Exhaust emissions from natural-gas school buses contain higher levels of air pollutants and toxic air contaminants than those in school buses powered by advanced-technology, low-emitting diesel engines.
That is the chief finding by an independent research laboratory under contract to International Truck and Engine Corp, presented to the Society of Automotive Engineers (SAE) conference. Research compares emissions from a popular model natural gas bus with emissions from diesel school buses.
International Truck and Engine has begun selling a low-emitting diesel engine certified to United States Environmental Protection Agency (EPA) and California Air Resources Board (ARB) 2007 particulate and hydrocarbon emission standards. The company sponsored the research along with ConocoPhillips, a producer of ultra-low-sulfur fuel.
Of 41 toxic air contaminants (TACs) listed as present in diesel exhaust by the California ARB, tests did not find 21 of them in the exhaust of any of three tested power system configurations — conventional diesel, low-emitting diesel, or natural gas.
The natural-gas bus exhaust had higher levels of six of California's listed TACs than exhaust from the low-emitting diesel bus. In the three tested bus configurations, the natural-gas bus had the highest emissions of nitrogen oxides (NOx), nitrogen oxide (NO), total hydrocarbons, non-methane hydrocarbons, methane, and carbon monoxide (CO).
The low-emitting diesel bus was found to be higher than both natural gas and conventional diesel in two other emissions — nitrogen dioxide and carbon dioxide — but the low-emitting diesel had the lowest emissions of the four engine exhaust “criteria pollutants” regulated by EPA and the ARB: NOx, CO, particulate matter, and hydrocarbons.