There is much debate about the cleanliness of school buses today, particularly surrounding diesel and propane engines. Both are touted as clean burning and emissions data being thrown around in the industry can make you dizzy and, quite frankly, confused. The truth is, based on EPA-regulated emissions, clean-diesel school buses today are comparable to or cleaner than other fuel types like propane, CNG and gasoline.
“Propane school buses are being lauded as the cleanest in the industry,” said Caley Edgerly, president and CEO of Thomas Built Buses. “We produce propane school buses, so of course we would stand behind that statement if it were true. Unfortunately, we can’t unequivocally say that propane is the cleanest fuel for school buses today. Based on EPA certifications, propane provides the lowest level of NOx in the industry. This is true. But when considering the other EPA-regulated emissions like particulate matter, carbon monoxide and non-methane hydrocarbon, propane is not the clear winner in all of the four critical emissions categories. All engine types have nearly the same emissions footprint, and this footprint has improved dramatically compared to just ten years ago.”
NOx is a Small Part of a Larger Story
In 1970, air quality became a hot issue in the United States, which resulted in the enactment of the Clean Air Act. Overseen by the EPA, air quality nationally is continually monitored and compared to EPA federal emissions standards for six primary pollutants: carbon monoxide, nitrogen dioxide, sulfur dioxide, particulate matter, hydrocarbons and photochemical oxidants. These pollutants are considered the worst of the worst based on their effect on both human health and the environment.
School buses contribute to four of these six pollutants: carbon monoxide (CO), nitrogen dioxide (NOx), particulate matter (PM) and hydrocarbons (NMHC). Over the years, EPA standards for these four emissions have become more stringent, and will likely continue to tighten in the future. Which is why all of these emissions are driving the future of school buses and engine manufacturing.
Today, all new clean-diesel, compressed natural gas, propane and gasoline-fueled school buses not only meet but exceed 2017 EPA federal emissions standards. Take a look for yourself.
As you can see, some engines are lower in some emissions and higher in others. But across the board, all are clean, based on EPA emission standards.
Why Can’t I Just Rely on NOx Levels?
There’s a reason the EPA regulates several emissions for the school bus industry and not just NOx. Each pollutant has its own unique but critical impact on the environment and human health. Therefore, all of these core pollutants must continue to be monitored and regulated. Here’s a quick snapshot of why each pollutant matters.
“With more than 480,000 school buses on the road every day, the impact of emissions can be great, which is why manufacturing safe and clean school buses is our number one priority,” said Edgerly. “All Thomas Built school buses including propane, CNG and clean diesel, not only meet EPA and GHG standards, but exceed them. This is true today and will continue to be the case well into the future.”