Impacts of MTBE in gasoline on the tailpipe ammonia emissions from China-6 certified passenger vehicles

Postcatalyst ammonia emissions from gasoline vehicles recently received attentions because of their contribution to the formation of urban secondary aerosols. To better understand whether fuel formulation would curb ammonia, the influence of methyl tertiary-butyl ether (MTBE) additive in gasoline on the tailpipe ammonia emissions from six China-6 compliant vehicles was investigated over the World Harmonized Light-duty Test Cycle (WLTC) at −7 °C and 23 °C. Ammonia emissions were measured with MTBE-free and 10 % MTBE-containing China-6 compliant gasoline fuels on the test vehicles. At – 7 °C, the ammonia emissions with and without MTBE addition ranged from 0.15 – 17.07 mg/km and 0.81 − 25.13 mg/km, respectively. At 23 °C, ammonia emissions were 0 − 7.4 mg/km for MTBE-containing fuel and 0 − 8.76 mg/km for MTBE-free fuel. More ammonia emissions were often observed as a result of maneuvering of enriched air-fuel mixtures to improve the combustion stability after cold-start and for de-NOx purpose. Due to the oxygen-containing nature of MTBE, its addition favors ammonia reduction in cold-start tests at – 7 and 23 °C. It is noted that “λ sweep”, an engine control strategy injecting extra fuel to deplete stored oxygen in the catalyst and suppress transient NOx formation during reacceleration after prolonged deceleration, may contribute to ammonia emissions due to the use of enriched air-fuel mixtures.