DAF 95XF. Manual - part 412

4

XE ENGINE FUEL SYSTEM

General

95

XF series

2-4

Pump unit

Principle of operation

i400226

7

6

5

4

3

2

1

A

B

Case A
In this case, the pump piston (3) performs the
suction stroke.
Due to the permanent overpressure in the
low-pressure area of the fuel system, the
high-pressure area above the pump piston (3) is
filled through the fuel supply pipe (4).

Case B
In this case, the pump piston (3) performs the
delivery stroke.
Because the valve (6) has not yet shut off the
connection with the fuel return connection (2),
the fuel is forced into the fuel return.

5

©

 0008

4

95

XF series

General

XE ENGINE FUEL SYSTEM

2-5

i400227

7

6

5

4

3

2

1

C

D

Case C
In this case, the injection timing is determined.
The solenoid (5) is activated by the electronic
unit.
As a result, the valve (6) closes the connection
between the high-pressure area above the
pump piston (3) and the fuel return
connection (2).
Pressure is built up above the pump piston (3),
which causes fuel to be injected through the
injector (7).

Case D
In this case, the quantity of injected fuel is
established.
The solenoid (5) is no longer activated by the
electronic unit.
As a result, the valve (6) opens the connection
between the high-pressure area above the
pump piston (3) and the fuel return
connection (2).
The fuel pressure above the pump piston (3) is
then reduced rapidly due to the expansion
space in the return pipe.

5

©

 0008

4

XE ENGINE FUEL SYSTEM

General

95

XF series

2-6

Operation of the pump unit
The valve (6) with back plate (8) and the pump
piston (14) are located at the top of the pump
unit.
These parts are lubricated by the fuel.
The roller (18) with rocker roller (16) and
spring (15) with spring retainer (17) are located
at the bottom of the pump unit.
These parts are lubricated by the engine
lubrication system.
The lower sealing ring of the pump unit
separates the fuel system from the engine
lubrication system.

A delivery valve (5) has been installed at the top
of the pump unit.
The delivery valve serves to prevent the
low-pressure area from being disturbed by the
pressure peaks from the injection line.

Using a magnetic valve (9) consisting of a
solenoid and valve (6) with back plate (8), the
injection timing and quantity of injected fuel are
controlled by the electronic control unit.

1

16

18

2

3

4

6

7

8

10

11

12

13

14

15

17

9

5

i 400411 

5

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 0008

4

95

XF series

General

XE ENGINE FUEL SYSTEM

2-7

When the magnetic valve (9) is not activated,
the valve (6) is pressed against the valve
stop (4) by valve spring force.
This creates a very small opening between the
high-pressure area above the pump piston (14)
and the low-pressure area of the return bore (2).

The fuel flows to the fuel return bore (2) via the
fuel supply bore (11) and the fuel filter (10). As a
result, the high-pressure area above the pump
piston (14) is also completely filled with fuel.
When the magnetic valve (9) is activated by the
electronic unit, the back plate (8) with valve (6)
will be attracted by the solenoid under the
influence of the magnetic field.
Valve (6) closes the connection between the
high-pressure and low-pressure areas, enabling
pressure to be built up above the pump
piston (14).
As a result, injection takes place.

Due to the high pressure in the high-pressure
area the valve (6) is not re-opened against the
magnetic force, because there is a state of
balance in the groove of valve (6).
This is because the areas to the left and right in
the groove of valve (6) are the same.

The time when valve (6) is closed depends on
the injection timing calculated by the electronic
unit.
The closure time of valve (6) and, consequently,
the quantity of injected fuel, is also calculated by
the electronic unit.

1

16

18

2

3

4

6

7

8

10

11

12

13

14

15

17

9

i 400412

5

5

©

 0008