1.9-ltr. TDI Engine with Pump Injection System - Volkswagen ...

The pre-injection cycle commencesInjector needle dampingDuring the pre-injection cycle, the stroke of the injectorneedle is dampened by a hydraulic 'cushion'. As a result, it ispossible to meter the injection quantity exactly.This is how it works:In the first third of the total stroke, the injector needle isopened undamped. The pre-injection quantity is injected intothe combustion chamber.UndampedstrokeAs soon as the damping piston plunges into the bore in theinjector housing, the fuel above the injector needle can onlybe displaced into the injector spring chamber through aleakage gap. This creates a hydraulic 'cushion' which limitsthe injector needle stroke during the pre-injection cycle.209_35Injector springchamberInjector housingLeakage gapHydrauliccushionDamping piston209_3613