Final Assembly

Inspiration

As a final individual project to send off my senior year, I decided to make a model steam engine. I intend to cover all phases of design, prototyping, manufacturing, and redesign. So far I have completed the first full working assembly in SolidWorks.

I wanted to end my senior year in mechanical engineering with a purely mechanical project. I chose a steam engine because it is one of the most fundamental symbols of engineering and the industrial revolution. If you can build a steam engine, everything else -- rotary motion, locomotion, electrical power -- becomes possible.

Full View Motion Study

Background & Research

Knowing I wanted to build a steam engine meant looking into how other people had done it first. There are a number of steam engine enthusiasts on youtube covering their own models, prototypes, and getting in depth on certain mechanisms. I had good examples of the flywheel and shaft design that would apply to essentially any piston-cylinder engine, as well as different model housing and display methods.

I was also introduced to the first design challenges that I would be facing. How would exhaust gas leave the piston cylinder? How would intake gas enter? Was I going to use a single cylinder, or more than one? Which pieces of the assembly should move, which ones had to move? What should the stroke length be? How would I create smooth action in the working cylinder?

In the slow process of answering these questions I eventually settled on a Piston-Valve design. It simplified both the motion and dimensional complexity of the model; I could work along just one axis. It also added an interesting timing problem, a second Valve cylinder is added to time the intake and exhaust of the working cylinder. These were the predominant reasons why I settled on this design over others, although I give the oscillating engine a close second place.

Piston-Valve Steam Engines

Source: https://animatedengines.com/locomotive.html

Piston-Valve steam engines work by simultaneously expanding and exhausting the working cylinder with a secondary Valve cylinder which controls access to intake and exhaust ports. Mechanical timing can be set by connecting both rods to the same shaft and varying either stroke length, phase, or both. In my case I reduced the stroke length and set the valve cylinder out of phase with the working cylinder.

Two-Cylinder Timing

As you can see, access to the intake and exhaust ports is precisely controlled. The two cylinders are both timed by the same shaft, so variations are only variations of phase, and each remains synchronized according to the initial conditions. For my Piston-Valve design, a 90 degree phase difference seemed to work the best.

One small detail is that the circular ports and edges of the Valve piston create dampening on the working piston. This is actually more efficient than rectangular ports and sudden on/off intake and exhaust, as the dampening prepares the piston for the next stroke. The reduction in jerk associated with the sudden change of direction also helps with part lifetimes.

If you'd like a more detailed explanation of Piston-Valve steam engines, this video was an extremely helpful resource for me.

Sub-Assembly Views

Each piston cylinder was modeled as a separate sub-assembly for modularity and redesign purposes. I started the design process with the working cylinder, before moving on to the rest of the assembly -- connecting the working cylinder with the shaft -- before finally aligning the valve cylinder and orienting its piston for alignment in the final assembly.

Google Test is integrated with ROS2 for C++ development, so I developed my tests there. The framework is pretty simple, and it allowed me to write tests for individual methods that sped up my development when I made refactors later. This was especially helpful near competition time, when I needed to rework the particle filter to accept information from our configuration system. With the tests I had written, all I had to do was refactor and ensure that my code still passed all my tests. The freedom to make changes like this has really paid off in my experience, so I will probably include testing in any large project I work on in the future.

Working Cylinder Views

Valve Cylinder Views

Engineering Drawings and Manufacturing

At this point in the project I am beginning the manufacturing and material selection process. My intention is to CNC lathe the piston-cylinder sub-assemblies first, before working on the driven shaft. I'll be updating this page with my progress as I continue.