Two-stage ignition behavior and octane sensitivity of toluene reference fuels as gasoline surrogate

Doohyun Kim, Charles K. Westbrook, Angela Violi

Research output: Contribution to journalArticlepeer-review

19 Scopus citations


Current approaches to improve the efficiency of Spark-Ignition (SI) gasoline engines have been focusing on turbocharging, increasing the compression ratio, and pursuing advanced low-temperature combustion concepts. In order to maximize these strategies, it is important to optimize the knock resistance of the fuel, and therefore knowledge of the sensitivity of the ignition process under a wide range of engine operating conditions is required. Octane sensitivity (OS), which is defined as the difference between Research Octane Number (RON) and Motored Octane Number (MON), has been introduced to represent how fuel's ignition reactivity changes relative to the primary reference fuels (n-heptane/iso-octane) within RON/MON conditions. Previous works have indicated that OS is intimately related to low temperature reactivity of the fuel, which can be revealed as two-stage heat release characteristics during an ignition event. Prompted by these findings, in this paper, we investigate the relationship between two-stage ignition behavior and OS, using chemical kinetic simulations of 24 Toluene Reference Fuels (TRFs)/ethanol blends. TRFs are ternary mixtures of n-heptane/iso-octane/toluene, which is capable of capturing aromatic content and positive values of OS of real gasoline fuels. Simulation results show that fuels with weak or no two-stage ignition behavior tend to have high OS, due to their lack of Negative Temperature Coefficient (NTC) effect and high sensitivity in ignition delay time. Leveraging such observations, we develop a correlation between two-stage behavior and OS as an OS prediction method. Two metrics that represent the strength of the two-stage ignition behavior are proposed and used as OS predictors, which are Low Temperature Heat Release percentage (LTHR%) and Heat Release Rate at the end of first stage (HRRinf) calculated from a simple kinetic simulation. Regression analysis shows a clear trend between decreases in the proposed two-stage behavior metrics and increases in the value of OS of the fuel. We also test the new metric (LTHR%) using simulation results of 0-D reactors with imposed pressure time histories obtained from engine experiments, as well as using different TRF kinetic mechanisms. The results demonstrate the effectiveness of the metric as a representation of the two-stage ignition behavior in practical combustion systems, highlighting the importance of the proposed relationship, and its potential as a simple and effective OS predictor.
Original languageEnglish (US)
Pages (from-to)100-113
Number of pages14
JournalCombustion and Flame
StatePublished - Aug 28 2019
Externally publishedYes

Bibliographical note

KAUST Repository Item: Exported on 2022-06-10
Acknowledgements: Authors would like to thank Dr. S. Goldsborough and Dr. D. Kang at Argonne National Laboratory for discussions on experimental aspects of the two-stage ignition behavior. Authors would also like to thank Dr. W. Pitz at Lawrence Livermore National Laboratory for providing the CoOptima mechanism before public release, and Dr. C. Naik at Ansys Inc. and Prof. M. Sarathy at KAUST for technical assistance with kinetic simulations. This work was supported by 2019 Hongik University Research Fund. This work was also supported in part by the US Army Research W911NF-14-1-0359. The portion of this work performed at LLNL was supported by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy (EERE), Vehicle Technology Office (VTO) under contract no. DE-AC52-07NA27344.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.


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