Introduction: Build a Better Stirling Engine
Following my experiments with the first LTD Stirling, I decided to try and make a better one. The main problems with the first had been the glued joint leaking air, and excessive requirements for machining. This one would have soldered joints to prevent the leaks and would have far less machining and could be made with just a drill press..
The main difference between this one and the first one I made was the diffuser cylinder. This one would use a longer, thinner cylinder with an aluminium piston. I also decided to use metal where possible on this one and have roller bearings. The diffuser cylinder would be bolted on allowing strip down of the machine and a rubber gasket to hold the seal.
Step 1: Materials
For this engine I used the following materials:
1 length of 22mm (1") copper pipe
1 length of 15mm (1/2") copper pipe
1 22mm copper end cap (not shown)
3 aluminium heatsinks from an old TV (scrapyard)
1 piece of resin stock
Paxolin copper clad PCB board
HDD actuating head arm with bearings and shaft
2.5mm brass tube
1.5mm brass rod (sliding fit)
One marker pen with an aluminium body (not shown)
Part of a plastic 3" pipe or similar (not shown)
The brass centre boss from a defunct CD drive
A short length of 10mm aluminium bar from a scrap printer
A short length of aluminium channel (not shown)
Various nuts and bolts rescued from scrap items (not shown)
A small piece of high density rubber (a piece of bicycle inner-tube)
4 15mm brass disks (not shown)
The reason for the not shown items is ..... at this point I haven't acquired them yet!
Step 2: The Pistons and Cylinders
The power cylinder on this engine is made from a piece of 15mm copper pipe, 1/2" will do if that's what you have. I cut a 40mm section using a cheap pipe cutter, then cleaned the ends up with a Dremel to remove any burr and lip at the ends. The bore was polished using a small wire brush in the Dremel, followed by wire wool and very fine grit wet and dry rolled into a tube.
I then cast the tube using the resin process in my other instructable.
Just a note here. I tried both the resin and some epoxy putty cast internally as per other 'ibles on the site. I found that in both cases the shrinkage (around 5%) made the piston too loose. Maybe it's the brand I was using.
The resin blank was then sanded down in the drill press and polished for a sliding fit. This gave a piston 13mm in diameter. I cut it down to 13mm in length then cross drilled 2mm and drilled it out 6mm at one end for the con-rod connection.
I decided to simplify the con-rods for this engine. I went down to the model shop and bought some more 2.5mm tube and some 1.5mm rod. These two can be slid into each other. The piston connecting rod is 2mm steel from an old printer. To make the connectors I simply crushed 4mm of the tube flat in the vice, then cut it off at 10mm. Drilled the flat 2mm and slid it on to the piston connecting rod.
The diffuser cylinder is a length of 22mm copper pipe (1" should do). The pipe needs to be cut to length as depending on the length of the piston and the stroke of the crankshaft.:
The piston must have a gap all around it in the cylinder, the idea is that as the piston moves, air flows around it from one end of the cylinder to the other heating and cooling as it goes. There should be a gap at the sides of at least a couple of millimetres. I wanted to use a metal piston so that I could use a tea light candle to run the engine.
I found an aluminium bodied marker pen that was around 16mm across. This left a gap of about 2mm either side when slid into the tube. Ideal. I cut the pen in half and removed the innerds which were discarded. The plastic end was broken off and cleaned with the Dremel to leave a nice round hole. Then I worked out the length of the required piston. A guess but I went for 65mm. To make the piston I cut the rear portion down to give the overall dimension I wanted, then I slid a short section of 15mm plastic plumbing tube into the end a glued it using super-glue gel. The other piece of the pen was then slid onto the other end of the plumb fitting and secured with super-glue gel. This gave a lightweight air-tight piston. The open end was fitted with an aluminium plug pre-drilled for the con-rod. That completed the diffuser piston.
Now I wanted to be able to strip and re-assemble this engine so as few glued joints as possible. You could use a plain solder end cap on one end of your cylinder, but because of the soldered bolts I didn't want to disturb with more heating I used a compression fitting. This will become the hot end. Next I sourced 4 brass motherboard mounting bolts from a scrap yard PC. These come in a number of thread sizes, most common sizes are M3 fine thread or M3 course thread but there are some imperial ones around. the trick is to find 4 the same (which should be the case if they came from the same PC). Find 4 bolts that fit the threads. These need to be at least 15mm long. If they are metric you should have no trouble finding bolts. If they are imperial suck it and see.
Cut off the threaded part of the bolt, leaving just the brass 'nut' which should be about 6mm long. We are going to solder these onto the cold end of our cylinder and then use them to bolt the cylinder to the chassis. If you manage to solder them at exactly 90 degrees each then you don't need to mark the cylinder. I didn't trust my accuracy so I scribed a top mark so that each time the cylinder is bolted up it will go back in the same place allowing for misalignment of the soldered stand-offs.
I made a simple wooden jig to hold everything in place. i tinned the bolts and one end of the pipe and then used a micro pen torch to solder the bolts on. I tested bolting it all up before going back to finish the chassis.
With the exception of the con-rods this completes the cylinders and pistons.
Step 3: Main Chassis
Assembling the main chassis.
This engine differs from the last in that the cool plate will house a separate diffuser and power cylinder one either side. Both will be sealed in their own right and linked by some tubing.
The chassis is made up of a sandwich of four plates. Two heat-sinks (they had power transistors on them) from a TV I found in the scrap yard. They have pre-drilled holes in them from the transistor mounts and are identical. This means that I can reverse and screw them together to form a single thick plate with the cooling fins on.
Each cylinder is formed from the copper tubing, the power cylinder is soldered to a Paxolin board that is bolted to the two heat-sinks. The diffuser cylinder will be bolted to a second Paxolin board and bolted to the chassis via a gasket. This should enable me to seal the two cylinders and then simply link them with some tubing. (I hope).
I cleaned up the two heat-sinks with the Dremel to ensure good mechanical contact between them. I cut and marked out the two Paxolin boards, then drilled the power cylinder board to bolt to the heat-sink.
The diffuser cylinder board was marked up and drilled for the bearing and the feeder tube, then tacked on with super-glue. The four mounting bolt holes were drilled through and then the two feeder tubes were soldered into place. The power cylinder was soldered on and the nylon bearing for the diffuser piston glued in.
Finally the feeder tube was connected up. That completes the main chassis assembly
Step 4: The Flywheel and Boss
I wanted to make the engine look nice, so I decided to try and make an elegant flywheel.
I had found an old brass centre from a defunct CD or DVD player, then I downloaded a protractor template from the net and marked lines at 0, 120 and 240 degrees to give three equal spokes. I still had the top ring of a food container I had cut down for my first Stirling, so I used that as the flywheel outer ring. You could cut a ring from 3" plastic drainpipe instead.
I placed the brass boss onto the protractor with double sided tape and then measured and cut 3 lengths of 1.5mm brass rod from my stock. These were then supported on some levelling materials and soldered to the centre boss. The outer ring was laid onto the protractor and marked then drilled 1.5mm to accept the spokes. Because the ring is slightly flexible it was easy to deform it enough to force the spokes through. Once it was all back in shape and trued up on the protractor, the spokes were super-glued into the ring.
A piece of 10mm aluminium was cut to size and drilled ready to accept the crankshaft. It was then super-glued onto the brass boss.
That completes the flywheel.
Step 5: The Support Pillar and Bearings
Recognition for the idea for the support pillar and bearings must go to eVolti which was an inspired bit of 'ible.
I decided to use a very similar system, just a bit simpler in execution.
Using the head arm and bearings from the HDD that I got the platters from for my LTD engine, I drilled the shaft 2.5mm to accept the crankshaft. The head had originally had four read arms, three of these were ground off with the Dremel and all the ancillary coils, wire and the actual heads were removed.
The arm was carefully ground to fit a piece of aluminium channel, then the channel was drilled and the arm bolted to it. A little epoxy glue was added for a really good strong joint.
The channel was cut 60mm long, but you can set the size to whatever you require.
The bottom of the channel was bolted to the third heat-sink I found in the old TV which had a good 90 degree bend in it. The bend will be bolted to the main chassis plate.
Step 6: The Crankshaft
The crankshaft consists of a short length of 2.5mm tube. This is passed through the bearing head and carries a flywheel on one side and the cranks on the other.
The cranks are formed from 15mm disks of brass. You could cut these off a piece of brass bar, as I intended to do, but whilst I was in the plumbing section of the hardware store buying the compression end cap for the diffuser cylinder I came across a pack of four 15mm 'blanking caps' for 79p. These will do the job very well.
I drilled the centres of three of the caps 2.5mm to accept the shaft, then I drilled 1.5mm offset by 5mm from the centre for the crank rods. This will give a stroke of 10mm.
The first disk was slid onto one end of the shaft and soldered in place. The distance between the cylinders (26mm) was marked onto the shaft and the pair of disks for the second crank were slid on and soldered in place.
The crank rods were slid into the disks, the con-rod connectors slid onto the rods and then the rods were soldered.
Finally the extra metal on the rods and the main shaft of the crank were ground out using the Dremel. The shaft was secured to the bearing using some super-glue gel and the flywheel fitted.
Step 7: Final Assembly
I made up a small connector for the diffuser piston, then the con-rods were measured and cut to length. These were then glued into position using super-glue gel.
The engine was tested and adjusted to ensure that everything turns freely, and for balance. The flywheel will need some work to get better balance but the engine should work.
I need to make a stand so that it can be positioned above a tea light candle but that can wait.
I may get around to posting a video at some stage.