Fuel Supply, Function - GF07.10-P-1003MNG
ENGINE 276.8 in MODEL 292
ENGINE 276.8/9 in MODEL 166
as of model year 2016
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 fuel quality monitoring (for code 494 (USA version) and for code 929 (Engine for ethanol fuel)
- 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 via the drive train CAN (CAN C) 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.
The fuel pump pressure is controlled dependent on the fuel temperature and the engine rpm between about 4.5 to 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 fuel system 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.
Hydraulic scheme for fuel low-pressure circuit
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 signal from the following sensor for regulation of the fuel high pressure:
- Fuel pressure and temperature sensor (B4/25)
The fuel from the fuel tank flows from the low-pressure fuel distributor to the fuel system high pressure pump. This delivers the fuel (according to the operating condition) under a pressure of up to 200 bar to the high-pressure line and the rail to the fuel injectors. The three fuel injectors per cylinder bank are supplied directly from the respective rail with fuel. Located on the fuel system high pressure pump is a quantity control valve (Y94) that regulates the fuel quantity fed to the pump element to match the specified fuel pressure.
The fuel pressure and temperature sensor detects the current fuel high pressure as well as the temperature of the fuel in left the rail. 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 150 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 mechanical 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 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.
Leak fuel lines are located on both rails which, in the case of a leak on the sealing rings for the fuel injectors to the rail, route the fuel into the cylinder head. This prevents fuel escaping and thus any possible ignition on hot engine parts.
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.
- The fuel pump pressure is about 4.5 up to 6.7 bar.
Normal mode
- The duty cycle is used by the quantity control valve to regulate the fuel quantity inflow to the fuel system high pressure pump.
- The fuel pump pressure is regulated dependent on the fuel temperature between about 3.0 and 5.5 bar.
- The fuel pre-delivery pressure is regulated dependent on the engine rpm and fuel temperature between 4.5 and 6.7 bar (absolute).
Low-pressure limp-home mode (fuel high pressure is not reached)
- Quantity control valve is deenergized and therefore opened.
- Fuel pump pressure is approx. 4.5 to 6.7 bar, fuel flows over the open quantity control valve into the rails.
- Actuation of the fuel injectors is extended.
- Stratified operation is blocked (with CODE 920 (Gasoline direct injection with stratified charge)).
- Performance reduction occurs (max. speed about 70 km/h).
Stop
- Quantity control valve is deenergized and opened.
- Fuel pump is 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 repair instructions.
View of the fuel high-pressure circuit
Function sequence for fuel quality monitoring (for code 494 (USA version) and for code 929 (Engine for ethanol fuel)
With the ever higher increase in the admixture of ethanol into the fuel it has become necessary to monitor the ethanol content or the fuel ethanol mixture. Variations in the stoichiometric fuel air/fuel ratio can occur due to the variable ethanol contents. This can lead to a loss of engine power.
Monitoring takes place by means of the fuel quality sensor (B4/31) which detects the relative conductivity of the gasoline ethanol mixture.
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 is transmitted with an appropriate voltage signal to the FSCU (N118). There it is converted and sent as information to the ME-SFI [ME] control unit in order to undertake the appropriate adaptation of the engine management.
The power supply of the fuel quality sensor takes place from the FSCU.
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
- Throttle valve positioner (M16/6), throttle valve position
- Supplemental restraint system control unit (N2/10), direct crash signal
- Supplemental restraint system control unit, indirect crash signal over chassis CAN 1 (CAN E1)
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 indirectly over the chassis CAN 1 or directly from the supplemental restraint system control unit, it switches off the fuel pump through the fuel system control unit (directly and over the drive train CAN) and the quantity control valve in order to depressurize the fuel system.
| Electrical function schematic for fuel supply | PE07.10-P-2703-97NBG | ||
| Electrical function schematic for safety fuel shutoff | PE07.10-P-2733-97NBG | ||
| Overview of system components for gasoline injection and ignition system with direct injection | ENGINE 276.8 in MODEL 166 (except 166.063) as of model year 2016 ENGINE 276.8 in MODEL 292 |
GF07.70-P-9998MMX | |
| ENGINE 276.9 in MODEL 166 as of model year 2016 | GF07.70-P-9998MNG |