Engine Stories : Wankel Engine

Automotive history is interspersed with innovations and new technologies. However some of them do not get the success expected or are major failures. ClassicCarNews.net will present you with an unusual engine, known by some well-informed technology-lovers. It was originally designed by a German engineer, right after World War II. Let’s talk about the Wankel engine, also called rotary engine, even if it applies to other layouts.

The principle of Wankel engine

How is this engine designed ? As stated by its name, the piston does not follow an up-and-down movement in a cylinder. The piston actually is roughly shaped like a triangle and rotates in an oval-like epitrochoid-shaped chamber, called housing. The corners of the piston ensure the separation of the different “areas” of the combustion cycle (intake, compression/expansion, exhaust). This layout allows to do without a significant amount of mobile parts. No need for connecting rods, the piston [A] rotates directly around the eccentric-shaft [B]. No need for valves either (and therefore no need for camshaft and all the related parts), ports, like in a two-stroke engine, are enough to deal with the gas flow in the engine.

Animation Wankel - Wikimedia

This engine was designed by Felix Wankel, a self-taught engineer, who layd the foundations of his concept since the 1920s. He even opened his own workshop in Heidelberg in 1924 to dedicate himself to the development of his revolutionary engine, patenting his first designs in 1929. These works lead him to join the Reich Ministry of Aviation which wasinterested in his works. After the Nazi capitulation, Felix Wankel was arrested by French authorities, who dismantled his workshop and confiscated all his documents.

The first steps of  the standardisation of the Wankel engine

In 1951, Felix Wankel opens a new Technical Development Center in his private house, in Lindau, thanks to the funding from Goetze AG company. He wis contacted by NSU Motorenwerke AG, in order to develop his rotary engine, foreseeing an industrial production. He has to wait until 1954 to develop an experimental car engine, the DKM54. This engine delivers promising figures : 29 hp at 17,000 rpm, for only 125 cm3 ! However, this engine will never reach serial production because its design, with both the piston and housing rotating, implies stripping the engine to change the spark plugs.

In the meantime, Hans Dieter Paschke, a NSU engineer develops an engine with a fixed housing, the KKM57. This different layout uses less parts than the DKM and is based on a simple movement (the piston rotates around a crankshaft), however, it doesn’t reach the high revolutions of the DKM. Felix Wankel  will even remark “you have turned my race horse into a plow mare”.

In 1960, a 250 cm3 version of this engine is put in a modified NSU Prinz which will be used as a road test car. This engine delivers 30 hp at 5,000 rpm, a far lower output compared to the DKM, but NSU still decides to build a production car, equipped with a rotary engine.

In 1961, the Wankel GmbH company sells licenses to several car/heavy goods vehicle building companies such as Deutz, MAN or Daimler-Benz, who start developing their own engines.

In 1964, NSU unveils the NSU Spider, based on the  NSU Prinz Sport Coupé, equipped with a single-rotor engine with a 497.5 cm3 displacement delivering 50 hp. 2,375 were delivered until the end of its production, in 1967. During this period, NSU founded a research centre, Comobil, in association with Citroën, based in Geneva to develop a Wankel-powered car. The project is dropped and a new joint venture between NSU and Citroën, Comotor is created, in charge of assemble rotary engines for the cars of both manufacturers. A plant is built in Luxemburg, and opens in 1967.

In 1967, NSU launches the Ro80, driven by the brand new two-rotor Comotor 626 engine (995 cm3, 115 hp at 5,500 rpm), coupled to a 3-speed semi-automatic gearbox. This car is the vision of what 80’s cars would be (as the name implies it). This saloon is fitted with ATE disc brakes, the front disc mounted inboard, reducing unsprung weight. The suspension was independent on the four wheels, using McPherson struts at the front and semi-trailing arm suspension at the rear. Today these features look usual, but back in those days, it was reserved for the high performance or luxury cars, in the best case !

Simultaneously, Citroën launches (in 1969) the M35, a customer test car (they had no lack of ideas to cut the expenses off), based on the Ami 8. This coupé was driven by a single-rotor prototype displacing 497.5 cm3. Those cars were lent to hand-picked customers who had to assure beforehand that they would drive over 20,000 miles a year, in exchange for full servicing. The 267 cars produced were reserved for destruction and today,only a few of them are still alive… In 1973, Citroën finally launches its Wankel powered production car, the GS Birotor, a luxury and high performance version of the GS. Powered by the Comotor 624 (two-rotor 995 cm3 engine, 107 hp at 6500 rpm), it is distinguished from the usual GS with wider tyres, a high-end interior, a rev alarm and a custom-designed front axle, which prefigures the front axle of the CX.

1973 is also the end of the negotiations between AMC and Wankel GmbH, allowing the smallest american car manufacturer to build its own Wankel engines for both their passenger cars and Jeeps. However, 1973 is a dark year for the automotive world. In October, car manufactures are caught in the turmoil generated by the energy crisis, and the gas-guzzlers are the most heavily affected. The Wankel engine is one of the perfect targets, its fuel consumption being hardly able to get under 22 mpg.

This iis why the GS Birotor production is stopped in 1975, after only 846 cars sold, and Citroën, lead to bankruptcy, is taken over by Peugeot. The new leadership encouraged the dealerships to swap the GS Birotors for CXs, in order to avoid the huge costs of maintenance.

NSU will keep making Ro80s until 1977, after 37,389 cars were built in the Neckarsulm plant. This year is marked by the closure of Comotor and the disappearance of NSU, taken by Audi. It also sounds the death knell of the Wankel engine, dropped by all manufacturers (even the development programs started by GM, Mercedes, Alfa Romeo, Ford Germany, Rolls-Royce and Nissan)

Mazda, alone against the world, sticks with the Wankel engine

End of an era ? Nay ! Tsuneji Matsuda, chairman of the Toyo Kogyo group fell in love with this revolutionary engine. As soon as 1961, he starts negotiating with Wankel GmbH, for its automotive division, Mazda, helped by the japanese government. His goal was to take advantage of the taxation in favor of the low displacement engines (under 1,000 cm3), while offering performances worthy of sports cars.

This is how starts the development of the 40A, a single-rotor Wankel engine, technically close to the KKM400 from NSU (90 mm rotor radius, 59 mm rotor thickness) which will never reach serial production. Really quickly, Mazda engineers are confronted with “chatter marks” appearing on the housing inner surface. These marks are due to the early design of the apex seals that reached a resonating vibration.

In 1963, Mazda introduces the Cosmo Sport prototype at Tokyo Motor Show, along with its L8A engine (two rotors, 98 mm radius, 56 mm thickness, that is to say 798 cm3 displacement). This brand new engine is fitted with new apex seals, made of hardened steel, that do not reach resonating vibration (removing the chatter marks issues). New rubber seals are fitted on the side of the rotors to lower oil consumption.

Cosmo 110 S - petroloicousAs soon as 1965, the L8A engine is replaced by the L10A (two-rotor, 982 cm3) in its 0810 version (110 hp at 7,000 rpm). It introduces twin side intake ports (perpendicular to the rotation plane of the rotor), each fed by one of four Stromberg caburetor barrels. The rotor housing was made of sand-cast aluminum plated with chrome, while the aluminum sides were sprayed with molten carbon steel for strength. Cast iron was used for the rotors themselves, and their eccentric shafts were of expensive chrome-molybdenum steel. In particular, this engine was used in the 80 pre-production cars that will wander the japanese roads for over 3 million kilometers in two years in order to improve reliability. The definitive Cosmo Sport 110S is unveiled in May 1967 and 343 cars are built until July 1968.

Actually, the materials chosen for the engine put a strain on the cost-efficiency of the car. Mazda then decides to get back to more conventional materials, The main changes of the 0813 version of the L10A engine are its weight (122 kg, instead of 102 for the 0810) and its power output, gaining 18 hp. This engine is then fitted in the new series of Cosmo 110S, which is now equipped with 15-inch wheels (instead of 14) and a 5-speed gearbox (instead of 4). 1176 cars will get out of the Hiroshima plant, until September 1972. In the meantime, a tuned down version (100hp, one carburettor) was put into the Familia saloon, named Familia Rotary, and in the coupé based on it, known as R100 in the West. The Familia Rotary and R100 production will stop in 1973.

The L10A engine will be upgraded one last time in 1971, giving birth to the 0866 version, specifically designed for the japanese market. It will propel the  Mazda Savanna S102 (exported as Mazda RX-3). This engine has reworked exhaust ports and a modified exhaust system, in order to comply with the new anti pollution regulations set up by the japanese government. It will disappear along with the 0810, in 1973.

Luce R130To extend its range, Mazda decides to put a rotary engine into the Luce coupé, designed by Giugiaro. However, the Luce is a front-wheel drive car, which implies developing a new engine. The Luce R130 is powered by the 13A, a two-rotor 1310 cm3 engine (rotor radius increased to 120mm) developing 126 hp. It will be the only appearance of a front-wheel drive rotary powered Mazda, the new generation of Luce going back to a rear-wheel drive layout in 1972. The R130 is then one of the most desireable off the beaten track rotary-powered Mazdas.

At the same time, Mazda develops the 12A engine, an “elongated” version of the 10A (rotor thickness increased from 60 to 70 mm, for a displacement of 1146 cm3) which will power roughly ten different cars from 1970 to 1985, with different settings. From 1975 to 1980, you can find it under the bonnets of RX-2s, RX-3s, RX-4s, RX-5s, Luce Rotary, Cosmo coupé (LA/HB) and the first RX-7s (SA/FB) developing 130 hp. For environmental issues, a 110hp version will be created for the RX-3s from 1972 to 1974, and 105hp version for the RX-7s sent to the US. It uses the thermal reactor introduced with the 0866 10A engine, and uses new materials to harden the rotor housings. The latest versions of this engine called 12A-6PI featured variable induction ports.

The ultimate 12A engine was the electronically fuel-injected turbocharged engine, increasing its power to 160 hp, then 165 hp for the “Impact Turbo” specs. These engines will be put in the SA/FB RX-7s, Luce and HB series Cosmo.

Quietly introduced in 1974 (and produced until 1978), the 12B was produced for the RX-2 and RX-3. It had increased reliability, compared to the previous versions and it was the first Mazda rotary engine to use a single distributor, instead of two.

Mazda_RX-5_Rotarystock_HollandLast step of the Wankel engine development, Mazda introduces the 13B engine, an “elongated” 12A (rotor thickness is increased from 70 to 80mm), which is not related to the 13A. The 13B displaces 1308 cm3 and will be the most produced Wankel engine in the automotive history. The first version of the engine produces 130 hp, but is more environment-friendly than the 12A. It is fitted since 1973 in the RX-4, then in the Rotary Pickup (the only pickup truck ever produced with a Wankel engine) in 1974. In 1975, the 13B-AP (anti-pollution) is put in the Roadpacer, RX-5 and Cosmo.

Cosmo HBFor the years 1984-85, the 13B engine becomes 13B-RESI (Rotary Engine Super Injection), it features a brand new intake manifold and a so-called Dynamic Effect Intake which generates (thanks to a two-level intake box) a supercharger-like effect due to the resonance of the opening and closing intake ports. Featured with a Boch L-Jetronic fuel-injection, this new engine produces 135 hp, reducing emissions even more. It will be put in the RX-7 GSL-GE, Luce and Cosmo HB.

The 13B-RESI engine is quickly replaced in 1986 by the 13B-DEI, featured with a variable-intake system, and a four-injector fuel-injection. This engine , put in the RX-7 FC, delivers 146 hp and even 160 since 1989.

That same year, the 13B is also boosted by a turbocharger, becoming the 13B-T. This engine keeps the four-injector fuel-injection, but is closer to the 74-78 layout. Its twin scroll turbocharger is fed by a mechanically-actuated valve until 1988, where the valve is replaced by a reworked exhaust manifold. This engine delivers 185 hp and is put into the Luce HC Turbo II and RX-7 Turbo. In 1989, its power is increased to 200 hp for the RX-7 Turbo II.

Between 1990 and 1995, the 13B-T becomes 13B-REn featuring the biggest intake side-ports ever developed for a Wankel engine and two sequential turbochargers (a large Hitachi HT15 as primary and a smaller HT-10, as secondary), the very first appearance of this layout in a mass produced car. This 235hp engine will propel the Eunos Cosmo coupé (JC series).

Since 1992, another twin-turbocharged 13B appears, the 13B-REW, featuring two Hitachi HT-12 sequential turbocharges (the primary offering boost from idle to 4,500 rpm, the secondary coming online afterwards). This engine, developing 250 to 285 hp, will be the heart of the RX-7 FD, from 1992 to 2002.

The 13B-REW will be the basis of the 20B-REW, the only triple-rotor mass-produced Wankel engine ever built. It displaces 1962 cm3 (three 654 cm3 rotors instead of two for the 13B). The sequential HT-12 turbochargers allowed it to deliver 300 hp, unfortunately linked to an automatic gearbox, for the touring flagship that the Eunos Cosmo coupé was.

The last Wankel engine that hit the road appeared under the bonnet of the RX-8, in 2003. It is the naturally aspirated Renesis, or 13B-MSP (Multi Side Port). It was developed to lower emissions and increase its output (which are the main drawbacks of the Wankel engine, due to its geometry). Compared to the other 13B engines, the major difference consists of the side exhaust ports (that are no longer peripheral) erasing  overlap and allowing to redesign the intake ports.

RX-8These upgrades allow a higher compression ratio, therefore higher power output. On the other hand, new side seals and new apex seals allow higher revs. This new engine develops 192 hp and even 231 for the high performance versions that feature a three stage intake and reve limiter set to 9,500 rpm instead of 9,000). This engine dissapears alongside the RX-8 in 2012, without any successor.

Is the Wankel engine dead ?

As you could notice, the Wankel engine never stopped being upgraded, even if a few manufacturers mass-produced it. However, its development cost a lot to those who tried : NSU disappeared, Citroën, nearing a new bankruptcy, was saved by Peugeot and Mazda almost died, because they made the Wankel engine their main engine, before dedicating it to sports cars. At the moment, it is no more mass-produced, it only appears in the Pro Mazda Championship. Regulations are severe against the polluting engines, and it would be pointless to deny the Wankel engine is one of them. However Mazda keeps having faith, developing a rotary-powered range extender, for which the lightness of its components and its compactness can be a major advantage, and, obviously with the RX-Vision Concept, powered by the SkyActiv-R 16X engine still in development. Wait and see…

For the petrolheads, the Wankel engine will always be unique, with an incredible smoothness, an indefinable sound (for those of you who would not know, look for a video of the 787B running Le Mans in 1991, which it won, you will be seduced, like i was). Rotary-powered cars are to be collected from now on. However, be careful with your car service history, because a lot of mechanics are clueless with this technology, and a lot of cars have to be junked, lacking proper maintenance. Save them !

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