A hybrid pneumatic power system (HPPS) comprises an internal combustion engine (ICE), an air compressor, a high-pressure air storage tank, and a turbine, which stores the flow work instead of a battery's electrochemical energy; moreover, this system can recycle the exhaust-gas energy and make the ICE operate at its optimal point. Therefore, it can be viewed as a promising solution to increase a system's thermal efficiency and greatly improving exhaust emissions. This paper presents experimental study results concerning the operating capabilities of the HPPS and the effect of the contraction of the cross-sectional area (CSA) at the merging region of the energy merger pipe for the change in the compressed airflow pressure (P-air,) on the exhaust-gas energy recycling of the HPPS. The experiments were performed on an HPPS that uses an innovative energy merger pipe with a total length of 530 mm, a diameter of 34 mm, and ail angle between the two pipes of 30 degrees, and the CSA was adjusted for the change in P-air. The experimental results show that the exhaust-gas energy recycling and the merger flow energy are significantly dependent on the CSA adjustment for the change in P-air. The optimum conditions for the best merging process can be achieved at a CSA of around 5-35% in the full range of P-air. Under these conditions, the exhaust-gas energy recycling efficiency reached approximately 75-81%; therefore, a vehicle equipped with an HPPS can achieve efficiency that is approximately 40%, higher than that of conventional vehicles. Copyright (C) 2009 John Wiley & Sons, Ltd.