| According to the fuel design concept,the application of alternative fuels is an effective way to reduce engine emissions while maintaining higher thermal efficiency.In this work,the influences of different oxygenated fuels on engine performance and emissions were studied experimentally using multi-cylinder engine,optical engine,and constant volume combustion vessel combined with optical diagnostic technique,and numerically based on Kiva-Chemkin simulation.A practical strategy of blending oxygenated fuels was developed for improving engine efficiency and significantly reducing criteria pollutant emissions.Firstly,the characteristics of particulate emissions from an engine fueled with different biodiesel blending ratios were studied using a China-IV diesel engine.Then the combustion and emission characteristics of pentanol in a direct-injection diesel engine were compared with the diesel fuel.Results showed that NO_x and soot emissions decrease simultaneously for pentanol.Moreover,experiments were conducted on a diesel engine to evaluate the effects of various fuel design schemes on combustion characteristics and emissions using biodiesel and pentanol.Biodiesel has higher cetane number,viscosity,and boiling point.While pentanol has lower viscosity and higher volatility that improves atomization and higher oxygen content that is beneficial for soot reduction.Experimental results show that biodiesel-pentanol-diesel with proper blending ratio can achieve fuel properties ideal for internal combustion engines.The NO_x and Soot emissions can be significantly reduced while achieving higher thermal efficiency than the conventional diesel fuel.Secondly,the spray characteristics for different oxygenated fuels were investigated in a constant volume vessel using Phase Doppler Particle Analyzer(PDPA).The results showed that higher ratio of pentanol led to shorter spray penetration,larger spray angle,and smaller sauter mean diameter(SMD),while biodiesel showed opposite trends on spray characteristics.The results showed that the addition of pentanol fuel can improve theatomization of the biodiesel-pentanol-diesel blends.Meanwhile,the soot formation characteristics of various blends were studied using laser induced incandescence in both constant volume vessel and optical engine.The results indicated that longer flame lift-off length and decreased peak soot concentration were achieved with increasing proportion of the pentanol.As a result,the biodiesel-pentanol-diesel blends is an ideal fuel for soot reduction due to the improved fuel atomization and fuel-air mixing.Finally,a multi-component biodiesel reaction mechanism combined with a multi-step phenomenological soot model was developed for KIVA-CHEMKIN simulation.This model was used to simulate the combustion and emission characteristics for biodiesel with different blending ratios and saturation levels.The impact of oxygen content and saturation level on soot formation was well predicted using this model.Number analysis showed that,compared with diesel,biodiesel reduced soot mass concentration primarily due to the suppressed soot formation process and improved oxidation rate.The soot mass concentration of biodiesel was proportional to the content of the unsaturated fatty acid methyl ester.The simulation results were in good agreement with the trends observed in the engine experiments.A reduced mechanism with 51 species and 152 reactions was developed to simulate the combustion process of pentanol.Extensive validation was carried out and the results showed that the mechanism well predicted the ignition delay timings,the major species concentration,and the laminar flame speeds of pentanol.The pentanol model was then integrated into KIVA,which predicted the combustion characteristics of HCCI engine with high accuracy.The proposed reduced reaction mechanism is helpful to develop the heptane-biodiesel-pentanol mechanism in the future and further investigate the impact of different oxygenated fuels on the soot emissions. |
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