cations. Diesel engines, particularly older models, tend to have higher NOx emissions due to the combustion process and the nature of the fuel,” citing a 2019 West Virginia University study. She added that propane’s lower NOx emissions offer


the advantage of a cleaner-burning fuel choice in many applications. “Diesel engines, due to their high com- pression ratios and combustion conditions, naturally produce more NOx,” she said. Schmidt noted while modern diesel engines equipped


with emissions control technologies can reduce NOx, they still typically produce higher levels than propane engines. The National Renewable Energy Laboratory published


a study last year of Class 6 medium-duty propane trucks operated by the U.S. Postal Service in Colorado com- pared with diesel mail trucks. NREL found “significantly lower” NOx and carbon dioxide emissions compared to diesel, though sulfur dioxide emissions increased by nearly 200 percent. “Propane’s lower NOx emissions contribute to better


air quality, as NOx is a precursor to ozone formation and smog,” she said, adding high NOx emissions from diesel engines can contribute to respiratory problems and acid rain formation. Propane buses are quieter and have cleaner burning


fuel, she added. “They warm up quickly, including inside the vehicle,


providing a comfortable ride for passengers on cold mornings and during inclement weather,” Schmidt said. “Windows stay defrosted for clear visibility. Diesel buses require a long time to idle to warm up, especially on a very cold day.” Cost savings mean savings go back into the classroom


for students, noted Schmidt. Student Transportation of America, an STN 2022


Green Bus Summit Fleet Award winner in the private fleet category, has a fleet of more than 18,000 buses, which run in 25 states and most Canadian provinces. “We run a variety of fuels: Diesel, biodiesel, CNG,


propane and electric,” noted Rachel Lane, vice president of electrification and sustainability. “Our fuel selection is


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