Study On Combustion And Emissions Of A Di Diesel Engine Fueled With The Diesel-oxygenate Blends

The researches on combustion theory and emission control of a diesel engine fuelled with the diesel-oxygenate blends were systematically conducted under the supports from the 973 Project, the National Natural Science Fund of China and the Doctoral Foundation of Xi'an Jiaotong University. Both the general characteristics and the individual behavior of the combustion and emissions of the diesel engine fuelled with the diesel-oxygenate blends were clarified. The relationships between the combustion and emission parameters and the oxygen fraction and/or the oxygenate type were analyzed. The quantitative values between the combustion and emission parameters and the oxygen fraction were obtained. The study on the diesel-oxygenate blends was developed from the qualitative aspect to the quantitative aspect. These results can provide the fundamental theory and the practical guidance for designing and developing high-efficiency low-emission alternative fuel engines.The characteristics and innovative aspects of the dissertation are as follows.(1) The combustion and emission characteristics of the diesel engine fuelled with diesel-dimethoxymethane blends, diesel-diglyme blends, diesel-dimethyl carbonate blends, diesel-diethyl carbonate blends, diesel-diethyl adipate blends and diesel-ethanol blends were investigated. The combustion theory and the formation mechanisms of pollutants of diesel engine fuelled with the diesel-oxygenate blends are clarified. The behaviors of the combustion and emission parameters versus the engine operating parameters, such as fuel delivery advance angle, engine speed and load were revealed. The quantitative relationships are obtained between the oxygen fraction and the parameters such as the combustion duration, the effective thermal efficiency, the brake specific fuel consumption, the smoke value, and the concentrations of NOx/CO/HC. The results showed that the combustion behaviors vary with the variation of oxygen mass fraction in the blends. Increasing the oxygen mass fraction in the blends will lead to the improvement of the diffusive combustion phase due to the oxygen-enrichment, shortening the diffusive combustion duration and the total combustion duration, making a fast combustion process, moving the center of heat release curve close to the top-dead-center and increasing the effective thermal efficiency. The addition of oxygenates can remarkably decrease the smoke concentration without increasing NOx concentration, presenting a plat trade-off curve for smoke and NOx. The reduction rate of smoke decreases with the increase of oxygen mass fraction in the blends. Combination by postponing fuel injection timing and using diesel-oxygenate blends can reduce smoke and NOx simultaneously. CO and HC concentrations decrease with the addition of oxygenate in the blends. The contribution of this study is to develop the combustion theory and to clarify the emission characteristics of the diesel engine fuelled with diesel-oxygenate blends through the systematic and quantitative research, and to provide the theoretical and experimental supporting for the development and the utilization of diesel-oxygenate blends.(2) The ignition delay of the diesel engine operating on the diesel-dimethoxymethane blends was studied. A new correlation of the ignition delay for the diesel-dimethoxymethane blends is obtained. The results revealed that the available correlations cannot correctly predict the ignition delay of the diesel-dimethoxymethane blends without taking into account the effect of oxygen fraction on the ignition delay. The new correlation by this study is useful to predict the ignition delay of the diesel-dimethoxymethane blends. The contribution of this study is to propose the correlation for calculating the ignition delay of the diesel-dimethoxymethane blends.(3) By using a constant volume bomb, the transient spray characteristics of the diesel-dimethoxymethane blends and the diesel-ethanol blends were investigated with the high-speed photography and the schlieren system. The spray developing process of the diesel-ethanol blends under high temperature and pressure was simulated by using commercial CFD software, AVL Fire. The relationships between the spray angle or penetration and the dimethoxymethane fraction in the blends, the ambient pressure and the diameter of nozzle orifice were clarified. The relationships between the spray characteristics under high temperature and pressure and/or the ethanol fraction in the blends were determined through the simulation study. The results showed that the spray angle increases and the spray penetration decreases with the increase of dimethoxymethane fraction in the blends. The spray angle increases while the spray penetration decreases with the increase of the ambient pressure. The correlation between the spray angle and the ambient pressure is in the form of 0.15θ∝Pa. The spray penetration has a liner behavior versus the time during the early stage of spray development, while an exponential relationship between the spray penetration and the time s∝tn is presented after break-up timing. Both the spray angle and the spray penetration increase with the increase of the diameter of nozzle orifice. The contribution of this study is to clarify the effect of the addition of dimethoxymethane in diesel on the spray characteristics, the relationships between spray penetration versus time for the unbroken spray phase and the break-up spray phase, and the relationship between the spray angle and the ambient pressure.(4) The thermal properties of the diesel-dimethoxymethane blends, the diesel-diglyme blends, the diesel-dimethyl carbonate blends, the diesel-diethyl carbonate blends and the diesel-diethyl adipate blends are studied. The effects by addition of the oxygenates on the thermal properties such as the density, the surface tension and the viscosity are clarified. The thermal properties of the diesel-oxygenate blends were measured over the whole composition range. Based on the experimental data, the excess thermodynamic properties were calculated. Polynomial correlations were fitted for the excess thermodynamic properties. The results show that the estimated values in densities, surface tensions and viscosities according to oxygenate mass fraction in the blends give lower values comparing with those of experimental data. The difference between the estimated values and the experimental data in density is smaller than that in surface tension and viscosity. The contribution of this study is to clarify the relationship between the thermal properties of the diesel-oxygenate blends and oxygenate mass fraction in the blend.