| Since the internal combustion engine was invented, it is still the main power source with the highest thermal efficiency and power per volume and weight, so it's applied very widely and the world wide engine amount already exceed 10 billion units until now. It's generally believed in each automobile manufacture and internal combustion engine supplier that the engine is still the main power source for vehicle and petroleum is still the main engine fuel in foreseeable future.Internal combustion engine, as the main power source of automobiles, motocycles, shipes, agricultural machinery, engineering machinery, military vehicles and other mobile vehicles, is the main oil consumer. Taking China as an example, Chinese oil import dependence reaches 47% in 2006, the ratio of the oil consumption in engine to the total oil consumption in China is over 66%, but the energy utilization efficiency is very low in the internal combustion engine in China compared to the energy utilization level of foreign advanced engine. Based on the corresponding statistics, the engine fuel consumption rate per kilowatt-hour in China is 0.22~0.35 kilogram, the energy utilization rate is 20% lower than the countervalue in foreign advanced engine, thus more than 30 million tons oil was consumed bonus. Therefore, engine energy-saving is one of the main battlefields of national energy-saving project, and that carrying out the internal combustion engine energy-saving research relates to the state energy security. Engine is also the main source of atmosphere environment pollution, especially metropolitan atmosphere environment. There are many combustion products in the emission including incomplete combustion and harmful products and CO2 and H2O.The incomplete combustion and harmful products are harmful to human being, biological and atmosphere environment. Sunlight would lead to unburned hydrocarbon (UHC) and NOx from the emission photochemical reaction and this would further lead to photochemical smog which irritates the eyes and throat and hinders plant growth. The PM diameter in emission is less than 1 micrometer and can be inhaled into pulmonary. There are many polycyclic aromatic hydrocarbons, such as benzopyrene, which is carcinogenic substances, and is harmful to human being's health. Although the complete combustion products are not harmful to human and environment, CO2 is generally regarded as the main source of green house. Hence, more and more strigent emission regulations are enacted and put into practice all over the world and in order to meet the corresponding requirements, emission purification devices are needed to improve the emission in the automobie, these devices are more and more complicated and expensive as the regulations are more and more strigent and the price of these devices is up to 40~90% of the engine total cost, especially most of the precious metal have been consumed in the emission purification devices. According to an authoritic statistics from USA, the world annual precious metal consumption in automobile industry is about 54% of the total global consumption, however, the precious metal resources are very limited; precious metals have already become the important substances in financial operation in major automobile manufactures.Concerning the studies on the new internal combustion engine combustion theory, homogeneous charge compression ignition (HCCI), are getting more and more attention, it has been validated that this new combustion theory can improve the thermal efficiency and decrease NOx and PM emission when the combustion mode is put into practice, which can decrease the dependency of the emission purification devices, therefore, the precious metals can be economized.The experiments of ethanol HCCI combustion by preheating intake charge were carried out in this paper, and the 0-D ethanol HCCI simulation was also performed with the Chemkin4.0.The ethanol spark ignition (SI) and HCCI combustion were realized on an experimental platform respectively which derived from a single-cylinder diesel engine with modifications in compression ratio, fuel supply system and ignition system etc. The ethanol HCCI combustion was achieved by preheating the freash intake charge. The investigation results show that HCCI can only be operated in a region, the upper boundary of the operation region is limited by knock and the lower boundary of the operation region is limited by misfire, knocking boundary and misfire boundary were defined in this paper respectively. In the HCCI operation region, the maximum fuel equivalence ratio exceeds 6, and can be up to 6.1, the minimum fuel equivalence ratio is 1.05, the maximum brake mean effective pressure (pe) is up to 0.545MPa, the nearer the load to the knocking boundary, the higher the combustion efficiency and effective thermal efficiency. The combustion efficiency and effective thermal efficiency decrease as the load was decreased. Compared to the conditions when the engine operated wth SI, the advantages of lower fuel consumption rate and extra low NOx emission of HCCI are very obvious, the maximum dropping percentage of brake fuel consumption rate is 30%, the average dropping percentage exceeds 10%, the NOx emission droppes greatly, the maximum dropping percentage of NOx emission is 98%, and the average dropping percentage exceeds 60%.How to put the HCCI combustion mode into practice is an important research topic in HCCI investigation, adopting hybrid combustion mode is one of the ways to utilize HCCI in practice. As the HCCI can only be operated in a narrow region, when there are load variations in the engine with hybrid combustion mode, it's unavoidable to perform combustion mode transition; the investigation of ethanol SI and HCCI combustion mode transition was carried out in this paper. The results under different operation points indciated that the transition between the two combustion modes can not be completed within one cycle under the current experimental conditions no matter the direction, engine speed and torque are. Misfire cycles might occur in the transition period and this would result in the flucturations in engine speed and torque. Compared to the results obtained in misfire and knocking boundaries, the fluctuations of engine speed and torque in misfire boundary are higher than the fluctuations in knocking boundary, the conclusion can be made that it's more difficult to achieve HCCI/SI combustion mode transition in the misfire boundary than in the knocking boundary. How to realize the smooth transition between the two combustion modes and avoid the fluctuations in engine speed and torque is one of the critical problems needed to be resolved before putting the HCCI combustion mode into practice.In order to improve the smoothness of the ethanol HCCI/SI transition, the effects of spark ignition on the ethanol HCCI stability in misfire boundary conditions were investigated firstly. The results show that the stability of ethanol HCCI combustion in critical temperature conditions can be improved greatly with the assistance of spark ignition in cylinder and therefore the smoothness of the SI/HCCI transition was improved. The results obtained in different engine speeds and different ignition advance angles show that there is an optimum ignition advance angle corresponding to different engine speed, and cycle numbers in the transitional period are less and the fluctuations of engine speed and brake mean effective pressure are lowest, but there is still fluctuations in engine speed and brake mean effective pressure although the spark ignition was adopted to stabilize the HCCI/SI transition, hence, the fluctuations in engine speed and brake mean effective pressure can not be eradicated only with the assistance of spark ignition. Through referring to the overseas corresponding articles and analysizing the obtained experimental datum, the key influenctial factors were found out, an optimized control strategy conjunction with ignition advance angle, main throttle movement guideline and fuel supply guideline was put forward in this paper. Adopting this strategy, the fluctuations of engine speed and brake mean effective pressure were almost eracdicated and smooth HCCI/SI transition was achieved with self-designed oil position and main throttle position motor driving circuits and control programs.The ethanol HCCI ignition mechanism and whole reaction process were investigated in detail with modified 0-D single zone model in Chemkin4.0. A simplified mechanism of ethanol HCCI combustion was summarized through analysizing the reactants occurred in the each cell reaction of the HCCI combustion. The investigation shows that the ignition and combustion can be separated into three phases which includes the reaction chain solicitation phase, reactant radicle accumulation phase and ignition and combustion phase. The different chain reactions were solicitated in chain solicitation phase and these reactions release small amount heat. The generation ratios of each reactant are bigger than the consumption ratios of every reactant in reactant accumulation phase and much heat was released in this phase and the cylinder pressure rises and blue flame occurs in this pahse. The main source of the heat comes from ethanol and aldehyde oxidation. During the ignition and combustion phase, ethanol was ignited to combuste and every reaction rate is of the state of avalanche, the combustion heat was released concentrately, the main source of heat release comes from aldehyde oxidation, CO oxidation and generation of HO2. Through analysizing the ethanol consumption channels, it was summarized that there are three channels to consume ethanol in the combustion process which includes the reactions C2H5OH+OH→products, C2H5OH+O→products and C2H5OH+CH3→products and the reaction of C2H5OH+OH→products plays dominant role in these three channels. When combustion boundaries were varied, the conclusions can be made that the whole reaction rate of ethanol oxidation accelerates and the reaction time shortens as the intake charge temperature rises when the fuel equivalence ratio was kept constant and there was no EGR. When the intake charge temperature was kept constant and there was no EGR, effects of equivalence ratio on ethanol oxidation mechanisim mainly comes from effects of ethanol concentration on ethanol oxidation and combustion. As the fuel equivalence ratio rises, the crank angles of middle reaction products fully oxidation delay, the ignition preparation duration prolongs and the combustion was retarded. When the intake charge temperature and fuel equiovalence ration were kept constant, as the EGR ratio raises, the concentration of OH and CO and the middle reactants including CH2O,CH3HCO,CH4,C2H4,C2H6,H2 decrease and the full oxidation time delayes which would result in the CO density raise in the latter of the reaction process and this is one of the reasons that the CO emission raises after the EGR is inducted into the cylinder.
|
PDF Full Text Request