| Modern high speed direct injection (HSDI) diesel engine for passenger car is one of the most effective technologies for energy saving and emission reduction. HSDI diesel engine combines the remarkable performances of both traditional diesel engine and gasoline engine, such as low fuel consumption, high torque at low speed, low noise and high power per liter, etc. As the issues of energy-saving and emission reducing have been becoming more prominent today, in China, most auto companies aim to develop HSDI diesel engine to cope with these problems. However, combustion path of traditional diesel engine passes through the regions which are easy to generate soot and NOx in T-φ plot, and it causes higher sources emission of soot and NOx in cylinder and can’t meet the requirements of Euro IV and Euro V emission standard unless adopt complex post-processing system. Therefore, seeking for feasible clean efficient combustion system, lightening post-processing system’s burden and reducing post-processing system cost become the most urgent tasks for diesel automobile industry development in the world.The research aim of this paper is to make the in-cylinder NOx emissions of HSDI diesel engine reach the Euro V standard and reduce soot emissions as much as possible. By the means of combining simulation and experiment, the paper explores optimization methods and technical route of diluted low-temperature clean-combustion technologies on HSDI diesel engine with cooled EGR after turbocharging, including turbocharging and EGR control technology. The paper focuses on how the parameters, including EGR rate and injection law, etc, affect the mechanism and characteristics of quality for cylinder spray and combustion process. The main research contents and innovations are summarized as follows:(1) Firstly, the emission characteristics of the original HSDI diesel engine (a single-stage turbocharging VGT mode) were analyzed. Then, by using the vehicle dynamics simulation platform (GT-drive), the HSDI diesel engine was added to a car which ran under the New European Driving Cycle (NEDC) and FTP75, and the operation areas and corresponding emission characteristics of this engine under the two road circular running conditions were studied. Results show that it should focus on improving emission characteristics of the HSDI diesel engine under the conditions of low speed and low load, which provide significant evidences for selecting operation point for the latter study.(2) Secondly, the relationship among EGR rate and boosting pressure as well as the HSDI diesel engine performance simulation model were established through the reverse method. The optimal VGT opening and corresponding EGR rate were obtained. The results show that when the EGR rate increases to50%, boosting pressure should be greater than0.4MPa. For the two levels of supercharger, equivalent intensification causes most working points locate in low efficiency area of compressor, and that is bad for the engine’s fuel economy. Unequal boosting pressure distribution can improve both the efficiency of pressurization system and the fuel economy of engine.(3) Thirdly, to solve the problems caused by the uniformity between fresh air and EGR quantities in each cylinder of IC engine, its steady state and transient state simulation platform based on1D&3D coupling was established. Fresh air and EGR uniformity of each cylinder were analyzed under steady and a variety of non-steady operation conditions, including uniformity of intake flow mass for similar ports but different cylinders and intake mass variation laws of each cylinder. Additionally, the variation laws and response time and response characteristics of total intake mass, fresh air, EGR, BMEP and air-fuel ratio after fuel injection variation ends under non-steady operation condition were also studied. On that basis, a new control method for the balance of EGR and fresh air in each cylinder was proposed, and it was authorized national invention patent. The research method established in this paper is very helpful for analyzing and optimizing intake uniformity and response characteristics of each cylinder. In addition, this method not only gives a important monitoring and analysis means to study engine transient behavior, but also provides a important technical support to realize high efficient clean-combustion in each cylinder at each cycle.(4) Next, a simulation platform of Kiva-3V for diluted low-temperature clean-combustion, including complex geometric structure topological method and hexahedral structure grid generation technology, was established. On this basis, the influences of fuel injection system on spray and combustion process under conventional combustion method (EGR rate is0) were studied, and finally the optimal geometrical parameters of nozzle were determined. Results show that too many holes is bad for spray and combustion improvement; the smaller fuel beam angle is, the more NOx and soot emissions would be; the nozzle bulge height, combustion chamber depth and jet angle should match reasonably. In order to facilitate data processing and analysis and evaluate performance parameters of the engine, an engine performance evaluation database and automatic data processing software was developed and granted software copyright.(5) Based on the HSDI diesel engine under operation condition of RPM=2500r/min BMEP=0.9MPa, the effects of fuel injection pattern, EGR rate, excess air coefficient, injection timing, inlet temperature and swirl ratio on the processes of spray and combustion with a high EGR rate were studied by simulation analysis and experiment. Furthermore, the formation environment and mechanism of NOx, soot and CO, the effects of parameters variations on ignition delay, heat release rate and emission pollutants were analyzed. Through the coupling optimization of multiple parameters, an optimized route was determined for improving combustion process and reducing emission pollutants, and it would be helpful guideline in the next stage for engine calibration to meet Euro V emission standard.Finally, the simulation platform of Kiva-3V was verified by experiment, and it was proved to be reliable and can predict in-cylinder spray combustion process and emission pollutants very well. The simulation platform is not only a favorable tool for engine combustion system development, but also provides important analysis method to determine EGR rate and supercharged pressure control strategy, at the same time gets more important information to propose optimization method and technical route for the development of clean low-temperature combustion system. |
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