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Homework Help: Diesel Engine driving a generator

  1. Aug 28, 2009 #1
    1. The problem statement, all variables and given/known data

    A diesel engine which drives a generator has an output of 60 hp operating at a n=31%. Then generator has a n=79% and supploes a load circuit at 240 V. The diesel engine uses light fuel oil (LFO) with a calorific value of 19,600 BTU/lb.
    1. The generator output in kW
    2. The current in the generator
    3. the fuel consumption in gallons per day

    (weight of LFO=8.3lb/gal; 1BTU=1055J)

    2. Relevant equations

    3. The attempt at a solution
    I am not sure if this is correct, its out of the book so I don't know.

    For the first part i simple turned the 60hp into W knowing that 1 hp=745.7 W, so that gave me 44,742 W. I then multiplied that number by .79(the effecicency of the generator to get 35,346.2W which is equal to 35.35 kW. Is this correct?

    For the second part I simpled used P=IV plugging in P=35.35kW, and V=240 to get 147.27 A. I believe this part is right aslong as the first part is right.

    The third and final part i used what I learned in chemistry, stochiometry in a way,

    35,346.2 W *(1 BTU/1055 W)*(1 lb/19,600 BTU)*(1 gal/8.3 lb)=2.06x10^-4 gals

    everything except gals cancels, but the question asks gals/day but i don't know how I would do that....

    Thanks for your help!
  2. jcsd
  3. Aug 28, 2009 #2
    any ideas?
  4. Aug 28, 2009 #3
    I am just confusing myself more and more.
  5. Aug 28, 2009 #4
    Why can't I understand circuits :-/ thank god my major is Civil.
  6. Aug 29, 2009 #5


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    In your equation: 35,346.2 W *(1 BTU/1055 W)*(1 lb/19,600 BTU)*(1 gal/8.3 lb)=2.06x10^-4 gals

    Where did (1 BTU/1055 W) come from?
  7. Aug 29, 2009 #6
    Your diesel engine puts out 60 hp but it is only 31% efficient. Therefore the input energy to the engine is 193.5 hp. That is the energy the oil must supply to get 60 hp output.

    1 hp=33475 btu/hr, 1 lb of oil provides 19,600 btu's, 1 gal of LFO weighs 8.3 lb, there are 24 hours in a day (assuming the engine 24 hours)

    This information will give you gallons of LFO used per day
  8. Aug 29, 2009 #7
    RTW69: Where did you find that 1 hp=33475 btu/hr?

    OmCheeto: I thought that it said 1 BTU/1055J but really it said 1 BTU=1055 J so I could just multiple change for BTU to J just by multipling. I think I am going to go with RTW69 said and see where that takes me.
  9. Aug 29, 2009 #8
    Were my other parts correct for this problem then?
  10. Aug 29, 2009 #9


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    It is the same thing in the context of your equation.
    Yes. But you converted 1 BTU = 1055 J to 1 BTU = 1055 W.
    1 Joule does not equal 1 Watt.
    My conversion table says 1 hp = 42.4 btu/min, which does not yield 33475 btu/hr.
  11. Aug 29, 2009 #10
    I think I was wrong before can someone tell me if I am right now? (i am making another post right now)
  12. Aug 29, 2009 #11
    Part A: The generator output in kW

    If the engine has a input of 193.55 hp that means the output of the generator has to be 193.55 correct? so then I just do 193.55*745.7 W/1000=144.33 kW?

    Part B: The current in the gnerator load ciruit

    If the generator has an output of 193.55 with a effeicency of 79% the input is 245 hp, so then I use P=VI solve for I=P/V I have 245 hp*745.7W/240 V=761.24 Amps

    Part C: The fuel consumption

    193.55hp*(33475BTU/hr)*(1 lb/ 19,600 BTU)* (1 gal/8.3 lb)*(24 hr/1 day)=955.85 gal/day
  13. Aug 29, 2009 #12
    or instead of the 193.55 hp should it be the input of the generator which is 245 hp?
  14. Aug 29, 2009 #13


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    No. The initial problem stated:
    ie. the output of the diesel engine is 60 hp.

    The output of the generator will be less since it is only operating at 79% efficiency.
  15. Aug 29, 2009 #14
    1 Boiler horsepower (Bohp)=33,475 Btu/hr. It is a term used to rate boilers and is the energy needed to evaporate 34.5 lbs of water at 212 degrees F in one hour. You should probably use electric horsepower instead. I electric horsepower is 746 watts. 1 watt is 3.4129 Btu/hr or 1 electric horsepower is 2545 btu/hr. Sorry for the confusion.
  16. Aug 30, 2009 #15
    The output of the engine=the input of the generator? is this correct
  17. Aug 30, 2009 #16
    When I find the current in the generator load circuit do I use the power output? or input?
  18. Aug 30, 2009 #17


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  19. Aug 30, 2009 #18
    Ok, and why is that?
  20. Aug 30, 2009 #19


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    Why what? Yes or output?
  21. Aug 31, 2009 #20
    Why is it the output? Sorry about that
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