Detonation Peninsula for TRF-Air Mixtures: Assessment for the Analysis of Auto-Ignition Events in Spark-Ignition Engines 2018-01-1721

Controlling abnormal auto-ignition processes in spark-ignition engines requires understanding how auto-ignition is triggered and how it propagates inside the combustion chamber. The original Zeldovich theory regarding auto-ignition propagation was further developed by Bradley and coworkers, who highlighted different modes by considering various hot spot characteristics and thermodynamic conditions around them. Dimensionless parameters (ε, ξ) were then proposed to classify these modes and to define a detonation peninsula for H₂-CO-air mixtures.

This article deals with numerical simulations undertaken to check the relevancy of this original detonation peninsula when considering realistic gasoline fuels. 1D calculations of auto-ignition propagation are performed using the Tabulated Kinetics for Ignition model. Chemical kinetics calculations are first carried out to build the needed look-up table for the auto-ignition delay time τ_i, and the excitation times τ_e of E10-air mixtures using a RON 95 TRF surrogate.

The dimensionless parameter ε is based on the hot spot radius and on the excitation time τ_e of the fuel. Previous chemical kinetics calculations confirm the impact of the fuel on this parameter as H₂-CO-air mixtures feature much longer excitation times than TRF-air mixtures. Focusing on the parameter ξ, its estimation depends on hot spots characteristics and thermodynamic conditions. The limits of the peninsula therefore vary depending on initial conditions and hot spot characteristics, that is why this paper focuses on several conditions to validate the dependency of the boundaries between the different auto-ignition modes. Hundreds of simulations are performed and due to the large amount of calculations, a specific post-processing methodology is defined to determine the auto-ignition propagation modes by automatically characterizing the coupling conditions between reaction and pressure waves. Several new detonation peninsulas are finally proposed depending on initial conditions in terms of temperature, pressure, fuel-air equivalence ratio and dilution. Limits of the detonation peninsula for TRF-air mixtures are more affected depending on each operating conditions. These new limits can finally be used to better understand abnormal auto-ignition events in spark-ignition engines.