DRAMATIC Emission Reductions From Propane Autogas School Buses

Emissions of nitrogen oxides (NOx), one of the biggest challenges to air quality in the United States, are as much as 96% lower when a school bus is fueled by propane autogas rather than diesel. That’s just one of the findings in a study recently published by West Virginia University’s Center for Alternative Fuels Engines and Emissions (CAFEE). Reduction of NOx is important because these emissions are regulated by the federal government and are a predominant non-attainment concern in many areas, the author of the study adds.

“NOx is one of the primary precursors to ground-level ozone, or smog,” Ross Ryskamp, associate director, testing and development, with CAFEE, tells BPN. “The whole nation is making a big push to bring down NOx emissions from vehicles.”

For the study, CAFEE completed two types of tests at different times on four Blue Bird school buses—two fueled by propane autogas and two fueled by diesel. The buses were nearly identical apart from the engine, transmission, and mileage.

When a 2015 propane autogas school bus and a 2014 diesel school bus were tested over a route that included both city and highway roads, the propane autogas bus’s NOx emissions were 95% lower than the diesel model’s. When the two buses were tested over a stop-and-go route, the propane autogas bus’s NOx emissions were 96% lower. “The stop-and-go route was designed to simulate how school buses are used,” Ryskamp says. The findings were confirmed in subsequent testing that used a 2017 propane autogas bus and a 2017 diesel bus.

“We made sure the aftertreatment on the newer diesel bus was conditioned so the test was fair,” Ryskamp notes. “The older buses and the newer ones showed similar results, with the propane buses having lower NOx emissions than diesel. That’s especially true on low-load, low-speed routes.”

“Diesel has an inherent disadvantage in reducing NOx emissions before they go out the tailpipe due to the lean air-to-fuel ratios they operate at and current exhaust aftertreatment technology,” Ryskamp explains. “Low-load, idle, and stop-and-go operation can make it difficult for diesel exhaust aftertreatment systems to mitigate these emissions, especially when the exhaust aftertreatment system is not fully warmed up.”

Reducing those emissions from diesel engines has another effect in the real world, he adds:

“All vehicles—propane, gasoline, and diesel—have come a long way in the last 20 years in terms of reducing harmful emissions. But when you look at the exhaust aftertreatment side and compare the single catalytic converter on propane or gasoline vehicles to the diesel exhaust aftertreatment system that includes a diesel oxidation catalyst, a diesel particulate filter, and a selective catalytic reduction system, propane and gasoline vehicles have a much simpler, but effective, technology to treat emissions,” he notes. “Diesel exhaust aftertreatment is much more vast; it is a system with many more parts and much greater cost and complexity.”

This study, “In-Use Emissions and Performance Testing of Propane-Fueled Engines,” was commissioned by the Propane Education & Research Council (PERC). A link to the study, together with other information about propane autogas school buses, can be found on PERC’s website at propane.com/propane-products/buses/. — Steve Relyea