Development of a Fast, Predictive Burn Rate Model for Gasoline-HCCI

Operating gasoline engines at part load in a so-called Gasoline-HCCI (gHCCI) combustion mode has shown promising results in terms of improved efficiency and reduced emissions. So far, research has primarily been focused on experimental investigations on the test bench, whereas fast, predictive burn rate models for use in process calculation have not been available. Such a phenomenological model is henceforth presented. It describes the current burn rate as the sum of a sequential self-ignition process on the one hand and a laminar-turbulent flame propagation on the other hand. The first mechanism is essentially represented by ignition delay calculation, in which the reaction rate is computed separately for some hundred groups of different temperatures based on the Arrhenius equation. Thermal inhomogeneity is described by a contaminated normal distribution which accounts for the influence of wall temperature on mixture close to the cylinder wall. The second mechanism is based on a modified entrainment model, accounting for the different boundary conditions in gHCCI mode, such as a term describing the increase of laminar burning speed at high pre-reaction levels in the unburned charge and a modified flame surface calculation. The new model correctly predicts burn rates for a wide range of control parameter variations, including PTDC combustion (“reforming”) and operating mode switches, using a single set of tuning parameters, while requiring low computational times. It is thus well suited for the development of operating and control strategies, a timely assessment of potential and hardware requirements or finding suitable engine and valve train layouts.