Sterling Engine for Students

    I built 6 of these sterling engines to help a professor to prepare for a class. This professor in the Industrial Engineering department was ramping up for a new class on CAD, CAM, and simulation. Typically the students learn Creo (or OnShape), but if they so choose, they can now take a class to learn SolidWorks modeling, MasterCAM, and SolidWorks simulation. A class project was selected by the professor on the criteria of it needing to be complex enough to last a semester, have moving parts for animation, have a thermal element for simulation, and have some pieces simple enough that a student could program and machine them.

A finished engine. The legs and flywheel are made by the class students.

   
    The idea was the students could progress through the project, modeling and assembling components in CAD, running motion and thermal analysis on the model, and programming and making a few parts. The remaining parts would be made be me and at the end of the year, the students could assemble the engine in real life with their parts and see it run. The parts of the engine the student didn't make would be reused year to year.

    Chosen was the "Vickie" Victorian styled sterling engine designed by Jerry Howell; a beautiful and elegant model engine. This project was really doomed from the start due to the choice of this engine. The plans from Jerry Howell contain no tolerances and the instructions intend a lot of hand fit and finish, creating bespoke parts. This doesn't jive well with the fact that 6 needed to be built, all with interchangeable parts, and several of the parts had to be made by inexperienced students. Add to this some on the fly design changes by the professor that override my better judgement and we ended up with 6, pretty, but non functional model engines.

Detail of the linear guide for the power cylinder.

    I came into this project half way though. The professor and another student worker had already made many of simpler parts of the engine. As I progressed through the remaining parts and sub-assemblies, I ran into fit issues. Going back and measuring the parts the professor and other student had completed, I found every single part was out of tolerance (self imposed +/- 50 um), or in some cases totally missing features. This revelation essentially doubled my workload on the project. While some parts were only off by a small amount or had a bad surface finish, I saw no excuse for missing features, or dimensions off by more than 1 mm. Some parts I reworked or shimmed to fit, most others I totally remade.

    This is what I believe to be one of the larger contributors to the engine not working. The other is on the fly design changes the professor wanted to make the engines easier to build. These included replacing some of the custom fasteners with OTS parts (minimal impact) and replacing some of the weight reduced parts with heavier ones. When I was reading through the assembly plans, a great deal of mention wad given to what components had to be very light weight. Many of those components involved soldered assemblies of machined parts and brass tubes. It was decided to replace the brass tubes with solid steel shafts to simplify. I think this extra weight contributed to the engines not working. 

    In the end, these engines are only good for display. Maybe some student or professor in the future will find these and be able to get them working.

6 completed engines. Note the oxidization on the hot ends from testing.