Two-stroke cycle

A constant-volume cycle of energy transformation that completes its operating cycle in two strikes of the piston, one up and one down. When the piston moves up, fuel is pulled into the crankcase, and at the same time the air/fuel mixture inside the cylinder is compressed. When the piston is near the top of its stroke, a spark plug ignites the compressed air/fuel mixture, and the burning and expanding gases force the piston down. Near the bottom of the stroke, the piston uncovers an exhaust port and the burned gases leave the cylinder. When the piston moves further down, it uncovers the intake port, and a fresh charge of fuel and air are forced from the crankcase into the cylinder.

Aviation Maintenance Technician Handbook — Powerplant, p. G-29