.
https://www.sbmar.com/articles/continuous-duty-a-different-perspective/

[Avenger]

I knew you guys could do it with the help of the internet. Who needs to listen to a guy with an engineering degree?

BTW, there are different blower drive ratios, but you're barking up the wrong tree. Engine life is measured by fuel burn. That 210 HP 6-71 has the same life expectancy as the 485 HP 6-71TIB.

[Pete]

Kelpy, and here I thought your word was golden and you were bowing out. On the 390 HP 671, you got me, I should have typed 310 HP as produced by J&T in the late 70's, used in a few Hatteras but mostly by the Jersey builders. I can see a 79 Viking out my window as I type with a pair installed.

As for the blowers, I will leave that to you to check out. Also, you may not want to call DD kit engines, but I suspect you are in the minority when talking to people who know something about them, including me who you believe knows so little. An engine that you turnaround the block and bolt on some different parts to make it rotate in the opposite direction is a kit to me.

For my money, DD have many attributes, but refined and precision are not included.

And as for your overall summary, for NA engines I believe we are close on our opinions. Prop for WOT, run them hard enough to get good combustion heat and scavenger air flow, regular service and I will add there are longevity benefits of slightly underproping. Those torque curves everyone loves are limit curves, and running engines at their limits of output or close to it is not how you get extra long hours to rebuild. But all DD are not created equal. Remember the 671TIB/485 HP, 1000 hours with luck engines.

Where I suspect we vary in opinion is the turbo variations of DD. While the scavenger blower is linear with speed, turbo are not. Running turbo engines slow, especially over propped produces rich fuel mixtures and is absolutely unhealthy for the engine.

Welcome back.

Pete

[kelpy]

Well, after seeing some errors and inconsistancies, I had to jump back in. But, with Irma quickly approaching and me being located in Fort Lauderdale, I will have to postpone further debate. Two yachts to get up the New River to Rosciolli tomorrow and my boat Thursday before lift bridges close will keep me busy.
See you all next week. Best of luck to the others here in south Florida and to those along the storm track. Feel free to PM me if you need some local assistance. I will be more than happy to help if I can.

[Pete]

And good luck to you too. Stay safe, boats are easier to fix than people. No new and very few used parts available in the human marketplace. Unlike DD, humans are not constructed from a catalog of kit parts.

As for this thread, I have concluded that this thread is making no contribution to the group, at least our exchanges. So, why continue?

When you return to this thread, and I hope Irma permits that in short order, you will find like Elvis, I have left the building. I will be moving on to more productive endeavours, including other HOF threads.

My word is golden, so enjoy, over and out.

Pete

[kelpy]

I agree. Not much more here. I accept your surrender. But, exercise caution on the stabilizer thread concerning the cooling water supply. You are on a slippery slope there!:cool:

[Avenger]

Kelpy, this mess kind of reminds me of Reagan's quote "It's not that they're ignorant. It's just that they know so much that isn't so."

Good luck to you and everybody in Irma's path. Be safe.

[kelpy]

Right on!

Let's hope Irma spares us a direct hit anywhere. Good luck to all.

[Seapig100]

They are predicting a direct hit on Miami.

[Pete]

Kelpy, no surrender involved, just recognition that your opponent with self destruct if just left alone.

On the stabilizer comment, did you pay attention in grammar school physics class?

Pete

[Pete]

Avenger, while I am an admirer of Ronald Reagan, I believe a much older quote from Aristophanes may be more approp: “Youth ages, immaturity is outgrown, ignorance can be educated, and drunkenness sobered, but stupid lasts forever.”

Pete

[Seapig100]

From what I can figure out there were two blowers for Detroit 71 series engines. There was one for the naturals and one for the turbo set up. The blower on the 1200 rpm generators is the same blower that's in my boat. The blower on the turbocharged engines used larger bearings than the natural setup.

[Avenger]

Who said "A little knowledge is a dangerous thing"?

Without redirecting your quote I'm going to point out that the recent portion of this thread is a classic example of what is wrong with this forum, and the reason why many of the people who are professionals in the engine and marine field with a lot of experience and knowledge have abandoned the place. Internet experts. You've made a number of incorrect statements and in the interests of trying to stop the spread of misinformation several people who do understand how engines work have tried to correct what is being presented here. But instead of seeking clarification you and seapig get into your echo chamber, reaffirm each other's mistakes and continue to argue about them. This place is supposed to be for us to help each other. It's no help at all when bad information is spread. But eventually one has to give up trying, if you'll pardon the expression, trying to teach the pig to sing.

[Pete]

Avenger, and exactly what are those mistakes? And be factual in your response. An "internet expert" not me, formal credential holding person in multiple fields, including DD, that's me.

My suggestion, know the facts before you pass inaccurate judgement.

Pete

[Avenger]

Really? What DD credentials do you hold? Because honestly I've worked with professionals in the engine rebuilding and service industries including dealers and in some cases factory people for over 40 years and I've never heard of a Detroit, or any other engine referred to as a "kit" engine by anybody or in any credible literature.

You want specifics, reread the thread from about page 9. krush tried to explain it, Craig tried to explain it, Glory tried to explain it, kelpy has tried to explain it, I've tried to explain it and now you want more explanation. No amount of explanation will do if just refuse to accept it.

[Pete]

NO! You made the claim, now back it up.

Pete

[Seapig100]

I went into this with an open mind. I have extensive knowledge about engines I have torn apart and rebuilt aviation turbines, I have torn apart and rebuilt ls1 chevy engines, Amc inline 6's done it, transmissions yes been there done that. Are any of these Detroits no, but do I have some knowledge about mechanics I rebuilt my first engine at 11 years old I am now 42. There has been different information given on this thread. First no Detroit was designed to run at 1200 rpm's that obviously is not true. Second After I proved the 71 series were put in generators designed and governed at 1200 rpm's I was told the blowers are different. Then someone chimed in and said the blowers are the same. Who do I believe? So I made some phone calls to several parts dealers. They all stated there are two blowers for the 71 series engine. One for naturals one for turbocharged. This makes sense to me I would find it very hard to believe that an engineer would not provide enough scavenging air for the engine to properly run at 1200 rpm's. It just makes no sense the high idle on the trucks we run with Detroits put the engines at 1100 rpm. Your telling me that running an engine at high idle is doing it harm or that the engineers would allow that. The question is am I loading the engine enough at 1200 rpm. At 4 gallons an hour per side I would argue I am. Unless someone can prove that there are indeed different blowers that really is the only argument left. We can agree to disagree as to whether or not 80 hp is enough to prevent lugging my opinion it is. It's 25 percent of total available power.

[Avenger]

NO! You made the claim, now back it up.

Pete

Huh?

You've got two industry pros and a mechanical engineer in this thread refuting your claims, plus a couple of other people who clearly have knowledge of the subject contradicting your arguments. But you claim you're willing to be persuaded? Hasn't worked yet. It's all there in the last few pages, go read.

Seapig: You're right, the only question is are you loading it enough at 1200 to provide proper combustion and operating temperature. There's instrumentation and specs for that. The only reason a generator runs at 1200 RPM is because that's the RPM the number of poles in the generator end require to maintain 60Hz. Load it insufficiently and it will have problems regardless of whether it's a 1200, 1800 or 3600 RPM generator. Forget the blower nonsense. Your situation requires the correct propping to achieve that loading which may require a compromise on top end. Invest in a pyrometer and find the correct power curves for your engine.

[Seapig100]

Huh? You've got two industry pros and a mechanical engineer in this thread refuting your claims, plus a couple of other people who clearly have knowledge of the subject contradicting your arguments. But you claim you're willing to be persuaded? Hasn't worked yet. It's all there in the last few pages, go read.Seapig: You're right, the only question is are you loading it enough at 1200 to provide proper combustion and operating temperature. There's instrumentation and specs for that. The only reason a generator runs at 1200 RPM is because that's the RPM the number of poles in the generator end require to maintain 60Hz. Load it insufficiently and it will have problems regardless of whether it's a 1200, 1800 or 3600 RPM generator. Forget the blower nonsense. Your situation requires the correct propping to achieve that loading which may require a compromise on top end. Invest in a pyrometer and find the correct power curves for your engine. I will do thanks. Now what temps am I looking for and where is the best place to take the reading from? Do you have a recommendation on what pyrometer to purchase? Thanks Duncan.

[Avenger]

The ideal place to put a pyrometer is pre-turbo. If you have to go post turbo you need a correction factor. Isspro makes some nice setups. As for temps my DD book only specs maximums for full load, and I don't know your application offhand. I would suggest you contact DD for specs specific to your engine.

[Seapig100]

I have no turbos, so the exhaust manifold just aft of the block?

[Avenger]

Then yes, that's the place.

[Pete]

Avenger, no backup yet, just another suggestion that others know. Sounds like Fake News to me, just a good headline and no facts. So, what are you going to do, backup your accusations with some facts or confirm your inability to do so. You are starting to resemble yelpy, you know the denial comments, character impugning, anything but a verifiable fact or two.

The world, well at least I, await your wealth of knowledge.

Pete

[Avenger]

No idea what facts you're expecting that haven't already been stated. It's all there in black and white exactly where and by who I stated. I'm not going to do it all over again because you didn't believe it the first time.

Wait, I get it,... Do you actually think you successfully refuted the points that were made correcting your statements?

[Pete]

My point is there were never any facts presented by anyone that refuted my understanding. You today stated I presented some erroneous info. Now you decline to identify the errors and the facts that back up such a statement.

BTW, I do admit to one error,actually a typo, where I stated a 671n was produced for marine use rated at 390 HP where the fact is it was rated at 310 HP.

With that one out of the way, bring on the rest of the errors and facts. I can hardly wait.

And BTW, I would be glad to review your and kelpy's facts, if either of you ever offered any.

Pete

[Avenger]

Thought so.

The 390, 310 thing is irrelevant. We all do things like that.

Your fixation on scavenging air is not.

Again, if you re-read, you state that scavenging air provides the air to drive the turbocharger. Several people stated that it does not, and that the turbocharger does work because of engine loading and thermal expansion and that RPM and blower volume produces no manifold pressure by itself.

Your reply is about how far the exhaust goes when the engine is at maximum RPM in no load.

Krush explained that that is volume and not pressure. And your response is basically that pressure vs volume is a semantic exercise.

I'm sure you know that pressure and volume are not the same thing, yet your engineering argument is that they are. This is the kind of thing where we have a problem.

[krush]

On DD engines, the volume of blower scavenger air is directly proportional to the RPM of the engine, as it is gear driven. The scavenger air volume is the primary contributor to the air volume exhausted through the turbo to increase the spool rpm. Thus, requiring more power at a lower engine RPM will have little influence on the turbo output to the blower and thus the scavenger air volume.



This statement is not correct.

[Pete]

Avenger, Krush, since both of you mention the comment I made about scavenger air being the primary air that drives the turbo, let address that.

Lets start with how the scavenger air portion of the cycle progresses. The blower is running proportional to the rpm of the engine, constantly sending air to the air box. When a piston gets to the lower part of its stroke, the cylinder sleeve vents are exposed and the blower air enters the cylinder and starts to aid in the exhaust of the spent combustion. This blower air continues to aid exhaust until the piston bottoms and start to travel upward in the cycle. Once the rings of the piston close off the air box vents, the exhaust valves close and the fresh air provided by the blower is compressed until just before top dead center when the injector fires and combustion occurs. The piston travels down again until the air box vents become exposed and while the exhaust valves open. At that instant, what is in the cylinder is the previous air box charge and the results of combustion. That is exhausted through the valves as previously explained. So the exhaust is the blower fresh charge, most of which has been converted to diesel exhaust byproducts, which drive the turbo hot side/exhaust side.

BTW, if anyone needs proofs that the blower air in the air box is required to get the engine to run and spin the turbo, try starting a detroit 2 cycle without the blower. As an easier way, just pull the emergency shutdown that closes the air intake to blower and try to start it. BTW, I do not recommend the second idea as the engine may just have enough residual air to fire one or two cylinders and damage the blower seals

A second BTW, I have seen mention of manifold pressure or the lack of it. What are these folks writing about. The only manifolds on a DD 2 cycle are exhaust. There is no intake manifold.

Well, that is the minute detail that leads me to state that the blower air charge is the primary air to drive the turbo out of the exhaust.

If I missed something, please point it out. I realize this does not address what Krush underlined in my previous post, but if we can all agree on this, I will address that next.

Pete

[Top Hatt Craig]

Avenger, while I am an admirer of Ronald Reagan, I believe a much older quote from Aristophanes may be more approp: “Youth ages, immaturity is outgrown, ignorance can be educated, and drunkenness sobered, but stupid lasts forever.”Pete I would like to acknowledge Avenger and Krush for their efforts to contribute facts to this forum, facts readers with expertise in other areas can use to learn more about their engines. It is unfortunate that an ego is preventing a proper exchange of knowledge. I find the included quote truly ironic. Regards, Craig

[krush]

Exhaust valve(s) open before intake port (or valves) open.

Remaining heat energy in the gasses in the cylinder further expand out the valve and spin the turbo. Assuming RPM is contant (say 1200 or 1800 RPM), the more fuel is injected per cycle to make more HP. The results is more heat energy in the gasses of the cylinder, which expands and spins the turbine on the turbo faster, which is connected to a centrifugal compressor...which pumps more mass with faster RPM.

http://marinediesels.info/Theory/timdiag1.gif

Step by step, includes PV diagrams and other stuff
http://marinediesels.info/Theory/actual_diesel_cycle.htm

[Avenger]

Thank you Craig. Your contributions are appreciated as well. I always try to remind myself that we are supposed to be helping each other and proceed accordingly.

With the consideration that this is probably borne of misunderstanding and not trolling I'm going to try to explain this in that vein. If I can figure out where to start.

And fortunately I can see as I'm typing that krush has most of it covered, so the only thing I'm going to add at this point, with a touch of sarcasm, is that manifold pressure is the proper technical term for what guys who think a DD is a "kit engine" would call boost.

[SEVEN]

I would like to acknowledge Avenger and Krush for their efforts to contribute facts to this forum, facts readers with expertise in other areas can use to learn more about their engines. It is unfortunate that an ego is preventing a proper exchange of knowledge. I find the included quote truly ironic. Regards, Craig

Well stated Craig.

[Pete]

I would add that to get the maximum efficiency of the fuel burn, the air to fuel relationship must be held close to 14.5 to 1. And since the air from the air box, supplied by the blower is controlled by engine RPM, dumping excess fuel into the cylinder on a lugging engine will result in incomplete combustion.

Krush, since you added no comment to your graphic I conclude that the purpose was to provide an increased level of precision on the timing of liner vents opening versus exhaust valves opening. If there is more to it, please explain.

Pete

[jrbrein]

First off, I am no diesel guru.

Can someone please tell me why fuel injection is designed to occur at (just before and after TDC) the moment of maximum compression inside the cylinder?

Would injector longevity or degree of complexity be different if they were able to inject fuel into a lower pressure environment somewhere say somewhere between when exhaust closes and TDC? Would compression of the fuel would still occur the same way?

Thank you!

Jon

[Seapig100]

I would say no the spray pattern is important for complete combustion. I would think that if you injected the fuel to early you would not have a perfect pattern. I am certainly no expert but that's my opinion.

[Avenger]

Oh Boy.

Let's start with Pete: You're still hung up on the scavenging thing I see. You are basically making the equivalent statement of "the intake stroke of a four-cycle engine drives the turbocharger." Scavenging providing the air charge for the cylinder does not drive the turbocharger. Any engine will not fire if you obstruct the intake so that is proof of nothing. And again, if it's all done by the blower how come an engine at maximum RPM in neutral develops zero manifold pressure (boost)? The engine has to be loaded for a turbocharger to make boost. If the mechanically driven, fixed ratio blower is running at the same speed in both scenarios how come an unloaded engine doesn't produce boost?

And please, dear God let's not go down the rabbit hole of mixture on a diesel.

Jon: Without getting into high tech engines like common rail diesels,... injection starts before TDC because of ignition delay. The fuel doesn't burn instantaneously so they allow time for the fire to get started. The actual pressure rise from combustion should start at TDC. Starting before TDC would be trying to run the engine backwards. Depending on load injection will continue to occur until after TDC until the full fuel charge has been delivered.

Jon and Seapig: Let's keep the term "Compression Ignition Engine" in mind. The fuel is not compressed in a mixture like a gasoline engine. Only air is compressed which compression brings it's temperature above the ignition temperature of diesel fuel. Then the fuel is injected at the precise time before TDC to start the ignition process as described above. Having fuel in the cylinder prior to that time will cause it to ignite too soon and try to drive the piston downward while the crankshaft is trying to push it upward. Not a good scenario.

Hope this helps.

[krush]

I would add that to get the maximum efficiency of the fuel burn, the air to fuel relationship must be held close to 14.5 to 1.

Krush, since you added no comment to your graphic I conclude that the purpose was to provide an increased level of precision on the timing of liner vents opening versus exhaust valves opening. If there is more to it, please explain.

Pete


Exhaust valve(s) open before intake port (or valves) open.


Air/fuel ratio talk is for gasoline engines (spark ignition), because it is a parameter that is monitored by oxygen sensor and controlled by the fuel system (carburetor, fuel injection). On a diesel engine, air fuel ratio is not explicitly controlled, nor is it monitored.

As for the second line, and purpose of the image I posted, see quote above.

[Top Hatt Craig]

I would add that to get the maximum efficiency of the fuel burn, the air to fuel relationship must be held close to 14.5 to 1. And since the air from the air box, supplied by the blower is controlled by engine RPM, dumping excess fuel into the cylinder on a lugging engine will result in incomplete combustion.Krush, since you added no comment to your graphic I conclude that the purpose was to provide an increased level of precision on the timing of liner vents opening versus exhaust valves opening. If there is more to it, please explain.Pete Now you're confusing nearly fixed ratio gas engines with wildly variable ratio Diesel engines. Diesels are throttled by changing their air/fuel ratio from very lean at idle to very rich at wide open throttle. From the Banks site.. "Gasoline engines operate within a narrow air/fuel ratio range of approximately 12:1 to 15:1, although some modern "lean-burn" technology engines have been able to achieve significantly leaner air/fuel ratios.Diesels can operate with a broader range as rich as 15:1 or as lean as 60:1, however, going richer than about 22:1 to 25:l produces excessive temperature, soot, smoke, and poor fuel economy." Regards, Craig

[Pete]

So, lets go on. The blower does provide the scavenger air charge, that provides the oxygen to support combustion with the diesel and it is this gaseous spent combustion vented through the exhaust valves that drives the turbo. No scavenger air charge compressed in the cylinder when the injector fires, no combustion, the turbo does get driven.

On the fuel air ratio, read what I wrote carefully, "to get the maximum efficiency of the fuel burn, the air to fuel relationship must be held close to 14.5 to 1" (not the 14.7 for gasoline engines). Not that it is the only ratio that is usable, or even most common, just the most efficient.

On the comment that during operation the exhaust valves must open before the piston clears the scavenger vents, absolutely. Otherwise the gaseous compressed spent combustion will vent into the air box. My post was a high level description of operational cycle, and was not trying to provide precision angular relationship, however if I inferred that the combustion somehow vented into the air box, that is incorrect.

And on the "kit" engine, perhaps some interaction with former 2 cycle DD engineering staff would support their common use of the term. The basis is you choose a basic block, and based on what fuel pump, fresh water pump, oil pump, governor, starter, cam shaft, etc. you bolt on you get a clockwise or counter clockwise rotating engine. It is simply a common parts strategy used in many low volume manufacturing operations. Another big advantage to the manufacturing business is inventory size that will need to be maintained for service parts.

Pete

[34Hatt]

Wow :D

I am still trying to figure out where a generator running at 1,200 rpm came from?
Except for a Phasor which is belt driving it is something like 2,800 rpm (or something like that)

All others are 1,800 or 3,600 for 60 hz some may think it is close enough but 1,200 to 1,800 is a good increase like 33%
Even if we go to 50 hz 3,600 runs at 3000 or 1,500 for a 4 pole gen.

[Avenger]

A six pole back end will produce 60 Hz at 1200 RPM. He's not talking about marine generators. He's trying to draw a parallel to running his DDs at 1200 RPM.

[krush]

1200rpm 3-53 DD: https://www.youtube.com/watch?v=MkGmVuaI4AQ

http://www.hardydiesel.com/diesel-generators/detroit-1200-rpm-diesel-generator-22-kw.html

[Avenger]

So, lets go on. The blower does provide the scavenger air charge, that provides the oxygen to support combustion with the diesel and it is this gaseous spent combustion vented through the exhaust valves that drives the turbo. No scavenger air charge compressed in the cylinder when the injector fires, no combustion, the turbo does get driven.

On the fuel air ratio, read what I wrote carefully, "to get the maximum efficiency of the fuel burn, the air to fuel relationship must be held close to 14.5 to 1" (not the 14.7 for gasoline engines). Not that it is the only ratio that is usable, or even most common, just the most efficient.

On the comment that during operation the exhaust valves must open before the piston clears the scavenger vents, absolutely. Otherwise the gaseous compressed spent combustion will vent into the air box. My post was a high level description of operational cycle, and was not trying to provide precision angular relationship, however if I inferred that the combustion somehow vented into the air box, that is incorrect.

And on the "kit" engine, perhaps some interaction with former 2 cycle DD engineering staff would support their common use of the term. The basis is you choose a basic block, and based on what fuel pump, fresh water pump, oil pump, governor, starter, cam shaft, etc. you bolt on you get a clockwise or counter clockwise rotating engine. It is simply a common parts strategy used in many low volume manufacturing operations. Another big advantage to the manufacturing business is inventory size that will need to be maintained for service parts.

Pete

If you spent any time with DD engineers you didn't learn a blessed thing from them.

Now you're getting to the point of using weasel words to back out of admitting you're wrong. I asked you some questions in my earlier post. Let's see if you can rationalize answers to them as well as you're rationalizing this BS.

[Pete]

Avenger, what is the question you asked that I did not respond to from your last post. The one I read was another denial that the scavenger air charge exits the exhaust valve and is a major player in driving the turbo. If this air charge as modified by the combustion process does not exit through the turbo input, where does it go?

And if you believe there are some weasel words, in your opinion, point them out.

You are slipping back into the yelpy model once again, being short on facts you resort to insults. I will refrain on this post, but am up for that game if you want to go down that path again. You decide!

Pete

[krush]

Avenger, what is the question you asked that I did not respond to from your last post. The one I read was another denial that the scavenger air charge exits the exhaust valve and is a major player in driving the turbo. If this air charge as modified by the combustion process does not exit through the turbo input, where does it go?



If no fuel in injected and the engine is turned over by the starter, does the scavenge air exhaust the cylinder and spin the turbo? What if the motor is in a truck and coasting down hill turning 1800rpm with the fuel shut off....does the turbo spin fast and raise the manifold pressure (aka make lots of boost)?

[Avenger]

Avenger, what is the question you asked that I did not respond to from your last post. The one I read was another denial that the scavenger air charge exits the exhaust valve and is a major player in driving the turbo. If this air charge as modified by the combustion process does not exit through the turbo input, where does it go?

And if you believe there are some weasel words, in your opinion, point them out.



First off, that's not what anybody said. So you're convoluting that to suit your rationalizations. Nobody denied that air goes in the intake and out the exhaust. "Major player" that's how it works? Really?

You're trying to equate the fact that an engine uses air with turbocharger operation. That's not what you said before and now you're trying to rationalize your way out of it.

Plenty of facts have been presented. It's your refusal to accept them, or admit that you're wrong that is the problem.

Here's the question again. See if you can do better:


if it's all done by the blower how come an engine at maximum RPM in neutral develops zero manifold pressure (boost)? The engine has to be loaded for a turbocharger to make boost. If the mechanically driven, fixed ratio blower is running at the same speed in both scenarios how come an unloaded engine doesn't produce boost?

[Pete]

Avenger,

An old wise man once wrote, "“Youth ages, immaturity is outgrown, ignorance can be educated, and drunkenness sobered, but stupid lasts forever.”

Thus I will not try.

Regards,

Pete

[johnjen50]

Really Pete? Who's reverting to insults now?

[Avenger]

As someone else here pointed out. How ironic.

Taking your ball and going home?

[Pete]

An old wise man once wrote, "“Youth ages, immaturity is outgrown, ignorance can be educated, and drunkenness sobered, but stupid lasts forever.”

Thus I will not try.

Regards,

Pete

[Pete]

JonJen50, Albert Einstein wrote, “The definition of insanity is doing the same thing over and over again, but expecting different results”.

Pete

[krush]

So if one is trying to move their boat up away from the storm in Florida that is soon to land, would running the engines at low rpm (1000-1200RPM) cause engine destruction and thus mobility which then would lead to dead-in-water while hurricane sneaks up and destroys/sinks vessel and crew dies after a heroic, but horrific fight to survive as the eye of the hurricane passes overhead?

Just think, they could've survived if they ran their engines at 1800RPM.

[Avenger]

Definition of insanity, is that why you keep repeating the quote?

All I'm pointing out is that you went from this:


The scavenger air volume is the primary contributor to the air volume exhausted through the turbo to increase the spool rpm.

To this:


If this air charge as modified by the combustion process does not exit through the turbo input, where does it go?

Interesting how the tune changed but the denial remains. I'd even let you take the politician's out of "I misspoke".


Another thing I found interesting while rereading this thread was these quotes:


Also, what is manifold pressure at WOT under load?




A second BTW, I have seen mention of manifold pressure or the lack of it. What are these folks writing about. The only manifolds on a DD 2 cycle are exhaust. There is no intake manifold.

Notice how he didn't blink at the first mention, but later was mystified by the term. Something about ol' Pete's story doesn't add up.

[Pete]

An old wise man once wrote, "“Youth ages, immaturity is outgrown, ignorance can be educated, and drunkenness sobered, but stupid lasts forever.”

Thus I will not try.

Regards,

Pete

[johnjen50]

Is this guy microlon in disguise??

[SEVEN]

I think you're onto something.

[Pete]

Krush, in your scenerio, those guys should and probably would run WOT, and hope that over propping their engines did not tear them apart and start throwing rods through the blocks and even the thick Hatteras fiberglass hull which would lead to the same fate.

Pete

[Pete]

[QUOTE=Avenger;318719]Definition of insanity, is that why you keep repeating the quote?

See, here we go again, a simple quote from our friend Albert Einstein, and you get it wrong.

Albert mentions two conditions required to establish insanity. First that you do something over and over, second that you expect different results.

Since I absolutely DO NOT expect different results, the test fails. Very simple, two conditions required, one not met, test fails.

Pete

[Avenger]

I think you misread my meaning there.

But funny thing is, I kept expecting different results too.

[jrbrein]

Avenger,

Your answer to my question re: fuel injection timing was very helpful...thank you!

You are the man.

Jon

[Avenger]

That's what I do. Literally.

Glad I could help.

[bobk]

So if one is trying to move their boat up away from the storm in Florida that is soon to land, would running the engines at low rpm (1000-1200RPM) cause engine destruction and thus mobility which then would lead to dead-in-water while hurricane sneaks up and destroys/sinks vessel and crew dies after a heroic, but horrific fight to survive as the eye of the hurricane passes overhead?

Just think, they could've survived if they ran their engines at 1800RPM.

I've done that trip 23 times with the 48 Hatteras/6V92s and they still purr.

Bobk

[Pete]

BobK, there are people who are heavy smokers that live to be 100. Not sure that makes it a recommended lifestyle?

Pete

[34Hatt]

A six pole back end will produce 60 Hz at 1200 RPM. He's not talking about marine generators. He's trying to draw a parallel to running his DDs at 1200 RPM.

Oh Now I see :D


From Krush Link

Most Diesel generators run at 1800 rpm or even 3600 RPM. Not these!!! They run at a slow slow 1200 rpm. This is a design that is hard to beat.They come with a delco brushless alternator that also turns at a slow 1200 RPM. There are not many Diesel generators that are built this tough.

[Avenger]

The biggest problem with that is cost. Obviously a more complex generator with more copper will cost more. The other problem is engine power. The power curves for engines like that are pretty low at 1200 RPM so you need a fairly large engine for the power you're getting. OTOH large engines run at lower RPM so more poles in the back end are required to get the frequency needed.

For example these Nordbergs don't spool very high. The reciprocating components tend to be a little too massive to crank up to 1800, so a more complex generator is required.

http://i44.tinypic.com/21bndcz.jpg

[sgharford]

North Shore Towers? I got to visit those up close once too.

[Avenger]

Rockville Centre. A little bigger than North Shore Towers.

But I have pics from NST also. We do work for them too. They have Chicago Pneumatics:

http://i41.tinypic.com/2danl8y.jpg

They recently switched to a mix of the diesels and CAT 3600 series natural gas engines.

[Pete]

"Notice how he didn't blink at the first mention, but later was mystified by the term. Something about ol' Pete's story doesn't add up."

Avenger, having duly noted your alarm that I was unaware of the existence of an air intake MANIFOLD between the turbo output and the blower input, I decided to educate myself. So having a few minutes available I broke out the 53 series, three volume parts catalog. Bottom line is I was unable to find any such part labelled manifold. So, how about some help, please point out the parts in this catalog. I only have the 53 series catalogs, having sold my 71 series parts catalog when we purchased our LRC. So please provide the reference for the 53 series engine to get me started.

Thanks

Pete

[Avenger]

So you're hung up on the use of the term "manifold pressure" because you've decided that since a DD doesn't have an intake manifold per se that that term cannot be correct. Manifold pressure is simply an industry term that refers to positive charge developed by forced induction. In automotive circles it's usually referred to as "boost". However when I'm working with the people who operate the engines you saw in the pictures nobody uses the term boost, they say manifold pressure. That's also generally true of industrial engines, generators and trucks.

Every manufacturer has variations in their terminology and unless you're a walking parts manual it's about impossible to get every one correct. Since I deal with engines of many sizes and stripes I tend to use the terminology that is most common and avoid getting hung up on semantics. As long as we can make ourselves understood it's good enough. i.e. if I'm going to get picky about semantics I'll point out that those slots in the liners are not "vents" as you said, they are ports. However I understood what you meant and that was good enough.

So if you want to be that specific; since the airbox on a DD functions as the intake manifold the term is airbox pressure. Same function, different words.

My issue with your comments was that when it first came up you asked what manifold pressure was at WOT which sounds like somebody with some experience and then later you come back with what do you mean manifold pressure, there is no manifold? Which doesn't sound like experience at all. Can you see where that looks suspicious?

[Pete]

So, in a hundred words or so you admit there is no such thing as an intake manifold on a Detroit Diesel after insulting my knowledge of the engines for not knowing about something so important.

Makes one wonder just how much of what you claim is equally a figment of your imagination. This was not a case of using a slightly different descriptive name, you made up a name and part that does not exist and criticized me for not knowing all about this common well known non existant part.

And the saddest circumstance of the whole event is how many other members accepted what you said as gospel, believing you were writing truth based on your superior knowledge. Please spare me your BS in the future.

Pete

[Avenger]

Wow!!!

[SEVEN]

GROW UP gentlemen. This is a forum for knowledge not pissing matches.

[krush]

Manifold pressure is a very common term. In fact, on many engine systems the MAP (manifold absolute pressure) is what reads the pressure of the air charge that is going into the engine (which may be under a vacuum, or above absolute pressure). On airplanes, there is often a gauge on the dash, "The manifold pressure gauge is an engine instrument typically used in piston aircraft engines to measure the pressure inside the induction system of an engine." http://www.askacfi.com/421/what-is-manifold-pressure.htm

http://expertaviator.com/wp-content/uploads/2011/10/ManifoldPressureGauge.jpg

In the UK, don't they call a wrench a spanner? Don't they call the hood, a bonnet...the trunk a boot?

By definition in a dictionary, a manifold is: "a pipe or chamber branching into several openings". Seems to fit what a DD airbox is, even if the manual doesn't call it that. Semantics isn't the best way to win a gun fight, is it?

[Avenger]

GROW UP gentlemen. This is a forum for knowledge not pissing matches.

And to think I was trying to raise the level of civility. But I guess he thinks he's found a "gotcha." I'm sure nobody's calling for warranty because DD left the intake manifold off the engine. This is just delusional. Pete can go on believing whatever he wants. As long as people who come here for knowledge aren't similarly deceived that's fine.

[Pete]

Avenger, just show us the air intake "manifold" between the turbo and blower where you measured zero pressure, that you were so amazed that Pete did not know about. Total BS, and nothing short of that.

Pete

[Pete]

Krush, perhaps you are missing my point. This is not about semantics, it is about inventing a non existing part, between the turbo and blower entry, measurements made using the part, and demeaning a members knowledge for not being familiar with the part. Appears some members may want to let it slide. Not me, what use is this forum if you can not count on accurate, sincere information? Just my thoughts.

Pete

[Seapig100]

Sometimes you just have to know when to let it go.

[yachtsmanbill]

WHEW... Sounds like this guy leans towards the Dimpleberger side of motive theory. Everyone
knows Detroits have aluminum intake manifolds just below the carburetor. I'm going back to my
man cave to prove this theory... ws

http://pad3.whstatic.com/images/thumb/0/0c/User-Completed-Image-Make-a-Bowl-%28Pipe%29-out-of-Aluminum-Foil-2016.01.30-20.04.29.0.jpg/670px-User-Completed-Image-Make-a-Bowl-%28Pipe%29-out-of-Aluminum-Foil-2016.01.30-20.04.29.0.jpg

[johnjen50]

Seven, although I do get your point, I also think its important for everyone else reading these threads to understand when incorrect or partially wrong information is put forth as fact. Derek is a pro working in the industry and clearly knowledgeable on these matters. I believe he is just trying to make sure that the many people reading this come away with a clear understanding of the discussion. I do think at this point things are clear though...........

[Pete]

Avenger, one unanswered question I have that only you can answer is on my 453N's would they benefit from the installation of one of your imaginary intake manifolds or do only the turbo models see the advantage?

Pete

[krush]

Krush, perhaps you are missing my point. This is not about semantics, it is about inventing a non existing part, between the turbo and blower entry, measurements made using the part, and demeaning a members knowledge for not being familiar with the part.


I have no idea what your point is, and whether semantics is involved or not. But I will provide two pictures that show the turbo on a DD connected to an intake pipe (could be called manifold, per my definition in previous post). Manifold pressure, or boost, would be measured here.



http://tqdieselengines.com/images/Industrial%20Engines/Detroit-6-71TB-1.jpg

https://i.ytimg.com/vi/X4Ua0a-OEuE/maxresdefault.jpg

[Pete]

Krush, appreciate the pictures and your thoughts on what could constitute a manifold. Before responding I did a search of definitions on manifolds as they apply to engine parts, and all include multiple ports as an element of a manifold. This aligns with my understanding of a manifold. It also is consistent with the naming conventions used by DD, as I understand them.

The actual part is not my primary point, that is one member demeaning the knowledge of another using an imaginary part and knowledge of this non part as the basis of the critique. That is not and should not be what this forum is all about.

Pete

[Avenger]

Pete, an old wise man once wrote, "“Youth ages, immaturity is outgrown, ignorance can be educated, and drunkenness sobered, but stupid lasts forever.”

I'm done trying to fix that.

Thank you everyone who made quality contributions to this thread and who has supported me here. Your confidence, backing and help is much appreciated.

[racclarkson@gmail.com]

GROW UP gentlemen. This is a forum for knowledge not pissing matches.Thank you, Rusty. Whatever points, counterpoints, corrections, clarifications, illuminations or elucidations have been made have likely been lost on most everyone seventeen pages ago. With so many of our friends in danger, you guys need to sign off. If for no other reasons, good taste and some semblance of compassion for what is really important.

[kelpy]

Avenger, having duly noted your alarm that I was unaware of the existence of an air intake MANIFOLD between the turbo output and the blower input, I decided to educate myself. So having a few minutes available I broke out the 53 series, three volume parts catalog. Bottom line is I was unable to find any such part labelled manifold. So, how about some help, please point out the parts in this catalog. I only have the 53 series catalogs, having sold my 71 series parts catalog when we purchased our LRC. So please provide the reference for the 53 series engine to get me started


Pete, I don't believe that you actually did this! Except for you, everybody on the face of the earth that knows anything about a 2 cycle Detroit knows that the "combustion air manifold" is cast integral to the block and is refered to as the air box? It comes with the engine block "kit", as you prefer to call it and the blower "kit" is bolted over the opening to it. It is the same as the old Ford six cylinder engines that had the intake manifold cast integral to the head. You knew it was there and what it was but there wasn't a part number for it.

I want you to know that I have been thoroughly amused by the answers you have given to Krush, Avenger, Seapig and the others that actually know what they are talking about. It has provided some levity to my drive north away from my home and boat in south Florida. Keep up the good work.