Materials
We studied the purpose of each part of the design and decided which material properties would help to fulfil this purpose. After comparing these with the properties of the materials used in the original design we found that several material could be changed to improve the design. The table below contains data for the conductivity, linear expansion and melting point for a series of materials we believed might be suitable for use for various parts in our engine design.
The insulating ring between the hot and cold caps was made of aluminium on the original design. The purpose of this ring is to prevent the transfer of heat between the hot and cold ends of the displacer piston. Aluminium has a relatively high thermal conductivity which is not a desirable property for this purpose. Therefore, we have decided that it should be made from steel for the new design. We have also decided to use the piston support as another form of insulator between the hot and cold ends. This is also to be made from steel due to its low thermal conductivity.
The hot cap material was also changed to improve the design. This part needs a high thermal conductivity and a relatively high melting point so that it will allow the heat source to efficiently conduct the heat energy into the cylinder. As a result the Stirling engine will initiate operation more quickly and hopefully sustain operation for a longer period of time. Another consideration when choosing the material is the linear expansion. The hot cap must have a low linear expansion so that the fit between it and the regenerator is not compromised.
The hot cap material was also changed to improve the design. This part needs a high thermal conductivity and a relatively high melting point so that it will allow the heat source to efficiently conduct the heat energy into the cylinder. As a result the Stirling engine will initiate operation more quickly and hopefully sustain operation for a longer period of time. Another consideration when choosing the material is the linear expansion. The hot cap must have a low linear expansion so that the fit between it and the regenerator is not compromised.