Author's personal copy Combustion and Flame 155 (2008) 739–755 www.elsevier.com/locate/combustﬂame A shock tube study of iso-octane ignition at elevated pressures: The inﬂuence of diluent gases Hsi-Ping S. Shen, Jeremy Vanderover, Matthew A. Oehlschlaeger ∗ Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, JEC 2049, Troy, NY 12180, USA Received 24 March 2008; received in revised form 2 June 2008; accepted 2 June 2008 Available online 22 July 2008 Abstract The ignition of iso-octane/air and iso-octane/O 2 /Ar (∼20% O 2 ) mixtures was studied in a shock tube at tem- peratures of 868–1300 K, pressures of 7–58 atm, and equivalence ratios Φ = 1.0, 0.5, and 0.25. Ignition times were determined using endwall OH ∗ emission and sidewall piezoelectric pressure measurements. Measured iso- octane/air ignition times agreed well with the previously published results. Mixtures with argon as the diluent exhibited ignition times 20% shorter, for most conditions, than those with nitrogen as the diluent (iso-octane/air mixtures). The difference in measured ignition times for mixtures containing argon and nitrogen as the diluent gas can be attributed to the differing heat capacities of the two diluent species and the level of induction period heat release prior to ignition. Kinetic model predictions of ignition time from three mechanisms are compared to the experimental data. The mechanisms overpredict the ignition times but accurately capture the inﬂuence of diluent gas on iso-octane ignition time, indicating that the mechanisms predict an appropriate amount of induction period heat release.  2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Shock tube; Ignition; Iso-octane; Diluent; Argon; Nitrogen 1. Introduction Iso-octane (2,2,4-trimethylpentane), a primary ref- erence fuel for octane rating in spark ignition engines, has drawn considerable interest as a model compound for branched alkane components found particularly in gasoline [1], but also those found in diesel [2] and jet fuels [3]. Due to its relevance to practical liquid fuels, iso-octane has been the subject of many experimental and kinetic modeling studies. Experimental investiga- tions of iso-octane oxidation and ignition have been * Corresponding author. Fax: +1 (518) 276 6025. E-mail address: oehlsm@rpi.edu (M.A. Oehlschlaeger). carried out in shock tubes [4–11], rapid compression machines (RCMs) [12–19], ﬂames [20–22], jet stirred reactors [23–26], and ﬂow reactors [27,28]. In these studies a variety of kinetic measurements have been made including ignition times, radical species con- centrations, stable species concentrations, and ﬂame speeds. Ignition delay times, the subject of this work, are an important validation target for the develop- ment of kinetic mechanisms and have been mea- sured for iso-octane in both shock tubes and RCMs. Nixon et al. [4], Vermeer et al. [5], Burcat et al. [6], Oehlschlaeger et al. [7], Fieweger et al. [8,9], David- son et al. [10], and Vasu et al. [11] have previously investigated iso-octane ignition in shock tubes for, in 0010-2180/$ – see front matter  2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustﬂame.2008.06.001