Fuel Supply, Function - GF07.10-D-1003TSB

Engine 274 in model 907 

Block diagram 

Function requirements for fuel supply, general points 

• Circuit 87M ON (Engine management ON)
• Circuit 15 ON (ignition 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 (19).

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 with fill level sensor (M3/6) is switched on if the signal "fuel pump ON" is received by the fuel system control unit (N118/9). This signal is sent redundantly from the ME-SFI [ME] control unit (N3/36) as a CAN signal via the drive train CAN (CAN C) and as a ground signal.

The fuel system control unit (N118/9) also receives the CAN signal "Fuel specified pressure" from the ME-SFI [ME] control unit (N3/36). The ME-SFI [ME] control unit (N3/36) detects the current fuel pressure via the fuel pressure and temperature sensor (B4/21) and transmits this information to the fuel system control unit (N1189) via the drive train CAN (CAN C).

The fuel system control unit (N1189) evaluates the current fuel pressure, compares it with the fuel specified pressure and actuates the fuel pump with fill level sensor (M3/6) in such a way that the actual value corresponds to the specified value.

In order to determine the fuel specified pressure (fuel requirements), the ME-SFI [ME] control unit (N3/36) evaluates the fuel pressure and the load requirement. This means that, at a fuel prefeed pressure of approx. 4 to 6.7 bar, the fuel delivery rate is regulated variably from 0 to 130 l/h depending on fuel requirements.

For actuation the fuel pump suctions with fill level sensor (M3/6) the fuel from the fuel feed module and pumps it through the fuel filter to the fuel system high pressure pump (19) (single line system without return line).

The overflow valve in the fuel filter opens at a fuel pressure of approx. 7 to 9 bar.

There is a check valve in the feed line to the fuel filter. This prevents reduction of the fuel prefeed pressure when a fuel pump with fill level sensor is switched off (M3/6) (to below approx. 4.5 bar).

Fuel high-pressure circuit 

Function sequence for fuel system high-pressure circuit 

In the fuel high-pressure circuit the required fuel pressure for spray guided direct injection is generated, regulated and in the rail (20). The ME-SFI [ME] control unit (N3/36) directly reads in the pressure and the temperature signal of the fuel pressure and temperature sensor (B4/21) to regulate the fuel pressure.

The description of the fuel high-pressure circuit is subdivided into:

• Fuel feed
• Fuel pressure control

Fuel feed 

The fuel system high pressure pump (19) compresses the fuel (depending on the operating condition) and routes it over the high-pressure line and the rail (20) to the fuel injectors (Y62).

At the fuel system high pressure pump (19) there is a quantity control valve (Y94) which regulates the fuel quantity which is fed to the pump element in the fuel system high pressure pump (19) dependent on the requirements and a characteristics map. The quantity control valve (Y94) is actuated via a pulse width modulated signal from the ME-SFI [ME] control unit (N3/36).

Fuel pressure control 

The fuel pressure and temperature sensor (B4/21) detects the current fuel pressure as well as the temperature of the fuel in the rail (20). The operating pressure is approx. 200 bar.

The pressure is only lowered to 130 bar when the vehicle is at a standstill in order to reduce the noise emissions from the fuel high-pressure pump (19).

If a vehicle with a hot engine is switched off, the fuel pressure may rise up to 250 bar (+ 17 bar) in the high-pressure fuel circuit. Upon reaching this threshold a pressure limiting valve opens in the fuel system high pressure pump (19) and the pressure is reduced. Upon starting the engine the pressure falls rapidly to the normal operating pressure of bar.

During regulation of the high-pressure fuel circuit, a distinction is made between the following operating states:

• Start
• Normal mode
• Low-pressure limp-home mode (fuel high pressure is not reached)
• Stop

Start 

• The quantity control valve (Y94) is energized and closed, so there is full delivery from the fuel system high pressure pump (19) and rapid pressure buildup.

Normal mode 

• The quantity control valve (Y94) regulates the supplied and the required fuel quantity during the duty cycle.

Low-pressure limp-home mode (fuel high pressure is not reached) 

• The quantity control valve (Y94) is deenergized and therefore opened.
• Actuation of fuel injectors (Y62) extended
• Power output limitation, max. speed approx. 70 km/h

Stop 

• The quantity control valve (Y94) is deenergized and opened.
• Fuel pump with fill level sensor (M3/6) is not actuated.

The high pressure fuel lines made out of stainless steel can be reused after checking. More information on this is available in the repair instructions.

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 (N3/36) controls the safety fuel shutoff system on the basis of the following assembly parts and signals:

• Crankshaft Hall sensor (B70/2), engine RPM
• Throttle valve actuator (M16/50), throttle valve position
• Control unit for the Supplemental Restraint System (SRS) (N2/5), direct crash signal or indirect signal via the user interface CAN (CAN HMI), EZS control unit (N73/8), suspension FlexRay (Flex E), powertrain control unit (N127) and engine CAN (CAN C1)
• Absence of the engine speed signal
• Crash signal

Mechanical fault in the throttle valve actuator 

If the ME-SFI [ME] control unit (N3/36) detects a mechanical fault in the throttle valve actuator (M16/50) when evaluating the throttle valve position, the engine speed is limited to approx. 1,200 RPM in idle mode and in driving mode by shutting off the fuel injectors (Y62).

Absence of the engine speed signal 

If the engine speed signal generated by the ME-SFI [ME] control unit (N3/36) is missing, the fuel pump with fill level sensor (M3/6) is switched off via the fuel system control unit (N1189).

The safety fuel shutoff is activated by the ME-SFI [ME] control unit (N3/36) under the following conditions:

• Mechanical fault in the throttle valve actuator

Crash signal 

If the ME-SFI [ME] control unit (N3/36) receives a crash signal from the control unit for the Supplemental Restraint System (SRS) (N2/5), it switches off the fuel pump with fill level sensor (M3/6) via the fuel system control unit (N118/9) and actuates the quantity control valve (Y94) in order to depressurize the fuel system.

Electrical function schematic for fuel supply	PE07.10-D-2703-97TSB
Overview of system components for gasoline injection and ignition system with direct injection	GF07.70-D-9998TSB