Multi-Point Fuel Injection - Basic Components

Control Unit
The control unit receives the following information through the multi-pin connector from:

Starter terminal 50 – start and end of the starting process.

Throttle valve potentiometer – current throttle valve position.

Hall sensor in the ignition distributor – engine speed.

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

Potentiometer in the air flow sensor – position of the air flap (amount of intake air).

Intake air temperature sensor in the air flow sensor.

Coolant temperature sensor in the coolant pipe.

Knock sensor on the cylinder block for detecting knock combustion; see also the Ignition System chapter.

Based on engine speed and load information, the control unit calculates the duration of opening of the solenoid fuel injectors and thereby the injected fuel quantity. For this purpose, the control unit has injection characteristics and, in addition, ignition characteristics available. These characteristics are data collections for all possible engine operating situations and the corresponding fuel quantities and ignition timing. The control unit can modify these values after receiving so-called correction signals (for example, intake air temperature and coolant temperature).

Throttle Valve
Two throttle valves are located in the housing in the intake manifold. The smaller valve is connected by the accelerator cable to the accelerator pedal in the passenger compartment. It meters the flow of intake air into the engine. If the accelerator pedal is pressed further, the linkage opens the second, larger valve; at full throttle both valves are open.

Throttle Valve Potentiometer
The throttle valve potentiometer detects the current throttle valve position from idle (throttle valve closed) to full load (throttle valve fully open). Based on its signals, idle speed stabilization, fuel cut-off during overrun, or enrichment of the mixture at full throttle is activated.

Pressure Regulator
It is located downstream of the fuel distributor and regulates the fuel supply pressure to the fuel injectors. To do this, it receives information about the vacuum level in the intake manifold. At idle with the throttle valve closed and high vacuum, it maintains a lower pressure. As vacuum decreases under higher engine load, the pressure regulator increases the fuel supply pressure. The fuel pump delivers fuel at a higher operating pressure, but thanks to the pressure regulator, the amount of fuel returned to the fuel tank is correspondingly increased or, when necessary, reduced.

Injectors
With each crankshaft revolution they inject fuel into the injection port in front of the intake valve of the corresponding cylinder; the duration of this process is determined by the control unit. The corresponding cylinder receives one portion in advance, and the second portion directly into the combustion chamber while the intake valve is open.

Intake Air Temperature Sensor
It is located on the side of the air flow sensor and can thus accurately determine the temperature of the incoming air. The intake air temperature is sent to the control unit in the form of a resistance value. This value is needed for optimal fuel metering.

Coolant Temperature Sensor
Coolant temperature is required to control many injection functions: start enrichment, post-start enrichment (across the entire temperature range), acceleration enrichment, and fuel cut-off during overrun (coasting). Coolant temperature information is also sent to the control unit in the form of a resistance value. The control unit calculates the required injection duration, which, with the engine at operating temperature, is in the range between two and eight milliseconds. This value can increase by up to 70% if the coolant temperature reaches –25°C.

Air Flow Sensor
The combustion air coming from the air filter pushes the air flap in the air flow sensor open to a greater or lesser degree, depending on the quantity drawn in. The damping flap connected to the air flap prevents air flap oscillation. Depending on the position of the air flap, the potentiometer sends a voltage signal to the control unit: high voltage at full opening and large air quantity, low voltage at small opening.

Cold Start Valve
This solenoid-operated injector sprays additional fuel into the intake manifold for 1–4 s, depending on engine temperature during starting. The duration of injection is determined by the Digifant injection system control unit.

Idle Speed Control Valve
During the warm-up phase, when the power steering is fully turned, when the air conditioning is operating, or when an automatic transmission is in gear, this valve opens an additional air passage, bypassing the throttle valve.

Increased airflow into the intake manifold simultaneously results in increased fuel supply. This compensates for increased engine friction when cold or for the higher engine load caused by the power steering pump, air conditioning compressor, and automatic transmission, which consume energy.

The idle speed control valve is also used during overrun idle. It allows the air required for driving to pass with the throttle valve closed.