Air fuelled zero emission road transportation: A comparative study

Road transportation using air as a fuel has attracted much attention over the past decade. The fuel (air) can be in two forms, compressed gas form and cryogenic liquid form and engines based on both forms of air have been investigated. Prototypes of air powered road vehicles are expected to emerge over the next few years. However, there have been debates over the advantages and disadvantages of the two technologies. This paper aims to compare the two technologies from the technological point of view. Engines for a typical small scale passenger car are used for the analyses and the comparison is based on the shaft work, coolth, efficiency and energy density. It is shown that the shaft work outputs and the coolth available to engines powered by both fuels increase with increasing working pressure and temperature. Given the working pressure and temperature, liquid air powered engines have a slightly lower specific work outputs than compressed air powered engines. The volumetric energy density of liquid air, however, is much higher than that of compressed air, and liquid air has much higher coolth than compressed air. On the other hand, the efficiency of the compressed air powered engines is higher than that of liquid air powered engines mainly because of the higher energy consumption of liquefaction plants. The analyses also suggest that an effective use of coolth be a key to improve the overall efficiency of liquid air powered engines.