Improvement Of A Phenomenological Soot Model For The Application In HCCI/PCCI Engine

One of the disadvantages which limit the wide use of diesel engine is the soot emissions. The soot emission is very harmful to human health. Therefore it is a pressing task to reduce the soot emissions from diesel engines. The traditional experimental method has the advantages in providing straightforward and accurate information, but it involves the problems of time consumption and high cost. It is therefore a strong desire to make full use of current powerful computer resources and develop mathematical models in order to accurately predict the soot formation and oxidation processes.Currently, semi-empirical soot model has been widely used. In this thesis, a six step semi-empirical soot model developed previously by the research group at Dalian University of Technology is improved. The primary work performed in this study is to optimize some empirical constants in the model to improve the prediction accuracy. The improved soot model includes six steps, namely, the formation of the precursor, the formation of soot particles, the surface growth and condensation of the soot particles, the oxidation of the particle surface and the oxidation of the precursor. The calculation is based on the engine computational fluid dynamic (CFD) code KIVA-3V coupled with the chemical kinetics package CHEMKIN. The chemical reaction model of the fuel oxidation is taken from the skeletal mechanism for primary reference fuel (PRF) developed by our group. The final soot model was validated extensively against available experimental data from a shock tube, a constant volume combustion chamber, two PCCI diesel engines, and two conventional diesel engines. Finally, the model is used to investigate the influences of the intake air temperature, pressure and engine speed on the soot emissions of two combustion modes in diesel engine.The results of the calculations in shock tube show that the model can well predict the formation process of soot, and the variation trend of the soot particle size and number density. The results for constant volume combustion bomb also show that the combustion, soot formation and soot distribution were also satisfactorily reproduced. Through the calculations for two PCCI diesel engines, it is found that the amount of soot formation and its evolution can be predicted reasonably. Moreover, the influence of operating parameters, such as intake air pressure, temperature and engine speed on soot emissions was discussed. It is shown that by modifying the empirical coefficient in the model for OH oxidation to1, the soot formation and its evolution can be well predicted in two conventional diesel engines under wide operating conditions. In addition, the influence of the intake air pressure, temperature and engine speed on soot emissions can be reproduced well. Moreover, it is revealed that the impact factors affecting soot formation and oxidation characteristics in PCCI engine and conventional diesel engines may be different due to the difference in their combustion mechanisms, which should be further investigated.