Most widely used form of internal-combustion engine, found in most automobiles and many other vehicles. Gasoline engines vary significantly in size, weight per unit of power generated, and arrangement of components. The principal type is the reciprocating-piston engine. In four-stroke engines, each cycle requires four strokes of the piston—intake, compression, power (expansion), and exhaust—and two revolutions of the crankshaft. In a two-stroke cycle, the compression and power strokes of the four-stroke cycle are carried out without the inlet and exhaust strokes, in one upstroke and one downstroke of the piston and one revolution of the crankshaft. The size, weight, and cost of the engine per horsepower are therefore less, and two-stroke-cycle engines are used in smaller motorcycles, most marine motors, and many handheld landscaping tools (e.g., hedge trimmers and chain saws). Seealso compression ratio; piston and cylinder; rotary engine.

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