Central Fuel Injection, Basic Components

The smallest Audi engine is equipped with a single-point fuel injection system with a throttle valve, the so-called Mono-Motronic system. It consists mainly of a housing similar to a carburetor body and equipped with a single injector. The fuel output from the injector is controlled by the electronic control unit, which also controls ignition timing.

Control Unit
It is located in the front passenger footwell in front of the heater duct and receives information via a multi-pin connector from the following components:

Hall sensor in the ignition distributor – engine speed.

Lambda sensor in the catalytic converter – residual oxygen content in the exhaust gases.

Potentiometer – throttle position and thus the amount of intake air.

Intake air temperature sensor at the air inlet of the mixture preparation unit – intake air temperature.

Coolant temperature sensor on the coolant outlet pipe on the left side of the cylinder head – coolant temperature.

Based on the incoming signals, the control unit calculates the opening duration of the solenoid-operated injector and thereby the quantity of injected fuel. The control unit uses engine multi-parameter maps – a file storing all possible engine operating states. Fuel quantity data corresponding to each situation are also stored there in the form of electrical signals. In addition, ignition timing values related to fuel quantity and operating conditions are stored in Mono-Motronic as maps.

Throttle Valve
It is located at the very bottom of the mixture preparation unit and regulates the amount of air supplied to the engine.

The throttle opening is set by the driver by pressing the accelerator pedal. The pedal and the throttle valve are directly connected by the accelerator cable.

Mixture Preparation Unit
Most components of the central fuel injection system are assembled in this housing. Intake air passes through it, and fuel is added here by the injector, as in a carburetor.

Fuel Injector
It opens by means of a solenoid. Thus fuel can or cannot be delivered depending on the signal from the control unit. For optimal fuel atomization, the injector has angled outlet openings through which gasoline strikes the conical wall of the outlet opening and swirls there.

The fuel injector can only open and close, but cannot meter the fuel quantity. Therefore, fuel quantity is regulated by injection time. This works as follows: the injector sprays once with each high-voltage current pulse from the Hall sensor. For a small fuel quantity, the injector opens only briefly for this pulse – often less than one thousandth of a second. If the engine needs more fuel (during cold start or full load), injection lasts longer. This is repeated with every Hall sensor pulse.

Intake Air Temperature Sensor
It is located on the side of the injector housing and can thus accurately determine the temperature of the air entering the mixture preparation unit.

Fuel Pressure Regulator
The fuel pump delivers fuel under pressure to the fuel pressure regulator, which keeps fuel supply to the injector at a constant pressure of 0.8 – 1.2 bar. To do this, it allows a greater or lesser portion of the fuel to return to the fuel tank through the return line. The supply through the fuel filter remains practically constant.

Throttle Valve Positioner
A small electric motor with an angular gear moves the actuating plunger closer to or farther from the idle stop of the throttle valve. Thus, depending on the situation, the throttle valve can be opened to a greater or lesser extent.

As a result, the idle speed remains constant under various loads. At the front of the actuating plunger of the throttle valve positioner is the so-called idle switch, which informs the control unit when the accelerator pedal is released. This signal is necessary to stabilize the idle speed.

Throttle Valve Potentiometer
It informs the control unit about the movements and position of the throttle valve. For example, rapid opening of the throttle valve is recognized as vehicle acceleration. When you release the accelerator pedal, the throttle-closed signal activates fuel cut-off during idle. The signal is transmitted as an electrical signal – the potentiometer is nothing more than a variable resistor.