The effect of closely coupled pilot injections on diesel engine emissions

Abstract

Modern common rail fuel injection systems allow for very sophisticated injection strategies to facilitate the compromise between low emissions, fuel consumption and combustion noise. One such injection strategy is the reduction of the dwell time between injections to extremely low values. In general the Diesel mixture formation is governed by the geometry of injection nozzle and combustion bowl. Common rail injection systems additionally offer the possibility to inject the fuel in a train of discrete injection pulses (multiple injection). The spacing between the pulses and the individual fuel mass per pulse allow for a “radial stratification” of the injected fuel, and this can be used to favourably control engine emissions and combustion noise. The current work focuses on optimum injection strategies as a function of engine speed and load within the NEDC. For this, two operating points were assessed, one representing the city driving and the other one standing for the acceleration to 120 kph motorway speed. For these points especially the “minimum dwell”, MD, injection strategies were investigated for a number of operating conditions. The low engine load and speed regime was explored at fully warmed up engine as well as for cold start and warm-up, in particular with respect to the reduction of unburnt hydro carbons, HC, and carbon monoxyde, CO. The high engine load and speed “acceleration regime” to 120 kph was studies for two operating provisos. One was that of purely engine internal reduction of nitrous oxides, NOx, via high EGR rates and retarded centre of combustion, HR50, the other one that of NOx reduction mainly from exhaust gas aftertreatment (DeNOx), which allows for an advance of the centre of combustion. This improves the thermodynamic efficiency to its best, but poses the challenge of high combustion noise, CN. The reduction of combustion noise can be achieved by suitable injection strategies and good results were found with MD. The MD injection strategy yielded good results throughout the engine map, compared to conventional dwell injection patterns, and it was attempted to elucidate the mechanisms causal for this advantage.