Fuel octane number measurement method based on high-frequency combustion pressure and its application to fuel octane number over 100

Increasing the compression ratio in spark-ignition (SI) engines improves thermal efficiency but raises knock risk. Knock resistance is measured by Research Octane Number (RON) in Cooperative Fuel Research (CFR) engines under standard conditions, which differ from modern SI engines. The CFR engine's knock sensor, fitted with a low-pass filter, fails to accurately detect high-frequency knock oscillations, making conventional RON inadequate for assessing knock resistance in modern engines. To address this, the CFR engine was modified by replacing the knock sensor with a cylinder pressure sensor for direct combustion pressure measurement and upgrading the carburetor to electronic fuel injection. Under stoichiometric conditions, the Maximum Amplitude of Pressure Oscillation (MAPO) and Integral of Modulus of the Pressure Oscillation (IMPO) methods were applied to evaluate PRF blends with RONs of 88–100. Strong correlations were observed between the fuels RON and both IMPO and MAPO measurements, with coefficients of determination of 0.98 and 0.99, respectively. This study also analyzed error sources and quantified measurement inaccuracies in both methods, with IMPO showing lower absolute error of RON and variability. To verify the reliability of the IMPO and MAPO methods in evaluating high-octane fuels, particularly those with RON >100, these methods were applied to test TRFs and oxygenated fuel blends with RONs of 94–108. Results showed deviations from conventional measurements; however, for fuels with RON >100, both methods effectively distinguished fuel knock resistance capacity. Additionally, IMPO and MAPO captured the knock resistance improvements for the addition of ethanol and 2-MF in gasoline blends.