EGR effect on ion signal in HCCI engines

Homogeneous Charge Compression Ignition (HCCI) engines offer promise due to low emissions and high efficiency. The control of start of combustion in a HCCI engine is a continuous challenge. Expensive pressure transducers determine the start of combustion in research engines, but these devices are likely impractical for most commercial applications. Recent publications by the authors as well as other investigators show the potential of utilization of inexpensive ion sensors in HCCI engines. For operation without EGR, the ion signal correlates strongly with peak cylinder temperature, and typical HCCI operation with low peak cylinder temperatures can result in weak ion signals. Exhaust Gas Recirculation (EGR) significantly enhances the ion signal in HCCI despite low temperatures, especially at high EGR levels with little excess oxygen available to the combustion process. In this paper, analysis of the EGR effect on ion signal is done with chemical kinetic models that include ion chemistry along with autoignition chemistry. Experimental data and numerical simulation results that look into detailed chemical kinetic origins of ions are presented. These results provide an understanding of how EGR leads to greater production of ions. The results show that, while the ion reactions rate is largely independent of temperature, the concentrations of the reactants that promote ions depend strongly on temperature. The EGR effect can be largely attributed to a substantial increase in concentration of available CH molecules when excess air is replaced by EGR in HCCI engines. Greater concentrations of CH promote the ion production reaction. Interestingly, there is a slight decrease in O-atom concentration as air is replaced by EGR. Understanding these EGR effects on the ion concentration helps guide the design of practical ion sensing devices for measurement of combustion timing in HCCI engines.