GF07.10-P-1003MMC Fuel Supply, Function
GF07.10-P-1003MMC Fuel Supply, Function
- ENGINES 157.9 in MODEL 216.3, 221.0 /1
- ENGINES 278.9 in MODEL 216.3, 221.0 /1
Function requirements for fuel supply, general points
- Circuit 15 ON (ignition ON)
- Circuit 87M ON (engine control 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 injectors (Y76).
Function sequence for fuel supply
The function sequence for fuel supply is described in the following steps:
- Function sequence for low-pressure fuel circuit
- Function sequence for high-pressure fuel circuit
- Function sequence for safety fuel shutoff
Function sequence for low-pressure fuel circuit
Switching on of the fuel pump (M3) takes place of a signal "fuel pump ON" is received by the fuel pump control unit (N118). This signal is sent redundantly from the ME-SFI [ME] control unit (N3/10) as a CAN signal via the drive train CAN (CAN C) and as a ground signal.
The fuel pump control unit also receives the CAN signal "nominal pressure of the fuel" from the ME-SFI [ME] control unit.
The fuel pump 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 with a pulse width modulated signal (PWM signal) as required such that the actual value corresponds to the specified value.
The fuel pump pressure is regulated between about 4.5 to a maximum of 6.7 bar depending on the fuel temperature and the motor rpm.
For actuation the fuel pump suctions the fuel from the fuel feed module and pumps it through the fuel filter to the high-pressure pumps (single line system without return line).
The overflow valve in the fuel filter opens at a fuel pressure from about 7 to 9 bar. Fuel is removed upstream of the filter over a T-piece which drives the suction jet pump with 20 to 40 l/h.
This suction jet pump delivers the fuel out of the left fuel tank chamber into the fuel feed module (in the right fuel tank chamber) and thus prevents single-sided emptying of the fuel tank.
In the feed to the fuel filter there is a check valve in the feed line to the fuel filter which prevents dropping of the fuel pressure (down to below 4.5 bar) for switched off fuel pump.
Hydraulic scheme for low-pressure fuel circuit
18 Low pressure fuel distributor
19a Left high-pressure pump
19b Right high-pressure pump
55 Fuel delivery module
55a Right suction spray pump
55/2a Fuel filter
55/2b Overflow valve
55/2c Check valve
75 Fuel tank
75a Left suction spray pump
B4/7 Fuel pressure sensor
M3 Fuel pump
A Fuel feed to fuel filter
B Fuel return from overflow valve
C Fuel for high-pressure pump
Function sequence for high-pressure fuel circuit
In the high-pressure fuel circuit the fuel pressure of about 170 bar required for is jet-guided direct injection is generated, regulated and the pressure is stored in both rails.
The ME-SFI [ME] control unit reads in the following sensor for regulating the high pressure fuel:
- Fuel pressure and temperature sensor (B4/25)
The fuel from the fuel tank flows from the low-pressure fuel distributor to the high-pressure pumps. This compresses the fuel (according to the operating status) up to 170 bar and directs it to the fuel injectors over the high-pressure fuel lines and the rails.
The four fuel injectors per cylinder bank are supplied directly from the respective rail with fuel.
The left quantity control valve (Y94/1) is located on the left high- pressure pump and the right quantity control valve (Y94/2) is located on the right high-pressure pump. The quantity control valves control the quantity of fuel which fed to the respective pump element for compression according to the fuel specified pressure.
The fuel pressure and temperature sensor detects the current fuel high pressure (rail pressure) as well as the temperature of the fuel in the left rail. The operating pressure is 170 bar. It is only for standstill of the vehicle and the selector lever position "N and P" that the pressure drops to 120 bar in order to reduce noise emissions from the high-pressure pumps.
If a vehicle is stopped with a hot engine the fuel pressure can increase up 250 bar (+17 bar) in the high-pressure circuit. Once this threshold is reached a valve opens in the high-pressure pump and the pressure is reduced. The pressure fall quickly when starting the engine to the normal operating pressure of 170 bar.
To control the rail pressure the respective quantity control valve is actuated with a pulse width modulation signal by the ME-SFI [ME] control unit until the fuel specified pressure is achieved in the rail.
There are leakage lines on both rails which lead the fuel in the cylinder head to the rail if there are any leaks on the fuel injector sealing rings. This prevents fuel escaping and therefore any possible ignition on hot engine parts.
During regulation of the high-pressure fuel circuit one differentiates between the following operating conditions:
- Start
- Normal mode
- Low-pressure limp-home mode (fuel high pressure not reached)
- Stop
Start
- The quantity control valves are energized and closed, therefore there is full delivery of high-pressure pumps and a rapid pressure buildup
- The fuel pump pressure is about 4.5 to 6.7 bar.
Normal mode
- The quantity control valves regulate the fuel high pressure via the duty cycle
- The fuel pump pressure is dependent on the fuel temperature between about 3.0 to 5.5 bar.
- The fuel predelivery pressure is varied dependent on the motor rpm and fuel temperature between 4.5 and 6.7 bar (absolute).
Low-pressure limp-home mode (fuel high pressure not reached)
- Quantity control valves are deenergized and therefore 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 extended
- Reduced performance, max. speed approx. 70 km/h
Stop
- Quantity control valves are deenergized and therefore open
- Fuel pump not actuated.
Assembly operations
The high pressure fuel lines made out of stainless steel can be reused after check. An appropriate test specification can be found in the WIS.
View of the high-pressure fuel circuit
18 Low pressure fuel distributor
18/1 Fuel feed (low pressure)
18/2 Pressure gauge connection with service valve
19a Left high-pressure pump
19b Right high-pressure pump
20a Left rail
20b Right rail
21 Leakage line
B4/25 Fuel pressure and temperature sensor
Y76y1 Cylinder 1 fuel injector
Y76y2 Cylinder 2 fuel injector
Y76y3 Cylinder 3 fuel injector
Y76y4 Cylinder 4 fuel injector
Y76y5 Cylinder 5 fuel injector
Y76y6 Cylinder 6 fuel injector
Y76y7 Cylinder 7 fuel injector
Y76y8 Cylinder 8 fuel injector
Y94/1 Left quantity control valve
Y94/2 Right quantity control valve
A Low pressure fuel
B High pressure fuel
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), engine speed
- Throttle valve actuator (M16/6), throttle valve position
- Restraint systems control unit (N2/7), direct crash signal
- Restraint system control unit, indirect crash signal over the chassis CAN (CAN E)
The safety fuel shutoff is activated by the ME-SFI [ME] control unit in the following conditions:
- Mechanical fault in the throttle valve actuator
- Absence of the engine speed signal
- Crash signal
Mechanical fault in the 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 about 1400 rpm at idle and about 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 crankshaft Hall sensor is missing, the fuel pump is shut off via the fuel pump control unit.
Crash signal
If the ME-SFI [ME] control unit receives a crash signal indirectly via the chassis CAN or directly from the restraint system control unit, it switches off the fuel pump and the quantity control valves indirectly via the fuel pump control unit (directly and via the drive train CAN) in order to depressurize the fuel system.
