U of M researchers have developed a way to identify the exact location of “hot spots” for air pollutants created by transit buses—work that could be used to create new strategies for addressing emission hot spots in the future.
The research team, led by Professor David Kittelson of the Department of Mechanical Engineering (ME), began by collecting data using two different instrumented buses, one with a standard diesel engine and automatic transmission and another with a hybrid engine and selectively enabled start-stop technology (both model year 2013). Nitrogen oxide (NOX) emissions and GPS data were recorded for each bus during spring, summer, and fall on an inner city route with frequent stops and slow speeds, a medium-speed route with longer distances between stops, and an express route that required little braking.
Analysis of the collected data resulted in a number of significant findings. First, researchers discovered that buses driving their routes often emit NOX emissions at much higher levels than during certification testing, particularly routes with frequent stops. On selected routes, bus stops resulted in 3.3 times the route-averaged NOX emissions.
“We believe this shows that the federal testing procedure does not accurately represent real-world driving conditions for in-use transit buses, which make up more than 7 percent of the heavy-duty vehicles this standard was intended for,” says Andrew Kotz, a graduate student on the research team.
In addition, researchers were able to pinpoint the conditions under which increased NOX emissions were most likely to occur. “Our results indicate that bus stops, cold starts, inclines, and accelerations had the most noticeable impact on elevated NOX emissions for the tested routes,” Kotz says.
The hot spot detection technique developed for this study will provide a new lens with which to view emissions hot spots.
“Identifying spatiotemporal hot spots can give researchers and vehicle manufacturers a better understanding on where to focus their emissions-reduction efforts and provide regulators with data for improved standards,” explains Will Northrop, ME assistant professor and project co-investigator. Ultimately, the technique could be used for emissions analysis of larger data sets, such as networks of connected vehicles, he says.
The project was funded by the U of M’s Initiative for Renewable Energy and the Environment and CTS.
The U of M researchers continue collaborations with Metro Transit and the transit bus engine manufacturer Cummins and have added a 2015 model year bus to assess improvements made from the initial study. Preliminary data show substantial reductions in real-world driving NOX emissions.