Analysis of Thermal Stratification Effects in HCCI engines using Large Eddy Simulations and Detailed Chemical Kinetics

Paper #:
  • 2018-01-0189

Published:
  • 2018-04-03
Abstract:
Paper Title: “Analysis of Thermal Stratification Effects in HCCI engines using Large Eddy Simulations and Detailed Chemical Kinetics” The widespread practical application of Homogeneous Charge Compression Ignition (HCCI) has limited controllability and narrow load range due to high heat release rates. It has been shown that thermal stratification affects ignition and heat release in HCCI and therefore can dictate its upper load limit. Thus, fundamental understanding of thermal stratification in HCCI combustion is necessary, along with the development of appropriate models to simulate it. A 3-D Computational Fluid Dynamics (CFD) model of single cylinder from a 2.0L production engine (LNF type) was developed using CONVERGE CFD, in which large eddy simulations (LES) are combined with combustion modeling using detailed chemical kinetics. The modeling framework is validated against experimental data of HCCI combustion in the modeled engine using negative valve overlap. The effects of thermal stratification on ignition is investigated. Multiple cycles are simulated to investigate the cyclic variability of thermal stratification and its resulting effect on ignition, combustion phasing, and heat release rate. Moreover, compositional stratification induced by high levels of residual gas trapping is examined in order to decouple the effects of thermal and compositional stratification. A new validation approach for thermal stratification of HCCI over multiple cycles is presented. Results indicated a competing effect between mixing of fresh charge and residuals and heat transfer induced thermal stratification during the compression stroke. The effect of local temperature on the onset of chemical kinetics is indicated to be more pronounced than that of local composition. Significant cyclic variability of thermal stratification was found, with more thermally stratified cycles exhibiting larger burn duration.
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