Fuel high pressure delivery function

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1 Unit pump

2 High pressure pipe

3 Pressure pipe connection

4 Nozzle holder combination

General information

The fuel high pressure delivery by the pump-line-nozzle system takes place by way of individual unit pumps (1). Each of these unit pumps is assigned a cylinder. The unit pumps (1) are driven by roller tappets of the camshaft and are connected to the injection nozzles in the nozzle holder combinations (4) via short high-pressure lines (2) and the pressure pipe connection (3).

Each unit pump (1) has a fast-acting solenoid valve. This solenoid valve regulates the start of delivery and controls the quantity delivered. The solenoid valves are actuated by the MR/PLD control unit that calculates the start of delivery and the quantity delivered according to the operating condition of the engine as soon as the engine is cranked and/or when the engine is running.

However, it is required that before actuation, the immobilizer (WSP) in the MR/PLD control unit detect a correct transponder code (from the ignition key) and be therefore deactivated. In addition, the engine stop push button switch must not be activated if the engine has been started with the engine start push button switch because in this case, even the solenoid valves have not been actuated.

Function

Depending on the position of the camshaft, each of the unit pumps (1) have the following power strokes:

 Induction stroke

 Pre-stroke

 Delivery stroke

 Residual stroke

Together these power strokes form a working cycle, which is repeated with every additional rotation of the camshaft at the respective unit pumps (1), till the engine is turned off.

As soon as the engine is switched off, the MR/PLD control unit stops the actuation of the solenoid valves. The unit pumps (1) deliver the almost depressurized fuel to the fuel return of the fuel low pressure circuit and the engine switches off.

 The following description of a unit pump's work cycle can be transferred to other unit pumps. A schematic representation is used for better understanding.

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Induction stroke

2 High pressure pipe

3 Pressure pipe connection

4 Nozzle holder combination

5 Pump element

6 Supply passage

7 Solenoid valve

8 High pressure chamber

B Fuel feed

D Fuel return

The induction stroke begins after the vertex of the cam is exceeded and the pump element (5) moves downwards under the force of the restoring spring.

Owing to the continuous fuel overpressure of about 2.0 to 6.5 bar in the fuel low pressure circuit, the high pressure chamber (8) is filled with fuel via the supply passage (6) and the opened solenoid valve (7).

Therefore, the pressure in the high-pressure chamber (8) filled with fuel is the same as the pressure in the fuel low pressure circuit.

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Pre-stroke

2 High pressure pipe

3 Pressure pipe connection

4 Nozzle holder combination

5 Pump element

6 Supply passage

7 Solenoid valve

8 High pressure chamber

9 Diversion chamber

10 Return flow passage

B Fuel feed

D Fuel return

After the induction stroke, the pump element (5) continues to remain in the low stop position, and the roller tappet of the unit pump rolls off on the base circle of the unit pump cam. The pre-stroke begins only when the camshaft rotates further and the pump element (5) begins to move in the upward direction owing to the rising unit pump cam.

Due to the upward movement of the pump element (5) and the still open (de-energized) solenoid valve (7), the fuel in the high pressure chamber (8) is first pushed into a diversion chamber (9) and then into the return flow passage (10).

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Delivery stroke

2 High pressure pipe

3 Pressure pipe connection

4 Nozzle holder combination

5 Pump element

6 Supply passage

7 Solenoid valve

8 High pressure chamber

9 Diversion chamber

10 Return flow passage

11 Solenoid

12 Armature plate

13 Float needle

14 Injection nozzle

B Fuel feed

C Fuel high pressure

D Fuel return

The pre-stroke ends, and the delivery stroke begins as soon as the MR/PLD control unit actuates the solenoid (11) in the unit pump. The solenoid (11) activates the anchor plate (12) with the float needle (13), which then locks the connection between the high-pressure chamber (8) and the return flow passage (10).

If the solenoid valve (7) is closed, the fuel in the high-pressure chamber (8) is compressed due to the upward movement of the pump element (5) (start of delivery).

With the further upward movement of the pump element (5), the pressure in the high-pressure chamber (8), and thereby the pressure in the high-pressure line (2), in the pressure pipe connection (3) and in the injection nozzle (14) also increases further.

At a pressure of about 330 bar, the injection nozzle (14) opens, and the fuel is injected into the combustion chamber (start of injection). The fuel pressure can rise up to 1800 bar during injection.

As soon as the pump element (5) is in the upward movement and solenoid valve (7) is closed, the unit pump is in the delivery stroke. The holding time of the solenoid valve (7) determines the quantity delivered and / or injected.

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Residual stroke

2 High pressure pipe

3 Pressure pipe connection

4 Nozzle holder combination

5 Pump element

6 Supply passage

7 Solenoid valve

8 High pressure chamber

9 Diversion chamber

10 Return flow passage

11 Solenoid

12 Armature plate

13 Float needle

14 Injection nozzle

B Fuel feed

D Fuel return

If the actuation of the solenoid (11) is stopped, the float needle (13) of the solenoid valve spring is pushed back in its starting position on the shut-off stop. The solenoid valve (7), and thereby the connection between the high-pressure chamber (8) and the return flow passage (10) is opened. As a result, the fuel pressure in the high-pressure chamber (8) and in the injection nozzle (14) falls sharply. The injection nozzle (14) closes and the process of injection concludes (end of delivery).

The remainder of the fuel delivered by the pump element (5) up to the vertex of the unit pump cam is again delivered to the fuel low pressure circuit through the diversion chamber (9) and the return flow passage (10).

The diversion chamber (9) is necessary as an expansion chamber for the pressure peaks of the unit pump during the residual stroke. This prevents the pressure behavior of the adjacent unit pumps being affected through the return flow passage (10).