Numerical Investigation On The Combustion And Emission Characteristics Of Hydrous Ethanol Gasoline On A SI Engine

With the growing scale of automobile industry and fuel consumption,the pollutions within the emissions of vehicles are one of the most dominating factors that harm the environment.Adding ethanol into gasoline at certain proportion can not only reduce the using of gasoline but also improve the octane number and flame ability of the mixture in turn.The more completed combustion of ethanol may lower the emission as well.Nevertheless,the removal of water in the production of pure ethanol increase the energy cost due to the strong hydrophilicity of ethanol.A more economical and promising way is making use of hydrous ethanol gasoline.To make up for the deficiency of engine bench test research,in this thesis,the three-dimensional numerical simulation method is adopted to investigate the combustion and emission characteristics of hydrous ethanol gasoline on a SI engine.A model based on three-dimensional CFD software CONVERGE was developed coupled with the skeleton mechanism of chemical reactions.This model was validated against experimental results and then applied for simulation of the performance of ethanol/hydrous ethanol gasoline with blending rate from 0 to 40%at speeds of 1200r/min,1600r/min and 2000r/min.The results showed that,at low speeds and low engine loads,the flame propagation speed dominated the cylinder pressure?P?and the heat release rate?HRR?which increased with the rate of ethanol in the fuel.At the same blending rate,the P and HRR seemed to reduce with hydrous ethanol addition.With the rising of speed and load,the effect of the low temperature reactions among the unburned mixture became obvious.The combustion rate of pure gasoline was faster than other ethanolic fuels with high intense low temperature reaction rate.At high blending ratio,hydrous ethanol gasoline accumulated more hydrogen peroxide?H2O2?through low temperature reactions which further decomposed into OH radical and stimulated the combustion process against the unhydrous fuels.Moreover,advanced spark timing and increased intake temperature turned out more effective improvement on the combustion of hydrous ethanol gasoline.The HC emission increased with the blending rate and became higher for hydrous ethanol gasoline.The rising load blurred the distinction between hydrous and unhydrous fuels.The CO emission increased with the blending rate at low and medium loads while reduced at high engine loads.The NOx emission first decreased and then lifted up with blending rate at all working conditions.Hydrous ethanol gasoline had higher CO emission but lower NOx emission.Moreover,with higher content of ethanol,the emission of formaldehyde?CH₂O?got down while a rising tendency was observed for unburned ethanol and acetaldehyde?CH₃CHO?.The emission of methanol kept lower and steady.All in all,hydrous ethanol addition increased all the unregulated emissions while lowered the NOx emission.In the end,the combustion stability of different fuels was tested both experimentally and numerically.The results showed that the LES model adopted in this study could reproduce the variations between cycles to a certain extent.Hydrous ethanol improved the stability of the engine performance which mainly attributed to the antiknock property and flame speed of ethanol.