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March 01, 2006  |   Comments (0)   |   Post a comment

Fleet Facts About Diesel Particulate Filters

What you need to know about these emission-control devices, which are being retrofitted into thousands of school buses and will be installed in all 2007 diesel models.

by Steve Hirano, Editor


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Whether you want them or not, diesel particulate filters (DPFs) will be a part of your future if you’re planning to buy school buses manufactured in 2007 and beyond. The filters will be one of the downstream components of the engine system, which will be required to meet the EPA’s 2007 standards for emissions of nitrogen oxide, hydrocarbons and particulate matter.

Some fleet managers, however, have already begun adding DPFs to their buses. Most of these fleets have received funding for the retrofits through the EPA’s Clean School Bus USA program or through the Clean Buses for Kids Program, which provided funds through an EPA enforcement action against Toyota Motor Corp.

Some of the funding can also be used to offset the cost differential of regular diesel fuel and ultra-low-sulfur diesel (ULSD). Later this year, the federally mandated phase-in of ULSD will begin nationwide.

According to the EPA, the cost of an installed DPF system, which includes the mounting brackets, filter and backpressure monitor, ranges from $6,500 to $7,500.

Ahead of the curve
Last summer, Baltimore County (Md.) Public Schools had 12 of the 850-plus buses in its fleet retrofitted with DPFs manufactured by Donaldson Co.

Fleet Manager Wayne Hopkins and Transportation Director Linda Fitchett took the initiative to apply for an EPA grant because they wanted to be proactive about reducing tailpipe emissions.

Subsequently, they received a $90,000 EPA grant for the DPFs and the related hardware. That translates into $7,500 per bus for the equipment. The district also received a $4,200 fuel subsidy to help cover the additional cost of the ULSD.

The buses were chosen for the retrofitting by data-logging their operating temperatures, says Ken Morosko, transportation maintenance supervisor. He explains that the engines need to generate enough heat to burn the soot that’s trapped by the DPF; otherwise, the filter could get clogged and create dangerous backpressure in the engine.

The buses selected for the retrofitting were manufactured by IC Corp. and have T 444 engines. The model years are 1998 (2), 1999 (3), 2001 (2), 2002 (4) and 2004 (1). “They’re all performing fine, except one,” Morosko says, adding that he believes the problem with the exception stems from a driver allowing the bus to idle too long. This can cause soot to collect in and eventually block the filter. “It was clogged enough to trigger the warning light,” he says.

The filter was removed from the bus and treated for two minutes with a device called a pulse cleaner. This device and another called a thermal regenerator are used to remove ash and unburned soot from the DPFs. Morosko says the district’s normal cleaning interval is annually or every 20,000 miles.

Morosko says the 12 buses equipped with the DPFs do not emit any black smoke, which he says is a double-edged sword. Although it’s pleasing to cut down on the cloud billowing from the back of the bus, it makes it more difficult for technicians to spot problems with injectors, o-rings and head gaskets, which, under normal circumstances, cause the bus to produce black smoke.

Clearing the air
The Huron (Mich.) School District also has concerns about air quality, especially since the bus yard backs up against the district’s kindergarten play center.

Transportation Director Joe Beaubien says he received a $250,000 EPA grant to have Donaldson DPFs installed in 27 of the fleet’s 35 buses. The retrofitting was performed in September by Michigan Bus Parts in Brownstown, Mich.

“We just thought it was a good idea,” Beaubien says, acknowledging that the proximity of the play center played a role in the decision.

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