# Heat Engines problem

1. Jan 29, 2012

### ChunkymonkeyI

1. The problem statement, all variables and given/known data
When gasoline(density=.7297 g/cm^3) is burned, it gives off 5.00 times 10^4 J/g(its heat of combustion). If a car's engine is 25% efficient, how much gasoline per hour must it burn in order to develop an output of 50.0hp. 1hp=746 W

2. Relevant equations
P=W/t
e=W/Qh

3. The attempt at a solution
I believed I was solving for time so what I did was I used P=W/t
then I rearranged it to t=W/P and plugged 5.00 times 10^4 for w and 746 times 50 for P and it didn't get me the right answer because the answer is 10.7 kg/h

2. Jan 29, 2012

### tiny-tim

Hi ChunkymonkeyI!

3. Jan 29, 2012

### ChunkymonkeyI

Last edited: Jan 29, 2012
4. Jan 29, 2012

### tiny-tim

aha!

everything in an exam question is there for a reason!

if you're totally stuck, just try either multiplying the answer by 0.25 (ie 25%), or dividing by it …

50% chance you'll get full marks!

in fact, the "car's engine is 25% efficient" means that the power output (to the wheels) is only 25% of the power input (from the gasoline)

5. Jan 29, 2012

### ChunkymonkeyI

Lol but I want to learn how to do the steps can u please show me them it's bothering me that I can't solve this physics heat engine problem pleaaaaaaaaaaase

6. Jan 29, 2012

### tiny-tim

the power output (to the wheels) is only 25% of the power input (from the gasoline) …

so put that into the equation

7. Jan 29, 2012

### ChunkymonkeyI

What equation

8. Jan 30, 2012

### tiny-tim

(just got up :zzz: …)
this one

9. Jan 30, 2012

### ChunkymonkeyI

10. Jan 31, 2012

### tiny-tim

find the energy from the gasoline,

then multiply by 0.25 to get the energy output