Can someone help me understand torque curves for diesels?

Discussion in 'Diesel Engines' started by bntii, Jul 11, 2008.

  1. bntii
    Joined: Jun 2006
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    bntii Senior Member

    As per thread below I have a new re-power in my sailboat.
    Everything is fine but I am looking at the torque curve for the engine and am not clear on the operational limits implied by the power curve provided by the engine manufacturer.

    See specification:


    My question is- by looking at the "net continuous" rating for this engine I one sees a line which represents hp at a given RPM. This line continues all the way up to the full rated RPM for this engine. This observation leads me to believe that the engine may be continuously run at the full rated RPM's if it does not exceed the accompanying load at this rating.
    This interpretation of the power curve goes against what I had understood about how one must run diesels in sailboats:
    "Run at cruising of ~70%, use ~85% to push to weather and only use full revs under emergency conditions for brief period."
    The power curve tells me that I can run to full rated RPM's at any time if I do not overload the engine. In other words- load not RPM's is the limitation?

    Whats the deal?

    Edit: While we are at it- how do I calculate expected fuel use from this graph?

    I see: 170= gal/hp-h at 2400 which is 23 hp used.

    so... gal= 170(23-1) for a hours run... this makes no sense.

    Thanks all
  2. Guest625101138

    Guest625101138 Previous Member

    You can run the motor at full rpm and continuous power rating. The curves show it will not overheat. It will not be quite as efficient as running at lower rpm. The most efficient continuous output is 23HP achieved at 2400rpm. This is where the specific fuel curve is at a minimum.

    If you were selecting a prop you would like to work a bit below this point for cruising speed. Say 20HP at 2200 to 2400 rpm. This would then give margin to hold the speed in slightly adverse conditions.

    Specific fuel burn is say 183g/HPhr. At 20HP the motor would burn around 1.13USgph.

    If you use a smaller pitch prop to get 20HP at 3000rpm then the fuel burn goes up to 1.17USgph. Also you have no opportunity to go faster in good conditions. (The specific fuel rate provided is at rated torque. You may in fact find that operating slightly below rated torque improves efficiency even more. You need a series of fuel curves to determine this.)

    So the typical design conditions you provide are based on the above considerations. They are not hard and fast rules.

    The reason that the fuel burn goes up is that the engine is less efficient. There is more wear and tear. The porting is getting breathless. Dropping back to 70 - 80% of rated rpm will extend engine life as well as save fuel.

    Rick W.
  3. bntii
    Joined: Jun 2006
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    bntii Senior Member

    Thanks Rick,

    In my installation I am seeing peak load RPM's with installed prop of 2950. At these revs the hull is at "hull speed" of 7.2 and seems to have no tendency to easily exceed this speed. I am seeing 3125 no load rpm.
    The engine seems to my ear to be running very comfortably at 2250 which is the rev's I am using as 'cruise' rpm. At these revs I am seeing just under 7 kt which is a good speed I think for my hull. At just over 2000 I am going 6.5 kt and this may be the better best range rpm. I have net yet run in tough conditions. In a 15-18 kt head wind I was holding speed nicely at 2300 and am expecting to use ~ 2500 or 2600 if needed for tougher conditions. So I am hearing that there is a cost- more wear and tear, though it is technically permissible to run up to the higher end if really needed.

    Edit- I see from your answer and the fuel curve that the best fuel efficiency is at ~2400 so am i incorrect to say I will conserve fuel by running at a lower rate?
    This seems counterintuitive.

    Thanks much for the reply
  4. Guest625101138

    Guest625101138 Previous Member

    The prop selection seems close to ideal.

    The specific fuel consumption curve is quite flat. There is a slight benefit in running in the range 2200 to 2400. Remember this curve only applies to rated continuous torque. From other engine data I have looked at, the best efficiency occurs below rated torque as well. If the engine is at rated torque the combustion is probably at its limit. That is why I suggested 20HP would be better than 23HP at 2400rpm. Combustion engines typically give the best efficiency when both revs and torque are around 80 to 90% of rating. This means power output around 70% of rating.

    Another point to realise is that the curve gives "specific" fuel consumption. So lower power output means lower fuel burn rate.

    Rick W.
  5. bntii
    Joined: Jun 2006
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    Location: MD

    bntii Senior Member

    Thanks Rick- that helps.

    Joined: Oct 2002
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    FAST FRED Senior Member

    I am seeing peak load RPM's with installed prop of 2950.

    In that case I would consider 300rpm LESS as max rpm for cont operation.2650 tom avoid overloading.

    You are in the "sweet spot" and will never be able to measure a difference in fuel use from 2000 to 2600,.

    Sounds like a good prop/ boat match , just be sure to use Diesel Lube oil, and diesel Antifreez if required by Mfg.

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