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This is a spring engine using a roughly made Walschaert valve gear as its timing mechanism. It's an inside-admission layout, that the timing valve (expansion valve) is connected between the ends of the 'Combination Lever'.
The springs' forces in this scene are simply being switched on and off.
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This scene:
Chooo Chooo 2023 might has more realistic power delivery; the springs' force gradually adds to a limited value, stays, then 'exhausts' back to zero, mimicking the 3 stages of a steam engine's power stroke of my understanding:
[1] Admission - A cylinder's timing valve moves and opens one of its ports, allowing steam to enter one side of the space between the piston and the cylinder head to a level that's controlled by the throttle. The pressure starts pushing the piston.
[2] Cut-off/Steam Expansion - Once the piston is moving, it drives the wheels to move the valve gear that actuates the timing valve to close the opened port, 'cutting-off' the steam admission. The steam entered the cylinder from the last stage is now trapped and acts like a spring pushing the piston away from the cylinder head. The pushing pressure gradually decreases as the space between the piston and the cylinder head increases and as the steam expands.
[3] Exhaust - The piston is near the end of this stroke. The timing valve opens the same port again but to an exhaust passage. The remaining pressure escapes the cylinder and shoots out of the chimney.
For the 3 stages to happen, the valve gear would need finer 'lead/lap' timing, controlled by the 'Combination Lever' in reality. When making the valve gear, this seems to be one of the things need to be set first. In the scene mentioned above, this is set by the piston lasers placement instead.
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Other Notes:
- To get a correct Main Rod length, have the pistons and wheel the position/angle at their top/bottom dead center since the 'half' point of the stroke is not 90° of the wheel's rotation.
- It's easier to have the Expansion Link (the curved thing)'s connection with the Eccentric Rod leveled with the wheel's center, so it matches Algodoo's grid.
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References:
http://trumpetb.net/loco/index.html - valve gear introduction
https://youtu.be/O6rlfXCsoPk - a deep explanation of Walschaerts Valve Gear
https://youtu.be/LU-KgJVlZ0M - a SoilidWorks design
https://youtu.be/JRdTPNApfuk - How Steam Engine Cylinders Work
https://youtu.be/C_YNn3ZkJmU - piston and rod relation
Feel free to edit
