The automotive manufacturing industry has been under tremendous pressure to develop high-efficiency power-train systems because of the considerations of protection of the global environment from vehicle pollution, reduction in fuel consumption, and maintenance of a high level of performance by end-user vehicles. Besides the conventional hybrid electric vehicles, an innovative hybrid pneumatic power system (HPPS) is undergoing research and considered to be a promising technology; it replaces the battery’s electrochemical energy with a high-pressure air storage tank and enables the internal combustion engine (ICE) to function at its sweet spot. The HPPS, which effectively merges both the high-pressure air flow from the storage tank and the recycled exhaust flow from the ICE, thereby increases the thermal efficiency of the ICE and transforms the merged flow energy into mechanical energy using a high efficiency turbine. This paper focuses on the major research process into HPPSs, including