Study On The Influence Of DOC+DPF System On PM And Gaseous Emissions Of Diesel Engine

Diesel engines have excellent power and economy,but their emission pollutants have limited the development of diesel applications.To meet the increasingly stringent emission regulations,aftertreatment devices have become indispensable.Various exhaust aftertreatment technologies such as Diesel Oxident Catalyst（DOC）and Diesel Particulate Filter（DPF）can effectively remove pollutants such as Particulate Matter（PM）,Hydrocarbon（HC）and carbon monoxide（CO）.In this paper,the experimental study of diesel engine emission characteristics was conducted on a four-cylinder diesel engine with DOC and DPF to investigate the changes of diesel engine carbon and oxygen compounds,NO_xand PM through DOC and DPF.The components,oxidation characteristics and microscopic morphology of PM after DOC and DPF were investigated by using Thermo-Gravimetric Analysis（TG）and Scanning Electron Microscope（SEM）.The main research contents of the paper are as follows.（1）A diesel engine test rig equipped with DOC+DPF after-treatment system was built to study CO,CO₂,O₂and NO_xemissions of diesel engine under different load conditions.The effects of DOC and DPF on CO,CO₂,O₂and NO_xemissions were investigated by using a flue gas analyzer at a rated speed of 2500 r/min,and the effects of load on exhaust temperature,internal temperature of DOC and internal temperature of DPF were investigated by using a temperature patrol meter.The results show that under diesel engine load greater than 50%working condition,the complete oxidation of CO can be achieved when the internal temperature of DOC reaches 287℃or above,and DOC can increase the oxidation rate of NO,thus promoting the passive regeneration of DPF.Diesel exhaust temperature and DOC internal temperature have a linear relationship,and the increase of diesel engine load will intensify the exotherm during the passive regeneration of DPF,so that the internal temperature of DPF is gradually higher than the internal temperature of DOC.（2）To investigate the effects of DOC and DPF on the quantity and mass emissions of PM from diesel engines,the diesel engine exhaust before DOC,after DOC and after DPF were diluted by diluter under different load conditions at rated diesel engine speed,and the engine exhaust particle size spectrometer（EEPS）was used to measure the quantity and mass emissions of PM before DOC,after DOC and after DPF.The quantitative and mass emissions of PM before DOC,after DOC and after DPF were measured using an Engine Exhaust Particle Sizer（EEPS）.The changes of PM quantity and mass before and after DOC were compared with those before and after DPF to investigate the removal pattern of PM emissions from different diesel engines by DOC and DPF.The results showed that the quantity emission of PM in the nucleated and aggregated states was normally distributed with particle size except for the working condition of12.5%diesel engine load;the lowest PM quantity emission was achieved when the diesel engine load was 50%;the removal rate of quantity emission of PM by DOC ranged from 17%to 27%;the removal rates of nucleated particles,aggregated particles and ultrafine particles by DOC and DPF were at the diesel engine load of 50%The lowest removal rate also reached more than 85%.the removal rate of total quantity concentration of PM by DPF exceeded 90%,and PM after DPF was dominated by nuclear particles.（3）In order to investigate the effects of DOC and DPF on the volatile fraction（VF）and elemental carbon（EC）in PM,the PM before DOC,after DOC and after DPF were sampled by filter paper sampler at the diesel engine speed of 2500 r/min and the load of 25%,50%,75%and100%,respectively.The PM before DOC,after DOC and after DPF were sampled by filter paper sampler,and the composition of PM components,volatile properties of VF and oxidation properties of EC were investigated by TG analysis.The microscopic morphological changes of PM after DOC and DPF were investigated by scanning electron microscopy.The results showed that the EC content in PM decreased significantly after DPF;the EC content in PM increased with the increase of diesel engine load;the starting temperature,maximum weight loss temperature and final combustion temperature of EC increased,the oxidation activity decreased and the oxidation difficulty of PM increased;the surface roughness of PM increased after DOC and the particle size of PM decreased after DPF.