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California ARB compares diesel and CNG bus emissions

19 April 2002

The California Air Resources Board (ARB) has announced results from a last-year study comparing 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 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,” thus confirming the fuel neutral approach in regulating emissions from urban buses.

The research study tested two buses: one run on ultra low sulfur diesel and the other on CNG. Both buses were standard, 40-passenger vehicles 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 CRT particulate filter (diesel/CRT). The tests, performed on a dynamometer, ran from March through June of 2001.

The study confirmed that both the CNG and diesel/CRT buses had lower emissions than the baseline diesel bus. The diesel/CRT bus produced lower mass emissions of particulate matter (PM) and toxic organic compounds than the CNG bus. “When the diesel bus was refitted with a trap and run on low sulfur fuel its performance was very promising,” Lloyd said. However, both diesel buses produced higher nitrogen oxides (NOx) emissions, and the diesel/CRT bus exhibited a substantial increase in the nitrogen dioxide (NO2) proportion in the total NOx emission.

According to the ARB, additional control of NO2 and total NOx emission is required for filter equipped diesel buses. It was also concluded that additional control is required for the CNG bus emissions. “We are working now to obtain better performance from CNG buses when they are equipped with state-of-the-art aftertreatment equipment,” Lloyd said. The in-use CNG bus tested was not equipped with any exhaust gas aftertreatment devices. Additional tests are now being conducted by the ARB 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 said it hopes to test a particle trap on a CNG fueled bus, and will do so when a suitable trap becomes available.

The study results have been known to the ARB for almost one year (compare ARB’s NO2 results in the DieselNet CRT Filter paper). The current announcement took place coincidentally with the discussion on the possible modification to the California Public Transit Bus Fleet Rule. The rule, which requires bus operators to achieve a 4.8 g/bhp-hr fleet NOx average by 1 October 2002, following by particulate filter retrofit requirements beginning in 2003, may not be feasible because control equipment is not available for all types of buses in operation. The rule allowed California bus operators to follow either a diesel or CNG fuel approach in new bus procurement, despite pressures from various groups of interest to ban diesel-fueled buses.

A number of organizations and lobbying groups issued statements and press releases trying to discount the recent ARB announcement to their respective benefit.

Source: California ARB


CRT Filter