Bridging Fundamental Combustion Analysis and Engine Knock Behavior - 2nd Report: Detailed Analyses of Knock Behavior Based on Fundamental 1D DNS and Its Relationship with Engine Knock

A collaborative study was conducted to bridge the gap between fundamental combustion research and engine-scale observations of knock in spark-ignition (SI) engines. In the first report, CFR engine tests using Primary Reference Fuels (PRFs) were conducted and knock onset conditions were identified for each fuel. In this study, one-dimensional direct numerical simulations (1D DNS) were performed for stoichiometric PRF80, 90, and 100/air mixtures to investigate fundamental characteristics of knocking with different Research Octane Number (RON) fuels. The 1D DNS reproduced knocking in the constant-volume chamber, and the knock onset conditions in terms of unburned gas pressure and temperature were derived. The 1D DNS results demonstrated that knock onset timing, pressure, and temperature conditions all increased with increasing RON. When comparing 1D DNS and CFR engine tests, differences in pressure-temperature trajectories of unburned gas were observed. However, a key similarity emerged in the relative positioning of knock onset conditions on pressure-temperature diagrams across fuels: higher RON fuels exhibit knock onset at higher pressures and tend to have higher unburned gas temperatures. Additionally, in both 1D DNS and CFR engine tests, knock onset conditions of PRF100 showed higher temperature and pressure values than those of PRF80 and PRF90. These results suggest that even a fundamental 1D DNS can represent knock phenomena in engines and allow investigation of the underlying physical mechanisms of knock onset.