Fuel injection system operation

Fuel is drawn from the fuel tank by the electric fuel pump and delivered through the fuel filter to the fuel rail and then to the injectors. The fuel pressure regulator at the fuel rail ensures that the fuel pressure is 3.5 bar. The injectors are electrically controlled and inject fuel in pulses into the intake manifold upstream of the intake valves.

Air is drawn into the engine through the air filter and passes through the mass air flow sensor. The sensor housing contains a thin, electrically heated plate that is cooled by the airflow. The control unit varies the heating current so that the plate temperature remains constant.

If, for example, the volume of intake air increases, the plate tends to cool down. As a result, the heating current of the plate increases. Based on the changes in heating current, the engine control system determines the air flow and therefore the engine load.

The control unit determines the injection time, and thus the quantity of injected fuel, based on the air mass and engine speed. The longer the injectors are open, the greater the quantity of injected fuel. Additional sensors monitor correct fuel supply in extreme driving situations.

Fuel injection is sequential. This means that the injectors are controlled individually, and the injection sequence corresponds to the firing order. By selecting the injection timing in relation to the opening timing of the intake valves, exhaust emissions and engine output can be adjusted.

The throttle position sensor is located on the throttle valve shaft. It sends throttle position information to the control unit. This allows, in particular, the deceleration fuel cut-off to be controlled: while the throttle valve is closed and engine speed is above a certain value, the control unit stops fuel supply to the engine.

The fuel pump relay is located in the relay box behind the left shock absorber strut. The relay supplies current to the fuel pump. The safety switch interrupts the fuel supply when the engine is not running, for example, if the engine has stalled.

The crankshaft position and engine speed are determined by two inductive sensors. The engine speed sensor is located at the crankshaft pulley. The cylinder identification sensor is located at the front of the engine, in the timing chain cover.

Two oxygen sensors measure the oxygen content in the exhaust gases and send a signal to the control unit. Based on this information, the control unit adjusts the air/fuel ratio, which ensures optimal combustion of the fuel mixture.

The idle air control valve controls the amount of air bypassing the throttle valve at idle. This maintains a constant idle speed regardless of the activation of additional consumers, such as the power steering pump or the air conditioning compressor.

The fuel tank vent solenoid valve is controlled depending on engine operating conditions. Fuel vapors generated in the fuel tank are stored in the charcoal canister and are supplied to the combustion chamber through the solenoid valve. In this way, fuel vapors are used effectively and do not escape into the atmosphere.

The variable camshaft timing system, abbreviated VANOS, rotates the intake camshaft relative to the drive chain by means of oil pressure, depending on engine speed and load. This allows optimal valve timing for idle comfort, torque characteristics and fuel economy. The DME control unit controls the oil flow to the adjusting element by means of a solenoid valve.

Ignition voltage monitoring switches off the DME control unit if the voltage is too low (for example, due to damaged cable). In this case the engine cannot be started. This prevents damage to the catalytic converter.

Communication with other control units (ABS, ASC, transmission control) takes place via CAN, a special data bus. This system has, among other things, the advantage that the engine wiring harness contains fewer wires. Such a bus provides a higher degree of vehicle protection in the event of failure of individual control units.