Experimental Study on Spark Strategy Effects on Knock Mechanism and Combustion Characteristics
Engine knock is an abnormal phenomenon, which places barriers for modern Spark-Ignition (SI) engines to achieve higher thermal efficiency and better performance. In order to trigger more controllable knock events for study while keeping the knock intensity at restricted range, various spark strategies (e.g. spark timing, spark number, spark location) are applied to investigate on their influences on knock combustion characteristics and pressure oscillations. The experiment is implemented on a modified single cylinder Compression-Ignition (CI) engine operated at SI mode with port fuel injection (PFI). A specialized liner with 4 side spark plugs and 4 pressure sensors is used to generate various flame propagation processes, which leads to different auto-ignition onsets and knock development. Based on multiple channels of pressure signals, a band-pass filter is applied to obtain the pressure oscillations with respect to different spark strategies. Finally, the relationships among in-cylinder pressure, knock intensity, pressure fluctuation, heat release and measurement location, are analyzed to get better understanding on knock mechanism, influence factors and measurement methods. The main results show that: Igniting two spark plugs simultaneously brings higher knock amplitude than single spark ignition, however, adding more spark sites could effectively suppress the knock strength and rate of recurrence. A function (Y = -0.25X + 2.82) is fitted to illustrate the relations between the crank angle of 1st peak of knock oscillation and MAPO. Besides, the correlations among MAPO and other influential factors are evaluated. Moreover, the pressure sensors installed around the liner give different pressure fluctuations, which indicate the directionality of pressure wave transmission in cylinder during knock process.