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.

A series of experiments using a single-cylinder direct injection diesel engine was conducted to investigate the smoke reduction effect of post injection while varying numerous parameters: the post-injection quantity, post-injection timing, injection pressure, main-injection timing, intake pressure, number of injection nozzle orifices, and combustion chamber shape. The experiments were performed under a fixed NOx emission condition by selecting the total injection quantities needed to obtain the predetermined smoke emission levels without post injection. The smoke reduction effects were compared when changing the post injection timing for different settings of the above parameters, and explanations were found for the measured smoke emission trends. The results indicate that close post injection provides lower smoke emission for a combination of a reentrant combustion chamber and sevenhole nozzle. The same trend was also found in the tests that varied the injection pressure, main-injection timing, and intake pressure. However, a lower sensitivity of the smoke emission to the post injection timing was observed when using an injection nozzle with a larger number of orifices and a toroidal combustion chamber. The smoke reduction rate at the best post injection timing was higher for a lower injection pressure, larger number of nozzle orifices, and toroidal combustion chamber. The reasons for these trends were investigated, giving attention to the relation between the main spray flames and post sprays.

A series of experiments was conducted using a singlecylinder small-bore (85 mm) diesel engine to investigate the smoke-reduction effect of post injection by varying the number of injection nozzle orifices and the injection pressure. The experiments were performed under a constant injection quantity condition and under a fixed NOx emission condition. The results indicated that the smoke emission of six-hole, sevenhole, and eight-hole nozzles decreased for advanced post injection, except that the smoke emission of the 10-hole nozzle increased as the post injection was advanced from a moderately late timing around 17 ATDC. However, the smoke emission of the 10hole nozzle with a higher injection pressure decreased for advanced post injection. These trends were explained considering the influence of the main-spray flames on post sprays based on CFD simulation results.

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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