Reciprocating Internal-combustion Engines
The modern automobile is usually driven by a water-cooled, piston-type internal-combustion engine, mounted in the front of the vehicle; its power may be transmittedeither to the front wheels, to the rear wheels, or to all four wheels. Some automobiles useair-cooled engines, but these are generally less efficient than the liquid-cooled type. Insome models the engine is carried just forward of the rear wheels；this arrangement, whilewasteful of space，has the advantage of better weight distribution. Although passengervehicles are usually gasoline fueled, diesel engines (which burn a heavier petroleum oil)are employed both for heavy vehicles，such as trucks and buses, and for a small number offamily sedans. Both diesel and gasoline engines generally employ a four-stroke cycle.
The Wankel Engine
For some years，it was hoped that the Wankel engine, a rotary internal-combustionengine developed by Felix Wankel of Germany in 1954，might provide an alternative to thereciprocating internal-combustion engine because of its low exhaust emissions andfeasibility for mass production. In this engine a three-sided rotor revolves within anepithrochoidal drum (combustion chamber) in which the free space contracts or expands asthe rotor turns. Fuel is inhaled, compressed, and fired by the ignition system. Theexpanding gas turns the rotor and the spent gas is expelled. The Wankel engine has novalves，pistons, connecting rods，reciprocating parts，or crankshaft. It develops a highhorsepower per cubic inch and per pound of engine weight, and it is essentiallyvibrationless，but its fuel consumption is higher than that of the conventional pistonengine.
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