Impact Of Butanol/Gasoline Blends On The Physical And Chemical Properties Of Particle Emissions Derived From Spark Ignition Direct Injectin Engines

PN(Particle Number)emission has become the primary concern for SIDI(Spark Ignition Direct Injection)engines,due to the low particle mass production but large number of ultrafine particles.In order to meet the requirement of increasingly stringent particle emission limit,butanol was selected as gasoline additive in this research for PN reduction.A brief summary of this research is shown below:Firstly,in view of the advantages in fuel properties and potential production of sec-butanol,iso-butanol and tert-butnaol compared with n-butnaol,the impact of the four butanols on PN emission on a SIDI engine were studied,which followed the trend:sec-Butanol/gasoline < n-butanol/gasoline < iso-butanol/gasoline < tert-butanol/gasoline < gasoline.And this trend remained constant at different blending ratios,loads,injection timings,spark timings,equivalence ratios and EGR rates.At a certain operating condition,variations in PN emissions with the addition of butanol isomers was mainly caused by their sooting tendencies and fuel-air mixing properties.Despite the higher sooting tendency of sec-butanol compared with n-butanol,its lower surface tension and latent heat could improve the fuel-air mixing,leading to lowest PN emission in engine test.Secondly,in consideration of the lack of particle toxicity index under present emission legislation,the impact of butanol isomers with EGR on the concentration and toxicity of PAHs were investigated.Total PAHs concentration was affected by the sooting tendency of fuel,the fuel-air mixing and the combustion temperature,which followed the trend: n-Butanol/gasoline < sec-butanol/gasoline < iso-butanol/gasoline < tert-butanol/gasoline < gasoline.The cooling effect of EGR was undesirable for PAHs reduction.The equivalent B[a]P concentration(μg/m3)followed the same trend as total PAHs concentration,indicating the lowest toxicity of n-butanol/gasoline.However,the equivalent B[a]P concentration per particle mass(%)was related to the fraction of five and six aromatic rings,meaning that tert-butanol addition,retarding injection timing and EGR was positive for reducing this toxicity index.Thirdly,in view of the low oxygen concentration and low exhaust temperature for GPF regeneration,the impact of butanol isomers on soot oxidative reactivity was further investigated.Soot particles with less ordered structures favored to form at lower combustion temperatures.Although the fractal dimension would increase at lower temperatures,however,soot oxidative reactivity shows more correlation with the nanostructure of soot primary particles rather than the fractal dimension.As a result,soot oxidative reactivit follows the trend: Gasoline < n-butanol/gasoline < iso-butanol/gasoline < sec-butanol/gasoline < tert-butanol/gasoline.According to the above results,the physical and chemical properties of particulate emissions are mainly affected by three factors of butanol isomers: Sooting tendancy,combustion temperature and fuel physical properties.For n-butanol,the low sooting tendancy may inhibit PAHs and soot formation,while the high combustion temperature promotes the oxidation of PAHs and soot,leading to low PN emission and low PAHs concentration,but high equivalent B[a]P concentration,ordered soot structure and low soot oxidative reactivity.On the other hand,the higher latent heat and higher surface tension of n-butanol lead to the less favored fuel-air mixing properties among the butanol isomers,which give rise to the higher PN emission compared with sec-butanol.PAHs emissions and soot structures are more sensitive to combustion temperature,and seem to be less affected by fuel physical properties.As a summary,sec-butanol has the most desirable emission control effect among the four butanol isomers,due to the reduction in PN emission,PAHs concentration,equivalent B[a]P concentration,and the improvement in soot oxidative reactivity.At optimized injection timings,the addition of sec-butanol coupled with EGR would achieve improved fuel economy and emission control durability,which are favorable for the high efficiency and clean combustion of SIDI engines.