| With increasing concerns over the environmental protection and the shortage of crude oil supply, much effort has been focused on the utilization of alternative fuels in engines. Natural gas is a potential alternative fuel due to its high octane number, low emissions, low price, and abundant reserve. Nowadays, natural gas engine has entered into the stage of commercial engine. The combustion of natural gas produces less emission than that of gasoline and diesel fuels due to its simple chemical structure and absence of fuel evaporation. The high octane number of natural gas gives the engine high anti-knocking capability and allows it to operate at high compression ratio, leading to the further improvement of both power output and thermal efficiency. However, because of the slow burning velocity of natural gas and the poor lean-burn capability, the natural gas spark-ignition engine has the disadvantages of large cycle-by-cycle variations, poor lean-burn capability, and poor EGR tolerant ability, and these will decrease the engine power output and increase fuel consumption. One effective method to solve the problem of the slow burning velocity of natural gas is to mix natural gas with the fuel that possesses fast burning velocity. Hydrogen is regarded as the best gaseous candidate due to its very fast burning velocity, much better lean-burn capability, and small quenching distance. This combination is expected to improve the lean-burn characteristics and decrease engine emissions. Many researchers studied the effect of the addition of hydrogen into natural gas on engine performances and emissions in the past several years. The previous work mainly concentrated on the engine performances and emissions fueled with natural gas-hydrogen mixtures using lean combustion and retarding ignition timing, but few literatures were found by combining EGR with natural gas-hydrogen blends to reduce NOx emission.The research on fundamental combustion and engine application fuelled with natural gas-hydrogen blends combined with EGR was systematically conducted under the supports from the 973 Project, 863 Project and National Natural Science Foundation of China. By using engine experiment, constant volume bomb experiment and chemical kinetics analysis, research work was developed from macro vision to micro vision, from physical aspect to chemical aspect and from experiment to simulation. These results can provide the fundamental theory and the practical guidance for designing and developing high-efficiency low-emission engines fueled with natural gas-hydrogen blends. The characteristics and innovative aspects of the dissertation are as follows:(1) Experimental study on combustion characteristics of a spark-ignition engine fueled with natural gas-hydrogen blends combined with EGR was conducted. The optimum ignition timing is advanced with the increase of EGR ratio and is retarded with the increase of hydrogen addition in natural gas-hydrogen blend. For a specified hydrogen fraction, the flame development duration, the rapid combustion duration and the total combustion duration increase with the increase of EGR ratio and they decrease with the increase of hydrogen fraction. Hydrogen addition gives larger influence on flame development duration than on rapid combustion duration.(2) Cycle-by-cycle variations in a spark ignition engine fueled with natural gas–hydrogen blends combined with EGR were investigated. Cylinder peak pressure and maximum rate of pressure rise are decreased with the increase of EGR ratio, while cycle-by-cycle variations of the two parameters are increased with the increase of EGR ratio. Weak interdependency between cylinder peak pressure, maximum rate of pressure rise and their corresponding crank angle is presented with the increase of EGR ratio. Cycle-by-cycle variations of indicated mean effective pressure are increased with the increase of EGR ratio. Partial burn cycles and/or misfire cycles will appear at large EGR ratio. CoVimep is slightly increased with the increase of EGR when EGR ratio is less than a certain value, and CoVimep is remarkably increased with the increase of EGR ratio when EGR ratio is over this certain value. Hydrogen addition has little influence on CoVimep at small EGR ratio while hydrogen addition can remarkably decrease the CoVimep at large EGR ratio. A nearly linear correlation between EGR ratio of CoVimep=10% and hydrogen fraction is presented.(3) Experimental study on performance and emissions of a spark-ignition engine fueled with natural gas-hydrogen blends combined with EGR was conducted. Brake mean effective pressure is decreased with the increase of the EGR ratio. Brake mean effective pressure is decreased at small hydrogen fraction and is increased with further increase of hydrogen fraction. Effective thermal efficiency is increased with the increase of EGR ratio when the EGR ratio is less than a certain value (10%), whereas it decreases with further increase of EGR rate when the EGR ratio is larger than this value. NOx concentration is decreased with the increase of EGR ratio, and this effectiveness becomes remarkably at large hydrogen fraction. NOx concentration shows an increasing trend with the increase of hydrogen fraction. HC emissions increase with the increase of EGR ratio and decrease with the increase of hydrogen fraction. EGR doesn't have significant influence on CO and CO2 emissions. CO and CO2 emissions are decreased with the increase of hydrogen addition. At engine speed of 2000 r/min, when the hydrogen fraction is in the range of 30%-40% and the EGR rate is in the range of 10%-20%, engine performance and emissions get the reasonable values. At engine speed of 3000 r/min, when the hydrogen fraction is in the range of 20%-40% and the EGR rati is in the range of 20%-30%, engine performance and emissions get the reasonable values. Engine fuelled with natural gas-hydrogen blends combined with EGR is a favorable approach to realize high-efficiency and low-emission combustion.(4) Experimental and numerical study on premixed combustion characteristics of methane-air flame and hydrogen-air flame was conducted by using a constant volume bomb and Chemkin package. The laminar burning velocity decreases with the increase of the dilution ratio. When dilution ratio is less than 30%, with the increase of dilution ratio, the reaction thickness and diffusive thickness increase slightly, but the preheating thickness increases remarkably. The laminar burning velocity increases exponentially with the increase of adiabatic flame temperature. The adiabatic flame temperature plays a dominant effect on the laminar burning velocity and the thermal diffusivity does a secondary effect. Lewis number decreases with increasing dilution ratio. Dilution enhances the diffusional-thermal instability of premixed flames. With the increase of dilution ratio, thermal expansion is decreased and flame thickness is increased, leading to the decrease of hydrodynamic instability. The combined influence of these two instabilities results in little variation in flame instability at different dilution ratios. Laminar burning velocity depends on the competition between chain branching reaction and chain recombination reaction. Numerical study indicates that the suppression of laminar burning velocity is closely linked to the decrease of H and OH mole fractions in the flames. When the mole fraction of H and OH exceed 10-3, the laminar burning velocity increases remarkably.(5) Experimental and numerical study on the effect of hydrogen addition on the laminar premixed combustion characteristics was conducted. The laminar burning velocity increases with the increase of initial temperature and hydrogen fraction, and they decrease with the increase of initial pressure. Markstein length shows an increase with the increase of equivalence ratio, and the behavior becomes more remarkably at high equivalence ratio. Markstein lengths of methane-hydrogen-air flames with large hydrogen fraction and hydrogen-air flames show a slow increase with the increase of equivalence ratio. Markstein length is decreased with the increase of hydrogen fraction. With the increase of initial pressure, advancement of onset of cellular instability is presented and the critical radius is decreased, indicating the increase of hydrodynamic instability with the increase of initial pressure. Initial temperature is insensitive to the flame instability. Suppression (or enhancement) of overall chemical reaction with the increase of initial pressure (or temperature and hydrogen fraction) is due to the decrease (or increase) of H, O and OH mole fractions in the flames. Strong correlation exists between burning velocity and maximum radical concentrations of H and OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone.
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