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  1. #21

    Re: Effects of Running Diesel at Low RPM

    Quote Originally Posted by MikeP View Post
    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

  2. Re: Effects of Running Diesel at Low RPM

    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.
    Chris
    1973 48' Yachtfish
    "Boss Lady" my other expensive girlfriend.
    Follow the refurb at www.starcarpentry.com

  3. Re: Effects of Running Diesel at Low RPM

    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.
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  4. #24

    Re: Effects of Running Diesel at Low RPM

    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

  5. #25

    Re: Effects of Running Diesel at Low RPM

    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

  6. Re: Effects of Running Diesel at Low RPM

    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....
    Rob Brueckner
    former 1972 48ft YF, 'Lazy Days'
    Boating isn't a matter of life and death: it's more important than that.

  7. #27

    Re: Effects of Running Diesel at Low RPM

    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

  8. Re: Effects of Running Diesel at Low RPM

    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.
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  9. #29

    Re: Effects of Running Diesel at Low RPM

    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.

  10. Re: Effects of Running Diesel at Low RPM

    A few minutes, 5-10 maybe.
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