Research On Combustion And Emission Characteristics Of DME Engine Based On Chemical Kinetics

Environmental and human health concerns over emissions from internal combustion engines continue to bring about increasingly stringent emission standards and drive research into the development of cleaner-burning fuels. Aiming at this current research situation together with the contents of National Basic Research Priorities Program (2001CB209207) administrated by the State Ministry of Science & Technology of China, this paper deals with combustion and exhaust gas emission characteristics in a small direct injection diesel engine fueled with pure dimethyl ether(DME).The author makes a summary on the experiments and simulations of DME engine after reading a great number of literatures on relevant research field. Then the existing problems and technical measures are analyzed. The performances and conventional emissions of DME engine are studied by experiments. The experiments according the physical chemistry characteristics of Dimethyl Ether (DME) to adjust the structure of diesel engine are done. The fuel leakage and pipe pressure waveforms are done by the fuel injection experiment under engine motored conditions. To DI DME engine, the influences of the nozzle opening pressure, the fuel delivery advance angle, the timing of the opening exhaust valve, the nozzle diameter and the compression ratio are analyzed. Compared with the original diesel engine, smokeless combustion is realized and oxides of nitrogen (NOX) emission are lower for the DME engine at rated speed condition. At medium or low speed condition, HC emissions are higher and decrease with the increase of load. To DI DME engine, NOX emission obviously decrease and HC emission obviously increase in HCCI DME engine. The experimental results show that it is necessary to specially study HC emission on DME engine.The unregulated emissions of engine are studied by experiments for the first time. In the experimental work the sample acquisition system and analysis method are constructed. Fourier Transform Infrared spectroscopy (FTIR) method is used to quantitatively investigate the characteristics of unregulated emissions from the tested DME engine and compared with those from diesel fuel. In order to ensure the reliability and accuracy of FTIR, gas chromatograph (GC) is also used in the experiments. Based on the optimization of column temperature, carrier gas flow rate, injection temperature, detector temperature for GC with hydrogen flame ionization detector (FID), the workstation of chromatogram collects data and draws the working curve of formaldehyde and methyl formate by using external standard method. Then the comparisons between FTIR and GC indicate FTIR is an effective means to study unregulated emission. Further work with FTIR is needed to investigate the formation and characteristic of unregulated emissions in DME engine. The results show there are formaldehyde, methyl formate and formic acid in DI DME engine emission. Among the three species, the content of formaldehyde is the highest, and the content of formic acid is the lowest which is less than 10×10-6. Comparing to DI DME engine, there are higher formaldehyde and methyl formate in HCCI DME engine emission.Based on the ignition reaction kinetics mechanism and thermodynamic combustion sub-model of the engine fuelled with dimethyl ether, an ignition delay database library and working process simulation of the engine are set up in this paper. The calculated results show the higher are the temperatures, pressures and fuel/air equivalence ratios, the shorter is the ignition delay of DME. Comparisons between computational and experimental data show that combustion sub-models such as Wiebe model, Watson model and Whitehouse-Way model are suit to predict performance characteristics of engines operated on DME. The coupling of the Watson model with the ignition delay database library can result in the better results for predicting the engine performances.The performances and unregulated emissions of DME DI engine by 3D simulation are studied. KIVA3V code is compiled by PC computer and library of liquid DME on thermophysical properties are established. A Partially Stirred Reactor model for combustion and a secondary breakup model combined KH (Kelvin-Helmhotlz) and RT (Rayleigh-Taylor) two unstable waves theories are implemented in KIVA3V code. Influence of turbulence on combustion is taken into account by using code coupled with detailed chemical kinetic models of pollutants formation. The calculation results of in-cylinder pressure and oxides of nitrogen agree well with those of experiments. Through analyzing in-cylinder flow velocity, temperature and species concentration changed with the crank angle in calculation, the results are listed below. Formaldehyde is relative stable in low and medium temperature condition, and then accelerates oxidation with increasing in-cylinder temperature. Due to wall transfer and flow and local poor oxygen caused by combustion, there are higher concentration emissions of partial oxidation products of formaldehyde and methyl formate. The choice of spray model and chemical reaction mechanisms is obvious to the ignition timing and emissions. Increasing the size of the nozzle hole or the intake charge temperature can obviously reduce the concentration of formaldehyde. Moreover, the swirl through the optimization on intake port and the shape of combustion chamber can reduce the the concentration of formaldehyde. The underestimate of the concentration of methyl formate can be analyzed,and the forming mechanisms of methyl formate are summarried.The forming mechanisms of the unregulated emission of DME HCCI engine are analyzed and measures are provided to reduce unregulated emissions. This paper presents a new detailed chemical kinetic model for DME combustion that consists of 97 species and 457 elementary reactions. The practicability of the new model is obtained by the simulation results agreed well with the measured data. Through extending the application ranges of the new model and then analyzing the relationship of key reactions and important species changed with the crank angle, the main course of DME combustion is derived and the mechanisms of CH2O and HCO2H formation are obtained. The results researched the mechanism of NOX formation indicated the quantity of NO in NOX exhaust changed less after the NO formation max concentration was reached. The emission quantity of N2O and NO2 were less and almost had no effect by the in-cylinder temperature. The proportion of NO in NOX emission increases with the increase of in-cylinder temperature. Based on the analysis of reaction rates and sensitivity analysis of chemical reactions, the major paths of NO emission on the DME combustion occurring in the HCCI engine are extra Zeldovich mechanism and N2O approach.A new simplified chemical kinetic model is obtained in this paper. The simplified chemical kinetic model consists of 36 species and 73 reactions, and includes three sub-models, i.e., a low temperature and negative temperature coefficient region sub-model, a cytolysis and oxidation sub model for high temperature, and a sub-model for NOX. In order to gain further insights of CH2O and HCO2H formation, the calculation using CFD software FLUENT coupled with low temperature and negative temperature coefficient region sub-model and high temperature pyrolysis and oxidation sub model was done. The result shows there are higher concentration emissions of partial oxidation products of CH2O and HCO2H in emission. The obvious influences of parameters, such as intake temperature, equivalence ratio, compression ratio and the clearance around the piston are calculated and analyzed. This approach is useful in predicting unregulated emission in HCCI combustion process.