position against the opening forces applied to its
pressure stage (Fig. 11).
Injector opens (start of injection)
The solenoid valve is energized with the pickup
current which serves to ensure that it open 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 possi-
ble 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 situ-
ated above it, and from there via the fuel return to
the tank. The bleed orifice prevents complete pres-
sure 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 open as a result, and injection starts (Fig. 11).
Injector opens fully
The control plunger reaches its upper stop where it
remains supported by a cushion of fuel which is gen-
erated by the flow of fuel between the bleed and feed
orifices. The injector nozzle has now opened fully,
and the fuel is injected into the combustion chamber
at a pressure almost equal to that in the fuel rail
(Fig. 11).
Injector closes (end of injection)
As soon as the solenoid valve is no longer trig-
gered, the valve spring forces the armature down-
wards 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 “overspring” with the
return spring so that it exerts no downwards-acting
forces on the armature and the ball. The closing of
the bleed orifice lead to pressure build up in the con-
trol chamber via the input from the feed orifice. This
pressure is the same as that in the rail and exerts an
increased force on the control plunger through its
end face. This force, together with that of the spring,
now exceeds the force exerted by the chamber volume
and the nozzle needle closes. Injection ceases as soon
as the nozzle needle comes up against its bottom stop
again (Fig. 11).
STANDARD PROCEDURE
STANDARD PROCEDURE - INJECTOR CLASSI-
FICATION
NOTE: Fuel Injectors have different flow rates.
When ALL injectors are removed, re-enter all injec-
tor six digit codes.
The
classification
of
injectors
into
3
classes
describes the quantity characteristic of the injector.
This will make it possible in the future to match the
engine software to the tolerances of the injector
within a more narrowly graduated range. Classifica-
tion can be clearly recognized, and assigned only by
means of a DRBIII
t.
Classified injectors can be recognized by the six-
digit alphanumeric code or part number and identifi-
cation on the magnetic head (circle with a number
between 1 and 3 inside) (Fig. 12). The number corre-
sponds to the classification stage.
These general conditions equally apply if, as a
result of replacing an engine, carrying out repairs to
the cylinder head etc., the cylinder selective assign-
ment of the injectors or the engine control module
assignment may have changed. If proper attention is
not paid to the classification on these vehicles drive-
ability and smoking concerns could result.
If an injector is replaced, it is then necessary to
assign the classification number to the corresponding
cylinder with the DRBIII
t in the control module.
Fig. 12 INJECTOR CLASSIFICATION MARKINGS
1 - ELECTRICAL CONNECTOR
2 - SIX-DIGIT ALPHANUMERIC CODE
VA
FUEL INJECTION
14 - 37