The typical fuel injection engine has a separate fuel injection mechanism for each cylinder. The mechanism consists of a high-pressure pump, a nozzle, and a valve fitted into the wall of the cylinder. During the compression stroke the pump forces a small amount of fuel through the nozzle into the cylinder, where it ignites and burns. The nozzle sprays the fuel evenly through the air in the cylinder. The valve allows the fuel to enter but prevents the compressed fuel-air mixture from backing up into the fuel system.
When a fuel injection system is used in a gasoline engine, it may completely replace the conventional carburetor. Alternatively, the fuel may be injected into the stream of air entering the cylinders, in a device called the injection carburetor. The principal advantage of a fuel injection system over the conventional carburetor and intake manifold system in a spark ignition engine is that it provides a more even distribution of fuel to the cylinders.
Reaction engines, such as the turbojet engine, also use the fuel injection technique. In such engines the fuel is injected into the combustion chamber, where it burns continuously.