Effects of Lambda on Knocking Characteristics and RON Rating

The knock resistance of fuels has been historically measured using the ASTM RON and MON methods. However, significant discrepancies between the fuel octane number and knock-limited performance in modern spark-ignited (SI) engines have been well-documented. Differences between the operating conditions of the Cooperative Fuels Research (CFR) engine during RON rating and those attained in modern SI engines have been highlighted in the literature. While octane ratings are performed for each fuel on the CFR engine at the lambda that provides the highest knockmeter reading, modern SI engines are generally operated at stoichiometry and knock intensity is based on the high frequency cylinder pressure oscillations associated with knocking combustion. In the present work, an instrumented CFR engine was used to analyze lambda effects on both the conventional knockmeter RON rating method and cylinder pressure transducer based knock intensity. It was found that both knock intensity metrics reduced significantly at stoichiometry for highly iso-paraffinic fuels (such as primary reference fuels), compared to test fuels composed of higher ethanol or aromatic content. This behavior had a significant impact on how the octane ratings of the fuels were perceived at stoichiometric conditions.