Effects of Running Diesel at Low RPM

[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.