Finished Regenerator, Vessel Lid and Heater                                              

The 2.25 inch thick regenerator is made from alternating layers of aluminum screen and nylon net fabric in a light-weight wood and aluminum frame. I am thinking that the nylon insulation between the sheets of aluminum screen will retard the heat trapped in the regenerator from creeping in the direction of airflow. The 3 lbs of aluminum (94 sq ft of screen) makes a regenerator volume that is three times smaller than the cold space volume, but air emerging from the regenerator should still be adequately warmed or cooled.

Air is kept from flowing around the regenerator by lightly packed polyester between the regenerator and the wax polished glass walls of the box. The regenerator will be moved the 6.25 inches up and down in the box by four 5/64” vertical rods that enter though #106 o-ring seals in the regenerator box lid corners. The box lid will be held tightly down on a weather stripping seal by eight draw catches.

The Heater

The terrapin is my first attempt at building a stirling engine. I have decided that I can more easily regulate temperature and keep track the amount of energy entering the engine, if I heat the working gas using an electric element rather than using a hot water pipe. Using the information gained from these first experiments, I could modify this same engine to use another heat source, or I could build a bigger sized engine, if it turns out that the basic concepts of this engine are correct.

I had planned on using one-way air valves to have air enter the heater only during the warming part of the engine cycle, as the cooling is done in the case of the cold heat exchanger. I can simplify the heater design by cycling the electric heating element on and off such that it warms the surrounding gas only during the warming part of the engine cycle. Gas can now take the same path in and out of the power piston but is heated only as air enters the piston. I could tinker with the timing of the heating but I want to stay close to the concept of having the power piston pumping gas through a hot heat exchanger, even though the power piston is somewhat out-of-phase with the regenerator movement.

The heating element comes from a thrift store toaster and is wrapped around glass scaffolding which sits in the lid space between the regenerator box and the power piston. Power to the heater is run though a dimmer switch. A power range of 27 watts to 732 watts is available by adjusting the dimmer. At full power the heater elements are glowing red.   I have found that a resting cylinder air temperature of 100C can be maintained with a power setting of 123 watts. I modified the circuit inside the dimmer switch by running a pair of wires from the triac’s gate circuit to a remote switch, such that the heater circuit can be turned on and off as needed.