Low Temperature Stirling Engine by Penn Clower
This article is reproduced from the July/August 1999 issue of Village Press: Home of Live Steam Magazine with the kind permission of Joe Rice of Village Press Publishing. The article is © Copyright 1999 Village Press Publishing All Rights Reserved.
By Penn Clower
Drawings by Author
Please note that the images on this page are low quality for display on screen.
This little engine can be built from scrap in about 20 hours. It's a loose copy of one described by Ian Stewart in the Melbourne Society of Model & Experimental Engineers Journal for 12 July, 1997 (available on the web at the Steam & Engine of Australia web site). This engine is smaller and simpler than the original and will operate with a temperature differential of only 70° Fahrenheit. A very basic model, it has only five moving parts but runs for 30 minutes on the heat lost by a cooling cup of coffee. The slow running speed (about 50 RPM) and visible displacer movement provide an excellent demonstration of the Stirling cycle. As a bonus, the simple heat source makes operation clean and easy.
The engine is pretty and fun to watch, but because of the small chamber size and low operating temperature there isn't enough energy converted to drive an external load. Low friction and good balance are the keys to successful construction, so getting it to run well requires attention to detail.
It's an excellent change-of-pace project for the model steam engineer and certainly not beyond the reach of a careful beginner.
THE STIRLING CYCLE
Unlike most engines, the Stirling runs with a sealed cycle and has no intake or exhaust. There are no valves and relatively few moving parts. The engine is very quiet in operation since power is delivered in two smooth pulses per cycle. The machine works by alternately expanding and contracting the air inside it. This causes the internal pressure to cycle between pressure and vacuum, so the small (3/8") power piston is alternately pushed out by internal pressure and then pushed back in by external pressure. The temperature cycling inside the engine is created by a displacer - a large and loose-fitting "piston" that takes up about half the internal space. The displacer moves up and down to shuttle the internal engine air from a cool area where it contracts to a warm area where it expands. The only moving parts are the displacer and power piston, their connecting rods, and the rotating shaft assembly.
Like Ian Stewart's engine, this one is built with a squat Plexiglas main chamber sealed on the top and bottom with aluminium disks. The clear Plexiglas makes the motion of the displacer visible and; more importantly, provides some thermal isolation between the cool top and warm bottom plates. Those aluminium covers provide the thermal surfaces used to alternately heat and cool the air inside the engine. In this model the chamber diameter is 4.5", thus making the bottom plate a comfortable fit for a coffee cup. Inside the chamber, the displacer is a solid cylinder cut from foam building insulation. Just 1/2" thick, it takes up half the available vertical height. The annular clearance between the displacer and the chambers inside wall is about 1/8", so there's plenty of room for the air to flow by as the displacer moves up and down.
The output shaft is horizontal and mounted several inches above the main chamber. The end of the shaft centred over the main chamber has a crank and connecting rod, which drive the displacer through a third vertical rod. This rod comes up from the chamber through a gland mounted in the middle of the top plate. A second crank on the other end of the output shaft connects to the power piston at the outside edge of the top plate. The piston crank disk doubles as the mounting hub for the 4" diameter fly- wheel. Between the two cranks, the shaft is hidden inside a cylinder that holds the two supporting ball bearings.
There's not much power available to drive this little engine. It's amazing that it runs at all, considering the small quantity of air that is heated and cooled slightly each cycle to produce what must be a very small pressure differential - and this is then applied to a piston having only .14 square inch of surface area. Not surprisingly, careful craftsmanship and a good set of ball bearings are necessary for successful duplication.
Given those, the engine is not difficult to build and certainly not beyond the skill of anyone who has had success with similar projects. The attached drawings show pretty well how to duplicate the engine, and the next several para- graphs may make the process a little clearer. The drawings aren't lavishly dimensioned because most of the dimensions aren't critical and will be modified anyway depending on the stock you may have.
Visible from this angle is the general arrangement of the crank disk, displacer gland, and bearing mount. Note how the crank disk is the same diameter as the end of the bearing mount, and the bearing mount flange is the same diameter as the top of the bearing standard. You can also see how the lower end of the connecting rod is captured by the hinge, and how the hinge wrist is glued onto the upper end of the displacer control rod.
MAIN CHAMBER (Assembly 1.0)
This is the place to start building the engine, since the chamber diameter will determine many of the other dimensions. My collection of stock included a length of 4.5" diameter Plexiglas that defined the overall engine size. I cut the plastic down to a thickness of 1" and bored out until the rim wall was about 5/16" thick. O-rings provide a tight seal between the chamber walls and the aluminium end plates.
One of the interesting challenges of home machining is planning the order of operations so pieces can be made easily and accurately with a limited selection of tools. Certainly this is true with the chamber assembly, especially when it came to drilling the six screw holes used for mounting the end plates. The Plexiglas cylinder (Part 1.1) was first machined by facing off the end surfaces, allowing for a finished height of just over an inch. I put a shallow pit in the centre of its smooth face with a small centre-drill. Out on the bench, this allowed scribing a clean line across the diameter of the part. Using this line and an inexpensive protractor, I then made index marks at 60° and 120° so two additional diameters could be scribed at those angles as well.
The end plates (Parts 1.2 and 1.3) will be attached with six 4-40 screws evenly spaced around the outside edge of the chamber. 1 spotted the hole locations on the scribed plastic face after marking the desired radius with a sharp pair of dividers pivoting on the centre pit. ! set the radius so the heads of the screws would be inboard of the chamber edge by just .025" or so. The holes themselves, sized for later threading with a 4-40 tap, are then carefully drilled on the drill press (or in the lathe using a tailstock drilling pad). Be sure to use a sharp drill, feed it slowly, and watch the plastic carefully for signs of melting while the holes are being made. A drop or two of oil on the drill will certainly help.
The chamber plates will be sealed with O-rings fitted to grooves turned in the plastic just inboard of the screw holes. I cut the O-rings to size from larger rings; a good square cut with a razor blade leaves a surface suitable for super gluing. The ring cross section isn't critical. I used .125" rings because they were available, but thinner ones would work just as well. The grooves for the rings should be slightly wider (maybe .025") than the ring cross-sectional diameter. This provides room for the ring to deform slightly when the cover plate screws are tightened and also makes ring length a little less critical when the rings are sized for the grooves. The groove depth should leave about .01" to .015" of the ring height to be squeezed against the mounted end plate. If O-rings aren't available, the chamber ends could be sealed with paper gaskets made from high quality writing paper that's been lightly oiled. A gasket width of 1/4" would probably work well.
My end plates (Parts 1.2 and 1.3) were made from 1/8" thick aluminium originally sold as a standard 19" wide electrical rack panel. This was cheap and available, but really didn't turn out to be the right material. If the paint is removed, the exposed metal has a rough brushed finish that is both unattractive and difficult to seal against the chamber ends. To avoid that sealing problem, I built my engine with tile original grey paint intact. I later prettied it up by repainting the visible surfaces with brass sign-painters paint. This aluminium is also soft and was difficult to turn cleanly. All in all, it's worth getting some harder 1/8" aluminium or brass for the end plates.
I first marked out the circular plates with a compass and then cut them slightly oversize with various hand tools. The six 4-40 hole positions were carefully spotted by clamping the Plexiglas blank over the aluminium disks, being careful to centre it with the scribed compass outlines. Using the as-yet unthreaded screw holes in the plastic to guide a No. 43 drill, the hole positions can be marked on the aluminium below. The holes in the cover plates are 4-40 body holes, so a No. 36 drill was used to make them. At this point, unless your work has been perfect, the top and bottom plates will have preferred mounting positions on the plastic cylinder. It's worth gently scratching some witness marks on the plastic and plates to ease future alignment.
The outer rims of the covers are then trued and turned to size. I did this by first screwing the rough covers to a scrap 1/2" aluminium plate bolted on my faceplate. This scrap piece, somewhat riddled with holes from previous use, was first faced off and its centre located. The top engine cover plate will have a hole in its centre to mount the displacer gland, so I picked one of my plates to be the top cover and drilled a 3/16" hole through the centre prick left by the compass that marked out the original disk. Sighting through this hole, it was easy to align the rough-cut disk with the centre of the faceplate fixture. The six rim hole locations were then transferred to the scrap aluminium, drilled, and tapped to receive the 4-40 screws. The end plates could now be mounted to the faceplate by those screws and the edges turned to size. I left the diameter large at this point by perhaps .05".
The next step is to finish the Plexiglas chamber (Part 1.1) by boring out the middle, turning down the outside edge, and tapping the plate mounting holes. The inside diameter of the chamber should reach to within .025" to .05" of the O-ring groove. The outside walls were turned down to a thickness of about 1/8" with the profile shown in the drawing. This helps reduce heat leakage between the end plates through the Plexiglas. The top and bottom shoulders are left thick enough so the plate mounting screws (mine were 5/16" long) don't protrude through. The last step is to thread the 12 screw holes with a 4-40 tap.
The cover plates can now be mounted onto the plastic chamber and turned to an exact fit in their final locations. Once again, several witness marks were discreetly scratched across the edges to assist future reassembly. At this point, the bottom plate is complete with its six mounting holes. The top plate also has a single 3/16" hole in its centre. More holes will be added to the top plate to mount the displacer gland, bearing standard, and power piston. That work is best left until those parts are completed and the holes can be accurately spotted.
DISPLACER (Part 1.7)
The displacer body was cut from closed-cell foam building insulation. This is similar to the Styrofoam used for inexpensive picnic coolers, except that the building foam is stronger, denser, and less likely to crumble while being worked. The foam can be cut with any fine- toothed saw or sharp knife, then sanded lightly to improve the surface finish. The displacer doesn't have to be perfectly round since there is an 1/8" radial gap between it and the chamber walls. The top and bottom surfaces should be 1/2" apart and reasonably parallel. That's required because the displacer stroke is set to give only about 1/32" clearance at each end of its travel.
The completed displacer is mounted on a 1.5" length of 1/16" brass rod. This rod isn't shown in the detailed drawings of the chamber since there is so little to it. Even the exact rod length doesn't matter because it gets adjusted during final engine assembly. To mount the rod to the displacer a 3/8" diameter hole is first carefully punched into the centre of the foam disk with a length of sharpened tubing. A 1/2" length of 3/8" wooden dowel is then glued into this hole to provide a sturdy centre core for the brass shaft. The wood plug is secured to the foam with white household glue and allowed to dry overnight. The hole for the shaft isn't drilled until the displacer gland is finished and mounted on the chamber top plate. A temporary assembly of the top plate and chamber wall, with the displacer held in place in the centre of the chamber, will allow the hole position to be carefully spotted through the gland bore. The displacer can then be removed and the necessary hole drilled into the wooden core. The brass rod is attached to the core with a gentle press fit.
DISPLACER GLAND (Part 1.6)
This is a simple piece of aluminium turned as shown in the diagrams. The hole through the gland is just big enough to provide a nice sliding fit for the brass displacer rod. The best way to get a straight smooth hole is to start with a drill two sizes smaller than required and then enlarge the hole carefully, using plenty of cutting oil and sharp drills for the final cuts. The overall height of the gland is a non-critical 11/16", and the fancy head shape I used is purely decorative. The gland is held to the cover plate with three 2-56 screws spaced evenly around its skirt. The radius for the holes and the skirt diameter are picked so the screw heads are attractively placed between the central column and outside.
When the piece is finished it can be used as a template to spot the threaded hole positions in the top cover. The gland body is located by a slight extension that just fits into the 3/16" hole in the cover centre. The joint between the gland and top plate should be sealed with either a small O-ring or paper gasket. I used an O- ring but didn't show the ring or its groove in the drawings. It's important to minimise air leakage from the chamber to maximise the size of the engine pressure cycle. For this reason, the three gland mounting screws are threaded into holes that do not completely penetrate the top cover. With 1/8" thick material this takes some care, but the 2-56 screws are fine enough that five to six full thread lengths can be cut with a bottoming tap.
POWER CYLINDER AND PISTON (Parts 1.4 and 1.5)
Many plans for small Stirling engines specify the use of glass syringe sections for the power piston and cylinder. Great stuff if you can get it. I couldn't, so the cylinder was made of aluminium and the piston from Delrin plastic. Teflon or a similar material would probably work as well. The piston diameter is 3/8" and, as can be imagined, the piston and cylinder are very carefully machined and fitted together. The piston, slightly longer than it is wide, is centre drilled to receive a short length of 1/16" brass rod for hinge attachment. A depressed ring was turned in the backside of the piston to lighten it a bit and help prevent the rod insertion from expanding the diameter. The rod is a light press fit, and no glue or cement was used to hold it in place.
The power cylinder (Part 1.4) is turned to the dimensions shown in the drawings and, like the displacer gland, used as a template to spot its attachment and air holes on the top cover (Part 1.2). Like the displacer gland, the cylinder is also held to the cover with three 2-56 screws and should be sealed with either an O-ring or paper gasket. The cylinder is mounted at the very edge of the top plate to maximise the length of the engine shaft and to provide adequate room for the bearing mount and flywheel. Most of the cylinder is mounted so far over the chamber wall that the hole communicating between the cylinder and air chamber must be off-centre with the cylinder bore. The outermost mounting screw is actually beyond the cover plate O-ring seal, so the hole it threads into can pass completely through the top plate. The other two holes, like the three under the displacer gland, do not penetrate through the plate.
The piston (Part 1.5) was first turned to a close fit, then lapped into the cylinder bore. This was done with an oil slurry made of a very fine abrasive called "rotten- stone" (commonly sold as a colourant for tile grouting). I lapped the piston and cylinder until the piston was a smooth sliding fit and the cylinder walls looked nicely polished. All the abrasive was then carefully removed with clean lightweight oil.
At this point, I assembled the completed chamber with the displacer on its shaft with the power piston free-floating inside the cylinder. When the bottom plate of the chamber was held in the hand for a few minutes, and the displacer operated by lifting and dropping the rod, the power piston would move up and down in the bore by itself. There was still work to be done, but the engine was coming to life!
BEARING STANDARD (Part 2.0)
The bearing standard holds the bearing mount and output shaft above the engine air chamber. A simple slab of 1/4" aluminium would work, but for the sake of appearance it was given the somewhat classier shape shown in the drawings and photographs. The most critical feature is that the fiat bottom surface be perpendicular to the vertical face. This insures that the engine output shaft (and thereby the crank- pins) will be perpendicular to the displacer and piston motions. The bearing mount is attached by screws passing through it and into five threaded 2-56 holes spaced around the large hole at the top of the standard.
The bearing standard is held to the engine by two flathead 4-40 screws coming up through the top plate. The holes for these have to be very carefully spotted because the countersunk depressions for the screw heads will not allow adjustment of the screw spacing. If you have any concerns about your ability to lay out these holes accurately it might be better to use regular pan head screws in holes counter- bored to keep the heads below the bottom surface of the plate (so they don't interfere with the displacer motion). These mounting screws are a potential source of air leakage, but the flathead variety would seem to offer a good chance for a seal against the countersunk seats. To help with alignment, the screws selected are 1/2" long and the receiving holes in the bottom of the standard are not threaded for the first 1/4". This allows at least a little wiggle room as the screws are tightened.
BEARING MOUNT AND BEARINGS (Parts 4.1 and 4.3)
The ball bearings used to support the main shaft must have very low friction. I have a collection of small bearings, and the best of the lot was a pair of sealed units with an OD of 1/2" and a 3/16" centre bore. The bearing mount is basically a 5/8" hollow aluminium cylinder bored at the ends to take these bearings with a "finger press" fit. A 3/16" wide flange runs around the middle of the bearing mount, and the five No. 42 holes in this flange pass the 2-56 screws that attach the bearing mount to the bearing standard. The overall length of the bearing mount depends on the diameter of the air chamber, so if your chamber is other than 4.5" in diameter you will want to make some adjustments.
MAIN SHAFT AND CRANKS (Parts 4.2, 4.3, and 4.5)
The main shaft (Part 4.3) is a 3/16" steel rod just long enough to pass through the bearings and receive the crank disks.
The displacer crank (Part 4.2) is a 5/32" thick aluminium disk of the same diameter as the end of the bearing mount. A radial hole for the 4-40 setscrew is directly opposite the crank pin position. The pin itself is made from 1/16" brass rod pressed into the disk and extending out from its surface for about 1/4". The pin offset is calculated to set the displacer travel so there's just 1/32" of clearance at each end of the stroke.
The piston crank disk (Part 4.4) is more complicated since it also forms the mount for the flywheel (Part 3.0). As shown in the drawings, the flywheel is mounted to this disk with five 2-56 screws that pass through the disk and thread into holes on the flywheel hub. The shape of the piston crank disk is such that the fly- wheel is mounted over the outboard main shaft bearing and clears the bearing standard by about 1/16".
On the inside of each crank disk I left a small shoulder to ride against the rotating inner race of the bearing. The two disks are mounted onto the shaft and capture the bearings between them with just enough tension to prevent shaft end play. The raised shoulder is a bit of a guarantee to prevent the outer edge of the disks from rubbing the non-rotating outer race of the ball bearings.
FLYWHEEL (Part 3.0)
The flywheel diameter is just over 4", though the mounting height would allow for 4.25". The larger wheel would be better, as inertia is needed to carry the engine over top and bottom centre. The first flywheel made for this engine used the same l/8" aluminium sheet as the chamber cover plates. This had insufficient inertia to run the engine smoothly, so a second and heavier wheel was fabricated from 3/16" steel plate.
The flywheel hub is bored out to 3/4" diameter so it can clear the bearing housing. The piston crank disk has a small shoulder that fits just inside this flywheel bore to align the two pieces. Attachment is by five 2-56 screws that fit into threaded holes in the flywheel hub.
Though it wasn't strictly necessary, the flywheel was lightened by thinning down the centre and boring five large holes to form wheel spokes. The main advantage of the holes is the visual impact they provide when the engine is running. These holes had to be bored into the fly- wheel while it was clamped off-centre on the faceplate. This was done after thinning down the central web of the wheel but before the centre hole was enlarged to its final .75". The process started by drilling a 10-32 clearance hole in the centre of the flywheel blank. Next, witness marks were carefully scratched (once again using the cheap plastic protractor) every 72° around the rim of the flywheel. Using the same piece of 1/2" aluminium plate employed when truing the chamber cover plates, a threaded 10-32 hole was carefully placed 1" off the centre of rotation. The centre of the flywheel was bolted to the faceplate through this hole (using a 10-32 screw, washer, and lock washer) and further held by several clamps spaced around its circumference. By means of the five witness marks made earlier it was easy to bore a hole, rotate by 72°, then bore the next hole. Notice that the five threaded 2-56 attachment holes in the hub are spaced symmetrically between the large spoke holes.
CONNECTING RODS and HINGES (Parts 5.1 and 5.2)
These parts were one of the biggest challenges in making the engine. The stock used was 1/16" brass rod and telescoping 1/16" ID brass tubing. These materials are widely available in hobby and art craft stores. A 3-foot length of each was purchased, along with a matching piece of aluminium tubing.
The goal was to make the links shown in the drawings (see Part 5.1) by soldering short pieces of tubing perpendicularly onto the ends of the rod sections. The challenge was to keep the tubing lengths parallel to each other and to solder the pieces neatly and accurately. The key to success was the simple fixturing jig sketched in the drawing (Part 5.4). This was made by clamping together two pieces of 1/8" aluminium. I drilled two holes of a diameter slightly less than the tubing OD perpendicularly to each other along the surface joint between the pieces. I then sawed an opening in the blocks as shown to create a U-shaped fixture.
The brass pieces to be joined were carefully prepared. For an example, assume that an end ring is to be soldered onto a connecting rod. The rod is first cut and filed carefully to the desired length. A round needle file is used to make a small depression across one end to provide a seat for the ring section. This rod length is mounted in the jig by sliding it into a section of aluminium tube clamped in the hole running vertically up the U between the two clamping screws.
Preparation of the end ring begins by carefully cutting off a section of brass tube slightly more than 1/16" long. I had trouble doing this cutting operation entirely in the lathe, so the surface of the tube was only scored on the lathe and then cut through by hand with a jewelers saw. The rough edges of the brass ring were polished smooth on 400 grit paper over a flat surface. Holding the ring against the paper is easy. Start with 1' lengths of scrap tubing and brass rod. Thread the ring and scrap tubing onto the rod, and hold the whole group vertically against the emery paper with the ring on the bottom. The scrap tubing provides a convenient handle and a way to press the ring against the paper, while the brass rod holds the collection together. The ring ends can be easily squared and polished this way while its length is adjusted to exactly 1/16".
To prepare the ring for soldering, it's first threaded onto a 2" long section of scrap rod between two 1/2" lengths of aluminium tubing. This collection is clamped through the fixture holes crossing the top of the U, with the brass ring carefully placed in the centre. The flanking sections of aluminium tubing hold the ring in position and also prevent solder from flowing around the ring ends and accidentally soldering the ring to the rod inside it.
To make life easy, the fixture is held in a bench vice while everything is care- fully positioned under a magnifying glass. When it's ready for soldering, the connecting rod is pulled back from the ring and coated lightly on the end with soldering flux. A paste material sold under the trade name NoKorode works well for this purpose. The solder used is a tiny pre-cut grain of standard wire solder. The piece required is only slightly larger than a grain of coarse table salt - experience will quickly show how much is needed. This bit of solder is placed with tweezers in the blob of flux on the end of the connecting rod, and the rod is then pushed up gently to pinch the solder against the brass ring. The paste flux will help keep it there while the assembly is carefully heated with a propane torch. As soon as the solder melts, the rod is pushed fully into position and the heat removed. The brass rod and aluminium spacers holding the ring in position can then be removed and the connecting rod taken out of the fixture. Examine the joint care- fully after removing all traces of flux with a paper towel dampened in alcohol. There should be a neat fillet of solder around the end of the rod with no excess flowing around the ring or into the bore.
A similar procedure is used to put a second ring on the opposite end of the connecting rod. To permit removal from the holding fixture it will be necessary to clamp the rod body in a split section of aluminium tubing. Parallel alignment of the two end rings can be insured by threading a 2" length of scrap rod through the previously soldered ring and carefully lining this up with the holding fixture during soldering.
The inside bores of the connecting rod ends were smoothed with a skinny strip of oiled 600 grit paper. The ends of the bores should first be chamfered slightly by hand with a small centre drill or countersink. As a final step, examine everything carefully under a magnifying glass or eye loupe to make sure no rough edges or stray whiskers of brass remain. These steps insure a minimum of friction in the assembled linkages.
Making the hinge ends is only a little more complicated and goes quite quickly after a few trials. Just be sure to make everything as smooth and square as possible. The wrist pins, short pieces of 1/16" brass rod, will fit loosely through the hinges and are held in place after final assembly by a drop of white household glue. This is strong enough to retain the pins but easy to remove if disassembly is required. The glue also dries clear and isn't apparent to the casual observer.
The hinge ends are mounted on the piston and displacer shafts with quick- drying epoxy cement. The piston hinge position isn't especially critical, though together with the other dimensions it will decide which portion of the cylinder length is swept by the piston. The displacer hinge position is used as the final adjustment, setting the clearance at the ends of the displacer travel. Ideally, this clearance should be as small as possible. Practically, the limits of crank offset, displacer thickness, chamber height, and displacer surface finish combine to make 1/32" of clearance a reasonable number. Obviously, it's important that the displacer not bump into either the top or bottom plate during its travel!
To position the displacer hinge, it's easiest to assemble the displacer in the chamber with a 1/32" thick piece of scrap temporarily between it and the bottom plate. That holds the displacer in the end- of-travel position while the rod is measured and trimmed to length. The hinge is then glued to the rod end as close to the top of the displacer gland as possible. After the glue cures, the spacer can be removed from the chamber. Be sure to make a dummy weight matching the displacer assembly, for use in balancing as described later, before you glue the hinge wrist! Once the glue is set, you won't be able to separate the displacer assembly from the gland and cover plate.
The dimensions of the main shaft and cranks are arranged so the exposed 1/4" lengths of the crank pins are centred over the power piston and displacer gland. This is shown in the overview of assembly 4.0 and in the side drawing of the completed engine. The connecting rod ends should position on the middle sections of the pins. The rods are held there by short aluminium tubing spacers that are placed on the cranks on either side of the rod ends. The outer spacer is adjusted to give the rod end .01 to .015" of slop and is then held in place with a drop of glue. The bore diameter of the brass tubing used for the rod ends is such that the rods turn quite freely on the pins and even, because of the short 1/16" ring length, can angle from side to side for perhaps 1/8" without binding. Keeping the ring length short reduces the running friction by limiting the bearing contact area. It also makes accurate alignment less important during rod construction. No oil is required or used on the connecting rod ends.
BALANCING, TIMING, TESTING, AND FINISHING
Only one adjustment is necessary after assembling the engine for the last time: the two cranks must be spaced 90(c) apart. This job can be done by eye before tightening the last setscrew. The direction of the 90(c) rotation will determine which way the engine runs. That's not really important; just pick a setting and stick with it! After timing, the only task left is balancing the rotating weights. Since the engine develops so little torque it won't run well, or at all, unless balanced.
Balancing is done after replacing the weights of the piston and displacer assemblies with dummies that allow the output shaft to spin with no drag other than the ball bearings. The weights used for this purpose were made from short sections of prototype connecting rods. Typically, these were sections of brass rod about 1" long with the cross rings mounted on at least one end. I wrapped wire solder around these rods to add weight until they massed the same as the parts they were replacing. That judgment was made with the assistance of a simple dual pan balance. The finished weights I used are shown in one of the photographs. They're not real pretty but they work just fine.
With the dummy weights hanging on the crank pins, any unbalance in the rotating parts will be evident as the heavy section rolls to the bottom. This is corrected by using hot melt glue to tack a small patch of brass sheet to the flywheel near the rim of the upper outside edge. The brass stock used for this purpose in my engine was .025" thick and sized about 1/4 × 3/4". It's not shown on the drawings but is visible in the photographs. Be sure to put it on the outside face of the flywheel; there isn't enough clearance between the inside flywheel face and the bearing standard! I didn't use the usual applicator gun for the glue. Instead, the steel flywheel was heated gently with a propane torch. Then a small sliver of cold glue was dropped in the correct spot and mashed under the balance weight. After cooling, any excess glue was removed with a pocket knife.
A little experimentation is required to get both the size and position of the weight just right. The unbalanced engine originally had a clear tendency for the fly- wheel to rotate to a preferred position. Once the balance weight was approximately correct, the error torque was so small that the flywheel would rest in almost any position. The final adjustment was made after spinning the wheel by hand several times and noting any tendency for it to stop in a particular spot. Only two spots on the engine were lubricated. A drop of 3-in-1 oil was placed on the displacer rod where it would be drawn down into the displacer gland bore. This spot is probably one of the largest friction sources in the engine. The oil serves a double purpose of reducing that drag and improving the gland seal slightly. A drop of lighter penetrating oil (I used Liquid Wrench) was also placed on the inside of the piston bore to provide sealing and lubrication. As mentioned previously, no oil was used on the connecting rod ends. Examination of the inside surface of the rod ends shows them to be pretty rough, however, so some experimentation with powdered graphite might prove worthwhile.
The steel flywheel and the top aluminium chamber cover on my engine were finished with brass paint. There was some concern that the steel flywheel might rust if left unpainted, and the chamber covers were made from that unattractive material described earlier. The brass paint gave the engine a nice finished look. If I were to build another engine of this type, I would probably spend the money for brass stock to begin with! As a final measure, three adhesive rubber feet were attached to the bottom engine plate to prevent the cover attachment screws from marring any surface the engine might find itself sitting on. The rubber feet are spaced so the engine can still sit fiat on top of a 3.5" diameter mug.
Operation normally begins with a cup of boiling water placed underneath the engine. It can take as long as a minute for the vapour to heat the lower plate enough for the engine to run. The engine won't start by itself, but given a little push in the proper direction (strong enough to coast the parts over for several cycles), the engine will take off and continue running on its own. Starting speed will be about 20 to 30 RPM. This increases to maybe 60 RPM as the engine heats up, and then slowly decreases over the next 30 to 35 minutes as the water cools.
There's an interesting unbalance visible during the first few minutes of operation. Recall that the Stirling cycle produces two smooth pulses of power per revolution. Key to this is the fact that, on the average, the internal and external pressures must be equal or there will be a small compensating leak. Most of that leakage must be out the displacer gland as all the other joints are sealed with gaskets or O-rings. Of course, the running pressure balance cannot exist when the engine first starts. For the first few minutes of operation the average internal pressure must be high because heat is being added to the engine and the internal temperature is increasing. During that time, the engine will produce a strong pressure stroke but no vacuum stroke. This is clearly visible as a significant change in flywheel speed over the rotational cycle. Equilibrium will occur after several minutes of running and the engine will settle down to a smooth cycle with a barely discernible speed change.