MAX PIANTA
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Year 1 · Design & Practice and Materials & Stress · 2023

Stirling engine: designed, analysed and machined by hand

A complete Stirling engine taken from SolidWorks model to a full set of manufacturing drawings, stress-checked for a space mission case study, then machined part by part and assembled in the university workshop.

  • ContextENGR4003 Design & Practice and ENGR4008 Materials & Stress Analysis, Oxford Brookes University
  • RoleCAD, drawings and machining individually; stress case study in a team of five (I owned the piston con rod)
SolidWorksTechnical drawingsFEAHand calculationsManual machining
SolidWorks render of the complete Stirling engine

Key numbers

22
manufacturing drawings, one for every part
−233 °C
cold side in the space case study (hot side 85 °C)
28.3 kN
critical buckling load of my con rod design
8.1
factor of safety on the titanium con rod

The project

A Stirling engine turns a temperature difference into motion: heating the gas in a sealed chamber drives a displacer and piston, which spin a flywheel. The brief was to design one completely in CAD, document it to a manufacturable standard, and then make it for real.

I machined every part myself in the university workshop and assembled the engine by hand, turning my own drawings into a working mechanism.

Assembly drawing with the full parts list: plates, column, crank wheels, piston, displacer and fasteners
Assembly drawing with the full parts list: plates, column, crank wheels, piston, displacer and fasteners

What I did

  • CAD: modelled every component and the full assembly in SolidWorks, from the top and bottom plates to the crank wheels, piston, displacer and fasteners.
  • Drawings: produced a 22-sheet drawing pack with dimensions, sections and detail views ready for the workshop.
  • Thermal FEA: simulated the M4 bolts at 85 °C in steel, aluminium, titanium and nylon to compare expansion against weight.
  • Machining: made each part by hand in the workshop, then assembled and set up the finished engine.
Top plate drawing: section view, hole pattern and tolerances
Top plate drawing: section view, hole pattern and tolerances

Stress case study

In a team of five we re-engineered the engine to survive on the James Webb Space Telescope, with a hot side at 85 °C and a cold side at −233 °C. Each of us analysed one component to a factor of safety of at least 3.

  • My part, the piston con rod: peak shear stress of 112.4 MPa at the end loops under a 320 N load.
  • Buckling: critical load of 28.3 kN from Euler's formula, far above the working load.
  • Material: selected Ti-8Al-1Mo-1V titanium (910 MPa yield, 2.5 g part), for a factor of safety of 8.1.
SolidWorks static study of the piston con rod
SolidWorks static study of the piston con rod

The finished engine

Photo of the machined engine
coming soon
Close-up of hand-machined parts
coming soon
Engine running
coming soon

Gallery

Piston con rod drawing with detail view at 10:1
Piston con rod drawing with detail view at 10:1
Thermal FEA of an M4 bolt at 85 °C (displacement in mm)
Thermal FEA of an M4 bolt at 85 °C (displacement in mm)
Render of the complete engine
Render of the complete engine

Looking back

What went well

Seeing a design go from SolidWorks to a real object I had machined myself. Producing a full set of 22 manufacturing drawings forced me to think about tolerances and fits, and the stress and buckling hand calculations gave clear, defensible safety factors.

Even better if

I'd design more closely around the workshop's real capabilities, as some features were harder to machine than they looked in CAD. I'd also back the hand calculations with more FEA and test the finished engine, for example measuring flywheel speed against temperature difference.

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