About Two-Cycle Engines

In the two-cycle engine, the fuel mixture is first moved via a diaphragm or suction feed carburetor into a crankcase and then into the combustion chamber. When the piston is initially moved upward by a pull of the starter rope, it changes the pressure in the crankcase, causing a thin rubber diaphragm to pulsate and draw fuel through a valve into the carburetor where it is mixed with air. That same upstroke of the piston also creates a low-pressure area below the piston, drawing the fuel mixture from the carburetor through a port or tube into the crankcase.

The fuel mixture that is already above the piston is compressed; the spark plug fires and ignites the mixture, which explodes, expands, and drives the piston down. As the piston drops, it exposes the exhaust port, and the spent gases are expelled. Almost at the same moment, the fresh fuel mixture in the crankcase is forced up into the combustion chamber above the piston. As the piston again moves up toward the spark plug, the fuel mix is compressed for the next spark and explosion. Simultaneously, the diaphragm pulsates and again draws fuel into the carburetor and the air-fuel mix from the carburetor into the crankcase.

Since every downstroke of a two-cycle engine is a power stroke, this engine sounds twice as fast as a four-cycle engine running at the same speed.