At the Mercedes-Benz headquarters here in Stuttgart, Germany, a showroom display illustrates the rapid evolution of the iconic luxury brand’s hydrogen-fueled cars. In a 1992 prototype, the fuel cell takes up the entire cargo area of a delivery van. Across the room, today's model — now merely the size of an average television set — is displayed in front of a neon-lit outline of a compact passenger car. This “zero emission” engine, it appears, is ready for prime time. With the aid of government spending and joint efforts by Ford and Nissan, Daimler — Mercedes’s parent corporation — aims to start selling hydrogen-fueled cars to the public as early as 2017.