Combustion and Flame 140 (2005) 267–286 www.elsevier.com/locate/combustflame Co-oxidation in the auto-ignition of primary reference fuels and n-heptane/toluene blends Johan Andrae a,∗ , David Johansson a , Pehr Björnbom a , Per Risberg b , Gautam Kalghatgi b,c a Department of Chemical Engineering and Technology, Chemical Reaction Engineering, Royal Institute of Technology, Teknikringen 42, SE-100 44 Stockholm, Sweden b Department of Machine Design, Internal Combustion Engines, Royal Institute of Technology, SE-100 44 Stockholm, Sweden c Shell Global Solutions, P.O. Box 1, Chester CH1 3SH, UK Received 30 June 2004; received in revised form 29 October 2004; accepted 24 November 2004 Available online 23 December 2004 Abstract Auto-ignition of fuel mixtures was investigated both theoretically and experimentally to gain further understand- ing of the fuel chemistry. A homogeneous charge compression ignition (HCCI) engine was run under different operating conditions with fuels of different RON and MON and different chemistries. Fuels considered were primary reference fuels and toluene/n-heptane blends. The experiments were modeled with a single-zone adi- abatic model together with detailed chemical kinetic models. In the model validation, co-oxidation reactions between the individual fuel components were found to be important in order to predict HCCI experiments, shock- tube ignition delay time data, and ignition delay times in rapid compression machines. The kinetic models with added co-oxidation reactions further predicted that an n-heptane/toluene fuel with the same RON as the cor- responding primary reference fuel had higher resistance to auto-ignition in HCCI combustion for lower intake temperatures and higher intake pressures. However, for higher intake temperatures and lower intake pressures the n-heptane/toluene fuel and the PRF fuel had similar combustion phasing.  2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: HCCI; Homogeneous charge compression ignition; Auto-ignition; Fuel chemistry; Primary reference fuels; n-heptane; Toluene; Co-oxidation; CHEMKIN 1. Introduction Homogeneous charge compression ignition (HCCI) combustion, first studied over 20 years ago [1,2], is a combustion process which utilizes a (more or less) homogeneous fuel/air mixture. Combustion is initiated by auto-ignition of the usually very fuel-lean * Corresponding author. Fax: +46-8-696-0007. E-mail address: johana@ket.kth.se (J. Andrae). mixture. It offers a number of benefits over conven- tional spark-ignition and diesel engines, such as much lower NO x emissions, higher combustion efficiency at part load than its SI counterpart, and zero par- ticulates [3,4]. A disadvantage is the relatively high emissions of unburned hydrocarbons [5], the sources being quenching wall boundary layers [6] and crevice regions [7,8]. Unlike the diesel and spark-ignition engines, where the combustion is directly controlled by the 0010-2180/$ – see front matter  2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2004.11.009