Propane vs. Diesel School Buses: Study Shows Clear Winner When it Comes to Emissions


After more than a year of testing, researchers from West Virginia University (WVU) recently concluded that emissions measured from propane autogas school buses are significantly lower than those from diesel buses.

In a study commissioned by the Propane Education & Research Council (PERC), WVU’s Center for Alternative Fuels Engines and Emissions (CAFEE) completed two tests at different times during 2018 on four Blue Bird school buses. Test routes included city and highway roads and a stop-and-go route similar to standard school bus operation. Researchers installed a portable emissions measurement system to calculate 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.

Focusing on nitrogen oxides (NOx), the study found that that distance-specific NOx 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, NOx emissions were 15 to 19 times higher for the diesel school bus. NOx was reduced by 95% with the propane bus. For the stop-and-go route, NOx emissions were 34 times higher for the diesel school bus. In addition, NOx 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.

“This study is monumental from an emissions and health perspective for students, schools and communities across the country,” says Tucker Perkins, president and CEO of PERC. “Children arriving to school in propane buses aren’t exposed to harmful NOx emissions; they feel better and are more prepared to learn.”

“In real-world applications, particularly those with significant low-speed or low-load operation, propane vehicles can provide dramatically lower NOx emissions compared to similar diesel vehicles,” notes Ross Ryskamp, associate director for testing and development at CAFEE. “These findings are significant due to the fact that NOx and ozone are major non-attainment concerns for many areas across the nation.”

As a nonprofit research center that works on emission reduction research, CAFEE also conducted research that exposed the Volkswagen emissions violations in 2015.

The full propane autogas study, entitled “In-Use Emissions and Performance Testing of Propane-Fueled Engines,” can be found here.

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