Two-stroke engine

The two-stroke internal combustion engine differs from the more common four-stroke engine by completing the same (thermodynamic) cycle 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 combustion stroke to simultaneously perform the intake and exhaust functions, which is called scavenging. 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. Invention of the two-stroke cycle is attributed to Dugald Clerk around 1880 whose engines had a separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Joseph Day (and Frederick Cock for the piston-controlled inlet port).


Throughout the 20th century, many small motorized devices such as chainsaws and outboard motors were powered by two-stroke designs. They are popular due to their simple design (and resulting low cost) and higher power-to-weight ratios. However, in most designs to date the lubricating oil is mixed with the fuel, which significantly increases the emission of pollutants (due to the oil's incomplete combustion). For this reason, two-stroke engines have been replaced with four-stroke engines in many applications, though some newer two-stroke designs are as clean as four-strokes.

Two-stroke engines are still commonly used in high-power, handheld applications where light weight is essential, primarily string trimmers and chainsaws.

To a lesser extent, these engines may still be used for small, portable, or specialized machine applications such as outboard motors, high-performance, small-capacity motorcycles, mopeds, underbones, scooters, tuk-tuks, snowmobiles, karts, ultralights, model airplanes (and other model vehicles), chainsaws and lawnmowers. The two-stroke cycle is used in many diesel engines, most notably large industrial and marine engines, as well as some trucks and heavy machinery.

Several automobiles used two-stroke engines in the past, including the Swedish Saab and German manufacturers DKW and Auto-Union. Production of two-stroke cars stopped in the 1960s in the West, but Eastern Bloc countries continued producing Syrena in Poland, Trabant and Wartburg in East Germany with two-stroke engines until as recently as 1991. Suzuki also produced them in the 1970s.

Mode of operation of the two-stroke engine

1st stroke: The piston is at the bottom of the cylinder. A pipe at the left side is opened and lets the fuel mixture, which is already compressed a bit, flow from the lower to the upper part of the cylinder. The fresh gases expulse now the exhaust through an ejection pipe, which is not closed by the piston at this moment.

2nd stroke: After being hurried upward, the piston now covers the pipe on the left side and the ejection pipe. Because there is no way out any more, the upper, fresh gas mixture gets compressed now. At the same time in the part below fresh gas is taken in by the piston driving upward through the open suction pipe. At the upper dead-center, the compressed fuel mixture is ignited by the sparking plug, the piston is pressed downward while he compresses at the same time the fresh gas below. The process begins again as soon as the piston arrives at its lowest point.