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 advanceNear 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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