Solar-powered Stirling engine prototype Part 3

Power cylinder
The above photo shows the power cylinder mounted on the cold plate. A tube brings the gas over to another fitting so that it enters the cold end of the displacer cylinder. The power cylinder is just a length of brass tubing. The brass tubing used on this engine is thin wall and I’m changing to a thicker wall because it is more rigid and is more likely to hold a perfectly round shape. The thin wall distorts easily even under gentle finger pressure enough to stop the engine (.0005 inch) although it springs back when you release it.

Power cylinder mounting
The photo shows where the brass tube mounts by sliding over a machined section of aluminum with an o-ring seal. The o-ring also holds it in place. A thicker wall brass tube is heavier and should be securely held in position for long term operation with more than just the o-ring. The aluminum fitting is threaded on top and held with a machine screw from above. The hole visible in the photo connects to the right angle brass tube. All gas connections should be air tight so that the only sources of leakage in the engine are around the power piston and between the displacer shaft and the brass bearing it slides in. Those two sources of leakage should be kept very small. I use a 3/16 ID vinyl tube for connections and a 7/32 OD brass tube (3/16 ID) for the gas connection.

Power Piston

The power piston is machined from an aluminum rod (6061-T6) to slide easily inside the brass tube but with very little excess clearance. The long piston is used to further reduce leakage. The piston must be able to fall freely through the brass tube when the ends are open and drop very slowly under its own weight when one end is sealed. Slowly is on the order of about 10 seconds per inch or slower. Test that the piston moves freely under any orientation (twisting the piston in the cylinder).

The engine will not run if oil is used on the piston, it causes too much friction. I have experimented with dry graphite lubricants that seem to work okay. I usually just leave the piston dry. This would not work with a horizontal piston but is fine for this lightly loaded vertical piston. I would only use the aluminum piston and brass cylinder in a vertical orientation. Also note that the connecting rod is 3.5 inches long. Shorter connecting rods introduce more side loads and should be avoided. This is true both for the power piston and the displacer.

Aluminum and brass have similar thermal expansion coefficients. I have never had any problems with the piston seizing over the temperatures I’ve used (about 40 degF to 125 degF). I can tell you that nylon and acetal pistons can work well over a narrow temperature range (delta T of around 30 degF). On my larger engines (2 inch piston) I have tried plastic pistons and found thermal expansion problems in as little as 10-20 degF with a steel cylinder. Thermal expansion mismatch leaves a fine line between seizing and excessive leakage around the piston.

If the piston is dropped on a hard surface (it happens!), the corner will probably be distorted and must be dressed with a file. Although it is possible to turn the piston to size before parting it from the stock, I often end up having to rework the piston. It’s better to start tight and turn or polish to just right. In that case it is usually convenient to turn a small section to reduced diameter (just a few thousandths) and then chuck the piston on this region while turning/polishing the rest of the piston. The photo above shows the reduced section at the bottom.

A .063 dia music wire is used for the wrist pin in the piston. It should be a light press fit in the piston with no play on the con rod. A thread locker like Locktite (or clear fingernail polish) can be used to secure the wrist pin if necessary.

Power piston and con rod
The photo above shows the connecting rod with another bronze bearing inside the power piston. This bearing is also made from SAE 841 bronze. Because the power piston does experience load reversals on each stroke, any play in the bearings will result in noise and probably increased wear. Try make the bearings on both ends of the connecting rod without play.

The crankshaft in the photo above is a precision ground stainless steel 3/16 shaft to fit the ball bearings. The shaft extends from the white crank disk on the left through the aluminum crank disk on the right. You can’t actually see the crankshaft because brass spacers cover what would be the visible portions. Phasing of the power piston and displacer is set by the angle between the crank pins in the crank disks on the crankshaft (90 degrees).

The central vertical aluminum piece is the support for the crankshaft with the two aluminum pieces bolted to it holding the radial ball bearings. This is a simple to make but fairly crude design. You might design the crankshaft support to house the bearings too as a one-piece or two-piece design.

It’s important that the crankshaft spin with very little friction. With the flywheel attached but without connecting rods attached, the crankshaft and flywheel should spin for a long time slowing down very gradually. I haven’t timed it but a good spin should last for maybe a minute or 30 seconds.

Brass tubes over the crankshaft serve as spacers to keep the shaft in position with respect to the bearings. The small aluminum disk on the left side is a spacer and could be eliminated. The assembly that serves as a flywheel is between the small aluminum disk and the white plastic crank disk.

Part 1
Part 2
Part 4