Analysis of Thermal Stratification Effects in HCCI Engines Using Large Eddy Simulations and Detailed Chemical Kinetics 2018-01-0189

The operating range of Homogeneous Charge Compression Ignition (HCCI) engines is limited to low and medium loads by high heat release rates. Negative Valve Overlap (NVO) can be used to facilitate ignition of high octane number fuels and control pressure rise rates by diluting the mixture with hot residual gas and introducing some thermal stratification. Controlling the thermal stratification results in sequential autoignition, reduced heat release rates, and operating range extension. Therefore, fundamental understanding of thermal stratification in HCCI combustion with high levels of internal residuals is necessary, along with the development of appropriate models to simulate thermal stratification and its effects on HCCI combustion.

A 3-D Computational Fluid Dynamics (CFD) model of a 2.0 L GM Ecotec engine (LNF type) engine cylinder, modified for HCCI combustion, was developed using CONVERGE CFD. Large Eddy Simulations (LES) were combined with combustion modeling using detailed chemical kinetics. Fifteen consecutive cycles were simulated and the results were validated against individual cycle data of 300 consecutive experimental cycles. The results showed a competing effect between mixing of fresh charge and residuals, and heat transfer-induced thermal stratification during the compression stroke. A large amount of thermal stratification was found at the onset of autoignition, resulting in a skewed temperature distribution. Compositional stratification was minimal despite the large residual gas fraction. Thermal stratification resulted in sequential autoignition, with the hotter regions igniting earlier. Significant spatial variability of thermal stratification on a cyclic basis was found, which did not affect the bulk thermal stratification. Heat release was found to depend predominantly on the bulk thermal stratification rather than the spatial distribution of thermal stratification.