It's much cleaner than the most environmentally friendly fossil fuels, and has the potential to be even cleaner than battery-powered vehicles. Used as fuel in conventional combustion engines, it emits only water vapor and small amounts of nitric oxides, and no carbon dioxide.

Used to power fuel cells -- which produce electricity through a chemical reaction -- it's the only true zero-emissions source. Battery-powered electric vehicles, of course, require power from polluting power plants.

Some day these positive images of hydrogen -- a major component of water and the most plentiful element in the universe -- may erase the more vivid negative ones: the Hindenburg dirigible exploding in flames, the hydrogen bomb, dangerous rocket fuel.

Odorless, tasteless, and -- if derived from water through the process of electrolysis -- an inexhaustible resource, many futurists and hydrogen proponents regard it as the ultimate fuel of the future. "People want the feeling there is a renewable energy source, and hydrogen is being recognized as the ultimate renewable energy," says Vahe Kludjian, manager-developmental research for Mazda Research and Development of North America.

But questions about safety, high costs, lack of a distribution infrastructure, on-board storage -- and the lack of a high-powered lobbying group -- have made hydrogen a minor player in the alternative fuels race. That may be changing. Environmental activists worried about carbon-dioxide-induced global warming and a few powerful friends in Washington, DC, are starting to pump up hydrogen's future as an alternative fuel. A steady stream of technical advances isn't hurting, either. At their worst, hydrogen-powered vehicles can drive as far and fast as EVs, and in some cases much farther.

Rep. Robert Walker (R-PA), the new chairman of the Federal Science, Space and Technology Committee, is a long-time hydrogen advocate and is using his power to push a bill, the Hydrogen Future Act, that would earmark $100 million in federal funds for hydrogen research and development projects over the next several years.

The House passed a similar bill last year, but it was killed in the Senate. This year, prospects for passage by both chambers is reportedly better, thanks in part to another powerful supporter, Sen. Tom Harkin (D-IA).

According to attendees of the 6th annual Hydrogen Association meeting in Virginia last month, Rep. Walker gave a passionate speech supporting more hydrogen research, and proponents now are aglow with the prospects of increased funding from what is known as the Walker Bill.

One industry insider says the bill could boost U.S. hydrogen research funding from about $10 million to $25 million per year.

Interest in hydrogen as a fuel is especially keen outside the U.S., where carbon dioxide and its potential to produce global warming is considered a serious environmental threat. Industry sources say funding for hydrogen-fueled vehicle research all over the world increased over 15% last year to about $55 million.

Japan's Ministry of international Trade and Industry (MITI) is currently engaged in a global hydrogen transportation and utilization systems project known as WENET (World Energy Network), and hydrogen production and utilization projects are currently under way in Canada and Germany.

California's demand for zero-emission vehicles beginning in 1998 also is driving increased interest. Three hydrogen-powered pickup trucks are expected to start work this year at a Xerox Corp. facility in California, and a bus manufacturer is expected to have a small number of electric buses equipped with hydrogen-powered fuel cells by 1998.

Automakers long have understood hydrogen's potential as a fuel, and most have had developmental projects in place since the 1970s. However, Daimler-Benz AG, BMW AG and Mazda Motor Corp. are typically the most open about their activities.

Last year Mazda initiated a hydrogen-vehicle testing program at Nippon Steel Corp. in Hirohata, Japan, which produces hydrogen for use in making sheet steel. Since the end of 1992, it has provided Mazda with hydrogen fuel and a test site to conduct durability tests on hydrogen-rotary engines.

In addition to hydrogen's environmental benefits, Mazda is intrigued by hydrogen because its combustion characteristics seem especially well suited to rotary engines.

Besides developing numerous prototype vehicles, Daimler-Benz AG ran a test fleet of hydrogen-powered taxis in Berlin from 1984 to 1988. Its conclusion: "Running vehicles on hydrogen is possible. However, some restrictions have to be taken into account compared to conventional petrol and diesel vehicles, such as shorter range, lower payload and longer filling times. Hydrogen is therefore ideal for vehicles that are not as restricted by high weight and large tank volume as is the case with passenger cars."

Those vehicles stored hydrogen in special metal-hydride tanks, where gaseous hydrogen is bonded chemically to metal powders contained in tubes. The hydrogen is released by supplying heat to the tank. Hydrogen also can be stored on-board in high-pressure cylinders or as a super-cooled liquid.

Mercedes is looking at a new concept where hydrogen is chemically bonded to liquid instead of metal powders. This of course would be preferable and allow the fuel to be distributed and stored much like gasoline and diesel fuel. Unfortunately, a Mercedes source says the only liquids capable of doing the job are highly toxic. Researchers now are hard at work trying to find more suitable liquid carriers.

Although some automakers totally reject the idea of cryogenically storing any fuel on-board, BMW is enjoying considerable success with the concept. Thanks to a new highly efficient tank design, its latest hydrogen-powered 7-Series test vehicle can drive as far as 250 miles (400 km) on one tankful of liquid hydrogen. That's double the best-case scenario for an EV. Plus, most hydrogen-powered vehicles can be refueled in 10 to 15 minutes. EVs require hours of recharging.

The BMW also can go back and forth between hydrogen and gasoline power with the flip of a switch. The 2.5L 6-cylinder engine makes about 190 hp with conventional fuel, about 109 hp with hydrogen.

One of the major criticisms of storing gases cooled to hundreds of degrees below zero is that they soon warm, expand and build up tank pressure so that they have to be released into the atmosphere through special valves. That means your car can "run out of gas" while sitting at the airport. BMW spokesman Thomas Zauber says, however, that the automakers' tanks now are so efficiently insulated that a vehicle can sit for weeks at a time with only tiny fuel losses. Driving a vehicle as little as twice a week ensures almost no fuel loss.

Furthermore, Mr. Zauber says the tanks have been extensively crash-tested and found to be very safe. A full tank placed in a hot fire turned out to be a very boring demonstration, he says: the hydrogen heated up and evaporated through the pressure vents uneventfully.

One of the most interesting paradoxes involving hydrogen fuel today is its cost. About 90% of hydrogen now is derived from fossil fuels such as natural gas, and is mainly used by the chemical industry as a raw material and by aerospace as a fuel. Sourced this way, it is relatively cheap. Producing it through the environmentally correct method of electrolysis -- using hydroelectric or solar power -- is much more expensive.

Cooling hydrogen to -482[degrees]F (-250[degrees]C) to use it as a more energy-dense liquid adds further to the cost. BMW's Mr. Zauber says an experimental Canadian project is producing hydrogen in a volume the equivalent of one liter of gasoline with hydroelectric power for $1, or roughly $4 per gallon. Producing liquid hydrogen with solar power bumps costs up to about $12 per gallon.

These high costs now have automakers working with outside companies to develop more efficient electrolysis processes.

Meanwhile, most automakers are working with natural gas as a prelude to hydrogen because the two behave similarly. Some researchers are even adding hydrogen to natural gas as a means of reducing it's allegedly harmful greenhouse emissions.

As experience using gaseous fuels mounts, engineers and vehicle buyers will be less intimidated by hydrogen's differences from traditional fuels, proponents argue. Yet, some big questions remain.

Even though hydroelectric power is called environmentally friendly, environmentalists resist building more hydroelectric dams, and in some cases demand that others be dismantled. As a BMW source notes, generating huge amounts of electricity utilizing massive wind, solar or hydroelectric power stations also "would ultimately leave severe marks on nature."