Two-stroke cycle

The two-stroke cycle of an internal combustion engine differs from the more common four-stroke cycle by completing the same four operations (intake, compression, power, exhaust) in only two strokes (linear movements of the piston) rather than four. Thus, there is a power stroke per piston for every engine revolution, instead of every second revolution. Wth proper design, a two-stroke engine can be arranged to start and run in either direction, and many engines have been built to do this. Engines not designed to run in reverse are still capable of doing it; however running one in reverse for long periods might cause internal damage. This is due to piston throw and piston pin offset, a design feature of all modern piston engines that reduces piston slap. Ignition timing will also be severely retarded in reverse and oil pumps will not function backwards.



Two-stroke engines are used mostly among the smallest and largest reciprocating powerplants, but less commonly among medium-sized ones.

The smallest gasoline engines are usually two-strokes. They are commonly used in outboard motors, high-performance, small-capacity motorcycles, mopeds, scooters, snowmobiles, karts, model airplanes (and other model vehicles) and motorized garden appliances like string trimmers, chainsaws and lawnmowers. In each application, they are popular because of their simple design (and therefore, low cost) and very high power-to-weight ratios (because the engine has twice as many combustions per second as a four-stroke engine revolving at the same speed). Two-stroke engines often have a simple lubrication system in which oil is mixed with the fuel, (then known as 'petroil' from "petrol" + "oil") and therefore reaches all moving parts of the engine. For this reason, for handheld devices, they have the advantage of working in any orientation, as there is no oil reservoir dependent upon gravity.

Two-stroke cycles have also been used in diesel engines, notably opposed piston designs, low speed units such as large marine engines, and V8 engines for trucks and heavy machinery.