Using natural gas in internal combustion engines (ICEs) is emerging as a promising strategy to improve thermal efficiency and reduce exhaust emissions. One of the main benefits related to the use of this fuel is that the engine can be run with lean mixtures without compromising its performances. However, as the mixture is leaned out beyond the Lean Misfire Limit (LML), several technical problems are more likely to occur. The flame propagation speed gradually decreases, leading to a slower heat release and a low combustion quality, thus increasing the occurrence of misfiring and incomplete combustions. This in turn results in a sharp increment in CO and UHC emissions, as well as in cycle-to-cycle variability. In order to limit the above-mentioned problems, different solutions have been proposed over the last decade. Among them, the stratification or the partial stratification of the charge has been shown to successfully extend the lean limit with respect to conventional lean burn engines.
During the development and optimization of such strategies, Computational Fluid Dynamics (CFD) is a fundamental tool to thoroughly understand the phenomena occurring during the mixing and combustion phases. In order to reliably simulate the combustion process, a proper model is required which takes account of the Turbulence-Chemistry Interaction (TCI).
In the present work the Partially Stirred Reactor (PaSR) model was used in the numerical simulation of a natural gas fueled single cylinder research engine. Several tests with different relative air-to-fuel ratios were carried on in order to evaluate the robustness of the model, without performing any tuning operation across the various test cases. For this purpose, both homogeneous and partially stratified charge (PSC) cases were run.
Results were compared against the experimental data gathered at the University of British Columbia by E.Chan et al., showing a good agreement between such data and the numerical ones in terms of pressure trace over time. The solver was able to correctly capture the performance enhancement of PSC engines with respect to the homogeneous counterparts, thus confirming the potential of CFD as a valid alternative to experimental investigation.