A study of the physics and chemistry of knock in modern SI engines and their relationship to the octane tests

Abstract

Avoiding knock is the major design constraint for spark ignition engines because of the unacceptable noise and engine damage associated with it. Hence, the Research and Motor Octane Number (RON and MON) tests were established in 1928 such that a fuel with a higher RON and MON is less likely to knock than a fuel with a lower value. However, engine and fuel technology has evolved since 1928, and thus the relevancy of these tests for modem engines needed to be evaluated. First, the study compared knock onset, knock metrics, reference fuels, and test conditions for the octane tests to those in modem engines. The results showed that in modem engines, for a given RON, fuels with lower MON values performed better than fuels with higher values, and this trend becomes stronger when engines are boosted and intercooled. Second, detailed chemical kinetics models were used to study fuel autoignition phenomena leading to knocking conditions. These models showed that the fuel autoignition chemistry in modem engine is different from that in the RON and MON tests. Based on these results, it was concluded that the RON and MON tests no longer represent modem engine operating conditions; therefore, modifications are suggested and evaluated to improve the tests' applicability.