Electronic Fuel Injection

There is a vacuum-powered pressure regulator at the end of the fuel rail which ensures that the fuel pressure in the rail stays constant. Fuel injectors are connected with rail, but their valves remain closed until the ECU sends injection signal.

Usually, the injectors contain two pins. One pin is connected to the ECU. The ECU sends a pulsing ground to the injector, closing circuit. This lets the injector's solenoid get charged. The magnet on the plunger is attracted, opening the valve. The high pressure in the rail  starts fuel injection at a high velocity when the valve opens.

When the plunger rises, it opens a valve and the injector sends fuel just upstream of the intake valve, or straight into the cylinder. The first system is called multiport fuel injection and the latter is called direct injection.

The ECU decides the amount of fuel to inject into the cylinders to make the mixture stoichiometric.