According to a study commissioned by the Washington, DC-based Propane Education & Research Council (PERC), NO_x emissions measured from propane autogas school buses were substantially lower than those measured from diesel school buses. The study was conducted in 2018 by West Virginia University’s Center of Alternatives Fuels, Engines, and Emissions (CAFEE).

NO_x emissions remain a significant challenge to air quality in the US, and are a predominant non-attainment concern for many areas, especially in California.

West Virginia University’s Center for Alternative Fuels Engines and Emissions (CAFEE) completed two types of tests at different times of the year during 2018 on four Blue Bird school buses. Test routes included both city and highway roads, and a stop-and-go route similar to standard school bus operation.

Researchers installed a portable emissions measurement system to measure exhaust emissions on each vehicle and performed test runs on each bus with both cold and hot starts, for a total of 36 test routes.

The study’s results demonstrated that distance-specific NO_x emissions measured from the diesel bus were significantly higher than those measured from the propane bus for tests conducted in early 2018. Specifically:

• For the city route, which included city and highway roads, NO_x emissions were 15 to 19 times higher for the diesel school bus.

• For the stop-and-go route, NO_x emissions were 34 times higher for the diesel school bus. NO_x was reduced by 96% and carbon dioxide by 13% with a propane bus.

Subsequent testing performed in late 2018 with newer model year and lower mileage propane and diesel buses validated the previous testing results.

In real-world applications, particularly those with significant low speed or low load operation, propane vehicles can provide dramatically lower NO_x emissions, compared to similar diesel vehicles. These findings are significant due to the fact that NO_x and ozone are major non-attainment concerns for many areas across the nation.

—Ross Ryskamp, Ph.D., associate director for testing and development at CAFEE

Ryskamp also said the findings from the in-use tests of high NNO_xOx emissions for medium- and heavy-duty diesel vehicles are supported by other studies in literature.

As a nonprofit research center that works extensively on emission reduction research, CAFEE also conducted research that exposed the Volkswagen emissions violations in 2015, resulting in a $14.7 billion settlement. Nearly $3 billion is set aside for the sole purpose of funding transportation projects in each state that reduce NO_x emissions—such as the adoption of propane school buses.