Solar-powered Stirling engine prototype Part 4


Flywheel
The flywheel doesn’t look like a normal flywheel but performs two basic functions. The first is to provide mass at some distance from the center of rotation. This is the normal flywheel function.

The second is to balance the mass of the power piston and displacer. This mass balance is important to make the engine able to operate at low RPM. At high RPM the flywheel stores plenty of energy to complete a cycle. At low RPM the flywheel doesn’t store much energy (unless you have a very large mass) and the weight of the piston being raised will slow and stop the engine if it is not balanced. The same is true for the displacer. Even if a perfectly round and symmetrical flywheel is used, you will still need to add small counter-balance weights for the power piston and displacer.

Attached to the flywheel with masking tape a is magnet for the Hall effect sensor to detect each revolution for the controller. The particular sensor I use only detects the south pole of a magnet so you have to orient the magnet correctly.

The position of the flywheel on the crankshaft is not too important. Ideally it should be close to the support bearings for the best structural integrity.

Starter motor engaged
The starter motor is only necessary for automatic operation. The engine is much simpler to build and operate if manual starting is satisfactory. It also avoids the need for the electronic controller.

The starter motor assembly requires attention to a few details. If you draw a line from the pivot shaft that supports the starter motor to the crankshaft, the starter motor shaft will be to the left of that line when the starter motor is engaged as shown in the above photo. The closer the motor shaft is to that line, the more force the motor will exert on the crank disk when it energizes to start the engine. This force is a good thing because it keeps the rubber o-ring wheel from slipping on the crank disk. Over the life of the engine some wear on the rubber will cause the motor to end up further to the right. If it gets all the way to the line it will fall past the crank disk and not be able to start the engine.

In this view you can also see how close the connecting rod on the crank disk is to contacting the stepper motor. Make sure to account for this clearance issue when designing the starter motor assembly.

The motor I used is a very inexpensive ($3 from Jameco) stepper with 48 steps per revolution. It is perfectly adequate and I think should be reliable. I’ve always thought it would look nicer to use a motor with around 200 steps/revolution to get smoother action.

Starter motor disengaged

The above photo shows the front of the starter motor assembly. You can see the pin on the face of the starter disk against the vertical pin in the starter base. The vertical pin is .093 music wire and the other pin is .063 music wire. The vertical pin must be short enough that the pin in the starter disk can pass over it. It only needs to be slightly higher than the center of the starter disk when it is vertical. The pin in the start disk needs to be long enough to engage the vertical pin but short enough to avoid touching any part of the crankshaft assembly or bearing support.

The photo above shows the completely disengaged starter position. When the starter motor turns counterclockwise to engage the starter, the pin must pass over the vertical pin and then engage it and pull the starter past vertical and into the crankshaft. The travel of the start arm must be set so that the starter arm doesn’t fall too far from the vertical pin or it will not engage it.

When the starter motor turns clockwise it starts the engine and then disengages, ending in the position shown in the above photo.

In the above photo you can see the white spacer that sets the travel on the starter arm when the starter disengages. A corresponding stop is not needed on the other side because the starter arm is stopped when the starter disk touches the crank disk. A spacer is used on both sides to let the starter arm swing freely but have minimal axial travel.

This photo also shows the o-ring on the starter disk. It is a standard 1/8 inch cross section o-ring but it sits in a groove that only comes to the midpoint of the o-ring.

Slightly visible in the photo are the flanged bronze bushings for the starter pivot shaft. The 1/4 inch pivot shaft is overkill for this application but allows the use of off-the-shelf bronze bearings.

It isn’t visible in these photos, but the starter assembly is mounted on an aluminum angle having slots so that the position of the starter assembly can be adjusted slightly for moving it closer or further from the crankshaft.

I’ve recommended a few changes in the above text. One more I’d like to add for a new design:

The entire engine assembly should be fastened structurally to the cold plate. I’d suggest 1/4 aluminum although you could go thicker if necessary. The cold plate will then be the only part of the engine that fastens structurally to the engine mount. This ensures that all the critical parts of the engine stay in alignment.

Part 1
Part 2
Part 3
engine controller