The improvement of DI diesel engines for passenger cars to fulfil pollutant emission limits and lower fuel consumption and noise is closely linked to continued development of the injection system. Today's injection systems demonstrate varying potential in terms of the flexibility of injection parameters for improving mixture formation and combustion.
DaimlerChrysler evaluated the potential of different injection systems, looking particularly at the distributor pump, unit injection system and Common Rail system. Based on the results of these investigations, the Common Rail system was selected. The tests presented in this paper were performed on a single-cylinder engine with Common Rail system. They focused on increased rail pressure in combination with different nozzle geometries. The results show significant benefits in NOx/smoke trade off at part load conditions with high EGR rate. The smoke intensity at constant NOx-emission decreases with high rail pressure and less hydraulic flow rate of the nozzle. At this operating point there was a potential to reduce smoke emissions by 40 - 60 %. Due to the flexibility of the Common Rail system, particularly its variable injection timing and its capability for pilot injection, noise emissions at increased rail pressure and reduced nozzle hole diameter are low.
At part load conditions without EGR the Common Rail system possesses the potential to adapt the injection parameters to the operating point in order to decrease the NOx-emission. Varying rail pressure indicates lower NOx-emission at decreased rail pressure without significant disadvantages in terms of smoke emissions. The results show progressive benefits with decreased hydraulic flow rates of the nozzle.
At full load conditions and low engine speed the mixture formation improves with increased rail pressure in combination with reduced hydraulic flow rate of the nozzle. The increased maximum rail pressure, the reduced nozzle hole diameter and the high flexibility show significant benefits in terms of smoke emission. The future power output of advanced DI diesel engines for passenger cars will also be reached.