The significance of octane numbers to drive cycle fuel efficiency

Abstract Modern spark-ignition engines are knock-limited over a significant fraction of their operating map. The efficiency loss induced by knock mitigation depends on both the fuel octane quality and driving conditions. This paper quantifies the relevance of the Research Octane Number (RON) and Motor Octane Number (MON) to the knock-limited fuel efficiency losses (KLFEL) of a mid-size sports utility vehicle operated in several standard drive cycles. The Octane Index model, OI = (1-K)·RON + K·MON, is used with K as the weighting factor to evaluate the relative importance of RON and MON. The K-map recently obtained for a gasoline turbocharged direction-injection engine (Zhou et al., Fuel 290 (2021) 120012) is used to determine the K distribution in these drive cycles. The analysis shows that the engine is not significantly limited by knock in the city and highway cycles. However, knock becomes a major constraint under more aggressive driving conditions such as in the US06 cycle and the hot-weather towing tests, where approximately 70% of fuel consumption occurs under knock-limited conditions and the KLFEL accounts for up to 6.3% of total fuel consumption. Among the drive cycles studied, the KLFEL-weighted K ranges from −0.44 to +0.12, indicating that increasing RON would significantly improve fuel efficiency and decreasing MON would have only a minor effect.