Abstract
A new approach to the numerical analysis of the auto-ignition of a premixed fuel-air mixture is presented. The model solves the spatially-averaged transport equations, integrating a reduced kinetic model for the low temperature oxidation reaction. It simulates each realization of spatial and temporal variations of velocity, temperature, species concentration of the mixture blobs in sizes of almost the integral length scale of turbulence. This approach is essential for calculating the reaction terms, which have the strong non-linearity to temperature and depend on the temperature-time history at each local point during the induction period of the auto-ignition. From this view point, the Reynolds averaged transport equations, which are commonly used in engine CFD simulations, can not be said appropriate, for they provide the phase-averaged or turbulence-averaged values.
Two different cases of the auto-ignition processes were simulated: one is the auto-ignition in the end gas in a spark ignition(SI) engine, and the other is the auto-ignition and combustion process in a homogeneous charge compression ignition(HCCI) engine. A pancake combustion chamber was used for both the computations. The initial velocity field included spatially fluctuating velocity components generated by random numbers.
The auto-ignition in the end gas in an SI engine was simulated by integrating a turbulent propagating flame model expressed in a spatially averaged form. The computations terminated when any local temperature of the unburned mixture reached 1100 K. The computed result for a central spark ignition case shows that the first local auto-ignition appears at the location between the wall and the flame front. The result also shows that there are several locations which almost come to the auto-ignition between the wall and the flame front. The computed pressure agrees well with the measurement in the time history and the time of the knock initiation. An off-center spark ignition case was also investigated. The result shows that the first local auto-ignition took place in close vicinity to the wall. The analysis of the local temperature history of the end gas reveals that the location of the first auto-ignition depends on which temperature region the local temperature of the end gas stays longer, in the negative temperature coefficient region or in the temperature regions outside of it.
The computations were performed for the HCCI engine with the initial fields of uniform distribution of fuel concentration and temperature and of random distribution of them. The result shows that several auto-ignition spots first appear sporadically near the walls and then many auto-ignition spots follow, even though the initial temperature and fuel concentration fields are uniform. The computed temperature fields indicate that many locations reach the auto-ignition and burn, but not the whole volume at the same time. Such appearance of auto-ignition sites and the moderate combustion are typical in high speed photographic observations of the compression ignition and in pressure-time histories presented in the literature.
It is demonstrated that the new approach is an analytical tool for detailed understanding of the auto-ignition in SI engines and in HCCI engines.