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Project Logs / Making a Prony Brake to Measure the Power of My Hot Air Engines
« Last post by vtsteam on May 19, 2025, 05:41:52 PM »It was a lot of fun when rehabilitating No. 83, my hot air engine to see how fast it would go after each change. But top RPM is not a very good measure of a hot air engine. Since I want to go further with modifications, I really need a way to look at power output. And I'd like to build other hot air engines as well (in fact I've already started on No. 84, a Rider type engine) so a tool for the job is needed.
Enter Professor Dennis Chaddock and his Prony brake dynamometer for hot air engines, featured in September 1976 Model Engineer. This simple device was used to compare model hot air engines submitted for a new competition slated for the upcoming 1977 Model Engineer Exhibition.
This device used a steel brake drum and brake shoes of hardwood, with a notched and graduated aluminum balance arm with moveable weights. The moment was used to figure momentary torque, and in conjunction with a clockworks tachometer the power output could be arrived at arithmetically.
I'm going to use some of Professor Chaddock's design, but use a digital tach, and scale, since I already have both. Somewhat less visually elegant, but a lot easier to work out the power curve -- particularly since I can do it in Watts with minimal units conversion, by comparison with his original inch and ounce torsionometer.
Another slight variation, his was designed for a standardized motor shaft of 5/32" diameter and mine will be for 8mm, since that's what No. 83 has, and is a standard I intend to continue. Also he modestly referred to his cool little dynamometer as "a bit of oak" because of the brake shoes. Mine will be "a bit of cherry," in that case since I have a lot of that hardwood, which I cut and milled on my own property.
Here's a start, parting off the brake drum on my homemade lathe:
[ You are not allowed to view attachments ]
And here's the cherry wood I'll be using, milled out. It's kind of surprising how small this all is, after having looked at the plans many times over the years in the magazine.
It really is just a little bit of... whatever!
[ You are not allowed to view attachments ]
Enter Professor Dennis Chaddock and his Prony brake dynamometer for hot air engines, featured in September 1976 Model Engineer. This simple device was used to compare model hot air engines submitted for a new competition slated for the upcoming 1977 Model Engineer Exhibition.
This device used a steel brake drum and brake shoes of hardwood, with a notched and graduated aluminum balance arm with moveable weights. The moment was used to figure momentary torque, and in conjunction with a clockworks tachometer the power output could be arrived at arithmetically.
I'm going to use some of Professor Chaddock's design, but use a digital tach, and scale, since I already have both. Somewhat less visually elegant, but a lot easier to work out the power curve -- particularly since I can do it in Watts with minimal units conversion, by comparison with his original inch and ounce torsionometer.
Another slight variation, his was designed for a standardized motor shaft of 5/32" diameter and mine will be for 8mm, since that's what No. 83 has, and is a standard I intend to continue. Also he modestly referred to his cool little dynamometer as "a bit of oak" because of the brake shoes. Mine will be "a bit of cherry," in that case since I have a lot of that hardwood, which I cut and milled on my own property.
Here's a start, parting off the brake drum on my homemade lathe:
[ You are not allowed to view attachments ]
And here's the cherry wood I'll be using, milled out. It's kind of surprising how small this all is, after having looked at the plans many times over the years in the magazine.
It really is just a little bit of... whatever! [ You are not allowed to view attachments ]
