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Free-piston Stirling displacer Ø29 × 75 and Ø39 × 100
What size displacer goes in a free-piston Stirling engine? · Hardware · basis: Beta free-piston layout (Sunpower RE-1000 / NASA): displacer = bore − 1.0 (0.5 mm radial gap, designed), length 2.5 × bore
Long Earth Society build part. The LES free-piston test bed: Ø40 core on flexures in a 2 in tri-clamp vessel. The engines
Exact dimensions. Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part. Mating features
- sizes
- 2 in table
Notes
- The displacer of the free-piston core: a hollow cylinder with a 1.2 mm wall and a 2 mm rounded top edge, Ø29 × 75 for the Ø30 bore or Ø39 × 100 for the Ø40 bore.
- The 0.5 mm radial gap to the liner is designed: it is a shuttle, not a seal.
- A Ø9 × 8 rod boss with a Ø5 bore (fit: slide) takes the Ø5 displacer rod. Its spring (flexure plus gas spring) is tuned so it leads the piston by about 60–90°.
Sizes
| file | diameter | length | bore it runs in | radial gap | wall | rod bore |
|---|---|---|---|---|---|---|
| displacer-30x75 | 29 | 75 | 30 | 0.5 | 1.2 | 5 |
| displacer-40x100 | 39 | 100 | 40 | 0.5 | 1.2 | 5 |
Mating features
Drawn at the exact size. Open press, slide, run and socket features by your process's clearance; leave designed gaps and clamp fits as drawn.
| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| rod bores | Ø5 | Ø5 rod | slide | all |
| displacer in the liner | liner bore − 1.0 (0.5 radial) | cylinder liner bore | designed | displacer-* |
Where the files are
Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.
| NASA (NTRS, public domain) | Free-piston Stirling engine design and analysis (RE-1000 test results) (1984) | US Government work, public domain | |
| NASA (NTRS, public domain) | Dynamics of free-piston Stirling engines (1987) | US Government work, public domain | |
| Wikipedia | Free-piston engine | web | CC BY-SA 4.0 (text) |
| Wikipedia | Beale number | web | CC BY-SA 4.0 (text) |
| Ohio University | Stirling cycle machine analysis (Urieli), free online | web | Author's site |
| Long Earth Society | Engine builds | web, video | LES site |
Free files
★ Free files 2 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json
| displacer-30x75 | displacer Ø29 (0.5 mm radial gap in a Ø30 bore) × 75, hollow 1.2 wall, Ø5 rod boss | STEP · STL |
| displacer-40x100 | displacer Ø39 (0.5 mm radial gap in a Ø40 bore) × 100, hollow 1.2 wall, Ø5 rod boss | STEP · STL |
Plain text: sheet.txt · JSON: /api/geometry?object=free-piston-displacer
How it fits
Goes in or on
Free-piston cylinder liner Ø30 × 120 and Ø40 × 160in the Ø30 / Ø40 bore, 0.5 mm radial gapConnects to
Ø5 rods with E-clip grooves, 120 and 150 mmØ5 × 150 rod in the boss
Planar spiral flexure bearing leavesrod suspended on a flexure stackAlternatives
Tri-clamp displacertri-clamp displacersYou might also need
Questions
How does a free-piston Stirling keep its phase without a crank?
The displacer is a tuned mass on a spring (flexure and gas spring) that resonates ahead of the piston, typically 60–90°.
What gap does the free-piston displacer have?
0.5 mm radial: Ø29 in the Ø30 liner, Ø39 in the Ø40.