W07.02-1006-79
Shown on unit pump of 4th. cylinder on engine 541.9
I Intake stroke
II Pre-stroke
III Delivery stroke
IV Residual stroke
1 Unit pump
2 High pressure pipe
3 Pressure pipe connection
11 Armature plate
12 Float needle
13 Injection nozzle
B Fuel feed
C Fuel high-pressure
D Fuel return flow
4 Nozzle holder combination
5 Pump element
6 Supply passage
7 Solenoid valve
8 High pressure chamber
9 Return flow passage
10 Solenoid
Each unit pump (1) is driven through a roller tappet from the camshaft and is connected to the injection nozzle in the nozzle holder-combination (4) by a short high-pressure line (2) and the pressure pipe connection (3).
Each unit pump (1) has a fast-acting solenoid valve (7). This solenoid valve regulates the start of delivery and controls the quantity delivered. The solenoid valves are actuated by the control unit of engine control (MR), which calculates the start of delivery and quantity delivered according to the operating condition of the engine as soon as the engine is cranked and / or the engine is running.
Depending on the position of the camshaft, each unit pump (1) will have the following power strokes:
Intake stroke (I)
Initial stroke (II)
Delivery stroke (III)
Residual stroke (IV)
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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 turned off, the control unit of engine control (MR) stops the actuation of the solenoid valves. The unit pumps (1) deliver the almost unpressurized fuel to the fuel return of the fuel low pressure circuit and the engine switches off.
The procedures described below within the individual power strokes of an operating cycle (shown on unit pump of 4th. cylinder on engine 541.9) can be transferred to all other unit pumps.
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Intake stroke (I)
The intake stroke (I) 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 4 to 6 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.
Initial stroke (II)
After the intake stroke (I), the pump element (5) continues to remain in the low stop position, and the roller tappet of the unit pump (1) rolls off on the base circle of the unit pump cam. The initial stroke (II) 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 and then into the return flow passage (9).
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Delivery stroke (III)
The initial stroke (II) concludes, and the delivery stroke (III) begins as soon as the control unit of engine control (MR) actuates the solenoid (10) in the unit pump (1). The solenoid (10) activates the anchor plate (11) with the float needle (12), which then locks the connection between the high pressure chamber (8) and the return flow passage (9).
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 (13) also increases further.
At a pressure of about 330 bar, the injection nozzle (13) opens, and the fuel is injected into the combustion chamber (start of injection). The fuel pressure can rise up to 1800 bar during injection.
The unit pump (1) is in the delivery stroke as long as the pump element (5) moves in the upward direction, and the solenoid valve (7) is closed. The holding time of the solenoid valve (7) determines the quantity delivered and / or injected.
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Residual stroke (IV)
If the actuation of the solenoid (10) is stopped, the float needle (12) 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 (9) is opened. As a result, the fuel pressure in the high-pressure chamber (8) and in the injection nozzle (13) falls sharply. The injection nozzle (13) 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 and the return flow passage (9).
The diversion chamber is necessary as the expansion chamber for the pressure peaks of the unit pump (1) in the residual stroke. This prevents the pressure behavior of the adjacent unit pumps being affected through the return flow passage (9).