The one-year anniversary of the arrival of ultra-low-sulfur diesel at U.S. fueling stations quietly came and went last month, but anecdotal information continues to surface that the clean-air fuel is causing headaches for many bus maintenance personnel.

Initial concerns about fuel availability and the performance of over-the-road buses and trucks transitioning to ultra-low-sulfur diesel have largely disappeared, according to the Diesel Technology Forum.

However, there remain sporadic reports in the motorcoach industry of fuel injector problems that operators and their technicians are blaming on the low-sulfur diesel. And some worry the issue may become more pronounced as engines accumulate more miles and years burning the fuel.

The injector failures are thought to be caused by two interrelated factors the reduced lubricity, or lubrication properties, of ultra-low-sulfur diesel, and the use of filters that fail to remove impurities and particles in the fuel that can destroy injectors.

It has long been held that the reduced lubricity of ultra-low-sulfur diesel causes accelerated wear of fuel system components. But its also possible it may make injectors more susceptible to damage from impurities in the fuel.

And replacing a set of fuel injectors on a modern diesel engine is no walk in the park; it typically is a $2,500 job.

Pay me now or pay me . . .

The fuel-injection system on a diesel engine is its most complex component and has been the subject of a great deal of design, redesign and experimentation. Injectors have to be able to withstand the tremendous temperatures and pressures inside the engine cylinder and still deliver fuel in a fine mist.

Additionally, injectors that are not working correctly may damage the exhaust after-treatment system. Therefore, proper functioning of the injectors is critical.

Detroit Diesel, the No. 1 supplier of engines to the coach industry, now requires that the primary and secondary fuel filters on its power plants be changed every 15,000 miles and, as of late last year, established higher fuel filtration requirements.

The new filters mandated by Detroit Diesel trap more foreign particles than filters used on previous Series 60 engines. And use of correct filters is mandatory to qualify for fuel injector warranty coverage. Use the wrong filters and the warranty isn't worth a thing.

"The new injectors have smaller orifices and thus the need for a finer filter," said one respected bus maintenance expert. "Many fleets are running the same Series 60 filter they have (been using) for years thinking everything was the same. Detroit Diesel released a (service) bulletin last year but it went unnoticed by most fleets."

"Detroit's standard answer on any injector issues for engines under warranty is "denied" if they have the wrong filters on, regardless of the cause," said this coach maintenance man.

Doing what's right

Operators should check the most recent fuel-filter requirements and service bulletins for the engines used in their coaches, and cease using conventional over-the-counter, spin-on fuel filters and switch to filters that meet the latest engine manufacturer specifications. And pitch em when theyre supposed to be changed.

Another factor in the equation may be the initial content of sulfur in the base crude oil from which the diesel fuel is refined, and which may account for some operators experiencing the problem and others not.

Crude oil from Alaska, for example, tends to be very high in sulfur, while Venezuelan crude oil very low. As a result, and because of different methods used by refiners to remove sulfur, the lubrication properties of the fuel could be different for each oil refinery and even change as a particular refinerys crude oil sources change.

One operator attending the California Bus Association meeting in Las Vegas last month reported experimenting with B5-biodiesel fuel to see if the increased lubricity of that fuel might reduce injector failures.

And about supply . . .

Meanwhile, the U.S. Energy Information Administration reports that more than 93 million barrels of ultra-low-sulfur diesel fuel were refined in July, compared with 54 million a year earlier.

Ultra-low-sulfur diesel now accounts for more than 75 percent of all distillate fuel production, and the U.S. Environmental Protection Agency estimates that more than 90 percent of all retail service stations that have diesel fuel are now carrying the cleaner fuel, exceeding the required 80 percent minimum level.

Since October 2006, more than 838 million barrels of clean diesel fuel have entered the U.S. distribution system.

"The transition to ULSD fuel has been nearly seamless, with virtually no supply interruptions nor technical glitches," said Allen Schaeffer, executive director of the Diesel Technology Forum. "Like the switch from leaded to unleaded gasoline, this transition to clean diesel fuel is fundamentally transforming diesel technology to be a leading solution for reducing energy consumption, lowering greenhouse gas emissions and meeting aggressive clean air standards."

While the EPA and the Diesel Technology Forum maintain there is no shortage of ultra-low-sulfur diesel, reports of spot shortages and supply problems persist.

What's going on?

Some believe the issue may be related to lack of proper decals identifying the type of fuel suppliers and retailers have in the ground.

However, one of the nation's largest coach fleet operators says that getting ultra-low-sulfur diesel is a major problem for their operation, with some suppliers still not able to supply in bulk.

"This is certainly different than what we are being told by EPA," said one industry executive.

Ultra-low-sulfur diesel fuel is required for all 2007 clean-diesel motorcoaches and trucks, plus new passenger cars and SUVs. Its use also allows for the modernizing of some existing engines and equipment with emissions control technology such as particulate filters.

The EPA estimates that this year, thanks to a combination of cleaner diesel fuel and new engine technology, sulfur oxide emissions (a contributor to acid rain) from over-the-road trucks and buses will decrease by more than 100,000 tons and carbon monoxide emissions by more than 70,000 tons.

However, the most significant benefits of clean diesel will be realized when new buses and trucks have largely replaced the existing fleet by 2020. At that time, EPA predicts 2 million fewer tons of nitrogen oxides (or NOx, a component of smog) and 83,000 fewer tons of fine particulate matter (or soot annually), thanks to clean diesel.

A 2007 diesel motorcoach emits one-sixtieth the soot exhaust of one produced in 1988.