The paper presents a theoretical analysis of possible thermodynamic cycles of a pulse jet engine. To clarify the type of thermodynamic cycle, equations for calculating the polytropic index from instantaneous gas parameters in the combustion chamber are derived. The classic Argus As 014 valved pulse-jet engine workflow, both at static conditions and at 800 km/h, has been simulated. The simulation results confirm the known data that the real operating cycle of a valved pulse jet engine during static conditions is the Lenoir cycle. However, for the engines with straight air intake and with increasing flight speed, the specific workflow nature, i.e. the presence of polytropic pre-compression before heat release, that corresponds to the Humphrey cycle rather than the Lenoir cycle, has been found. Unlike a straight intake engine, a valved pulse jet engine with a side air intake that does not have air pre-compression has also been found to exhibit increased performance with increasing flight speed. The obtained data confirm that, in general, the thermodynamic cycle of a valved pulse jet engine can be represented as the Humphrey cycle, which in specific cases, such as zero flight speed and/or a side air intake, does not experience polytropic pre-compression before heat release and corresponds to the Lenoir cycle.