Using Multiple Injection Strategies in Diesel PCCI Combustion: Potential to Extend Engine Load, Improve Trade-off of Emissions and Efficiency

The Premixed Charge Compression Ignition (PCCI) engine has the potential to reduce soot and NO_x emissions while maintaining high thermal efficiency at part load conditions. However, several technical barriers must be overcome. Notably ways must be found to control ignition timing, expand its limited operation range and limit the rate of heat release. In this paper, comparing with single fuel injection, the superiority of multiple-pulse fuel injection in extending engine load, improve emissions and thermal efficiency trade-off using high exhaust gas recirculation (EGR) and boost in diesel PCCI combustion is studied by engine experiments and simulation study.

It was found that EGR can delay the start of hot temperature reactions, reduce the reaction speed to avoid knock combustion in high load, is a very useful method to expand high load limit of PCCI. EGR can reduce the NO_x emission to a very small value in PCCI. At low engine load using high EGR, low soot, CO, UHC and NO_x emissions can be simultaneously achieved. There are no significant differences in mixture formation before ignition, emissions and thermal efficiency using single and multi-injection at low load. At low load, with the increase of EGR rate, smoke emission of PCCI doesn't increase obviously because the homogeneous mixture avoids fuel-rich. At higher engine load with high EGR, the localization of single injection in extending operation range is the high soot, CO and UHC emissions; using multi-injection mode, the CO and UHC emissions are decreased with little expense of NO_x emissions. Comparing with single injection, multi-injection is more beneficial for lean and homogenous mixture formation, especially at high load. With intake boost using multi-injection, the engine load can be further extended; despite the ignition time is advanced, the leanness and homogeneity of the mixture are improved; the indicated thermal efficiency is increased, and the CO and soot emissions are greatly decreased with not large expense of NO_x emissions. Using PCCI with high EGR and boost, the IMEP of the engine can be further extended to 1.01 MPa, while keeping near to zero emissions and high thermal efficiency up to 47%.