Two-stroke engine

An internal-combustion engine that completes a power cycle in two piston strokes, combining intake, compression, combustion, and exhaust processes.

Crankcase compression: A method that uses the crankcase as a pump to compress fresh fuel–air mixture before transfer to the cylinder. Many small two-stroke engines use this pumping step during the piston’s upward stroke.
Four-stroke engine: An internal-combustion engine that completes a cycle over four piston strokes. Its separate intake and exhaust strokes avoid the two-stroke’s simultaneous gas exchange.
Two-stroke diesel engine: A compression-ignition engine that completes its cycle in two piston strokes, commonly used in large marine propulsion. Its uniflow scavenging and high efficiency suit ships requiring continuous, high-power output.
Two-stroke oil: Lubricant formulated to burn with fuel in engines where oil is mixed with fuel or separately injected. Many small two-strokes lack a sealed sump lubrication system, so oil enters the combustion process.
Scavenging: The removal of exhaust gas from a cylinder and its replacement with fresh charge. Scavenging must clear the cylinder while fresh mixture enters, often through open ports.
Direct injection: A fuel-delivery method that sprays fuel directly into an engine’s combustion chamber. It can reduce unburned fuel loss through the exhaust ports.
Two-stroke motorcycle: A motorcycle powered by a two-stroke internal-combustion engine. Lightweight engines with frequent power strokes shaped many competition and road motorcycles.
Oil injection system: A metering system that delivers lubricant separately from fuel to an engine’s moving parts. It replaces premixed fuel-and-oil while maintaining lubrication in many two-strokes.
Piston-controlled port: A cylinder opening whose timing is controlled by the piston uncovering and covering it. Port timing determines when gases enter and leave many two-stroke cylinders.
Uniflow scavenging: A scavenging arrangement in which gas flows through a cylinder predominantly in one direction. Large two-stroke diesels use it to improve exhaust removal and charge replacement.

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