What is a two-stroke engine? And how does it work?

A two-stroke engine refers to the number of strokes the piston needs to complete in one cycle or one rotation of the crankshaft. A stroke is one up or down movement of the piston inside the cylinder. Thus, in a two-stroke cylinder, the piston moves up and down once in one cycle. More specifically, the cycle is comprised of two strokes: the "compression stroke" and the "combustion stroke" (Brain), also known as the power stroke.

During the compression stroke (the upward movement of the piston), the piston's "underside pulls a partial vacuum in the crankcase" (Cameron) and seals the intake port and the exhaust port, and compresses the air/fuel mixture before being "ignited by a spark plug" (PetroWiki). Once the spark plug has ignited the compressed air/fuel, the combustion "of the air/fuel mixture pushes the piston through the power stroke" (PetroWiki). Moreover, "the descending piston is... compressing the fuel-air mixture trapped beneath it in the crankcase" (Cameron). During its power stroke, the piston uncovers the exhaust port to expel gases and then uncovers the intake port to let fresh fuel/air into the cylinder (PetroWiki). The fresh fuel/air mixture rushes into the cylinder because the pressure in the cylinder is lower than the pressure in the crankcase (Cameron). The crankcase acts "as a pressurization chamber to force air/fuel into the cylinder" (Brain).


In comparison to four-stroke engines, two-stroke engines have a "great power-to-weight ratio" because they have more power strokes and thus potentially "pack about twice the power into the same space" (Brain). In addition, they "do not have valves, which simplifies their construction and lowers their weight", and "can work in any orientation" (Brain). Also, two-stroke engines are "easier to start in cold temperatures" (AMSOIL Action News).


However, two-stroke engines have their disadvantages. For instance, they are not fuel efficient as every time new air/fuel enters the cylinder "part of it leaks out through the exhaust port" (Brain). In fact, "up to 30 percent of the unburned fuel/air mixture" can exit via the exhaust (AMSOIL Action News). This loss of unused fuel/air creates two challenges for the two-stroke engine. One is the loss of efficiency and power, which is why the two-stroke engine is not able to double the amount of power outputted despite having a power stroke every crankshaft rotation (AMSOIL Action News). The other challenge is that the lost fuel/air mixture pollutes the environment with its "leaking hydrocarbons" (Brain).

In addition, two-stroke engines do not have "a dedicated lubrication system", making them wear out quicker (Brain.) This is because two-stroke engines do not "have a traditional valve arrangement that separates the crankcase from the combustion chamber" (PetroWiki).To accommodate for this, two-stroke engines are lubricated "by mixing oil with the fuel" (AMSOIL Action News). However, this is also a source of pollution when the oil in the fuel/air mixture is ignited (Brain).

Do you have a preference for two-stroke engines? Let us know what you think in the comments.


Brain, Marshall. "How Two-stroke Engines Work." HowStuffWorks. InfoSpace LLC, 1 April, 2000. Web. 21 August, 2015.

Cameron, Kevin. "Tech Talk: Two-Strokes, Explained." Cycle World. Bonnier Corporation, 6 April, 2015. Web. 21 August, 2015.

"Reciprocating engines." PetroWiki. Society of Petroleum Engineers, n.d. Web. 21 August, 2015.

"Two-cycle Engine Applications and Lubrication Needs." AMSOIL Action News. AMSOIL INC., 1 July, 2001. Web. 24 August, 2015.

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