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They all say rotary engine is gorgeous, but what exactly is a rotary engine?

They all say rotary engine is gorgeous, but what exactly is a rotary engine?

Sep 21,2023

They all say totary engine is gorgeous, but what exactly is a rotary engine?


In fact, Mazda was not the first to make a rotary engine. Everyone knows that the rotary engine is also called the Wankel engine. This person is Felix Wankel, the founder of the rotary engine.

In the early 1950s, German engineer Felix Wankel joined NSU Motorenwerke AG (full name NSU Motorenwerke AG, acquired by Volkswagen Group in 1969 and merged with Auto Union to form today's Audi Motor Company) to lead the new the first result of the engine development work was the rotor valve and rotary engine supercharger, which was installed on a 50c.c. motorcycle by the NSU car factory. It can generate an air pressure of 45p.si, making the horsepower reach 13.5 bhp, this locomotive set a world record of 120mph for its class on the Bonneville salt flats (a large salt flat located in northwest Utah, USA. The flat is often used for vehicle speed tests).

Next, some dkm54 engines were developed one after another (DKM is the abbreviation of German Drehkolbenmotor).

Because the mechanical structure was too complex, the NSU depot abandoned the design concept of the DKM engine in 1957 and instead used only the rotor to rotate while the rotor chamber was fixed, called the KKM type (abbreviation of German Kreiskolbenmotor). But this has encountered a problem: the air sealing performance of the three tops of the rotor will gradually decrease as the rotor rotates. This is the flaw of the Wankel engine - it is called the "devil's fingernails" (chatter marks, serious scratches on the cylinder wall caused by the rotor during operation)


It is Mazda that really promotes the rotary engine~

Since February 27, 1961, Japan's Toyo Industries (now known as Mazda Motor Corporation) signed a contract with the German NSU car factory and Felix Wankel to obtain the Wankel engine license.

In order to sign the contract and obtain authorization, Mazda accepted the following almost harsh conditions:

The 10-year contract amount was 280 million yen, which at that time could cover approximately one month's salary for Mazda's 8,000 employees.

Mazda must unconditionally provide product patent rights to NSU car manufacturers.

After each car equipped with a Wankel engine is put on the market, a royalty must be paid to the NSU car factory.

It basically means that if you use my technology, you will pay the patent fee first, and then the technology you develop based on my patent will be given to me unconditionally. Finally, if your car sells well, I will get a share of the profit. Simply heartless

In July of the same year, the company assigned eight technicians to the NSU depot in West Germany at the time to receive training on technical details. NSU depot personnel placed a coin on the running Wankel engine but it remained standing. They demonstrate the quietness of this engine. However, they later discovered that after running for a period of time, corrugated wear marks caused by the diamond seal at the rotor apex would appear on the cylinder wall of the rotor chamber; only by solving this problem can it be officially put into practical mass production.

Wear of the diamond seal may cause air leakage, and excessive hardness of the diamond seal may cause scratches.

Due to the unique structure of the Wankel engine, the rotor rotates in an elliptical space in an eccentric circle. In order to maintain a certain airtightness between its three surfaces and the cylinder wall, its three end points must be equipped with a three-dimensional seal consisting of an "apex seal" and a "corner seal" mechanism. The function of the diamond seal is similar to that of a piston ring, and the internal spring leaf will expand and contract according to the gap between the diamond seal and the cylinder wall. After years of eccentric rotation, the diamond seal will cause wavy scratches on the cylinder wall (chatter mark, jokingly called "the devil's fingernails"). In other words, the process of improving the seal material has become the history of the improvement and development of the Wankel engine.

                                                                                                    Devil's fingernails

After continuous testing, the Mazda team found that the gap between the corrugated wear marks was the same as the inherent vibration value of the diamond seal. Therefore, they changed the shape of the diamond seal, made a horizontal hole near the top, and trepanning longitudinally in the cross direction, called cross hollow diamond seal. The experiment confirmed that the resonance frequency of the diamond seal and the cylinder wall was changed, and no corrugated wear marks would appear within 300 hours. This problem was finally solved initially. Then in the summer of 1964, Nippon Carbon Co., Ltd. developed a new carbon material, and Mazda replaced the original diamond seal material with an aluminum alloy material mixed with carbon. The Cosmo Sport, launched in May 1967, was the first mass-produced car in the world to put a rotary engine into practical use.


The basic structure of a rotary engine is roughly as follows:

1. Engine inlet: Introduces mixed oil and gas.

2. Exhaust outlet: Exhaust waste gas.

3.  Rotor housing: Made of aluminum alloy, the inner wall is surface-hardened, and a spark plug insert and exhaust port are provided. This part is equivalent to the cylinder head of a reciprocating piston. Side shell: assembled on the side of the cylindrical rotor shell to form a sealed combustion chamber. There is an air inlet port on the top surface, and in the center, there is a sun gear passing through an eccentric shaft bearing.

4. Combustion chamber:Rero triangleThe rotor divides this sealed space into three independent working spaces: air intake compression, explosive expansion, and exhaust.

5. Stationary gear: meshes with the inner ring gear of the rotor and controls the rotor ​​of maneuver.

6.  Rotor: Its function is equivalent to the piston and connecting rod of a reciprocating piston engine. The cross-section isRero triangle, the size of the side groove (combustion cavity) is related to the engine compression ratio. Depending on the position of the swing, the intake and exhaust ports will be closed or opened accordingly, so they also have the function of exhaust valves.

7. Internal gear: meshes with the center gear and controls the rotor​​of maneuver.

8. Eccentric shaft: Also called the output shaft, its function is equivalent to the crankshaft of a reciprocating piston engine. The rotation of this part creates an eccentric effect on the rotor bearing, driving the rotor to rotate.

9. Spark plug: generates an electric spark to ignite the atomized gasoline and air mixture to achieve explosive expansion.

Working principle:

The basic structure of the Wankel engine is to place an Rero triangle Shaped rotor, the three faces of the rotor divide the oval space into three independent combustion chambers. Since the rotor operates eccentrically, the volumes of these separated independent combustion chambers will continue to change during operation. This type of engine uses the changing characteristics of the closed space to achieve the intake, compression, ignition and exhaust process required for four-stroke operation.

A traditional four-stroke reciprocating piston engine rotates twice before each cylinder completes the intake, compression, ignition and exhaust processes. As for the Wankel engine, the number of teeth of the inner ring gear of the rotor is 51, and the number of teeth of the center gear is 34, 51-34=17, 17÷51=1/3. The three faces of the rotor perform different four-stroke cycles simultaneously, so when the first face returns to the origin (that is, the rotor rotates once), three four-stroke cycles are completed.


High output power: The rotor of the Wankel engine does work three times every time it rotates. Compared with the general four-stroke engine which does work once every two revolutions, the entire engine has only two rotating parts. Unlike the general four-stroke reciprocating engine, which has two Compared with more than ten moving parts, the simplified structure makes the engine smaller in size, lighter in weight, and has a lower failure rate. In addition, due to the axial running characteristics of the rotary engine, it does not require precise crankshaft balancing to achieve higher operating speeds. Its speed rises faster than that of the reciprocating engine, and it has a high horsepower-to-volume ratio (the engine has a smaller volume but can output more power).

Smooth operation: The circular motion of the rotor rotation runs more smoothly than the horizontal linear motion of the reciprocating piston engine (referring to the connecting rod and crankshaft driving the piston), so the vibration and noise of the Wankel engine are relatively small. Since the Wankel engine is small in size and runs smoothly, and there is no local high heat caused by the exhaust valve of a reciprocating engine, there are few hydrocarbons in the exhaust gas, which is also one of its advantages.


Under torque at low speed: The method to increase the horsepower of a reciprocating piston engine is very simple, but the Wankel engine is not. For example, changing the angle or lift of the camshaft of a reciprocating piston engine can change the valve timing or increase the overlap angle to increase horsepower; on a Wankel engine, the intake and exhaust ports must be moved or expanded to achieve the same effect. However, the convex-top piston assembled into a reciprocating engine increases the compression ratio to increase the mixture density of oil and gas and adjust the air-fuel ratio to increase horsepower. This method cannot be achieved on the Wankel engine. Even if the groove size on the side of the rotor is increased, the amount of air intake cannot be changed. In other words, if you want to modify the Wankel engine body to increase horsepower, you must sacrifice low-speed torque for high-speed horsepower. Therefore, Mazda recommends that it is best to shift at red line. . .

The ignition system is complex: The ignition system was also one of the weak points of the Wankel engine. Because its combustion chamber moves and operates, the flame propagation pattern during the explosion must be poor. A complex dual ignition system and a more powerful electric spark must be used, so the ignition timing and the position of the spark plug are very important.

High fuel consumption: Due to the short combustion time, the combustion of the mixed oil and gas is incomplete, resulting in fuel consumption about 10% more than that of a reciprocating engine. The Wankel engine emits a large number of hydrocarbons at startup and low speed, twice as much as a reciprocating engine. However, after accelerating and increasing the speed, the exhaust volume drops significantly, so there are generally concerns about the air pollution problem of Wankel engines. In order to solve this problem, equipment such as thermal reactors, catalytic reactors and afterburners are generally installed. In contrast, since the three combustion chambers of a rotary engine are not completely isolated, after a period of use, it is easy to cause air leakage due to wear of the sealing material and cylinder wall, significantly increasing fuel consumption and pollution.


Although the rotary engine has the characteristics of producing high output with a small displacement and high speed, its working principle is different from that of a reciprocating engine. When formulating tax regulations on engine displacement, countries around the world all use the actual emissions of the rotary engine. The air volume is multiplied by two to provide a baseline for comparison with a reciprocating engine. For example, although the actual displacement of the RX-8 sports car produced by Mazda is only 1,308cc, in Japan, the displacement of 2,616cc is used as the basis for tax bracket calculation.