A motorcycle engine propels a motorcycle. The engine typically sits immediately under the fuel tank, in between and just forward of the rider's legs.
Almost all commercially available motorcycles are driven by conventional gasoline internal combustion engines, increasingly four-strokes in all size ranges. The mid-range and large two-strokes seen in the 1970s and 1980s have almost disappeared, particularly as emission laws were introduced. There are a few small scooter-type models using batteries and an electric motor. Two manufacturers in the 1980s produced quite small numbers of motorcycles propelled by Wankel rotary engines, but these were neither clean, nor economical nor particularly reliable.
Displacement is defined as the total volume of air/fuel mixture an engine can draw in during one complete engine cycle. In a piston engine, this is the volume that is swept as the pistons are moved from top dead center to bottom dead center. This is the "size" of the engine. Motorcycle engines range from less than 50 cc (cubic centimetres), commonly found in many mopeds and small scooters, to a 6,000 cc engine used by Boss Hoss in its cruiser style motorcycle BHC-3 LS2. Many state laws in the U.S. define a motorcycle as having an engine larger than 50cc, and a moped as a vehicle with an engine smaller than 60cc.
Small motorcycles normally have a single cylinder, many smaller and mid-range motorcycles have twin cylinders and most medium to large motorcycles have triple cylinders and four cylinders. However, no generalizations can be made, as there are a few large singles and twins. Three cylinders have been widely used and there have been some six-cylinder machines. Many different layouts have been used with vertical cylinders the most popular. There are some horizontally opposed and V layouts.