A 4-Stroke is an engine that uses four strokes to complete a complete cycle. The four strokes are: Intake, Compression, Combustion, and Exhaust. During the Intake Stroke, the intake valve opens and an air-fuel mixture enters the chamber above the piston. The piston moves down drawing the mixture in. The intake valve then closes and the piston moves up for the Compression Stroke compressing the mixture. The spark plug then ignites the mixture causing an explosion. The resulting force moves the piston down again for the Combustion Stroke. Then the exhaust valve opens as the piston moves upward for the Exhaust Stroke and all the exhaust gases are ported through the exhaust system.