This work introduces a novel expansion engine that integrates heat exchange with thermal energy storage and operates on a Quasi-Carnot cycle, combining quasi-isothermal and quasi-adiabatic processes. Unlike conventional combustion, Stirling, or Organic Rankine systems, the proposed design minimizes entropy generation and improves efficiency by using alternating hot and cold water sprays to sustain isothermal compression and expansion. A prototype (1-liter chamber) demonstrated ~10% efficiency, leveraging water’s high specific heat for effective energy storage. The system operates under low temperature differences (ΔT < 60 K) and low pressures, allowing for cost-effective materials, simplified mechanics, and silent operation.