An investigation of an innovative power pack and motor for a compressed air car of practical performance
Creators: name: Ditmore, Henry
Publisher: name: University of Westminster
Year: 2011

description: This thesis investigates the design and potential of an automobile that is powered by compressed air supplied from an on board store charged by a separate compressor. That store is a carbon fibre sphere of 250 litres capacity and, once charged with compressed air to 30MPa, has a specific energy density the equal of conventional Ni-Mh electrochemical batteries. It is shown that this store can be built to a safe international standard and it is reasonable that the store can be expected to have a life equivalent to the life of the vehicle. In contrast, all electric vehicle batteries can be expected to have a life of 3 – 5 years. The range and efficiency of the vehicle are contributed to by a lightweight heat pipe based heat exchanger that takes heat from ambient air and uses it to raise the temperature of the compressed air stream that is initially at 60K below ambient temperatures. The compressed air stream’s temperature is raised to within 10K of the ambient temperature and that increase in enthalpy makes a significant difference to the performance of the system. It is shown that this is equivalent to at least a 13% increase in storage capacity. The inclusion of this heat exchanger increases the system energy density to equal the best Ni-Mh electrochemical batteries. Almost all electric battery vehicles have a regenerative braking system that collects a proportion of the vehicles braking energy, normally lost to the environment. This project has investigated a unique design of regenerative braking system that uses a heat pipe heat exchanger to collect the heat resulting from braking. Those heat pipes then transfer the heat to a water/alcohol based heat store. Further heat pipes transfer the heat to the compressed air flow once the vehicle is moving and re-uses that energy to further increase the energy available to the compressed air motor/prime mover. The first system tested was an open system that allowed the hot air to escape to the atmosphere after one pass through the heat exchanger. Whilst being an easily implemented system it proved to be inefficient at capturing the heat of braking and it was replaced by a closed system in which the hot air was recirculated through the brake and heat exchanger. This final system achieved an efficiency of 70% with potential for further improvements. It is believed that this is the first system of its type to be used in this kind of application. In order to maximise the range of any future compressed air driven vehicle it was necessary to design a new kind motor. The basic design is that of a rotary multi-vane expander. In order to maximise the motor’s efficiency, whilst dealing with the wide range of motoring demands that a commuting vehicle can expect to make upon its motor, it was necessary to make several elements of the motor’s geometry adaptable. Some of these can vary their geometry whilst the motor is in operation. Initial tests suggest an efficiency of 88% has been achieved but higher efficiencies are expected with further work.