Parts of a 2 Stroke Engine

by Amrita Chuasiriporn
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El Caganer:

While most modern motorcycles no longer use two-stroke engines, many scooters still do. A lot of outdoor power equipment (such as chainsaws and snow-blowers) also uses two-stroke engines as well. While their construction is simpler than a four-stroke engine, it's still important to know what the function of each of the parts is, so you can get a better understanding of how the engine works as a whole.


Like other types of engines, a two-stroke engine has a crankcase that surrounds and protects all other parts of the engine. Inside, it has a crankshaft, connecting rod and single piston. It's also got an intake port, a reed valve, an exhaust port, and a cylinder---all in addition to the combustion chamber, where the power is produced that moves whatever the engine is powering.


The crankshaft in a two-stroke engine rotates, moving the piston by means of the connecting rod. These three parts are the only moving parts in a two-stroke engine. All power produced is a direct result of the action of these three moving parts.

Connecting Rod

The connecting rod is connected to the crankshaft at one end, and to the piston at the other. It translates the movement of the crankshaft so that the piston is moved up and down.


The piston is moved up and down inside the cylinder by the crankshaft, which is connected to it via the connecting rod. A vacuum is formed as it takes its upward stroke, drawing air and fuel down through the reed valve. When the piston reaches the top, the spark plug then lights the air/fuel mixture, burning it and sending the piston back down. On the downward stroke, the reed valve gets closed because of the increased pressure of the fuel and air mixture within, which is being compressed. New fuel and air travel via the intake port into the cylinder, ready to be burnt. The exhaust is expelled through the exhaust port, and an unpleasant side effect is that it usually takes some of the unburned fuel mixture with it.


A two-stroke engine fires once every revolution, unlike a four-stroke engine. Theoretically, this means that two-stroke engines should be more powerful than four-stroke engines with the same displacement. However, because some unburned fuel invariably escapes during the combustion process, they are not as efficient as they could be.


Different two-stroke engines may have different means of transferring exhaust and unburned fuel and air through them, using various ports and valves. This process is referred to as "scavenge phase," and you can find more information about the main scavenge phase types by following the "Outdoor Power Equipment" link in References.

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