Ford Transit 2002 Owner's Manual page 851

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After a time determined by the PCM the current supply to the solenoid valve is interrupted,
and the needle valve closes the fuel return as a result of the solenoid valve spring force .
The pressure in the control chamber is built up again through the control chamber upper
and lower inlet chokes.
At the same time the inlet chokes of the nozzle chamber prevent a sudden back pressure of
fuel in the nozzle chamber. This causes a high pressure level in the control chamber and the
nozzle needle closes the fuel injector nozzles.
Control of system pressure
The engine management system is able to make the optimum injection pressure available
for every operating state.
Maximum fuel pressure is controlled so that it is only available above a certain engine speed
(about 3000 rev/min). Below this engine speed the fuel pressure must be reduced.
In this way the noise emissions during fuel combustion are lowered and the engine
management system can perform fuel metering more easily and more precisely. This has a
positive effect on the exhaust emissions and on fuel consumption.
For this reason, controlled pressure reduction in the common rail is necessary.
The pressure drop occurs when the solenoid valve applies current to the injector valves at
short intervals.
Each time the pull-in current is enough to open the control valve, but remains small enough
so that the needle valve does not lift and therefore does not permit an undesired fuel
injection.
Pressure drop when the engine is switched off
Pressure drop also occurs after the engine is switched off, in the same way as when the
system pressure is controlled.
As soon as the solenoid valves are actuated the pressure is fully depleted within the space
of a few seconds.
Afterwards current is no longer applied to the solenoid valves of the injectors. This results in
zero pressure difference between the control chamber and the nozzle chamber. No more
fuel injection can occur.
Before opening the high-pressure system, use WDS to make sure that the fuel
pressure has been completely depleted and that the fuel temperature is below 30
°C.
Fuel return
Needle valve
Control chamber
Nozzle needle
Nozzle chamber
Nozzle chamber inlet choke
Control chamber lower inlet choke
Control chamber upper inlet choke
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