Date of Award
Doctor of Philosophy (PhD)
Computational fluid dynamics (CFD) simulations of internal combustion engines (ICEs) are becoming an increasingly popular tool in the automotive industry to either explain experimentally observed trends or perform lower cost design iterations. The convenience of commercially available CFD software and advancements made in computing hardware have been the impetus behind this growing popularity. However, obtaining accurate results using these software packages is not a trivial process and requires an in-depth understanding of the underlying numerical methodology and sub models for various physical phenomena. Specific to the ICEs, CFD simulation often entails the use of models for detailed chemistry and combustion, heat transfer, liquid spray injection, and, most importantly, turbulence. Direct resolution of the governing equations is still extremely impractical for ICE simulations which implores the need for lower computational expense turbulence modeling like Reynold’s Averaged Navier Strokes (RANS) or Large Eddy Simulations (LES) to generate accurate results. RANS is the most common turbulence model type for CFD of ICEs given its low computational cost compared to LES, but it is inherently less accurate. The practicality of LES for engine simulations continues to improve as computational power grows indicating that the future of engine CFD simulations lies with the more accurate turbulence model. But, in the current landscape, new engines and combustion strategies are adding further thermophysical complexity that were not as prevalent in conventional spark-ignition or diesel combustion. Previous and current work suggests that LES has the potential to improve the predictive capability of these new designs and strategies, and it is imperative that this approach is investigated thoroughly.
This work highlights the application of LES to three different engines operating with different combustion strategies including a medium-duty four-stroke engine operating with Low Temperature Gasoline Combustion (LTGC), a light-duty four-stroke engine using homogeneous charge compression ignition (HCCI) with wet ethanol, and an opposed-piston two-stroke (OP-2S) engine operating on mixing-controlled compression ignition (MCCI) with diesel, wet ethanol, and hydrogen. Computational models with detailed chemistry and combustion were developed for each engine architecture and validated to collected experimental data. Each model is used to analyze the effects of certain design parameters on multiple aspects of engine performance while discussing how LES can be used to improve the prediction of this performance. Results indicate that simulations with LES have implications for combustion performance, in-cylinder stratification, and heat loss relative to a RANS. The claim is made throughout the results that LES may be better suited for advanced combustion given its ability to generate more realistic flow fields and better prediction of in-cylinder stratification. Additionally, the ability of LES to predict inhomogeneities in cylinder stratification on a cycle-to-cycle basis is demonstrated in the higher cyclic variation over the RANS simulations.
An understanding of the implications of LES turbulence modeling for advanced combustion is further explored throughout this work with the simulation of two promising fuels: wet ethanol and hydrogen. It was found that the enhanced turbulent mixing from an LES simulation can improve the fuel-air mixing relative to these two new fuels. The results of MCCI with diesel and wet ethanol are compared and it was determined that wet ethanol is effective at reducing nitrous oxide (NOx) emissions but incurs an efficiency penalty when direct injected in MCCI. An advanced compression ignition (ACI) strategy is then proposed to improve the indicated efficiency of the wet ethanol while still maintaining significant reductions in NOx. While for MCCI of hydrogen, a baseline understanding of this fuel in a compression-ignited strategy is presented. It was found that there are challenges associated with the fuel injection and combustion of a gaseous fuel that is extremely resistant to autoignition.
O'Donnell, Patrick, "Applications of Large Eddy Simulations to Novel Internal Combustion Concepts" (2023). All Dissertations. 3374.