DV11
Operation and Maintenance
126
Common Rail Fuel-injection System
Printed in Mar. 2005 PS-MMA0608-E1A
2) Injector closed (at rest status)
In the at rest state, the solenoid valve is not energized and is therefore closed.
With the bleed orifice closed, the valves spring forces the armature's ball onto the
bleed-orifice seat. The rail's high pressure builds up in the control valve, and the
same pressure is also present in the nozzle's chamber volume. The rail pressure
applied at the control plunger's end face, together with the force of the nozzle
spring, maintain the nozzle in the closed position against the opening forces
applied to its pressure stage.
3) Injector opens (start of injection)
The injector is in its at-rest position. The solenoid valve is energized with the pick-
up current which serves to ensure that it opens quickly. The force exerted by the
triggered solenoid now exceeds that of the valve spring and the armature opens
the bleed orifice. Almost immediately, the high-level pick-up current is reduced to
the lower holding current required for the electromagnet. This is possible due to
the magnetic circuit's air gap now being smaller. When the bleed orifice opens,
fuel can flow from the valve-control chamber into the cavity situated above it, and
from there via the fuel return to the fuel tank. The bleed orifice prevents complete
pressure balance, and the pressure in the valve control chamber sinks as a result.
This leads to the pressure in the valve-control chamber being lower than that in
the nozzle's chamber volume which is still at the same pressure level as the rail.
The reduced pressure in the valve-control chamber causes a reduction in the
force exerted on the control plunger, the nozzle needle opens as a result, and
injection starts.
The nozzle needle's opening speed is determined by the difference in the flow
rate through the bleed and feed orifices. The control plunger reaches its upper
stop where it remains supported by a cushion of fuel which is generated by the
flow of fuel between the bleed and feed orifices. The injector nozzle has now
opened fully, and fuel is injected into the combustion chamber at a pressure
almost equal to that in the fuel rail. Force distribution in the injector is similar to
that during the opening phase.
4) Injector closes (end of injection)
As soon as the solenoid valve is no longer triggered, the valve spring forces the
armature downwards and the ball closes the bleed orifice. The armature is a 2-
piece design. Here, although the armature plate is guided by a driver shoulder in
its downward movement, it can "over spring" with the return spring so that it
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