Study On Influence Of Fuel Injection Strategy On Performance And Exhaust Emission Of Diesel Engine

With the increasingly shortage of petroleum resource and aggravation of environmental pollution, as well as stringent exhaust emission standards, the development of Internal Combustion Engine is facing with more and more severe challenge. The fuel economy of diesel engine is 20～30% better than gasoline engine, but its major problems are the higher level of NO_x and PM emission. Therefore, the popularization and application of diesel engine with reducing pollutants emission by effective measures will be of important theoretical significance and practical value to delay energy crisis and improve environmental problems.The fuel injection system has direct and decisive effects on the combustion and emission of diesel engine. The traditional mechanical fuel supply system has already been no way to satisfy the requirement of more stringent fuel consumption and emission legislation. The invention and development of Electronic Controlled High Pressure Common Rail Fuel Injection System solves the problem. In Europe, America, and Japan that is advanced at engine technologies, it has been the research hot topic to improve performance of diesel engine combustion and emission through optimizing fuel injection rate, using the function of multiple injections of CRS. Since the CRS was introduced in domestic later, and its development was restricted by productivity, the research on this aspect has not been reported in detail. Based on EuroIII diesel engine, using the capability of multiple injection of CRS, adopting numerical simulation combined with bench test, the thesis aims at discovering potential of diesel engine to meet the demand of more stringent emission standards, at the same time to explore the effect rule of multiple injection on diesel engine performance and emission. The study provides theoretical foundation and experimental basis for the low emission development work of similar type diesel engines.In order to investigate deeply the injection characteristics of CRS, the EFS common rail fuel injection test bench was used to measure injection rate under conditions of different rail pressure, different injection advanced angles, and different multiple injections in this dissertation. Through study and analysis it was found that the typical rectangle injection rate curve was realized because of the stability of rail pressure. Keeping the start of injection invariable, the injection rate is larger and the injection duration is shorter with the rail pressure rising. When the rail pressure is the same, with the varying of start of injection the shape of injection rate curve did not change, only the position of which shifted along with x axis. When the pilot injection fuel quantity was invariable, the variety of pilot injection position was equal to shift along with x coordinate axis. When the pilot injection timing kept the same with different pilot injection quantities, it was displayed different peak value of pilot injection rate on the same start time of pilot injection. Similar to pilot injection, the injection rate position of post injection shifted along with x axis when the post injection interval varying, but its main injection rate shape was unchangeable nearly. From the curves post injection rate was displayed by different peak values on the same position with the variable post injection fuel quantities.The best advantage of CRS was that the injection pressure, injection timing, and injection quantity was independent relatively, and was not effected by engine speed and torque. So it became possible that the diesel engine was provided with the best fuel injection process under different operating conditions. In the dissertation, the experiment of engine performance and emission were carried out on the test bench through adjusting rail pressures and injection advanced angles. The study shows that the raise of fuel injection pressure made the quality of fuel atomization raise, and the mixture formation more homogeneous, furthermore improving combustion process, resulting in PM emission reducing and BSFC decreasing. The increasing of injection advanced angle caused fuel combustion releasing heat closer up to TDC, the combustion process ending in advance, which resulted in raising heat efficiency and improving fuel economy. But it was also found that although the combination of higher injection pressure with less injection advance angle could improve the emission of diesel engine to a certain extent, it wouldn't make fuel economy and emission performance to be the best at same time when facing more stringent emission standards and legislation. Through referring to relative literatures, it was decided to apply multiple injection technologies to improve emission performance of diesel engine. Finally the feasibility was proved by Pi-3.0-25 pilot injection scheme and Po-5.0-10 post injection scheme through bench test.The appearance of numerical simulation technologies brought a new way to the design and development of modern diesel engine. Meanwhile due to the new technologies of diesel engine applying continuously, the origin mathematical model hadn't satisfied the requirement of simulation and prediction completely. For example, when adopting multiple injection function of CRS, the combustion process was changed for the different fuel injection modes. So the formation and oxidation of soot were influenced, at this time the origin soot model was exposed with the shortages. Based on this, lots of literatures were collected and summarized in the dissertation, a soot oxidation model was presented basis of mean reaction rate, aiming at the post injection study. The presented soot model considers both turbulent flow mixing control and chemical reaction kinetics as the integrated effect factors on soot oxidation. With the interface supplied by CFD code, the presented model was integrated into the software using Fortran 90 program. Coupled with other submodels the diesel engine combustion and emission process were simulated with different pilot and post injection conditions. The accuracy and rationality of the model were validated combined with experimental results finally.To obtain the influence rule of different pilot and post injection conditions on the combustion and emission of diesel engine, and to offer theoretical guidance for experimental study, the combustion and emission process of diesel engine were numerical simulated in detail with presented soot model and other submodels in FIRE software. Through comparison and analysis of computation results, it was concluded as following. When pilot injection was applied, the fuel/air mixing was more homogenous than that without pilot injection during the main injection. The heat release of pilot injection fuel combustion resulted in shortening the ignition delay of main injection fuel. With the pilot injection fuel quantity increasing and pilot injection dwell reducing, the ignition delay of main injection fuel was shorter. Little pilot injection fuel combustion released heat, which decreased proportion of premix combustion of main injection fuel, reduced the T_max in cylinder, so as to improve the NO_x emission. With the fuel injection delaying and pilot injection fuel combustion increasing, the combustion performance deteriorated quickly. The pilot injection interval should be optimized adequately. If the interval was too long, the pilot injection combustion didn't heat main injection fuel well. While the interval was too short, the main injection fuel would carry excess pilot combustion products, which resulted in soot formation largely for lack of oxygen. As for post injection, little post injection fuel combustion didn't influence main combustion process nearly. But the post injection spray jet enhanced the turbulent disturbance of mixture in cylinder. Meanwhile the post injection fuel released heat to raise the temperature in cylinder. All of which enhanced the oxidation rate of soot, resulting in the final soot emission reduction. Through analysis of many factors influencing soot oxidation, it was found that the turbulent mixing induced by post injection spray jet was a key factor to enhance soot oxidation.In order to further analyze and reveal the combustion process and emission performance of diesel engine, the acquisition and analysis of indicator diagram were carried out through bench test respectively under different multiple injection conditions. At the same time the NO_x and PM emission were measured and their variable rule were analyzed and discussed. The study shows that there was a good agreement between simulation results and experimental data, which indicated that the adopted computation model reflected the combustion process of diesel engine factually.Through optimizing simulation and experimental study, it can be concluded as following firstly. 1) When optimizing single injection, in order not to bring fuel consumption increasing largely, it prefers to higher injection pressure combined with small injection advanced angle. Finally the common rail pressure 65MPa with 1℃A BTDC injection advanced angle was selected as the baseline. Meanwhile it was found that only adjusting injection pressure and injection advanced angles would not meet the demand of stringent standards for NOx and PM emission. So it was decided to adopt multiple injection technology, including pilot injection and post injection. 2) When optimizing pilot injection, it prefers to select small pilot injection quantity, and the optimum of pilot injection timing was combined NO_x emission with fuel consumption. It was selected as pilot injection scheme by injection quantity of 1.0mg, injection intervals of 15℃A or 35℃A finally. When optimizing post injection, in order not to inducing fuel economy deteriorating, it prefers to select larger post injection quantity with shorter injection interval. Finally the scheme of post injection was selected with injection quantity of 5.0mg and injection interval of 10℃A. With all the schemes mentioned above the diesel engine emissions were reduced further.At the same time, it was also found the influence relation of different multiple injection schemes on performance of diesel engine. 1) In case of pilot injection schemes, NOx emission was in inverse proportion to injection intervals when keeping injection quantity the same. When keeping injection interval the same and less than 35℃A, the NOx emissions were in proportion to injection quantity, but more than 35℃A the direction was on the other way round. The soot emission variety was similar to NOx emission, but the only difference was that the variable regulation before and after injection interval 35℃A was coincident. 2) Under post injection conditions, on keeping the same injection interval, NOx emission was in inverse proportion to injection quantity. When keeping the same injection quantity, NO_x emission was in inverse proportion to injection intervals. As for soot emission, when keeping injection interval as the same, its variable trend was similar to NO_x emission. When keeping the same injection quantity, the soot emission increased firstly and decreased then with the injection interval increasing. But excess post injection would induce fuel consumption increasing and exhaust temperature rising. Therefore the post injection quantity and intervals must be regulated and optimizing carefully. In a word, simulation optimum and experimental study of a DI diesel engine were carried out under different multiple injection schemes in the thesis. The performance and emission of engine were improved effectively, and the better results were achieved. Since the numerical simulation and experimental results agreed well, numerical simulation can be used for the design and optimization of diesel engine low emission development. One hand, the investigating results can be used to improve diesel engine performance and emission continually. On the other hand, it provides theoretical and experimental foundation for the further research and wide application.