OXNARD, Calif. — Instead of the far-off promise of cars propelled by pollution-free hydrogen fuel cells, BMW said last week that it plans to field test cars that burn liquid hydrogen in their engines next year in the U.S.

There's just one big catch.

When BMW launches a fleet of about 25 modified 7-Series sedans in the U.S. — among about 100 worldwide — the liquid hydrogen will have to be stored at more than 400 degrees below zero. And there are only a couple of liquid hydrogen pumps in the country.

The cars will be doled out to agencies or government officials for testing. None will be sold.

The cars will run on hydrogen or gasoline in their modified internal-combustion engines. They will be able to switch between fuels at the push of a button mounted on the steering wheel.

The bolted-in, 45-gallon liquid hydrogen tank will provide for a range of about 125 miles. The tank is designed to keep hydrogen cold for up to 17 hours. After that, it vents away as water vapor.

Other automakers are focusing on building fuel-cell vehicles. They are powered by a chemical reaction involving hydrogen inside the cell, not burning the hydrogen in an engine as in the BMW.

"Everybody else has (gone) in another direction with gas hydrogen," says Ron Cogan, publisher of the Green Car Journal. "BMW is pretty much standing alone." But he adds, "Who is to say they are wrong?"

Instead of liquid as in the BMW, fuel-cell vehicles usually use hydrogen in a gaseous form. There are more places to refuel those vehicles.

BMW officials know of only two stations in the U.S. dispensing liquid hydrogen. One is at its own testing center in Oxnard. The other is in Washington, D.C.