We are interested in developing combustion and ignition models for a range of internal combustion engines. In the past our research efforts were mostly focused on diesel engines, as they have been shown to have higher efficiencies than gasoline engines. However, in recent years we have focused on developing models for Homogeneous-Charge Compression Ignition (HCCI) engines because they have been shown to have higher thermal efficiencies and lower NOx and soot emissions than Spark Ignition (SI) engines. While a promising technology, HCCI engines experience high levels of carbon monoxide (CO) and unburnt hydrocarbon (UHC) emissions. These pollutants are formed in regions of the cylinder where wall heat loss is significant and are strongly impacted by the auto-ignition and combustion process. Improving CO and UHC emissions in HCCI engines requires a fundamental understanding of the interactions of chemical kinetics, fuel/air/EGR mixing, heat loss, and transport between near wall and cylinder core regions. Therefore, we have developed a ignition and combustion model based on detailed chemical kinetics for HCCI engines. The model considers inhomogeneities in both local equivalence ratio as well as local enthalpy distribution. These inhomogeneities might be caused by incomplete mixing and wall heat losses, respectively. The basis of the model is the solution of scalar equations in a two-dimensional mixture fraction/enthalpy space. The proper coupling with an engine simulation is ensured by the solution of the Reynolds averaged mixture fraction and normalized enthalpy equations.

We have conducted several validation studies of the model will. First, we applied the model to a Rapid Compression Machine operated under HCCI conditions with fully homogeneous fuel/air ratio, but strong variations in local temperature. Results are compared to experimental data from Murase and Hanada (2002). The simulations correctly predict ignition timing trends as a function of initial mixture temperature. Additionally, the effect of modeled transport across enthalpies on the ignition characteristics were quantified and the importance of this effect was demonstrated in comparison with a multi-zone model.

To gain a more fundamental understanding of the underlying physics of auto-ignition in systems with initial temperature fluctuations but initially homogeneous mixture composition, we also applied our model to the Direct Numerical Simulations (DNS) of Hawkes et al. (2005). Simulations using the model show substantial improvement over multi-zone modeling also performed by Hawkes et al. (2005).

Most recently, we applied the model in a fully coupled CFD simulation to an experimental HCCI configuration with inhomogeneities in both enthalpy and mixture composition. The test case is a laboratory 4-stroke HCCI engine. Experiments for this engine have been conducted by Bosch RTC. Numerical results have been compared against experimental data. Our current research efforts are focused on improving our models to provide accurate predictions of pollutant emissions for HCCI engines.

Simulation of Temperature Contours in an HCCI Engine

(Provided by David Cook)

Unsteady flow around a circular cylinder

Unsteady flow around a circular cylinder for a Reynolds number of 333 showing the von Karman vortex street.

(Provided by Prahallad Iyengar)