With the universal utilization of the common-rail injection system in automotive diesel engines, the multistage injection strategies have become typical approaches to satisfy the increasingly stringent emission regulations, and especially the post injection has received considerable attention as an effective way for reducing the smoke emissions. Normally the post injection is applied in combination with the pilot injection to restrain the NOê missions, smoke emissions, and combustion noise simultaneously, and the pilot injection condition affects the combustion process ofthe main injection and might affect the smoke reduction effect of the post injection. Thus this study aims at obtaining the post injection strategy to reduce smoke emissions in a diesel engine, where post injection is employed in combination with pilot injection. The experiments were performed using a singlecylinder diesel engine under various conditions of pilot and post injection with a constant load at an IMEP of 1.01 MPa, flxed speed of 1500 rpm, and NO^ emissions concentration of ¡50 ± 5 ppm that was maintained by adjusting the EGR ratio. The injection pressure was set at 90 MPa at first, and then it was varied to 125 MPa to evaluate the effects of post injection on the smoke reduction in the case of higher injection pressure. The experimental results show that small post injection quantity with a short intei-val from the end of main injection causes less smoke emissions. And larger pilot injection quantity and later pilot injection timing lead to higher smoke emissions. And then, to explore and interpret the smoke emissions tendencies with vaiying pilot and post injection conditions, the experimental results of three-stage injection conditions were compared to those of two reference cases, which only included the pilot and main injection, and the interaction between main spray flames and post sprays was applied for analysis. Based on the comparative analysis, the larger smoke reduction effect of post injection was obsei-ved with the larger pilot injection quantity, while it is not greatly influenced by pilot injection timing. In addition, the smoke emissions can be reduced considerably by increasing the injection pressure, however the smoke reduction effect of post injection was attenuated. And all of these tendencies were able to be interpreted by considering the intensity variation ofthe interaction between main spray flames and post sprays.

In this study, a phenomenological combustion model, which had been developed based on a stochastic combustion model for simulating combustion with multi-stage injection in diesel engines, was improved to estimate the NOx and soot emissions with modifications in the model for the interaction of sprays from sequent injection stages. Model validation was performed by comparison with experimental data from a single-cylinder diesel engine. To understand the variation of NOx and soot production with injection conditions, analysis of combustion process was conducted based on the calculation results from different injection parameters including injection timing and quantity.

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