Experiments and Modeling of the Autoignition of Methyl-Cyclohexane at High Pressure

Cycloalkanes and alkyl-cycloalkanes are well known major components in several transportation fuels, including gasoline, diesel and jet fuels. Since these transportation fuels have hundreds or thousands of individual chemical components, incorporating all these components in the kinetic model would make it very difficult to build and computationally expensive to use. To facilitate modeling such real fuels, it is necessary to formulate a surrogate mixture by selectively choosing a much smaller set of neat components that will reproduce the physical and chemical behavior of the target fuel. Methyl-cyclohexane (MCH) is frequently suggested as a candidate component in these formulations to represent the cycloalkane content of real fuels. Furthermore, an understanding of MCH kinetics can provide the base from which to build models of the combustion of other cycloalkanes or alkyl-cycloalkanes.

If chemical kinetic models are to be used in engine design, it is critical that they are able to reproduce the combustion behavior of fuels under the thermodynamic conditions prevalent in engines. New engines, using advanced concepts such homogeneous charge compression ignition (HCCI) and reactivity controlled compression ignition (RCCI) incorporate Low Temperature Combustion (LTC) to help achieve goals of improved fuel efficiency and lower emissions. However, a detailed understanding of LTC reaction pathways is often required to properly predict combustion phasing, heat release rates and engine-out emissions. Also, RCCI and diesel engines operate at high pressures and the chemical kinetic models need to be validated at these conditions. Therefore, experimental data acquired at engine-relevant conditions are of critical importance for validating chemical kinetic mechanisms behavior.

Previous work conducted in our Rapid Compression Machine (RCM) measured the ignition delays of MCH at pressures of 15.1 and 25.5 bar and for three equivalence ratios between ϕ=0.5 and 1.5. Over the temperature range 680-840 K, substantial discrepancies between the experimental data and the Lawrence Livermore National Laboratories (LLNL) kinetic model were found. Thus, the objectives of this work are twofold. First, new autoignition data collected in an RCM at higher pressures (50 bar) that include LTC range (690-900 K) are presented. The datasets include the pressure history that relates to the heat release rates in the RCM, the first stage ignition delay time that is characteristic of the low temperature combustion and the total ignition delay time that corresponds to hot ignition in the engine. The second objective of this paper is to update the reaction paths and reaction rate constants in the LLNL kinetic model and use the previously and newly obtained experimental data to validate the model. Updates were made that substantially improved the ability of the model to predict ignition delay times of MCH under the high pressure, low temperature conditions studied in the RCM.