Evaluation And Analysis Of The Combustion Flow Characteristic For High-Pressure Common-Rail Diesel Engine

Diesel with high thermal efficiency, low fuel consumption, high torque, easy to implement exhaust-driven turbocharger, and reliability, long life and other advantages is being the trend of the vehicle engine. The HC and CO emissions of conventional diesel are lower than Otto engine, but the NOx and PM emissions are higher. Because of theemergency of environment and energy, the NOx and PM emissions of the Diesel are more and more restricted by the developing emission standard. Now so many ways are taking active parts in reducing the NOx and PM, such as inter-cooling, multi-phase injection, after-treatment, etc. And there is a trade-off relation of the two emissions. The pre-mixed combustion has a direct relation not only with the mixture conditions but also the spray quality. NOx is mainly formed in pre-mixed combustion and PM is mainly formed in diffused combustion. In order to reduced the PM amount, the method of organizing stronger air flow, making good mixture, applying more oxygen are taken, although it can shorten the combustion phase with low fuel consumption and high power, it can also increase the maximum temperature and the NOx emission. In contrast, in order to reduce NOx emission, the method of weakening the flow intensity in pre-mixed combustion is taken, but it forms the area with high temperature and lack of oxygen, and makes no means of diffusion combustion phase, and increase the PM emission.The core problem of the combustion phase in diesel is to make a good matching of the intake flow, chamber geometry and the spray. Different chamber geometry results in different flow and its intensity maintenance. And the intensity maintenance makes great effect on the diffused combustion phase. The turbulence status in diffused combustion makes direct effect on the PM emission. So in order to take the advantage of the reentrantchamber, make good control on heat release and reduce the NOx and PM emissions in premise of the approximately the same fuel consumption and power, it's still a crucial step to thoroughly study and optimize the chamber geometry.The policy of the paper is'to limit the pre-mixed combustion and strengthen the diffused combustion'. Here, we simulated a kind of reentrant low-emission chamber of DI diesel engine with 2.5 liter volume with software FIRE, and analyzed the effect on the flow field and flow intensity within the chamber during combustion phase with different conditions and different structural parameters. Then the software MATLAB is used in order to calculate the maintenance of the flow intensity with a integral formula predefined.The main contents are as follows:1. Simulated a reentrant low-emission chamber of DI diesel engine with 2.5 liter volume with software FIRE. Analyzed the characteristics of the flow field, equivalence-ratio field and temperature field in different intake swirl ratio and different engine speed. Calculated the squeeze and swirl intensity in different conditions.2. Calculated the flow field, equivalence-ratio field and temperature field on the structural parameter changed chamber, including the different reentrant ratio, TDC clearance height and piston cup center height, in premise of the same compression ratio. Analyzed the squeeze and swirl intensity.3. Calculated the maintenance of the flow intensity with an integral formula predefined with MATLAB in pre-described conditions.4. Analyzed the reasons for the trend of different curves compared in order to serve the optimized design of the reentrant low-emission chamber of DI diesel engine.The main results of the study are as follows:1. Full load, defined engine speed, defined spray parameters. Higher intake swirl ratio causes weaker squeeze intensity after the spray end. Smaller the intake swirl ratio is, better the maintenance of the squeeze intensity will be. Higher the intake swirl ratio is, stronger the intake swirl ratio will be. There is little relation between the swirl intensity maintenance and the intake swirl ratio. Smaller intake swirl ratio causes shorter pre-mixed combustion phase and lower maximum heat release value.2. Full load, defined intake swirl ratio. Different higher engine speed(3000r/min and 3600r/min),the lower speed, higher is the squeeze intensity before and after the spray end. Higher engine speed (3600r/min) causes better squeeze intensity maintenance. There is little relation between the swirl intensity and swirl intensity maintenance and the engine speed. Higher engine speed (3600r/min) causes shorter pre-mixed combustion phase and lower maximum heat release value.3. The change of the reentrant and TDC clearance height make apparent influence on the flow characteristics with the chamber.4. Smaller reentrant lead to not only stronger squeeze intensity but also better squeeze intensity maintenance. Smaller reentrant causes shorter pre-mixed combustion phase and lower maximum heat release value. It also enhances the diffusion combustion phase.5. It is the smaller TDC clearance height that causes the stronger squeeze intensity. The TDC clearance height makes little effects on the squeeze intensity maintenance. There is a proper TDC clearance height causes suitable pre-mixed combustion phase and maximum heat release value to reduce the NOX emission.6. The shallow piston cup center height that causes the stronger squeeze intensity. The piston cup center height makes little effects on the squeeze intensity maintenance. The 4.6mm piston cup center height causes shorter pre-mixed combustion phase and lower maximum heat release value.7. Smaller TDC clearance height leads to stronger swirl intensity. The reentrant and the piston cup center height make little effects on the swirl intensity. The three kinds of structural parameters make little effects on the swirl intensity maintenance.