Carmakers also are concerned that Americans may still associate diesel with models in the early 1970s that were noisy, smelly and hard to start in cold weather.

"It's unclear how well we'd do in the car side of the market," said Jim Padilla, Ford Motor Co.'s chief operating officer.

Speculation about diesel car sales in the United States has largely been spurred by their popularity in western Europe, where roughly 45 percent of all new cars are fueled by diesel engines.

In North America, diesel engines currently account for less than 1 percent of the light new vehicle market, according to the Diesel Technology Forum, a trade group formed to promote diesel.

The U.S. Department of Energy sees use of diesel engines in light vehicles increasing between 4 percent and 7 percent by 2012. While the numbers aren't huge, executives are encouraged the climate is improving.

"There's more dialogue and more discussion, more positive verbiage going on about diesel engine use in North America than I can remember in the last 10 years," said Tim Manganello, chairman and chief executive of BorgWarner Inc..

Diesel engines also have continued to be popular in U.S. pickup trucks over the years. And the latest car and SUV models with diesels have proven popular.

Volkswagen AG estimates about 10 percent of all the new cars its sells in the United States are now diesel.

DaimlerChrysler AG said last week that its new Mercedes E320 model met its 2004 sales target of 3,000 units in the first five months on the market. It also has high hopes for a Jeep Liberty SUV with a diesel engine that will be introduced in November.

"Modern clean diesel is an excellent product for certain applications," said Dieter Zetsche, CEO of Chrysler, the U.S. arm of DaimlerChrysler. "I'm very optimistic that there will be more and more customers seeking those advantages." Diesel supporters note low-sulfur fuel required by the U.S. government in late 2006 will help cut diesel emissions, making diesel a more viable option.

Some also are hoping the U.S. Environmental Protection Agency may review their emissions regulations as diesel cuts the so-called greenhouse gases like carbon dioxide.

Diesel engines currently are only sold in 45 states because they don't meet requirements for nitrous oxide, a component of acid rain, and particulates or soot, which has been linked to respiratory problems.

The more stringent emissions standards, now effective in California, New York and three other states, will be extended nationwide in 2007. It's unclear yet whether those standards can be met in a cost-effective way.

"We've had good results in the past with those kinds of challenges. And I hope we'll see that again this time," Zetsche said.

Many car executives said they are concerned that the economics of diesel are less favorable in the United States than in Europe, making it harder for consumers to justify the higher cost of a diesel engine.

Ford's Padilla noted that gasoline in Europe is much more expensive than diesel fuel. That, plus diesel's greater fuel efficiency, makes it attractive overseas.

He and other executives worry they may have to overcome consumers' lingering memories of the chugging 1970s diesel engines.

"I've lived in Europe, driven a diesel and I'm very high on the performance of diesel," Charles "Chip" McClure, CEO of parts supplier ArvinMeritor Inc., said at the summit. "But I also drove a diesel in the early '70s and it was noisy, loud, belched black smoke and I had to go to a truck stop to get fuel."

McClure, whose company makes filters to treat diesel exhaust, said he believes people who are willing to test-drive modern diesel engines will buy them.

"When you look at the fuel economy improvement (of diesel engines over gasoline engines), which is anywhere from 30 to 70 percent, and you look at the cost of gas going up, it quickly pays for itself," he said.