Study On Particle Morphology And Mechanical Characteristics Of Diesel Engine Exhaust Process

Diesel particulate emission is one of the main air pollutants,which is harmful to the ecological environment and human health.In the sixth stage of China's pollutant emission regulations,the quality?PM?and quantity?PN?of the emitted particles were put forward more stringent requirements and limits.In the process of diesel engine exhaust,particles went through collision condensation,condensation adsorption and other processes,resulting in changes in particle morphology and size distribution,which directly affects the number of particles in diesel engine.The process of particle collision and coalescence is the result of the interaction between air flow and particles,which is related to the exhaust pressure,air flow speed,temperature and other environmental conditions.The research on the particle morphology and mechanical characteristics of diesel engine exhaust process has certain academic value for revealing the particle coalescence process,providing basis for optimizing the structure of DPF and reducing the number of particles emitted by diesel engine.In this paper,the morphology and mechanical characteristics of the particles formed by diesel combustion were studied.The morphology and mechanical parameters of the particles were measured by means of SEM,TEM and AFM,and the change rules of the morphology,microstructure and mechanical parameters of the particles were analyzed.Through the establishment of CFD-DEM collision simulation model,the exhaust environment of diesel engine was discussed.The change of parameters and the dynamic characteristic parameters of particle impact process in exhaust stage.The main research work was as follows:A diesel engine test-bed was set up to collect particles under different loads,rotating speeds and exhaust temperatures.The micro morphology,particle size distribution and box dimension of particles were measured and calculated by SEM.By means of high power transmission electron microscope,the variation rules of the parameters such as layer spacing,crystallite size and curvature of particles were analyzed.The results show that with the increase of diesel engine load,the cluster particles increase obviously,the arrangement structure of particles is more compact,the average particle size,box counting dimens ion and average stacking height of particles increase,the average microcrystalline size of particles increases,and the layer spacing and curvature decrease.With the increase of diesel engine speed,the chain length of particles increases,the box dimension and the average stack height of particles increase;the microcrystalline size of particles decreases,the average layer spacing and curvature increase,but the increase amplitude is not obvious.With the decrease of exhaust temperature,the average particle size,counting box dimension and average stacking height of particles increase;the microcrystalline size of particles decreases,and the average layer spacing and curvature increase.The changes of the mechanical characteristic parameters such as adsorption force,adhesion force,adhesion energy and Young's modulus of the particles in different working conditions of diesel engine were analyzed by AFM.The results show that with the increase of load,the adsorption force,adhesion force,adhesion energy and Young's modulus of particles increase;the distribution of adsorption force is different,mainly in the range of 1.3³.5nN.The distribution range of adhesive force tends to be higher and wider.With the increase of rotational speed,the adsorption force,adhesion force,adhesion energy and Young's modulus of particles all increased;the adhesion force mainly distributed in the range of 13²4nN,and the adhesion energy mainly distributed in the range of 8×10^-1720×10^-17J.The ability of diesel engine emission particles to resist deformation increases with the increase of load.With the decrease of exhaust temperature,the adsorption,adhesion,adhesion energy and Young's modulus increase.The distribution range of adsorption force is gradually narrowe d,ranging from 1.5 to 4.0nN;with the decrease of exhaust temperature,the adhesion of particles is enhanced,the degree of agglomeration is more obvious,and the adhesion between particles is more stable.By using Fluent software,the model of exhaust channel was established,and the change rule of environmental parameters such as pressure and velocity of particles in the process of exhaust was simulated and analyzed.The results show that the pressure is higher at the elbow of exhaust duct.Along the direction from the exhaust valve to the air port,the pressure decreases gradually.When the exhaust valve was opened,the air flow reaches the maximum speed at the elbow.After impinging on the outer wall of the airway,the air flow rebounds in the direction of motion.Based on the similarity theory and the collision simulation model,the collision of particles at the outlet of the diesel engine was simulated,and the dynamic characteristic parameters of particles in the collision process were analyzed under different exhaust pressure,air velocity and particle size.The results show that when the flow velocity is fixed and the particle size is the same,the larger the exhaust pressure is,the larger the angular velocity,turbulent kinetic energy and torque of particles will be,and the larger the normal/tangential collision force of particles will be;when the pressure difference is fixed and the particle size is the same,the larger the air velocity is,the larger the particle's motion velocity will be,and the normal/tangential collision force of particles will be,and the angular velocity,turbulent kinetic energy and The torque increases obviously.With the increase of particle size,the angular velocity,rotational kinetic energy,torque and normal/tangential collision force increase.It is important to reduce the number of particles in exhaust gas by shortening the coalescence time and increasing the coalescence rate.The results show that the exhaust process of diesel engine has an important influence on the morphology,mechanics and collision process of particles.Increasing the exhaust pressure,air flow speed and other environmental parameters makes particles collide more frequently;reducing the exhaust temperature can increase the adsorption force and adhesion force of particles,and makes particles agglomerate more closely,which is conducive to reducing the number of exhaust particles.