Almost all commercially available motorcycles are driven by conventional gasoline internal
 combustion engines, increasingly four-strokes in all size ranges. Some are still
air-cooled (forced with a fan in some cases) but water-cooling is more common.
 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. Van Veen, Hercules, Norton, and Suzuki produced
quite small numbers of motorcycles propelled by Wankel rotary engines. The 2009 TT races
 included a new category 'TTX' for electric bikes using either fuel-cells or batteries

Most motorcycle engines have the primary working member or crankshaft across the
 frame (transverse mounting). Others are arranged to turn a shaft-drive to the rear
 wheel and the crankshaft is longitudinal, along the frame.

A sub-type of motorcycle, the scooter, has the engine as part of the rear
 suspension, so it is not fixed to the main frame. Such engines pivot to
follow the road surface and are partly "unsprung weight".
 The final drive of scooters is much shorter than that of regular motorcycles
 and is contained within the engine casings in an oil-bath, a design that is
only suitable for machines with small wheels[verification needed], or is fully
automatic using belts and expanding/contracting pulleys, ala DAF variomatic cars.
 The engines of the motorcycles known as underbones or "step-throughs" may be of either kind.
[edit] Two-stroke and four-stroke

Two-stroke engines have fewer moving parts than four-stroke
engines, and produce twice the number of power strokes; consequently,
two-stroke engines are more powerful for their mass. Two-strokes offer stronger
 acceleration, but similar top speed compared to a four-stroke engine. They are
also easier to start. However, two-stroke engines have shorter life due to
 poorer piston lubrication, since lubrication comes from the fuel-oil mix.

Four-stroke engines are generally associated with a wider power band
making for somewhat gentler power delivery, but technology such as reed
valves and exhaust power-valve systems has improved ride-ability on two-strokes.
[citation needed] Fuel economy is also better in four-strokes due to more complete
combustion of the intake charge in four-stroke engines.

Nevertheless, two-strokes have been largely replaced on motorcycles in
 developed nations due to their environmental disadvantages. Cylinder
lubrication is necessarily total-loss and this inevitably leads to a smokey exhaust,
 particularly on wide throttle openings. Two-stroke engined motorcycles continue to
 be made in large numbers, but mostly low power mopeds, small scooters and step-through
underbones where they still compete strongly with four-strokes (including the highest
 selling motorcycle of all time, the 50 cc Honda Super Cub). The major markets of
two-stroke motorcycles are in developing nations