Fuel Supply, Function - GF07.10-P-1003MRA
Engine 274.9 in model 205 (except 205.047/053/054/147/247/253)
Engine 274.9 in model 253 (except 253.354/954)
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 pump.
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 fuel specified pressure and actuates the fuel pump in such a way that the actual pressure corresponds to the specified pressure.
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 according to 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.
Function sequence for fuel system high-pressure circuit
In the fuel high-pressure circuit, the fuel high pressure of approx. 200 bar as required for spray guided direct injection is generated, regulated and the pressure is then stored in the rail. The ME-SFI [ME] control unit reads in the following sensor for regulation of the fuel high pressure:
- Fuel pressure and temperature sensor (B4/25)
Y76The fuel system high pressure pump compresses the fuel (depending on the operating condition) up to approx. 200 bar and routes it over the high-pressure line and the rail to the cylinder 1 fuel injector to the cylinder 4 fuel injectors (Y76/1 to 4).
At the fuel system high pressure pump there is a quantity control valve (Y94) which regulates the fuel quantity which is fed to the pump element in the fuel system high pressure pump dependent on the requirements and a characteristics map. Actuation of the quantity control valve takes place via a PWM signal from the ME-SFI [ME] control unit.
The fuel pressure and temperature sensor detects the current fuel high pressure as well as the temperature of the fuel in the rail. The operating pressure is about 200 bar.
It is only at vehicle standstill and for a selector lever position "N" or "P" (for an A/T) that the pressure drops to 130 bar in order to reduce the noise emissions from the fuel system high pressure pump.
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 a pressure limiting valve opens 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
- The quantity control valve is energized and closed, so there is full delivery from the fuel system high pressure pump and rapid pressure buildup.
- Fuel pump pressure is approx. 4.5 up to 6.7 bar.
Normal mode
- The quantity control valve regulates 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)
- The quantity control valve is de-energized and therefore opened.
- Fuel pump pressure about 4.5 to 6.7 bar, fuel flows over the open quantity control valve into the rail.
- Actuation of fuel injectors extended
- Reduced performance, max. speed approx. 70 km/h
Stop
- Quantity control valve is de-energized and opened.
- 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.
Fuel high-pressure system
Function sequence for safety fuel shutoff
The safety fuel shutoff system is designed to ensure traffic and occupant 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) (with rotational recognition), engine RPM
- Throttle valve actuator (M16/6), throttle valve position
- 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), powertrain control unit (N127) and the engine CAN (CAN C)
The safety fuel shutoff is activated by the ME-SFI [ME] control unit for the following conditions:
- Mechanical fault in throttle valve actuator
- Absence of the engine speed signal
- Crash signal
Mechanical fault in throttle valve actuator
When evaluating the throttle valve position, if the ME-SFI [ME] control unit detects a mechanical fault in the throttle valve actuator, 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 directly from the supplemental restraint system control unit, it switches off the fuel pump via the fuel system control unit (directly and via the drive train CAN) and the quantity control valves in order to depressurize the fuel system.
| Electrical function schematic for fuel supply | PE07.10-P-2703-97FBA | ||
| Electrical function schematic for safety fuel shutoff | PE07.10-P-2733-97FBA | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-P-9998MRA |