How to Find the Number of Liters of Gas Used in a Gasoline Engine

[Benb591]

Homework Statement

The problem is "A multicylinder gasoline engine in an airplane, operating at 2.40

10^3 rev/min, takes in energy 7.98

10^3 J and exhausts 4.50

10^3 J for each revolution of the crankshaft." and it wants me to find the liters of fuel consumed in 1 hr of operation if the heat of combustion of the fuel is 4.03

10^7 J/L.

Homework Equations

Work done by an engine is Energy in - energy out

I then just tried to manipulate units since I was very confused...

The Attempt at a Solution

I did (7.98*10^3)-(4.5*10^3)*(2.4*10^3) to get the work done per minute. Then converted to work per hour by multiplying by 60. Then divided by the heat of combustion. This made my units come out as L/h revolutions, and as revolutions are unit-less I thought it would work but it did not...help!

Thank you,
Ben

 

[Asymptotic]

How many liters of fuel are used per engine revolution?

 

[phyzguy]

Two comments:

(1) The energies are given in J/revolution, so when you multiply by rev/minute the revolutions cancel and units come out right.

(2) Why subtract the exhausted energy? The fuel used depends on the input energy only.

 

[Benb591]

Oh...ok it makes sense now thank you so much!