Exploitation of injection fusion strategies in diesel engines equipped with solenoid injectors

Pilot-main injection strategies are investigated in the short dwell-time range at medium load and speed conditions for a low-temperature combustion diesel engine endowed with indirect-acting solenoid injectors. Dwell-time sweeps have been carried out for pilot injections with different energizing times under steady-state working conditions and constant combustion phasing. The experimental results on the engine are combined with in-cylinder analyses of pressure, heat release rate, temperature and emission time histories. As the electric dwell time is reduced toward zero, combustion noise reaches a minimum at a dwell time of 120-140 mu s, and a reduction of 1-4.5dB is possible compared to levels at dwell time=500 mu s. This decrease in combustion noise is obtained in conjunction with a local reduction in engine-out nitrogen oxide emissions, but penalties occur for engine-out soot emissions and brake-specific fuel consumption. The pilot and main injection shots feature fusion phenomena at the shortest dwell-time values, thus giving rise to rate-shaped single injections, which resemble the boot-shaped profiles in direct-acting piezoelectric injectors. The increase in the velocity of the needle, which occurs during the nozzle opening phase pertaining to the main injection for closely coupled injections as dwell time is reduced, is shown to be not the primary cause responsible for the observed trends in combustion noise and soot emissions as dwell-time changes. The combination of simulation and experimental data supports the theory that the main injection interacts with the pilot mixture field: the relative phasing between pilot and main combustion and the in-cylinder equivalent ratio distribution are the dominant aspects in the definition of the engine performance in the short dwell-time range.