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

Question

As a gasoline engine is running, an amount of gasoline containing 10,200 J of chemical potential energy is burned in 1 s. During that second, the engine does 3,400 J of work.

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

(b)The burning gasoline has a temperature of about 4,200°F (2,600 K). The waste heat from the engine flows into air at about 88°F (304 K). What is the Carnot efficiency (in percent) of a heat engine operating between these two temperatures?

Answer 1

The efficiency is the amount of work output divided by the amount of work input. The work output is given as 3400 J/s. The work input is given as 10200 J/s. The efficiency is 3400/10200 = 0.33 or 33%.

Carnot efficiency is defined as (T1 -T2)/T1 x 100 where T1 is the temperature of the burning gasoline and T2 is the temperature of the air; therefore, the Carnot efficiency is (2600K - 304K) /2600K x 100 = 88.3%.