If the estimated Power (Energy/time) usage in a car traveling at 65 mph in a hig

Question

If the estimated Power (Energy/time) usage in a car traveling at 65 mph in a highway is distributed as follows: 20 kJ/s Thermal power in exhaust Thermal power due to road friction Mechanical power lost to internal engine 4 kJ/s 1 k/s friction Thermal energy lost to braking Thermal power lost through radiator Mechanical power used to overcome air 5 kJ/s 40 kJ/s resistance Mechanical power used to empower car1 k/s accessories Total power usage (traveling at 65 mph) 80 ki/s If A gallon of gasoline can provide about 1.30x105kJ of chemical energy Calculate the highway miles per gallon (mpg) consumed by the vehicle

Explanation / Answer

No. of miles/gallon required = (No. of miles / kJ) * (No. of kJ / gallon of chemical energy)
?= [ (No. of miles / sec) / (No. of kJ / sec) ] * [1.3 x 105 ?kJ / gallon]
= [ (65/3600 miles / sec) / (80 kJ / sec) ] * [1.3 x 105? kJ / gallon]
?= (65 x 1.3 x 105? ) / (3600 x 80)
= 29.34 mpg

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