Gasoline engine can be divided into carburetor type and injection type (or electronic injection type) according to the different ways of fuel supply. Carburetor is commonly used in the engines of old models. In the injection type gasoline engine, gasoline can be injected at the air inlet or directly into the cylinder during the intake stroke. The injection process can be controlled by a computer program, and thus the fuel can be more evenly distributed to each cylinder; At the same time, the average effective pressure and thermal efficiency in the cylinder can be improved by reducing the resistance of the intake air due to the absence of the throat. In addition, knock combustion can be reduced or avoided.
The highest position that the piston can reach in the upward direction of the cylinder is called "top dead center", and the lowest position that the piston can reach in the downward direction is called "bottom dead center". In many engines, at TDC, the top of the piston is flush with the top of the cylinder block. The volume of the combustion chamber is the volume of the cavity in the cylinder head above the piston, but this volume will slightly change due to the shape of the top of the piston. Therefore, the precise definition of compression ratio should be the ratio of total cylinder volume at BDC to total combustion chamber volume at top dead point. Compression ratio is an important index of engine performance. The linear distance from TDC to BDC is called stroke.
