Solar-powered Stirling engine prototype Part 2

Heat collector plate insulation

Another foamcore insulating disk is used beneath the heat collector plate to insulate the bottom of the heat collector plate that is outside of the displacer cylinder. A secondary piece of foamcore is used to cover the cutout necessary for the temperature sensor. Nuts and washers beneath this foamcore square are used to hold it in place against the bottom of the heat collector plate.

¼ inch thick foamcore insulation was used on the prototype but should be replaced by an insulating product without the paper laminations used on foamcore. This will avoid problems with moisture. A foam type product capable of withstanding at least 185 deg F should be used or other material with similar insulating properties. If crushing of the insulating material is a problem, small insulating disks near the four bolts can be made from PTFE (Teflon) that will withstand the compression and the temperature. For example, a 1/4 inch diameter disk with the same thickness as the insulating foam could be used between the top cover and the heat collector plate. The top insulation could be thicker as long as it does not shade the collector plate excessively when the sun is at a low angle.

The above photo also shows the cold temperature sensor attached to the underside of the cold plate.

Temperature sensors

Both the hot and cold temperature sensor were epoxied to small U-shaped aluminum plates that were bolted to the heat collector plate and the cold plates respectively. An epoxy rated for 250 degF should be used (for the heat collector end). Typical 5 minute epoxy is not adequate for this temperature. A better design would be to bore a hole in a piece of aluminum and epoxy the temperature sensor inside the hole. The sensor could be mounted inside the actual hot or cold plate if it is thick enough, otherwise it should be bolted tightly to the plate for good heat transfer.

Displacer cylinder and displacer

The polycarbonate displacer cylinder is clearly visible here surrounding the expanded polystyrene displacer. This displacer is approximately 3.10 inches in diameter. The polycarbonate tube is approximately 3.50 inches outside diameter and 3.25 inches inside diameter (1/8 inch wall thickness). It is desirable to keep the clearance between the displacer and inside diameter of the displacer cylinder as small as possible, but the two must not touch because the friction will slow or stop the engine. The reasons for the small clearance are twofold: (1) To minimize dead volume in the engine. (2) The increased gas velocities provide better heat transfer from increased turbulence when the gas impinges on the hot or cold plates. There may also be some heat transfer benefit to the to the polycarbonate cylinder which has a thermal gradient along its length.

Polycarbonate was selected as the material for the displacer cylinder because it can tolerate higher temperatures than many other plastics such as acrylic or ABS. Plastics such as polycarbonate have low thermal conductivity which is also a benefit here. Glass could also be used. Any metal, even stainless steel, would be a poor choice for the displacer cylinder due to the high thermal conductivity.

Displacer Shaft
The displacer shaft is .063 music wire that slides up and down on brass bearings. The brass bearing is threaded on the outside near the top so that it can screw into a threaded hole in the center of the cold plate. A backup nut and washer secure it in position. This makes the brass bearing easily removable from the cold plate. The music wire should be buffed and polished where it is against the bearing to minimize friction and wear. No oil should be used on the bearing surface. The bearing is designed so that the shaft only slides on the bearing near the top and bottom for approximately 1/8 inch near each end. The center section of the brass bearing is drilled larger. You can see in the photo that a small brass end piece is pressed into the main brass bearing. This end piece provides the lower bearing surface. The music wire should be a very easy slip fit but not excessive or too much gas will leak out around the music wire. Drilling the brass about .001″ larger than the music wire should work. I use both inch and metric drills to get one with the desired fit. More than .002″ larger will drastically reduce the performance and probably make low rpm operation poor.

The above photo also shows the top of the displacer connecting rod and fork fitting. The displacer shaft is secured to the fork fitting with a set screw. I often punch out a small disk of neoprene to put under the set screw to avoid marking the shaft with the set screw. Any shaft marking can damage the bearing when the shaft is removed.

The fork is made from acetal but could be made from other materials such a brass or aluminum. The pivot pin that secures the fork to the connecting rod is also .063 music wire. Note that a short brass tube is pressed into the top of the connecting rod to both act as a bearing surface and to keep the connecting rod centered in the fork so that it doesn’t rub. The pin is a press fit in the fork. If there is any doubt about the security of the press fit you can add collars on both ends of the pin to lock it in place.

Displacer connecting rod and crank
The left side of the above photo shows the lower end of the connecting rod joining the crankshaft. The connecting rod was also made from acetal but I think it would look better if made from aluminum. The pin-to-pin length of the connecting rod is 4 inches. A flanged bearing was machined and pressed into the connecting rod. The bearing is machined from SAE 841 oil impregnated bronze and should last for years. The connecting rod seemed to only need a stop collar on the right side to hold it in position. For long term use I would use either a stop collar on both sides or a tube spacer against the crank disk and a stop collar on the outside. The shaft and bearings on both ends of the connecting rod should have minimal play to avoid noise and probably lead to a shorter life. This is not as critical on the displacer as on the power piston because load reversals only happen at high speed if at all.

Power piston connecting rod and crank
The right side in the above photo shows the crank end of the power piston connecting rod. The connecting rod is held in position by a brass tube over the crank pin on the left side and a stop collar on the right side. The bearing is the same as was used on the displacer end. The crank disk on the power piston end is made from aluminum. The crank disk on the displacer end was made from acetal. I use a set screw with rubber pad on both to lock the crank disk to the crankshaft (with 90 degree phasing).

Part 1
Part 3
Part 4