Jackrabbit Engine                                                                                 

Jackrabbit Layout.

I am making an engine for the Jackrabbit out of a five gallon open-head HDPE bucket. I wanted to use five gallon steel pails but none are available locally and I cannot afford the online shipping fees. HDPE is pliable when over 100ºC and 82ºC is listed as the maximum safe working temperature. I am going to try to keep temperatures under 80ºC. Most of the heat is concentrated in the top ¼ of the bucket. In that part, I have doubled the wall thickness of the plastic and I have also reinforced the plastic lid  and bucket bottom with sheets of wood. 

Regenerator and fan in bucket.

Sitting ¾ of an inch below the lip of the bucket is a donut-like regenerator made from alternating layers of aluminum screen and nylon netting. Poking through the center of the regenerator is a three inch diameter metal tube containing an electric hairdryer motor and heating element.

 

Engine animation.

The polarity of the energized fan, determines the direction of the air blowing through the regenerator. Hot air blows though the center fan tube into the top of the regenerator and cold air comes out the bottom of the regenerator. In the bottom ¾ of the bucket, the hot air and the cold air volumes are kept separated by a plastic bag partition. The fan repeatedly fills and empties the bag with hot air within the surrounding space of cold air. The amount of air in the bucket is fixed, so the pressure of the air in the bucket increases and decreases each cycle. A flexible hose runs from the cold side of the bucket to the piston.

One engine simulation

I have not, as of yet, added water tubing for the heating and the cooling of the engine air, but I have left room for copper coils above and below the regenerator. For my initial tests, the electric heating element and the ambient air temperature should give me a temporary temperature gradient of about 60ºC.

Bucket clamped down.

A pressure test of the HDPE bucket fails to keep even one PSI of pressure from leaking out from under the lid within a few seconds. I am thinking that I can make the bucket airtight if I seal the lid with aquarium glue. Each PSI of over-pressure inside the bucket produces 94 pounds of upward force on the lid. I am using 3/8” allthread clamps to hold the lid tightly down on the bucket with about 150 pounds of force. During the vacuum part of the engine cycle, an additional 118 pounds of downward force is felt by the lid. I am hoping the hot HDPE bucket can withstand the stress.