Off Topic — HabitableZone

Off Topic » in reply to That include losses?

Yes.
SDF — (archived)

I used the Kia Rio as an example, with about 30 mpg fuel economy at highway speed. My estimate was based on "input energy" from fuel of 66.5 kilowatts, or 89.2 horsepower -- that is total input of fuel energy content for that particular constant-speed example. Much higher power input would be required during acceleration. Much of that energy would be wasted by the IC engine, maybe 65% given off as heat, and 35% for mechanical use (have to look that up to confirm sometime), including the alternator that runs the A/C. So the power output from a car engine is probaby only about 23 kW (31 hp) at highway speeds. This is the mechanical power output after all the carnot losses are subtracted. Mechanical power during acceleration is upward of 120 hp or more. This is all implied by the fuel economy.

Interesting to note that an electric car would not be limited by carnot efficiency since it is not a heat engine, and therefore would not give off alot of waste heat prior to converting electicity to mechanical uses. So, power output would roughly equal power input. An electric car motor might therefore consume just 8 kW (10.8 hp) of electrical energy at highway speeds, assuming it weighs roughly the same as the Kia Rio, and has the same aerodynamic drag coefficient, rolling resistance, etc.

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