Rotary piston engine

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The Wankel cycle, animated
Wankel rotary piston engine, manufactured in 1970

A rotary piston engine (RKM) is a prime mover in which the parts that do mechanical work only perform rotary movements.

This design offers various possibilities to convert energy into rotary motion. If the energy is available in the form of hydraulic or pneumatic pressure, you can z. B. Use vane motors.

Is the energy chemically bound available, e.g. B. in the form of liquid fuel ( gasoline , diesel oil , alcohol, etc.), you can run the engine as a heat engine .

In contrast to reciprocating piston engines , the parts that perform mechanical work perform a periodic rotary movement. The energy conversion process can take place using different cycle sequences. These different cycles (filling, blowing out, etc.) take place during the rotary movement (s).

Principal advantages and disadvantages

advantages

  • Simplicity: Potentially fewer moving parts than in a reciprocating engine, resulting in a simpler, more robust design. In addition, in contrast to the reciprocating engine, there is generally no power transmission via the crankshaft .
  • Quieter running: Many rotary piston engines can be fully balanced , which results in significantly lower vibrations than in reciprocating engines.
  • Lower power-to-weight ratio : Since many rotary piston engines are very compact and can be built with few parts, the power-to-weight ratio is reduced to a fraction of that of reciprocating piston engines.

disadvantage

  • Problematic sealing : Rotary piston engines have to struggle with sealing problems to varying degrees, depending on the type. While the sealing of the combustion chamber of reciprocating piston engines can be achieved relatively easily and reliably by means of piston rings (but represents a speed-limiting factor because piston rings collapse and fail at high speeds), different sealing systems sometimes have to be used in rotary piston engines. In addition, the sealing strips always run over the same places on the walls. This can create a grinding pattern that leads to leaks.
  • Unfavorable crescent-shaped combustion chamber of the rotary piston engine compared to the hemispherical combustion chamber of the reciprocating piston engine formed by the cylinder wall, cylinder head and piston crown. Although this offers an optimal ratio of (minimum) surface to (maximum) volume, one must also take into account the displacement volume, cycle duration and actual lossy areas. As an aside, it should be noted here that the ideally hemispherical combustion chamber is nowadays due to its narrowly limited geometric design options (a multi-valve arrangement is difficult to implement) and an increased tendency to glow ignition and ringing, caused by the rotational symmetry (causes focusing), which quickly leads to holes in the piston crown is rarely used.
  • Often, due to the design, very special parts have to be manufactured, which increases manufacturing costs. These parts are u. U. even complicated to manufacture, which further increases the cost and effort for the production of a rotary piston engine.
  • Difficult lubrication : As a rule, lubricating oils and greases should not penetrate certain areas of the engine so that they do not burn and the work process is negatively affected. However, this can usually not be prevented at all or not completely or it is associated with great effort. Coatings made of Teflon , ceramic or graphite may offer an alternative here, but these are sometimes more expensive than conventional lubrication and more complex to manufacture and maintain.

Construction methods

Rotary piston engine

All moving parts of a rotary piston engine rotate around a fixed point (usually around its own center of gravity ). Although there are a large number of patents for rotary piston engines, none of them have proven to be competitive or practical.

Continuously working rotary piston engine

Continuously working rotary piston engine

A rotary piston engine rotating without dead center cannot work in cycles, but must work continuously; with continuous constant pressure combustion, such as gas turbines. This is possible because the combustion and the increase in volume do not increase the working gas pressure, but instead act on a larger piston area in a working chamber that is larger in accordance with the isobaric expansion. That brings the same higher piston pressure. A machine for carrying out the process is described in patent specification EP 0 805 916 1B "piston combustion turbine". An annular cylinder screwed together from two halves, the content of which is divided by the rotor into five circumferential annular spaces, from which ten working chambers are created by two shut-off parts, the inlet and outlet openings of which are appropriately connected to one another by lines. The chambers are traversed by three rotating disc pistons, so that they continuously convey and work. Without the damaging high peak pressure, all that is needed to seal the working chambers is to use moveable labyrinth seals in the shut-off parts, which take part in the thermal expansion and elastic deformation and seal. The consumption lubrication of the bearings and piston rings and plan sides takes place from the rotor bearing, which is supplied by a line from an oil container. The combustion to increase the volume takes place in an external combustion chamber, with multiple excess air. The compressed working air can also be heated by concentrated solar heat by means of an air heater. And if the working air is compressed in several stages with intermediate cooling, if possible isothermally, it only needs to be heated from around 62 ° C (335 K) to 397 ° C (670 K) to double the volume. (Instead of 600 K to 1200 K after adiabatic compression to 12 bar.)

Rotary piston engine

With a rotary piston engine, all the parts that do the work rotate on a circular path. Here, too, there is an abundance of patents that have not been realized. The only exception (and thus the only rotary piston engine ready for series production) is the rotary engine . In the design as a rotary piston Wankel, it represents the kinematic reversal of the rotary piston Wankel. With it, the rotor and the envelope figure (trochoid) no longer rotate, but only the rotor. The shell figure is fixed to the outside world as a housing. The runner no longer only rotates around its own center of gravity, but in such a way that the center of gravity also circles on a circular path.

See also

literature

  • Martin Werdich: Stirling machines. Basics - technology - application. 11th revised and expanded edition. Ökobuch-Verlag, Staufen near Freiburg 2007, ISBN 3-922964-96-6 (detailed description of hot gas engines with a separate chapter on rotary piston Stirling engines).
  • EP 0805916 B1 continuously operating rotary piston engine.

Web links

Individual evidence

  1. ↑ page 28, Recognizing and repairing piston damage; Kolbenschmidt Pierburg AG (PDF; 3.4 MB).