| In this research, regulated pollutants from buses with different fuel (diesel, CNG) andsatisfying different emission standards, including hybrid diesel buses were tested on roadusing Portable Emission Measurement System (PEMS). CO2-based averaging windowmethods were used to assess whether these buses meet the emission standards in real-worldconditions.As a calculating methods for real-world emission standard, Not-To-Exceed (NTE)method needs the engines to operate with higher speed and higher load, in order to creatNTE events. But this requirement was difficult to be satisfied in Beijing city, because of thelower average speed of city buses. In addition, instantaneous torque and speed of enginewere difficult to get in real-world conditions in China. Hence, CO2-based averagingwindow method was used in this research as a judgement whether buses passed the real-world emission standards, and emission limits were established.CO, THC and PM emissions from China National-â…£ and National-â…¤ diesel buseswere lower than those from National-â…¢ diesel buses. But because of the increase of fuelinjection pressure and decrease of accumulation mode particles, particle numbers fromNational-â…¤ buses were higher. The lower average speed in Beijing city lead to lowerexhaust temperature of diesel engines and lower efficiency of SCR system. As a result, NOxemissions from National-â…£ diesel buses were higher than National-â…¢ diesel buses inurban. The NOxemissions from National-â…£ diesel buses were lower than National-â…¢diesel buses only when Vehicle speeds were higher than30km/h. National-â…¤ busesemitted lower NOxthan National-â…£ buses, but their NOxemissions were higher thanstandard limit when vehicle speeds were lower.Because of the presence of electric motor in hybrid buses, power output of dieselengines was lower, which decreased the exhaust temperature and the efficiency of SCRsystem. This lead to higher NOxemissions from parallel-hybrid buses. For complex-hybridbuses, because of the start-stop control, the exhaust temperature were even lower.The National-â…£ CNG buses which used stoichiometric combustion mixture and TWCwere able to stafisty the Jing-â…£ real-world emission standard, but the inaccuracy of theair-fuel ratio and the aging of the catalyst lead to different emission performance betweenbuses and higher NOxemissions from some buses. The EGR system in stoichiometricengine reduced the NOxemissions and made the buses had the ability to meet the Euro-â…¥ standard. Lean-burn CNG engine with only Oxidation Catalyst (OC) can meet the Jing-Vemission standard on road, but they emitted more NOxemissions than diesel buses whenvehicle speeds were high. Lean-burn CNG buses with OC and SCR systems emitted verylow NOxemissions because of the higher efficiency of SCR systems, which can beattributed to the higher exhaust temperature and higher NO2/NOxratio in CNG exhaust.The presence of thermal insulation layer raised the temperature and SCR catalyst andhence increased the efficiency, which reduced the NOxemissions from National-â…£ dieselbuses. Compared with Urea-SCR system, Solid SCR system effectively reduced NOxemissions in low-temperature and low-speed conditions. In addition, the low temperatureefficiency of SCR system can be further increased using Cu-Fe zeolite catalyst. NOxemission from diesel buses with SCR system can be reduced when specific power (enginemaximum power divided by vehicle mass) was reduced.Beijing bus typical cycle was extracted from the vehicle speed got in the tests.Emission rates from different kinds of buses were calculated and numerical model wasestablished using VSP and vehicle speed as parameters. Emissions factors of different kindsof buses in Beijing city can be calculated using the cycle and numerical mode... |
PDF Full Text Request