Clean Diesel Engines

You don't often see the words "clean" and "diesel" put together like that, but get ready for some big changes to the diesel landscape. Starting in October 2006, gas stations and truck stops will sell a whole new kind of diesel, one that burns far cleaner than the diesel we're using today. And when that fuel is run through the next generation of diesel engines and emissions-control systems (which will begin to show up in model year 2007), they'll be in some ways cleaner than gasoline engines.

That's all well and good if you're planning to buy a brand-new diesel truck next year. But what about the thousands of you driving older diesels? Will this new fuel work in your rigs too?

Since we hate surprises as much as the next guy, we won't leave you hanging. The short answer is yes, though you may need to keep an eye on your fuel system for leaks. Keep reading to learn more about this new clean diesel and how it will run in current and older diesel engines.

Starting this summer, refineries are going to change the way they produce diesel, and by doing so will reduce the amount of sulfur in the fuel. Current standards allow a sulfur content of 500 parts per million (ppm) by weight; the new rule hacks that standard down to 15 ppm. By itself, this ultralow sulfur diesel (ULSD) will have a significant effect on diesel engine emissions, reducing sulfur oxides by 88 percent and particulate matter by 4 percent, reports the California Air Resources Board.

This diesel gets really clean when it's paired with new emissions control equipment that will be added to diesel trucks starting in 2007. Traps in the exhaust system will capture particulate matter, cutting those emissions by 80 to 90 percent, according to the Diesel Technology Forum. Catalytic converters will focus on nitrogen oxides (NOx), reducing those emissions by 25 to 50 percent. The net result is that when these systems take effect, "clean diesel trucks and buses will have reduced both particulate matter and NOx by 98 percent from 1988 levels-a virtual elimination of these emissions from on-highway engines," says the Forum.

This dramatic decrease in diesel's sulfur content has some side effects, as you may imagine. "Among the ways diesel is different from gasoline is that fuel is used to lubricate the fuel pump, not oil," said Loren Beard, senior manager of environmental and energy planning for DaimlerChrysler. "When you take out all the sulfur compounds, the hydro-treating process also takes out compounds that give diesel its natural lubricity, which protects the fuel pump and injectors." To prevent any lubricating problems with the new fuel, the American Society for Testing and Materials added a lubricity specification for all diesel fuels in 2005. "That additive will actually make diesel's lubricity better than it was before," said Beard.

The sulfur-reducing process also lowers the "aromatics content and density of diesel fuel, resulting in a reduction in energy content," said a report from Chevron, a reduction that could affect power production and energy efficiency. However, Chevron and other refiners peg this energy content loss at approximately 1 percent, so unless you own an over-the-road trucking company that tracks its fuel economy very carefully, any loss of mpg or power most likely won't be felt.

On the upside, the sulfur reduction causes an increase in diesel fuel's cetane rating, which is sort of like gasoline's octane. The cetane rating is a measure of the fuel's ignitability; increasing it improves the fuel's efficiency and also reduces its black, smoky emissions.

Our upcoming ULSD has been compared to the diesel fuel currently used in Europe, but the two aren't identical. While Europe's diesel has a sulfur content that's even lower than our new fuel-at 10 ppm-its cetane rating is in the mid-50s, much higher than the low-40s ratings for U.S. diesel. Plus, according to Andrew Buczynsky, a senior fuels engineer for General Motors, European emissions laws are more lax than ours regarding NOx. "Those engines will have to be tweaked before they can be used in the U.S." That tweaking may have already begun, as you'll see later in this story.

None of the refiners we spoke to wanted to make specific estimates of what the new diesel will cost at the pump, but expect an increase. "As an industry, we've invested $8 billion to make the change," said John Leon, who works in fuels marketing for Shell.

Some of the rise in the pump price will be to offset the new diesel's refining process, which is more expensive than the current system. There will be additional transportation costs too, especially at the fuel's introduction. Diesel fuel is very susceptible to sulfur gain, said Leon. Any exposure to the old, 500-ppm fuel-in pipes, tanks or transport trucks-could cause the new fuel's sulfur content to rise out of spec. So the refiners will be taking a lot of care to clean their transport means and keep the new fuel away from the old.

The closest we could get to a cost estimate was from a study of ULSD by Washington State University, which found that areas already using the fuel were paying anywhere from 11 to 15 cents per gallon over the cost of conventional diesel.

Daimler Chrysler's Beard looked at it another way: "It's hard to predict changes in retail price. If people believe there will be a shortage and rush to stations to fill up, there will be shortages. Shortages will cause prices to spike. But the actual cost to produce this new fuel is low, pennies per gallon, which is less than seasonal price fluctuations we experience now."

Keep in mind, too, that diesel engines are inherently 20 to 40 percent more efficient than gasoline, and should return a comparable increase in fuel efficiency, which should help offset the higher per-gallon price.