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Study finds propane school buses significantly decrease NOx emissions compared to diesel

According to a study commissioned by the Washington, DC-based Propane Education & Research Council (PERC), NOx 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).

NOx 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 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.

  • For the stop-and-go route, NOx emissions were 34 times higher for the diesel school bus. 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.

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. These findings are significant due to the fact that NOx 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 NNOxOx 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 NOx emissions—such as the adoption of propane school buses.



CNG/DME would be better, more abundant supply and cleaner.


The propane buses now used in the US national parks are both clean and quiet.  I rode a bicycle behind some (drafting them to get good speed uphill) and found them to be very pleasant road companions.

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If Propane looks good as a fuel why not Ethane?
We have a surplus of Ethane and mostly export it (the US is the largest exporter of Ethane in the world). It is mostly used to make Ethylene though we could use Methane for that purpose if necessary.
Ethane is a good automotive fuel, it is like Propane in it's properties and much better than Methane.


Both fuels burn cleanly, but natural gas is a cleaner burning fuel...


Ethane's boiling point of -88°C probably has something to do with that, gryf.


It's too bad someone doesn't offer well engineered BEV conversion kits for diesel buses, so we can skip over the interim polluting CNG transition phase.

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