GF07.10-P-1003MM Fuel Supply, Function
GF07.10-P-1003MM Fuel Supply, Function
ENGINES 276.9 in MODEL 204.0 /2 /3 /9, 207.3 /4, 212.0 /2, 218
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 Circuit 87M ON (engine control ON) 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 system control unit detects the current fuel pressure based on a voltage signal from the fuel pressure sensor (B4/7) (model 204, model 207, model 212), fuel pressure sensor (B4/7) (model 218) 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 dependent on the fuel temperature and the engine rpm between about 4.5 and a maximum of 6.7 bar.
For actuation the fuel pump suctions the fuel from the fuel feed module and pumps it through the fuel filter to the high-pressure pump (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.
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 a switched off fuel pump.
Low-pressure fuel circuit
18 Low pressure fuel distributor
19 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 or 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 200 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 regulation of 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 pump. This compresses the fuel (according to the operating status) up to 200 bar and directs it to the fuel injectors via the high-pressure line and the rails.
The three fuel injectors per cylinder bank are supplied directly from the associated rail with fuel.
At the high-pressure pump there is a quantity control valve (Y94) which regulates the fuel quantity according to the fuel specified pressure, which is fed to the pump element for compression.
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 200 bar. It is only at standstill of the vehicle and the selector lever position "N and P" that the pressure drops to 150 bar in order to reduce the noise emissions of the high-pressure pump.
In the case of stopping of a vehicle with a hot engine, the fuel pressure can increase up to 250 bar (+17 bar) in the high-pressure circuit. Upon reaching this threshold a valve opens in the high-pressure pump and the pressure is reduced. Upon starting the engine the pressure falls rapidly to the normal operating pressure of 200 bar.
In order to achieve rail pressure regulation, the quantity control valve is actuated by means of a PWM signal by the ME-SFI [ME] control unit until the fuel specified pressure is set up in the rail.
There are leakage lines on both rails which, in the case of a leak on the sealing rings for the fuel injectors to the rail, lead the fuel into the cylinder head. This prevents fuel escaping and thus 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 valve is energized and closed, so there is full delivery from the 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 over the duty cycle of the fuel high pressure
- 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 not reached)
- The quantity control valve is deenergized and therefore opened
- A 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
- Stratified operation locked out (for an engine with stratified operation)
- Reduced performance, max. speed approx. 70 km/h
Stop
- The quantity control valve is deenergized and opened
- Fuel pump not actuated.
Assembly operations
The high pressure fuel lines made out of stainless steel can be reused after checking. An appropriate test specification can be found in the WIS.
Fuel high-pressure circuit
18 Low pressure fuel distributor
18/1 Fuel feed (low pressure)
18/2 Pressure gauge connection with service valve
19 High-pressure pump
20a Left rail
20b Right rail
21 Leakage line
B4/25 Fuel pressure and temperature sensor
Y76/1 Cylinder 1 fuel injector
Y76/2 Cylinder 2 fuel injector
Y76/3 Cylinder 3 fuel injector
Y76/4 Cylinder 4 fuel injector
Y76/5 Cylinder 5 fuel injector
Y76/6 Cylinder 6 fuel injector
Y94 Quantity control valve
A Fuel low pressure
B Fuel high pressure
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 positioner (M16/6), throttle valve position
- Supplemental restraint system control unit (N2/10) (model 204, 207, 212), supplemental restraint system control unit (N2/10) (model 218), direct crash signal
- Supplemental restraint system control unit or supplemental restraint system control unit, indirect crash signal via the chassis CAN (CAN E)
The safety fuel shutoff is activated by the ME-SFI [ME] control unit in the following conditions:
D Mechanical fault in the throttle valve actuator
D Absence of the engine speed signal
D Crash signal
Function sequence for fuel quality monitoring (for code (494) USA version and for code (929) Flexible Fuel Vehicle (FFV))
With increasing increase of admixture of ethanol in the fuel it has become a necessity to monitor the ethanol contents or the fuel ethanol mixture. Changes in the stoichiometric fuel air/fuel ratio can occur due to the variable ethanol contents. This can lead to losses in the engine power.
Monitoring occurs by the fuel quality sensor (B4/31) which detects the relative conductivity of the gasoline ethanol mixture.
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 supplemental restraint system control unit or supplemental restraint system control unit, it switches off the fuel pump via the fuel pump control unit (directly and via the drive train CAN) and the quantity control valves, in order to depressurize the fuel system.
The value of the relative conductivity is dependent on the gasoline ethanol mixture and the fuel temperature. This value is measured by the fuel quality sensor and transmitted with an appropriate voltage signal to the FSCU (N118). Here it is converted and sent as information to the ME-SFI [ME] control unit in order to perform the appropriate adaptation of the engine timing.
The power supply for the fuel quality sensor occurs from the FSCU.
