When gasoline is burned, it gives off 46,000 J/g of heat energy. If a car uses 13kg of gas per hour... (cont.)

with an efficiency of 21% what is the average horsepower output of the engine? (1 hp=746 W)

I know the answer is 47 hp, I'm looking for how to solve the problem. A step by step solution would be amazing!

I take it you got the answer out of the back of the book and need to show your work, so here 'ya go.

46,000 joules per gram X 13,000 grams = 598,000,000 joules
598,000,000/3600 seconds per hour =166,111 watts
166,111 watts/746 = 222.67 hp


Joules to watts calculation formula

The power P in watts is equal to the energy E in joules, divided by the time period t in seconds:
P = E / t
So watt = joule / second
Or W = J/s

Example
What is the power consumption of an electrical circuit that has energy consumption of 90 joules for time duration of 3 seconds?
P = 90J / 3s = 30W