Two-stroke cycle

The two-stroke cycle of an internal combustion engine differs from the more common four-stroke cycle by completing the same four processes (intake, compression, power, exhaust) in only two strokes of the piston rather than four.

This is accomplished by using the beginning of the compression stroke and the end of the power stroke to perform the intake and exhaust functions.

This allows a power stroke for every revolution of the crank, instead of every second revolution as in a four-stroke engine.

For this reason, two-stroke engines provide high specific power, so they are valued for use in portable, lightweight applications such as chainsaws as well as large-scale industrial applications like locomotives.

The two-stroke spark-ignition engine's invention is generally credited to Joseph Day (and Frederick Cock for the transfer-port), whereas the two-stroke valved compression-ignition engine is attributed to Dugald Clark.

A two-stroke engine, in this case with an expansion pipe illustrating the effect of a reflected pressure wave on the fuel charge. This feature is not present in most engine designs.

The following animation shows a two-stroke engine in action. You can compare this animation to the animations in the car engine and diesel engine section to see the differences. The biggest difference to notice when comparing figures is that the spark-plug fires once every revolution in a two-stroke engine.

This figure shows a typical cross flow design. You can see that two-stroke engines are ingenious little devices that overlap operations in order to reduce the part count.

The two-stroke cycle

Two-stroke cycle engines operate in two strokes, instead of the four strokes of the more common Otto cycle.

Power/exhaust: This stroke occurs immediately after the ignition of the charge. The piston is forced down. After a certain point, the top of the piston passes the exhaust port, and most of the pressurized exhaust gases escape. As the piston continues down, it compresses the air/fuel/oil mixture in the crankcase. Once the top of the piston passes the transfer port, the compressed charge enters the cylinder from the crankcase and any remaining exhaust is forced out.
Compression/intake: The air/fuel/oil mixture has entered the cylinder, and the piston begins to move up. This compresses the charge in the cylinder and draws a vacuum in the crankcase, pulling in more air, fuel, and oil from the carburetor. The compressed charge is ignited by the spark plug, and the cycle begins again.

In engines like the one described above, where some of the exhaust and intake charge are in the cylinder simultaneously the gasses are kept separate by careful timing and aiming of the transfer ports such that the fresh gas has minimal contact with the exiting exhaust which it is pushing ahead of itself.

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