| Various biomass fuels have been widely used. The length of carbon chain and oxygen function groups will bring about different physical and chemical properties of fuels, which greatly affect the combustion progress and emission characteristics. Ethanol and dimethyl carbonate（DMC） of short chain were used as substitute fuels to study the impact of carbonyl and hydroxyl on combustion process in this paper. Based on the molecular structure, and polarity of ethanol, DMC and diesel and the principle of similar compatibility, the diesel fuel blends with different mixing proportions were prepared. The fuel blends are as follows: E0D15（15% DMC+ 15% diesel）, E7.5D15（7.5% ethanol+ 15%DMC+ 77.5% diesel）, E15D15（15% ethanol+ 15% DMC+ 70% diesel）. The combustion and emission characteristics of a diesel engine fueled with DMC/ethanol/diesel blends were studied. The equivalent specific fuel consumption, cylinder pressure, heat release rate and exhaust emissions were analyzed at various loads. Besides, the free action radicals profile at the low temperature reaction stage and the characteristics of some important intermediates were studied based on detailed chemical kinetics. The main work and achievements are as follows:（1） The combustion process and emissions characteristics of oxygenated DMC/ethanol/diesel fuel blends at various loads have been analyzed. At low loads, the temperature in cylinder was reduced because elthanol and DMC would absorb more heat for vaporization. Therefore the equivalent specific fuel consumption was increased slightly. At high loads, more combustible mixtures were formed because fuel blends with ethanol and DMC of low boiling point started to evaporate at lower temperature, which results in rapid combustion and high thermal efficiency. Therefore, the equivalent specific fuel consumption was declined. At the full load, the temperature in cylinder was declined with the increase of the proportion of ethanol and DMC because the oxygenated feul blends adsorb more heat for vaporization, which leads to longer ignition delay. Then the pressure and heat release rate profiles gradually move backward. At the same time, pressure peak was declined while the heat release peak rised progressively.（2） The smoke emissions were decreased obviously with the increase of ethanol and DMC. At low loads, the NOx emissions of different fuels were closed. But at high loads, the NOx emissions of blended fuels were increased obviously compared with diesel. The HC and CO emissions of E0D15 were less than that of diesel. At different loads, the HC and CO emissions of E7.5D15 and E15D15 were increased. And HC and CO emissions were increased more obviously at low loads.（3） The combustion process of blended fuels were simulated based on Chemkin. With the increase of ethanol and DMC, the peak pressure and maximum temperature in cylinder were decreased. Besides, the pressure and temperature profiles gradually moved backward. According to detailed reaction mechanisms, at the low temperature reaction stage, OH· were generated by dehydrogenation of n-heptane. However, the active OH· was absorbed by ethanol and DMC, and then turned into H2O2, which hold the chemical reaction process slowly. As a result, the ignition delay was prolonged.（4） The finial CO emissions were greatly affected by the temperature in cylinder and the concentration of OH·. On one hand, with the increase of ethanol and DMC, the temperature in cylinder declines, which produces more CO emissions. On the other hand, although more H2O2 were generated in cylinder for ethanol-DMC-diesel fuel blends, they were dissociated into active OH· at high temperature. Then CO can be oxidized by OH· and transformed into CO2. Blended fuels can bring about more O in the reaction system, which triggers to more thermal NO. Because the bond energy of carbonyl with double bond is greater than that of hydroxyl, the hydroxyl can be broken out from the ethanol molecular and assists to produce more NO emissions than carbonyl. |
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