An assessment of the lean flammability limits of CH4/air and C3H8/air mixtures at engine-like conditions

F. N. Egolfopoulos, A. T. Holley, Chung King Law

Research output: Contribution to journalConference articlepeer-review

30 Scopus citations


The lean flammability limits of CH4/air and C3H 8/air mixtures were numerically determined for a wide range of pressures and unburned mixture temperatures in order to assess the near-limit flame behavior under conditions of relevance to internal combustion engines. The study included the simulation of freely propagating flames with the inclusion of detailed descriptions of chemical kinetics and molecular transport, radiative loss, and a one-point continuation method to solve around singular points as the flammability limit is approached. Results revealed that both pressure and unburned mixture temperature have significant effects on the lean flammability limit as well as the attendant limit flame temperature. Specifically, the lean limit was found to first increase and then decrease with pressure, while the limit temperature decreases with pressure in general, and can be reduced to values as low as 900 K under engine-like conditions. Through sensitivity and species consumption path analyses it was further shown that the chain mechanisms that control the near-limit flame response critically depend on the thermodynamic state of the mixture. Thus, mechanisms that are identified as important at near-atmospheric conditions may not be relevant at higher pressures and unburned mixture temperatures. In particular, the response of near-limit flames was found to resemble the homogeneous explosion limits of hydrogen/oxygen mixtures in that while at low pressures the main branching and termination reactions are respectively H + O2 → OH + O and H + O2 + M → HO2 + M, at the elevated pressures relevant to internal combustion engines the system branching is controlled by the HO 2-H2O2 kinetics. Potential avenues for extending the lean operation limits of internal combustion engines are suggested based on the understanding gained herein.

Original languageEnglish (US)
Pages (from-to)3015-3022
Number of pages8
JournalProceedings of the Combustion Institute
Volume31 II
Issue number2
StatePublished - 2007
Event31st International Symposium on Combustion - Heidelberg, Germany
Duration: Aug 5 2006Aug 11 2006

All Science Journal Classification (ASJC) codes

  • General Chemical Engineering
  • Mechanical Engineering
  • Physical and Theoretical Chemistry


  • Flame extinction
  • Flammability
  • Premixed flames


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