MODEL ENGINEERING TIPS > WHY WON'T MY STIRLING ENGINE RUN?

If you are new stirling engine refuses to run, here are some things to check:

Power Cylinder - The cylinder must be true and straight, no taper, bell mouth or barrel shapes allowed! If it was accurately machined with a nice surface finish then all that is needed is a nice polish. The closer to a mirror finish the better.

Power Piston - The piston must also be true with no taper, etc. as the cylinder above. The graphite piston must be within .0005" of the cylinder diameter. Pistons over .750" can be a little smaller than that and pistons smaller than .600" should be a little larger than that. The correct fit is when the piston will fall through the cylinder of it's own weight, but when the piston is pushed into the cylinder with the bottom closed it feels like there is a spring under it. Both cylinder and piston must be clean, dry and absolutely oil free.

Mechanical Tightness or Binding - Model stirling engines produce little power. Because of that if they are to run properly, or at all, the mechanical aspects must not rob power. If there is any tightness or binding it must be tracked down and corrected. Even a small amount of tightness or binding will rob much more power than you would expect. Better a little loose that too tight.

Displacer Timing - For all practical purposes, the displacer movement should be 1/4 crankshaft revolution (90 degrees) ahead of the power piston - ie crank pin to crank pin. This is not critical and can vary a few degrees one way or the other. Not all engines will be at optimum performance at 90 degrees but it is the best test setting for a new engine.

Engine Balance - If the engine is a "Miser" or other low temperature difference engine, balance is important. With compression relieved by loosening or removing the bottom plate, adjust the balance disk so that the engine will stop at random places when given a spin. If the engine can't be balanced, gradually enlarge or plug the balance disk holes as needed. If the engine is unbalanced, it will require more heat and operate at a higher RPM than it would if balanced.

Air Leakage - Other than minute leakage around the piston and displacer rod bushing, there should not be any other air leaks. When given a spin, the engine should exhibit some compression by coming to a stop at about the 3:00 o'clock or 9:00 o'clock power piston crankpin positions (vertical engine example). If it exhibits no compression and all else above is well, there is an air leak somewhere that must be found and corrected. Don't overlook the displacer itself. It must be a sealed air tight can. If you put it in the freezer and get it very cold and then submerge it into near boiling hot water it should not show any bubbles coming from it. Low temperature "Miser" type engines should have non pourus foam displacers and are exempt from this test.

If your engine is of sound basic design and it passes all the above tests it will be nearly impossible for it NOT to run! One last caveat - be careful not to use too large a flame to operate your engine as small models can easily be damaged by overheating. An alcohol or propane flame of 1/4" diameter and around 1/2" high (or less) will easily operate any of my engine designs. Miser should NEVER be operated over any flame.



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