The current state-of-the-art Stirling engines use sinusoidal piston and displacer motion to drive the thermodynamic cycle and produce power. Research performed at NASA Glenn has shown that non-sinusoidal waveforms have the potential to increase Stirling engine power density, and could possibly be used to tailor engine performance to the needs of a specific application. However, the state-of-the-art Stirling engine design uses gas springs or planar springs that are very nearly linear, resulting in a system that resonates at a single frequency. This means that imposing non-sinusoidal waveforms, consisting of multiple frequencies, requires large forces from the drive mechanism (either the alternator or the crank shaft). These large forces increase losses, and increase the size and requirements of the control system. This innovation aims to reduce the external forcing requirements by introducing internal mechanical components that provide the forces necessary to achieve the desired waveforms.
A simple linear mass spring system has a single resonant frequency. It is relatively easy to add energy into the system at this frequency, and relatively difficult to add energy at other frequencies. These systems operate sinusoidally at the operating frequency unless large forces are introduced to impose non-sinusoidal motion.
Multi-degree-of-freedom systems and systems that use nonlinear springs are capable of achieving non-sinusoidal waveforms with relatively little external forcing. Adding more mass spring systems, tailored for the desired harmonics, could reduce the amount of external force required to achieve non-sinusoidal waveforms that have been shown to increase Stirling engine power output. Alternatively, nonlinear springs could be tailored to provide the force required to achieve non-sinusoidal motion without relying on external forces. Reducing external forces can reduce wear mechanisms, frictional losses, resistive losses, and controller complexity in both kinematic and free-piston Stirling engines.
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 http://technology.grc.nasa.gov. LEW-19325-1