Compensation Strategies for Aging Effects of Common-Rail Injector Nozzles

The thermal and emission efficiency of diesel engines depends to a large extent on the quality of fuel injection. However, over engine lifetime, injection rate and quality will change due to adverse nozzle aging effects, such as coking or cavitation. In this study, we discuss the influences of these effects on injection and heat release rate. The injection rates of previously unused nozzles and a nozzle that had been operated in a vehicle engine were compared in order to clarify the impact of aging effects. The key to the detection of alterations of injection nozzles is the identification of strongly correlating parameters. As a first step, an instrumented injector was set up to measure fuel pressure inside the feed line of the injector and the lift of the control piston. Different nozzles showed a distinguishable control piston motion depending on their different geometric specifications, which also affect the injection rates. In a second step, engine simulations were performed to investigate the impact of nozzle aging on heat release rate and engine performance of a single-cylinder engine. The predictive combustion model employed simulated injection rates to calculate the heat release rates. The geometric parameters of the different nozzles strongly influenced the respective heat release rates. Different compensating strategies for these effects were compared regarding their emission potential. Adapting the rail pressure to compensate for load losses due to high coking levels led to very high necessary rail pressures. The combined adaptation of rail pressure and energizing time to compensate for nozzle wear or erosion resulted in the best NO_x emissions. This study closes the knowledge gap between the effects of nozzle aging and the best method to compensate for them.