Emissions Characteristics And Evolution Principle Of Carbonaceous Particles From Diesel Engines

With the increase in the vehicle population in China,carbonaceous particles emitted from the vehicles become a major source of the atmospheric PM,so that effective methods are strongly needed for the control of the carbonaceous particles pollution.Meanwhile,the non-renewable nature of the fossil fuels requires the usage of the alternative fuels,and thereby,to seek effective and clean alternative fuels also becomes the main task in order to solve the current energy and environmental issues.The present study has fully investigated the effects of oxygenated fuels on the combustion and emissions characteristics of a diesel engine,and has further revealed the physical revolution and chemical aging mechanisms of the exhaust carbonaceous particles near the tailpipe so as to provide abundant data and theoretical support for the atmospheric PM control as well as the usage of the alternative fuels.The present thesis is mainly composed of the following parts:?1?Effects of the two types of the oxygenated fuels on the combustion and emissions characteristics of the diesel engine have been investigated based on the engine bench experiments and the results have also been compared with those obtained from the neat diesel fuel.Meanwhile,a new on-line method to estimate the concentration of the total organic compounds has been proposed by using three independent measurements,namely,TEOM,OC/EC analyzer and TD-SMPS.It is observed that,particle number and mass concentration from the oxygenated fuels are decreased as compared with the results obtained from the neat diesel fuel.However,the volatile organic compounds are increased with the addition of the oxygenated fuel,indicating that the carbonaceous particles emitted from the oxygenated fuels have a higher volatility.The three measurements on the particle emissions are found consistent with each other,and the combination of the above three measurements can realize the on-line measurement on the concentration of the particle-phase organic compounds.?2?Identification and emissions of the carbonaceous organic aerosols emitted from the above oxygenated fuels have been studied and compared based on the particles sampling method,and the toxicity of the exhausted carbonaceous particles has also been evaluated based on the B?a?P toxicity equivalency factor.It is observed that,both of the DB blends and the DBE blends have an effective suppression impact on almost all the measured particle-phase n-alkanes,but present a promotional impact on the short carbon chains compounds.Particle-phase PAHs and nitro-PAHs are found suppressed by using the oxygenated fuels,but a promotional impact on the total measured oxy-PAHs is observed,on which more concerns will be needed.Both of the DB blends and the DBE blends can effectively reduce the toxicity equivalency concentration of the exhausted carbonaceous particles.Additionally,ethanol fuels are found more effective on reducing the toxicity than the biodiesel fuels,and hence,when using the alternative fuels in the future,a suitable amount of ethanol fuels can be added so as to reduce the toxicity of the carbonaceous particles.?3?Based on the dilution system,effects of the engine load,the dilution ratio,the TD temperature and the fuel type on the gas-particle partitioning of the carbonaceous particles at the near tailpipe region have been investigated with the help of absorption and adsorption mechanisms.Meanwhile,the X_P-DR and the MFR-TD models have been proposed to reveal the principle of the phase partitioning of the diesel carbonaceous particles.It is observed that,the engine load that can affect the total organic carbon emissions and the OC/EC ratio is considered as the dominated factor for the gas-particle partitioning of the semivolatile organic compounds.At the near tailpipe dilution condition,the phase partitioning of the organic aerosol is dominated by the adsorption mechanism and the adsorption model of the X_P-DR can well predict the partitioning results.The partitioning principle of the carbonaceous particles emitted from the DB blends and the DBE blends is similar with that obtained from the neat diesel fuel,but the specific adsorption results of the oxygenated fuels are significantly different from those of the neat diesel fuel.The volatility distribution of the organic compounds is the major factor to determine the temperature sensitivity of the carbonaceous particles and the carbonaceous particles with a high volatility feature are found more sensitive to the temperature.The MFR-TD model can predict the temperature sensitivity of the carbonaceous particles to some extent,but is still partially different from the actual experimental results.?4?Based on an atmosphere-pressure flow reactor,the surface chemical aging process of the carbonaceous particles in the presence of NO₂ has been investigated to reveal the heterogeneous nitration mechanisms occurring on the carbonaceous particles in the dark and under the sunlight.It is observed that,the reaction order of NO₂ for the surface aging reactions of the carbonaceous particles is close to first-order,with HONO and NO as the major gaseous products.Our findings support the‘red-ox'mechanism that the carbonaceous particle can itself provide an H atom to support the reaction,inducing the formation of HONO.Moreover,the introduction of the visible light can additionally provide energy of hv,which can excite the majority of the active sites on the surface of the carbonaceous particles to a higher adsorption and reaction reactivity.In the meanwhile,PAHs can also absorb the energy to enhance their reactivity,but the newly-formed nitro-compounds will be decomposed under the light with more HONO formation.Overall,the photochemistry of the carbonaceous particles in the presence of NO₂ is of great importance during the surface aging process of the carbonaceous particles,because it is not only a major photochemical source of HONO,but also a dominated aspect to determine the fates of PAHs and nitro-PAHs on the carbonaceous particles,further affecting the atmospheric chemistry balance as well as the toxicity feature of the carbonaceous particles.