Can Euro V heavy-duty diesel engines, diesel hybrid and alternative fuel technologies mitigate NO_X emissions? New evidence from on-road tests of buses in China

摘要

Nitrogen oxides (NO_X) emissions are creating significant air quality challenges in China's megacities. Since Euro IV diesel buses equipped with selective catalyst reduction (SCR) systems failed to mitigate on-road NO_X emissions as expected, real-world NO_X emissions from newly introduced Euro V diesel buses, diesel hybrid and alternative fuel (e.g., natural gas) buses are of great concern to policymakers in China. In this study, NO_X emissions from two Euro V diesel, two Euro IV diesel hybrid, nine compressed natural gas (CNG) and two liquefied natural gas (LNG) buses were measured on-road by using portable emission measurement systems (PEMS). The average NO_X emission factor of the Euro V diesel buses was 7.5 ± 0.1 g km~(-1) for a typical driving cycle, 37% lower than the Euro IV diesel buses. However, the average brake-specific emission factor still exceeded the Euro V standard by 180%. The diesel hybrid buses had an average NOX emission factor of 4.4 ± 1.1 g km~(-1), much lower than their conventional diesel counterparts. CNG and LNG buses also had lower NO_X emission factors. The average NO_X emission factor of the LNG buses was 3.2 ± 0.7 g km~(-1), due to the performance of the SCR systems under higher exhaust temperatures. Furthermore, real-world NO_X emission factors for all tested vehicle categories except diesel hybrids were significantly sensitive to changes of average speed. Operation of air conditioning in the bus reduced average NO_X emissions by 38% for diesel hybrid buses although fuel consumption increased. These results suggest hybrid and CNG/LNG technologies are better options than the Euro V diesel engines to mitigate NO_X emissions from urban buses.