Measure the power and efficiency of your Stirling engine; it’s easy

In the world of small Stirling engines everyone seems to talk about power and efficiency—but you seldom see the facts and the data. Just a couple days ago I saw the following Stirling engine on Youtube:

The promoter claims it must be putting out 100 to 150 watts. It was the usual unloaded engine spinning a flywheel. I thought it looked like a good engine. It was powered by a huge concentrating Fresnel lens. Maybe it was capable of 100 watts, or maybe 5 or 10. The promoter held a stick up against the flywheel and was trying to show how powerfully it rubbed on the stick.

Why be so crude when it’s really not that hard to test the power output of an engine? The video below demonstrates the setup I used to test my model 3F engine. It uses simple, inexpensive equipment. In this video I’ve gone to some extra effort to measure the efficiency too. For that you need an accurate measure of the heat you are using to run the engine.

I’m not proud of the power output (1.05 watts) or efficiency (1.5%) of my engine. On the other hand, it’s nice to know what I’ve got and what I need to work on to improve it. Those that haven’t tested their engines and just make wild guesses may be surprised by the reality.

In a future post I’ll go into where all that heat goes that doesn’t end up coming out the crankshaft of my engine. A lot of it goes from the heat source into the surrounding environment—even with all the insulation.

More information on the torque measurement and the tachometer is available on this website.