Real-World Testing Proves Propane Autogas School Bus Emissions Are Significantly Lower than Diesel

October 16, 2019

Rubber is meeting the road when it comes to school bus emissions in real-world tests.

With more than 18,000 propane autogas school buses on the road from coast to coast, propane autogas has long been accepted as a viable, clean, convenient, and reliable fuel choice for districts looking to significantly reduce emissions while offering a safe, healthy ride for students.

Real-World Testing Confirms Propane is a Low NOx Leader

But now, a new study by West Virginia University has reinforced propane’s unmatched emissions profile in real-world testing by proving that propane autogas school bus emissions are as much as 96% lower than those from diesel buses.

In a study commissioned by the Propane Education & Research Council, West Virginia University’s Center for Alternative Fuels Engines and Emissions (CAFEE) completed two types of tests at different times during 2018. CAFEE tested four Blue Bird school buses, two propane buses (model years 2015 and 2017), and two diesel buses (model years 2014 and 2017). 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.

Distance-specific NOx emissions measured from the diesel bus were significantly higher than those measured from the propane autogas bus.

The results demonstrated that distance-specific NOx emissions measured from the diesel bus were significantly higher than those measured from the propane autogas bus. On the city route, which included city and highway roads, NOx emissions were 15-19 times higher for the diesel school bus and NOx emissions were reduced by 95% with the propane autogas bus. On the stop-and-go route, NOx emissions were 34 times higher for the diesel school bus. NOx emissions were reduced by 96% and carbon dioxide by 13% with the propane autogas bus.

Why Targeting NOx Emissions is Critical for Clean Air

This is profoundly important because NOx emissions are federally regulated due to their negative impact on human health and are a known trigger for issues like asthma, bronchitis, and other respiratory problems, according to the Environmental Protection Agency. This is especially significant in the school transportation industry where students are coming face-to-face with tailpipe emissions every day, but the lesson here can be translated to nearly any fleet in any industry.

Recent propane autogas innovations are continuing to raise the bar on what it means to be a clean fuel for fleets.

While this test proves that propane autogas buses are significantly lowering emissions, that hasn’t stopped the industry from trying to make the ride to school even cleaner. Recent propane autogas innovations, including the ultra-low NOx engine, are continuing to raise the bar on what it means to be a clean fuel for fleets.

The ultra-low NOx engines have emissions that are 90% lower than any EPA standards and are certified to the optional ultra-low NOx emissions standard as defined by the California Air Resources Board (CARB) for heavy-duty engines with .02 grams per brake horsepower-house.

Funding Available to Transition Away from Diesel

Fleets looking to take advantage of propane autogas’s clean benefits can apply and qualify for funding to replace their current diesel buses through the $2.7 billion Volkswagen Mitigation Settlement. Mitigation plans in most states are finalized and the application and awards process is well under way. For more information about state mitigation plans, visit PERC’s Volkswagen Trust Resource Center at

Not only is propane autogas improving thousands of school district’s emissions profiles and creating a cleaner and safer ride to school for students, it’s also the most cost-effective fuel solution for reducing NOx emissions. To learn more about how propane autogas is cleaning up communities with reduced emissions, visit or reach out to the Propane Education & Research Council for additional information.