THERMODYNAMIC ANALYSIS OF COMPRESSED AIR VEHICLE PROPULSION

The fist compressed air vehicles were built by Andraud and Tessie du Motay in Paris between 1838 and 1840. Since then the idea has been tried again and again, but has never reached commercialization. In recent years the French developer MDI has demonstrated advanced compressed air vehicles. However, the claimed performance has been questioned by car manufacturers and automobile expert. Basically, when referred to ambient conditions, the relatively low energy content of the compressed air in a tank of acceptable volume is claimed to be insufficient to move even small cars over meaningful distances. On the other hand, another air car developer claims to have driven 184 km on one 300 Litre filled with air at initially 300 bar pressure. Obviously, there are issues to be resolved, not by heated debates, but by an analysis of the thermodynamic processes involved. This is the aim of this study. The results indicate that both sides are correct. At 20°C a 300 Litre tank filled with air at 300 bar carries 51 MJ of energy. Under ideal reversible isothermal conditions, this energy could be entirely converted to mechanical work. However, even under isentropic conditions (no heat is exchanged with the environment or generated by internal friction) not more than 25 MJ become useful. By multi-stage expansion with inter-stage heating the expansion process is brought closer to the isothermal ideal. The analysis is extended to the compression of air. Again, the ideal isothermal compression is approached by multi-stage processes with inter-cooling. By this approach compression energy requirements are reduced to acceptable levels and system pressure and temperature are kept within safe limits. The results of this analysis seem to indicate that the efficiency of the four-stage expansion process is acceptable, while even a four-stage air compression with inter-cooling is associated with significant losses. However, the overall energy utilization could be increased if the waste heat generated during the air compression process would be used for domestic water and space heating. It seems that there is some justification for continuing the development of compressed air cars. However, it would be useful to establish the performance of such vehicles by an endurance race under controlled conditions in the presence of the general public.