The current concern over engine emission motivates us to reconsider the continuous-combustion option, which was first adopted by Brayton in the 1870's, with disappointing fuel economy in comparison with the Otto four-stroke.The two-part cycle principle of the gas generator engine was proposed by Wang & Jeng (1992). In this paper, a continuous-combustion gas generator engine is investigated using zero-dimensional modeling. The result confirms the validity of the two-part cycle principle producing simultaneous increase in power output and thermal efficiency. However, the various irreversible losses of the continuous-combustion piston unit result in unsatisfactory thermal efficiency. The continuous-combustion gas generator engine is, therefore, not a viable concept in its basic form. The alternative, intermittent-combustion gas generator engine is expected to sustain smaller irreversible losses; its performance prediction remains to be studied.