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In the last decades, ion current sensing has been widely recognized as a promising approach to the engine knock/super knock detection. However, ion current formating mechanism under engine knocking conditions is yet to be revealed, which substantially hinder the further development of ion current based knock detecting technology. In this paper, the ion current signals detected under engine knocking conditions are investigated both experimentally and numerically. In a spark ignition engine, the ion current signal and combustion pressure are analyzed in both time and frequency domain. Besides, a multi-physics CFD model, which incorporates compressible fluid dynamics, neutral/charged species reaction kinetics and flame plasma hydrodynamics, is constructed in this work. Then the combustion process, the flame - pressure wave interaction and the motion of the charged species are analyzed, and the key features of the ion current signal under knocking conditions are studied. The multi-physics model indicates that the in-cylinder distribution of positive and negative charged species is almost the same due to the ambipolar diffusion of the electrons, and then the ion current can be analyzed via observing the positive ion flux solely. By combining the experimental and numerical results, it is revealed that the ion concentration is correlative to the temperature in a non-linear way, and the ion current mainly responds to the low frequency pressure oscillation mode. Correspondingly, the relation between the ion current oscillating amplitude and the knock intensity is non-linear. For knocking at high load conditions, both the amplitude of ion current signal and the ion concentration can respond to the oscillation of local pressure and knocking intensity. In particular, the signal oscillation becomes insignificant when knocking intensity KI < 0.85. [ABSTRACT FROM AUTHOR]

• A multi-physics model for flames plasmas coupling electric field is constructed. • A statistical physics model for describing electron transport behaviors is applied. • The effect of gas composition on the in-flame electron–ion diffusion is studied. • The mechanism how the gas composition affects ion current signal is investigated. • Ion current features in the excess air/ EGR diluted methane flames are analyzed. The real-time detection of the key combustion status such as combustion timing, misfire and knocking will be crucial for the future internal combustion engines. Ion sensing technology can be potentially applied to develop the ion current based combustion detecting sensor on mass-produced engines. However, the low signal to noise ratio (SNR) of the ion current signal still be a major obstacle for the large-scale application of this technology, especially when the lean mixtures or exhaust gas recirculation (EGR) is applied on the engines. In order to improve the SNR, a fundamental study on the ion current formatting process in air/EGR diluted methane flames in studied in this paper. A kinetics analysis of the electron transport behaviors in the electric field is conducted. By solving the Boltzmann's equation, the electron transporting parameters can be predicted. The in-flame electron motion predicted through this method is more accurate than through the model based on thermal equilibrium assumption, especially when an external electric field is applied. Besides, a multi-physics CFD model of the methane flame coupling electrohydrodynamic of the flame plasma is established. Combined the numerical results and the experimental results achieved in a constant volume combustion chamber, the electron–ion diffusion process and their distributions with different diluting conditions is investigated. Consequently, the characteristics of the ion current signals in the excess air/ EGR diluted methane flames can be well predicted. A further improvement of the ion sensing technology could be achieved based on the above analyzing results. [ABSTRACT FROM AUTHOR]

Ion sensing technology has the potential to be an in-cylinder combustion diagnostic solution. However, the large-scale application of this technology still be difficult in the last decades due to the absence of fundamental level research on the ion current formatting mechanism. In this study, a multi-physics analysis of the ion formatting process in a premixed methane flame is conducted, and the effect of electron diffusion process on the ion current signal is explicitly studied. Through the incorporation of the flame plasma physics, ionization reaction mechanisms and ion-electric field interactions, a numerical flame ionization model is constructed. By comparing the flame schlieren imaging data and the modelling results, the ion current signal formatting mechanism is elaborated, and the effect of electron/ion motions is studied. The results show that the ion current signal amplitude is in good agreement with the flux of the charged species. More importantly, the model reveals that the ambipolar diffusion of the electrons play a dominate role in the ion current formatting process, and it will significantly affect the ion current waveform. Due to this diffusion process, electrons cannot transport outside of the flame front zone, and its distribution will be decided by the heavy ions in the flame. Consequently, a serial of experimental results about the ion current signal can be analyzed based on the constructed multi-physics model, and the findings in this work could be crucial for the development of the future ion sensing system. [ABSTRACT FROM AUTHOR]