Effects of inversed-delta injection rate on late combustion and soot emissions from diesel combustion in a constant-volume chamber

The aim of this study is to clarify the effect of an inversed-delta injection rate, which is a ramp-down injection rate achieved with a tandem injector, on the late combustion period, which affects thermal efficiency and soot emissions. Diesel combustion was simulated in a constant-volume chamber combined with a rapid compression machine. Two types of injection rate profile, namely rectangular and ramp-down, were tested in our experimental trials. To eliminate the significant effect of the maximum injection rate on mixture formation, we compared the results for the two types of injection rate profile under a given rail pressure. The time histories of the heat release rate demonstrated that the ramp-down fuel injection profile shortened the late combustion period by 0.2 ms. As a result, combustion was maintained despite the longer injection duration. The non-dimensional fuel consumption rate, defined in terms of the heat release rate and the fuel injection rate, revealed that high fuel consumption in the injection period reduced the quantity of fuel carried over to the late combustion period, resulting in a short duration of late combustion. When the ramp-down injection profile was employed, the temporal variation of total soot (spatially integrated KL factor) exhibited a rapid decrease after the end of injection. This is due to the abundant oxygen in soot regions that originated from the significant quantity of ambient air entrained into the spray during the injection period. This result indicates that ramp-down injection may reduce engine-out soot emissions.