# Study of System Integral Energy Efficiency of a Hybrid Pneumatic Power System
## 1 Introduction  
In recent years, in addition to the technical advancement of the internal combustion engine (ICE), a significant amount of research effort has been given to designing a new type of vehicle that utilizes the ICE, called the hybrid electric vehicle (HEV); it uses both an electric motor and an ICE to generate power for propulsion. The major advantages of the HEV are increased energy efficiency and a reduced greenhouse effect compared with conventional vehicles. The new technology has overcome several challenges of HEV development, such as system integration [1], difficulty in recharging the battery in a short time [2; 3], matching of the best power conditions [4], and plausible alternative energy sources [5; 6; 7]. However, product design and product sustainability are crucial to the success of HEVs in the future [8; 9; 10]. Combined with reductions in energy and environmental pollution for future use, air-powered vehicles present several superiorities [11; 12]. First, only air is emitted during operation; therefore, production of compressed air appears to be the only possible pollution source. Also, the cost of air power is much lower than that of fossil fuels, such as in the case of electric, battery, and fuel-cell vehicles. Finally, the container for the compressed air can be designed such that it can be assembled and disassembled conveniently. The vehicle can be refilled with power faster when compared to electric vehicles in particular. [PERSON] and [PERSON] demonstrated that the pneumatic driveline was up to 67% lighter than the lightest competing hybrids, and that fuel consumption could be reduced from 35% to 30% [13]. The concept of the pneumatic car was proposed in the early 20 th century. [PERSON] [14] obtained a patent named \"Combined internal combustion and compressed air engine\". In this concept, the compressed air is used to ignite and start an ICE or vehicle brakes. Compressed air is produced from two of four engine cylinders while the other two cylinders perform their typical functions. The concept of applying a multicylinder engine to generate compressed air was proposed by [PERSON] et al. [15]. This concept is similar to that of [PERSON]'s patent. The only difference is that [PERSON]'s concept was intended for driving pneumatic tools such as spray guns and drill hammers. [PERSON] [16] filed for a patent that utilized a compressor with an electric motor to generate compressed air to drive an air engine. [PERSON] [17] proposed an invention wherein ICE cylinders were used for compressing air and subsequently allow this air to drive a vehicle. [PERSON]'s invention could also perform regenerative braking, which was not feasible in [PERSON]'s invention. [PERSON] [18] defined the pneumatic hybrid, which is currently available. He utilized supercharging to store gas into a pressure tank such that the pressure inside the tank was greater than the outside environment. In 1998, Motor Development International, SA (MDI), first introduced air vehicles for commercial purposes. Currently, MDI can support single- to six-seater minibuses. In collaboration with MDI, Tata Motors released an air-powered vehicle named AIRPOD in 2012. The Korean company Energine created a pneumatic hybrid electric vehicle in 2005. The engine of this vehicle operated with an electric motor to generate the power source. The Lucksky Group from China displayed an airbus at the Beijing International High-Tech Expo in 2015. Additionally, several research institutes and car companies worldwide have recently focused on air-powered vehicle development [19]. In addition to purely air-powered vehicles, a part of the research is related to the combination of the systems with conventional internal combustion engine. [PERSON] et al. showed that the pneumatic power system uses heat of cooling water to optimize the performance of air engine. It can increase power by 38.22% and efficiency from 26.18% to 41.22%, and save 50% of fan power [20]. [PERSON] et al.