Experimental Investigation and Numerical Modeling of Spray Combustion and Emission in a Diesel Engine

Abstract

An improved multi-dimensional CFD code has been employed to simulate the spray, combustion and pollution formation process within a diesel engine cylinder. The computational results are compared with experimental data from an optical high-speed research engine equipped with a high-pressure injection system. Several spray sub-models have been implemented into the code, and their influence on the predicted droplet characteristic was evaluated. These models account for liquid core atomization, droplet secondary breakup, spray/wall interaction, droplet turbulent dispersion and evaporation. These models improve the prediction of the droplet sizes within a diesel spray and provides a more accurate initial condition for the evaporation, combustion models. The combustion sub-model employed has two components: one for predicting auto-ignition and one for computing the subsequent combustion of the ignited gas. Thermal NOx formation is calculated according to the extended Zeldovich mechanism, which gives the NOx formation as a function of temperature and O, H and OH radical concentrations. Soot formation process adopted in present study is modeled according to a hybrid chemical kinetics/turbulent mixing controlled rate expression. For the engine configurations and operating conditions considered, in most case the calculated cylinder averaged results show good agreement between measured and global pressure, heat release rate and emission data, but in some case they have limitations. Discrepancies are highlighted and possible reasons suggested. The major influences of the injection timing and combustion chamber geometry on the pollutant formation processes have been identified. The calculated results provide a detailed insight into the processes governing combustion and pollutant formation in spray flames under diesel engine conditions. The good agreement indicates that computer models are available for use by the engine industry to provide directions for engine design.