Regenerator Material

We had taken the decision at the material selection stage to have the regenerator tube manufactured from brass. The reason for this was that the coefficient of sliding friction is lower between the aluminium displacer piston and a brass tube rather than an aluminium tube. We wished to decrease any frictional losses as much as possible to allow the engine to produce more power. However, during the manufacturing stage we were made aware of the limitations associated with the machining of brass. The regenerator tube was to be made as thin as possible (0.5mm) to allow some heat to pass through it. This was deemed to be too thin to be made from brass so the decision was made to change it to aluminium because any dimension alterations would affect the operation of the regenerator. The coefficient of dry sliding friction between two aluminium surfaces is 1.4 whilst between brass and aluminium is 0.3. This is a considerable difference which did not help reduce the friction within the engine, but it was a compromise we had to make to maintain the regenerator design.

Regenerator Design

After some disassembly of the engine to assess the lack of performance, it was concluded that the positioning to the holes at one end of the regenerator tube were incorrect. On the original design there was supposed to be a series of slots at each end of the regenerator tube to allow the air to pass from each end of the cylinder through the regenerative material. Due to the fact that the tube was so thin, the slots could not be made so an array of holes were drilled around the circumference at each end of the tube. However, the holes were not drilled in the same positions as the slots and it was discovered that those at the bottom end of the piston's stroke were creating a vacuum which resisted the movement of the piston. This was rectified by covering the original holes and creating new ones in the correct position.

As a result of this same problem with the hole positioning the air to flow around the top of the regenerator tube was restricted. The air should have been able to flow freely around the top of the tube as it was compressed by the movement of the piston but could not as the holes were in the wrong place. This was corrected by shortening the length of the tube so that it was not flush with the cylinder wall.