Two-stroke engine

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A two-stroke engine is an internal combustion engine that completes the process cycle in one revolution of the crank shaft (an up stroke and a down stroke of the piston, compared to twice that number for a four-stroke engine). This is accomplished by using the beginning of the compression stroke and the end of the combustion stroke to perform simultaneously the intake and exhaust (or scavenging) functions. In this way two-stroke engines often provide strikingly high specific power, at least in a narrow range of rotations speeds. The functions of some or all of the valves of a four stroke engine are usually served by ports that are opened and closed by the motion of the pistons, greatly reducing the number of moving parts. Gasoline (spark ignition) versions are particularly useful in lightweight (portable) applications such as chainsaws and the concept is also used in diesel compression ignition engines in large and non-weight sensitive applications such as ships and locomotives.

Invention of the two-stroke cycle is attributed to Scottish engineer Dugald Clerk who in 1881 patented his design, his engine having a separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Englishman Joseph Day (and Frederick Cock for the piston-controlled inlet port).

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Applications

The two-stroke engine was very popular throughout the 20th century in motorcycles, small engined devices such as chainsaws and outboard motors and was also used in some cars, a few tractors and many ships. Part of their appeal was due to their simple design (and resulting low cost) and often high power-to-weight ratio. Many designs use total-loss lubrication, with the oil being burnt in the combustion chamber, causing "blue smoke" and other types of exhaust pollution. This is a major reason for two-stroke engines losing out to and being replaced by four-stroke engines in many applications.

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