Fuel Injector: Description and Operation

Digifant II Fuel Injector:

The Fuel Injectors are located on the intake manifold near the cylinder head intake ports, this allows the electric solenoid valve injectors to spray fuel directly into the intake port just behind the intake valve. The Fuel Injectors are held in the intake manifold with an injector insert and O-rings that insulate them from engine heat and vibration. The four injectors use safety mounting clips to connect them to a common fuel supply known as the fuel rail. Each injector is opened by an electrical signal from the Motronic control unit, and closed by spring force when that signal stops. When current is sent through the injector electrical connection to the winding, electromagnetic force lifts the solenoid, and fuel is delivered. The lift is about 0.006 in. (0.15 mm), and takes about 1 millisecond. When the needle valve is closed by the spring, no fuel flows. The injectors have a pintle on the tip of the needle valve which helps atomize and distribute the fuel.

The Fuel Injectors are electrically-hot whenever the ignition key is ON and the injection relay is closed. For fuel delivery, they are grounded by the Motronic control unit. All four injectors are open at the same time and are synchronized to engine rpm. With Motronic fuel injection the metering of fuel takes place at the tip of the injector as the fuel flows past the pintle. Since the lift of the needle valve from its seat is always the same distance, and the fuel pressure is regulated at a constant level, the amount of fuel injected into the engine depends solely on the amount of time the control unit applies the ground signal to open the injector.

This opening time is known as injection duration or Pulse Time, and can vary anywhere from 2 milliseconds to as much as 15 milliseconds or more. The Fuel Injector takes about 1 millisecond to open, which is counted in the injection time. The closing time is not counted, but it averages out, so pulse time is the effective injector open/delivery time. Various operating factors determine pulse time, the Motronic control unit monitors several sensors to decide correct pulse time for all driving conditions.