Function requirements for fuel supply, general points
Circuit 15 ON (ignition ON)
Circuit 87M ON (Engine management ON)
Fuel supply, general points
The fuel supply provides an adequate quantity of filtered fuel from the fuel tank to the fuel injectors for cylinders 1 to 6 (Y76/1 to Y76/6) at an adequate pressure and under all operating conditions.
Function sequence for fuel supply
The function sequence for fuel supply is described in the following steps:
Function sequence for fuel pump actuation
Function sequence for fuel low-pressure circuit
Function sequence for fuel system high-pressure circuit
Function sequence for safety fuel shutoff
Function sequence for fuel pump actuation
Switching on of the fuel pump (M3) takes place when the signal "fuel pump ON" is received by the fuel system control unit (N118). This signal is sent redundantly from the CDI control unit (N3/9) as a CAN signal over the drive train CAN (CAN C1) and as a ground signal. The fuel system control unit also receives the CAN signal "Fuel specified pressure" from the CDI control unit.
Function sequence for fuel low-pressure circuit
The fuel low-pressure circuit consists of the following components:
Fuel tank
Fuel pump (FP)
Fuel lines
Fuel filter
Quantity control valve (Y94)
Fuel system high pressure pump, low-pressure side
Fuel temperature sensor (B50)
Pressure regulating valve (Y74), low-pressure side
The fuel system control unit detects the current fuel pressure by means of a voltage signal from the fuel pressure sensor (B4/7) and sends this information via drive train CAN to the CDI control unit. The fuel system control unit evaluates the current fuel pressure, compares it with the fuel specified pressure and actuates the fuel pump accordingly by means of a pulse width modulated signal in such a way that the actual value corresponds to the specified value. During actuation the fuel pump suctions the fuel out of the fuel feed module and pumps it through the fuel filter to the fuel system high pressure pump. Depending on the fuel requirements, the fuel pressure is regulated to about 4.0 bar through the pressure limiting valve integrated into the fuel system high pressure pump.
The quantity control valve also regulates the supply of fuel to the fuel system high pressure pump (feed quantity) depending on the engine operating point in order to keep the fuel temperature low.
Fuel preheating system
In order to achieve flowability of the fuel at low outside temperatures, a fuel filter condensation sensor with heating element (B76/1) for water level monitoring is installed. The heating element is energized by the glow output stage (N14/3) and heats the fuel in the fuel filter. Condensation level monitoring in the fuel filter is by means of the condensation sensor.
The condensation level is read in by the glow output stage and passed on via the drive train LIN (LIN C1) to the CDI control unit. As from a condensation level of approx. 100 ml, the CDI control unit sends a message over the engine CAN (CAN C), powertrain control unit (N127), chassis FlexRay (Flex E), electronic ignition lock control unit (N73) and over the user interface CAN (CAN HMI) to the instrument cluster (A1) where it is shown as a warning message.
Function sequence for fuel system high-pressure circuit
The fuel high-pressure circuit consists of the following assembly parts:
Fuel system high pressure pump (high-pressure side)
Rails
High-pressure lines
High pressure fuel pressure sensor (B4/6)
Pressure regulating valve
Fuel injectors
Quantity control valve
The fuel system high pressure pump pumps the fuel into the rails as a function of the signal from the accelerator pedal sensor (B37), the engine rpm and the fuel temperature. From the rails, the fuel flows through the high-pressure lines to the fuel injectors. The fuel injectors spray fuel in a fine atomized form.
The CDI control unit calculates the injection quantity selectively for each cylinder based on a characteristics map for the respective operating condition. The injection quantity is dependent upon the fuel pressure in the rail and the actuation duration of fuel injectors.
The fuel pressure in the rail is controlled by the pressure regulating valve and quantity control valve. The actual applied fuel pressure in the rails is detected by the high pressure fuel pressure sensor and is limited by the pressure regulating valve to a maximum of 1800 bar. The fuel temperature is detected over the fuel temperature sensor. Both pieces of information are passed on permanently to the CDI control unit.
Function sequence for safety fuel shutoff
The safety fuel shutoff system is designed to ensure traffic and occupant safety.
The CDI control unit controls the safety fuel shutoff on the basis of the following signals:
Crankshaft Hall sensor (B70), engine rpm
Supplemental restraint system control unit (N2/10), direct crash signal
SRS control unit, indirect crash signal via the user interface CAN, electronic ignition lock control unit, chassis FlexRay, powertrain control unit and engine CAN
The safety fuel shutoff is activated by the CDI control unit for the following conditions:
Absence of the engine speed signal
Crash signal
Absence of the engine speed signal
If the engine speed signal generated by the CDI control unit is missing the fuel pump will be switched off.
Crash signal
If the CDI control unit receives a crash signal indirectly via user interface CAN to the electronic ignition lock control unit and the engine CAN or directly from the supplemental restraint system control unit, it switches off the fuel pump, the quantity control valve and the pressure regulating valve and then briefly actuates the fuel injectors to depressurize the fuel system.