The Terrapin: Vessel, Cold Heat Exchanger and Cold Air Pump.                                                                                                  

When building robotic projects, I have a tradition of naming them after the scientific name of the animals they resemble. This engine may well be feeble and slow and it has a boxy look about it. The scientific name of the box turtle is terrapene carolina. I was thinking of naming this engine, the terrapin. Hey, I have to call it something.

I chose the largest regenerator vessel volume (1.25 cu feet) that would still be in the proper compression range for a high temperature of up to 100C. It is about twice the size of the model I had originally envisioned, so I am going to be over-budget.

The regenerator vessel was made from a big plywood lined wood box whose floor is sealed with resin and whose walls are covered in glass.

The water cooled heat exchanger is a 13 foot coil of ¼ inch OD copper tubing alternating with one inch strips of ¼” wire mesh.

The cold coil fits snugly under a 3 ½ gallon HDPE paint bucket lid that was modified into a cold air pump with the help of some sheet metal work and some PVC backed polyester material from a cheap rain jacket. The pump draws air across the cool coil only during the cooling part of the engine cycle as the regenerator is moving up. During the warming part of the engine cycle, as the regenerator moves down, the already cooled air contained in the pump, moves into the regenerator to be warmed.