Research On The Mechanism Of Low-temperature Premixed Combustion And Emission Control For Light-duty Vehicle Diesel Engines

The combustion process of diesel engines has a significant effect on the power, economy and emission performance. As internal combustion engines tend to be highly effective, energy-saving and environmentally friendly, all the scholars of the world in this area concentrate on creatively developing the combustion theories and techniques for the oncoming generation of engines in order to break through the emission limits of conventional diesel engines. Aiming at the difficulties of forming the homogeneous charge and organizing the combustion progress for the compact structure of light-duty vehicle diesel engines, the research in this dissertation is based upon advanced injection combined with high proportion cooling EGR to achieve the low-temperature premixed combustion. Taking advantage of in-cylinder working process visualization, pressure information acquisition, performance bench test and3-D combustion process simulation as well, the effects of fuel injection strategies, intake system parameters and space distribution of fuel inside the chamber on the low-temperature premixed combustion process and pollutants emissions are studied systematically, which explores the effective ways for forming the homogeneous charge and controlling the low-temperature combustion progress so that the theoretical principle of low-temperature premixed combustion techniques for engineering application is built.At first, visualization test for in-cylinder combustion process is conducted to investigate the mechanism of action that advancing injection and decreasing the intake oxygen concentration act on realizing premixed combustion and reducing combustion temperature, and to verify the possibility of actualizing low-temperature premixed combustion. The test indicates that advancing the injection timing is an effective way to realizing premixed combustion for it can prolong the time for mixing the fuel and gas so that the diffusion flames will be evidently reduced. With the advanced injection strategy, the proportion of premixed combustion increases, the zone where Soot is generated lessens, and the area ratio of high concentration decreases. The high proportion EGR makes further efforts to extend the fuel-gas mixing time, which cuts down the diffusion combustion and improves the quality of the mixture. With the decrease of the intake oxygen concentration, the diffusion flames darken, the areas of diffusion flames and Soot generating zone are both likely to descend, but the area of high concentration Soot ascends a little.Based on in-cylinder pressure information acquisition and performance bench test, the effects of combustion control parameters such as injection timing, injection pressure, intake oxygen concentration and intake temperature on low-temperature premixed combustion process and emission performance are studied. It reveals that taking the advanced injection strategy can prolong the ignition delay period, and improving the fuel-air mixture is the key to reduce the Soot emission. Diminishing the intake oxygen concentration can control the temperature of premixed combustion effectively, restrain the production of the NOX emission, and put the ignition phase of premixed combustion which is too early off, which is beneficial to improving the combustion efficiency and enhancing the fuel economy. Increasing the fuel injection pressure makes the NOx and Soot emissions reduced in the low-temperature combustion mode when the load ratios are10%and25%, while it brings about the increment of the NOx emission and the decrement of the fuel efficiency in a moderate duty. In the low-temperature combustion mode, the combustion efficiency can be dramatically improved by decreasing the intake temperature and putting off the ignition phase, the in-cylinder temperature can be decreased, and the production of NOx is restrained at the same time. The Soot emission can be reduced by raising the intake temperature appropriately when the load ratio is10%, while intake cooling plays an important role in cutting the Soot emission down at the load ratios of25%and50%.Multi-injection strategy is an effective method to extend the operating scope of low-temperature combustion and ameliorate the Trade-off relationship between the NOx and Soot emissions. Pre-injection can reduce the main injection amount and shorten the ignition delay period of the main injection fuel. As a result, the exothermic phase advances and the peak value of the heat release rate descend so that the Soot is restrained successfully. What’s more, different injection parameters have different effects on the Soot emission. Post injection can reduce the combustion heat release rate of the main injection fuel and promote the in-cylinder temperature after the main injection fuel ignites. The NOx emission can be cut down by adopting post injection, while the Soot emission depends on the parameters of the post injection, which means the NOx and Soot emissions can be modified simultaneously only when appropriate post injection is adopted at a right timing. The primary control parameters and strategies to realize effectively clean combustion are preliminarily ascertained by optimizing the low-temperature premixed combustion process and emissions at different load rates. Compared with the original engine, the NOx emissions decrease by97.8%,80.7%and72.6%respectively when the load rates are10%,25%and50%, and the Soot emissions decrease by76%,93.9%and74.3%, while the brake specific fuel consumptions increase only by3.6%,4.7%and2.8%.To improve the research efficiency, numerical calculation model of low-temperature premixed combustion for diesel engines is built by means of AVL Fire, which creates the condition to explore the basic theories and application of low-temperature premixed combustion further. The influence of the axial distribution of the fuel beam inside the chamber on the mixing of the fuel and gas as well as the pollutant emissions is researched via3-D numerical simulation for low-temperature premixed combustion of diesel engines. AS a consequence, when the revolution is1450m/min, the road rate25%and the three injection vertebral angle respectively155°,150°and145°, the NOx emission increases first and then decreases with the diminishing of the injection vertebral angle, while the situation of Soot is opposite. The emissions of the NOx and Soot adhere to the change of the effective volume ratio to some degree. Compromising both the NOx and Soot emissions, it is better to reduce the effective volume ratio in the condition of the low-temperature premixed combustion mode which is based on the advanced injection strategy.