Fuel presupply presupply control

The fuel supply on the N47 4-cylinder diesel engine is ensured by a pressure-controlled system.

Brief description of components

The following components of the fuel supply are described:

DDE control unit with presupply control function

The function of the fuel presupply control is integrated in the DDE control unit. The DDE uses the fuel pressure-temperature sensor to determine the current actual pressure in the presupply system. The DDE uses a number of input variables to determine the adjusting value signal factor to regulate the pre-delivery pressure. The pre-delivery pressure is regulated to a pressure of 4.8 - 5.0 bar.

The input variables relevant to determining the adjusting value are:

Actual pressure in the presupply system
Engine speed
Fuel injection rate

The adjusting value is sent to the fuel pump control in the form of a CAN message. The adjusting value is sent in the unit l/h. The value range is 0 - 254 l/h.

Fig. 1: overview of fuel supply with presupply control

1	DDE control unit	8	Input signal engine speed
2	EKPS control unit	9	Input signal fuel injection rate
3	Fuel tank	10	Function of the presupply control
4	Fuel feed line	11	Activation of the fuel heater
5	Fuel filter with fuel heater	12	Fuel temperature signal
6	Fuel pressure / temperature sensor	13	Pre-delivery pressure signal
7	High-pressure pump

Fuel pressure / temperature sensor

The fuel pressure / temperature sensor is located in the fuel feed line just before the high-pressure pump. The fuel pressure / temperature sensor measures the current actual pressure in the presupply system and sends the signal to the DDE control unit. For other functions in the DDE, the fuel pressure-temperature sensor also measures the fuel temperature.

Fuel pump control

The fuel pump control receives the adjusting value of the regulating circuit from the DDE as a CAN message. The value of the CAN message is converted via a characteristic curve into a voltage in the fuel pump control. This voltage is used by the fuel pump control to activate the electric fuel pump.

Electrical fuel pump

The electric fuel pump is activated by the fuel pump control with a voltage of 6.5 - 15 V.

For correct monitoring of the power consumption of the electrical fuel pump, an adaptation value is stored in the DDE control unit. The electrical power consumption of the fuel pump at minimum activation is stored with this adaptation value. The lower limit value for monitoring the power consumption of the fuel pump is calculated with this adaptation value.

Fuel filter heating

The electrical fuel filter heating is integrated in the fuel filter and is activated by the DDE control unit.

The fuel filter heating is switched on when:

Fuel temperature < 0 °C OR
Current power consumption of the electrical fuel pump > total of the adaptation value of the electrical fuel pump and the offset value. The offset value is determined from a characteristic map and depends on the engine speed and fuel injection rate.

Notes for Service department

Diagnosis instructions

The control of the pre-delivery pressure is monitored by the DDE. Faults that occur due to control deviations are stored in the fault code memory of the DDE.

Activation of the electrical fuel pump is monitored by the fuel pump control. Faults that occur here are stored in the fault code memory of the fuel pump control.

When the DDE control unit is wakened by a wake-up signal, the electric fuel pump is briefly activated for 10 ms.

When terminal 15 is switched on, the electric fuel pump is not activated. The electric fuel pump is only activated on actuating the starter motor.

Resetting the adaptation for electric fuel pump

If the electric fuel pump adaptation must be reset if the high-pressure pump is replaced.

The reason is that the high-pressure pump has a major influence on the power requirement of the electrical fuel pump.