An Investigation into the Onset of Knock in a CFR Engine

Internal combustion engine knock has limited compression ratios of spark ignition engines for most of the history of gasoline engines. This limitation continues to exist today. While knock is generally a low engine speed, high load phenomenon, this operating condition is infrequently used by many vehicle operators, and if the engine is brought to this operating condition generally little time is spent in this knock prone condition. This study seeks to investigate the transition into knock due to throttle changes from part to full load. The experimental results using a CFR engine operating on iso-octane fuel show that knock is delayed by at least one high load engine cycle after the throttle is opened. Optimization of spark timing to account for this effect provides for the best increase of engine load without audible knock occurring. Detailed analysis of the data with companion modeling strongly suggests that lower peak cylinder pressures from the cycle before the first Wide Open Throttle (WOT) cycle contribute to a cooler first WOT cycle charge temperature that hinders knock occurrence.