Tabulated chemistry method for fast calculations of detailed chemical kinetics in system level simulations of internal combustion engines

Abstract

To reduce the emissions from internal combustion engines, new combustion strategies along with alternate fuels blends are currently under investigation. The engine performance and emissions depend primarily on the chemical reactions occurring inside the engine. However, the modeling of reactions in engine simulation models using a detailed reaction mechanism is computationally very expensive, especially for very large mechanisms. Hence, a tabulated chemistry solver is developed in this work to reduce the computational time. The thermochemical states of the mixture are pre-calculated at different engine operating conditions based on homogeneous reactor simulations and stored in a lookup table which can be interpolated during the engine simulations. The present work introduces a novel method for thermochemical mapping between detailed chemistry and tabulated chemistry along with automatic identification of important intermediate species during table generation. The evolution of the thermochemical state of the mixture is described using a progress variable and its derivative. Different progress variables are investigated, and a novel formulation is proposed. The method is then applied to predict the reactions and knock phenomenon as well as the emissions in spark-ignited engines. The results using the tabulated chemistry are in good agreement with the detailed chemistry. The crank angle at the knock onset is predicted with a root-mean-squared error of 0.6 degrees crank angle. The cylinder pressure and temperature are in excellent agreement with the full detailed chemistry solution. The concentration of emissions (CO, NOx etc.) are also in good agreement for a wide range of engine operating conditions. The tabulated chemistry method provided a speedup of ~750 times as compared to detailed chemistry for a mechanism with 679 species. The computational time of the tabulated chemistry method remains constant and independent of the mechanism size. The method can be applied to large parametric studies for engine design, optimization, and calibration.