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Concept
Two-stroke engine
Summary
A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of the crankshaft. A four-stroke engine requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust (or scavenging) functions occurring at the same time.
Two-stroke engines often have a high power-to-weight ratio, power being available in a narrow range of rotational speeds called the power band. Two-stroke engines have fewer moving parts than four-stroke engines, and thus are cheaper to manufacture. In countries and regions with stringent emmissions regulation, two-stroke engines have been phased out in automotive and motorcycle uses. In regions where regulations are less stringent, or non-existent, small displacement two-stroke engines remain popular in mopeds and motorcycles.
The first commercial two-stroke engine involving cylinder compression is attributed to Scottish engineer Dugald Clerk, who patented his design in 1881. However, unlike most later two-stroke engines, his had 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. On 31 December 1879, German inventor Karl Benz produced a two-stroke gas engine, for which he received a patent in 1880 in Germany. The first truly practical two-stroke engine is attributed to Yorkshireman Alfred Angas Scott, who started producing twin-cylinder water-cooled motorcycles in 1908.
Two-stroke gasoline engines with electrical spark ignition are particularly useful in lightweight or portable applications such as chainsaws and motorcycles.
Official source
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