Effect of in-cylinder heat transfer and surface temperatures on knock in an SI engine

A single-cylinder, four-stroke, spark-ignition research engine instrumented with heat flux probes in the cylinder head, cylinder liner and piston was tested at two engine speeds and two loads to analyze the effect of in-cylinder heat transfer and surface temperatures on engine knock. During the intake stroke and part of the compression stroke the metal temperature is higher than the charge temperature. Heat transfer from the metal to the air-fuel mixture increases the temperature of the charge resulting in higher end gas temperatures, which can exacerbate knock. In the initial phase of testing, the heat transfer and surface temperatures were varied by changing oil temperature, coolant temperature, and coolant flow rate. In the second phase of testing, individual cycles from the same operating condition were binned according to their knock levels and correlations with the heat transfer before the occurrence of knock were analyzed. The results showed that in-cylinder heat transfer is not a dominant factor impacting engine knock, but an increase in surface temperatures does increase knock intensity. The results also confirmed that knock has a significant impact on the heat transfer that occurs from the gas to the metal after knock onset.