Influence of the wall temperatures of the combustion chamber and intake ports on the charge temperature and knock characteristics in a spark-ignited engine

•The efficiency improvement by knock suppression with cooling was elucidated.•In-depth analyses using experiments and simulations were performed.•The critical time window to mitigate knock under cooling condition was identified.•A strategic cooling beyond intake port insulation and intensification was proposed. Reducing wall temperatures is a promising method to suppress knocking behavior in spark-ignited engine. However, this may increase undesirable heat loss which acts as countereffect, so a strategic cooling approach is required. In this study, a multidisciplinary investigation of the wall temperature effect was demonstrated using experiments and simulations. By experiments under full load and part load conditions, improvements in the indicated thermal efficiency achieved by knock-limited spark advancement were obtained, and detailed analyses were incorporated. Under cooled conditions, it was found that an improved thermal efficiency was achieved by not only the advanced combustion phasing but also the reduced compression work obtained from increased gas density, particularly under part load conditions. By categorizing and evaluating the heat transfer phases using simulations, it was found that the cooled wall temperature did not provide a significant gas temperature drop via compression and combustion processes. Unexpectedly, a notable contribution to gas heat transfer reduction arose during the early gas induction stage because of not only the extended period of heat transfer but also the large surface area and initial low temperature before compression. An enhanced cooling on cylinder head resulted in a larger effect on knock mitigation than enhanced liner cooling under normal conditions, attributed to the large heat transfer at the intake port wall. From the assessment, as the liner coolant dominated the piston surface, it was found that the contribution of the liner wall temperature to the gas temperature reduction was significantly influential, even showing a higher knock mitigation effect after intake port insulation was applied. Intensified tumble flow showed a high potential of gas temperature decrease by increasing the heat transfer from gas to wall during the compression stroke, and the effect of the enhanced cooling on the liner was more significant than that of the normal intake port due to the high velocity and turbulence of the air. The simulation results revealed that enhanced liner cooling could decrease the in-cylinder temperature by