KEN W. PURDY
TWENTY-FIVE years ago, a German book salesman named Felix Wankel entertained a revolutionary, if not heretical notion: he fell to wondering if the standard gasoline engine, man’s commonest prime mover, might not be replaced by something better. The announcement a few months ago that the Wankel engine is in being may presage a period of notable excitement in the automotive field.
The internal-combustion reciprocating engine is basically so unsuited to its task that its universal acceptance and success are a source of wonderment. Driving behind a Rolls-Royce or a Cadillac engine, one may marvel at the oily-smooth, almost silent flow of power, but the sensation is illusory: in the heart of the beast, volcanic violence is being done, and quiet is achieved only by almost infinite refinements of muffling and insulation. It is when a gasoline engine is operated under conditions of optimum design efficiency, as in a racing model, that the nature of the animal reveals itself in the howling of gears, the continuous roar of the exhaust, 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, with the fuel charge the gunpowder, the piston the projectile, the spark plug the primer, or the trigger, to oversimplify the matter. The charge explodes; the piston, driving its connecting rod, starts to fly out the barrel; but at the end of only two or three inches of flight, it must stop, reverse itself, and come flying back toward the breech of the barrel. It is this repeated reversal of movement, taking place thousands of times a minute, that is at the root of the reciprocating engine’s savagery. A high-performance engine may move each of its pistons a total of 4000 feet up and down the cylinder bores in one minute. Every engine has a speed limit beyond which the strain of beginning, stopping, and reversing the piston movement becomes too great, and the engine destroys itself.
Because the piston movement is so violent, and because each stroke, for all its violence, produces little power — all the energy used in accelerating and stopping the piston is wasted — the internalcombustion engine can deliver useful effort only when it is running at high speed. At low speeds, as everyone who has stalled an automobile knows, it produces inadequate twisting effort, or torque. Thus, in order to drive a vehicle, the internalcombustion engine must be harnessed to a system of gears and there must be some form of clutch to connect and disconnect the flow of power to the road wheels. It was this necessity for gears and clutches, plus the gasoline engine’s inability to start itself, that convinced early steam-automobile builders that victory would surely he theirs in the contest between the two forms of propulsion. That it was not was no fault of the steam engine, which could start itself, could produce tremendous torque when turning almost imperceptibly slowly, and in itself was almost the ideal automotive power plant. Lack of efficient steam-producing equipment, coupled with the stubbornness and shortsightedness of its masters, betrayed the steam engine.
From the beginning, internal-combustion engine designers have striven to achieve the smooth power flow characteristic of the steam engine. The steam engine was a reciprocating engine, true, but a smooth, easy-running one, because the expansion of steam is slow and inexorable, while the expansion of burning gasoline and air is sharp and violent. The rotary movement of the universe has appeared to be the ideal. The steam turbine and the water turbine produce wonderfully constant, even power flow, but neither is suitable for automotive use. The gas turbine, in which the expansive force of burning kerosene and air is directed against the many tiny paddle-wheel blades of a turbine, is a possibility. The first gas-turbine car was run by the Rover company of England in 1951, and the major Detroit houses have since then produced their own versions, but the device suffers in some particulars: to produce adequate power it must turn at approximately 40,000 revolutions per minute, with consequent hazard and severe wear; the extreme temperature of the burning gases enforces the use of exotic metals in the turbine, the blades of which may have to run red-hot without losing strength or shape; it lags on acceleration, offers no enginebraking power when the foot is lifted from the gas pedal, and its fuel consumption is relatively high.
The idea of an engine rotary in basic motion but fueled and fired in the same manner as a reciprocating engine is most attractive, and many designers have struggled with the problem of how to make a paddle wheel turn by exploding gunpowder against its blades. The conundrum is an intriguing one. The turbine wheel must be sealed in an airtight casing to contain the explosion; the explosion must be directed against one segment of the paddle wheel, not its whole circumference; the fuel mixture must be drawn in at one place and its residue exhausted at another; since the charge must be compressed before being fired and allowed to expand afterward, the shape of the space provided between paddle wheel and casing must be altered as the wheel turns, but even as it is being altered, it must be tightly sealed. These considerations have defeated some good men, and in consequence the announcement by CurtissWright last November that it had a workable internal-combustion rotary engine was held to be news of the first importance. Here was a 29horsepower engine weighing only 26¾ pounds. It had no valves, no pistons, no connecting rods, no reciprocating parts of any kind, no crankshaft. It was comparatively cheap and easy to make.
THE Curtiss-Wright announcement conveyed the impression that the engine is largely an American development, and not until the last sentence was the name of the inventor revealed. Felix Wankel is a German associated with the NSU Werke of Germany, a firm notable heretofore as maker of sewing machines, motorcycles, and ultrasmall automobiles. Dr. Wankel is a designer who has specialized in sealing problems, and since the sealing of a rotary internal-combustion engine is crucial, it was logical that he should attack the problem. His solution was ingenious in the extreme.
Externally the engine is round, or drum shaped. Within is a rotor, triangular in configuration, but with its three sides belled outward in gentle curves. The rotor is fastened to a shaft, as a water wheel is fastened. It turns within a chamber that can be described as a circle slightly squashed at top and bottom, shaped as an inflated balloon would be if held lightly between one’s hands. It is the epitrochoidal shape of this combustion chamber, together with the gearing of the rotor to its shaft in such a fashion that it revolves off center, that enables the Wankel engine to work, for as the rotor revolves, its tips or corners constantly follow the contours of the chamber, and the spaces they enclose serially expand and contract. They must: when a flat side of the rotor faces one of the outward bulges in the end of the chamber, the space is greater; when a sharp side of the rotor faces the bulge, the space is smaller. Precisely, the revolving rotor does what a reciprocating piston does. It inhales fuel, compresses it into a small space, fires it, and allows it to expand in a big space. The fuel mixture is drawn in at one side of the engine, compressed as the rotor turns, carried past a spark plug which fires it, allowed to exert its expansive effort on one flat side of the rotor, thus spinning that component, and then carried to the exhaust side of the casing and allowed to escape into the atmosphere. Thus the Wankel engine produces three power strokes for each revolution it makes.
At first glance, the Wankel engine is so simple and appears to work so well that it was understandably hailed as revolutionary. Said the magazine Sports Cars Illustrated: “Tomorrow is here. It came on November 23rd, 1959. This design is truly the work of a genius.” The Curtiss-Wright announcement of that dale envisioned the development of units ranging from 100 to 5000 horsepower and foresaw their use in every kind of application, from stationary work engines to power plants for vertical take-off and landing aircraft. It was said that the engine produced 3.8 horsepower to the cubic inch of capacity, against the long-held ideal of one horsepower to the inch. It could produce one horsepower to .83 pounds of weight. One horsepower to the pound has long been held to be a ratio realizable only by the best aircraft and automobile racing engines.
The rotary internal-combustion engine will run well on low grades of gasoline and is very quiet. Since it is fully balanced, it produces minimal mechanical vibration. As an automobile engine, it would seem to possess attractive virtues, and the unqualified enthusiasm with which it was greeted would seem to have been fully justified.
The voice of the skeptic was soon heard, however. John Bond, publisher of Road and Track, was vehement: “Our opinion on the CurtissWright-NSU engine?” he wrote. “It will never be heard from again.” Bond, an automotive engineer of more than two decades’ experience, based his attitude of skepticism on the engine’s sealing. Since the rotor’s tips must accommodate themselves to die varying chamber wall, they carry sliding, spring-loaded vanes which bear against this wall. “If you have good sealing and adequate lubrication,” Bond said, “the oil consumption is extremely high. Cut down on oil flow and the seal scuffs and fails to seal. Wankel reduces his problem by a unique and clever configuration that may work in very small sizes where the rubbing velocity at the sealing point or points is kept low. However, I doubt very much if the oil consumption can be anywhere near ‘commercial’ even in the small size unit . . . furthermore it is significant that there is no mention of internal cooling for the rotor. Any large unit would definitely have to have internal cooling and this would be very difficult to arrange.”
Bond’s point about tip speed in large rotary engines is significant. A large-diameter rotor can achieve speeds of a very high order, as in the ease of the blades of a helicopter, which appear to the eye to be turning lazily but are, at their tips, moving at tremendous speed. However, GurtissWright engineers, who envision horsepower in the higher ranges as produced by comparatively small engines coupled together rather than by one bigone, say that their models, of which nine have been built, show a satisfactory oil consumption in 100-hour tests. One C-W engine which produces 100 horsepower at 5500 revolutions per minute has a rotor about twelve inches in diameter. It consumes, according to factory figures, a little less than half a pound of fuel per horsepower hour, an economical rating. Gurtiss-Wright anticipates production for industrial use within a year or so.
The suitability of the Wankel engine for propulsion of a small automobile was cited by the NSU people in demonstration runs of one of their Prinz models. The Prinz has a 79-inch wheelbase and weighs 1200 pounds. Gordon Wilkins, a noted British motoring journalist, reported the engine to be one of the very small ones, a drum nine inches in diameter and six inches deep. He said that the performance of the car was slightly better than that of the same model equipped with the regular engine. “The complete lack of vibration was uncanny,” he wrote. NSU has since announced that the engine may be used in two models of the Prinz in 1962.
In March, 1954, hard thinking and much paperwork had convinced Dr. Wankel that his engine would work on the classic four-stroke Otto cycle (intake, compression, power, exhaust). Since 1926, when the problem had first intrigued him, he had made a complete survey of the field of the rotary engine. Hundreds of designs had preceded the Wankel, and they had all failed. He and his assistant, Ernst Hoeppner, had classified the rotaries into species and subspecies. Thus, he knew that his design was no Simple restatement of an old idea, but a wholly new conception. In February, 1957, the first engine ran in Germany. In October, 1959, the first Gurtiss-Wright version of the Wankel was tested.
These dates may have marked a revolution in men’s means of moving across the earth. We may have moved closer to what seems now to be the ultimate: a fist-sized engine. The bulky, vibrating, noisy reciprocating engine that has, despite its flaws, served so well for half a century may have begun, barely begun, its journey to the industrial museum to join the paddle-wheel steamer, the electric streetcar, and the hydraulic elevator.