Carmakers' plug-in plans

Ford's HySeries system is similar to that used in GM's Chevrolet Volt. The biggest difference is that the HySeries uses a hydrogen fuel cell, not a gasoline engine, as an on-board back-up generator.

But either vehicle could use any type of on-board power generator. That's one of the benefits of this type of system. It can be produced economically with different sorts of engines or with fuel cells.

The wheels of these vehicles are powered by electricity alone at all times. The fuel cell, in this case, is used only to recharge the batteries if they run low.

Ford claims a plug-in-power-only range of 25 miles before the hydrogen is needed. After that, the Edge with HySeries can go another 200 miles on a tank of compressed hydrogen.

NEXT: Toyota Prius plug-in