The Study On Combustion Mechanism And Emissions Characteristics Of CI Engine Operated With DME/Methanol Dual Fuel

With stronger the voice of environmental protection and oil crisis, the application of alternative fuels and new combustion mode have became hot research points. Taking into account China's abundant coal resources, methanol and DME can obtained from coal are good alternative fuels. The research project is to utilize the fuel of DME and methanol in diesel engines for new combustion models. Combined with the National Natural Science Fund, Doctoral Fund and the content of the research works from home and abroad, make the following results of simulation and experimental validation in the combustion:Reviewed the utilization of DME and methanol in the engine experiments and numerical simulations. got a comprehensive analysis of physical and chemical properties and potential applications in the internal combustion engine of two fuels.On the basis of original works, a more efficient exhaust HC emissions testing technology was established. A capillary FFAP column was used to replace the original packed column in gas chromatography, the increasing column capacity improved emissions separation and chromatographic working conditions. Though optimizing the separation column temperature, carrier gas flow rate, vaporization temperature and detector temperature, GC was used for emissions of DME, methanol, formaldehyde and methyl formate. Fourier Transform Infrared spectroscopy method was also used to quantitatively investigate the characteristics of formic acid emissions. The emissions test obtained a series of precise emissions data of formaldehyde, formic acid, methyl formate, unburned DME and methanol, provided a scientific basis of pollutant emission control strategy and comprehensive management technology.The performances and emissions of the DME/methanol HCCI mode were studied by experiments. Experimental results showed that with the addition of combustion inhibitor methanol, the dual-fuel HCCI mode process has changed. The addition of methanol can inhibit reactions at low temperature, reduce the maximum cylinder pressure and combustion temperature, postpone the moment of main combustion reaction, solve the excessive and premature DME combustion. By adjusting the amount of methanol can effectively control the dual-fuel combustion process to achieve optimal control of HCCI mode, while adding methanol can significantly extend HCCI mode working range. Dual-fuel HCCI mode can reach the highest indicated thermal efficiency of about 49%, but at the burning border the indicating thermal efficiency reduced because of the detonation phenomenon. Dual-fuel HCCI engine had very low NOx emissions, but HC was significantly higher than the original disel engine. Inlet temperature raised to 90 degrees, HC emissions would decline 5%～20%, but in the case of low methanol concentration, the improvement became not obvious. DME accounted for approximately 60% of HC emissions, methanol was about 30%, formaldehyde, methyl formate and other substances was about 10%. Emission of CO had similar trend with HC, CO emission reduced with the increase of load and inlet air temperature.By changing the engine speed and load, fuel supply advance angle, dimethyl ether/ methanol injection ratio, and adding different proportions of CO2 into air, the tests of the PCCI mode with different boundary conditions were studied on PCCI test bench.PCCI mode consisted of three stages:low temperature reaction of DME, high-temperature reaction of DME and diffusion combustion reaction of methanol. DME as auxiliary lighting agent should be kept relatively low concentration, and with the decrease of methanol, its concentration need to be reduced. Utilized the EGR mode, engine output torque can exceed 80% of the maximum output torque of the original machine. Through the optimization of fuel supply advance angle, the indicated thermal efficiency can be significantly improved, and in the whole range of working conditions, the NOx emissions were not more than 400ppm, the value is only 30% amount of the original machine fueled with diesel. By precise control of DME/methanol injection ratio, the NOx emissions of PCCI mode were lower than the original machine.The HC emissions of PCCI mode were significantly less than the HCCI mode, methanol and formaldehyde were important parts of HC emission, their volume fraction was about 70%. Emissions of formic acid was very low, less than 20ppm. Methyl formate had higher concentration at the low load condition, power rise and DME concentration reduce can significantly reduce emissions of methyl formate.According to the single detailed mechanism of DME and methanol, through the CHEMKIN software can obtained DME/methanol detailed chemical kinetic mechanism, including 97 species and 508 elementary reactions. Through zero-dimensional simulation of HCCI combustion, the change of the mixed fuel reaction characteristics occurred mainly in low-temperature oxidation stage. The addition of methanol made the cylinder temperature decreased, low temperature reaction rate became slower and high temperature oxidation reaction delayed. In order to save computing time, a new simplified chemical kinetic model of DME/methanol HCCI mode was obtained, includes 38 species and 99 elementary reactions.Established two models of DME/methanol HCCI mode:1) a three-dimensional model with intake and exhaust channel; 2) two-dimensional rotation model ignored the airway and asymmetry structure of combustion chamber. The results showed that the indicator diagrams of multi-dimensional models were closer to the experimental results.3D model can predict cylinder pressure, temperature, fuel concentration in combustion process, but because of ignoring the gap effect,3D model had poor simulation of unburned HC. In the 2D model calculation, the addition of crevice zone made the simulation of HC emissions more accuracy, had well agreement with the experimental results. The 2D model simulation results showed that increasing the inlet temperature and the volume of crevice zone can reduce HC emissions, but when the temperature rise to a certain range, HC emissions decrease started slowing down.