This paper presents calculations of the theoretical cycle of engines with an active combustion chamber depending on energy delivery and dissipation. In the case of ACC engines, a different calculation approach is required to account for the possibility of additional volume change, independent of the piston-crank system. The introduction presents a schematic diagram of volume change, accomplished by two independent piston-crank systems and an active combustion chamber, as proposed by the authors. Moreover, the diagram, which is the basis for analysis in this paper, illustrates characteristic points of the cycle. In existing theoretical cycles of combustion, this issue does not present any difficulties, since the solution is exact and based on known equations. In the case of theoretical ACC engines, however, the situation is different, since this engine can perform not only in typical cycles, but also in new ones. To explain the challenges of these new cycles, authors present a few of the most probable calculation variants, taking into account the new kinematic capabilities of ACC engines. Each comment justifying the choice of a certain calculation variant is illustrated with a theoretical cycle figure and closest approximation of induced pressure course of the ACC engine. At the same time, however, the authors show that this problem can have many interpretations. It has been concluded that the solution depends on the assumptions made about the active combustion chamber, namely its principle of operation.