Spinning Dreams
Wankel Engine Development by Mercedes-Benz 1960-1975
Article by Graham Robson
Images from Daimler Archives
Scroll back to the 1970s, and we find one – perhaps just the one – technical innovation from Mercedes-Benz that was not a long-term success. In a 125-year life, that’s nothing of which to be ashamed. Even so, the company has drawn a veil over the development work it carried out on the revolutionary Wankel engine in the 1960s and 1970s.
No matter. As far as I am concerned, the company did a remarkable job in progressing as far as it did before closing down the operation. There’s no question that if the energy crisis had not erupted in 1973, and if the engine’s heavy fuel consumption could have been tamed, then the C111 – or a descendant of that car – could have replaced the 300SL Gullwing as a real icon of the breed.
However, the company had its own standards of excellence that it was not willing to abandon, or even to compromise. Even after much work, the engine did not deliver everything that designers originally hoped it would do. Not only that, but the rest of Europe’s engineers also found the technology challenging. One very distinguished technical observer summed it up for everyone: “Not until the Mercedes engineers pass the Wankel engine for serious production will it be really respectable in Europe. ...”
Progress, but slow progress
As far as the company was concerned, Wankel engines had been studied, developed, built and refined in Stuttgart for several years before the C111 coupe was officially previewed in Germany. There never seemed to be a rush on this project because the engineers wanted to get it right before they persuaded the directors to commit several millions to invest in a production line.
It was a story that began way back when Felix Wankel secured enough backing from the German automotive manufacturer NSU to produce prototypes of what became the famous rotary engine that bore his name. The first such engine ran in 1957. Curtiss-Wright Corporation, based in the United States, soon obtained the rights to develop such engines, and Walter Benzinger (the head of engine development at Mercedes-Benz and Wankel’s former assistant) became a disciple in 1960.
Progress thereafter was steady, but unhurried. By the time the first Mercedes-Benz prototype engine was built in 1961, the company had secured a manufacturing license from NSU. However, by the time an engine was publicly introduced in the C111 coupe in 1969, the company was already into its fourth generation of designs.
The principle of the Wankel engine, it has often been said, could no more be described without weaving ones’ hands and arms in the air than one could summarize the physical attributes of a beautiful girl with the same constraints. The basis was a complexly shaped epitrochoidal chamber, inside of which was a triangular-shaped rotary member (the rotor) geared to what we might even call the crankshaft. To withstand combustion pressures, sealing strips are fitted to the three edges of the rotor. A combination of the shape of the trochoidal cylinder chamber and the rotor itself, synchronized to the positioning of inlet and exhaust ports, enabled the use of a four-stroke cycle. One spark plug – sometimes two on more modern applications – is provided. One or more rotor chambers could be clamped together; for instance, the original NSU Wankel Spider with its single-rotor unit, and the NSU 80 sedan that followed had a twin-rotor engine.
The advantages of such an engine were that it was well-balanced and felt smooth, and could rev very high indeed, but the disadvantages were that fuel consumption tended to be excessive, rotor sealing tip wear was always considerable, and manufacturing costs were usually high.
The first Wankel engine developed by Mercedes-Benz was the KP (Kreiskolbenmotor Prototyp) Series, with just 700cc per chamber. By 1961, the company was spending at least $1 million each year on the project and it went on to become the first of the engine’s licensee’s to solve the problem of building durable rotor tip seals. The KA that followed the KP became the first such engine specifically intended for fitting into to a passenger car, and was ready in 1965-1966.
At this point, the company was seriously hoping to put a limited-, high-priced car into production. It was confident that it had solved the rotor tip seal problem (particularly guaranteeing a long life), and at the time it did not see either the high fuel consumption or the environmentally “dirty” state of the combustion process as issues: Wealthy customers, it was reasoned, would accept the fuel-mileage problem, and to secure such a novelty they would choose to ignore the exhaust-emissions levels.
It was typical of the company that it pushed every technological boundary on the way to developing better and better units. Original KAs, with 450cc chamber volumes and with two or even three rotors, had conventional carburetors, but most development engines (and all the subsequent units) were fitted with Bosch fuel injection. These injected fuel into the intake/compression side of the chamber: Not only that, there was not one, but two separate injector nozzles in each chamber, the second placed approximately 90 degrees further around the rotary “cycle” than the first. This was a real advance compared with any other company that was developing this principle.
However, the proposal to impose strict exhaust emission limits on road-car engines (the “Muskie” laws were already being formulated) was already beginning to raise its head in the United States, this being the major market which Mercedes-Benz was developing at the time. It was this period in which not one, but two, further technological leaps forward were taken. In 1967, the company designed the third-generation engine, the KC Series, which had 560cc chambers, and then in late 1968 came the definitive KE Series, which used 600cc chambers. This KE made its public debut later in 1969.
A Moving Test Bed
At this time, therefore, the company went ahead with two related ambitious projects – one to produce ultra-powerful, 3-rotor (and soon, 4-rotor) Wankel engines for a new car, and the other to design a magnificent mid-engined, 2-seater sports coupe that would use Wankel power. Work on a new high-performance 2-seater coded C101 began officially in November 1968 (though, unofficially, and strictly off-the-record as far as the public domain was concerned, work had gone ahead some time before this).
As it happened, the C101 would be renamed C111 in 1969, the style would prove to be extremely attractive, and the performance startling. By any standards, the C111 was the world’s fastest and most powerful Wankel-engined car at this stage. The full story of this machine was told in the January-February 2010 edition of The Star. The original C111 was built, tested, and proven remarkably quickly – a chassis mule ran in April 1969, the final style of the glass-fibre body shell was first seen in July of the same year, and the running car was launched in September.
Although the ever-pragmatic development team – led, of course, by the now-famous Rudolf Uhlenhaut – knew that much more work was needed, the general public greeted the arrival of the C111 with real reverence. Millionaires arrived waving open checkbooks, and the company even investigated where, how, and in what limited quantities it might make the cars themselves; in the meantime, work on the flagship Wankel engines pushed ahead.
At this time, make no mistake, the possibility of producing several different Wankel-engined models was considered, for – as developed in Stuttgart – the company made sure that this was a “modular” engine. Whereas the original C111 had a triple-rotor engine developing 280 horsepower, within months there would be a 4-rotor version of it with 350 brake horsepower, which endowed the C111 with a top speed of more than 180 mph.
That was only one feature, for such Wankel engines – even the 4-rotor variety – also were remarkably compact and relatively light. The 4-rotor engine of the C111 weighed only 308 pounds, and figures put this engine as at least 200-250 pounds lighter than the just-announced M116 3.5-liter, V-8 engine of the day. Not only that, but there were proposals to build these engines in a milder state of tune for installation in front-engined sedans – some with two rotors, some with three, in place of 4- or 6-cylinder reciprocating engines – which would mean that substantial production tooling and assembly facilities would have to be commissioned.
But that was just about the peak of the Wankel’s popularity, at least within Mercedes-Benz. Although the company had conquered challenge after technical challenge along the way (Have you ever known of a Mercedes-Benz project where this was not done?) other obstacles were developing. Within the company itself, it was clear that there was a reluctance to be pioneers once again, that the already accumulated development cost of $50 million (and rising) was beginning to hurt, and that the potential cost of putting the engine into series production looked frightening.
There was more, too. Although the big 4-rotor engine had endowed the C111 with formidable high performance, the company was nervously looking over its shoulder at all the obstacles. Not only was the longevity of its own engine still unproven – no high-mileage endurance running had yet been completed – but the in-service record of the contemporaneous twin-rotor NSU Ro80 was still appalling.
Then, of course, there were the probable costs and implications – of the burgeoning exhaust emission, safety and related regulations that were already flooding out of Capitol Hill. Not only would the Wankel engine find it difficult to meet the exhaust emission laws, but at the same time the company would have to divert massive resources to clean up the piston-engined cars that it was already selling, or proposing to sell, in the United States.
The writing was already on the wall before the energy crisis struck in 1973, for the company now knew – to quote Dr. Kurt Oblander – “Due to the elongated, not exactly compact, combustion chambers, fuel economy was poor, resulting in high fuel consumption. …”
No sooner had the second C111 – the 4-rotor/350-horsepower C111-II model – appeared in 1970, than activity at Stuttgart began to wind down, and by the mid-1970s, it was all over. Work, it was claimed, went on for some years after that, but it was never at a serious level. Many of the other companies that had secured development licenses were also discouraged by the high costs, and abandoned their work. It is significant that no new licenses were granted after the first energy crisis erupted in the fall of 1973.
Although Mercedes-Benz may be proud of what it achieved with what became a dead-end project, officials had to admit that for once – just once in all those years – they had not brought it triumphantly to market. And now they probably never will.
Felix Wankel
Born in Lahr, Germany, in 1902, Felix Wankel was a self-taught engineer, with no original connections to Mercedes-Benz. Having previously worked on German military machinery, he joined NSU and started work on a new type of rotary engine in 1951. This became what is now known as the Wankel engine, which first ran in 1957, and was soon licensed to a string of engineering concerns for development.
Curtiss Wright Corporation of the USA was the first to procure a license from NSU, after which many European and several Japanese concerns all hurried to join in. NSU was the first to put a Wankel-engined car on sale (this was the cute little Wankel Spider roadster of 1965). However, it was Mazda of Japan that eventually began true series production, placing a whole range of Wankel-engined cars on sale – sedans, wagons, and sports coupes – and even today the RX-8 coupe is Wankel-powered.
Throughout this period, Wankel remained independent, received an honorary doctorate in engineering, and won many other awards and recognition of his merits. Although his engine never achieved universal acceptance, patent and licensing fees made him rich. He died in 1988.
Background: Cross-section drawing of the 4-rotor Wankel engine, as fitted in the C111 coupe of 1970. Opposite page, center: This was the display unit which Mercedes-Benz produced of the original three-rotor Wankel engine if 1969-1970. By comparison with the V-8 engines of the day being produced in Stuttgart, it was an amazingly compact power unit. Right: A view of the spotless development workshops being used for the development and refinement of large-capacity Wankel engines within Mercedes-Benz in the early 1970s. We would expect nothing less from the Three-Pointed Star.
Above: This side-by-side comparison shows how neatly the original 280-horsepower triple-rotor engine (left) could be expanded to a 350-horsepower 4-rotor unit. Opposite page, top: Except for the handicap of its heavy fuel consumption, the Wankel engine as developed by Mercedes-Benz was a formidably powerful, compact and lightweight unit. This was the display unit which Mercedes-Benz produced of the original three-rotor Wankel engine if 1969-1970. By comparison with the V8 engines of the day being produced in Stuttgart, it was an amazingly compact power unit. Bottom: If it had gone on sale in 1970, even at a high price, would you have bought a C111/II? With 350 brake horsepower from its 4-rotor Wankel engine, it would have been as fast as any Ferrari, Lamborghini, or Maserati on the market. This car still exists, by the way, in the company’s heritage collection in Germany.