Why are Rotary Engines Banned? | Stirlingkit

Why are Rotary Engines Banned? | Stirlingkit

Why are Rotary Engines Banned? | Stirlingkit


Where did rotary engines come from?

When it comes to rotary engines, Mazda will come into people's minds. Many people say that the rotary engine is equivalent to the Mazda. In fact, it's not true. The person who really invented the rotary engine was an engineer from the German NSU company, Felix Wankel, born on August 13, 1902, in Lahr, Germany. And Mazda just carried forward the rotary engine. In the history of the rotary engine, Mazda had a bigger contribution to the rotary engine than that of any other company, and so did the rotary engine to Mazda. The first rotary engine was called the "Wankel engine". In 1954, the first rotary engine was successfully developed and used in race cars. Later, Mazda purchased the patent for the rotary engine, began to improve and follow-up research and development of the rotary engine, and assembled the rotary engine on the mass-produced car.


 Kenichi Yamamoto, Father of the Mazda Rotary Engine

Founded by Jujiro Matsuda in Hiroshima, Japan in 1920, a company called "Toyo Cork Kogyo Co. Ltd." mainly manufactured cork products. In 1923, Japanese experienced the worst earthquake, the Great Kanto Earthquake. In 1927, the company changed its name to "Toyo Kogyo Co., Ltd." due to urban reconstruction issues and began to develop industrial machinery. Mazda's first model came into being in that era, called the Mazda Go, which was a motorized tricycle.

“Mazda” comes from Ahura Mazda, the god of harmony, intelligence and wisdom from the earliest civilization in West Asia. Mazda” was also a perfect way to pay respect to the substantial founder Jujiro Matsuda , whose family name is pronounced very close to “Mazda”. 

Thirty years ago today, on June 23, 1991, Mazda was the only rotary engine manufacturer and the first Japanese manufacturer who has taken overall victory at the 24 Hours of Le Mans.

The Benefits of a Rotary Engine

When a conventional engine works, the piston reciprocates linearly in the cylinder. In order to convert the linear motion of the piston into a rotary motion, a crank-slider mechanism must be used, which undoubtedly increases the complexity of the engine. The rotary engine is different, the output shaft connected to the rotor will rotate with the rotation of the rotor itself. There are many benefits brought by the cancellation of useless linear motion. Take a two-cylinder gasoline rotary engine as an example. Compared with a traditional 6-cylinder gasoline engine of the same power, its total number of parts is about 20% less, its volume is about 30% smaller, and its dead weight is reduced. nearly half.

Since the rotor is so important, its design naturally attracts attention, so why was the Lelo triangle chosen in the end? It all starts with the characteristics of the Lelo triangle.

The Leroy triangle is the simplest and easiest to understand Leroy polygon other than the circle, a curve of constant width. The fixed width is most directly explained by the following figure. We will find that after the circle is rotated by a certain angle, it can still be tangent to the upper and lower lines, that is to say, its width will not change.

Although the rotary engine has lower weight and higher thermal efficiency, in the 1991 Le Mans 24 Hours, the Mazda team equipped with the rotary engine won the championship by leading the second place by 2 laps. Debut is the pinnacle, proficient in singing, dancing, rap and basketball... (but the good times did not last long, probably because the shadow of the second place was too great, and the rotary engine was suspended in the second year)

Although it is so powerful, the fuel consumption, rotor life, and cost of rotary engines have not been widely used. The well-known rotor models are RX-7 and RX-8 ( Project D, Keisuke Takahashi)

What is a characteristic of a rotary engine?

The movement characteristics of the rotary engine are: while the center of the triangular rotor revolves around the center of the output shaft, the triangular rotor itself rotates around its center. When the triangular rotor rotates, the ring gear centered on the center of the triangular rotor meshes with the gear centered on the center of the output shaft, the gear is fixed on the cylinder block and does not rotate, and the ratio of the number of teeth between the ring gear and the gear is 3:2. The above motion relationship makes the motion trajectory of the vertex of the triangular rotor (that is, the shape of the cylinder wall) resemble an "8" shape.

The triangular rotor divides the cylinder into three independent spaces, and each of the three spaces completes the intake, compression, work and exhaust successively. Due to the above motion relationship, the rotational speed of the output shaft is three times the rotational speed of the rotor, which is completely different from the 1:1 motion relationship between the piston and the crankshaft of a reciprocating engine.

What makes a rotary engine different?

Rotary engines show a simpler, more efficient transfer of energy from burning the petrol to turning the wheels because all of the main internal components of rotary engine spin in a largely circular motion.  In this case, the rotary engine has less moving parts, is smaller, lighter and more powerful for its capacity.

A Youtuber used a transparent acrylic plate to replace the "side cylinder" of the engine, which well demonstrated the working state of the rotary engine.

This rotary engine is dedicated to model aircraft. Although it has become extinct in the civil automobile industry, its excellent performance makes the rotary engine shine in the world of model aircraft and kart racing.


From the above figure, we can see that the rotary engine cylinder is oval, or figure-eight or peanut-shaped. The triangular rotor moves inside the cylinder.

The side wall of the cylinder, that is, there are two openings on the plane, one for intake and one for exhaust, and the pit on the rotor is the combustion chamber.


Through a brief introduction you will find that there is no place to store oil. So how to lubricate it? It is very simple, that is, the gasoline and oil are mixed and burned.

Only mixed combustion can ensure lubrication, and there is no other way, which means that the rotary engine has been burning oil, no wonder the environmental protection fails.

We all know that the cylinder needs to be sealed so that the gas after combustion can push the piston, so the sealing depends on the piston ring, and the rotor also has a piston ring, which is called a scraper.

The rotor has a wiper at each corner, as well as the side of the rotor. At this point, you will definitely say: God! Isn't a rotary engine a single gas ring!

 That's right, the pistons on the piston engine are all, two gas rings plus an oil ring, which are properly sealed, and the rotor is completely operated by a single ring, no wonder the life is low.

The single ring will wear out very quickly, so the rotary engine needs to be overhauled once every 100,000 kilometers. Even if the scraper is replaced, it is a drop in the bucket. After all, the cylinder is also worn out.

The traditional round cylinder can be refurbished by boring the cylinder. The cylinder is slightly enlarged and restored to its original state. Can you tell me how to grind the special-shaped cylinder of the rotor? hard, right?

In addition, the rotor does not have a valve, and the intake and exhaust are completely driven by the rotor, or "scavenging", so not to say of variable valves and variable timing.

You will find that a rotary engine is very similar to a two-stroke engine. You can think of the rotor as: a four-stroke engine with a two-stroke intake and exhaust structure.

Since the rotary engine has no valve and no mechanical valve structure, the intake and exhaust are all random, and no one cares.

The piston engine has a mechanical air distribution structure, so when and how much air is taken in, it is planned.

Then I will imitate the exhaust sound of the two engines. Note that it is the exhaust sound at idle.

Piston engine:
brap... brap... brap... brap... brap... brap...
Rotary engine:
brap brap... brap... brap brap... brap... brap... brap...

The exhaust sound of the rotor is irregular at low speed. If the engine speed increases, the sound of the two engines will gradually approach, and the exhaust sound of the rotor will become more and more regular.

Goodbye! Rotary Engine

 The rotary engine seems like a dream. The inherent advantages of the rotary engine have deeply attracted car companies from all over the world, but its problems are also shrouded in researchers like a spell. No company win this game who study rotary engine.

The rotary engine has a congenital defect - high oil comsimption. Because Mazda successfully put the rotary engine into practical use after 1973. But after three oil crises, as oil prices continue to rise, this problem is fatal for Mazda. So Mazda gradually reduced the scope of application of the rotary engine, and in the end, it was only seen in sports cars that pursued high performance. Finally, increasingly stringent emissions regulations led to the cessation of production of the RX-8 in 2012 and the official withdrawal of the rotary engine from Mazda's product lineup.

The durability problem of the rotary engine is still not well solved. NSU is not allowed to improve the rotor sealing material and provide an extended warranty policy. This policy also exacerbates the financial crisis of NSU. In 1969 NSU was acquired by Volkswagen and merged into the Auto Union.

Now that new energy vehicles have a tendency to replace traditional fuel vehicles. That means there will be less possibility for rotary engines return to the stage of history.



  • Greg: March 19, 2023

    Seals replaced by teflon.

  • peter Veldman: February 23, 2022

    a few notes the rings on a rotary are called apex seals
    and they wear the camber does not wear with correct use
    also a correctly used rotery can run with like no wear at all
    the 13B out of the rx-8 (its know as the worst one) has records of above 200.000 k/m
    the 12A when build and used correctly wont even break
    but good job on the article!!

    ps: we call the sound of a rotary brap
    so when stationair brap brap brap

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