Underloading a Diesel

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

  1. FAST FRED
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    FAST FRED Senior Member

    The question of weather an engine is "underloaded" .

    LOAD , or lack of it is the question.

    Prime gen sets will frequently run at 1200rpm, the 50cps folks will use 1500, and the stand by gensets will chose 1800.

    Prime usually is on 24/7,, standby only a day or week as needed,

    The "proper " load will vary with the rpm chosen , a heavily but correctly loaded engine will not slobber to an early death

    "What is the difference with similar Diesel engines used in marine applications which apparently have problems with similar usage?"

    The tiny gas replacements like a BMW car now Yannmar do not suffer as car engines because the rpm and LOAD vary.Hills help cars to live longer.

    The big problem with underloading is the compression pressure of the cylinder firing is what gets behind the compression ring , forcing it to seal.

    With low pressure from tiny loading the cylinder will not seal,blowby results, diluting the oil, and causing crud to form in the ring lands.

    The low ring pressure burnishes the cylinder walls , you loose the honing that holds oil.for proper lubrication, more loss of compression.

    For a light duty truck or tractor where under loading could be a problem , usually no turbo, and square cut rings are installed by the engine maker.

    FF
     
  2. Easy Rider
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    Easy Rider Senior Member

    Running at a full load at slow engine speeds is like making a truck pull a hill that it can barely pull at WOT. Even truckers downshift. The issue of underloading has mostly to do w heat. A diesel engine runs w far less fuel at half load than a gasoline engine and produces much less heat whereas a gasoline engine engine runs w basically a constant combustion temperature. And Frosty the coolant temperature (that is thermostatically controlled) only controls the coolant. The engine itself is relatively independent ...temperature wise. In a diesel engine running at 30% load the exhaust valves, guides, pistons and rings are much much cooler than in the same engine running at 75% load. I think an engine will last longer running at a lighter load and faster. If you run an engine at slower speeds and require it to produce the same amount of power at lower speeds much greater forces on the piston will result in much greater forces exerted on the cylinder wall and the side of the piston. Higher speeds and less forces should result in less wear. My grandmother had a 1956 Volkswagen and the manual said to shift into 3rd gear at 30mph. She lived in the city where the speed limit was 30 mph and grandmother followed directions very well. She wen 29mph in 2nd gear most of the time. Relatively speaking the little VW made a lot of noise but was in excellent condition when she sold it. I think it's best and I think most or all engine manufacturers will recommend propping for rated rpm at WOT. Engine noise in this case is not a bad thing.
     
  3. Frosty

    Frosty Previous Member

    If we want to go down this pressure behind the rings theory which I dont hold much emphasis to then you have to consider that the pressure to do that is only for a few degrees on the top of the compression stroke and ignition stroke . The rest of the time there is none.




    Quote
    And Frosty the coolant temperature (that is thermostatically controlled) only controls the coolant. The engine itself is relatively independent Quote

    And the coolant cools the engine, that is the only way we have of cooling an engine isnt it?
     
  4. powerabout
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    powerabout Senior Member

    Old post but even northern lights had 50kw 1200 60hz rpm gen sets
    they were very quiet
    Cat 3412 500kw @1200 are everywhere its just that they are not clean emission wise so gone from all catalogues these days
     
  5. DCockey
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    DCockey Senior Member

    Four different situations which seem to be frequently confused:

    1) Engine running at rated power at max rated speed. This would be the situation on a boat with a prop which gives the highest speed at max rated speed with the throttle wide open.

    2) Engine running at less than max rated speed with the throttle less than wide open. This would be the situation with a prop as described in 1) but running at less than wide open throttle.

    3) Engine running at less than max rated speed but with the throttle wide open.

    4) Engine running at max rated speed but with the throttle less than wide open.

    It also seems that some of the information about Diesel engines and what's good for them and what's not is not applicable to all engines old and new.
     
  6. powerabout
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    powerabout Senior Member

    and I would say light load on a turbo engine is also more of a problem than a NA engine due to the vast difference in cyl temp and more importantly pressure.
     
  7. Easy Rider
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    Easy Rider Senior Member

    Frosty,
    Engines are cooled by air, liquid coolant, circulating oil and sea water in our boating applications. My Marine diesel runs it's coolant at over 185 degrees at 1100rpm w no load at all in neutral gear and at 190 degrees in gear at 70% load. The internal parts of the engine like pistons and valves run at MUCH MUCH higher temps at higher loads while the coolant temp remains the same. Lube oil temperature is a much better measure of the temperature of internal engine parts than coolant and exhaust gas temp is most likely even better yet but coolant temp does not reflect properly the actual temperature of the engine.
     
  8. Frosty

    Frosty Previous Member

    All you need to consider is if you need to load the engine do it at an RPM that it produces power.

    That may not be max RPM

    If I race my Mazda diesel 2.2 12 valve from the lights(ha ha) I keep it at 1800 to 2200 and not the red line 5000.

    It pulls much harder at these RPM where it makes power, it has nothing at over 4000.
     
  9. DCockey
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    DCockey Senior Member

    Lots of confusion about the difference between power and torque. The fundamental, simple relationship is:
    Power = Torque x Rotational Speed
    Torque = Power / Rotational Speee

    What matters in a vehicle is power and torque at the drive wheels. What matters in a boat is power and torque at the propeller shaft. If engine A has 50% more torque than engine B but at half the speed of engine B, then engine A has 75% of the power of Engine B. Pick transmission/reverse gear ratios so that the wheel/propeller rotational speeds are the same and vehicle/boat with engine A will only have 75% of the torque at the wheel/propeller as the one with engine B. (Neglecting any differences in drivetrain losses. With gears the differences should be very small to negligable.)
     
  10. DCockey
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    DCockey Senior Member

    Cars and truck Diesel engines have turbos, and only run at a fraction of rated load most of the time. Why don't they have problems?
     
  11. michael pierzga
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    michael pierzga Senior Member

    The typical gen sets I see are 1500 or 1800 . You could spin very slowly if the electricity generating componenet was designed for it. A tramsmission may also be used to change the rpm powering the unit.
     
  12. FAST FRED
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    FAST FRED Senior Member

    "The typical gen sets I see are 1500 or 1800 . You could spin very slowly if the electricity generating componenet was designed for it. "

    Minimum rpm is the design of many modern small gen sets.
    They produce power at the lowest RPM which is converted to the local flavor 50cps or 60cps.

    Higher load simply increases the RPM.
     
  13. powerabout
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    powerabout Senior Member

    Because thats what they are designed to do
    Boat engine could be running at full power all day +
     
  14. powerabout
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    powerabout Senior Member

    I would hazard a guess that the future of gen sets will be have them variable speed and the hz sorted out by electronics
    There would be a fuel saving so hence overall emission gain which would be the driver
     

  15. michael pierzga
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    michael pierzga Senior Member

    They have Frequency converters and also Continuously variable transmissions for generator installations. Ive never seen one on a typical small craft gen installation.
     
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