Selection of a diesel fuel surrogate for the prediction of auto-ignition under HCCI engine conditions J.J. Hernandez a, * , J. Sanz-Argent a , J. Benajes b , S. Molina b a E.T.S.I. Industriales (Edificio Polite ´cnico), Universidad de Castilla-La Mancha, Avenida Camilo Jose Cela s/n. 13071, Ciudad Real, Spain b CMT-Motores Te ´rmicos, Universidad Polite ´cnica de Valencia, Camino de Vera s/n. 46022, Valencia, Spain Received 21 February 2007; received in revised form 9 May 2007; accepted 9 May 2007 Available online 11 June 2007 Abstract Homogeneous charged compression ignition (HCCI) is a promising combustion concept able to provide very low NO x and PM diesel engine emissions while keeping good fuel economy. Since HCCI combustion is a kinetically controlled process, the availability of a kinetic reaction mechanism to simulate the oxidation (low and high temperature regimes) of a diesel fuel is necessary for the optimisa- tion, control and design of HCCI engines. Motivated by the lack of information regarding reliable diesel fuel ignition values under real HCCI diesel engine conditions, a diesel fuel surrogate has been proposed in this work by merging n-heptane and toluene kinetic mech- anisms. The surrogate composition has been selected by comparing modelled ignition delay angles with experimental ones obtained from a single cylinder DI diesel engine tests. Modelled ignition angle results are in agreement with the experimental ones, both results follow- ing the same trends when changing the engine operating conditions (engine load and speed, start of injection and EGR rate). The effect of EGR, which is one of the most promising techniques to control HCCI combustion, depends on the engine load. High EGR rates decrease the n-heptane/toluene mixture reactivity when increasing the engine load but the opposite effect has been observed for lower EGR rates. A chemical kinetic analysis has shown that the influence of toluene on the ignition time is significant only at low initial temperature. More delayed combustion processes have been found when toluene is added, the dehydrogenation of toluene by OH (termination reac- tion) being the main kinetic path involved during toluene oxidation. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Auto-ignition; Diesel HCCI combustion; Fuel surrogate; Chemical kinetic analysis 1. Introduction The increase of the environmental concern and the more stringent regulations about pollutants emissions from inter- nal combustion engines [1,2] make necessary the search of alternatives for current automotive engine combustion pro- cesses. One of the technologies which is receiving attention is the homogeneous charge compression ignition (HCCI), based on the combination of traditional spark ignition (SI) and compression ignition (CI) processes, and providing very low NO x and particulate matter (PM) emissions while keeping a good fuel economy [3–5]. Unlike diesel combus- tion, where the fuel injection and in-cylinder turbulence phenomena are of great importance, or gasoline engine combustion, where a flame front propagates from the spark towards the cylinder walls, during the HCCI combustion process a fresh oxidant/fuel charge is compressed until it auto-ignites in the combustion chamber due to the high pressure and temperature reached. In SI and CI engines, the physical phenomena, such as the flame development (SI) or the fuel atomization or evaporation (CI), dominate the combustion process, so a proper design and optimiza- tion of traditional engines does not require an exhaustive knowledge of the chemical kinetic behaviour of the fuel, but just the fulfilment of some fuel requirements, such as the octane number (ON) or the cetane number (CN). How- ever, in HCCI engines, auto-ignition and combustion rate are mainly controlled by the fuel chemical kinetics, which 0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.05.019 * Corresponding author. Tel.: +34 926295300x3880; fax: +34 926295361. E-mail address: JuanJose.Hernandez@uclm.es (J.J. Hernandez). www.fuelfirst.com Available online at www.sciencedirect.com Fuel 87 (2008) 655–665