Applicability of ionization current sensing technique with plasma jet ignition using pre-chamber spark plug in a heavy duty natural gas engine

This article deals with study of ionization current sensing technique's signal characteristics while operating with pre-chamber spark plug to achieve plasma jet ignition in a 6 cylinder 9 liter turbo-charged natural gas engine under EGR and excess air dilution. Unlike the signal with conventional spark plug which can be divided into distinct chemical and thermal ionization peaks, the signal with pre-chamber spark plug shows a much larger first peak and a negligible second peak thereafter. Many studies in past have found the time of second peak coinciding with the time of maximum cylinder pressure and this correlation has been used as an input to combustion control systems but the absence of second peak makes application of this concept difficult with pre-chamber spark plug. However, it has been observed that the first peak is very strong and does not deteriorate much even under lean operation and hence ion current signal integral can be used to calculate real time combustion stability parameters for combustion control systems. The correlation between the coefficient of variation (COV) of indicated mean effective pressure (IMEP) and ion current integral for operation at 5, 12 and 18 bar IMEP load under stoichiometric and diluted operation with EGR and excess air is presented in this article.