Since the appearance of the first automotive internal-combustion engine, there's been an ongoing quest to obtain readily available fuel to pour in and burn.

Going into the second century of internal combustion, nothing's really changed except our priorities. In the latest portion of the 19th century, before gasoline was readily available, searching for a consistent supply of fuel made early motorists the hunter-gatherers of the industrial revolution. The oil barons of the early 20th century invested sagely in infrastructure to ensure there wouldn't be much incentive to forage for anything but their wares, and they priced gasoline attractively.

Now we enjoy Exxon and Mobil and BP on every corner to supply us with two energy-rich fuels: gasoline and diesel. Cheap, efficient and available. The three chief qualities we've historically demanded of motor fuel. "Clean" didn't really matter.

Now it does. Thirty years ago, long before "Kyoto" entered the automotive lexicon, concerned interests strolled to the back of their cars and determined the stuff billowing from that dirty pipe might not be good for you.

We got exhaust catalysts. It took California regulators - as you'd expect - to start thinking about what was going into the engine as having an impact on what comes out. Around 1978, the California Energy Commission initiated its own alternative fuel vehicle (AFV) program; Volkswagen Rabbits got a diet of ethanol or methanol (more on fuel specifics comes below).

The big alternative fuels push came with passage of 1990's Clean Air Act Amendments. Besides securing legislation requiring 150,000 "clean fuel" vehicles be sold in California in 1996 - and double that amount by '99 - the CAAA insisted that the smoggiest U.S. cities get seasonally reformulated gasoline.

The rest of the country got into the alternative fuels action with the National Energy Policy Act in 1992. The NEP set down guidelines requiring the federal government to buy AFVs and provided for tax incentives for individuals and fleets purchasing, or converting to, AFVs.

What constitutes an alternative fuel or the more ambiguous "clean fuel?" Here's what the California Energy Commission says:

n Compressed natural gas (CNG).

n Liquified natural gas (LNG)

n Liquified petroleum gas (LPG)

n Coal-derived liquid fuels

n Alcohol-based fuels (mainly ethanol or methanol)

n Bio-derived fuels

n Hydrogen

n Electricity

For everything about electrically powered vehicles, see the EV overview on page 79. But forget about electricity here, because it's not really an "alternative" fuel in the sense of the others - electricity largely is generated by burning something else.

Regulators think these alternative fuels are inherently cleaner than gasoline or diesel because they've talked to chemists. The alternative fuels are made up of reasonably simple molecules (we all know water as a simple molecule of two parts hydrogen and one part oxygen), while liquid petroleum molecules are chemically complex. Simple molecules, when burned, don't leave as much residue to be spewed out as emissions. The alternative fuels generally present drastic emissions advantages over gasoline and diesel fuels.

You don't need to be a chemist to think that this all sounds pretty good. And it is. But there's no free lunch. Because they're simpler, alternative fuels also contain less energy - basically in the form of carbon and hydrogen; that means less mileage when compared to gasoline. Also, the physical state of some of the alternative fuels at normal atmospheric temperature or pressure is not as "benign" as liquid gasoline and diesel fuel.

So you practically need a scorecard to keep it straight. That's a doable thing:

Compressed natural gas is the genuine front-runner among the non-petroleum-based alternatives. There are abundant domestic supplies, and conventional internal-combustion engines are easy to adapt for CNG use.

Energy density vs. gasoline: -70%.

Emissions: As much as 96% cleaner for some pollutants, although oxides of nitrogen (NOx) production varies widely.

Drawbacks: Requires pressurization in expensive, heavy storage tanks. Filling up at residential-delivery pressures takes a long time. Some early storage-tank blowups.

Infrastructure: For automotive use, perhaps the best of the gaseous fuels.

On the used-car lot: Early '90s Big Three pickups; Chrysler and Ford vans.

In the showroom: Yes, from Ford, Chrysler and Honda - although most are bought by government and municipal fleets.

CNG is inexpensive, but its energy is half that of gasoline, so driving range typically suffers. Forget liquefied natural gas (LNG); it has to be kept cool at -372 degrees.

Liquefied petroleum gas (LPG), or propane, is a liquid at its mild storage pressure and turns to gas when depressurized. It's been used forever as a heating fuel in rural areas. The California Energy Commission estimates there are more than 300,000 propane-powered vehicles in the U.S., most in fleet use. Selling propane and propane accessories keeps the Fox Network's King of the Hill in business.

Energy density vs. gasoline: -27%

Emissions: Roughly one-third of gasoline, except for NOx.

Drawbacks: Just as expensive as gasoline, subject to spot-shortages.

Infrastructure: Good, provided one stays on the same routes traveled by motor homes.

On the used-car lot: Maybe, but it's a conversion.

In the showrooms: No OEM models.

Let's face it: Propane ain't too sexy.

Ethanol (commonly E85) is a blend of 85% ethyl alcohol and 15% unleaded gasoline. Ethanol is primarily derived from corn or similar starchy grains. Flexible Fuel Vehicles (FFVs) can be easily calibrated to run on E85 or straight gasoline or any mixture of the two. E85 is considered a renewable energy source, which the bureaucrats love.

Energy density vs. gasoline: -27%

Emissions: 30% to 50% less than gasoline, but burning alcohol forms nasty aldehydes.

Drawbacks: Critics say heavy subsidies for corn growers eliminate any economic advantages; growing corn just to make fuel is a costly endeavor that requires its own energies. Questions remain about fuel-system component durability with alcohol fuels.

Infrastructure: 40 stations. In the entire country. Bring your (large) gas can.

On the used-car lot: Maybe; early-'90s Chevrolet Lumina VFVs, but only a few hundred. Some Ford Taurus's might be around.

In the showroom: All '98 Chrysler Voyager, Caravan, Town & Country minivans with 3.3L OHV V-6. All '99 Ford Rangers with 3L V-6 engines.

The real deal with E85 is a dodge: with somewhere between 500,000 and 1 million E85-capable vehicles soon to be in individuals' hands, there's going to be a helluva line at those 40 E85 stations. The reality is that the automakers get generous Corporate Average Fuel Economy (CAFE) credits for each FFV sold. Everybody knows darn well the "flexible" capability of these vehicles will never be used.

Methanol is made from natural gas. It's a high-octane liquid that's been used in high-performance race cars, but the stuff at the pumps is 85% methanol blended with 15% gasoline.

Energy density vs. gasoline: -40%.

Emissions: Right around 50% reduction for most pollutants.

Drawbacks: Prices vary widely and lean to the high side.

Infrastructure: Not much for vehicular use.

On the used-car lot: You name it, from '82 pilot-program Ford Escorts to Volkswagen Jettas to Big Three midsize cars.

In the showroom: Yes, from the Big Three.

The hottest news in alternative fuels comes from the area least expected: diesel. Early developers are calling the new-age diesel derivatives "superfuels."

There has been talk for several years about dimethyl ether, or DME. It is a gaseous diesel fuel derived from natural gas. But the need to pressurize DME is the same distinct problem that plagues the other "traditional" gaseous alternative fuels.

A bit more refining, however, can create dimethoxy methane, DMM, a liquid diesel that retains almost no bad portions - like sulfur and aromatics - of conventional diesel fuel. And it doesn't require pressurization.

Perhaps better yet is another natural-gas diesel produced through the Fischer-Tropsch gas-to-liquid (GTL) refining method that produces a water-clear diesel fuel (Germany used Fischer-Tropsch to make "synthetic" fuel during World War II).

GTL diesel has a high cetane value - roughly comparable to gasoline octane - and there's a Shell refinery in Malaysia making the stuff right now. The best part is that GTL could be made from vast amounts of worldwide natural gas that, horrifyingly, is simply vented to the atmosphere as a by-product of other manufacturing.

British Petroleum reportedly is looking at Alaska's vast natural gas reserves for GTL production, and other oil companies are scrambling to get involved.

DMM and GTL just might be the acronyms of future alternative fuels. If these clean fuels come to fruition, they might send the traditional alternative fuels packing.