Oxidation catalyst puts CNG bus nearly on par with DPF-equipped clean-diesel PM emissions. Workshop Highlights

Diesel Fuel News, Sept 16, 2002

California Air Resources Board tests show that a CNG bus equipped with oxidation catalyst is almost as good as a filter-equipped clean-diesel bus on particulate matter (PM) emissions. "Our study suggests that total PM emissions from CNG with or without [oxidation catalyst] after-treatment are on the same order of magnitude as PM emissions from clean diesel (DPF-equipped vehicle fueled by ULSD)," CARB researcher Alberto Ayala explains.

"In average terms, the CNG emissions [for PM] are slightly higher within a factor of two or less." The CNG oxidation catalyst greatly reduced toxic formaldehyde emissions found on the baseline CNG bus (see Diesel Fuel News 4/29/02, p7). Not all CNG engines are suitable for retrofit with oxidation catalysts, however, as some spark-ignited CNG systems run too rich, and can thus burn up an oxidation catalyst, explains Johnson-Matthey exhaust catalyst expert Sougato Chatterjee. So it's ironic that "greens" attack DPF-equipped buses for small increases in N[O.sub.2] emissions, when toxic emissions from uuretrofittable CNG vehicles are far worse than the emissions of DPF vehicles, he said. But Natural Gas Vehicle Coalition claimed CNG vehicles "are, and will always be, cleaner than diesel vehicles."