Fuel Supply, Function - GF07.10-P-1003MNC

Engine 276.8 in model 172.4 

Function requirements for fuel supply, general points 

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

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 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. In order to determine the fuel specified pressure (fuel requirements), the ME-SFI [ME] control unit evaluates the fuel pressure and load requirements. A variable fuel delivery volume of 0 to 130 l/h is regulated at a fuel pressure of 4.0 to 6.7 bar to match the fuel requirements.

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 fuel pressure is limited to a maximum of 9 bar by the fuel pressure regulator. The fuel filter is also located in the fuel feed module in the fuel tank. The fuel tank has to be removed to replace the fuel filter.

There is a check valve at the feed point of the fuel filter which prevents removal of the fuel pressure when the fuel pump is switched off.

Function sequence for fuel system high-pressure circuit 

In the fuel high-pressure circuit, the fuel high pressure of approx. 200 bar as required for spray guided direct injection is generated, regulated and the pressure is then stored in the rail. 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)

The fuel from the fuel tank flows from the low-pressure fuel distributor to the fuel system high pressure pump. Y76This delivers the fuel (according to the operating condition) at a pressure of up to 200 bar and directs it to the fuel injectors of cylinders 1 to 6 (Y76/1 to 6) over the high-pressure line and the rail. At the fuel system high pressure pump 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 dependent on the requirements and a characteristics map. The quantity control valve is actuated by a pulse width modulated signal from the ME-SFI [ME] control unit.

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 about 200 bar. It is only at vehicle standstill and for a selector lever position "N" or "P" (for an A/T) that the pressure drops to 100 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 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 about 200 bar.

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 is approx. 4.0 up to 6.7 bar.

Normal mode 

• The quantity control valve regulates the supplied and the required fuel quantity over the duty cycle.
• Fuel pump pressure is dependent on the fuel temperature between about 3.0 to 5.5 bar.
• The fuel pre delivery pressure varies according to the engine speed and fuel temperature between 4.0 and 6.7 bar (absolute).

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

• The quantity control valve is de-energized and therefore opened.
• Fuel pump pressure about 4.0 to 6.7 bar, fuel flows over the open quantity control valve into the rail.
• Actuation of the fuel injectors for cylinder 1 to 6 extended
• Reduced performance, max. speed approx. 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. More information on this is available in the repair instructions.

View of the fuel high-pressure circuit 

Courtesy of MERCEDES-BENZ USA

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) (with rotational recognition), engine RPM
• Actual value potentiometer 1 and 2 (M16/6r1, M16/6r2), 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 (M16/6), engine speed is limited to about 1400 RPM at idle and about 1800 RPM in driving mode by shutting off the fuel injectors for cylinder 1 to 6.

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 the chassis CAN 1 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, and briefly actuates the fuel injectors for cylinder 1 to 6 to depressurize the fuel system.

	Electrical function schematic for fuel supply		PE07.10-P-2703-97TAH
	Electrical function schematic for safety fuel shutoff		PE07.10-P-2733-97TAH
	Overview of system components for gasoline injection and ignition system with direct injection		GF07.70-P-9998MNC