Characterization of Knock and Its Effect on Surface Temperatures

Knock in a spark-ignition engine was characterized in terms of its occurrence and magnitude or intensity. Cylinder pressure data from 90 consecutive individual cycles were generated from a single-cylinder engine of disc chamber design at about 72kHz sampling rate over a range of operating conditions between no knock and 100% of the cycles knocking. Mean values and distribution of following parameters were analysed: knock occurrence crank angle, knock intensity, combustion rate and the end-gas thermodynamic state. The effects of fuel octane number and inlet air temperature on these parameters were studied. The thermal imaging technique has been applied to record two-dimensional surface temperatures of cylinder head and piston simultaneously. The change in surface temperatures during knocking and non-knocking cycles was thus studied.

As expected, increase in the inlet air temperature or decrease in the fuel octane number caused the knock onset to occur at less advanced spark timing. For a given octane number fuel and inlet air temperature, the difference in spark timing between no knock and 100% of the cycles knocking was about 10 degree crank angle. On the individual cycle basis, the non-knocking cycles were among the slower burning cycles and knock occurrence crank angle correlated well with the combustion rate, especially the pressure at individual 50% burn crank angle. However they did not correlate well with end gas temperature in present experiments. The individual cycle knock intensity did not correlate well with any parameters used though it showed a trend of increase as the combustion rate and pressure at 50% burn crank angle increased and knok occurred earlier. The thermal images recorded showed the increased surface temperatures during knocking operations.