Numerical Study On Spray/Wall Interaction Models And Cold Start Of A Diesel Engine

Multi-dimensional numerical simulations of the in-cylinder process in internal combustion(IC) engines are widely accepted as an important tool for modern engine product design and development. Compared with 1-D simulations, spatial information is included in multi-dimensional simulation results, through which more details about fuel injection, mixture preparation, combustion, and emissions formation can be investigated. Reliable multi-dimensional simulations require accurate modeling of a variety of physical and chemical processes in engines, like the injection, atomization and vaporization of the fules. Thus, different numerical models are needed, one of which is the spray/wall interaction model. In the last few years, with the depletion of petroleum resources and more emphasis being put on environmental problems around the world, energy conservation and emission reduction become increasingly demanded and direct-injection(DI) technologies are also being widely used in vehicle engines.For both diesel and DI gasoline engines cold start is one of the conditions in which serious emissions are generated. Due to the poor thermal conditons, the atomization and vaporization of liquid fuel sprays are not satisfactory so that excessive fuel is injected to ignite the engine successfully, yielding a great amount of wall film. This wall film is one of the main sources of emissions in DI engines. In order to accurately reproduce the spray/wall interactions in DI engines and investigate their influence on the emissions under cold start conditions, improved spray impingement models are needed. This is also the key objective of the present work.The structure of KIVA-3V Release2, a widely used open source code in the multi-dimensional simulation of IC engines, is introduced at the beginning of this research and the spray related submodels are discussed. Then some previous spray/wall interaction models are analyzed and their advantages and disadvantages are compared. The results show that these previous models were developed decades ago and the experimental sources on which they were developed are mostly droplet and drop train impingement experiments. The conditions in these experiments are far from those in modern DI engines, so the performances of the previous models are not satisfactory when they are used in modern DI engines. An improved model is needed for better prediction.According to the above analysis and the characteristics of high pressure, spray impingement modeling in this work are improved in three aspects. At first, a wall-jet submodel is proposed to correct the gas phase velocity in the near wall region, thus the droplet-gas relative velocity and the drag force on the droplets can be estimated more accurately. Accordingly, the mesh dependency of the proposed model is also reduced. Secondly, the life force is considered for the post-impingement droplets. The vortex at the leading edge of the wall spray is then reproduced and the movement of the wall spray is improved. Lastly, a new correlation is proposed, which is based on spray/wall impingement measurements instead of drop wall impingement results, to predict the adhered film ratio when the impingement happens. Validations against diesel spray impingement data show that the predictions of the new model match experimental data well and good agreement is achieved.Another process occurring in the spray/wall interaction process is the heat and mass transfer between the impinging spray and the wall. This process is short but also important. In the previous models, the effect of spray impingement is not considered when heat transfer between the wall film and wall is calculated. A new heat transfer model is developed based on diesel spray impingement data in this research, in which the influence of the kinetic energy of impinging droplets on the heat transfer is considered and the heat transfer between the wall film and ambient gas is estimated by using the correlations from gas jet impingement experiments. Simulation results indicate that the proposed model performs well and can be used as a helpful supplement to the currently used wall film heat transfer models.In the last part of this research the proposed kinetic and thermodynamic models for high pressure spray impingements are used in the predictions of the cold start of a small DI diesel engine. The mechanisms of emissions formation and the influence of different injection schematics are discussed. Results show that a great amount of wall film is formed under the cold start conditions, which is the main source of HC and CO emissions. After combustion the maximal ratio of remaining wall films is about 25%, which is close to experimental measurements. A split fuel injection can help to improve the vaporization of spray thus reducing emissions compared with a single injection.