Spectre5
Feb29-04, 06:50 PM
Hey, I need some help on this problem, I get an answer, but it I know it is wrong because an automobile does not get .76 miles to the gallon!
Problem:
"An automobile engine with a compression ration of 10 produces 30 hp while being driven at 50 mph on a level highway. In those circumstances, the engine power is used to overcome air drag, rolling resistance between the tires and the road, and friction wihtin the drivetrain. Esitmate the fuel economy in miles per gallon. The density of gasoline is 1.32 slug/cubic foot."
I found the otto cycle effiency using n(otto)=1-(1/r^(.4)) where r is the compression ratio.
this gives a 60.2% effiency
I then tried to find the pounds of force per gallon using the density.
I had 1.32 slug/ft^3 and converted it to lbm (pounds mass)/gal and since 1 lbm = 1 lbf, the answer was the same as 1lbf/gal (I got the answer to be 5.68 lbf/gal after using the correct conversions)
I then used the hp and the mph to get lbf using conversion again (I got 225 lbf)
Then I just mulitplied 50 miles times 5.68 lfb/gal times 1/225 lbf and times the effiency of .602
The answer I got was .76 mpg which is obviously much lower than it should be, and I do not know the correct answer
Any help is aprecieated, thanks!
Problem:
"An automobile engine with a compression ration of 10 produces 30 hp while being driven at 50 mph on a level highway. In those circumstances, the engine power is used to overcome air drag, rolling resistance between the tires and the road, and friction wihtin the drivetrain. Esitmate the fuel economy in miles per gallon. The density of gasoline is 1.32 slug/cubic foot."
I found the otto cycle effiency using n(otto)=1-(1/r^(.4)) where r is the compression ratio.
this gives a 60.2% effiency
I then tried to find the pounds of force per gallon using the density.
I had 1.32 slug/ft^3 and converted it to lbm (pounds mass)/gal and since 1 lbm = 1 lbf, the answer was the same as 1lbf/gal (I got the answer to be 5.68 lbf/gal after using the correct conversions)
I then used the hp and the mph to get lbf using conversion again (I got 225 lbf)
Then I just mulitplied 50 miles times 5.68 lfb/gal times 1/225 lbf and times the effiency of .602
The answer I got was .76 mpg which is obviously much lower than it should be, and I do not know the correct answer
Any help is aprecieated, thanks!