What is rotary engine


The only manufactured model of the rotary engine on an industrial scale today is the Wankel engine, which is a type of rotary engine with a planetary circular motion of the main operating element. This structural arrangement of the rotary engine is undoubtedly simplest by its technical unit, but not the most optimal method of organization of work processes and therefore has its own inherent and serious limitations.

There are many varieties of the rotary engines with a planetary movement of the main operating element, but in fact they differ only in the number of faces of the rotor and the appropriate shape of the inner surface of the housing.

What is rotary engine


  • Increase of the power-weight ratio(hp / kg), it is almost two times higher than that of the piston 4-stroke engines. The weight of unevenly moving parts in the Wankel engine is far less than in similar power piston engines, and the amplitude of such unstable motions significantly less. This is due to the fact that piston engines operating reciprocating motion and the Wankel engine has rotational ones, planetary scheme. In addition, there are no crankshaft and connecting rods in Wankel engine.
    For these reasons, the power per combustion chamber unit in the Wankel rotary engine is significantly more. When the volume of the working chamber of 1.3L Mazda RX-8 has a power of 200 hp – 250 hp.
  • Wankel engine much easier to mechanically balance and get rid of the vibration, which improves the comfort of light vehicles such as microcars;
  • Dimensions of the rotary engine is 1.5-2 times less of comparable power piston motor. The Wankel engine has up to 35 – 40% fewer parts.

What is rotary engine


  • Low length of stroke brink of the triangular rotor, it’s one-fifth less than the length of the stroke. There is one fundamental difference from the piston engines – it goes in the direction of an increase in a linear direction, which coincides with the direction of travel. While Wankel has a complicated movement, and only a portion of the path of the triangular rotor with planetary motion becomes a proper line of travel. That’s why the Wankel engine fuel efficiency is worse than that of piston engines. Therefore, because of the short length of the stroke exhaust temperature is very high – working gases have no time to pass the main pressure on its rotor, as the window is opened and the high pressure hot exhaust gases with bulky moieties parts of working mixture get out into the exhaust pipe. Therefore, the exhaust gas temperature at the Wankel engine is very high.

wankel engine

  • The complex “sickle” shape of the combustion chamber. In such a combustor gases have large contact surface with the walls of the housing and the rotor. So much part of the heat goes to heating the engine parts, which reduces the thermal efficiency and increases the heating of the motor. Moreover, such shape of the combustion chamber leads to a deterioration of mixing and deceleration of the working mixture combustion. Therefore Mazda RX-8 engine has two spark plugs on one rotor section. These features also adversely affect the level of thermodynamic efficiency.
  • A potentially low level of torque for rotary engine. In order to remove the rotation with the moving rotor rotation center which itself continually carries planetary rotation on a circular path around the geometrical center of the combustion chamber, in which the engine is used eccentrically located on the main shaft drives. In fact – these are elements of the crank device. That is the Wankel engine was not able to completely get rid of the major disadvantage of classical piston internal combustion engine – crank mechanism. Although it submitted to the Wankel engine in a lightweight form – in the form of the eccentric shaft, but the most important defects of this mechanism: a ragged, pulsating torque mode and the small shoulder of the main element, the sensing torque – and were not “cured.” (RIS). That is why the single-section Wankel engine doesn’t work properly and manufacturers are using 2 or 3 rotor section to obtain a normal performance, it’s desirable to put on a flywheel on the shaft.

wankel engine

  • Presence of vibrations in the housing. The fact that the system is a rotary motor with planetary motion of the operating element assumes a non-equilibrium of the body movement. So center of mass during rotation of the rotor performs a continuous rotary motion around the center of body mass and radius of this rotation is the shoulder of the eccentric main shaft motor. That is why the inside of the motor housing acts constantly rotating force vector is equal to the centrifugal force arising in the rotor. That is, when the rotor rotates in the rotating turn the eccentric shaft is in the nature of their movement, and unavoidable elements expressed oscillatory motion, which leads to the inevitable vibrations.

wankel engine

  • Rapid deterioration of the end of the radial seals on the corners of the triangle rotor, because they are going strong radial load, inevitable in the Wankel engine on its own principles.
  • The constant threat of a high-pressure gases breakthrough from the cavity of the power stroke in the cavity of another stroke. This occurs because the radial seal contact edges of the rotor and the chamber wall caused by one thin line. At the same time there is still a problem of blow-through socket installation of plugs when they pass over the edge of the rotor.
  • A complex system of lubrication of the rotating rotor. In the Mazda RX-8 engine special nozzles inject oil into the combustion chamber to lubricate the rubbing during rotation of the chamber wall edges of the rotor. This reinforces the exhaust emissions and at the same time makes the engine very demanding to the quality of the oil. In addition, at high speeds there is an increased requirement for lubrication of the cylindrical surface of the eccentric portion of a main shaft around which the rotor rotates, and which takes power from the main rotor and translates into rotation of the shaft. These two technical difficulties, which is very difficult to solve, leading to insufficient lubrication at high speeds the most loaded friction parts of the engine, and this, consequently, dramatically reduced engine life.


  • High accuracy requirements of performance parts with complex shapes make this difficult in the motor industry. Such production requires high precision and expensive equipment – machines that can create complex volumes of the working chambers with a curved surface. The rotor itself also has a complex shape of a triangle with convex surfaces.
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