starts with the piston at the top of the cylinder (TDC) and the intake valve open to admit air and fuel. At bottom-dead-center (BDC) the inlet valve is closed and the compression stroke (b) starts.

A two-stroke engine is fueled by a mixture of gas and oil and has no camshaft or valves. It fires every time the piston reaches the top. A four-stroke engine uses four separate strokes—two up ...