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The greening of diesel involves the new ultra-low-sulfur fuel, cleaner-burning engines and a suite of emissions equipment.

Filters trap sooty particulates while catalysts use ammonia to convert nitrogen oxides into harmless nitrogen and water in the exhaust.

“There’s a little chemical processing plant in there, and some pretty amazing chemistry,” said Thomas Hinman, vice president for diesel technologies at Corning, a leading supplier of cellular ceramic filters for diesel engines.

For many models, including those from BMW, Mercedes and Audi, there is a catch: their S.U.V.’s will carry six- to eight-gallon tanks of urea, an ammonia-rich solution injected into the exhaust to neutralize smog-forming pollution.

And to ensure that consumers don’t let the urea run dry, Mercedes is installing a dashboard alert that warns consumers when the urea level drops below one gallon. From there, owners will be on a countdown until the tank is topped off: the cars will start just 20 more times before they cannot be operated. That countdown is a concession to federal regulators, who demanded technical assurances that these groundbreaking systems would work continuously to keep emissions below legal levels.

The smaller 4-cylinder VW and Honda diesels, in contrast, meet 50-state standards without requiring urea tanks that would have to be replenished every 12,000 miles or so.

Yet as automakers dress up diesel for its coming-out party, one unexpected development is threatening to spoil it. For decades, diesel fuel cost less than gasoline, amplifying the advantage of its higher mileage. But over the last year, diesel has soared to a record average of $4.33 a gallon nationwide, compared with $3.72 for regular gasoline.

George Peterson, vice president of the AutoPacific consulting firm, said that diesel cars traditionally offset their higher prices through both fuel savings and higher resale value. But higher-price diesel fuel puts both those financial incentives at risk.

“Given the price of diesel, you can’t get the cars to pay you back, so it doesn’t make as much sense,” he said.

While diesel currently costs 16 percent more than gasoline, that premium is more than offset by mileage gains of 25 to 40 percent. Consumers would still save money with a diesel car, and they would fill it less frequently.

The Mercedes E320 diesel sedan, for example, can cover roughly 700 highway miles on a tank. Clean-diesel models may also become eligible for federal tax credits of up to $3,400.

Consumers will also pay more for diesel technology, with manufacturers estimating that diesel engines and emissions gear add from $1,500 to $3,500 to their costs for each car. Mercedes is charging only $1,000 extra for its diesel models, compared with the equivalent gasoline versions, though some analysts suggest that Mercedes is partly subsidizing diesels to win converts. Steve Keyes, a spokesman for Volkswagen, said the Jetta diesel sedan and sport wagon would cost less than $2,000 over the gas versions, a price that he said would cover the additional costs.

Automakers also note major differences between European and American markets. European nations have long subsidized diesel by taxing gasoline at higher rates. Additional taxes on large engines also drove consumers into small but relatively powerful diesels.

Finally, diesel isn’t as widely available as gasoline, though 42 percent of service stations nationwide offer the fuel, according to the Diesel Technology Forum, a trade group.

Many analysts expect diesels to blow past hybrids in popularity. J. D. Power & Associates estimates that diesel will explode from its 3 percent market share to 11.5 percent by 2015, exceeding hybrids at 7 percent. Continued high diesel prices could force an adjustment in that projection.

“People will definitely get sticker shock at over $4 a gallon,” said Mike Omotoso, the powertrain analyst at J. D. Power. “But we see the huge price gap between gasoline and diesel as a relatively short-term spike.”

And as the industry hedges its bets on which fuels and technologies — including gas-electric hybrids, diesels, plug-in hybrids and ethanol — will catch on, some automakers are publicly at odds over diesel’s chances.

General Motors’ advanced propulsion strategy is to develop a full range of alternatives to gasoline, including hybrids, ethanol, hydrogen and its Chevrolet Volt plug-in electric car. As part of that strategy, G.M. is developing new diesels, including a 4.5-liter V-8 that it will offer on 2010 models of the Chevy Silverado and GMC Sierra pickups.

Yet while GM is already selling 1.3 million diesel models a year worldwide — and is readying a diesel-powered Cadillac CTS for Europe — it sees diesel’s American future in pickups and S.U.V.s, not in affordable cars.

G.M. engineers say diesel can raise the mileage of a trailer-towing truck by 70 percent, making it a smart buy. But, they say, for a gasoline car that already gets 35 m.p.g., diesel’s gains don’t justify the added costs.

Some automakers prefer to squeeze higher mileage from gasoline-burning engines without the expense of diesel engines and emissions gear. “There’s no question that we can get to the 35 m.p.g. standard with gasoline,” said John Krafcik, Hyundai’s vice president for product development.

As technologies vie for supremacy, the diesel-versus-hybrid debate has been especially fierce. But diesel devotees don’t have to be hybrid haters, or vice versa. With petroleum expected to dominate the automotive landscape for several more decades, the hybrids and diesels that burn it are central technologies in the transition to alternative fuels and the drive against global warming.

As if to prove the point, some automakers are marrying diesel and hybrid for the best of both worlds. Mercedes has shown a diesel-hybrid prototype of its big S-Class sedan that the company estimates would achieve 44 m.p.g. VW has shown a 69 m.p.g. diesel-hybrid Golf, though Mr. Keyes said the technology was years away from production.

Johannes-Joerg Rueger, vice president for diesel engineering at Robert Bosch, a major manufacturer of diesel systems, said: “If you’re looking at the carbon dioxide and mileage goals that have to be met, it doesn’t really matter whether it’s diesel or hybrid. Let the consumer choose.”