twostrokes

Two-stroke engines, often written as two-strokes or twostrokes, are internal combustion engines that complete a power cycle in two strokes of the piston for every crankshaft revolution. This contrasts with four-stroke engines, which require four strokes over two revolutions. The two-stroke design is typically lighter and simpler, providing a higher power-to-weight ratio, but it generally offers lower fuel efficiency and higher emissions.

Operation and design features vary, but common elements include ports instead of valves and a scavenging process to replace spent combustion gases with a fresh air–fuel charge. As the piston moves, intake ports are opened to admit the charge, which is then compressed and ignited to produce a power stroke. On the return stroke, exhaust gases exit through exhaust ports while new charge enters the cylinder. Some layouts use crankcase scavenging or reed valves to regulate the incoming air-fuel mixture. Lubrication is usually achieved by premixing oil with fuel or by a separate oil-injection system, depending on the design.

History and applications: Two-stroke engines were widely adopted in lightweight and compact equipment, including motorcycles and mopeds, chainsaws, lawn and garden equipment, and some outboard marine motors. Their compact size and simplicity made them economical for small displacement engines and rapid production.

Environmental and efficiency considerations: Because fresh charge can escape with exhaust, two-stroke engines tend to burn more fuel and emit more hydrocarbons and particulates than many four-stroke designs. Modern developments aim to reduce emissions with direct fuel injection, improved scavenging, and catalytic aftertreatment, and some markets increasingly favor four-stroke engines or advanced two-stroke technologies.