Dead center: piston is connected with crank and rod assembly. The piston has top and bottom limit positions in the cylinder. The top limit position is called top dead center, which is the farthest from the crankshaft centerline. The bottom limit position is called bottom dead center, which is the nearest to the crankshaft centerline.

Piston stroke: The distance of piston movement from top dead center to bottom dead center is called stroke. Cylinder working capacity: piston swept volume in the cylinder when piston moves for a stroke.

Working volume of combustion chamber: the room that is between the piston top and the top of the central combustion chamber of the cylinder head when the piston is at top dead center

Total cylinder volume: the sum of cylinder working volume and working volume of combustion chamber

Compression ratio: an important structural parameter of the engine, which directly influence the power of the engine

Compression ratio（ε）= Total cylinder volume/ working volume of the combustion chamber= 1+ cylinder working volume/ working volume of the combustion chamber

In general conditions, for gasoline engine, ε=6～10