The engine of a motorcycle is suspended within the vehicle frame between the front and rear wheels. Like internal combustion engines that power automobiles, motorcycle engines transform chemical energy into mechanical energy by igniting a volatile mixture of fuel and air within a cylinder, causing gases to expand suddenly. The expanding gases push down on a piston, which turns a crankshaft. The crankshaft transforms the energy from the piston into rotary motion. The rotational force of the engine’s crankshaft turns other shafts and gears that eventually cause the rear wheel to rotate.
Engines with larger cylinders—or more of them—are more powerful and consume greater amounts of fuel. An engine’s displacement, or size, is expressed in terms of the number of cylinders it has and the total volume, in cubic centimeters (cc), displaced by each cylinder. Motorcycles may have single-cylinder, twin-cylinder, four-cylinder or even six-cylinder engines with displacements that range from 250 cc to 1500 cc or higher.
The cylinders in two- and four-cylinder engines may be arranged parallel to one another. Engines with this cylinder configuration, called inline engines, are usually mounted sideways in the motorcycle frame. In other engines, cylinders are canted at a 45-degree angle, in what has come to be known as a V-configuration. In two-, four-, or six-cylinder engines, cylinders may also be positioned horizontally opposite one another. The horizontal configuration produces less vibration than V-configurations or inline configurations do. It also lowers the center of gravity of the engine, improving motorcycle handling.
Motorcycle engines are also distinguished by the number of movements, or strokes, a piston makes per cycle. In four-stroke engines, the piston moves four strokes, igniting on the third stroke and expelling the spent gases on the fourth. Two-stroke engines have a simpler design that enables them to fire in two strokes. However, two-stroke engines burn a combination of oil and fuel, thus producing more pollutants than four-stroke engines, which separate the oil and the fuel. Some motorcycles have single cylinder two-stroke engines, but all motorcycle engines that have multiple cylinders are four-stroke to reduce exhaust emissions.