A car engine whose output power is 140 hpoperates at about 13 % efficiency. Assu

Question

A car engine whose output power is 140 hpoperates at about 13 % efficiency. Assume the engine's water temperature of 87 ?C is its cold-temperature (exhaust) reservoir and 510 ?C is its thermal "intake" temperature (the temperature of the exploding gas-air mixture).

Part A

What is the ratio of its efficiency relative to its maximum possible (Carnot) efficiency?

Part B

Estimate how much power (in watts) goes into moving the car

Part C

Estimate how much heat, in joules, is exhausted to the air in 1.0 h.

Explanation / Answer

a)

carnot efficiency is

e = 1 - T_c / T_h

= 1 - ( 87+273 ) / (510+273)

=54%

ratio of efficient to carnot efficiency is

13% / 54 % = 0.24

B)

the input power of the car is

P_in = P_out / 13%

= 140 hp / 0.13

=1076.92 hp

= 8.03*10^ 5 W

c)

heat exhaust is

E = P t

= ( 8.03*10^ 5 W ) ( 3600 s)

=2.89*10^9 J

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