Modeling of Combustion and Emission Characteristics in a Dual-fuel Engine by Combining Diffusion and Premixed Combustion

Abstract

Dual-fuel combustion is a combustion concept which uses a split injection strategy where the second injected fuel is used as an ignition trigger and the first injected fuel is used as heat release. Classical dual-fuel combustion has been studied for a long time to reduce the usage of liquid fuel. Recently, the dual-fuel combustion concept is coupled with the HCCI combustion concept and it receives much attention. However, the numerical approach to the dual-fuel combustion needs a great deal of calculation costs because there is no dual-fuel combustion models so they solve chemical kinetic directly. Classical dual-fuel combustion uses liquid fuel and RCCI combustion uses higher reactivity fuel as an ignition trigger and combustion propagates from the ignition source although the specific combustion behaviors are different to each other. Therefore, the dual-fuel combustion could be arranged into the couple of diffusion flame characteristics and premixed flame characteristics. In this study, dual-fuel combustion and emission models were developed from the laminar flamelet model which could describe the diffusion flame and from the level-set model which could describe the premixed flame. At first, the laminar flamelet model was applied to describe the diffusion combustion in dual-fuel combustion. The second injected fuel is ignited by the high temperature and pressure without spark plug during the compression stroke. In addition, the first injected fuel has a possibility to auto-ignite by the high temperature, pressure and radicals. By the early injection, multiple ignition points could be generated and it was also described in this study. Secondly, the level-set model was applied to the combustion model to describe the combustion propagation. The flame propagation speed was determined by competition between ignition propagation speed by the flamelet model and flame propagation speed by the level-set model. The burned gas composition and flame brush species composition were pre-calculated by the flamelet solution database. This new model was preliminary applied to a simple planar geometry to investigate the fundamentals of model behavior. In this two-dimensional mesh, the combustion model described the ignition of the higher reactivity fuel and flame propagation to the lower reactivity fuel. Then, three-dimensional CFD simulations were performed in a practical engine mesh. The simulation results were compared with the experimental data and showed a very good agreement with experimental cylinder pressure curve. The predicted levels of NOx, soot, and THC emissions showed reasonable agreement to the experimental data.