Stirling engines typically achieve high efficiency, but lack power density. Low power density prevents them from being used in many applications where internal combustion engines are viable competitors, and increases system costs in applications that require Stirling engines. This limits their operating envelope in both terrestrial and space applications. Sinusoidal piston and displacer motion is one of the causes of low power density. Previous work proposed solving this problem by replacing sinusoidal waveforms with waveforms that more closely approximate those of the ideal Stirling cycle. However, when working with real engines, imposing ideal waveforms has been shown to reduce power density and efficiency due to increased pressure drop through the regenerator and heat exchangers.

This innovation proposes to improve engine performance by replacing sinusoidal piston and displacer waveforms with optimized waveforms tailored to a specific engine and design goal. For example, different piston/displacer waveforms can be identified for maximum power density, maximum efficiency, or a weighted combination of the two. Piston/displacer waveforms could also be altered to meet other engineering requirements such as reducing peak pressure or reducing vibration. These optimized waveforms can be identified using numerical optimization with a high-fidelity Stirling model, or through trial and error in the lab using a suitable controller. Piston-only waveforms optimized for maximum power output have been identified and demonstrated on engines in the Stirling Research Lab at NASA Glenn, resulting in an increase in power output of 14%, with further improvement possible if both piston and displacer waveforms can be achieved.

This work was done by Maxwell Briggs of Glenn Research Center. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact . LEW-19324-1

NASA Tech Briefs Magazine

This article first appeared in the January, 2017 issue of NASA Tech Briefs Magazine.

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