DAF LF45, LF55 Series. Manual — part 290

©

200416

3-3

Description of components

BE ENGINE FUEL SYSTEM

ΛΦ45/55 series

4

2

3.2 FUEL PUMP CONTROL SOLENOID VALVE

The high-pressure pump has an over-capacity for

normal operating conditions. This could lead to

large quantities of fuel being forced at high

pressure to the fuel rail and then directly being

drained out to the return pipe via the pressure-

limiting valve on the fuel rail. This produces too

much unnecessary heat and loss of capacity

because large amounts of fuel are flowing at high

pressure.

The system has been designed such that only

fuel that will be used will be forced under high

pressure to the rail. For this reason, a fuel pump

control solenoid valve has been fitted at the

suction side of the high-pressure pump. This

solenoid valve is opened without being

energised, so that the high-pressure pump

elements can be filled in the normal manner.

If the fuel rail pressure becomes too high, for

example because of lower fuel off-take on the rail,

the solenoid valve will be energised by the

electronic unit with a higher duty cycle, so that the

plunger is pressed with a greater force against

the spring pressure and the valve reveals a more

constricted opening. This reduces the fuel supply

to the pump elements and the pump output will

thus fall. The fuel rail pressure will also fall as a

consequence.

If the rail pressure is too low, the reverse is true.

The current is supplied to the coil (3) via the

connector (1). The current pushes the core (2)

with the plunger (4) against the pressure of the

spring (5). This controls the fuel current from

input A to output B.

Due to this valve, under normal circumstances

very little fuel flows back from the rail. This

improves performance and reduces the

generation of heat.

B

A

B

i400591

1

2

3

5

4

BE ENGINE FUEL SYSTEM

3-4

©

200416

Description of components

2

ΛΦ45/55 series

4

3.3 INJECTOR

The injectors can be opened and closed

electrically. The injector is normally closed. The

load of the spring (2) and the fuel rail pressure

that controls chamber C ensure this.

The fuel is supplied from the fuel rail via A. The

return fuel can flow back to the fuel tank via B.

When the coil (7) is energised, the armature

housing (5) moves up. As a result the pressure in

chamber C drops slightly and the injector needle

(1) is forced up to chamber D by the rail pressure.

The fuel can then be injected.

7

C

E

A

B

F

D

6

5

4

3

2

1

I400548

©

200416

3-5

Description of components

BE ENGINE FUEL SYSTEM

ΛΦ45/55 series

4

2

3.4 FUEL RAIL PRESSURE-LIMITING VALVE

The fuel rail has a pressure-limiting valve so that

the rail pressure is maintained at a safe value in

emergency situations.

The fuel rail pressure control circuit is normally

active. This consists of the fuel lift pump, fuel

pump control solenoid valve, high-pressure

pump, fuel rail, rail pressure sensor and

electronic unit. If a fault occurs, the rail pressure

can no longer be controlled. This can rise to the

actuating pressure of the fuel rail pressure-

limiting valve. This pressure is approx. 1650 bar.

In the open position all surplus fuel flows without

pressure to the fuel tank return connection.

The valve includes a sealing cone (1), a valve

body (2), a spring (3) and a return connection with

quick-release coupling (4).

i400592

1

2

3

4

BE ENGINE FUEL SYSTEM

3-6

©

200416

Description of components

2

ΛΦ45/55 series

4

3.5 FUEL RETURN OVERFLOW VALVE

The injectors are mounted in the cylinder head.

The injectors do not have a separate return

connection. In place of this there is a longitudinal

bore in the cylinder head that meets the injector

bores of all cylinders coinciding with the height of

the return ports of the injectors. A pressure relief

valve is mounted at the end of the bore to prevent

vapour bubbles forming in it and to prevent it

emptying after the engine is turned off. This valve

maintains a residual pressure in relation to the

return pressure of 1.2 to 2.0 bar.

The pressure relief valve consists of a spring-

loaded plunger (1) and a housing (2) with quick-

release inlet and outlet connections (A and B

respectively).

A

B

1

2

i400861