Management & Operations

CARB study measures CNG, diesel bus emissions

Posted on May 1, 2002

Study results released in April by the California Air Resources Board (CARB) assert that buses running on both compressed natural gas (CNG) and ultra-low-sulfur diesel fuel, with the proper emission controls in place, can attain significant reductions in toxic emissions. The testing, which was conducted from March 2001 to June 2001, compared exhaust levels of a diesel transit bus equipped with emission control technology to another diesel bus and a CNG bus that had no particulate filter or after-treatment equipment. Test results confirmed that CNG buses emit lower levels of particulate matter and nitrogen oxide than diesel fumes. However, information gathered in the study also demonstrated that the level of some toxic pollutants given off by CNG buses can be substantially higher than those found in “clean diesel” fumes. The study produced favorable results for diesel buses using a particulate filter. The diesel bus running on ultra-low-sulfur fuel had lower emissions than the unequipped diesel and CNG buses in the amount of toxic organic compounds and total mass of particulate matter. “When the diesel bus was retrofitted with a trap and run on low-sulfur fuel, its performance was very promising,” said Dr. Alan Lloyd, chairman of CARB. “We are working now to obtain better performance from CNG buses when they are equipped with state-of-the-art after-treatment equipment.” Currently, tests are being conducted using CNG buses with new manufacturer-installed oxidation catalysts. Results of those tests will be released in mid-2002. A joint statement from several organizations in response to CARB’s results claimed that the study’s comparison of buses was unfair because the CNG bus tested was not equipped with the most up-to-date clean-air technology. The statement did offer an endorsement by the groups of the perceivable improvements in diesel fuel emissions as well as CARB’s efforts to improve bus emissions. The organizations that drafted the statement include the American Lung Association, Coalition for Clean Air, Natural Resources Defense Council, Planning and Conservation League, Sierra Club and Union of Concerned Scientists.

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