The alternative-fuel debate has carmakers and lawmakers struggling to meet the challenge of improving air quality with automotive technology that is both affordable and convenient for consumers.

Not only does controversy swirl around the likely high costs, but there is also rampant misunderstanding because of conflicting goals. For example, the most vocal environmentalists in the U.S. all seem to be focused on regional problems, such as the Los Angeles basin, while Europe has adopted a more global outlook.

Today's state-of-the-industry gasoline powerplant runs much, much cleaner than just a few years ago, but it is nearing the practical limit for low exhaust emissions. The concept of even cleaner propulsion, for some time now, has included thoughts of non-gasoline fuels and non-conventional engine designs.

The ideal alternative-fuel engine would be noticeably cleaner than current gasoline powerplants and yet still use the existing fuel infrastructure - in other words, gas stations. Interestingly, an impressively efficient, relatively low-emissions alternative-fuel engine is already here: the diesel.

On the island of Rugen off the North Sea coast of Germany, a fleet of quiet, smooth-running "biodiesel" test cars is running around on something akin to sunflower seed oil - an infinitely renewable fuel. The idea of operating efficient, practical motor vehicles on a fuel source that literally grows in the ground is certainly appealing. However, while the modern diesel shows great promise for the environmentally-conscious (and more value-conscious) future, it - like the gasoline engine - still isn't perfect.

What about compressed natural gas, propane, hydrogen, methanol or ethanol fuels? While they burn somewhat cleaner than gasoline, all fuels involve tradeoffs. For starters, because they take up more space per mile driven, these alternatives provide uncomfortably short distances between refueling stops.

The ultimate fuel and powerplant would perform exactly like the human body (and all animal life, for that matter) by converting energy using the oxygen in our air and the carbon and hydrogen in our "fuel," then breathing out exhaust laden only with water vapor and extra carbon dioxide. Is even that good enough? Doesn't extra [CO.sub.2] hold in heat and contribute to our so-called global warming?

The new paradigm suggests that perfect propulsion should behave more like a tree, actually converting some of that [CO.sub.2] back into oxygen. Although we're not aware of any current technology that can do that, a car powered by an ultra-low emissions engine (soon to be available) and equipped with a catalytic radiator (which purifies certain pollutants in the slipstream of air passing through it) could come close.

What about the electric car? Moving emissions from the tailpipe of a car to the smokestack of an out-of-town electric plant will be worthwhile in some areas, but let's not kid ourselves about any real reduction in pollutants unless your electricity comes from a hydroelectric or nuclear plant.

Battery-powered electric cars may well prove to be a good short-term solution for areas with special environmental problems such as Los Angeles. (We shouldn't forget that records of early Spanish explorers noted natural "smog" in the L.A. basin over 200 years before Gottlieb Daimler and Karl Benz invented the first motor car.) Much development work remains to be done before large numbers of consumers are likely to embrace the idea of actually buying and living with electric cars. Today's battery technology means that electric cars - even the Mercedes research vehicles with long-life, high-capacity sodium nickel chloride batteries - are limited to about 100 miles (161 km) of driving.

The most promising technology seems to be the fuel cell - a generic-sounding name for an elegant device that makes electrical current on board without producing pollutants. Fuel cells use special plastic membranes that allow hydrogen and oxygen to combine chemically without combustion, to make electric current efficiently. The only by-product is water.

Limited so far to exotic uses such as powering satellites and space stations, the fuel cell could combine the driving range of a gasoline car with the advantages of an electric car without the need for batteries. Right now, fuel-cell technology is very expensive and probably a decade or so away from practical use. Unlike conventional batteries, however, fuel cells are at the beginning of their development curve and are expected to drop in price (and bulk) dramatically in the next few years.

What about fuel for the fuel cell? Most promising is an onboard system that "cracks" hydrogen atoms from methanol - a relatively safe, liquid fuel that could be dispensed at any gas station. A methanol reformer can provide fuel cells with long-legged driving range equal to today's gasoline cars. That could result in near-zero-emissions that meet everyone's goals.