Underloading a Diesel

Discussion in 'Diesel Engines' started by Paul F, Oct 3, 2011.

  1. Paul F
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    Paul F Junior Member

    Can anyone please explain what problems may occur if a diesel engine is used for long periods with little load.

    My boat is about 2700 Kg, a Bayliner planing hull, with Isuzu 2.8 turbo diesel, 113 HP. Much of my use is on a river with a speed limit of 6mph, therefor my normal RPM is 1300 - 1400.

    Regards, Paul
  2. CatBuilder

    CatBuilder Previous Member

    And on the same topic...

    What would be the difference between running a Kubota engine at 1200RPM to run a generator head and running a Yanmar engine at 1200RPM for the same reason?

    Is the RPM only one factor? As in... if you are close to full throttle at 1200RPM, is this OK on an engine that's rated for 3600 MAX RPM?
  3. michael pierzga
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    michael pierzga Senior Member

    When an engine is light loaded for long periods of time the engine will be operating cold. Oil and fuel in the cylinder is not fully burnt . Your exhaust system will soon clog up and inside your engine... the piston rings and cylinder liner will accumulate this unburnt residue...they call it glazing.

    If your engine oil is not HOT...and marine engines naturally run COOL...its lube oil will not be allowed to boil off atmospheric impurities. A lightly loaded diesel engine would benefit from a much shorted oil change cycle to eliminate this contamination..

    Whether a recreational boat...trollling or moving slow ...can generate enough engine hours to cause damage is a good question. For a marine generator that will run hundreds or thousands of hours per year the damage is real.

    At the end of this season when you perform routine maintenance, remove the water injection section of your exhaust system and check for clogging. Low temp exhaust carries unburnt fuel and will accumulate at the water injection point in your exhaust.
  4. CatBuilder

    CatBuilder Previous Member

    Good post. But still a follow on question...

    What if you have the engine at or near full throttle but have it governed down to 1200RPM? As in... a generator with a full load, but running at 1200RPMs to allow for a longer engine life.

    Does it operate hot enough if at full throttle but lower RPMs?
  5. Stumble
    Joined: Oct 2008
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    Stumble Senior Member


    While Michael is correct on the theoretical problems in under-revving a diesel, as he points out most recreational boats don't operate at anywhere near the amount required for this to be an issue.

    Take for instance a heavily used recreational sailboat engine that sees 200 hours a year. This really is a lot for a sailboat, but commercial engines my be used 16 hours a day, 7 days a week, 50-52 weeks a year, or about 5500 hours a year. Roughly 30 times the use of a recreational engine.

    Keep in mind that the service interval for oil changes for a diesel is usually every 200-300 hours OR 3 months. So our recreation engine will need 4 changes a year at 50 hours on average, while the commercial is getting an oil change every 18 days.

    The reality is that the build up of soot, and unburned fuel, and glazing in a recreational engine just isn't that big of a deal since there isn't enough fuel being used to really worry about, and the oil is being changed so frequently/gallon burned it never gets high enough concentrations to do damage.

    The real problem is when you use a commercial engine at a low load rate. So that 5500 hours a year, and an oil change is every 15 days, but the engine is operating at 25% of designed power can lead to problems.

    Cat the issue isn't rev's it is operating temperature, and temperature variation. So long as the engine is within it's designed temps it will be fine.
  6. Paul F
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    Paul F Junior Member

    Thankyou Michael,

    That does help me understand as I am new to the workings of a diesel engine. I do like the fuel consumption when compared to the V8 petrol engine the boat used to have.

    I hope to bring the boat to Spain in the future, then I will be able to use it at higher speed with more load.

    Regards, Paul

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  7. michael pierzga
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    michael pierzga Senior Member

    For a generator...RPMs and loading are different. The RPM always stays the same via its govenor..the load is what generates the heat to burn off contaminates.

    As for 1200 rpm ? Ive never had a unit that runs so slow. ...I dont know what kind of generating unit you have ?

    With a 4-pole generator, 1,800 rpm produces output of 60 Hz, 1,500 rpm produces an output of 50 Hz.

    If you changed the rpms you would somehow have to change the frequency with a complw converter.

  8. Paul F
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    Paul F Junior Member

    Thanks Stumble,

    So far this year I have logged 165 hours, and it is close to the end of the season here. In this time I have changed the oil 3 times, and will change it again before winter lay-up. Perhaps a few hours use at higher speeds will help clear the engine out befoore the winter ?

    Regards, Paul
  9. CatBuilder

    CatBuilder Previous Member

    1200... I'm a little slow. :D I meant 1800. Still, well under the 3600RPM Yanmar max or 3200RPM Yanmar constant duty RPM.
  10. michael pierzga
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    michael pierzga Senior Member

    Ya...1800 sounds correct. In general if you want a 20,000 hr service life generator you must go low rpm. 1800. Those turbo jobs run roll over and die fast
  11. Brian@BNE
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    Brian@BNE Senior Member

    My understanding is that load is the key - load the diesel so it does get to normal operating temperature and you will be ok. But, most manufacturers also have a minimum rpm for full load, so you need to confirm you aren't trying to operate below that.

    Yanmar have a range of liquid cooled industrial engines in the size range you will need that run at fixed 1800 rpm. That would be better than de-tuning a high rpm motor.
  12. DCockey
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    DCockey Senior Member

    Some cars and trucks have Diesel engines which usually operate at a small fraction of rated power, and run for thousands or even tens of thousands of hours without any problems resulting. What is the difference with similar Diesel engines used in marine applications which apparently have problems with similar usage? The cars and trucks have thermostatically controlled cooling systems which keep the engines at the desired temperature. Is this one of the differences?
  13. Frosty

    Frosty Previous Member

    A modern diesel engine with a thermostat to keep the requied temperature does not suffer. The engine will run the thermostats temperature at all RPM and loads. Cylinder glazing is more or less a thing of the past. Many truckers would idle all night to keep the cab warm without damage.

    A rule of thumb is 25% this would be the minimum load.

    A blast of full throttle for 1 minute per hour would keep things all in order.

    However its not a problem --unless your looking for one.
  14. TeddyDiver
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    TeddyDiver Gollywobbler

    Mostly true what's said here, thou cylinder glazing is a bit different issue. It's mostly due running idle (to load batteries) or any revs without a load. This means there's not enough pressure in the cylinders to press the piston rings agains the cylinder walls and so there's a lot of excess oil buildup what glazes later on. To avoid this the engine should be run at full load after a couple hours of idling to burn this oil before it glazes..

  15. CDK
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    CDK retired engineer

    First of all, my complements for the cleanest engine bay I've ever seen: not a trace of oil after 3 changes!

    Concerning your initial question, consider the many industrial engines that never felt the full throttle experience. The cell phone masts and TV transponders in remote locations are often powered by small diesels running at 1500 rpm.
    They make 8760 engine hours per year and get serviced once every 2 months after doing almost 1500 hours of non-stop operation. At that occasion the giant fuel tank is filled, oil and filters are replaced and vital functions are checked. Such engines have a service life of more than 30.000 hours without any serious repairs. After 4 years they are replaced and sold to farmers, fishermen etc.
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