What is a Two Stroke Engine?

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  • Written By: H.R. Childress
  • Edited By: J.T. Gale
  • Last Modified Date: 12 October 2019
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A two stroke engine is a type of internal combustion engine that combines the four operations of a typical gas or diesel engine into two strokes. Gasoline-powered two stroke engines can be found in items such as chainsaws and outboard motors, while diesel two stroke engines are generally used to power things like locomotives and ships. Typical four stroke engines go through a cycle of intake, compression, combustion, and exhaust. A two stroke engine performs the same operations, but has only a compression stroke and a combustion stroke — intake and exhaust are incorporated into the compression and combustion strokes.

Basically, a two stroke engine consists of a piston within a cylindrical chamber. A compression chamber is on one side of the piston and a crankcase is on the other. The spark plug is located at the end of the compression chamber. Intake and exhaust ports are located in the walls of the cylinder.

During the compression stroke, the piston compresses a mixture of air and fuel while moving toward the spark plug. Simultaneously, another mixture of air, fuel, and oil from the carburetor is being drawn into the crankcase through the intake port. When the piston reaches the end of the compression stroke, the spark plug fires and the combustion stroke begins.


When the spark plug fires, the compressed air and fuel in the compression chamber ignite, which forces the piston downward for the combustion stroke. As the piston moves downward, the exhaust port is uncovered and the pressure in the cylinder pushes exhaust gases out; at the same time, the piston's movement is compressing the air and fuel in the crankcase. When the piston reaches the bottom of its stroke, the intake valve is uncovered and the pressurized fuel mixture in the crankcase escapes into the cylinder. Then the compression stroke begins again.

A diesel two stroke engine operates the same way, with one major difference: only air is drawn into the chambers and compressed. Fuel is injected directly into the compression chamber and immediately ignites because of the pressure and temperature. Diesel two stroke engines must also have a turbocharger and a supercharger to compress the air coming into the engine, making them larger and more expensive than gasoline two stroke engines.

Because of the difference in how fuel is used, gasoline two stroke engines are used for small tools and vehicles, such as lawn equipment and recreational water vehicles, while diesel two stroke engines are used for much larger applications, such as ships and trains. Neither is typically used in automobiles, as gasoline two stroke engines wear out more quickly, are less fuel efficient, and cause more pollution than four stroke engines. Generally, diesel two stroke engines are too large for a standard automobile, as well.


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