The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Englishman Joseph Day. However, unlike most later two-stroke engines, his had a separate charging cylinder. The first commercial two-stroke engine involving cylinder compression is attributed to Scottish engineer Dugald Clerk, who patented his design in 1881. In regions where regulations are less stringent, or non-existent, small displacement two-stroke engines remain popular in mopeds and motorcycles. In countries and regions with stringent emmissions regulation, two-stroke engines have been phased out in automotive and motorcycle uses.

Two-stroke engines have fewer moving parts than four-stroke engines, and thus are cheaper to manufacture. 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. 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. A four-stroke engine requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. Internal combustion engine type Animation of a two-stroke engineĪ 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.

4 cycle internal combustion engine