engine efficiency

Question

As a gasoline engine is running, an amount of gasoline containing 9,900 J of chemical potential energy is burned in 1 s. During that second, the engine does 3,300 J of work.

(a)What is the engine's efficiency (in percent)?

(b) The burning gasoline has a temperature of about 4,600°F (2,800 K). The waste heat from the engine flows into air at about 82°F (301 K). What is the Carnot efficiency (in percent) of a heat engine operating between these two temperatures?

Answer 1

The efficiency of an engine is

W/Q

where W is the work output,

and Q is the energy input.

Carnot efficiency, which is the optimal efficiency obtainable from an engine

with input temperature T_hot and output temperature T_cold, is

1 - T_cold/T_hot

Substituting actual numbers

(a) efficiency = 3300/9900 = 1/3 = 33.33%

(b) Carnot efficiency = 1 - 301/2800 = 0.8925 = 89.25%