Modeling the Effect of Thermal Barrier Coatings on HCCI Engine Combustion using CFD Simulations with Conjugate Heat Transfer 2019-01-0956
Low Temperature Combustion (LTC) engines hold promise to reduce fuel consumption and emissions of oxides of nitrogen compared to conventional spark ignited (SI) engines. LTC regimes tend to be very sensitive to temperature thus wall temperatures have a large influence on the combustion process. Thermal barrier coatings (TBC) have been investigated in the past for spark ignited and diesel engines and were found to result in increased emissions of soot and oxides of nitrogen, reduced volumetric efficiency, and increased occurrence of knock in SI engines. However, these TBCs tended to be thick resulting in an increase in the wall temperature over the entire engine cycle. Lately thinner coatings with low conductivity and heat capacity have been investigated; such coatings result in reduced cycle average wall temperatures and only high wall temperatures within a short period of the combustion event resulting in reduced heat loss. These benefits may be greater in LTC engines due to the temperature sensitive nature of LTC.
In this work, we explore the effect of TBC thickness and thermal properties on homogeneous charge compression ignition (HCCI) combustion using computational fluid dynamic (CFD) simulations. In the simulations we model heat transfer between the in-cylinder gas and solid piston using a conjugate heat transfer model. Combustion is modeled with a detailed chemical kinetics model of toluene reference fuel (TRF), which is used as a surrogate for the test gasoline used in validation experiments. Temperature and heat flux data are experimentally measured in a single cylinder HCCI engine equipped with fast-response thermocouples. Simulations over multiple engine cycles were needed to reach steady state because the combustion process of one cycle is influenced by previous cycles through the exhaust gas temperature and composition.
Nick Killingsworth, Tom Powell, Ryan O'Donnell, Zoran Filipi, Mark Hoffman
LLNL, Clemson-ICAR, Clemson Univ, Auburn University