A Study On Enhancing Mixture Formation And Combustion Of DI Diesel Engine By Preheated Fuel Injection

With severe worldwide energy crisis and stricter exhaust emission law and regulations, as one of the main fuel consumption and environment pollution sources, internal combustion engine is being paid more and more attention. As a new type engine combustion, homogeneous charge compression ignition (HCCI) and low temperature combustion(LTC) technology is being highly regarded as a research focus.There are some problems for conventional diesel fuel to achieve a uniform mixture distribution in the cylinder because of its high viscosity and poor fuel vaporization. So the homogenous mixture preparation is the key issue for diesel fuel to achieve HCCI combustion. With the support of National Basic Research Priorities Program administrated by the State Ministry of Science & Technology of China(Project 2007CB210001), this paper develops a technique to enhance diesel spray atomization, evaporation and mixture formation in the combustion chamber by fuel preheated after High-Pressure Fuel Pump. As we know when the fuel is heated to a very high temperature, the fuel boils when it is injected into the combustion chamber. This can significantly speed up burning due to better dispersing of the fuel drops.The mechanism of improved mixture formation and combustion process by preheated fuel injection is investigated by advanced optical visualization diagnostic technology, test-bed experiments and multi-dimensional numerical simulations. The new combustion technology can achieve the high efficiency and low exhaust emissions by optimum injection strategy with preheated fuel. This paper gives a novel way to speed up air-fuel mixture formation for HCCI engine. The main contents and achievements of this research are as follows:This paper reviews the research of the experiment and numerical simulation about HCCI, and then analyzes the bottleneck problems of application of HCCI. Through deep-going analyses of the latest research progress about the key technological issues of combustion boundary conditions combined with control of fuel design, it is realized that the difference of the fuel's physicochemical properties play an important part in mixture preparation and combustion process.To solve the diesel mixture formation problems, the mechanism relevant to strengthen and speed up mixture formation of the conventional diesel fuel by preheated fuel injection is studied. A multi-dimensional numerical model is developed based on KIVA-3 V, which includes two zone evaporation model, PaSR turbulent combustion model, Hiroyasu-Nagle soot emission model. Based on this model, the effects of fuel injection timing, injection strategy and fuel temperature on the fuel-air equivalence ratio and temperature distribution in cylinder are studied. The results show that the pre-injection fuel temperature plays a very important role in fuel spray atomization, evaporation and mixture formation of diesel fuelled direct injection engine. As fuel temperature increases, the more uniform distributions of fuel-air mixture and temperature in the cylinder are achieved, the volume of prepared combustible mixture increases, and the wall impingement of fuel spray is almost disappeared. The influence of pilot injection timing on mixture formation weakens with high temperature injection.The effect of temperature on spray microscopic features of injection in constant volume chamber was measured by a Laser Particle Size Analyzer. The results show that the SMD of droplet in the axial and radial direction decrease, and the difference is small with fuel temperature increased. Based on the structural and operating parameters of the tested engine and the experimental conditions, a numerical simulation is developed and applied to the diesel engine. Results of calculation show that fuel temperature plays a very important role in macroscopic and microscopic features of the fuel spray. AS the fuel temperature increases, the penetrations of liquid/vapor phase and the SMD decrease, the volumetric quantity of vapor phase increases quickly. The higher the fuel temperature is, the more significant influence on the equivalence ratio of vapor phase, and the fraction of rich mixture increases. At fuel temperature of 530K, the mass fraction of fuel vapor with fuel-air equivalence ratio larger than 2.0 increases to about 70% of the whole evaporated fuel.A Model ZS195 single cylinder direct injection diesel engine is used to test the HCCI combustion. The electric heating and temperature controlling system of high pressure fuel supply pipe is installed to the rear of the high-pressure fuel injection pump which has been retrofitted for HCCI fuel supply system. The fuel leakage of pump with high fuel temperature injection was measured. The results show that leakage rate increases as fuel temperature increases, and the leakage rate decreases as engine speed increases. However, even though the fuel temperature is preheated up to 513K and the engine speed is at 1000 r/min., the fuel leakage rate is only 1.2%.Using the preheated fuel injection technology, the EGR influence on engine combustion and emission characteristics is studied with special multi-hole injector. The results show that the combustion and emission characteristics have closely related to the combustion boundary conditions (such as engine load, the fuel injection parameters, EGR, etc.). Through optimizing the special multi-hole injector, the fuel temperature, the injection timing and EGR, the processes of spraying, mixing and combustion of d. i. diesel engine can be improved obviously. There exists an optimum temperature range for preheated fuel temperature and around the optimum fuel temperature the trade-off between fuel economy and lower emission performance can be obtained.By making use of the improved KIVA-3V code and considering the preheated fuel with the different inject strategy, the numerical simulation of DI, PCCI-DI, PCCI combustion modes have been carried out. The effects of the main injection timing, the pilot injection timing, the pilot injection proportion and EGR on the mixture formation, combustion and exhaust emissions are studied quantitatively. The results show that at the middle load of DI model, the emissions of NOx and soot can be decreased only by using fuel early injection. But at the high load conditions, the emission of NOx increases obviously. And if only using the early injection, the IMEP of the engine decreases obviously. For the PCCI-DI combustion mode, if the pilot injection timing is retarded, the distribution of fuel manifests heterogeneity and more serious rich mixing zone at the beginning of the main fuel injection. Because of this, some parts of cylinder have the area of overheating. So the ignition timing occurs earlier. When the pilot injection timing is 65BTDC and the main injection timing is 5～10BTDC, the engine can get the best balance of fuel economy and exhaust emissions. For the PCCI mode with the addition of CO2 in the intake air, the engine can work well when the pilot injection timing is 65BTDC and the main injection timing can be changed in a wide range of adjustment. Comparing the three combustion modes, it can be concluded that it is not good for economy only by using the early injection strategy. Regarding to the PCCI-DI combustion mode, the fuel economy can be improved and the part of the emission can significantly reduced. But in the middle load the CO emission increases obviously and in the high load the NOx emission increases obviously. While in the high load and using PCCI combustion mode, the soot, NOx and CO emissions can be significantly reduced and fuel economy improved simultaneously.