Abstract: The new type engines without the combustion process have to be researched and manufactured，especially for China，because of urban pollution and the short of fossil oil. Air powered engines，for example pressed air engine，liquid nitrogen engine and liquid air engine，are definitely zero emission engines，and have attracted the attention of government and the researcher. The theoretical analysis of the work cycle is made for the pressed air engine and liquid nitrogen engine. According to the calculation results of the available energy，the influence of the initial pressure and temperature of the expansion process to output energy is discussed，and the optimism method of the work cycle of the liquid nitrogen engine is described.

Key words: Work cycle, Available energy, Theoretical analysis, Air powered engine