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Flame propagation and auto-ignition behavior of iso-octane across the negative temperature coefficient (NTC) region on a rapid compression machine

Liu, Wei and Qi, Yunliang and Zhang, Ridong and Wang, Zhi (2022) Flame propagation and auto-ignition behavior of iso-octane across the negative temperature coefficient (NTC) region on a rapid compression machine. Combustion and Flame, 235 . Art. No. 111688. ISSN 0010-2180. doi:10.1016/j.combustflame.2021.111688.

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Strict regulations on fuel economy are driving modern gasoline engines to adopt more advanced technologies to improve thermal efficiency. Some of the technologies, for example ultra-high compression ratio and spark assisted compression ignition, will elevate the thermodynamic condition near top dead center (TDC) to a considerably high level, even up to or beyond the negative temperature coefficient (NTC) region. This will definitely increase knock tendency when the combustion is not well controlled. Previous knock-related research mainly focused on temperature ranges in/below the NTC region, while the knock combustion beyond the NTC region has rarely been studied. To understand the knock behavior beyond the NTC region, in this study the flame propagation process and end-gas auto-ignition of iso-octane under wide thermodynamic conditions across the NTC region were optically studied using dual-camera photography. The results showed that the flame propagation speed increased with increasing initial temperature and decreasing initial pressure, exhibiting no NTC characteristic. With the intervention of flame propagation, the residence time of the end-gas was shortened as the initial thermodynamic conditions were promoted, indicating no NTC behavior in the overall ignition delay time of the end-gas. Two kinds of detonation initiation processes were identified. In the cases strongly affected by low temperature chemistry (LTC), the auto-ignition showed a two-stage characteristic during which a widespread but relatively weak auto-ignition (first-stage) was observed prior to the final detonation initiation. In contrast, when the LTC was absent, the detonation was initiated directly in a single auto-ignition event. Lower initial energy densities were needed to initiate detonation in the cases less affected by LTC. Thermodynamic analyses based on Bradley's ε-ξ diagram showed that, for the LTC-affected cases, the pressure rise which resulted from the widespread weak first-stage auto-ignition had vital impacts on the final detonation initiation by shifting the ε-ξ location into or away from the detonation region. Finally, thermal diffusivity was demonstrated to be capable of distinguishing detonation from other combustion modes as detonation tended to occur with lower thermal diffusivities of the mixture.

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Wang, Zhi0000-0001-5932-9124
Additional Information:© 2021 The Combustion Institute. Published by Elsevier. Received 11 March 2021, Revised 29 July 2021, Accepted 6 August 2021, Available online 31 August 2021. This study was supported by the National Natural Science Foundation of China (Grant No. 52076118). The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding AgencyGrant Number
National Natural Science Foundation of China52076118
Subject Keywords:Flame propagation speed; Auto-ignition mode; Detonation initiation; Low-temperature chemistry; Thermal diffusivity
Record Number:CaltechAUTHORS:20210908-171123419
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Official Citation:Wei Liu, Yunliang Qi, Ridong Zhang, Zhi Wang, Flame propagation and auto-ignition behavior of iso-octane across the negative temperature coefficient (NTC) region on a rapid compression machine, Combustion and Flame, Volume 235, 2022, 111688, ISSN 0010-2180,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:110769
Deposited By: George Porter
Deposited On:08 Sep 2021 22:10
Last Modified:16 Dec 2021 19:33

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