Honda Motor Co. said Friday that it has developed a fuel-cell power generator, known as a stack, that enables a hydrogen-powered vehicle to start up at temperatures below freezing.

Making such a stack, which generates electricity to power an electric motor that actually propels the vehicle, has been a major technological goal for automakers hoping to promote the environmentally friendly cars.

The new stack can operate at temperatures as low as -20 by using a stamped metal separator and plastic aromatic electrolyte membranes as key components to generate electricity.

Its previous stack, which was developed in 2001 by using carbon separators and fluorine electrolyte membranes, operates only at temperatures above zero, company officials said.

The new stack can also generate 50 kw, while the previous stack generated 35 kw. The high power helps a fuel-cell vehicle smoothly accelerate.

Honda plans to conduct road tests this winter on its fuel-cell vehicles in Japan and the U.S. Northeast, using the new stacks.

The company has yet to decide when it will market a hydrogen vehicle with the new stack, company officials said.

Honda began leasing the fuel-cell vehicle FCX using stacks supplied by Canadian auto parts maker Ballard Power Systems Inc., to government authorities in Japan and California last December. The details of the Ballard stack's components are not disclosed.

Compared with the FCX, however, a similar vehicle using Honda's new stacks has 10 percent better fuel efficiency, according to Honda.

A fuel-cell vehicle is powered by electricity generated through a chemical reaction between hydrogen and oxygen in the stack, which is power generator. Because they emit water vapor instead of harmful exhaust gases, they are expected to replace gasoline-powered vehicles a few decades from now.