SUPERENGINE® | The compression-wave engine
                             by Karl Obermoser

SUPERENGINE ®  | The compression-wave engine

Acoustic waves can be transmitted through compressible media because these have, at all points, a “mass” characteristic and a “spring” characteristic. An equivalent to the acoustic wave that can be used in mechanical engineering is the waveguide. This can be, for example, a cylindrical tube in which equally spaced pistons (= the mass) can slide. The spaces between the pistons are filled by a volume of gas (= the spring). If the tube is closed at one end and a periodic force is applied to the piston at the open end, a standing wave is generated at certain resonant frequencies.

In order to generate a wave capable of propagating itself, such as is needed for the Stirling cycle, the closed end of the tube must be coupled to the open end. The easiest way to achieve this is for the piston at the open end to be of a stepped construction, with the smaller diameter extending through to the closed end of the pipe, and all the other pistons to be drilled out to the diameter that will enable them to slide over the smaller diameter of the stepped piston. Using this design, a Stirling cycle takes place in each of the gas volumes trapped between the pistons, provided that a regenerator is located in them.

SUPERENGINE® | Acoustic wave -> waveguide -> Stirling cycle
                             by Karl Obermoser

In the simplest version, the engine has a stepped piston and only one drilled-out piston, but even then operates with two simultaneous Stirling cycles, for instance one for the drive and one as a heat pump. Since no such engine exists at present, I have included a schematic illustration with one drilled-out piston and only one built-in heater - regenerator - cooler assembly. (Click the picture above to enlarge it)

The neutral position, to either side of which the drilled-out piston oscillates, defines the temperature difference at which the engine develops zero power. Depending on the direction in which this neutral position is displaced at a given difference in temperature, the engine either delivers power or operates as a heat pump (i.e. exerts a braking effect). By displacing the neutral position during operation, the engine’s power output can be regulated with a more rapid response than that of an internal combustion engine.

In mechanical engineering terms, this engine can be classified as a free-piston alpha-type Stirling engine – a previously unoccupied category. If the stepped piston is connected to a crank drive mechanism, the power-to-weight ratio is more favourable and the range of possible applications wider than for a pure free-piston version. In view of the absence of a suitable category, the engine has no previous history for me to quote here. It is interesting to note, however, that the drawings to be found in published literature are based on a serious misconception, and are incorrectly defined as illustrations of free-piston alpha-type Stirling engines. At SUPERENGINE®, we have now begun to develop machines of this type.

To conclude this topic, here is a quotation from a well-known authority on Stirling engines: “Yet the general knowledge and understanding of Stirling engines is still at such a low level that even among experts a wide divergence of opinion can be found, not only as to their basic applications or desirable features, but even as to the analytical approach appropriate to their design and optimisation.” T. Finkelstein, 1992 (Foreword to Organ, A.J., Thermodynamics and Gas Dynamics of the Stirling-cycle Machine)

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