ARTICLE IN PRESS JID: PROCI [mNS;October 6, 2020;19:37] Available online at www.sciencedirect.com Proceedings of the Combustion Institute 000 (2020) 1–8 www.elsevier.com/locate/proci Effects of octane sensitivity on knocking combustion under modern SI engine operating conditions Xumin Zhao, Hu Wang ∗ , Daojian Liu, Zunqing Zheng, Mingfa Yao State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China Received 1 November 2019; accepted 4 September 2020 Available online xxx Abstract Octane sensitivity (OS), as one of the fuel anti-knock quality indexes, is critical with respect to the effective design of next-generation spark ignition (SI) engines. This simulation study focuses on the effects of OS on knock behavior as a function of spark timing (ST) and compression ratio (CR) under boosted high load condition. Eight fuels with identical Research Octane Number (RON) and varying OS were selected to specify the relationship between OS and fuel-specifc properties, including a primary reference fuel (PRF), Ethanol Reference Fuels (ERFs), Toluene Reference Fuels (TRFs). It was found that increasing OS to decrease end- gas reactivity is conditional. The end-gas reactivity becomes less sensitive to OS with advancing ST. Analysis for the relationship between OS and fuel-specifc properties illustrate that fuel-specifc variations beyond OS play an important role in knock tendency when increasing CR, where OS value is insuffcient to describe the fuel anti-knock performance. ERF yields better knock resistance than the corresponding OS TRF at high CR conditions. The cause for this behavior is that the decrease in the autoignition temperature moves the end-gas of high OS fuel into a long-ignition-delay region. Comparable chemical and charge cooling effects are effective to retard auto-ignition more dramatically for ERFs. © 2020 Published by Elsevier Inc. on behalf of The Combustion Institute. Keywords: Octane sensitivity; Knock; Fuel-specifc; End-gas autoignition; Modern SI engines 1. Introduction The high-effciency spark ignition (SI) gasoline engine design and operation strategies, such as high compression ratio (CR), downsizing and down- speeding, could drive engines toward higher power density and higher load duty cycles, where knock is more problematic. Utilization of more highly knock-resistant fuels could be helpful to pursue ∗ Corresponding author. E-mail address: wang_hu@tju.edu.cn (H. Wang). high effciency of engines more aggressively by ap- plying higher CR and optimum spark timing (ST). Therefore, improving the understanding of fuel- specifc effects on knock remains a high priority in engine and combustion research community. Presently, the effciency of a fuel in a SI engine is regulated via its octane numbers, namely, the Re- search Octane Number (RON) and Motor Octane Number (MON), which are measured at two differ- ent temperature and pressure conditions in a CFR engine according to ASTM methods [1]. However, the relevance of the CFR-primary reference fuel (PRF) approach to understanding more complex https://doi.org/10.1016/j.proci.2020.09.007 1540-7489 © 2020 Published by Elsevier Inc. on behalf of The Combustion Institute. Please cite this article as: X. Zhao, H. Wang, D. Liu et al., Effects of octane sensitivity on knocking combustion under modern SI engine operating conditions, Proceedings of the Combustion Institute, https://doi.org/10.1016/j.proci.2020. 09.007