The History of Mazda's Rotary Engine

This article was first published in 2008.

Despite nearly all the major car manufacturers taking our licenses for rotary engine technology, Mazda has been the only car company to achieve commercial success with the design. In this story – sourced from Mazda – we take a look at the development of the engine, invented by German Felix Wankel.

The rotary engine began with an improbable dream one summer in 1919 by a 17-year-old German boy named Felix Wankel (1902 – 1988).

In the dream, he went to a concert in his own handmade car. He even remembers boasting, in the dream, to his friends. "My car has a new type of engine: a half-turbine, half-reciprocated engine. I invented it!" When he woke up in the morning, he was convinced that the dream was a premonition of the birth of a new type of gasoline engine.

He had at the time no fundamental knowledge about internal combustion engines, but he intuitively believed that the engine could achieve four cycles - intake, compression, combustion, and exhaust - while rotating. This intuition triggered the birth of the rotary engine, which had been attempted countless times by people all over the world since the 16th century.

Wankel’s dream and intuition steered his entire life.

In 1924, at the age of 22, Felix Wankel established a small laboratory for the development of the rotary engine, where he engaged in research and development. During World War II, he continued his work with the support of the German Aviation Ministry and large civil corporations, both of which believed that the rotary engine would serve the national interest once it was fully developed. They held that the rotary engine could move the German nation and its industries toward greatness.

After the war, Wankel established the Technical Institute of Engineering Study (TES) and continued his work on the research and development of the rotary engine and the rotary compressor.

One prominent motorcycle manufacturer, NSU, showed a strong interest in Wankel's research. NSU generated a great deal of enthusiasm among motor-sports fans; they were repeat winners of many World Grand Prix championships. NSU was also attracted by the ideal concept of the rotary engine. After creating a partnership with Wankel, NSU promoted Wankel's research and focused on the rotary engine design that uses a trochoid-shaped housing.

The First Wankel

Before that, however, NSU completed development of the rotary compressor and applied it to the Wankel-type supercharger. With this supercharger, an NSU motorcycle set a new world speed record in the 50cc class, reaching a top speed of 192.5 km/h. In 1957, Wankel and NSU completed a prototype of the DKM rotary engine, which combined a cocoon-shaped housing with a triangular rotor. This was the first rotary engine.

The DKM proved that the rotary engine was not just a dream. The structure, however, was complicated because the trochoid housing itself rotated; that made this type of rotary engine impractical. A more practical KKM-type with a fixed housing was completed a year later, in 1958. Although it had a rather complicated cooling system that included a trochoid with an oil-cooled rotor, the new KKM was a prototype of the current Wankel rotary engine.

By this time, no less than 39 years had already passed since young Felix Wankel dreamed of the rotary engine...

In November 1959, NSU officially announced the completion of the Wankel rotary engine. Approximately 100 companies throughout the world scrambled to license the technology; 34 of them were Japanese.

Chatter Marks

Mazda's president, Mr. Tsuneji Matsuda, immediately recognized the great potential of the rotary engine, and began direct negotiations with NSU. Those negotiations resulted in the formal signing of a contract in July 1961. The Japanese government gave its approval. The first technical study group was immediately dispatched to NSU, while an in-house development committee was organized in Mazda.

The technical study group obtained a prototype of a 400cc single-rotor rotary engine and related drawings, and realised that "chatter marks" - traces of wavy abnormal wear on the rotor housing that caused the durability of the housing to significantly deteriorate - was the most critical barrier to full development. It remained a problem even inside NSU.

Mazda, while testing the NSU-built rotary engine, made its own prototype rotary engine in November, 1961. The engine was independently designed in-house. Both engines, however, were adversely affected by chatter marks. Practical use of the engine was not possible without solving that problem first.

In April 1963, Mazda newly organized its RE (Rotary Engine) Research Department. Under Mr. Kenichi Yamamoto, chief of the department, 47 engineers in four sections - investigation, design, testing, and material-research - began exhaustive efforts in research and development. Its main objective was the practical use of the rotary engine: namely, mass production and market sales. The most critical engineering issue, the chatter mark problem, had to be solved.

The chatter marks were made inside the trochoid housing at the wall, where the apex seals on the three vertexes of the triangular rotor glided while juddering.

The apex seal itself caused abrasive vibration and the inside wall of the trochoid housing was marked through this abnormal wear. The RE Research Division called them “Devil's Nail Marks” and found that they were made when the apex seal vibrated at its inherent natural frequency.

To eliminate this phenomenon, a cross-hollow seal was developed, which helped a prototype engine to complete 300 hours of high-speed continuous operation. This technique, however, was not adopted in the mass-produced rotary engines, but served to promote further research of the apex seal in the areas of materials and structure. However, in the initial stage of rotary engine development, another problem caused thick white smoke to pour out through engine oil consumption. This was regarded as another barrier against commercialization. The cause of the problem was inadequate sealing. With cooperation of the Nippon Piston Ring Co and the Nippon Oil Seal Co, Mazda designed a special oil seal which proved to be a solution.

Early Prototypes

In the early 1960s, during the initial development stage of the rotary engine, Mazda designed and investigated three types of rotary engine: those with two rotors, three rotors, and four rotors. The single rotor version, prototypes of which were completed by NSU, could run smoothly at high speeds, but in the low speed range, it tended to be unstable, causing vibrations and a lacking of torque. This was due to the fundamental characteristic of single rotor engines, which had large torque fluctuations.

Mazda then decided to develop a two-rotor engine, in which the torque fluctuations were expected to be at the same level as a 6-cylinder 4-cycle reciprocating engine. The rotary engine could also further enhance the smoothness of revolution.

The first two-rotor test engine, type L8A (399cc unit chamber volume), was Mazda's original design, and mounted on a prototype sports car (type L402A, early prototype of the Cosmo Sport) exclusively designed for the rotary engine. Test drives began soon afterward. In December 1964, another two-rotor test engine, type 3820 (491cc unit chamber volume) was designed. It soon evolved to the mass-production trial-type L10A.

In recognition of the large potential of the rotary engine, Mazda invested heavily in imported and exclusive machine tools and proceeded with the trial manufacturing of multi-rotor rotary engines, including three and four-rotor versions. Those prototypes were installed on a prototype mid-engine sports car, Mazda R16A; test drives began soon afterward. Those driving tests were performed on a high speed test circuit at Miyoshi Proving Ground that was completed in 1965. The course was the most advanced in Asia at that time.

First Mazda Rotary Car

On May 30th 1967, Mazda began selling the world's first two-rotor rotary engine car, the Cosmo Sport.

It featured a 110-horsepower type 10A engine (491cc unit chamber volume) equipped with newly developed apex seals made with pyrographite, a high-strength carbon material, and specially processed aluminium sintering. This type of apex seal resulted from Mazda's independent development work and was proven durable through 1,000 hours of continuous testing. Even after a 100,000 km test drive, it showed only slight wear and an absence of chatter marks.

For the intake system, the side-port configuration, coupled with a two-stage four-barrel carburettor, was adopted to keep combustion stable at all speeds. For the ignition system, each rotor was equipped with spark plugs so that stable combustion could be maintained in cold and hot weather conditions alike, as well as on urban streets and expressways. The Cosmo Sport recorded more than 3 million kilometres of test drives in six years.

Emissions

After starting mass-production of its two-rotor rotary engine, type 10A, in 1967, Mazda did not limit its application to just the Cosmo Sport (which represented, after all, a relatively small market): it expanded its installation into other sedan and coupe models for larger volume production, acquiring a larger number of customers along the way.

Mazda also planned to export rotary engine cars to the world market.

In 1970 it started exporting to the United States, whose government was actively preparing the introduction of Muskie Act, the most stringent automobile emissions standards the country had yet devised.

In 1966, Mazda started development for the reduction of exhaust emissions while continuing early-stage developmental work of the rotary engine itself. Compared with the reciprocating engine, the rotary engine tended to emit less NOx (oxides of nitrogen) but more HC (hydrocarbons). For clearing the automobile emission standards under the Muskie Act, Mazda promoted the development of an ideal catalyst system but as a more realistic solution, developed a thermal reactor system that could be soon introduced. The thermal reactor was a device that burned HC in the exhaust gas, reducing HC emissions. This thermal reactor system came equipped in the first U.S.-bound export car with a rotary engine, Model R100 (Japanese name: Familia Rotary Coupe), which met the U.S.standards of that year.

Later, while other car manufacturers all over the world expressed that early compliance of the Muskie Act standards was impossible, Mazda reported in a public hearing with the U.S.government that the Mazda rotary engine could meet the standards. In February 1973, the Mazda rotary engine cleared the U.S. EPA Muskie Act test. In November 1972, in Japan, Mazda launched the first low emission series-production car in the domestic market, which came equipped with a Rotary Engine Anti-Pollution System (REAPS).

Fuel Crisis

In 1970s, the world went through a stormy period in international political relations. Many developing nations were gaining stature and power by using their oil resources as a political weapon. The "Oil Crisis" was the result of this political wrangling.

Most Middle-Eastern oil-producing countries during that time restricted their exports of oil; oil prices on the world market soared because of the supply shortage.
Automotive manufacturers, responding to those situations, started to develop mass-produced cars with dramatically improved fuel efficiency. Mazda realized that a drastic reduction in fuel consumption was a decisive factor for the survival of the thirsty rotary engine and initiated the "Phoenix Project" that targeted a 20 percent improvement in fuel economy for the first year of research and development, followed by a 40 percent rise as an ultimate goal.

After challenging the engineering development to improve the fundamentals of the engines and, among other measures, to improve their thermal reactor systems and carburettors, the company concluded that fuel economy could be raised by 20 percent as targeted. Further development, including enhancing reaction efficiencies by incorporating a heat exchanger in the exhaust system, finally led to a 40 percent rise, the ultimate goal.

The success of the Phoenix Project was reflected in the sporty Savanna RX-7, launched in 1978, which proved once and for all that the rotary engine was here to stay. Thereafter, the world's first catalytic converter system for the rotary engine was successfully developed, and fuel economy was even further improved. Soon afterward, fundamental engine improvements like the reaction-type exhaust manifold, the high-energy ignition system, the split secondary air control, and the two-stage pellet catalyst system, were developed in succession. The manifestation of all those developments was the Lean-Burn rotary engine that soon appeared on the market.

Six-Port Induction

After completing two key projects - the development of a low emission system and fuel economy improvement - Mazda adopted the six-port induction system and the two-stage monolithic catalyst system for its type 12A engine (573cc unit chamber volume). The six-port induction system had three intake ports for one rotor chamber. Through controlling the three intake port openings in three stages, fuel economy could be improved without sacrificing performance at high speeds.

This system, coupled with the two-stage monolithic catalyst system, would further advance the rotary engine.

Turbo!

The Cosmo RE Turbo, which went on sale in 1982, was the world's first rotary engine car with a turbocharger. The rotary engine's exhaust system inherently had more exhaust energy to drive the turbocharger turbine compared with the reciprocating engine; the rotary engine was better suited to the turbocharger. Moreover, the Cosmo RE Turbo was the world's first series-production rotary engine car equipped with an electronically controlled fuel injection system.

The Cosmo RE Turbo was the fastest commercial car in Japanat that time. It clearly demonstrated the attractiveness of the rotary engine. Thereafter, the "Impact-Turbo," developed exclusively for the rotary engine, made its debut. It was responsible for even further improvements in response and output.

The "Dynamic Supercharging" system was adopted in 1983 for the naturally aspirated (NA) rotary engine, type 13B. This system dynamically increased the intake air volume without turbo or mechanical supercharger, by utilizing the induction characteristics peculiar to the two-rotor rotary engine.

With the six-port induction system and the dual injector system, which had two fuel injectors in the chamber for each rotor, the 13B rotary engine came equipped with this dynamic supercharging system and achieved significant output increases regardless of the speed range. The dynamic supercharging system was further improved in 1985 through changes in intake plenum configuration.

Twin Scroll Turbo

To improve the driving performance of the turbo rotary engine, the second generation Savanna RX-7 adopted the type 13B engine with a Twin-Scroll Turbo which minimized turbo lag. The Twin-Scroll Turbo divided the exhaust intake scroll of the turbine into two passages so that exhaust could be supplied step-wise. With this configuration, the single turbocharger acted as a variable turbo and covered a wide range of speeds. This system helps reduce the turbo-lag, a traditional drawback of the turbo-charged engine. The duct leading the exhaust gas to the turbine was split into two passages, one of which was closed by a valve to accelerate exhaust gas flow at low speeds.

In 1989, The Twin-Scroll Turbo evolved into the Twin-Independent-Scroll Turbo, which had a simplified configuration. When this new turbocharger was coupled with improvements in the engine internals, it provided better low-speed torque, improved responsiveness, and upgraded driving performance.

Dual Fuel Injectors

Since 1983, the electronically controlled fuel injection system for Mazda rotary engines have adopted two injectors in each rotor chamber. Generally speaking, a large nozzle is most suitable for high-performance output because it can provide increased amounts of fuel. For more stable combustion at low speeds, however, a small size nozzle is more suitable because it can atomize the fuel better.

The dual injector was developed to cover such requirements in controlling the fuel injection over a wide range of operations. The two-rotor 13B-REW and the three-rotor 20B-REW rotary engines adopted air-mixture injectors that underwent further evolution of the dual fuel injectors, and achieved radical improvements in fuel atomization.

Triple Rotor

In 1990, the Eunos Cosmo, with its three-rotor rotary engine 20B-REW, went on sale after steady continuation of research and development for a quarter-century that had passed since the beginning of the rotary engine project. While the two-rotor rotary engine produced a smooth operation equivalent to the six-cylinder reciprocating engine, the three-rotor rotary engine exceeded that of the V8 engine; it even approached the level of the V12 engine.

However, a difficult engineering barrier existed for manufacturing the multi-rotor rotary engines. When the rotary engine was planned with an inline multi-rotor configuration, only two choices in designing the eccentric shaft were feasible: coupling it through joints, or making one of the fixed gears on the rotors split-assembled. Since the early stages of development, from the 1960s, Mazda had focused on the coupled eccentric shaft layout because the fixed gear split layout was considered too complicated for mass production. It then considered how to design the joints. The successful solution discovered in the 1980s was to use tapered joints in connecting the shafts. When the three-rotor rotary engine was developed, extensive driving tests for performance and durability were carried out, including participation in international sports car racing activities like the famous Le Mans 24 Hours race.

Sequential Twin Turbo

The Sequential Twin-Turbo, first adopted in type 20B-REW and type 13B-REW rotary engines in 1990, was based on the unique engineering concept of utilizing two turbochargers in sequence. At low speeds, only the first turbocharger works, but in the high speed range, the second turbocharger kicks in. Using both turbochargers enabled sufficient supercharging capacity and yielded high output. Running two turbochargers simultaneously also had the added benefit of reducing the exhaust resistance, which in turn contributed to even higher performance.

As the base engine to install the turbocharger, the rotary engine had several inherent superior characteristics, including a stronger exhaust pulse caused by the sudden opening of the exhaust port, and a short and smooth manifold. To fully utilize such features, the uniquely shaped Dynamic Pressure Manifold was adopted to guide the exhaust gas into the turbocharger in a minimum distance.