Felix Wankel's radical rotary engine never fulfilled its promise of a smooth ride for all. David Wilkins tells the story

Fifty years ago this month, the first prototype of an exciting new type of internal combustion engine sprang to life on a German testbed. The Wankel power unit threatened briefly to sweep away the piston engines that powered almost every car on the road, but in the end, never quite lived up to its promise.

The inventor of the rotary engine, Felix Wankel, born in 1902, was a remarkable fellow too. As an engineer, he was largely self-taught - no rarity in Britain, where the motor industry was traditionally dominated by "practical men", but in Germany, academic titles have always commanded enormous, perhaps exaggerated respect. Felix Wankel didn't even possess the one paper qualification that almost everyone acquires sooner or later - a driving licence.

Wankel's engine design used a curved-sided, triangular rotor (or rotors) revolving orbitally around a central shaft within a housing shaped, in cross-section, like an elongated circle; it therefore ran much more smoothly than a conventional reciprocating engine in which the pistons constantly change direction, violently accelerating and decelerating as they turn the crankshaft.

Wankels thrived on revs and were light and small in size in relation to their power outputs. They also had mechanical simplicity on their side, as they eliminated most of the complicated valve-gear associated with piston engines.

Wankel established links with NSU in 1950 and had already been working with the German car and motorcycle manufacturer for some time when the first test of the rotary engine took place in February 1957; the first car powered by the Wankel, a converted NSU Prinz, ran in 1960 and the first production model was the convertible NSU Wankel Spider of 1964.

At first, the rotary engine attracted enormous interest, and by 1967 it looked as if it would be taken up in large numbers. That year saw the launch of NSU's sleek Ro80 saloon; if the Wankel power unit was an impressive piece of engineering, the Ro80 was a fitting showcase for it, incorporating several other advanced features such as inboard mounting of the front disc brakes and a semi-automatic gearbox.

Another development in 1967 was the formation of Comotor, a joint venture between NSU and Citroën for the production of Wankel engines. Citroën had - and has - a long history of producing highly advanced cars that looked the part but were hobbled by ancient engines; Wankel's designs promised to give Citroën's cars the modern power units they deserved.

By this time, several other companies had also acquired Wankel licences. Car manufacturers such as Daimler-Benz, Alfa-Romeo and Toyo Kogyo (Mazda) signed up, but some licencees clearly had other applications in mind.

One was Curtiss-Wright, an American aerospace company; another was the German truck maker MAN. Everyone, from Britain's Rolls-Royce, down to Graupner, a manufacturer of tiny engines fitted to model aeroplanes, seemed to be interested. Mazda had launched its first rotary-engined car, the Cosmo, in 1965, and Mercedes pressed ahead with its spectacular mid-engined Wankel-powered C111 prototypes, the first of which ran in 1969, the year in which Citroën started to produce small numbers of the Ami-based, rotary-engined M35.

Then it all started to go wrong. The first engines fitted to the Ro80 proved to be unreliable; this was usually blamed on the rotor tip seals, although maintenance problems caused by mechanics' and owners' unfamiliarity with the ways of the Wankel ways may have played a part too. The costs involved caused NSU to collapse into Audi's arms in 1969. Citroën, meanwhile, was readying the Wankel version of the GS, the Birotor, which it ended up launching into the teeth of the first oil crisis in 1973. This highlighted the main weakness of the Wankel - its high fuel consumption, the factor that, more than any other, prevented its widespread adoption in the end. Birotor production ended and Citroën had to be rescued by Peugeot in 1974. Most other carmakers dropped the Wankel engine, too, and Mercedes's later C111 prototypes were used to test diesel piston engines instead.

Under Audi's ownership, production of the Ro80 continued, in small numbers, until 1977. The Ro80 was not directly replaced, but perhaps something of its revolutionary spirit lives on in later Audi innovations, such as aluminium bodywork and the quattro drivetrain; several of Audi's larger cars incorporating these features have subsequently been built at the former NSU plant at Neckarsulm.

Meanwhile, on the other side of the world, Mazda persisted with the rotary engine, quietly solving, or at least mitigating, the problems that had done for it in Europe. Felix Wankel died in 1988, so he lived to see his engine establish itself as the power unit for Mazda's sporty models, such as the RX-7, launched in 1978, and today's RX-8. The Wankel may not, as once seemed likely, have swept the world. But for those who prize its rare blend of qualities, it is at least still available in a car that works.

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