车用内燃机新概念燃烧过程的研究

For a conventional diesel engine, the combustion is controlled by stoichiometric diffusion flame. It is found that the heterogeneous feature of local high temperature and high fuel concentration in engine combustion chamber results in formation of soot and NOx. Therefore, it is significant to seek for new approach to mixture preparation in diesel engines for complying with the requirement of high efficiency and low emissions of future engines.It is found in this study that a thin and rich mixture layer on a wall is formed after impingement of a gas jet of fuel on the wall by means of. The measured thickness of the mixture layer is about 2 mm. and its dispersion rate after the end of injection is much lower, compared to that of a space gas jet. This phenomenon in a small D.I. diesel engine is known as "wall setting" or "wall fuel accumulation" which has an important influence on engine fuel consumption and emissions.This study develops a technique for enhancing the near wall mixing of an impinging jet by means of a bump on the wall. A wall jet formed after an impingement of a gas jet has been investigated by means of simultaneously measuring the near wall velocity and concentration distribution. It has been found that a wall jet is stripped off the wall and ejected as a secondary-jet when the wall jet encounters a bump of the wall. Effects of the bump height, the location of the 'bump and the impingement angle are studied at a number of-typical conditions. In addition, the concept of concentration mean dispersion rate is employed in data processing in order to understand the effects of the bump geometry on mixing rate. It has experimentally confirmed that a controllable mixing process of "space-wall-space" for an impinging jet may be realized by means of a bump setting on the wall.Besides, an optical experimental facility of the method of laser induced fluoresce is set up. The diesel fuel is injected by through a pressure intensified fuel injector. The experimental results of spray mixing are qualitatively agreed well with the finding by gas jet simulation. A study of airflow and mixture formation in a BUMP chamber were also studied by means a CFD model. Based on the knowledge raised by above research a kind of so-called BUMP combustion chambers was developed. In a wide engine operation conditions, reductions yield in 80 % of soot emission and 50 % NOx emission.Finally, a compound combustion technology of Premixed Combustion and "Lean Diffusion Combustion" for realizing the concept of HCCI combustion in a D I diesel engine was put forward. The premixed combustion is achieved by the technology of multi-pulse fuel injection based on advanced common rail fuel system developed by the research team. The start of pulse injection, injection-pulse number, injection period of each pulse and the dwell time between the injection pulses are controlled. The objective of controlling the pulse injection is to limit the spray enetration of the pulse injection so that the fuel will not impinge on the cylinder liner, and to enhance the mixing rate of each fuel parcel by promoting the disturbance to the fuel parcels. The last or main injection pulse is set around TDC. A flash mixing technology is developed from the development of the BUMP combustion chamber, which is designed with some special bump rings. The combustion of fuel injected in the main injection proceeds under the effect of the BUMP combustion chamber at much higher air/fuel-mixing rate than does in a conventional DI diesel engine, which leads to "lean diffusion combustion". The effects of multi-pulse injection and BUMP combustion chamber on auto-ignition, rate of heat release and engine emissions are investigated in detail. In the compound combustion mode very low soot and NOx emissions were obtained.

申请者先前已发现，燃烧室壁面上德特殊BUMP和凹槽可使燃油壁面射流剥离并形成强湍混合率二次射流，使燃油喷雾迅速转化为高空燃比的预混合气。另外在点火式发动机上通过二次喷油技术可实现准均质混合气，消除过浓和过稀区，二种新概念燃烧过程均可实现超低污染，本项目旨在深入研究其自身规律，为向实用技术转化提供依据。