Commercial Bus Emissions and Fuel Use

This paper summarizes the results of emissions testing conducted on six motor coaches with engines between model years 1997 and 2004. Each bus was tested while idling and also while being driven in simulated low-speed urban traffic. The intent of this program was to evaluate the potential effects of idle restriction policies on coach buses, particularly when coaches are forced to circulate in urban traffic to maintain appropriate cabin temperatures if restricted from idling. All tested buses used more fuel and emitted more nitrogen oxides (NO_x) when being driven in simulated urban traffic than when idling. Fuel use at least doubled for all tested buses when driven compared with idling. For older coaches, NO_x also doubled, whereas for newer coaches NO_x emissions increased by 40%. If a coach bus is forced to circulate in traffic to maintain appropriate cabin temperatures, rather than idling while stationary, it will use up to 375 more gallons of fuel and emit up to 22 lb of excess NO_x annually for only 1 h/day of circulating. Both fuel use and NO_x emissions generally increased for all buses when the air conditioning was on, compared with when it was not, both while idling and while being driven. In this test program, the increase was less during urban driving than while idling, but given the relatively low ambient temperatures during the testing, the results may not be fully reflective of actual summer results in many parts of the country. For both idling and urban driving, the two newest buses (2004 engines) produced significantly less NO_x than the four older buses. This finding is consistent with more stringent U.S. Environmental Protection Agency emissions standards for newer engines.