If the Orbital Engine Co. U.S.A. Inc. has its way, today's four-stroke gasoline engine soon could disappear. Taking its place: a simpler, lighter design offering far more horsepower per liter. But more important, Orbital's two-stroke engines promise up to 30 percent more fuel economy-a boast that's getting lots of attention from Congress as it debates a big jump in auto mileage standards.

Eleven carmakers, including General Motors Corp., Ford Motor Co., Fiat Auto S.p.A. and Honda Motor Co., have signed up for licenses from Orbital's Australian parent, Orbital Engine Corp. Ltd. So has India's Bajaj, the world's third-largest motorcycle manufacturer, and a number of boat engine makers, including Outboard Marine of Waukegan and Brunswick-Mercury Marine, a unit of Skokie-based Brunswick Corp.

Ford recently introduced two two-stroke-powered prototypes, the Zig and the Zag. And in a speech at the Geneva Auto Show this year, Lindsay Halstead, chairman of Ford of Europe, said, somewhat cryptically, the two-stroke engine "will become familiar to you . . . before you are 10 years older."