Combustion and evaporation processes occurring in a closed chamber can result in signie cant pressurerise and direct work transfer. The pressure and volumetric changes that accompany such processes allow substantial work potential to be achieved in cyclic nonsteady devices, such as internal combustion engines and pulsed combustion or detonation engines. The ideal pressure gain or work production is a function of the prescribed ine ow and oute ow conditions, volumetric cone nement, e uid properties, and other parameters. The generalized thermodynamic limits of pressure gain and work production in such devices are investigated. Analytic and iterative methods are provided to evaluate cyclic combustion and evaporation processes for enhancing airbreathing combustion engine performance.