Abstract： The emission test on four different vehicles was conducted by using onboard test system. Based on the theory of SCR, the reason of higher NOx emission from State Ⅳ conventional bus and hybrid bus equiped with SCR was analysed. By combining the data from the test and the methods of ′idle′ and ′segment′, the relationship between exhaust temperature and NOx emission was studied. The results show that the use of SCR technology can reduce NOx emissions to a certain extent. When the exhaust temperature is between 130 ℃ and 230 ℃, the NOx conversion efficiency increases about 6％ with temperature increase of about 10 ℃. For transit bus with SCR, especially HEV, the proportion of the exhaust temperature above 230 ℃ is very small, which causes the lower conversion efficiency and higher NOx emission. Some technology (e.g. exhaust heating system) should be used or the transformation efficiency under low temperature should be increased.
2011, Vol. 32 
