Refers to the pump system (used in a fuel injection system) that squirts or injects a measured amount of gasoline into the intake manifold in the vicinity of the intake valve. In the diesel engine fuel is injected directly into the cylinder. In a Continuous Injection System (CIS), the injectors atomize the continuous flow of fuel injected under pressure into the intake ports of the engine; a valve in the injector nozzle stops the flow of fuel when fuel pressure drops below a certain point; in a CIS, the quantity of fuel is regulated by the fuel distributor's metering unit.

In other fuel injection systems, the fuel system delivers a constant supply of fuel at a constant pressure to the injector, and an electronic sensing and control system produces electrical current pulses of appropriate duration to hold open the injector solenoid valves; as fuel pressure is held constant, varying the pulse duration increases or decreases the amount of fuel passed through the injectors.