The Wankel Motor: 1960’s Engine of the Future

by James Kraus

World’s first Wankel-powered production car: the NSU Spider, 1964

Automobiles can trace their reciprocating-piston engines back to the early days of steam power. As internal combustion replaced steam as the preferred method of powering transport, the concept of using reciprocating pistons to convert energy into motion was carried over.

As the automobile matured, the efficiency and operating smoothness of the reciprocating piston engine gradually improved through the use of a multiplicity of smaller cylinders, shorter piston strokes, counterbalanced crankshafts and other refinements. By the dawn of the 1960s however; the automobile was seemingly falling behind aviation, which had switched to smooth continuous-combustion jet engines. A number of auto manufacturers experimented with gas turbine engines, but none entered mass production.

Nevertheless there was another alternative engine on the horizon; one that likewise promised velvety smoothness, high rpm capability and significant size and weight advantages.

Felix Wankel, a self-taught German engineer had been working on the idea of a rotary internal combustion engine since his teen years, and was awarded his initial patent in 1929. Interrupted by the war years; he resumed his rotary development at NSU Motorenwerke, a German motorcycle manufacturer who were laying the groundwork for launching their first post-war automobiles.

Felix Wankel with DKM 54 Prototype

The initial prototype NSU-Wankel DKM 54 motor began testing in January of 1957. This design used a trochoidal piston spinning within a epitrochoidal housing, which itself spun at a slightly higher speed inside the engine case.

Later that year, further research and development would center on a simplified version of the engine with the trochoidal piston orbiting around a stationary gear in the engine case without the concentric circulating inner housing. This simplified version was largely the work of Dr. Fröde.

In 1958, the simplified engine concept was proven to be workable and NSU began licensing the design in 1958. Several automotive firms bought manufacturing rights, attracted to the compact form factor, light weight, smoothness and simplicity of the Wankel. By the end of the 1960’s, Alfa Romeo, Citroën, Daimler-Benz, Toyo Kogyo (Mazda) and Porsche had signed contracts.

Ski-Craft ski tow

The first rotary for sale to the public was not actually in an automobile at all. It could be found inside the Ski-Craft Wasserskischleppgerät ski tug launched in October of 1962. This remote-controlled unmanned unit pulled a water skier along without the need for a boat and skipper. It was powered by a NSU 150 cc Type 61 single-rotor Wankel.

NSU Wankel Spider

At the 1963 Frankfurt Auto Show, NSU displayed the very first rotary-powered automobile; the NSU Spider. It went on sale in 1964, fitted with a single-rotor 500 cc Wankel engine. The chamber housing was water-cooled, while the inside of the trochoidal piston was cooled with engine oil. To keep the oil temperature in check, the engine was fitted with a large oil/water intercooler and the sump held a generous 4 litres of oil.

NSU Spider engine and transaxle. Dark object in foreground is oil/coolant heat exchanger.

The Spider produced 50 hp; providing 100 hp/litre. This compared quite favourably with NSU’s own conventionally-powered four-cylinder 1.0 litre Prinz 1000, which produced 42 hp/litre. The engine had very little torque below 3000 rpm, behaving much like a highly-tuned conventional engine with fairly aggressive valve timing. This would prove fairly typical of all Wankels to follow, and was not unexpected given the high specific power output.

Although NSU never campaigned a works car, several privateers entered into competition with their own Spiders. Karl-Heinz Pannowitz and Rainer Struntz won the 1966 German GT Rallye Championship with a slightly modified Spider that produced 65 HP. In the U.S., Al Auger successfully raced the NSU in SCCA road racing. In 1967, Günther Irmcher drove a Spider to victory in the Tour d’Europe Rallye.

Christine Beckers

Also that year, 23 year old NSU rally driver Christine Beckers took second in class and 11th overall at the 12 Heures du Huy.

Christine behind the wheel of her Number 25 NSU Spider, Huy, 1967

In ‘67 and ‘68, legendary NSU tuner Siegfried Spiess won the German Hill Climb Championship with his personally tuned Spider that delivered nearly 100 hp at an astronomical 11,000 rpm.

Almost 2400 Spiders were built before the model was phased out in 1967.

Mazda Cosmo Sports 110S

Meanwhile, Mazda was quickly developing their own licensed version of the Wankel, displaying prototypes at the 1964 Tokyo Motor Show. In May of 1967, Mazda unveiled the fruits of their labour, the Cosmo Sports 110S.

In development since 1962, the Cosmo’s dual-rotor 1.0 litre 0810 engine was all aluminium with iron rotors. Induction was via a four-throat dual-stage carburettor. The efficiency was 10% better than the NSU Spider, with an output of 110 hp/litre. To put that in perspective, it was 5 hp/litre greater than the specific output of the legendary limited-production 1967 Porsche 2.0 litre 911R engine.

The Cosmo introduced dual ignition to the Wankel motor. With this system, each rotor housing incorporated two sequential spark plugs, each controlled by its own distributor. The leading (primary) plug fired first, with the trailing plug firing 5 to 15 degrees later.

Mazda Cosmo 110S at the Nürburgring, August, 1968

Mazda put together a works team and fielded two Cosmo Sports in the Marathon de la Route endurance race at the Nürburgring, with the lead car finishing fourth overall.

Slightly over 1500 first-generation Cosmos were built from 1967 until production ceased in 1972.

NSU Ro80

In September of 1967, NSU introduced the futuristic Ro80 sedan, powered by a dual-rotor 1.0 litre Wankel coupled to a 3-speed clutchless semi-automatic gearbox, similar to Porsche’s 4-speed Sportomatic. The Ro80 also featured 4-wheel disc brakes and fully independent suspension. Its most striking feature was the exterior design by Claus Luthe. It would still appear contemporary several decades later. The Ro80 garnered the 1968 European Car of the Year award.

NSU Ro 80: Cutaway of twin-rotor Wankel engine and torque convertor. Note inboard front disc brakes.

Despite initial problems with tip seal degradation leading to enough warranty engine replacements to force the financially stretched firm into the arms of Volkswagen, over 37,000 were built until the car was discontinued in 1977. The tip wear issue was eventually sorted and many of the later cars are still driving around on their original engines.

Mazda Familia R100 Rotary

In July of 1968, Mazda offered for sale the first rotary for the masses, the Familia R100 Rotary Coupé. Previous Wankel-powered cars were either two-place sports cars or the relatively costly Ro80 sedan. The popular-priced four-seater R100 was powered by the 1.0 litre 10A Wankel from the Cosmo. To reduce production costs, the aluminum side plates of the rotor housings were replaced with cast iron. A smaller carburettor reduced peak power but allowed for improved fuel mileage.

Familia Rotary at Spa piloted by Yves Deprez and Yoshimi Katayama, July, 1969

Mazda entered three Familias R100′s in the 1969 Spa-Francorchamps 24 Hour endurance race in Belgium, with two of the cars taking fifth and sixth overall. A month later a Familia Rotary came home fifth in the Marathon de la Route.

Mazda R130 Luce Coupé (top) and Familia R100 prototypes at the Tokyo Motor Show, 1967

In addition to the Cosmo and Familia, Mazda brought out a luxurious rotary front-wheel drive Coupé in autumn of 1969, the R130 Luce.

Mercedes-Benz C111, early three-rotor prototype

If the NSU Ro80 was the Queen of ‘60s Rotaries, the Mercedes-Benz C-111 was undisputedly the King. Its major drawback was that you could not buy one, and only a handful were built. The C111 was a strange beast; launched in the spring of 1969, it was built as an engineering prototype and test bed, yet was completely roadworthy down to the fitment of a Becker AM/FM radio. Mercedes-Benz Development Director Rudolf Uhlenhaut put several road kilometres on the cars personally, even taking one to the Alps on a skiing holiday.

Mercedes-Benz C111, final three-rotor version

The C111 started life with a 1.8 litre 3-rotor Wankel fed by Bosch high-pressure direct fuel injection that producing an amazing 155 hp/litre. The mid-mounted engine sent power to the rear wheels via a 5-speed transaxle and ZF limited-slip differential. A second version was fitted with a 2.4 litre 4-rotor producing a more sedate 146 hp/litre with greater low-end torque thanks to a variable intake system.

The majority of the C111s were finished in metallic orange; an interesting choice as no contemporary production Mercedes-Benz was offered in orange despite its popularity at the time. The fully-padded large-hub four-spoke steering wheel and Bundt Cake alloys wheels introduced on the C111 did make it into series production however.

Citroën M35

Citroën and NSU formed the Comotor partnership in 1967 to manufacture Wankel engines in Altforweiler Germany. The first car to receive a Comotor powerplant was the experimental Citroën M35 fitted with a single-rotor 1.0 litre Wankel. Like Mazda, the M35 engine used leading and trailing spark plugs. The M35 was the second and final automobile to be equipped with a singe-rotor Wankel. Production began in late 1969. A total of 267 were built for testing and issued to selected loyal Citroën owners for long-term evaluation.

Citroën GS Birotor

Satisfied by the performance and feedback received on the M35, Citroën continued development of the Wankel and launched the GS Birotor in 1973. The Birotor featured a 1.0 litre twin-rotor Comotor engine with single-plug combustion chambers. The engine produced 107 hp/litre. The cars  reception was marred by the near-simultaneous oil crisis of 1973, and popularity suffered as a result due to the relatively high fuel consumption. Less than 1000 were built.

By the mid-seventies, excitement for the Wankel among automotive engineering staffs and the general public began diminishing due to increasing concerns over comparatively high emission levels and fuel consumption; issues which were gaining importance as the decade began drawing to a close. Mercedes-Benz halted development as did NSU, which by then had become part of the Volkswagen Group.

Citroën would prove to be the last automobile manufacturer to introduce a Wankel powered model, leaving only Mazda to continue developing and producing the engine for automotive applications.

Nevertheless, throughout the 1970s, Wankel engines appeared in other applications including lawn mowers, snowmobiles and motorcycles.

Hercules W-2000

The first available Wankel motorcycle was the Sachs DKW/Hercules 300 cc single-rotor air-cooled W-2000 of 1974, followed later in the year by the 500 cc single-rotor water-cooled Suzuki RE5.

The wildest rotary-powered motorcycle was the Van Veen OCR 1000 introduced in 1978. The OCR 1000 used the same 1.0 water-cooled twin-rotor Comotor engine as the Citroën GS Birotor, sending power to the rear wheel via a Porsche-designed gearbox and drive shaft. The forks and rear suspension were developed by Koni, and braking was via triple Brembo discs. This meticulously hand-crafted machine sold for the same price as a contemporary Mercedes-Benz 280 SL. A total of 38 were built.

The last Wankel-powered car, the Mazda RX-8, was recently discontinued as the rotary engine was unable to meet Euro 5 emission regulations.

One of the key advantages of the rotary engine when introduced was the simplicity of having no valves. That lack of valves now works against it from an efficiency standpoint in comparison to conventional piston designs. Today’s infinitely variable valve timing and lift systems, notably BMW’s Valvetronic and Fiat’s MultiAir, eliminate pumping losses and broaden the power band of conventional engines. The intake and exhaust ports of a Wankel double as valves and of course are not movable nor adjustable in size.

The Wankel does have a possible future as a constant-speed battery charging power source as an electric car range extender. Precisely tuning a Wankel to run at a constant speed or within a narrow range of rpm greatly reduces emissions and fuel consumption issues and the inherent smoothness of a rotary is a major advantage in an engine that needs to cycle on and off intermittently during vehicle operation.

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3 thoughts on “The Wankel Motor: 1960’s Engine of the Future

  1. And now, Auto Universum with sound and picture:

    This video (http://www.youtube.com/watch?v=ol8oDlF9FkQ) shows the cold start of a Citroen M35. I am not a Wankel motor specialist, but this sounds very much like a model-kit airplane to me, although the starter sounds like the ordinary Citroen starter fitted on its twin-cylinder cars. This other video (http://www.youtube.com/watch?v=8kzRisZ7VMw&feature=related) is also interesting. You may check other M35-related videos if you have a whole afternoon, it’s endless fun when you get started on YouTube. The blue M35 was repainted by coachbuilder, Heuliez (http://leroux.andre.free.fr/h89.htm), one of the rare cars to have survived the organized scrap of M35 arranged by Citroen, just as it did with the GS Birotor. M35s were indeed sold to selected customers who drove at least 30 000 kms per yea, for FRF 14,000 (the DSpécial, the entry range DS, cost FRF 13,800…). Each car came in Nacre Grey only, was numbered and proudly sported the mention ‘prototype’ on the front right wing. There should have been 500 of them, so the numbers were doctored to give the impression that the planned target had been reached. For more information in French about the M35: http://www.rotativementvotre.org/citroen-m35.php. Cutting this URL after ‘.org’ will give you access to a website dedicated to Wankel-powered Citroens.

  2. Certainly today’s variable valve timing and lift are key to meeting current and future emission and fuel economy and performance targets, but the rotary engine is still lacking in comparison to piston engines that do not employ those features. It comes down to thermal efficiency. The rotary engine combustion chamber has a very large surface area relative to it’s size so heat loss to the cooling system is not insubstantial as compared to a piston engine. The same thermal efficiency (i.e. fuel economy) penalty occurs with an L-head design piston engine with respect to combustion chamber surface area relative to displacement. That is also why Mazda and others use twin spark plugs in their rotary designs. The combustion chamber is very long and thin in cross section, which means the flame travel may not make it to the ends of the chamber, resulting in much more unburned fuel (high emissions) in the exhaust.

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