MUNICH, GERMANY – Long lists of restrictions are familiar to journalists who drive press cars from automakers’ fleets: no smoking, no racing, no loaning the keys to your fugitive brother-in-law. But one prohibition placed on the BMW Hydrogen 7, a 760Li luxury liner modified to run on hydrogen in addition to its normal gasoline diet, was an eye-opener: The Port Authority of New York and New Jersey would not permit the car to be driven through the Lincoln or Holland Tunnels or on the lower level of the George Washington Bridge, according to www.nytimes.com.

It seems that BMW drew the Port Authority’s attention when it began pumping liquid hydrogen into its small test fleet of dual-fuel sedans in Port Jersey, not far from the docks where BMWs disembark after their voyage from Germany. But while BMW designed the Hydrogen 7 to be as explosion-resistant as any gasoline car, memories of the Hindenburg zeppelin cause misunderstandings among consumers and bureaucrats, a company spokesman acknowledged.

As with hydrogen cars from Ford, General Motors, Honda and others, showcasing hydrogen’s carbon-free potential is the BMW’s reason for being. But unlike far-costlier fuel-cell cars — which generate electricity through a chemical reaction of gaseous hydrogen and oxygen — the BMW runs on either liquid hydrogen or gasoline in a familiar internal-combustion engine.

When it is running on hydrogen, water vapor is the main byproduct, as shown by a damp spot beneath the bumper when the car idles. Emissions of carbon dioxide and nitrogen oxides are nearly zero, with just traces resulting from the engine’s lubricating oil and the heat of combustion.

BMW built its demonstration fleet of 100 Hydrogen 7s beginning in 2006 and keeps eight in the New York area and a dozen in Los Angeles, where testers have included the celebrities Leonardo DiCaprio, Brad Pitt, Will Ferrell, and Edward Norton. All told, the 100 BMWs have logged more than a million miles in testing around the world.

Automakers like Ford and Mazda are also experimenting with the technology of using hydrogen in conventional engines. Ford has built roughly 30 hydrogen powered V-10 buses for commercial customers in the United States and Canada, including major airports, the city of Las Vegas, and SeaWorld Orlando. In Japan, Mazda is road-testing a Premacy hydrogen hybrid minivan that combines a rotary engine with an electric motor and lithium-ion batteries.

BMW’s Hydrogen 7 is powered by the same 6-liter V-12 found in the 760Li, re-engineered to handle the special qualities of hydrogen, including separate injection systems for each fuel. But the real gee-whiz factor is at the rear, where tanks hold 17.5 pounds of hydrogen and 16 gallons of gasoline.

Inside that hydrogen tank is nature’s lightest element, super-chilled to 423 degrees below zero. At that temperature, nearing the frigid cold of outer space, hydrogen becomes a liquid and shrinks to about one-thousandth the volume, allowing the tank to pack in more fuel. Yet the vacuum-insulated tank felt room temperature to the touch, so well sealed that a block of ice inside would take 13 years to fully melt, BMW engineers say, and a fill-up of coffee would remain hot enough to drink three months later, according to www.nytimes.com.

The hydrogen tank leaves only enough trunk space for a pair of golf bags; a second-generation tank that fits the space more efficiently has been designed. The company is working with other automakers to create a standardized refueling system. When the car is parked for extended periods, the evaporating hydrogen builds pressure that must be safely released. A boil-off system mixes the hydrogen with air, runs it through a catalytic converter and releases water vapor through a rear-bumper vent.

Redundant safety systems abound. If the pressure inside the tank rises too high, a vent in the roof can release gaseous hydrogen directly. And if the car happened to roll over and block the roof opening, hydrogen would reroute through the underbody. A hydrogen detection system makes the car’s four door locks glow red to warn of leaking fuel in the trunk, fuel nozzle area or under the hood; windows automatically open if hydrogen should enter the cabin.

BMW conservatively estimates a 125-mile range on hydrogen, with another 300 miles on gasoline.

If the hydrogen economy fails to take off, BMW is hedging its bets with other technologies. The company plans to unveil an all-electric Mini Cooper at this year’s Los Angeles auto show; no date has been announced for sales.