Fuel injection and EGR control strategy on smooth switching of CI/HCCI mode in a diesel engine

Abstract

Homogeneous charge compression ignition(HCCI) combustion has been demonstrated abilities to lower engine emissions due to its nature of even mixture and low temperature combustion. This combustion mode is only applicable in part load and low speed area so far. This range limitation results in frequently combustion mode switching from CI mode to HCCI mode(CI–HCCI) and HCCI mode to CI mode(HCCI–CI) in real applications. Large variation of IMEP and combustion phase occurs during mode switching process. This is unfavorable to the performance of engine. In the research, diesel HCCI realized by early injection with negative valve overlap(NVO) is investigated.Fuel injection control strategy is studied to improve the smoothness of IMEP during mode switching firstly. Based on the concluded fuel injection control strategy, EGR strategy is studied to improve the smoothness of CA50 during mode switching. In the analysis, fuel compensation coefficient(η) and dimensionless cycle number(σ) are defined. Results of this study show fuel compensation of transition cycle is necessary in CI–HCCI mode switching, but direct mode switching is adequate in HCCI–CI mode switching. The optimum fuel compensation coefficient for CI–HCCI mode switching is at about 1.06, and the COVIMEP is about 5.6. Too late and early switching of EGR are both unfavorable to the smoothness of CA50 during mode switching. The optimum dimensionless cycle number of CI–HCCI and HCCI–CI mode switching are 4.8 and 0.6 respectively.