Investigation of thermal stratification in premixed homogeneous charge compression ignition engines: A Large Eddy Simulation study

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 can be used to control ignition timing and heat release by diluting the mixture with residual gas and introducing thermal stratification. Cyclic variability in HCCI engines with NVO can result in reduced efficiency, unstable operation, and excessive pressure rise rates. Contrary to spark-ignition engines, where the sources of cyclic variability are well understood, there is a lack of understanding of the effects of turbulence on cyclic variability in HCCI engines and the dependence of cyclic variability on thermal stratification. A three-dimensional computational fluid dynamics (CFD) model of a 2.0L GM Ecotec engine cylinder, modified for HCCI combustion, was developed using Converge. Large Eddy Simulations (LES) were combined with detailed chemical kinetics for simulating the combustion process. Twenty consecutive cycles were simulated and the results were compared with individual cycle data of 300 consecutive experimental cycles. A verification approach based on the LES quality index indicated that this modeling framework can resolve more than 80% of the kinetic energy of the working fluid in the combustion chamber at the pre-ignition region. Lower cyclic variability was predicted by the LES model compared to the experiments. This difference is attributed to the resolution of the sub-grid velocity field, time averaging of the intake manifold pressure boundary conditions, and different variability in the equivalence ratio compared to the experimental data. Combustion phasing of each cycle was found to depend primarily on the bulk cylinder temperature, which agrees with established findings in the literature. Large cyclic variability of turbulent mixing and spatial distribution of temperature was predicted. However, both of these parameters were found to have a small effect on the cyclic variability of combustion phasing.