I thought I understood the basic principles of thermodynamics, but my latest experiment has proven otherwise in a fairly spectacular way. I built a small Stirling engine from plans I found online, trying to demonstrate a simple heat-to-work conversion for a classroom project.
The design looked straightforward: a displacer piston, a power piston, a temperature differential between a hot plate and a cold sink. I followed every dimension to the millimeter, used brass for the cylinders and Teflon for the seals, checked for leaks with soapy water. The hot end sits at about 200°C, the cold end at roughly 25°C with a water jacket. That should give me a decent temperature gradient, right? I calculated the theoretical efficiency using Carnot’s formula, and for a 175 K difference I expected something sensible, even with real-world losses.
I fired it up, and nothing. The power piston twitched once, then stopped. I adjusted the flywheel, added a bit of lubrication, checked the seal clearances, even tried a different heat source to ensure steady temperature. Still no sustained motion. I read that regenerators can improve performance, so I stuffed some copper wool into the displacer path, thinking it might store and release heat more effectively. That just made the whole thing sluggish. I am now wondering if my fundamental understanding of the cycle timing is off, or if I am missing something about the relationship between pressure volume changes and the displacer movement. I know the displacer should shift the air between hot and cold zones, but maybe the phase angle between the two pistons is wrong, or my swept volume ratio is out of whack for the temperature delta I have.
What I really need is someone to look at my numbers and tell me whether the engine should theoretically run with a 175 K difference and a given bore and stroke, or if I am asking for an impossible pressure change. I am looking for a clear explanation of why it fails, not just another tweak to try, because I have already tried tweaks. I want to know specifically how to calculate whether my configuration yields a net positive work loop.
The design looked straightforward: a displacer piston, a power piston, a temperature differential between a hot plate and a cold sink. I followed every dimension to the millimeter, used brass for the cylinders and Teflon for the seals, checked for leaks with soapy water. The hot end sits at about 200°C, the cold end at roughly 25°C with a water jacket. That should give me a decent temperature gradient, right? I calculated the theoretical efficiency using Carnot’s formula, and for a 175 K difference I expected something sensible, even with real-world losses.
I fired it up, and nothing. The power piston twitched once, then stopped. I adjusted the flywheel, added a bit of lubrication, checked the seal clearances, even tried a different heat source to ensure steady temperature. Still no sustained motion. I read that regenerators can improve performance, so I stuffed some copper wool into the displacer path, thinking it might store and release heat more effectively. That just made the whole thing sluggish. I am now wondering if my fundamental understanding of the cycle timing is off, or if I am missing something about the relationship between pressure volume changes and the displacer movement. I know the displacer should shift the air between hot and cold zones, but maybe the phase angle between the two pistons is wrong, or my swept volume ratio is out of whack for the temperature delta I have.
What I really need is someone to look at my numbers and tell me whether the engine should theoretically run with a 175 K difference and a given bore and stroke, or if I am asking for an impossible pressure change. I am looking for a clear explanation of why it fails, not just another tweak to try, because I have already tried tweaks. I want to know specifically how to calculate whether my configuration yields a net positive work loop.
