Fuel octane number has long been used as an indicator for anti-knock capability of gasolines. However, whether octane number could capture the real knocking combustion process in practical engines remains unknown. In this work, the knocking combustion characteristics of methanol/PRF (Primary Reference Fuels), ethanol/PRF, and iso-propanol/PRF fuel blends with either the same research octane number (RON) or the motor octane number (MON) were studied using a three-dimensional numerical model of a Cooperative Fuel Research (CFR) engine. It is observed that the three fuel blends with the same octane rating present different knocking combustion characteristics. The methanol/PRF blend shows an advanced knock onset and higher knock intensity compared to the ethanol/PRF and iso-propanol/PRF blends at the RON test condition, while the iso-propanol/PRF blend is observed to have the earliest knock onset at the MON test condition. It is discovered that even with the same octane number, deviations in the end-gas auto-ignition and flame characteristics of the three fuel blends lead to different knock onsets, which in turn influences their knock intensity.
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