Study on the in-flame electron transport behaviors towards early-stage engine knocking detection

Abstract

In future high-efficiency engines that implement high compression ratios, the occurrence of knock/super knock phenomena, primarily triggered by the end gas auto-ignition (EGAI), will pose a significant challenge. To address this issue, ion sensing technology has emerged as a highly promising approach for real-time detection of knock events. However, the reliability of ion sensing based EGAI detection is still poor due to the electron ambipolar diffusion process. When EGAI occurs at the far end of the engine combustion chamber, the electrons produced in the EGAI zone are bound to positive ions due to electrostatic force. In such case, the electrons can hardly diffuse out of the zone and be collected by the anode of spark plugs (which functioned as the ion probes in production engines), consequently leading to the fault of ion sensing method. To overcome this challenge, the behaviors of electron transport under various external electric field configurations are analyzed by simulating the end gas ignition process in a constant volume combustion chamber (CVCC). The findings demonstrate that an external electric field can surmount the electrostatic force acting on electrons and positive ions. Specifically, when the voltage applied from the DC power source in the ion sensing circuit exceeds 10 kV, numerical analysis suggests a transition from electron ambipolar diffusion to unipolar diffusion. Consequently, electrons can successfully diffuse out of the EGAI zone in the CVCC. This enables the collection of ion current signals by an ion probe positioned outside of that zone. Thus, the potential of early-stage engine knocking detection based on ion sensing is highlighted, particularly since high voltages can be configured by incorporating engine ignition modules into the ion sensing systems.