Fuel Supply, Function - GF07.10-P-1003MMV
Engine 276.8 in model 205, 253
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 quantities under all operating conditions at an adequate pressure to the fuel injectors (Y761 to Y766).
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
The fuel pump (M3) is switched on when a "Fuel pump ON" signal 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 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 following signals for regulation of the fuel high pressure:
- Fuel pressure and temperature sensor (B4/25), fuel pressure and temperature
The fuel from the fuel tank flows from the low-pressure fuel distributor to the fuel system high pressure pump. This pumps the fuel (depending on the operating condition) via high-pressure lines and the rails to the fuel injectors. A pressure of up to approx. 200 bar is generated here.
The three fuel injectors per cylinder head are supplied directly from the associated rail with fuel.
Located on the fuel system high pressure pump is a quantity control valve (Y94) that regulates the fuel quantity that is fed to the pump element in the fuel system high pressure pump according to the fuel specified 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 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.
If a vehicle is switched off with a hot engine, the fuel pressure may rise up to approx. 250 bar (+17 bar) in the high-pressure circuit. Upon reaching this threshold a mechanical pressure relief valve opens in the fuel system high pressure pump and the pressure is removed. Upon starting the engine the pressure falls rapidly to the normal operating pressure of about 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.
- Fuel pump pressure of approx. 4.5 to 6.7 bar.
Normal mode
- The quantity control valve regulates over the duty cycle of the fuel high pressure.
- The fuel pump pressure is regulated to between approx. 3.0 and 5.5 bar depending on the fuel temperature.
- The fuel predelivery pressure is regulated to between 4.5 and 6.7 bar depending on the engine speed and the fuel temperature.
Low-pressure limp-home mode (fuel high pressure is not reached)
- The quantity control valve is de-energized and therefore opened.
- The fuel pump pressure is about 4.5 to 6.7 bar; fuel flows over the open quantity control valve into the rails.
- Actuation of fuel injectors is extended.
- Performance reduction occurs, max. speed about 70 km/h.
Stop
- Quantity control valve is de-energized and opened.
- Fuel pump is not actuated.
Assembly operations
The high pressure fuel lines made out of stainless steel can be reused after checking. The corresponding instructions are available in the repair instructions.
View of fuel high-pressure circuit with connection to low-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 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), chassis FlexRay (Flex E) and the engine CAN (CAN C) to the ME-SFI [ME] control unit
The safety fuel shutoff is activated by the ME-SFI [ME] control unit for the following conditions:
- Mechanical fault in throttle valve actuator (M16/6)
- 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 crankshaft Hall sensor 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 the user interface CAN to the electronic ignition lock control unit, chassis FlexRay (Flex E) and the engine CAN or directly from the Supplemental Restraint System control unit, it switches off the fuel pump over the fuel system control unit (directly and via the drive train CAN) and the quantity control valve to depressurize the fuel system.
| Electrical function schematic for fuel supply | PE07.10-P-2703-97FBB | ||
| Electrical function schematic for safety fuel shutoff | PE07.10-P-2733-97FBB | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-P-9998MMV |