WASHINGTON Starting June 1, motorcoach and trucking companies that operate in-house central refueling facilities will be required to label their pumps as either ultra-low-sulfur diesel or low-sulfur diesel.

U.S. Environmental Protection Agency regulations that go into effect on the first day of next month say operators that buy their own fuel in bulk must segregate ULSD, which must contain no more than 15 parts per million of sulfur, from so-called low-sulfur diesel, which contains anywhere from 16 ppm to 500 ppm of sulfur.

The regulation is part of EPAs 2006 ULSD rule, due to take complete effect later this year.

Under the rule, 80 percent of highway diesel fuel refined after June and sold after Oct. 15 may have no more than 15 ppm sulfur.

The reduced-sulfur fuel is needed for the 2007-model bus and truck engines to operate properly and produce fewer emissions.

Meanwhile, engineers from the federal government and oil companies say ultra-low-sulfur diesel fuel should be ready on time this fall, but that introduction of the fuel will present challenges.

The engineers told a trucking industry conference in San Diego last month that ULSD could be difficult for fuel carriers to transport, at least at first, and other difficulties are possible before the end of the transition.

"We will learn our lessons and then move forward," said Mike Leister, manager of fuels technology for Marathon Petroleum Co. of Findlay, Ohio.

Transporting the early batches of ULSD to retail fueling stations will provide lots of information on what works in the distribution system and what needs to be fixed, according to Leister.

Still, he predicted refiners and distributors will work out any kinks and motor carriers will see the new diesel fuel by Labor Day, well before the retail deadline of Oct. 15.