"If god had meant us to drive electric cars," engineers joke, "then He wouldn't have made lead so heavy." Lead-acid batteries pack rather little energy into their bulky selves, but their high peak power has long made them the reluctant choice of automotive tinkerers.

To get the lead out and still power a full-size, air-conditioned city bus, the Electric Fuel Corporation has resorted to zinc, in the form of a zinc-air fuel cell. The zinc bus recently took its maiden voyage in Las Vegas, where it was remarkable for what it lacked: carbon monoxide, particulates, and noise.

The company, based in New York but growing out of an Israeli venture, already incorporates the technology in a line of cell phone rechargers. To maximize output, the fuel cell has a highly permeable membrane that speeds oxygen on its way to the electricity-conducting liquid inside. It also has a specially fluffed-up zinc, grown in microscopic Christmas-tree form, that increases the surface area available for the reaction.

Electric Fuel's bus uses a second, relatively small, nickel-cadmium battery to provide the surges of power necessary for acceleration. The battery, designed by the company's partner, General Electric, stores electricity the cell puts out when the bus is standing still. It also recovers braking energy, a trick accomplished by making the motor act as a generator. Such economies, combined with the low maintenance needs of a transmission-free vehicle, should keep costs for a large, centrally managed fleet just 10 percent more than for a fleet of diesel buses, the company claims. That seems a small price for sweeter-smelling air.