Study on End Gas Auto-Ignition and Ion Current Formation Mechanism Based on One-Dimensional Flame Ionization Model

<div class="section abstract"><div class="htmlview paragraph">In future spark-ignition internal combustion engines, characterized by high compression ratios, issues such as knocking and super-knocking have increasingly emerged as major factors limiting thermal efficiency improvements. Ion current detection technology, with its advantages of not altering engine structure, low cost, and maintenance-free operation, is considered as one of the most promising methods for in-cylinder combustion detection. However, the mechanism of ion current formation under end gas auto-ignition conditions remains unclear, and the matching law between the ion current signal and the combustion state can only be obtained by experimental and statistical methods so far, posing challenges for abnormal combustion diagnostics and control based on ion current detection technology. To analyze the signal characteristics of ion current under abnormal combustion from a more intrinsic perspective, this paper develops a one-dimensional flame ionization model using MATLAB. The model focuses on electromagnetics, chemistry, and dynamics, while excluding the influence of geometric factors on pressure oscillation wave systems. Through this solver, we found that the temperature gradient within hot spots significantly affects the subsequent development of the combustion front. A -70000K∙m<sup>-1</sup> temperature gradient of hot spot will lead to a coupling of the flame front with the pressure wave to form a super knock, which is caused by the parameter coupling boundary velocity <i>u<sub>a</sub></i> = (d<i>T<sub>0</sub></i>/d<i>x</i>)<sup>-1</sup>·(d<i>τ</i>/d<i>T<sub>0</sub></i>)<sup>-1</sup> approaching the local sound speed. In addition, the reason why knocking causes violent oscillation on the ion current signal lies in the oscillation of the strong pressure wave inside the cylinder.</div></div>