How to Trouble Shoot a Stirling Engine
When they are working well, Low Temperature Differential (LTD) engines have just enough power to overcome their own friction. It is very common to experience difficulty when trying to get a new engine to work for the first time. If you are having trouble getting your Stirling engine to run, here are some troubleshooting tips.
Do you have sufficient temperature differential? You do not need to overheat your engine to get it to run. You need both hot and cold. Your heat source may be a candle or a cup of hot water, depending on your engine design. Not every candle is hot enough. If it does not harm your engine, try a candle with a larger flame, or perhaps more than one candle. An alcohol lamp may provide even more heat than a candle. Never heat your engine above the operating temperature it was designed for.
Keep the cool side as cool as possible. This usually means adding ice. Don’t let the ice interfere with the operation of the engine. And never let water get inside the engine. If ice melts and the water runs in through the pressure chamber gland, you will have to disassemble your engine and dry it out.
The key to the temperature differential is the DIFFERENCE in temperature between the warm side and the cold side.
I have seen some people try to rotate their engine in the wrong direction! The rotation direction is determined by the phase angle between the displacer and the drive mechanism. In most small Stirling motors the section of the crankshaft with the drive mechanism is following 90 degrees behind the rotation of the displacer piston. So, when the displacer piston reaches the top, the drive piston is half way up. When the displacer piston reaches the bottom, the drive piston is half way down.
If your displacer is not moving far enough, it may not be creating enough “displacement” to move the air inside your engine.
A teeny tiny pressure leak is usually a good thing. I have made several engines with air tight pressure chambers, and they actually need to have vents added to them so that the pressures can occasionally be equalized. However, it is very easy to get too much air leaking out. Since there is barely enough power to move the flywheel, it doesn’t take much of a leak to stop it.
One place pressure often leaks is around the drive piston (if you have one). If you use a diaphragm drive, leaking is not a problem here. Sometimes a little oil will help to lubricate a loose fitting piston while also slowing a leak.
If you are using a diaphragm drive, it must be set up so that it is not too loose. If the diaphragm is too loose, it is the same as a leak. All your pressure changes will be consumed in moving the loose fabric of the diaphragm, and the motion will not go to your flywheel. The diaphragm has to be set just right, so that all the slack is gone at each end of the stroke. But do not make it too tight, or the motor will have to fight against the fabric in order to rotate.
As a general rule, if it makes noise, it is making friction.
If the displacer is hitting, touching, or rubbing the inside of the pressure chamber, that is increasing your friction. Minimize this the best that you can.
If your displacer is coming to rest against a flat surface, it is increasing the “pull off” friction by creating a vacuum between the two flat surfaces. This is one reason why you want your displacer to stop 1/16” to 1/8” before touching a flat surface.
If your displacer does not have enough clearance for air to pass around it, this is increasing friction.
Every place where two moving parts touch has friction. You cannot eliminate friction, but you must minimize the friction at every point where moving parts come together. If you can create a design with fewer moving parts, you will also have fewer friction points.
Check the displacer motion and make sure it is not rubbing the inside of the engine. Make sure it is not hitting the top or the bottom.
Check the motion of the displacer shaft through the gland. This is a common trouble spot for both friction and leaking.
Check both ends of the connection between the displacer and the crank shaft. The direction of push/pull must always be perpendicular to the axis of the drive shaft. If the connecting rods are pushing at an angle other than 90 degrees, this will cause binding. Install keepers in necessary to maintain the angle of the connecting rod.
Does your crankshaft and flywheel turn freely? If you disconnect the displacer and the drive mechanism and spin the flywheel, how long will it coast? The pop can motor on my desk right now will coast for 15 seconds with a flywheel made from two CDs. If yours spins for less than 5 seconds, you have a serious friction problem.
Check the connecting rod between the crank shaft and the drive mechanism. This connecting rod must also always push and pull at 90 degrees to the crank shaft axis. If this is pushing to the side, it will cause friction and bind. You may need to install a keeper of some sort to get it to stay at the correct angle.
If you have a piston, make sure it does not bind. It must fit loose enough to fall through the cylinder under its own weight when there is no air pressure holding it up. And it must fit tight enough that it will not drop (or it will not drop quickly) when there is air pressure to cause resistance. If it is a tight fit, you may need to make it looser by polishing. If it is a loose fit, you may want to try oiling it.
Check everything thoroughly, but modify only one thing at a time. If your modification improves performance, keep it. If your modification makes things worse, or makes no change, set it back the way it was. If you change too many things at the same time and it degrades your performance, you won’t know which change caused the problem.
It is very normal and natural for these engines to require some tweaking and tuning to get them “in the zone”. Once you have them tuned up, they will run for a long time. Remember: Low Temperature Differential (LTD) engines have just enough power to overcome their own friction. You must pay great attention to detail to maximize power (by managing leaks and temperature differential) and minimize friction to make these motors run well.