It has been a long time since anyone had to settle for the retrofitted gasoline engines that General Motors Corp. passed off as diesels in the 1970s fuel crisis, but that image lingers.

The technology has taken such a black eye from its dirty past that many now shun the word "diesel" for a more politically correct (and more obtuse) "compression ignition," in reference to the process of using pressure to ignite the fuel instead of the spark system in a gasoline engine.

And even though just about every major automaker is counting on some form of diesel hybrid/electric system to help them bridge the gap between conventional gasoline engines and the anticipated age of fuel cells, it is by no means clear that diesels will be able to do the job.

While a diesel engine does a great job on energy efficiency, besting even gasoline for energy density, current technology lags behind gas engines for controlling oxides of nitrogen (NOx) and particulates (PM).

In fact, when Vice President Al Gore participated in a highly publicized press conference expressing confidence in the diesel engine as a future powertrain, the environmental community roundly chastised him because expected future emissions rules would allow diesels to produce four times as much PM emissions as gasoline.

In technical terms, gasoline engines are expected to be required to emit no more than 0.01 grams per mile of PM per mile by 2004, while diesels are expected to be allowed to hit 0.04 gpm. The new guidelines are likely to be part of the new Tier II emission rules to be rolled out this summer. The U.S. Environmental Protection Agency can't enforce the new emission rules until 2004.

Mr. Gore "clarified" his position with the automakers and let developers know that if they want to use a diesel in the car of the future, it has to produce the same level or fewer PM emissions than gasoline engines. That means diesels have to hit 0.01 gpm, too.

And while Europe has been willing to cut the diesel a little bit of emission slack in exchange for the fuel economy, it is doubtful the powertrain will get any such break in the U.S. even if 0.01 proves to be an especially elusive goal.

The California Air Resources Board, which tends to set trends for the rest of the world on emissions, is leading the fight to make diesel emissions a "toxic substance" - hardly an environment ripe for compromising. So it looks like 0.01 it will be.

The problem is, no one really knows how to get there right now. And many of the things developers do to control PM can make NOx worse. The clock is ticking because many of the tougher rules are expected to take effect in time for the 2004 model year.

That's when the Partnership for the New Generation of Vehicles (PNGV) will roll out its production prototype for an alternative sedan that gets 80 mpg without sacrificing performance or passenger and storage space.

There's nary a mention of diesel in the latest PNGV press release, which spells out the future direction of the program. Compression ignition is mentioned once, almost as an aside.

Although the latest PNGV release doesn't narrow the powertrain to a diesel-electric hybrid, most experts agree there's no way to get 80 mpg unless a diesel is part of the final solution. Even PNGV apparently is avoiding the diesel stigma.

Most participants agree the oil industry is going to have to talk about some cleaner fuels to get part of the trade-off. But with the oil lobby apparently carrying more clout than the auto lobby in the hallowed halls of Washington, DC, don't hold your breath for anything earth shattering to happen in fuels.

Meanwhile, the U.S. government and automakers and suppliers around the world are spending big wads of money trying to create a diesel comeback in the U.S.

So is diesel research just throwing good money after bad?

Murray Busato at Siemens AG hopes not.

His company is banking on a new common rail fuel delivery system for diesels to help them get at least part way to the goals.

"I don't think anyone can say it is or isn't possible," he says. "The regulators want to see what is realistic. They set the regulations and industry is left to develop the technology."

As one General Motors engineer describes it, "If you look at what's on the road right now, no one is close. If you go into their back rooms and look at what they're working on, the newest technology, you're still not there.

"It's going to take something new."

And Mr. Busato adds that the target is not really even 0.01 gpm.

"It always seems to be a target too far to reach. But the mandate is not really 0.01, but zero. That's where this is heading," he says. "But I certainly wouldn't take the attitude that this can't be done. There certainly is some light at the end of the tunnel."

Hopefully it's not a train full of new regulations bearing down.

At press time, General Motors Corp. and Amoco announced the two companies have entered into a partnership to produce less-polluting diesel engines -- and the cleaner diesel and gasoline fuels many industry experts insist are vital to achieving near-zero emissions from internal-combustion engines.

GM vice-chairman Harry Pearce says in a prepared statement that the auto and oil industries have been "too distant in the past, and this kind of joint research is what really allows you to move the ball ahead much more quickly."

Amoco president Bill Lowrie says the joint effort will examine ways to reduce some of the nastier elements in gasoline and diesel such as sulfur. Mr. Lowrie acknowledges that sulfur levels must be reduced, and "The issue is how much and how far, and when. We've had some disagreement between our industries on that."

Mr. Lowrie adds that the GM/Amoco effort will be broad in its examination of engine technology, fuel composition and infrastructure to distribute (and presumably, market) advanced fuels.

If fuels are to be more highly refined, the cost will elevate accordingly. The idea of raising prices markedly may frighten an oil industry that peddles to U.S. drivers "hooked" on $1-per-gallon fuel -- but if cleaner fuels carry a stiffer price, consumers may find incentive to seek out more fuel-efficient vehicles, completing the desired cycle.

The GM/Amoco agreement may pave the way for the a mutual understanding between the auto and oil industries that existing fuel specifications cannot satisfy the requirements of new-age powertrain technology -- and the emissions standards they're expected to meet.