Response of different injector typologies to dwell time variations and a hydraulic analysis of closely coupled and continuous rate shaping injection schedules

The multiple injection performance of Common Rail injectors has been analyzed at a hydraulic test rig as the dwell time was varied. The dependence of the injected volume on the dwell time has been investi- gated for direct acting piezoelectric and hydraulically-controlled (or indirect-acting) servo injectors. The injected fuel volumes in the long dwell-time range have been shown to be affected by the pressure waves that travel along the high pressure circuit for hydraulically-controlled servo injectors. On the other hand, the influence of pressure-wave-induced disturbances on multiple injection performance has been shown to be negligible for direct acting piezoelectric injectors. An analysis of closely-coupled injections has been conducted on a solenoid injector. When the dwell time is progressively reduced below a critical value, an increase in the fuel quantity that is injected in the second shot is observed. Injection fusion phenomena occur as the dwell time is diminished below a certain threshold and a maximum in the fuel volume, which is injected during the joint injections, is eventually detected for a very short electric dwell time value close to 100 l s. The cycle-to-cycle disper- sion around this dwell time value results to be reduced significantly. A previously developed 1D model of the fuel injection system has been applied to analyze the injector transients. Detailed knowledge of the injection dynamics in the short dwell time region is of fundamental importance to optimize the implementation of both discontinuous and continuous rate shaping strategies. At present, there is a great deal of interest in these advanced schedules, since they can allow a minimization of soot emissions, fuel consumption and combustion noise to be achieved in diesel engines equipped with indirect-acting injectors.