PD Compression: A Quasi-Static or Dynamic Process?

Gas compression in a positive displacement (PD) type piston engine, or piston compressor, has long been modeled as a quasi-static process in which the piston is assumed to move infinitely slow inside a cylinder. The gas compression mechanism is therefore regarded as being governed by the basic thermodynamic principles. Energy transfer is accomplished quasi-statically with a piston reducing the volume a little at a time, small perturbation, while increasing gas pressure simultaneously. This is in direct contrast to a dynamic type gas compressor such as turbo compressor that achieves energy transfer by initially imparting kinetic energy to the gas and then converting that kinetic energy to gas pressure. Is the quasi-static hypothesis still valid today with the quantum leaps made in piston speeds since the Watt’s days? Moreover, exactly how is the work done by the moving piston converted to gas pressure? This paper attempts to answer these questions by applying the classic shock tube theory to the transient dynamic process of high speed piston compression. The results reveal that the nature of the compression is in essence a dynamic process by actions of a pair of moving shock waves triggered by fast piston movement. A transient dynamic stage of a piston compression consists of a rotor with incident shock wave (ISW) compression and a stator with reflected shock wave (RSW) compression during a finite time interval; or a stage cycle of ∆t. The resulting energy transfer process is thus similar to that of a turbo compressor characterized by initially imparting kinetic energy to the gas through ISW action and then converting the kinetic energy to pressure by RSW reaction.