A two-stroke or two-stroke cycle engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston one up and one down movement in one revolution of the crankshaft in contrast to a four-stroke engine which requires four strokes of the piston in two crankshaft revolutions to complete a power cycle.
The second stroke encompasses the combustion of the mixture, the expansion of the burnt mixture and, near bottom dead center, the beginning of the scavenging flows. Two-stroke engines often have a higher power-to-weight ratio than a four-stroke engine, since their power stroke occurs twice as often.
Two-stroke engines can also have fewer moving parts , and thus be cheaper to manufacture and weigh less. In countries and regions with stringent emissions regulation, two-stroke engines have been phased out in automotive and motorcycle uses.
In regions where regulations are less stringent, 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 The crankcase -scavenged engine, employing the area below the piston as a charging pump, is generally credited to Englishman Joseph Day.
The first truly practical two-stroke engine is attributed to Yorkshireman Alfred Angas Scott , who started producing twin-cylinder water-cooled motorcycles in Two-stroke gasoline engines with electrical spark ignition are particularly useful in lightweight or portable applications such as chainsaws and motorcycles. However, when weight and size are not an issue, the cycle's potential for high thermodynamic efficiency makes it ideal for diesel compression ignition engines operating in large, weight-insensitive applications, such as marine propulsion , railway locomotives , and electricity generation.
