  Soft heat engines are poised to play a vital role in future soft robots due
to their easy integration into soft structures and low-voltage power
requirements. Recent works have demonstrated soft heat engines relying on
liquid-to-gas phase change materials. However, despite the fact that many soft
robots have air as a primary component, soft air cycles are not a focus of the
field. In this paper, we develop theory for air-based soft heat engines design
and efficiency, and demonstrate experimentally that efficiency can be improved
through careful cycle design. We compare a simple constant-load cycle to a
designed decreasing-load cycle, inspired by the Otto cycle. While both
efficiencies are relatively low, the Otto-like cycle improves efficiency by a
factor of 11.3, demonstrating the promise of this approach. Our results lay the
foundation for the development of air-based soft heat engines as a new option
for powering soft robots.
