Research On F-T Diesel Fuel Combustion And Particulate Matter Affected By EGR

The emission of pollutants from diesel engines is one of the main causes of air pollution.There are some technologies effective for reduction of emission from diesel engines such as clean alternative fuels,in-flight purification,exhaust gas recirculation?EGR?,and exhaust aftertreatment.Fischer-Tropsch diesel?F-T diesel?is a liquid fuel prepared by indirect liquefaction synthesis of coal.F-T diesel fuel is almost free of aromatic hydrocarbons and sulfur.F-T diesel fueled in the diesel engine can reduce particulate matter?PM?.The diesel engine adopts EGR,which can effectively reduce the NO_x of the diesel engine.The use of EGR technology also has an impact on the combustion of F-T diesel and formation of particle matter.Taking F-T diesel fueled in the diesel engine adopting EGR as the research object,the combustion process of diesel engine,the characteristics and formation of particulate matter was studied through numerical simulation and experiment.The influence of EGR on the combustion and formation of particle from the diesel engine fueled with F-T diesel was analyzed.The test bench based on a single-cylinder diesel engine was built.The dynamometer and gas emissions were measured at different EGR rates at 25%,50%,and 75%loads.The effect of EGR on the combustion process and gas emission of diesel engine fueled with F-T diesel was analyzed.The application of EGR resulted in a slight decrease in the maximum of burst pressure,rate of pressure rise and instantaneous heat release rate when diesel engine was fueled with F-T diesel.The ignition delay period was prolonged gradually,and the combustion center of gravity lagged behind,and the combustion duration had no significant changes with the increase of EGR rate.Compared with the non-EGR,the peak of burst pressure,pressure rise rate and the instantaneous heat release rate were reduced by 6.5%,26.7%and 5.7%respectively,and maximum combustion temperature in cylinder was reduced by 200 K,and ignition delay period was prolonged by 15.4%,and the combustion center of gravity of was shifted by 2.4°CA when the diesel engine was at 75%load with 30%EGR rate.The maximum decrease of NOx was 61.6%,and the maximum increase of HC and CO were 13.2%and 13.3%respectively when the EGR rate increased from 0 to 30%.The size and number density of the particles were measured by particle size spectrometer.The test result showed that the particle size of F-T diesel combustion particles is a single-peak logarithmic normal distribution.The peak particle size decreased at 25%load and increased at50%and 75%loads when the diesel engine adopted EGR.The number density of nuclear particle between 5nm and 50nm increased at 25%load and the number density of aggregated particle between 50nm and 1000nm increased at 50%and 75%loads.The peak particle size increased,and the number density of nuclear particle decreased and that of aggregated particle increased because of the increase of N₂.There was an opposite law of N₂ in the particle size and number density of modal particle caused by the increase of CO₂.The morphology and elemental composition of the particles was studied by scanning electron microscopy and X-ray energy spectrometer and the box-count dimension was calculated to study the density of the particle.The result showed that the number of chain-like particles in aggregated particles and the element of oxygen?O?increase gradually,and the count of box count and the content of carbon?C?decrease with the increase of EGR rate at 25%load,which was contrary to the law of particle at medium and high loads.The increase of N₂ in the exhaust gas resulted in an increase of carbon element and an decrease of oxygen element.There was an opposite law by the increase of CO₂.The weight loss curve of the particles with temperature was measured by thermogravimetric analyzer.The test result showed that the content of SOF increased and the content of solid soot decreased gradually,and oxidation activity increased gradually with the increase of EGR rate at 25%load.The content of SOF decreased and the content of soot increased gradually,and oxidation activity decreased gradually with the increase of EGR rate at 50%and 75%loads.The increased proportion of N₂ in the exhaust gas component reduced the oxidation activity of the particles,that of CO₂ resulted in the increase of the oxidation activity of the particles.Based on the theoretical analysis method and the physical and chemical properties of F-T diesel,the chemical reaction mechanism of PAHs generated by F-T diesel was constructed,which was verified by the shock tube reactor model in the CHEMKIN software.The main elementary reaction of PAHs was determined by the generation rate analysis method.The effects of equivalent ratio,EGR rate and exhaust gas composition on the formation of typical PAHs such as benzene,naphthalene,phenanthrene and anthracene were analyzed.The effect of EGR on the chemical reaction pathway of PAHs was discussed.The results showed that EGR mainly promotes the hydrogenation reaction of phenyl,naphthyl,phenanthryl and anthracenyl groups by diluting oxygen in the mixture,and inhibits the oxidation reaction of PAHs and OH,which promoted the generation of PAHs.The CO₂ in the exhaust gas promoted the oxidation reaction of benzene,naphthalene,phenanthrene and anthracene with OH and H.The N₂ in the exhaust gas promoted the hydrogenation reaction of phenyl,naphthyl,phenanthryl and anthracenyl groups.The research work provided a reference for the application of F-T diesel in diesel engines to reduce NO_x and particles from diesel engines.