Effect Of Fuel Volatility On Mixture Formation And Combustion Process Of Diesel Engine

Recently, with the more and more serious air pollution, the emission regulations of world are becoming stricter and stricter. As the main environmental pollution source, how to control the car's exhaust emission have become urgent affairs. The stricter emission regulations prompt lucubrate of the emission control technologies. There are many factors effecting the diesel emissions, mainly including intake system, fuel supply system, combustion chamber configuration and fuel property. Since 1980s, a lot of researches proved that fuel properties and components have direct affection to the engine emissions. To meet the stricter emission regulations, a lot of countries have carried legislation of fuel ingredient into execution. The future requirements of fuel property refers to cetane number, distillation range, aroma content, sulfur content, viscosity, density and so on. Thus it can be seen, improving fuel qualities has great sense.The process of the combustion has great influence to all kinds of the engine performances, so researching the combustion process is absolutely necessarily. For a long time, the combustion research way is by the experiment, but experiment research has a lot of shortage. With the computer technology and computer science quickly developed, numerical calculation has becomes the effective means to study the combustion process of the engine. There are many advantages of numerical simulation by computational fluid dynamic (CFD) software such as low cost, time economy and can show some information which can't be provided by experiment. The research method used in this thesis is simulation with FIRE order which is developed by Austrian AVL Company. From the fuel spray,mixture formation and combustion process search the micro mechanism which affect the emission, and look for the means which can improve the emission and control emission.With the help of numerical calculation software, the main tasks of this thesis included these aspects:1> First, building three-dimension-geometry in CATIA and transmit it into FIRE with the format of stl.2> Meshing the geometric model of the combustion chamber in the FIRE. The quality and number of the grid are very important to the precision of simulation, so meshing grid process is very important. After meshed the grid, advisable choosing sub-models such as gas flow, spray, evaporation, collision combustion and so on, and using these sub-models describe the transient status of the chamber and simulate the working process of the diesel engine.3> By adjusting the model parameters, it was validated by in-cylinder pressure curve comparison between computation and experiment.4> Studying the influence disciplinarian of fuel volatility on mixture formation,combustion and emission, and offer guidance to improve combustion and emission. Fuels of this thesis are diesel and n-heptane, by comparing the characteristics of these two kinds of fuels found that the catane of the two fuels are similar, but the volatility of the two fuels has great difference. Then, spray process, mixture formation process, fuel temperature field, gas concentration, velocity field and turbulent kinemics energy distribution were analyzed, which enable us studying the mechanism of fuel property effect on combustion process microscopically. Something found from computational simulation are as follows:a) The penetration distance decrease and the Weber number increase of the oil bind with improvement of volatility, which make better fuel oil atomizing, break-up capability boost up and the equilibrium to be better.b) With the improvement of fuel volatility, at the beginning of the spray process, fuel oil atomization becomes better, and the ignition delay becomes shorter. In the phase of pre-mixed combustion, a lot of fuel oil was vaporized to take part in combustion, which lead to at the initial stages of the combustion, in the chamber have higher temperature and thicker gas concentration. The produce qualifications of NOX are high temperature, rich oxygen and the proportion of premixed combustion has great influence to the produce of NOX. So fuel with better volatility will produce more NOX. At the stage of diffusion combustion, fuel with bad volatility will have higher temperature and thicker gas concentration. Meanwhile, it also showed that the fuel with better volatility own higher turbulent kinemics energy which is helpful to reduce soot emission.5> Researching the influence disciplinarian of fuel volatility on combustion and emission in different boundary conditions, and searching potential to improve the mixture formation and combustion which cooperated with by fuel volatility and combustion boundary conditions. Some things found from computational simulation are as follows:a) Fuel with better volatility has shorter ignition delay and the emission of NOX is more, so, it can adopt fuel injection timing retard to reduce NOX emission while it can't reduce fuel economy.b) Fuel injection system parameters have great influence to the combustion and emission. To the homology of the mixture, the fuel with better volatility to the changeable of the injection system parameters isn't sensitivity. When the homology of the mixture is well, the ignition delay will become shorter; When the homology of the mixture is bad, the homology will to be worse to the fuel with worse volatility and the ignition delay will become longer.c) EGR (Exhaust Gas Recirculation) is a preference technology to reduce NOX emission. When introducing EGR, the ignition delay of the fuel with better volatility has little influence, but to the fuel with worse volatility, it will prolong the ignition delay. Based on, it can take into account to suitable increase the ratio of EGR when use the fuel with better volatility. Also, it can found that when introducing EGR, it has a greater restrain function to produce NOX for the fuel with better volatility.The combustion and emission characteristic of the engine mainly lie on the physics process of fuel atomization, evaporation, mix and the chemical reaction kinetics process of the in-cylinder states, which has consanguineous connection with the physical and chemical properties of fuel. Recently, a lot of researchers devoted themselves to the improvement of physical and chemical properties of fuel, and they have acquired delightful achievements. In the future study, strengthen the research of fuel properties on combustion and emission must offer powerful help to improve combustion and emission of the engine.