The two-stroke cycle differs from the four-stroke cycle by having only two strokes instead of four.

A typical simple two-stroke engine contains a piston whose face is shaped, an exhaust port on one side of the cylinder, and an intake port on the other side. The piston opens the exhaust port first, then sucks air-fuel mixture (the fuel normally has some oil mixed in) through the intake port, the exhaust port not allowing any air in by means of a valve. It then compresses the mixture and a spark plug ignites it.

There are more elaborate possible two-stroke engine configurations, but these often have enough complications that they do not outperform comparable four-stroke engines.

Two stroke engines mix lubricant with their fuel (either manually at refueling or by injecting oil into the fuel stream) rather than an independent lubrication system as is used in four-stroke designs. The lubricant is subsequently burned.

The very smallest engines are usually two-stroke engines. They are often used in lawnmowers and mopeds. They are also standard in outboard motors, high-performance, small-capacity motorcycles, and hand-held motorized garden appliances like chainsaws. In each application, they are popular because of their simple design (and consequent 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). For handheld devices, they also have the advantage of working in any orientation, as there is no oil reservoir.

Two-stroke engines have several marked disadvantages that has largely precluded their use in automobiles and is reducing their prevalence in the above applications. Firstly, they require much more fuel than a comparably powerful four-stroke engine due to less efficient combustion. The burning oil, and the less efficient combustion, makes their exhaust far smellier and more damaging than a four-stroke engine, thus struggling to meet current emission control laws. They are perceived as noisier, partly due to the more penetrating high-frequency buzzing and partly due to the fact that muffling them reduces engine power far more than on a four-stroke engine (high-performance two-stroke engine exhausts are tuned by computing their resonant frequencies, essentially). Finally, they are considered less reliable and durable than four stroke engines.

New two-stroke designs rely on electronically-controlled fuel injection, oil injection and other design tweaks to reduce pollution and increase fuel efficiency. However, such systems increase the cost of the engines to the point that for small systems simple four-stroke engines are most cost-effective.

Two-stroke Diesel engines

A two-stroke cycle is also used on many large diesel engines. These differ from the simple two-stroke cycle described above in that they have normal intake and exhaust valves in the cylinder head, like a four-stroke engine. In these engines, the two-stroke cycle is used to improve power-to-weight ratio and/or reduce the engine speed to increase reliability.

The simpler stroke in the diesel two-stroke cycle is the compression stroke; both valves are closed, and the rising piston compresses the air, heating it. At the top of the stroke, diesel fuel is injected into the cylinder, where it ignites and burns. The hot, high pressure gases produced by the combustion push against the piston as it descends in the initial part of the second stroke, delivering power. At this point, both valves are still closed. When the piston nears the bottom of the stroke, the exhaust valve opens, and the exhaust gases, still under pressure, rush out. The intake valve then opens. Air under pressure rushes into the cylinder, blowing out the remainder of the exhaust gases. The exhaust valve closes at that point, and shortly after that, and at about bottom dead center, so does the intake valve.

The diesel two-stroke requires forced induction - the intake air must be under pressure, since the engine does not have an induction stroke and cannot suck the air in by itself. A low-pressure supercharger (blower) is needed at minimum, but many are turbocharged.

The diesel two-stroke lacks the inefficiency and pollution problems of the gasoline two-stroke, since no unburned fuel, only air, can get blown out of the exhaust valve before it closes.

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