California Air Resources Board

SACRAMENTO – The California Air Resources Board (ARB) has results from a study that compares emissions from current in-use diesel and compressed natural gas (CNG) fueled buses to emissions from a similar diesel-fueled bus equipped with advanced pollution controls.

ARB Chairman Dr. Alan Lloyd said, "The data suggest that both CNG and diesel engines need additional emission controls and with those controls both can achieve substantial and beneficial emission reductions."

Although it is known that CNG total particulate matter and nitrogen oxide emissions are lower than diesel emissions without exhaust aftertreatment, the data suggest that the levels of some toxic pollutants in CNG exhaust require further study and may warrant additional control. The in-use CNG bus tested was not equipped with a particulate filter or other proven aftertreatment equipment, such as an oxidation catalyst.

Additional tests are now being conducted that will use the same CNG bus refitted with an original equipment manufacturer's oxidation catalyst and a new, state-of-the-art CNG bus equipped with a manufacturer installed oxidation catalyst. Results of these tests should be available in mid-2002. The ARB also hopes to test a particle trap on a CNG fueled bus, and will do so when a suitable trap becomes available.

A diesel bus equipped with a new particulate filter produced promising test results for several pollutants. This bus, running on low sulfur fuel, produced lower emissions than either the diesel or CNG 'baseline' buses in terms of the total mass of particulate matter (PM) and the amount of toxic organic compounds. However, both diesel buses had higher nitrogen oxide (NOx) emissions, and the filter equipped diesel bus exhibited a substantial increase in the amount of NOx that is emitted as nitrogen dioxide (NO2). Heightened emissions of NO2 often leads to more rapid formation of atmospheric ozone and nitrate PM.  Particulate filter manufacturers have acknowledged this issue, and have expressed confidence that the conversion of NOx to NO2 can be reduced.

The research study tested these buses on a dynamometer from March through June of 2001, one run on low sulfur diesel and the other on CNG. Both buses were standard, 40-passenger vehicles.  Both were equipped with Detroit Diesel Series 50 engines. The diesel bus was run in two configurations; one a "baseline" with a Nelson's catalyzed muffler, and the second with the muffler removed and replaced by a new Johnson Matthey Continuously Regenerating Technology particulate filter (Diesel / CRT).

"Both the CNG bus and the trap-equipped diesel bus produced lower emissions compared to the conventional diesel bus," Dr. Lloyd said. "When the diesel bus was refitted with a trap and run on low sulfur fuel its performance was very promising. We are working now to obtain better performance from CNG buses when they are equipped with state-of-the-art aftertreatment equipment."