New federal standards for diesel fuel and vehicle emissions are both a boon and bust for the resurgence of diesel-powered vehicles in the U.S.

Ultra-low sulfur diesel fuel (ULSD), which has been available in Europe for some time and will be offered at U.S. pumps beginning Oct. 15, allows a new generation of clean-diesel vehicles with advanced exhaust aftertreatment systems, such as soot-reducing particulate-matter (PM) filters, to operate on American roads.

However, the rollout of ULSD, which is integral to the U.S. Environmental Protection Agency’s effort to reduce greenhouse-gas emissions, also ushers in a new challenge for diesel engines: burning as cleanly as gasoline engines.

Current Tier 1 federal emissions regulations place different requirements on gasoline and diesel vehicles, with diesels being allowed to emit slightly more oxides of nitrogen, a major contributor to smog and a disproportionate byproduct of diesel combustion vs. spark-ignition engines.

California, New York, Massachusetts, Vermont and New Jersey have adopted more stringent standards that group gasoline and diesel vehicles together and require slightly lower emissions of NOx, PM and carbon dioxide.

New federal Tier 2 standards, which begin to take effect Jan. 1, group all vehicles together, regardless of fuel type; apply to vehicles up to a gross vehicle weight rating of 10,000 lbs. (4,536 kg), up from 8,500 lbs. (3,856 kg); and cover a 120,000-mile (193,121-km) vehicle life, up from the previous 100,000-mile (160,934-km) limit, the EPA says.

Of the 10 emissions levels – what the EPA calls bins – comprising the Tier 2 standards, Bin 5 is of the greatest concern to auto makers because its levels represent the overall average each company’s fleet must meet.

Although this average allows some vehicles to pollute more than others, California’s Low Emission Vehicle II (LEV II) standards, effective Jan. 1, 2007, require every vehicle sold to meet emissions levels equivalent to Tier 2 Bin 5.

In other words, for an ’07 vehicle to be sold in California, the U.S.’s largest market, it must be Bin 5-compliant.

This stratified structure for cleaner-burning vehicles is expected to create a temporary hiccup in the light-duty diesel market, with a limited amount of new diesel vehicles being offered for sale in California and other states conforming to its regulations.

Most diesel vehicles available in the U.S. next year, with the exception of the largest light-duty diesel pickups, will be badged as carryover ’06 models or ’07 models that will be sold in only 45 states.

The sticking point for diesels in the new Tier 2 regulations is NOx, which diesels produce in greater quantity than gasoline engines due to the higher temperatures and pressures in their combustion process.

To meet the new NOx requirements, most auto makers planning to bring clean diesels to the U.S. will rely on several exhaust system after-treatment technologies, including PM filters, oxidation catalysts and selective catalyst reduction (SCR), also known as urea injection.

Urea is an ammonia-based fluid that, when diluted with water and injected into a diesel engine’s exhaust stream, helps a special catalytic converter to process NOx emissions into a harmless mix of nitrogen and water.

Although the specifics vary by auto maker, several gallons of a urea solution should last about 15,000 to 20,000 miles (24,140-32,186 km), or roughly the distance between routine engine service intervals.

“We know it (urea injection) works, but we want to make sure an infrastructure is in place,” Margo Oge, EPA director-Office of Transportation and Air Quality, says of the need to ensure an adequate supply of the fluid is kept onboard the vehicle at all times.

Most proponents believe the industry will overcome the hurdle of ensuring compliance with urea usage.

“We just need a device to prevent (people) from cheating,” DaimlerChrysler AG Chairman Dieter Zetsche says, referring to ways a vehicle could notify its driver that its urea tank is empty.

One idea for urea compliance is a system of warning lights on the dash to indicate the set number of times the vehicle will start without urea before being immobilized.

This would alert drivers the tank is empty, while ensuring the vehicle is returned for service before its aftertreatment systems lose their effectiveness.

The EPA says it will begin issuing individual rulings on urea injection systems later this year or early in 2007.

“We don’t dictate technology,” Oge says of the EPA. “We’re looking for (an auto maker’s) game plan for a urea infrastructure and a game plan for compliance with that infrastructure.”

– with Alisa Priddle