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Homework Help: Formula for working out power produced by petrol being burnt in an engine

  1. Jun 24, 2012 #1
    Iam not looking for the answer, I would just like to pointed in the right direction with the formula which would help me work this out. Its been a number of years since I have had to do this. (AN EXTREMELY MATURE STUDENT) Thank you

    If the fuel consumption of a car, travelling at a steady 60 mph, is 20 mpg what is the power produced by the petrol being burnt in the engine? [Assume that the density of petrol is 0.84 kg/l and the heat of combustion of petrol is 47 302 kJ/kg]
  2. jcsd
  3. Jun 24, 2012 #2
    Hi clifftifer! Welcome to PF :smile:

    Power is the rate of energy used up or work done in unit time. Can you find this from the given information??
  4. Jun 24, 2012 #3


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    Gold Member

    And proper PF members wouldn't give it to you, even if you asked politely.

    I had to throw in a few of other bits of information before I could come up with the answer:

    1 watt = 1 joule/second
    3.785 liters = 1 gallon
    3600 seconds = 1 hour
    745.7 watts = 1 hp​

    to yield the answer in hp.

    To start, my equation looked like:
    x watts = 47,302,000 joules/kg * .............................​

    I ended up with joules/second, and then converted to hp.

    Simple plug and chug, as we used to call it. hmmm... Do people still use that phrase; "plug and chug"?
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