How Two Stroke Engine Works?

Have you been wondering how 2 stroke engine works? Despite being a somewhat sensitive issue, two-stroke engines are not as complicated as many people believe. The engine performs the four phases of the thermodynamic cycle (intake, compression, combustion, exhaust) in 2 linear movements of the piston. In fact, they have the same steps as a four-stroke engine, but with only two piston movements.

Parts of a two-stroke engine

In a two-stroke engine, as in other engines, we can distinguish between fixed parts and moving parts.

Fixed part

  • Cylinder head. – It is the cover that closes the cylinder. It supports the spark plug and is part of the combustion chamber.
  • Spark plug. – It is a device located in the upper part of the cylinder that blows the electric spark from the ignition.
  • Cylinder. – It is a cast piece made of iron or aluminum, inside which the piston moves.
  • Crankcase. – Also known as a pre-compression chamber. It is the structural metal housing that houses the engine’s operating mechanisms.

Mobile parts

  • Piston. – It is the cylindrical piece of aluminum alloy that moves alternately inside the cylinder. This allows the compression of the fluid and performs the movement.
  • Connecting rod. – It is the mechanical element subjected to tensile and compression stresses. It is responsible for transmitting the articulated movement to the crankshaft.
  • Crankshaft. – It is the axis layered with counterweights that allow taking inertia responsible for transforming the rectilinear movement into a circular one, or vice versa.

Two-stroke engine operation

As said earlier, the two-stroke engine has a very simple operation that consists of a four-phase cycle, as in the four-stroke engine. However, this performs the four phases, as mentioned above in just two times. Let’s see what happens in each of them:

Phase 1: Intake – Compression

In this first phase, the piston moves vertically towards the cylinder head from its BDC (Bottom Dead Center). During its upward travel, it opens the intake port at the height of the crankcase, allowing the air mixture to enter. At the same time, compression of the mixture begins at the top of the piston. For this process to be effective, the crankcase must be sealed. In this way, intake and compression are performed simultaneously.

Phase 2: Combustion – exhaust

When the piston reaches it’s TDC (Top Dead Center), maximum compression of the combustion gas mixture is produced. An explosion occurs thanks to the electric spark produced by the spark plug. This explosion produces an expansion of the gases that pushes down the piston. The piston communicates the movement to the crankshaft through the connecting rod.

In the downward path, the piston opens the exhaust port, which allows the combustion gases to escape. At the same time, the transfer port is opened, where a mixture of air and fuel enters the cylinder again. This occupies the vacuum caused by the gas outlet. When the piston reaches its BDC again, the cylinder is again full of gases, and the cycle starts again.

In this phase, the correct functioning of the machine’s exhaust system is very important. This is because its main work is to maintain the compression of the gases inside the combustion chamber, favoring a correct explosion. Also, a correct exhaust system favors the rapid evacuation of gases, optimizing the beginning of a new cycle. Once the cycle is finished, the gases from the combustion must be released for the entry of the new mixture.


The 2-stroke engines continue to be useful for a wide variety of uses, such as clippers and other garden elements. However, thanks to the practicality and ease of use of 4-stroke engines, it is that the latter has replaced two-stroke engines for cars. Lighter equipment with lower fuel consumption continue to take advantage of 2-stroke engines.

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