A Novel Stirling Cycle Hot Air Engine To Build
This article has been reproduced from Melbourne Society of Model & Experimental Engineers Journal with the kind permission of the Secretary. The article is written by Ian Stewart, and the drawings by Paul Higgott. 12 July 97 - I have rescanned the drawings to provide better detail for intending builders.
If constructed carefully this engine will run on the heat of your hand, cup of coffee, or fax machine. If you wish, place it on a block of ice and it will run in the opposite direction! Stirling Cycle engines, perhaps better known as Hot Air Engines were invented in 1816. They were widely used in industry and for domestic purposes before the internal combustion engine superseded them around the turn of the century. They operate from an external heat source, and are scaled.
As air is heated it expands, this hot air is then transferred from the heated end to a cooler chamber. As the air cools the internal pressure reduces and atmospheric pressure acting on the exposed piston pushes it inwards transferring the cool air back to the hot end. Hot air engines need a relatively heavy flywheel to maintain momentum and are not self starting.
Although this novel engine is most unlikely to perform any useful work it can provide an interesting insight into the workings of these devices. Construction is straight forward and could be done without a lathe. The design is fairly flexible and can be simplified, eg. the lost motion link can be replaced by a simple crank. Use the drawings as a guide to construction, few of the dimensions are critical. The critical points are keeping friction to a minimum and thermally isolating the top and bottom plates.
Extensive use was made of brass tubing 1.5mm. OD x 0.8mm. bore and brass rod selected to be good sliding fit inside the tube. Obtainable from model shops the brass was used for pivots, links and the all important displacer gland.
The top and bottom engine plates are made from 3mm. aluminium plate sanded to a matte finish and sprayed with a lihght coat of matte black epoxy paint to assist in heat transfer, a black surface both absorbs and radiates heat more efficiently than a reflective surface. A 25mm. long piece of clear acrylic tubing l50mm. OD. was used for the displacer chamber allowing a clear view of the displacer motion. This can be expensive to buy, an off cut of PVC sewer pipe will work, however I feel a better alternative is to buy a cheap acrylic salad bowl and cut it up. Mount a block of scrap timber on a faceplate turn it to a loose fit for the tube. Wrap the spiggot with masking tape so the tube is a tight fit and secure with more tape, part off with a narrow parting tool and light machine oil as a lubricant, wash off the oil immediately with warm soapy water.
At this point I also recessed the ends of the tube to take an O ring seal. The O ring will have to be cut to size and glued with super glue. You can also use silicon sealer to make a gasket although this can be messy. The seal between the tube and the top and bottom plates is critical and warrants some time spent on careful work.
The fly wheel is a piece of 6mm perspex shaped to put most of the mass near the perimeter and to impart a sense of movement.
The power piston and its cylinder require care. A piece of precision glass tube can be cut or ground to length and a piston made from Teflon or machinable graphite. Friction must be kept to a minimum but still maintain a good seal.
The power piston stroke is approximately 11mm with an 8mm bore.
The bell crank on my engine is adjustable in all directions this is not necessary but can be used for experimentation; adjustment of the polystyrene displacer stroke and position may be necessary. The displacer is made from 12mm. thick polystyrene and has about 3mm. clearance from the chamber walls, stroke is 1Omm.
The phase angle between the power piston and the displacer was set by placing the power piston at top dead centre and the displacer at bottom dead centre: the displacer crank was then set by rotating the displacer crank 90 degrees clockwise (moving up from the bottom plate). This has proven to be best setting for clockwise movement viewed from the power piston side and cooling the bottom. BUT if you warm the bottom it will rotate in an anti clockwise direction quite happily.
Remember the engine like all Stirling Cycle engines is not self starting.