Investigations into the Effects of Spark Plug Location on Knock Initiation by using Multiple Pressure Transducers

Despite a long history of development, modern spark-ignition (SI) engines are still restricted in obtaining higher thermal efficiency and better performance by knock. Knocking combustion is an abnormal combustion phenomenon caused by the autoignition of unburned air-fuel mixture ahead of the propagating flame front. This work describes investigations into the significance of spark plug location (with respect to inlet and exhaust valve position) on the knock formation mechanism. To facilitate the investigation, four spark plugs were installed in a specialized liner at four equispaced distinct locations to propagate flames from those locations, which provoked a distinct flame propagation from each and thus individual autoignition profiles. Six pressure transducers were arranged to precisely record the pressure oscillations, knock intensities, and combustion characteristics. Four of the six transducers were mounted on the circumference of the liner (each next to one of the spark plugs), one was placed at the center of the cylinder head, and one at a slight offset from the center of cylinder head. The results showed that the spark plug which was close to the exhaust valves triggered higher knock intensity along with earlier CA50, but the spark plug near the inlet valves caused weaker knock intensities for the same operating conditions. In addition, the study also covered the effect of swirl direction to suppress knock. A band pass filtering analysis was applied to estimate the pressure oscillations with respect to the spark plug locations, using data from the multiple pressure sensors. Furthermore, Fast Fourier Transform (FFT) analyses were implemented to estimate the frequency of the pressure oscillation resulting from knock. It was found that firing the spark plugs, near the inlet and between the inlet and exhaust valves promoted the (1, 0) acoustic mode effectively, while the spark plug near the exhaust valves caused the (1, 0) mode along with the (2, 0) acoustic mode for the same operating conditions, indicating that the autoignition was initiated near the cylinder walls.