DAF XF105. Manual - part 86

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System description

DMCI ENGINE MANAGEMENT SYSTEM

XF105 series

1.3 SYSTEM DESCRIPTION OF FUEL SYSTEM

The fuel lift pump (3a) draws fuel out of the fuel 

tank (1) via a shut-off valve (2a) in the supply 

pipe. The fuel goes to the fuel pump (3) via the 

cylinder block (A). The fuel lift pump (3a) pumps 

the fuel via the fuel filter (4) to the fuel gallery in 

the cylinder block (A). The pressure in the fuel 

Fuel system

M

M

M

M

M

M

7

B

A

6

8

6

6

9

P

U

T

R

6

6

2a

2b

1

a

6

5

4

a

c

b

b

a

c

3

M

7

7

7

7

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b

d

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Cylinder block

4

Fuel filter

B

Cylinder head

4a

Filter element

1

Fuel tank

4b

Bleed restriction

1a

Fuel-tank coarse filter

5

Fuel pressure control valve

2a

Shut-off valve, supply

5a

Pressure control flap

2b

Shut-off valve, return

5b

Throttle bleed/idling speed

3

Fuel pump

5c

Fuel pressure measuring point

3a

Lift pump

6

Pump units

3b

Primer pump

7

Injectors

3c

Pressure release valve

8

Non-return valve

3d

Circulation valve

9

Fuel pressure and temperature sensor

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DMCI ENGINE MANAGEMENT SYSTEM

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System description

XF105 series

gallery is controlled by the pressure control flap 

(5a) in the fuel pressure control valve (5). The 

valve (5a) opens at a specific pressure and 

the fuel is led back to the intake side of the 

fuel pump (3).

There is a calibrated opening (5b) in the fuel 

pressure control valve (5) that ensures cooling of 

the fuel system, by means of fuel flow-back, at 

low engine speeds and in situations where fuel is 

not injected (e.g. DEB in use). This calibrated 

opening (5b) also ensures that pressure in the 

fuel gallery is stable at low speeds and therefore 

that pump pressures are low.

There is a test connection (5c) on the fuel 

pressure control valve (5) for measuring the fuel 

gallery pressure.

The fuel flows to the pump units (6) from the fuel 

gallery. If the solenoid valve in the pump unit is 

not activated, the pump unit pumps the fuel back 

into the fuel gallery. If the solenoid valve in the 

pump unit is activated, the fuel is pumped at high 

pressure to the injectors (7) via the fuel injection 

pipes. 

If the solenoid valve in the injector is activated, an 

injection takes place.

The lubricating and leak-off fuel from the pump 

unit plunger is discharged to the return gallery in 

the cylinder block via a bore. The return and leak-

off fuel from the injectors flows to the return 

gallery in the cylinder block via a bore in the 

cylinder head and the fuel return pipe with non-

return valve (8).

The return fuel flows back to the tank via a shut-

off valve (2b).

The shut-off valves (2a and 2b) are opened when 

the fuel pipes are connected. When the fuel pipes 

between engine and chassis are disconnected, 

the valves close the opening to the cylinder block.

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System description

DMCI ENGINE MANAGEMENT SYSTEM

XF105 series

Fuel filter
There is a coarse filter (1a) in the fuel tank (1), 

which prevents any larger impurities from the 

bottom of the fuel tank getting into the fuel lift 

pump. From the fuel lift pump, fuel is pumped to 

the fuel gallery through the fuel fine filter (4).

The fuel filter is self-bleeding. There is a throttle 

(3b) at the highest point in the fuel fine filter (4)

through which air in the system is discharged to 

the fuel tank.

Basic function of injection
The pump unit and injector work in tandem to 

bring about fuel injection. A complete fuel 

injection cycle is described in the following steps.

Filling
The fuel lift pump (3) draws fuel out of the tank (5) 

via the inlet pipe (6) and pumps it to the pump unit 

(1). The pump unit has not been activated and the 

pump unit valve (1a) is open. The space above 

the pump unit plunger (1b) is filled. Because the 

camshaft moves the pump unit plunger up, the 

fuel can now flow back to the supply side. 

Pressure has not yet built up in the fuel injection 

pipe.

Filling

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2

c

b

b

d

e

a

a

X

Y

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System description

XF105 series

Pressure increase
When the pump unit (1) is activated, the pump 

unit valve (1a) shuts off the fuel supply. The pump 

unit plunger (1b) continues to move up, but now 

pressure builds up because the fuel can no 

longer flow back to the supply side. The fuel is 

now pumped to the injector (2). The chamber 

above the injector plunger (2d) now fills slowly via 

the throttle (2c). Because the injector valve (2a) 

has not been activated, the fuel cannot flow out to 

the return (7). The fuel pressure and the spring 

above the injector plunger (2d) ensure that the 

injector needle (2e) cannot be lifted.

Curve x indicates the fuel pressure in the injector.

Curve y indicates the movement of the injector 

needle.

The fuel pressure (x) now increases. The injector 

needle has not yet lifted (y). 

Pressure increase

1

2

c

b

b

d

e

a

X

Y

3

P

5

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4

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