11. The most important factor in engine performance is the actual power output at the end of the crankshaft available for doing work. This is known as __________.

Brake horsepower is the correct answer; it represents the usable power measured at the crankshaft and available for external work, accounting for internal mechanical losses. Indicated horsepower is theoretical power, friction horsepower represents power losses, and 'net horsepower' is an imprecise term not referring to crankshaft output.