Fuel supply, function

Function requirements for fuel supply, general points

 Circuit 15 (ignition ON)

 Circuit 87M (engine control 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 operation steps:

 Function sequence for low-pressure fuel circuit

 Function sequence for high-pressure fuel circuit

 Function sequence for safety fuel shutoff

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Function sequence for low-pressure fuel circuit

Switching on of the fuel pump (M3) takes place if a signal "fuel pump ON" is received by the fuel pump 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 pump control unit also receives the CAN signal "nominal pressure of the fuel" from the ME-SFI [ME] control unit.

The fuel pump 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 in such a way that the actual pressure corresponds to the specified pressure.

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In order to determine the nominal fuel pressure (fuel requirements), the ME-SFI [ME] control unit evaluates the fuel pressure and load requirements. The fuel delivery volume is arises variably from 0 to 103 l/h for a fuel pressure of 4.5 to 6.7 bar according to 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 high-pressure pump (single line system without return line).

The fuel pressure is limited to 7 bar by the fuel pressure regulator.

The fuel filter is also located in the fuel feed module in the fuel tank. Removal of the fuel tank is required to exchange the fuel filter.

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

 Natural gas operation

In natural gas operation a fuel specified pressure with hysteresis is prescribed by the ME-SFI [ME] control unit. The fuel pump is switched off over the FSCU as soon as the upper value is reached. The fuel pressure can increase for short time due to the warming.

The fuel pump is switched on again over the FSCU just as soon as the fuel pressure drops below the lower hysteresis threshold.

Switch over takes place into gasoline operation if there is failure of the CAN system, a faulty fuel priming pressure or a faulty value of the fuel pressure sensor from the ME-SFI [ME] control unit.

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Function sequence for high-pressure fuel circuit

In the high-pressure fuel circuit the fuel pressure of about 200 bar required for jet-guided direct injection is generated, regulated and the pressure is stored in both rails.

The ME-SFI [ME] control unit reads in the following sensor for regulation of the high pressure fuel:

- Fuel pressure and temperature sensor (B4/25)

The fuel system high pressure pump compacts the fuel (according to the operating condition) up to about 200 bar and directs it via the high-pressure line and the rail to the fuel injectors (Y76).

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There is a quantity control valve (Y94) on the fuel system high pressure pump. It regulates the fuel quantity according to demand and to characteristic map which is fed to the pump element in the fuel system high pressure pump.

Actuation of the quantity control valve takes place over a pulse width modulated signal (PWM signal) from the ME-SFI [ME] control unit.

The fuel pressure and temperature sensor detects the current fuel high pressure (rail pressure) as well as the temperature of the fuel in the rail.

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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 130 bar in order to reduce the noise emissions from the fuel high-pressure pump.

In the case of stopping a vehicle with a hot engine, the fuel pressure can increase up to 250 bar (+17 bar) in the high-pressure circuit. Upon reaching this threshold a valve opens in the fuel 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.

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 Natural gas operation

In natural gas operation a fuel specified pressure with hysteresis is prescribed downwards by the ME-SFI [ME] control unit. The quantity control valve is actuated by the ME-SFI [ME] control unit over a PWM signal just as soon as the lower value is reached and the fuel pressure is built up to 200 bar.

Due to warming of the closed fuel system, the fuel pressure can increase by up to 250 bar (+17 bar) in the high-pressure circuit. Upon reaching this threshold a valve opens in the fuel high-pressure pump and the pressure is reduced.

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During regulation of the high-pressure fuel circuit one differentiates between the following operating conditions:

 Start

 Normal mode

 Low-pressure limp-home mode (fuel high pressure 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. 4.5 up to 6.7 bar

Normal mode

- The quantity control valve regulates the fed in and 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 predelivery pressure is dependent on the engine speed and fuel temperature and varies between 4.5 and 6.7 bar (absolute)

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

- The quantity control valve is deenergized and therefore opened

- A fuel pump pressure about 4.5 to 6.7 bar, fuel flows over the open quantity control valve into the rail

- Actuation of fuel injectors extended

- Reduced performance, max. speed approx. 70 km/h

Stop

- The quantity control valve is deenergized and opened

- Fuel pump not actuated.

 Assembly operations

The high pressure fuel lines made out of stainless steel can be reused after checking.

More information about this can be found in the repair instructions.

P07.02-2138-82

Design of the high-pressure fuel circuit

19 Fuel system high pressure pump

19/1 Drive plate (drive)

20 Rail

B4/25 Fuel pressure and temperature sensor

Y76/1 Cylinder 1 fuel injector

Y76/2 Cylinder 2 fuel injector

Y76/3 Cylinder 3 fuel injector

Y76/4 Cylinder 4 fuel injector

Y94 Quantity control valve

A Fuel feed from fuel tank
(low pressure fuel)

B Fuel feed to the rail (high pressure fuel)

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 controls the safety fuel shutoff on the basis of the following sensors and signals:

- Crankshaft Hall sensor (B70) (with direction of rotation recognition), motor 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 via chassis CAN (CAN E)

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The safety fuel shutoff is activated by the ME-SFI [ME] control unit for the following conditions:

 Mechanical fault in the throttle valve actuator

 Absence of the engine speed signal

 Crash signal

Mechanical fault in the throttle valve actuator

If, when evaluating the signals from the actual value potentiometer 1 and 2 (M16/6r2,M16/6r1), the ME-SFI [ME] control unit detects a mechanical fault in the throttle valve actuator, the engine speed is limited to about 1400 rpm at idle and about 1800 rpm in driving mode by shutting off the fuel injectors.

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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 pump control unit.

Crash signal

If the ME-SFI [ME] control unit receives a "crash signal" indirectly via the chassis CAN or directly from the supplemental restraint system control unit, the signal is passed on via the drive train sensor CAN (CAN I) to the CNG control unit (N118/2) and both of the gas/gasoline systems are switched off.

The ME-SFI [ME] control unit switches off the fuel pump over the FSCU (directly and also via the drive train CAN) and the quantity control valve and briefly actuates the injection valves in order to depressurize the fuel system.