Abstract: A hot gas engine is provided that supports internal combustion in a gas mixture maintained at high pressure and in a controlled temperature range. With air as the essential working fluid, an oxygen rich mixture is cycled in a semi-closed loop system supportive of combustion but nonetheless providing work in the fashion of the known Stirling cycle. The arrangement includes a high area, short path length regenerator that contributes to high work efficiency despite the use of a relatively high density working fluid. The high temperature chamber of the system includes an interior layer of self supporting permeable insulation, and confines the working fluid without significant heat loss. An air compressor-expander system is coupled to replenish air in the internal gas mixture at compatible pressures and temperatures and without imposing substantial work loads. Catalytic agents may be disposed in the system for greater burning efficiency.