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. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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