Study of the phase-varying mechanisms of ion current signals for combustion phasing in a gasoline HCCI engine

[Display omitted] •Free ion formation and recombination mechanism was investigated.•A multi-zone ion current predicting model was designed.•The predicted phase varying tendency of ion current is same with combustion phase.•The phase of ion current signals and heat release curves are not always the same.•The ion current phase will be retarded eve later when fuel concentration decreased. The phase-varying mechanism of the ion current observed in a Homogeneous Charge Compression Ignition (HCCI) engine is investigated to achieve ion current-based combustion phasing. By integrating the gasoline flame ionization mechanism with the HCCI combustion model, the mechanisms affecting the ion formation and recombination processes are analyzed, and the relationship between the phases of ion current and combustion event is studied. Modeling results indicate that the formation rate of H3O+ ions is mainly affected by the combustion boundary conditions. However, the ion recombination rate of H3O+ ions is mainly dependent on the concentration of these ions. In the presence of the above mechanisms, the phase-varying tendency of the ion current is found to be similar to the variations in the combustion phase, but the offset between these phases will vary when the combustion boundary condition changes. As the equivalence ratio becomes low, the rate of H3O+ formation is decreased and the ion recombination rate decreases even more, due to the reduced ion concentration. Therefore, the inflection point of the ion current curve, dImax, is retarded even further compared to the combustion phase CA50. In addition, a larger phase offset between dImax and CA50 is observed when the intake temperature is lower. All of the above modeling predictions agree well with the experimental results.