Numerical study of auto-ignition propagation modes in toluene reference fuel-air mixtures: Toward a better understanding of abnormal combustion in spark-ignition engines

摘要

Two main abnormal combustions are observed in spark-ignition engines: knock and low-speed pre-ignition. Controlling these abnormal processes requires understanding how auto-ignition is triggered at the "hot spot" but also how it propagates inside the combustion chamber. The original theory regarding the auto-ignition propagation modes was defined by Zeldovich and developed by Bradley who highlighted different modes by considering various hot spot characteristics and thermodynamic conditions around the hot spot. Two dimensionless parameters (epsilon, xi) were then defined to classify these modes and a so-called detonation peninsula was obtained for H-2-CO-air mixtures. Similar simulations as those performed by Bradley et al. are undertaken to check the relevancy of the original detonation peninsula when considering realistic fuels used in modern gasoline engines. First, chemical kinetics calculations in homogeneous reactor are performed to determine the auto-ignition delay time tau(i), and the excitation time tau(e) of E10-air mixtures in various conditions. These calculations are performed for a Research Octane Number (RON 95) toluene reference fuel surrogate with 42.8% isooctane, 13.7% n-heptane, 43.5% toluene, and using the Lawrence Livermore National Laboratory (LLNL) kinetic mechanism considering 1388 species and 5935 reactions. Results point out that H-2-CO-air mixtures are much more reactive than E10-air mixtures featuring much lower excitation times tau(e). The resulting maximal hot spot reactivity epsilon is thus limited which also restrains the use of the detonation peninsula for the analysis of practical occurrences of auto-ignition in gasoline engines. The tabulated (tau(i), tau(e)) values are then used to perform one-dimensional Large Eddy Simulations (LES) of auto-ignition propagation considering different hot spots and thermodynamic conditions around them. The detailed analysis of the coupling conditions between the reaction and pressure waves shows thus that the different propagation modes can appear with gasoline, and that the original detonation peninsula can be reproduced, confirming for the first time that the propagation mode can be well defined by the two non-dimensional parameters for more realistic fuels.