Wankel

KEN W. PURDY, who is now living in London, is widely known as an editor and writer and as an authority on the automobile.

The news that the Ford Motor Company had acquired a license to produce a new German automobile engine may have announced as profound a revolution as that heralded by the appearance of the Model T itself in 1909, an event which did indeed upend the world. The other signatory to the Ford contract is the NSU Motorenwerke Aktiengesellschaft of Neckarsulm, West Germany, and the engine concerned is the Wankel, a radical and astonishing device. Technically an internalcombustion rotary engine, the Wankel is much smaller and lighter than standard engines producing comparable horsepower, has far fewer parts, is easier to manufacture, and delivers smooth, vibration-free power on ordinary gasoline.

The Wankel is not shiny-new. What is new is the prospect that its success will be, as such things are reckoned, almost immediate. I wrote an article on the Wankel for the Atlantic in July, 1960. There were then authorities who thought that something might possibly come of it, and there were reputable engineers willing to say flatly that the thing would never be heard of again.

Now, however, the Wankel engine is running an automobile, the NSU Spyder; the car is in production and on sale, and when Ford acquired its manufacturing license, it was listed on NSU’s books as the twelfth licensee, behind such notables as Curtiss-Wright, Alfa-Romeo, Daimler-Benz, Citroën, Toyo Kogyo of Japan, and Perkins of England. It is Ford’s massive weight in prestige and wealth, and its present remarkably active and forward-looking orientation, that make its interest in the Wankel so important.

The Wankel engine is a solution — it appears to be the only practical solution so far discovered — of a problem that has challenged and fascinated engineers since the birth of the gasoline engine: how to make an engine run with a rotary rather than reciprocating motion. Because rotary motion, that of such primitive engines as the waterwheel and the windmill, is infinitely desirable.

The standard internal-combustion engine is not at all well suited to the task of driving an automobile. Indeed, it is inherently so unsuited that one must marvel, seeing how well it has been made to work. A fine automobile runs smoothly and almost soundlessly, but the sensation is illusory: in the heart of the vehicle, the engine, great violence is being done, and quiet is achieved by most sophisticated and subtle refinements of muffling and insulation. The nature of the beast reveals itself when it is operating at optimum efficiency, unmuffled on a laboratory test stand — the cannon fire of the exhaust, the howling of gears and the roar of inrushing air, wild clatterings and bone-jarring vibrations. A reciprocating engine seems always to be trying to destroy itself, and so it is.

The word “reciprocating” is the key. A reciprocating engine is in effect a multibarreled cannon: the fuel charge the gunpowder, the piston the projectile, the spark plug the detonator. The charge of vaporized gasoline and air explodes; the piston, driving its connecting rod, starts to fly out the barrel, but after only two or three inches it must stop, reverse itself, and come flying back toward the breech. It is this repeated reversal of movement, thousands of times a minute, that is at the root of the savagery. An electric motor, smoothly spinning, can be run sitting on a table, but an internal-combustion engine must be firmly fastened down. And to achieve the unwieldy up-and-down, in-and-out movement, a train of parts parasitic to the working pistons are needed: one spark plug per cylinder, at least two valves per cylinder, with a camshaft to run each, connecting rods, rockers, pushrods, and so on. Then there must be the huge and heavy crankshaft (it has weighed 250 pounds in some engines) to transform the reciprocating motion of the connecting rods to the only kind that will turn the wheels: rotary. Obviously it would be far better if the motion were rotary to begin with.

Steam and water turbines deliver rotary motion, but neither is suitable for the automobile. A gas turbine, in which the expansive force of burning fuel and air is directed against many tiny paddle-wheel blades, was built by the Rover Motor Company of England in 1950. In 1951 the Rover gas-turbine car ran 151 miles an hour; two years ago, a Rover won the special prize for the first gas-turbine car to finish the twentyfour-hour race at Le Mans. The major Detroit houses all have produced their own gas turbines now, and Chrysler has fifty gas-turbine cars running on the roads in consumer use at this moment. But automobile engineers are not wholly enthusiastic about the future of the gas-turbine engine, It is basically a constant-speed engine, best suited to aircraft use. To produce acceptable power, it must turn at speeds in the range of 40,000 to 65,000 revolutions per minute, ten times the ordinary working speed of an internalcombustion engine, with consequent hazard and wear; working in temperatures on the order of 1500° F, it requires exotic metals; it lags on acceleration, and it is difficult to make it produce an engine-braking effect when the accelerator is lifted. Fuel consumption has been improved, but it is still high. One early drawback has been overcome: a gas turbine’s exhaust will no longer cook one to a crisp at ten feet; indeed, it is no hotter than any other.

The idea of combining the advantages of the reciprocating engine — cheapness and ease of manufacture, good power at relatively low speed, reliability, and so on —with the turbine’s smoothness has always been attractive, and before Felix Wankel came on the scene, apparently unworkable. As long ago as 1769 James Watt registered patents on a rotary steam engine, but the technology of his day could not provide the methods and materials he needed. In recent years many other rotary engines have appeared (Omega, Tschudi, Rajakaruma, Renault), but only the Wankel has achieved full development.

The root difficulty in creating an internal-combustion rotary engine lies here: a device akin to a turbine wheel must be sealed in an airtight casing to contain the near-explosive burning of the fuel charge; force must be directed against only one segment of this spinning form, not its whole circumference; the charge must be compressed before being fired, and allowed to expand afterward — thus the shape of the space between wheel and casing must be altered as it turns, while the casing always remains tightly sealed.

Felix Wankel’s solution to this problem was an intellectual tour de force, and an extraordinary technological accomplishment. He created a round, drum-shaped engine. Within the engine is a triangular rotor, geared to its shaft so that it revolves off center and turns the shaft three revolutions for each one of its own. This triangular rotor, serving as piston, turns within a chamber of epitrochoidal form, a circle slightly squashed at top and bottom. As the rotor revolves, its corners follow the contours of tlie chamber, and the spaces they enclose serially expand and contract to go through the classic Otto cycle: intake, compression, expansion, exhaust. The rotor bulges outward slightly on its three sides. It is the precise form of tine triangular rotor and the chamber in which it spins that attests Wankel’s extraordinary ability. In the years the engine has been under development at NSU, research engineers have tried more than a thousand other configurations. Not one produced efficiency comparable with Wunkel’s forms.

Looking into a cutaway Wankel engine at rest, hardly bigger than a teakettle, one needs good spatial imagination to understand how the tips of the triangular rotor maintain contact, all the way around, with the walls of the chamber. It appears impossible, but it works. The rotor does exactly what a piston does, inhales fuel, compresses it into a small space, fires it, and allows it to expand. The fuel mixture is drawn in at one side of the engine, compressed as the rotor turns, carried past a single spark plug which fires it, allowed to exert its expansive effort on one side of the rotor, thus spinning the rotor, and is then swept to the exhaust side of the casing and allowed to escape. The output shaft carries the power to the wheels of the car through a standard clutch and gearbox.

Wankel began thinking about the engine more than thirty years ago. After the war he persuaded NSU to undertake it, and serious work began about 1952. As many as 150 engineers have been concerned with the project.

In September, I flew to Neckarsulm to drive the car. The engine is mounted in the rear, and there is nothing in the appearance of the car, or in its exhaust note, to indicate that the engine is radical. Except for the lack of vibration, the Wankel-Spyder feels like any other car of its class. The engine is not wholly happy at low rotational speeds, but from 3000 revolutions per minute on up it is smooth and powerful. It will take the car to ninety-five miles an hour, delivers forty miles or so to the gallon of gasoline, and has an acceptable oil consumption. It is rated at 53 horsepower. The only serious problem still remaining is a comparatively short life (15,000 to 20,000 miles) of the sealing strips on the rotor tips, which are equivalent to piston rings, and this will certainly yield to continuing research. NSU intends to build a thousand this year and five thousand next year, to sell at DM8500.

The sensation of the Tokyo Motor Show in October was a Wankelpowered two-seater by Mazda. It has a two-rotor engine: in effect, two Wankels joined. On another stand, Mazda showed a four-rotor engine. In ten years, Wankel engineers believe, there will be rotary internal - combustion engines rated at 50 to 500 horsepower. If so, by 1980 the automobile piston engine as we have known it will be where the airplane piston engine is today — obsolete for first-class service.