CDI fuel supply, function

Function requirements, general

 Circuit 15 ON, for feed line on the fuel pump (M3) for approx. 20 s

 Circuit 50 ON

Common rail diesel injection (CDI) fuel supply - general

The fuel supply must provide sufficient fuel at sufficient pressure in all operating conditions.

The fuel supply is subdivided into the following 2 function sequences:

 Low-pressure circuit function sequence

 High-pressure circuit function sequence

The following components are involved in the fuel supply process:

- Rail pressure sensor (B4/6)

- Fuel temperature sensor (B50)

- Fuel tank

- Fuel pump

- Fuel lines

- Fuel filter

- Pressure regulator valve (Y74)

- Fuel injectors (Y76)

- Quantity control valve (Y94)

- High-pressure pump

- High-pressure line

- Left rail

- Right rail

P07.16-2794-82

Low pressure circuit

19 High-pressure pump

70 Fuel main filter

75 Fuel tank

B50 Fuel temperature sensor

M3 Fuel pump

Y74 Pressure control valve

Y76 Fuel injectors

Y94 Quantity control valve

Low-pressure circuit function sequence

The fuel low-pressure circuit consists of the following components:

- Fuel tank

- Fuel pump

- Fuel lines

- Fuel main filter

- Quantity control valve

- High-pressure pump, low-pressure side

- Fuel temperature sensor

- Pressure regulator valve

- Fuel injectors, return flow

 The driver is informed of the fill level in the fuel tank via the fuel level indicator sensor in the left tank half (B4/1), via the fuel level indicator sensor in the right tank half (B4/2), the fuel reserve warning lamp (A1e4) and the fuel level and reserve indicator (A1p2).

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 In cold temperatures, on vehicles except code (U41) fuel/water separator and lubrication pack, the flowability of the fuel is ensured by the fuel preheating system heating element (R54) and, on vehicles with code (U41) fuel/water separator and lubrication pack, by a heating element in the Condensation sensor for fuel filter with heating element (B76/1). The power supply for the fuel preheating system heating element, or the heating element of the Condensation sensor for fuel filter with heating element, is provided by the front SAM control unit with fuse and relay module (N10/1).

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 The condensation level in the fuel main filter is monitored by the condensation sensor for fuel filter with heating element. Above a condensation level of ca. 100 ml the CDI control unit (N3/9) sends a message to the instrument cluster (A1) which is displayed as a warning message in the multifunction display (A1p13).

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 Fuel pre-heating for code (71o) Retrofitted heated fuel filter
A condensation sensor for fuel filter with heating element is installed for condensate level monitoring in order to achieve flowability of the fuel at low outside temperatures. The heating element is actuated by the control unit for condensation sensor for fuel filter with heating element (N33/7) and heats the fuel in the fuel filter. The condensation level in the fuel main filter is monitored by means of the condensation sensor for fuel filter with heating element. The condensation level is read in by the control unit for condensation sensor for fuel filter with heating element and transmitted to the CDI control unit . Above a condensation level of ca. 100 ml the CDI control unit sends a message to the instrument cluster which is displayed as a warning message in the multifunction display.

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For "circuit 15 ON", the fuel pump is actuated by the fuel pump relay (N10/2kS) integrated into the rear SAM control unit with fuse and relay module (N10/2) for approx. 20 to 25 s.

When the engine is started ("tml. 50 ON") and when engine running is detected ("tml. 61 ON"), the CDI control unit actuates the fuel pump continuously through the fuel pump relay integrated in the rear SAM control unit with fuse and relay module, and the fuel pump delivers the fuel through the fuel main filter to the high-pressure pump.

The fuel delivery pressure is limited to about 4.0 to 4.5 bar by the pressure relief valve integrated in the high-pressure pump, depending on fuel pump actuation (pulse width modulation (PWM) signal) and the loading in the fuel main filter.

The quantity control valve also regulates the supply of fuel depending on the engine operating point to the high-pressure pump, in order to increase engine efficiency and to keep the fuel temperature low.

The leak fuel from fuel injectors is led back before the main fuel filter into the system. Fuel diverted by the pressure regulator valve passes into the return line. The total return flow quantity comes from the pressure regulator valve and the high-pressure pump return line.

 If the CDI control unit receives a crash signal from the restraint system control unit (N2/7), the fuel pump actuation is interrupted through the fuel pump relay by the CDI control unit.

P07.16-2768-11

High-pressure circuit

19 High-pressure pump

21 Left rail

22 Right rail

23 High-pressure lines

B4/6 Rail pressure sensor

B50 Fuel temperature sensor

Y74 Pressure control valve

Y76 Fuel injectors

Y94 Quantity control valve

Function sequence for high-pressure circuit

The high-pressure circuit consists of the following components:

- High-pressure pump

- Left rail

- Right rail

- High-pressure line

- Rail pressure sensor

- Fuel temperature sensor

- Pressure regulator valve

- Fuel injectors

- Quantity control valve

The high-pressure circuit function sequence consists of the following subfunctions:

- Fuel distribution function sequence

- Regulation function sequence

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Fuel distribution function sequence

The high-pressure pump delivers fuel depending on the coolant temperature, position of the accelerator pedal, the engine speed and the fuel temperature into the left rail and from there simultaneously into the right rail. The fuel passes from the respective rail via the high pressure lines to the fuel injectors. The fuel injectors inject finely atomized fuel selectively into each cylinder.

The CDI control unit calculates the injection quantity selectively for each cylinder based on a characteristics map for the respective operating condition. The injection quantity is dependent upon the fuel pressure in the rail and the actuation duration of fuel injectors.

The fuel pressure in the rail is controlled by the pressure regulating valve and quantity control valve. The effectively applied fuel pressure in the rail is detected by the rail pressure sensor, passed on permanently to the CDI control unit and limited to a maximum of 1800 bar by the pressure regulator valve.

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The fuel temperature is detected by the fuel temperature sensor and permanently passed on to the CDI control unit.

Regulation function sequence

Regulation is subdivided into the following regulation strategies:

- Pressure regulator valve regulation

- 2-regulator concept regulation

- Quantity control valve regulation

Pressure regulator valve regulation

The rail pressure is regulated via the pressure regulating valve during the starting procedure and for fuel heating. The quantity control valve is opened in a controlled manner. Regulation by the pressure regulator valve occurs in the following conditions:

- Up to 30 s after the engine start in idle

- Up to a fuel temperature of 20°C

2-regulator concept regulation

The rail pressure is jointly regulated in idle and in overrun mode by the pressure regulator valve and quantity control valve.

Quantity control valve regulation

The rail pressure is regulated by the quantity control valve from 30 seconds after engine start. The pressure regulator valve is controlled closed.

 With the 2-regulator design and regulation by quantity control valve, the fuel is heated less strongly than is the case with control by pressure regulator valve.

 After switching off the engine, there is a residual pressure of about 50 to 80 bar in the high-pressure control circuit. The high-pressure system may only be opened after completely relieving the pressure on safety grounds.