Prediction of combustion and emissions in gas engines

Gaseous fuels provide an attractive option for internal combustion engines in terms of low emissions and costs. However, on one hand natural gas allows CO2 reduction but on the other hand natural gas engines emit unburned hydrocarbons (UHC), mainly methane, which is also a greenhouse gas. In the present work mathematical models are developed and incorporated in a computational fluid dynamics (CFD) code to predict the charge formation, consumption and emissions in gas engines. The charge formation in gas engines and the influence of different geometric and operating conditions on the mixture formation are investigated. Numerical simulations are performed using detailed chemistry to investigate the effects of blending of natural gas with higher alkanes and hydrogen at gas engine conditions and relevant correlations for the laminar flame speeds and ignition delay times are developed. A charge consumption model is developed to incorporate different modes of charge consumption in gas engines by means of exothermic reactions, namely combustion and auto-ignition at homogeneous and inhomogeneous conditions. Finally a hybrid model is developed to predict the unburned hydrocarbon emissions (UHC), wherein the different sources of UHC, such as flame-wall quench, valve overlap and crevices are modelled and incorporated in the 3D-CFD code. In order to consider the post-oxidation of the unburned hydrocarbons, an efficient single step model with detailed chemistry is developed. Further, a NOx model from the literature using tabulated chemistry is incorporated into the hybrid model. The numerical simulations are validated with single cylinder engine experiments performed by the research group at LVK, TU-Munich. The developed models in this work can help in developing technologies to achieve the goals of higher efficiency and lower emissions for the next generation gas engines.