The TDI Diesel Engine 25 DFI ECM J248 Cam Pressure Roller Cold Start Injector N108 Injection Timing Piston Cold Start Injector N108 Injection Timing Piston Spring Pressure Roller Pin Fuel Pressure in Interior of Pump Retarded Advanced 841103-026 Injection timing advance Near the end of each compression stroke, diesel fuel is injected directly into the combustion chamber. Injection must continue well past piston TDC to burn the necessary amount of fuel to provide engine power. As engine speed increases, stroke time becomes shorter and injection time becomes longer. Therefore, combustion must begin sooner to ensure that peak pressure still occurs at the most efficient point after TDC.
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The TDI Diesel Engine
25
DFI ECM J248
Cam
PressureRoller
Cold StartInjector N108
Injection TimingPiston
Cold StartInjector N108
Injection TimingPistonSpring
PressureRoller
PinFuel Pressurein Interior ofPump
Retarded
Advanced
841103-026
Injection timing advance
Near the end of each compression stroke,diesel fuel is injected directly into thecombustion chamber. Injection mustcontinue well past piston TDC to burn thenecessary amount of fuel to provide enginepower. As engine speed increases, stroketime becomes shorter and injection timebecomes longer. Therefore, combustionmust begin sooner to ensure that peakpressure still occurs at the most efficientpoint after TDC.
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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