The Significance of Octane Numbers to Hybrid Electric Vehicles with Turbocharged Direct Injection Engines

Hybrid powertrains reduce engine operation time at light loads where the engine is less efficient. For spark-ignition engines, this means that a larger fraction of operation time could be knock-limited, and the fuel octane quality could be more important for knock control and vehicle fuel efficiency. Built on our recent work on a conventional vehicle (Zhou et al., Fuel 302, 121,095), this paper extends the investigation of fuel octane and vehicle fuel efficiency to hybrid electric vehicles using a same turbocharged, direct-injection engine with the number of cylinders reduced from 4 to 3. The impact of the Research Octane Number (RON) and the Motor Octane Number (MON) on knock-limited fuel efficiency loss (KLFEL) is investigated using the Octane Index model, OI = (1-K)·RON + K·MON, where K is the weighting factor of the two octane numbers. Vehicle modelling is conducted to investigate the K and KLFEL distributions in three EPA drive cycles and the impact of octane numbers on these distributions. With increasing hybridization, the K distribution is shifted to more negative range and is accompanied with larger knock-limited fuel losses (in both relative and absolute terms). Increasing the RON and decreasing the MON are found to both improve the fuel efficiency, but the former is significantly more effective than the latter for a given change in octane number. The increasing importance of octane specifications for drive cycle fuel economy is therefore demonstrated for hybrid vehicles with different degrees of electrification.