Direct Versus Indirect Acting Piezoelectric CR Injectors: Comparison of Hydraulic Performance, Pollutant Emissions, Combustion Noise, and Fuel Consumption

A comprehensive comparison between a direct acting and an indirect acting piezoelectric injector has been carried out both at the hydraulic rig and at the dynamometer cell. The working principle of these injector typologies is illustrated, and their hydraulic performance has been analyzed and discussed on the basis of experimental data collected at a hydraulic test rig. The injector characteristics, nozzle opening and closure delays, injector leakages, injected flow-rate profiles, injector-to-injector variability in the injected mass, injected volume fluctuations with the dwell time (DT), and minimum DT for fusion-free multiple injections have been compared in order to evaluate the impact of the injector driving system on the injection apparatus performance. The direct acting and indirect acting piezoelectric injectors have been installed on a Euro 5 diesel engine, which has been tested at a dynamometer cell. Optimized double and triple injection strategies have been considered at some representative key points of the New European Driving Cycle (NEDC). Experimental data on engine-out emissions, brake-specific fuel consumption (bsfc), and combustion noise (CN) are presented and discussed with the support of a three-zone diesel combustion diagnostic model. The research has focused on the cause-and-effect relationships between hydraulic performance of the injectors and results of the engine tests. The main objective is to assess the possible differences in engine performance between the direct acting and indirect acting piezoelectric injector setups and to provide a clear evaluation of the benefits of the direct acting technology. In particular, the potential of the boot injection in direct acting injectors is assessed by comparison with engine results, which refer to efficient multiple injection schedules implemented on indirect acting piezoelectric injectors.