As Congress debates whether to force America`s automakers to improve the average fuel economy of their fleets, Chrysler Corp. announced Thursday that it will bring high-efficiency, lightweight two-stroke engines to the market within five years.

Chrysler will offer a two-stroke V-6 in the 1996 model subcompact PL body car, to be built at its Belvidere assembly plant outside Rockford.

Two-stroke engines are more common in lawnmowers, snowmobiles, motorcycles and boats, but the automaker said it hopes to obtain up to 10 percent better mileage than its current 2.5-liter, 4-cylinder engine.

A two-stroke engine requires two piston cycles to produce power; a four-stroke requires twice as many. And because a two-stroke typically is half the size and weight of today`s traditional four-stroke car and truck engines, it not only means fewer moving parts, but also the production of smaller, lighter-weight and more fuel-efficient cars.

The two-stroke will be displayed on the auto-show circuit starting in January in a concept car dubbed the Cirrus, but the PL car will not go into production until the 1994 1/2 model year, and then in a 4-cylinder, sedan version engines only. Chrysler said it will build 25,000 copies of the PL car with the new V-6 two-stroke engine starting with the 1996 model year.

The two-stroke is not new technology. Years ago, Saab used such an engine, and the widely ridiculed and now discontinued Trabant, made in the former East Germany, also was powered by a two-stroke (though it emitted so much blue smoke from the oil mix it was dubbed ''little stinker'').

The drawback of the two-stroke, which burns a mixture of oil and gasoline, is high levels of emissions. A four-stroke engine separates the two liquids and is less of a polluter.

But the two-stroke`s popularity is increasing because of the debate over proposed legislation that would force automakers to improve their corporate average fuel economy mileage by 40 percent by the late `90s from the current 27.5 m.p.g. average.

Chrysler said it continues to work on the emissions problem. Under federal clean-air laws, Chrysler could request a government waiver to build cars with test engines while it develops the technology to solve the emissions problem, and observers say the company`s likely to request such a waiver.

Chrysler has displayed two-stroke engines in the past, most notably in the Dodge Neon concept car at auto shows this year. That car featured a 1.1-liter two-stroke engine capable of developing 100 horsepower, though it was only half the size of and weighed 40 percent less than Chrysler`s 2.5-liter, 100-horsepower 4-cylinder engine.

This year, Chrysler and Mercury Marine entered an agreement for Chrysler to develop the marine company`s two-stroke design for use in autos.

It`s possible the Fond du Lac, Wis., boat-engine maker could produce the initial run of engines for the Chrysler cars, officials said.

General Motors Corp. and Ford Motor Co. also have been developing two-stroke engines.