Simple Stirling 1 performance with and without regenerator

I’ve tried a variety of regenerators on the Simple Stirling 1 engine and the one shown in the photo is simple to make and performs as well as or better than all of the other ones I’ve tried. The test results on the engine with the original displacer and with the modified displacer containing the regenerator are shown in the plot. As you can see for the same delta T (temperature difference between the hot and cold plates), the displacer with a regenerator provides much higher RPM.

Ideally, a regenerator makes a Stirling engine more efficient because it performs part of the heating and cooling of the working gas as the displacer cycles it back and forth between the hot and cold chambers. After the gas leaves the regenerator it enters the active heating or cooling regions. A regenerator is a passive component. It cools the hot gas as it flows in one direction through the regenerator and heats the gas when it returns back the other direction. The heat is transferred to the steel wool in the regenerator I’m using and is transferred back on the return trip.

The regenerator isn’t totally free. The steel wool material causes some friction with the air, causing a larger pressure differential on the two sides of the displacer that makes additional work for the engine. The volume taken up by the regenerator also adds dead space to the engine, making the engine slightly less efficient. Despite these disadvantages, the net gain is substantial.

Incidentally, making more clearance around the side of the displacer to make the air flow easier (with no regenerator) actually caused such a large drop in power the engine wouldn’t run at any reasonable temperature. I cut the displacer down from approximately 3.4″ diameter to 3.25″. The reason it causes such a drop in power I believe is because the tight clearance between the displacer and the cylinder wall accelerates the air as it flows past. The air speed is high enough to make the air swirl around in the hot or cold chamber and have good heat transfer with the hot or cold plate. The larger clearance reduces the air speed, possibly causing laminar flow instead of turbulent flow, and reducing the heat transfer.

Modifying the displacer to add the regenerator is probably self-explanatory if you look at the photos. I used a spade bit to drill the four 7/8″ diameter holes. You could probably use a 3/4″ hole instead of 7/8″. To keep the steel wool in place I used 5 minute epoxy to attach a disk of aluminum window screen on the bottom of the displacer and then divided up 0.6 grams of #0000 steel wool among the 4 holes. Try to fluff it up to fill the volume and make sure there are no straight through holes where the air can go without going through the steel wool. Test run the displacer to make sure you’re getting reasonable performance and then epoxy a screen on the top of the displacer to lock the steel wool in. In the photos you can see that I put masking tape on the screen to mark the circle and hold the screen in a circular shape.

If you test your engine without the regenerator and then add it, you’ll probably be as blown away as I was that 0.6 grams of steel wool (this is almost nothing) can make such a huge difference.

It’s quite possible you may be able to come up with a regenerator that works even better. I’m still planning to try some fine copper wool.



02/24/2013

A more recent and detailed article on regenerators discusses the regenerator and performance improvements on the 3D printed Stirling engine.