Two-stroke cycle
The two-stroke cycle of an internal combustion engine differs from the more common four-stroke cycle by having only two strokes instead of four, although the same four operations (intake, compression, power, exhaust) still occur. Two stroke engines are distinguished by having one power stroke for every engine revolution, whereas the four stroke engine has one power stroke for every two engine revolutions.
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.
The two-stroke engine is simple in construction, but complex dynamics are employed in its operation.
A typical simple two-stroke 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 downward movement of the piston first uncovers the exhaust port, allowing most of the exhaust to be expelled, and then uncovers the intake port through which an air-fuel mixture (the fuel normally has some oil mixed in) is let into the cylinder. The exhaust port does not allow any air in by means of a valve. The piston then moves upwards, compressing the mixture which is ignited by a spark plug.
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.
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 two stroke diesel does not use the crankcase as an air pump because it 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. Also, there is no need to mix lubricant with the fuel.
Two-stroke engines have several marked disadvantages that have largely precluded their use in automobiles (although there was some use, such as in historic Saabs and DKWs) and are 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 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.
There are more elaborate possible two-stroke engine configurations, but these often have enough complications that they do not outperform comparable 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. Many large manufacturers, including Ford and Honda are still actively researching ways to build practical and clean two strokes for automotive use.Basic operation
intake & compression
The rising piston creates a partial vacuum in the sealed crankcase, a port between the crankcase and the carburettor (inlet port) is uncovered by the piston as it rises, and the air-fuel mixture enters the crankcase. As the piston descends, it closes the inlet port and pressurizes the crankcase. The air fuel mixture is forced into passageways that connect the crankcase to the cylinder. Holes connecting these passages to the upper cylinder (transfer ports) are uncovered by the descending piston and air-fuel mixture is forced into the upper cylinder. As the piston reaches the bottom and then starts to rise again, the transfer ports are closed by the piston and the air/fuel mixture is compressed.power & exhaust
When the piston reaches the top of its stroke, the mixture is ignited, and the piston is forced down by the rapidly expanding gases of combustion. As the piston descends, a hole in the side of the cylinder connected to the exhaust pipe (exhaust port) is opened, and the burned gases can escape. The transfer ports are just a bit lower than the top of the exhaust port, so there is a period of time when fresh air-fuel mixture is coming in while exhaust is leaving. The major components of two-stroke engines are tuned so that optimum airflow results. Intake and exhaust pipes are tuned so that resonances in airflow give better flow. The cylinder ports and piston top are shaped to minimise the mixing of the intake and exhaust flows. The incoming fresh charge assists in forcing the exhaust gas out, and a tuned pipe provides back pressure at just the right time to push fresh air-fuel mixture sneaking out the exhaust back in again. A major problem with the two-stroke engine is however the short-circuiting of fresh charge from intake to exhaust which increases fuel consumption and emissions of unburned hydrocarbons.Two-stroke diesel engines
Compared with four-stroke engines