May 24-26, 2021: Five papers are presented at the 12th US National Combustion Meeting:(1) Flame initiation and propagation in mixtures with various reaction progress under elevated temperature conditions(2) Effects of stratification and charge cooling on combustion in gasoline direct-injection compression ignition (GDCI) engine(3) A binary soot model for engine combustion(4) Optimization of Pre-chamber Geometry Using CFD, Machine Learning, Bayesian Updating, and Genetic Algorithm(5) Combustion, Knock and Emissions in a Port-fueled Spark-ignition Hydrogen Engine: a CFD Study