A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed: Intake, Compression, Combustion, Exhaust.
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The piston make two complete passes in the cylinder to complete one operating cycle. An operating cycle requires two revolutions (720°) of the crankshaft. A 4-stroke motorcycle has a more compact and complex design that relies on the same oil to lubricate the engine, clutch and gearbox.
Cylinder head design has a significant effect on the efficiency of combustion, and hence the power output of the engine. The head may be flat, in which case the combustion chamber resides within the cylinder and/or a depression in the piston crown, but usually a “dome” within the cylinder head provides most of the combustion volume.
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Fuel economy is better in four-strokes due to more complete combustion of the intake charge in four-stroke engines.A motorcycle engine normally drives the rear wheel, power being sent to the driven wheel by belt, chain or shaft. Most engines have a gearbox with up to six ratios. The rider changes gears on most motorcycles using a foot-pedal and manual clutch, but early models had hand-levers./wikipedia
