Between the fuel cells that will power the cars of the future and today's petrol and diesel engines lie hybrid internal -combustion/electric vehicles. No one now believes that battery-driven electric cars will take over:  the batteries are too heavy and run down too quickly. But cars that get their electricity from and run down too quickly. But cars that get their electricity from an internal -combustion engine acting as a generator are a reality. Two such hybrid vehicles are already selling in America and Japan. However, they cost more than normal models and weigh more than conventionally powered cars.