The Humphrey cycle is a thermodynamic cycle similar to the pulse detonation engine cycle. It may be considered to be a modification of the Brayton cycle in which the constant-pressure heat addition process of the Brayton cycle is replaced by a constant-volume heat addition process.
Hence, the ideal Humphrey cycle consists of 4 processes:
- Reversible, adiabatic (isentropic) compression of the incoming gas. During this step incoming gas is compressed, usually by turbomachinery. Stagnation pressure and temperature increase because of the work done on the gas by the compressor. Entropy is unchanged. Static pressure and density of the gas increase.
- Constant-volume heat addition. In this step, heat is added while the gas is kept at constant volume. In most cases, Humphrey-cycle engines are considered open cycles (meaning that air flows through continuously), so this means that the specific volume (or density) remains constant throughout the heat addition process. Heat is usually added by combustion.
- Reversible, adiabatic (isentropic) expansion of the gas. During this step incoming gas is expanded, usually by turbomachinery. Stagnation pressure and temperature decrease because of the work extracted from the gas by the turbine. Entropy is unchanged. Static pressure and density of the gas decrease.
- Constant-pressure heat rejection. In this step, heat is removed from the working fluid while the fluid remains at constant pressure. In open-cycle engines this process usually represents expulsion of the gas from the engine, where it quickly equalizes to ambient pressure and slowly loses heat to the atmosphere, which is considered to be an infinitely large reservoir for heat storage, with constant pressure and temperature.
- Heiser, W. H. and Pratt D. T. "Thermodynamic Cycle Analysis of Pulse Detonation Engines," Journal of Propulsion and Power, Vol. 18, No. 1, January-February 2002