Experimental Study On The Improvement Of Engine Combustion And Emission Characteristics By Partially Hydrogenated Biodiesel

Biodiesel,as a kind of renewable and clean fuel,has received extensive attention of many experts and scholars.However,because of the higher unsaturation of biodiesel,its oxidation stability is worse and storage period is affected.The partial hydrogenation of biodiesel,which the high unsaturated fatty acid esters are selectively converted to the low unsaturated or saturated fatty acid esters,has been used to improve and enhance the oxidation stability and cetane number of biodiesel effectively.The partially hydrogenated soybean methyl ester(PHSME)is produced from soybean methyl ester(SME)via catalytic transfer hydrogenation.The fuel properties of before and after hydrogenated biodiesel are measured or calculated by suitable methods.Based on the analysis of thermogravimetry-differential scanning calorimeter(TG-DSC),the oxidation and combustion dynamic characteristics of upgraded biodiesel are investigated.In addition,the combustion and emission properties of partially hydrogenated biodiesel are studied based on engine tests.The main works and conclusions are as follows:(1)In the water environment,the catalytic transfer hydrogenation of SME has been implemented while using isopropanol as the hydrogen donor and Raney-Ni as the catalyst.It is clearly revealed that when the reaction temperature is(85±1)°C,the stirring speed is 600r/min,the reaction time is 100 min and the amount of catalyst is 13%(w/w)of SME,the hydrogenated degree of SME has reached maximum,and the unsaturated degree of SME is decreased by 46.2%.Meanwhile,the relative deviation between the calculated iodine value by gas chromatography/mass spectrometry and the measured iodine value by chemical titration method is less than 2.5%,so the chemical titration method for the determination of iodine value has good reliability and accuracy.In addition,the conversion rate of hydrogenation reaction is 37.8%.(2)The peroxide value,cetane number,low heat value,kinematic viscosity and melting point of SME(before and after hydrogenation)are measured or calculated by suitable methods.After moderate hydrogenation of SME,the cetane number and low heat value of biodiesel are increased,and its oxidation stability also has been improved significantly.Although the kinematic viscosity and low temperature flow property of biodiesel have been slightly worse,they can still meet the engine requirements if they are used as blending fuels.(3)Under air atmosphere,the oxidation and combustion characteristics of SME and PHSME3 are investigated with comprehensive thermal analyzer(TGA/DSC1).The results show that because the molecular structure and kinematic viscosity of SME after moderate hydrogenation increase,the initial weightlessness temperature of PHSME3 is 20.6°C later than that of SME.However,the finial weightlessness temperature of PHSME3 is 7.2°C earlier than that of SME.It indicates that PHSME3 has the higher average oxidation rate,and it is easier oxidation and combustion.Meanwhile,in the low temperature stage,the DSC curves of SME and PHSME3 present the endothermic peaks because of their volatilization,while the exothermic peaks could be found under higher temperature due to their oxidation and combustion.As compared with SME,PHSME3 has moderately high cetane number and excellent ignition characteristic,so the exothermic onset temperature of PHSME3 can be advanced by 10.7°C.(4)The fuel economy and combustion process of Diesel,B20,PHB10 and PHB20 are explored in diesel engine tests.The results indicate that the brake specific fuel consumptions(BSFC)are increased by 2.9%,1.5% and 2.6% for B20,PHB10 and PHB20 respectively at 25% load,as compared to the reference Diesel,while their BSFC are basically same at high load.Biodiesel has a higher cetane number before hydrogenation,its cetane number is further increase after hydrogenation,which can effectively shorten the ignition delay period and the formation of combination gas is much less before ignition.So the moments of maximum explosion pressure in cylinder and the peak moments of instantaneous heat release rate can be advanced.In the rated operating condition,the moments of maximum explosion pressure in cylinder are advanced by 1.0°CA,0.8°CA and 1.6°CA for B20,PHB10 and PHB20 respectively,as compared with Diesel,and the maximum explosion pressures are reduced by 0.74%,0.73% and 1.69%,and the peak moments of instantaneous heat release rate are advanced by 0.8°CA,0.6°CA and 1.6°CA,and the peaks of instantaneous heat release rate are reduced by 4.05%,4.01% and 7.72%.(5)The emissions of Diesel,B20,PHB10 and PHB20 are explored in diesel engine tests.The maximum emissions of HC are reduced by 9.9%,8.0% and 12.4% for B20,PHB10 and PHB20 in 3600r/min respectively,as compared with Diesel,and the maximum emissions of CO are reduced by 9.3%,4.1% and 13.5%,and the maximum emissions of smoke are reduced by 15.2%,9.5% and 17.1%,and the maximum emissions of NOx are increased by 8.5%,3.1% and 6.7%.In this paper,the physicochemical properties and oxidation characteristics of partially hydrogenated biodiesel are studied.And the fuel economy,combustion process and emission characteristics of partially hydrogenated biodiesel are explored in diesel engine tests.It has a certain guiding value for the preparation and application of new fuels.Meanwhile,it also provide theoretical basis and experimental support to meet clean and efficient combustion and emission reduction of diesel engine.