The TDI Diesel Engine
26
Injection TimingPiston 841103-027
Timing AdvancePiston
Base Timing
Full Advanced
Cold StartInjector N108
Cold StartInjector N108
TimingAdvancePiston
Cam Plate
Roller
Spring
RollerHousing
Vane PumpFuel Pressure
841103-029
A hydraulic piston in the injection pumpadvances injection
timing.
As engine speed increases, fuel pressurefrom the vane pump also
increases. Vanepump pressure pushes the injectionadvance piston to
the left against the springso that the roller housing turns
slightly.
Since the cam plate is turning in theopposite direction, the
ramps on the camplate engage the roller sooner whenever
theinjection advance piston moves to the left.This means that the
distributor plungerbegins injection sooner.
When the engine is cold, the DFI ECM J248will look at its cold
start map to determinethe injection timing advance. Based onengine
load, rpm, and temperature inputs,the DFI ECM J248 will actuate the
ColdStart Injector N108 to achieve the propercold-start
advance.
The Cold Start Injector N108 is an electricalsolenoid that
operates a pintle that, whenopen, allows fuel pressure to act on
oneside of the injection timing piston. Thispressure is opposed by
spring pressure. Asthe hydraulic pressure overcomes thespring
pressure, the movement rotates therollers and pump housing
clockwise amaximum of 5 degrees. This injectionadvance provides
more time for the fuel toburn, which improves cold
engineperformance and prevents smoking.
The Cold Start Injector N108 does notcontrol the complete range
of injectiontiming. Above 2200 rpm, the piston is nolonger
controlled by the DFI ECM J248.
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