Experiments have shown that an assembly of multiple free-piston Stirling engines can be designed and constructed in such a way as to both (1) make the vibrations of the engines balance each other to minimize the overall level of vibration, and (2) enable the engines to operate independently of each other, so that if one fails, the other(s) can continue to provide power. Prior to these experiments and to the research and development effort that preceded them, it was not possible to achieve both redundancy and suppression of vibrations: The only previously demonstrated method to balance out vibrations of multiple Stirling engines was by use of counter-oscillating pistons coupled to each other via a common thermodynamic hot space, with the engines driving linear alternators connected electrically in series. This older scheme precludes redundancy because the common thermodynamic interaction and the series electrical connection causes both engines to fail when one fails.
In the present scheme, the multiple Stirling engines in a given assembly are thermodynamically independent of each other. For coupling of vibrations between the engines, the housings of the engines are rigidly connected to each other at their cold ends in an opposed arrangement (see figure). A tuning capacitor is connected in series with the linear alternator of each engine to compensate for the alternator inductance in order to obtain a near unity power factor.
The experiments were performed on a two-engine assembly operating at a frequency ≈60 Hz and output-power levels ≤250 W per engine. When the electrical outputs of the engines were ac-coupled in parallel downstream of the tuning capacitors, the net vibration level was reduced to as little as 1/50 of that of a single engine operating alone. (DC coupling was found to exert no effect on vibrations.) It was observed that the ac connection between the two engines can be opened or closed at will with no adverse consequences other than that when the connection is opened, the engines gradually drift out of synchronization and thus vibrations are no longer suppressed.
When the ac connection is closed after having been open, the engines come back into synchronization with opposing piston motions, so that vibrations are once again suppressed. When so synchronized, the engines remain synchronized even in the presence of wide variations in charge pressure or hot-end temperature of one engine relative to the other. No significant transient overstrokes or other potentially damaging behavior was observed.
Housings of Multiple Free-Piston Stirling Engines are rigidly connected at their cold ends so that vibrations are coupled among them. The arrangement is symmetrical so that their piston motions will balance if properly synchronized. Experiments have shown that synchronization is achieved when the engine electrical outputs are ac-coupled.
This work was done by Maurice A. White, Laurence B. Penswick, and Songgang Qiu of Stirling Technology Co. for Glenn Research Center.
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