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
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.
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
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
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.
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
First Mazda Rotary Car
On May 30th 1967, Mazda
began selling the world's first two-rotor rotary engine car, the Cosmo
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.
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
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
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
launched the first low emission series-production car in the domestic market,
which came equipped with a Rotary Engine Anti-Pollution System (REAPS).
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
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
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.
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
This system, coupled with the two-stage monolithic catalyst system,
would further advance the rotary engine.
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
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
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
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.
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
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