Fuel supply, function - GF07.10-P-1003MMR
ENGINE 157.9, 278.9 in MODEL 166 up to 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 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 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 fuel specified 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 controls the fuel high pressure on the basis of the following sensor:
- 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 pumps. These 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).
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 fuel system high pressure pump and the right quantity control valve (Y94/2) on right fuel system high pressure pump. The quantity control valves control the quantity of fuel fed to the respective pump element for delivery depending on the specified fuel pressure.
The fuel pressure and temperature sensor detects the current fuel high pressure (pressure in the rail) as well as the temperature of the fuel in the rail. The operating pressure is 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 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 a 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 respective 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 leak fuel lines on both rails which, in the case of a leak at the sealing rings for the fuel injectors to the rails, lead 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 valves are energized and closed, so there is full delivery to the fuel system high pressure pumps and rapid pressure buildup.
- The fuel pump pressure is about 4.5 up to 6.7 bar.
Normal mode
- The quantity control valves regulate the fuel quantity feed to the fuel system high pressure pump over the duty cycle.
- 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 valves deenergized and therefore opened.
- 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.
- Performance reduction, max. speed about 70 km/h.
Stop
- Quantity control valve deenergized and open
- Fuel pump not actuated
Assembly operations
The high pressure fuel lines made out of stainless steel can be reused. An appropriate test specification can be found 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
- 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 via chassis CAN 1 or directly from the supplemental restraint system control unit, it switches off the fuel pump via the fuel system control unit (directly and via drive train CAN) and the quantity control valves in order to depressurize the fuel system.
| Electrical function schematic for fuel supply | PE07.10-P-2703-97NAF | ||
| Electrical function schematic for safety fuel shutoff | PE07.10-P-2733-97NAF | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-P-9998MMG |