When the Berlin wall opened last fall, one recurring image of a society where technology had stood still for 40 years was the little East German Trabant automobile, with its noisy two-stroke engine spewing pollutants and fumes. Westerners are more accustomed to two-strokes in lawnmowers than in cars.
But if two-stroke engines seem like yesterday's technology, they may also be tomorrow's. Limited in appeal for decades because of high emissions, poor gas mileage and a rough idle that prompted the nickname ''corn popper,'' two-strokes also have inherent advantages of light weight, small size and simplicity. This has prompted nearly every major automobile manufacturer to look at them anew.
At recent car shows, the General Motors Corporation displayed a sleek sports car and Toyota a luxury coupe equipped with prototype two-stroke engines.
Both manufacturers say cars with such motors could be in production by 1995. Automobile engineers say advances in electronics and fuel-injection systems have made possible smooth-running, powerful two-stroke engines that meet current and anticipated emission-control requirements.
''We're interested because the fundamentals are just so good,'' said Donald Runkle, General Motors' vice president of advanced engineering. Two-stroke ''engines are lightweight and they're small so the horsepower per pound and the horsepower per size are excellent,'' he said.
Two-stroke engines differ fundamentally from the four-strokes in most cars in that they develop power once for every two strokes of the piston - up and down. That equals once for every complete revolution of the engine crankshaft.
Four-stroke engines require two complete turns of the crankshaft - two up and two down strokes of the piston - to develop power from each cylinder. A three-cylinder two-stroke has the same firing frequency as a six-cylinder four-stroke.
Typical two-strokes also have far fewer moving parts. Four-stroke engines have one or two intake valves to let gas and air into the cylinder, plus one or two exhaust valves to let exhaust out, which must be carefully timed, requiring a complex system of camshafts driven by chains or belts. In a two-stroke, the motion of the piston itself uncovers an intake port and an exhaust port, simultaneously letting air and fuel in and the exhaust out.
The problem with this arrangement is that a lot of unburned and partially burned fuel is left in the exhaust, giving two-strokes a level of hydrocarbon emissions about 100 times that of a comparably sized four-stroke engine. That statistic alone spelled the death of two-stroke automobiles, at least in the United States, over 20 years ago. Sweden's Saab sold a small two-stroke engine car here until 1968, the same year emissions control laws went into effect.
To overcome emissions problems, General Motors' new engine uses fuel-injection technology developed by Ralph Sarich, an Australian inventor and founder of the Orbital Engine Company of Perth. Mr. Sarich's design, also licensed by Ford, improves on typical fuel injection, which squirts liquid fuel into the intake manifold by blasting the fuel directly into the cylinder with compressed air.
The air helps vaporize the fuel quickly, prompting more complete burning, while electronic control of the injection and spark allowsvery precise timing of fuel delivery and firing of the piston. By injecting the fuel after the exhaust ports have closed, the General Motors engine avoids passing unburned gasoline out with the exhaust.
Many modern two-stroke engines use some kind of blower, like a turbocharger, to force out exhaust gases, but the General Motors' design relies on pressures created in the engine's crankcase, a simpler approach.
''We're looking for the elegant solution - no valves, no turbocharger,'' said Mr. Runkle. The three-cylinder engine has a displacement of 1.5 liters, weighs just 165 pounds and develops 110 horsepower.
General Motors' engine needs further development to establish its reliability, but already, automotive engineers are enthusiastic about the design's potential. Because the engine is so much smaller than a four-stroke of comparable power, a car's engine compartment could be smaller too, reducing aerodynamic drag. With reduced weight, power steering would be unnecessary, providing further gains in efficiency.
''If you could propagate the size and weight savings throughout the vehicle, there might be in excess of a 20 percent fuel economy gain,'' said David E. Cole, director of the Office for the Study of Automotive Transportation at the University of Michigan. ''It's a carrot of considerable magnitude.'' he said.
Where General Motors has opted for the ultimate in simplicity, Toyota has taken a far different approach. Toyota's six-cylinder two-stroke engine also uses direct fuel injection, but it uses double overhead camshafts and four valves per cylinder, as in the four-stroke Supra or Cressida engines, which it closely resembles. A supercharger forces pressurized air through the intake valves to blow spent exhaust gases out of the cylinder.
While this engine is no lighter or less complex than its four-stroke brethren, it does offer the advantage of smoother running - a two-stroke six-cylinder engine is as smooth as a four-stroke 12-cylinder engine - as well as greater power and torque at lower engine speeds. Toyota also claims conventional valves - called poppet valves - and supercharging allow a cleaner running engine.
''The use of poppet valves gives us unusually precise control over the combustion sequence; it keeps the mixture in the combustion chamber longer and allows us to burn it more completely,'' said Michael Michels, product news manager for Toyota Motor Sales U.S.A. Toyota believes the engine's low-r.p.m. power and inherent smoothness lend themselves to luxury car applications, and is currently testing it in a Soarer, a large two-door coupe not sold in the United States. Automotive journalists have been impressed. ''It was everything they claimed,'' said Dennis Simanaitis, engineering editor of Road & Track, after driving the two-stroke equipped Soarer in Japan. ''The engine has a lot of bottom-end torque, a lot of power,'' he said. ''It was quite smooth; it did not have the popcorn sound of the old Saab,'' he said.