Particulate matter emissions have become a concern for the development of DISI engines. EGR has been extensively demonstrated as a beneficial technology to migrate knock performance, improve fuel economy and reduce NO_X emissions. Recently, the effect of EGR on particulate matter emissions is attracting increased attention. This work investigates the effects of EGR on PN emissions with the variations of engine operating parameters and aims to understand the role of EGR in PN emissions for DISI engines. A 1.8liter turbocharged engine with cooled EGR is used for this study. The engine is operated at steady-state conditions with EGR under various operating parameters including injection timing, excess air ratio, and spark timing to characterize the particle number emissions. The results indicates that there is a high sensitivity of PN emissions to EGR with the variations of those parameters. With middle and late injections during the intake stroke, EGR is beneficial to improve PN emissions. Conversely, EGR has an adverse effect for early injection. Also, with different air excess ratios, the influence is different. When air excess ratio is greater than 1, EGR can efficiently reduce PN emissions. However, for rich mixture, EGR results in a significant deterioration in PN emissions. Besides, as EGR rate is increased, the sensitivity of PN emissions to spark timing is weakened. In general, this work reveals the sensitivity of PN emissions to EGR with a range of operating parameters and indicates that the effects of EGR on PN emissions has a strong dependence on engine operating conditions.
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