# Formula for working out power produced by petrol being burnt in an engine

1. Jun 24, 2012

### clifftifer

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. Jun 24, 2012

### Infinitum

Hi clifftifer! Welcome to PF

Power is the rate of energy used up or work done in unit time. Can you find this from the given information??

3. Jun 24, 2012

### OmCheeto

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"?