Fuel Supply, Function - GF07.10-P-1003MNB
Engine 178.9 In Model 190
Function requirements for fuel supply, general points
- Circuit 15 (ignition ON)
- Circuit 87M (engine management ON)
Fuel supply, general points
The fuel supply supplies filtered fuel out of the fuel tank in adequate amounts under all operating conditions at an adequate pressure to the fuel system high pressure pumps.
Function sequence for fuel supply
The function sequence for fuel supply is described in the following steps:
- 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 low-pressure circuit
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 ME-SFI [ME] control unit (N3/10) 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 "specified pressure of the fuel" from the ME-SFI [ME] control unit. The fuel system control unit detects the current fuel pressure by means of a voltage signal from the fuel pressure sensor (B4/7) and transmits this information via the drive train CAN to the ME-SFI [ME] control unit.
The fuel system control unit evaluates the current fuel pressure, compares it with the specified fuel 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.
In order to determine the fuel specified pressure (fuel requirements), the ME-SFI [ME] control unit evaluates the fuel pressure and load requirements. The fuel delivery volume is regulated variably from 0 to 130 l/h for a fuel pressure of about 4 to 6.7 bar to meet the fuel requirements.
For actuation the fuel pump suctions the fuel from the fuel feed module and pumps it through the fuel filter to the fuel system high pressure pump (single line system without return line).
The overflow valve in the fuel filter opens at a fuel pressure from approx. 7 to 9 bar.
There is a check valve in the feed line to the fuel filter which prevents removal of the fuel pressure (down to below approx. 4.5 bar) for a switched off fuel pump.
ENGINE 178.9 in MODEL 190
with CODE 494 (USA version)
For vehicles in a USA version the fuel temperature is also detected by the fuel pressure sensor. This is read in by the fuel system control unit which forwards the information over the drive train CAN to the ME-SFI [ME] control unit.
Function sequence for fuel system high-pressure circuit
In the fuel high-pressure circuit, the fuel high pressure of about 200 bar required for spray guided direct injection is generated, regulated and stored in the rails. The ME-SFI [ME] control unit reads in the following signals for regulation of the fuel high pressure:
- Right fuel pressure and temperature sensor (B42/1), fuel pressure and temperature
- Left fuel pressure and temperature sensor (B42/2), fuel pressure and temperature
The fuel system high pressure pumps deliver the fuel (according to the operating condition) at a pressure of about 200 bar and guide it over the high-pressure lines and the rails to the fuel injectors (Y76).
At the fuel system high pressure pumps there are the left and right quantity control valves (Y94/1, Y94/2) that control the quantity of the fuel, based on the requirement and the characteristics map, that is fed to the pump elements in the fuel system high pressure pumps. The quantity control valve is actuated by means of a pulse width modulation signal from the ME-SFI [ME] control unit.
The fuel pressure and temperature sensors record the current fuel high pressure as well as the temperature of the fuel in the rails. The operating pressure is about 200 bar.
It is only at vehicle standstill and for a selector lever position "N" or "P" that the pressure drops to 130 bar in order to reduce the noise emissions from the fuel system high pressure pumps.
In the case of shutting off a vehicle with a hot engine, the fuel pressure can increase up to 250 bar (+17 bar) in the fuel high-pressure circuit. Upon reaching this threshold, mechanical pressure limiting valves open in the fuel system high pressure pump and the pressure is reduced. Upon starting the engine the pressure falls rapidly to the normal operating pressure of about 200 bar.
During regulation of the fuel high-pressure circuit one differentiates between the following operating conditions:
- Start
- Normal mode
- Low-pressure limp-home mode (fuel high pressure is not reached)
- Stop
Start
- Quantity control valves are energized and closed so there is full delivery of the fuel system high pressure pumps and rapid pressure buildup.
- Fuel pump pressure is approx. 4.5 up to 6.7 bar.
Normal mode
- The quantity control valves regulate the supplied and the required fuel quantity over the duty cycle.
- Fuel pump pressure is dependent on the fuel temperature between about 3.0 to 5.5 bar.
- The fuel predelivery pressure is dependent on the engine speed and fuel temperature and varies between 4.5 and 6.7 bar (absolute).
Low-pressure limp-home mode (fuel high pressure is not reached)
- Quantity control valves are de-energized and thus open.
- A fuel pump pressure about 4.5 to 6.7 bar, fuel flows over the open quantity control valves into the rails.
- Actuation of fuel injectors (Y76) extended.
- Performance reduction, max. speed about 70 km/h.
Stop
- Quantity control valves are de-energized and open.
- Fuel pump not actuated.
Assembly operations
The high pressure fuel lines made out of stainless steel can be reused after checking. More information on this is available in the repair instructions.
View of the fuel high-pressure circuit
Function sequence for safety fuel shutoff
The safety fuel shutoff system is designed to ensure traffic and passenger safety.
The ME-SFI [ME] control unit controls the safety fuel shutoff on the basis of the following sensors and signals:
- Crankshaft Hall sensor (B70), engine RPM
- Left and right throttle valve actuator (M16/60, M16/61), throttle valve positions
- Supplemental restraint system control unit (N2/10), direct crash signal
- Supplemental restraint system control unit, indirect crash signal via user interface CAN (CAN HMI) to the electronic ignition lock control unit (N73) and the engine CAN (CAN C)
The safety fuel shutoff is activated by the ME-SFI [ME] control unit for the following conditions:
- Mechanical faults in the throttle valve actuators
- Absence of the engine speed signal
- Crash signal
Mechanical faults in the throttle valve actuators
When evaluating the throttle valve positions, if the ME-SFI [ME] control unit detects a mechanical fault in the throttle valve actuator, the engine speed is limited to approx. 1400 RPM at idle and approx. 1800 RPM in driving mode by shutting off the fuel injectors.
Absence of the engine speed signal
If the engine speed signal generated by the ME-SFI [ME] control unit is missing, the fuel pump is shut off via the fuel system control unit.
Crash signal
If the ME-SFI [ME] control unit receives a crash signal indirectly via the user interface CAN, 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 over the fuel system control unit (directly and via the drive train CAN) and the quantity control valves to depressurize the fuel system.
| Electrical function schematic for fuel supply | PE07.10-P-2703-97HBA | ||
| Electrical function schematic for safety fuel shutoff | PE07.10-P-2733-97HBA | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-P-9998MNB |