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DAF LF45, LF55 Series. Manual - part 313

200416

3-1

Description of components

CE ENGINE FUEL SYSTEM

ΛΦ45/55 series

3. DESCRIPTION OF COMPONENTS

3.1 HIGH-PRESSURE PUMP

The high-pressure pump is driven by the

crankshaft by means of the camshaft gear. The

pump shaft rotates at a speed 1.33 times the

speed of the crankshaft. The pump shaft drives

the fuel lift pump. The fuel drawn from the suction

side by the gears is discharged through the exit

by the gears due to the reduction in volume. The

fuel lift pump has two added valves. The first

valve (1) limits the pump pressure, for example if

the fine filter is blocked. The second valve (2)

opens the by-pass over the fuel lift pump when

the primer pump is being used.

The high-pressure pump shaft (1) has an

eccentric. The eccentric drives the eccentric ring

(4). The pump plungers (3) are driven by the

eccentric ring and pushed back by the spring (2).

The fuel is supplied via the connection (5) and

then internally distributed among the three pump

elements. Before it is distributed among the pump

elements, the supplied fuel is dosed by means of

the fuel pump control solenoid valve (6).

The fuel pump control solenoid valve is controlled

by the electronic unit on the basis of the rail

pressure sensor signal. This creates a closed

control circuit. The fuel pump control solenoid

valve is "normally open" and is activated via duty

cycle control. The value when the engine is

started is fixed. When the measured pressure

deviates from the desired (programmed)

pressure, the duty cycle value will be adapted

until the measured and the desired values

correspond. If this is impossible, the electronic

unit will generate a warning and record a fault

code.

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CE ENGINE FUEL SYSTEM

3-2

200416

Description of components

ΛΦ45/55 series

During the downward stroke, the fuel is supplied

by the fuel lift pump via the suction valve (9) (see

illustration on left). During the delivery stroke

(illustration on right), the suction valve closes

under the load of the spring (8) and the fuel

pressure that has built up. The fuel leaves the

pump element via the delivery valve (10). The

compressed fuel from the three pump elements is

collected in the pump housing and leaves the

pump via a joint high-pressure connection.

Leaking fuel from the pump elements is

discharged via the return connection.

The fuel lift pump has over-capacity in relation to

the high-pressure pump. If there is only limited

fuel off-take on the rail, the fuel pump control

solenoid valve will be virtually closed. The over-

supplied fuel will cause an increase in pressure at

the inlet of the fuel pump control solenoid valve.

The pressure is limited by the circulation valve

(7). If the valve is open, the excess supplied fuel

will be returned to the suction side of the fuel lift

pump. A small amount of fuel is diverted via a

restriction to the shaft and the eccentric of the

high-pressure pump for lubrication.

When bleeding with the primer pump, any air

bubbles will be carried along by the fuel through

the low-pressure circuit. Fuel is forced inwards at

the connection (5). The fuel pump control

solenoid valve is opened fully without being

energised, so that the fuel can pass. The pump

elements create a high resistance to the fuel, so

that the fuel will escape via the lubricating

restriction in the pump. In this way the fuel, along

with any air bubbles, is discharged to the return

pipe.

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200416

3-3

Description of components

CE ENGINE FUEL SYSTEM

ΛΦ45/55 series

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.

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CE ENGINE FUEL SYSTEM

3-4

200416

Description of components

ΛΦ45/55 series

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.

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