# Build list: Modular free-piston thermoacoustic engine, 2 in tri-clamp (stack sections and a clamp-in linear alternator)

> What parts do I need to build a Modular free-piston thermoacoustic engine, 2 in tri-clamp (stack sections and a clamp-in linear alternator)?

- Category: Stirling engines › Assembled engines
- Kind: List / guide
- Basis: Every part is a free STEP/STL on this site or a standard part with its own page; quantities are the full count, every screw, nut, gasket and clamp. High-pressure clamps, the load resistor and the series capacitor have no drawing. Numbers from the linear network model in tools/gen_ta_modular.py (Rott / Swift equations, written from the published method).
- Page: https://xemstudio.com/objects/list-ta-modular-free-piston-2in/ · JSON: https://xemstudio.com/api/geometry?object=list-ta-modular-free-piston-2in · Plain text: https://xemstudio.com/objects/list-ta-modular-free-piston-2in/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- A standing-wave thermoacoustic engine built from 2 in sanitary sections like Legos: closed hot section with its exchanger, hot joint, stack section with the ambient exchanger, optional thermal break, and the clamp-in linear alternator module whose sprung piston replaces the resonator tube.
- Numbers: He 10 bar, 550 °C hot gas, piston ±5 mm: 47 Hz, 10 W electric, 13 % of 81 W into the hot exchanger (20 % of Carnot) · onset 296 °C · air 10 bar 1.7 W · air 1 bar 0.10 W (model) · sections 60 + 68.65 + module 147

## Sizes

| subsystem | object | parts | qty | note |
|---|---|---|---|---|
| None | 2020-aluminum-extrusion | 2020-300 | 2 | uprights |
| None | 2020-aluminum-extrusion | 2020-200 | 2 | rails on the bench behind the uprights, clamped or screwed to the bench |
| None | t-slot-corner-brackets | corner-bracket-20 | 4 | two per upright |
| None | button-head-screws-metric | button-m5x10 | 8 | bracket screws |
| None | flat-washers-metric | washer-m5 | 8 |  |
| None | drop-in-t-nuts | tnut-dropin-20-m5 | 10 | 8 brackets, 2 for the hanger |
| None | spool-hanger-collars | hanger-2in-tab | 1 | on the stack section, right under the jacket (the cool end of the stack) |
| None | spool-hanger-collars | hanger-2in-plain | 1 |  |
| None | socket-head-cap-screws-metric | shcs-m5x16 | 2 | hanger bar into the uprights' T-nuts |
| None | socket-head-cap-screws-metric | shcs-m4x20 | 2 | close the hanger on the tube |
| None | hex-nuts-metric | nut-m4 | 2 | hanger ears |
| None | tri-clamp-torch-stand | torch-stand-40 | 1 | on the bench under the shroud's flue, 25 below it; the heat source is a propane or butane torch head in the ring |
| None | set-screws-metric | set-m4x10 | 3 | centre the torch head in the ring |
| None | thermoacoustic-hot-shroud | ta-shroud-2in-front | 1 | flame shroud round the hot exchanger, its band 10.35 to 50.35 below the hot section's face; flue down to the torch |
| None | thermoacoustic-hot-shroud | ta-shroud-2in-back | 1 |  |
| None | socket-head-cap-screws-metric | shcs-m4x25 | 2 | shroud ears |
| None | hex-nuts-metric | nut-m4 | 2 | shroud ears |
| None | thermoacoustic-shroud-fin-sleeve | ta-fin-sleeve-half-2in-34-r36 | 2 | optional heat exchanger: fin sleeve over the hot exchanger, inside the shroud band |
| None | socket-head-cap-screws-metric | shcs-m4x12 | 2 | optional, with the fin sleeve: its ears |
| None | hex-nuts-metric | nut-m4 | 2 | optional, with the fin sleeve: its ears |
| None | tri-clamp-cups | cup-2in-60 | 1 | HOT SECTION: closed end down; the hot cavity and the hot exchanger |
| None | thermoacoustic-hot-cavity-liner | ta-hot-liner-2in-17 | 1 | hot section: stands on the closed end |
| None | thermoacoustic-heat-exchanger | heat-exchanger-hot-2in | 4 | hot section: four discs on the liner, plates all the same way, the top one flush with the face |
| None | tri-clamp-hot-gasket | hot-gasket-2in | 1 | J1, the hot joint: flat ring between the hot and stack sections; it runs at about the hot-exchanger temperature (550 °C design): rated for at least 600 °C |
| None | sanitary-high-pressure-clamp | hp-clamp-2in | 1 | J1: high-pressure bolted 2 in clamp, rated at the J1 temperature for the charge used (not drawn; published ratings stop at 121 °C) |
| None | thermoacoustic-stack-section-spools | spool-2in-68.65mm | 1 | STACK SECTION: plain 2 in spool cut to length |
| None | thermoacoustic-stack | stack-spiral-2in-50-g1.0 | 1 | stack section: stands on the hot exchanger, its lower 1.7 inside J1's ring; it must take 550 °C at its hot end |
| None | thermoacoustic-heat-exchanger | heat-exchanger-cold-2in | 1 | stack section: ambient exchanger on the stack's cold end |
| None | thermoacoustic-bore-spacer | ta-spacer-2in-9.8 | 1 | stack section: ambient exchanger to the J2 gasket lip |
| None | thermoacoustic-water-jacket | ta-jacket-half-2in-14 | 2 | optional heat exchanger: water jacket over the ambient exchanger, 9.35 below the stack section's top face; or cool that band any way you like |
| None | socket-head-cap-screws-metric | shcs-m4x25 | 2 | optional: jacket ears (only with the jacket) |
| None | hex-nuts-metric | nut-m4 | 2 | optional: jacket ears (only with the jacket) |
| None | tri-clamp-thermal-break-spacer | break-spacer-2in-25.4 | 1 | choose one cold joint, B (thermal-break spacer): between the stack section and the alternator's face A, at the cooling water's temperature; costs power (dead volume). A (the build): face A straight on the stack section |
| None | tri-clamp-gasket | tc-gasket-2in | 1 | choose one cold joint, B (thermal-break spacer): its second Type B joint |
| None | sanitary-high-pressure-clamp | hp-clamp-2in | 1 | choose one cold joint, B (thermal-break spacer): its second high-pressure clamp (not drawn) |
| None | linear-alternator-module-spools-2in | spool-2in-42mm-bored-48.00 | 1 | ALTERNATOR MODULE (closed, standard build): face A (to the engine) and the piston's cylinder |
| None | tri-clamp-spool-bored-honed | spool-2in-3in-bored-48.00 | 1 | module: the alternator section: magnet carrier inside, stator outside |
| None | linear-alternator-module-spools-2in | spool-2in-25.4mm | 1 | module: face B end: leaf 2's hub travel |
| None | tri-clamp-flexure-leaf | leaf-2in-piston-t0.8 | 2 | module: joints 1 and 2; leaf 2 turned over (opposite hand) |
| None | tri-clamp-leaf-half-gasket | leaf-half-gasket-2in-w0.45 | 4 | module: two per leaf, beads outward |
| None | linear-alternator-module-mover-2in | la-piston-cup-48x24 | 1 | module: in the 42 bored spool, crown toward face A |
| None | linear-alternator-module-mover-2in | la-tube-rod-18x14x108.15 | 1 | module: pressed into the cup's socket; through both leaves |
| None | linear-alternator-module-mover-2in | la-magnet-carrier-47x56 | 1 | module: pressed on the tube rod, centred under the stator |
| None | neodymium-disc-magnets | magnet-12x3 | 40 | module: 4 rows of 10 in the carrier, faces out: S, N, N, S from face A |
| None | fpse-hub-collars | fpse-collar-18x28 | 4 | module: either side of each leaf's hub on the tube rod |
| None | set-screws-metric | set-m3x4 | 4 | module: one grub per collar |
| None | fpse-linear-alternator-stator | fpse-stator-pack-43deg | 8 | module: round the 3 in bored spool at the magnets' centre |
| None | fpse-linear-alternator-stator | fpse-bobbin-65 | 1 | module: between the teeth; slid over the spool's flange first |
| None | fpse-linear-alternator-stator | fpse-coil-300t | 1 | module: 300 turns of 0.71 mm enamelled wire on the bobbin; the leads leave through the housing |
| None | fpse-linear-alternator-stator | fpse-stator-housing-half | 2 | module: clamp the packs onto the tube |
| None | socket-head-cap-screws-metric | shcs-m4x20 | 4 | module: stator housing ears |
| None | hex-nuts-metric | nut-m4 | 4 | module: stator housing ears |
| None | sanitary-high-pressure-clamp | hp-clamp-2in | 4 | module: high-pressure bolted 2 in clamps: joints 1 and 2, face B under the cap, and face A on the stack section (J2) (not drawn); rated above the 15 bar gauge relief setting |
| None | tri-clamp-gasket | tc-gasket-2in | 2 | module: Type B at face A (J2, on the stack section) and face B (under the cap) |
| None | tri-clamp-port-cap | port-cap-2in-4x-npt18 | 1 | module: closes face B (the bounce space): fill valve, pressure gauge, relief valve, and a plug or pressure transducer; off when face B clamps into a loop |
| None | npt-thread-envelopes | npt-1-8-male | 4 | module: the 1/8 NPT stems of the fill valve, gauge, relief valve and plug (bodies are the builder's choice) |

## Sections

| section | job | parts | length_mm | reused in |
|---|---|---|---|---|
| hot section | takes the heat: hot cavity (the gas spring at the hot end) and the 40 mm hot exchanger under the flame shroud | cup-2in-60 + ta-hot-liner-2in-17 + 4 × heat-exchanger-hot-2in | 60.0 | list-free-piston-2in and list-beta-rhombic-2in (the same cup is their hot end); any 2 in engine with a closed hot end |
| J1 hot joint | seals at about 550 °C; the stack passes into its bore | hot-gasket-2in + high-pressure clamp | 1.7 | every flame-side joint on the site |
| stack section | holds the temperature gradient (stack) and rejects heat (ambient exchanger, jacket) | spool-2in-68.65mm + stack-spiral-2in-50-g1.0 + heat-exchanger-cold-2in + ta-spacer-2in-9.8 | 68.64999999999999 | list-ta-standing-half-wave-2in and -quarter-wave (same stack, exchangers and spacer, there inside the hot cup) |
| thermal break (optional) | insulates the alternator from the stack section; costs power | break-spacer-2in-25.4 | 25.4 | between any warm 2 in section and a part that must stay cool |
| piston (in the module) | the module's Ø48 clearance-seal piston cup in its 42 mm bored spool is the engine piston | spool-2in-42mm-bored-48.00 + la-piston-cup-48x24 | 42.0 | list-linear-alternator-module-2in-closed / -open (face A) |
| linear alternator module | mass-spring of the free piston (flexures, bounce gas) and the generator | the closed module's standard build | 147.0 | list-linear-alternator-module-2in-closed: any 2 in pressure-wave source (free-piston hot ends, thermoacoustic loops and rings) |

## Model cases

| gas and module | f_Hz | onset (coil open) | P_e_W | eta_pct | Q_h_W | p1_pct_of_mean | R_load_ohm |
|---|---|---|---|---|---|---|---|
| He 10 bar | 46.9 | 296 °C | 10.19 | 12.6 | 81 | 8.9 | 81.6 |
| air 10 bar | 43.0 | 436 °C | 1.67 | 2.4 | 70 | 6.9 | 448.0 |
| air 1 bar | 21.0 | 392 °C | 0.1 | 0.2 | 52 | 9.1 | 1737.2 |
| air 1 bar, open-frame module | 18.5 | 278 °C | 0.17 | 0.3 | 52 | 9.1 | 836.4 |

## Swap and extend

| change (He 10 bar) | f_Hz | onset | P_e_W | eta_pct | Q_h_W | p1_pct |
|---|---|---|---|---|---|---|
| hot section cup-2in-60 (the build) | 46.9 | 296 °C | 10.2 | 12.6 | 81 | 8.9 |
| hot section cup-2in-90 (liner 47) | 45.7 | 248 °C | 11.3 | 11.5 | 98 | 7.8 |
| hot section cup-2in-120 (liner 77) | 44.7 | 220 °C | 11.6 | 10.2 | 114 | 7.0 |
| hot section closed-spool-2in-152.4 (liner 109.4) | 43.8 | 204 °C | 11.3 | 8.9 | 128 | 6.3 |
| stack section 30 (48.65 spool) | 48.8 | 265 °C | 14.2 | 8.9 | 160 | 10.5 |
| stack section 40 (58.65 spool) | 47.8 | 281 °C | 12.0 | 10.9 | 109 | 9.6 |
| thermal-break spacer added (J2 → 25.4 + one joint) | 44.8 | 344 °C | 4.3 | 6.3 | 69 | 7.0 |
| 3 in spool added on the cold side | 42.3 | 549 °C | 0.0 | 0.0 | 61 | 4.9 |
| 6 in spool added on the cold side | 40.4 | no onset | 0.0 | 0.0 | 57 | 3.4 |

## Where the files are

- G. W. Swift (Acoustical Society of America): [Thermoacoustics: A Unifying Perspective for Some Engines and Refrigerators (Rott's equations, the short-stack critical gradient, the energy equation: the method behind DeltaEC)](https://doi.org/10.1007/978-3-319-66933-5) — book — cited (verify at link)
- J. Acoust. Soc. Am. (G. W. Swift): [Thermoacoustic engines, JASA 84, 1145 (1988): standing-wave stack, critical temperature gradient, boundary-layer power estimates](https://doi.org/10.1121/1.396617) — web — journal article; cited (verify at link)
- Nature (S. Backhaus, G. W. Swift): [A thermoacoustic Stirling heat engine, Nature 399, 335 (1999): a regenerator and a feedback inertance give traveling-wave phasing, 30 % heat to sound, against ~10–20 % of Carnot for standing-wave stacks](https://doi.org/10.1038/20624) — web — journal article; cited (verify at link)
- Appl. Phys. Lett. (S. Backhaus, E. Tward, M. Petach): [Traveling-wave thermoacoustic electric generator, APL 85, 1085 (2004): a thermoacoustic-Stirling engine driving linear alternators, 18 % heat to electricity](https://doi.org/10.1063/1.1781739) — web — journal article; cited (verify at link)
- Steel & O'Brien: [Clamp ratings (clamp + gasket assembly): 2 in high-pressure bolted 1000 psi at 70 °F, 800 psi at 250 °F; no rating published above 250 °F](https://www.steelobrien.com/wp-content/uploads/2020/06/SOB-Clamp-Ratings.pdf) — PDF — manufacturer's rating sheet; numbers cited
- NIST Chemistry WebBook: [Thermophysical properties of fluid systems (helium, air): viscosity and conductivity behind the model's fits](https://webbook.nist.gov/chemistry/fluid/) — web — US Government data
- Xem Studio: [tools/gen_ta_modular.py: the section geometry, the network model and every number on these pages](https://xemstudio.com/) — code — CC BY 4.0
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Build order

1. Stand: rails, uprights, brackets; torch stand on the bench in front, ring toward the back; torch head in the ring with its three M4 set screws.
2. Hot section, closed end down: liner on the closed end, four hot exchanger discs on it (plates all the same way), the top one flush with the face.
3. J1: flat hot ring on the hot section's face. Stack section on top: stack into its lower end first (1.7 mm proud of the face), ambient exchanger, 9.8 spacer; lower the section onto the ring so the stack stands on the hot exchanger; high-pressure clamp.
4. Optional: fin sleeve halves round the hot exchanger band (2 × M4 × 12); shroud halves round it with the flue to the front, down to the torch (2 × M4 × 25).
5. Hanger on the stack section right under where the jacket goes, bar to the uprights. Optional water jacket 9.35 below the stack section's top face (2 × M4 × 25).
6. Cold joint B only (thermal-break spacer): Type B gasket, the 25.4 spacer, clamp.
7. The closed linear alternator module, built and checked on its own list (crown 9.00 below face A): Type B gasket on the stack section's top face (or the break spacer's), module face A down onto it, high-pressure clamp. Port cap, fill valve, gauge and relief valve on face B.
8. Load resistor and series capacitor on the coil before the burner is lit. Charge slowly through the fill valve (or run at 1 bar until J1 is verified), cooling water on, then light the torch.

All printed STLs: https://xemstudio.com/objects/list-ta-modular-free-piston-2in/stl-list.txt

## Questions

**What does each section do, and how long is it?**

Hot section: takes the heat: hot cavity (the gas spring at the hot end) and the 40 mm hot exchanger under the flame shroud (60 mm); J1 hot joint: seals at about 550 °C; the stack passes into its bore (1.7 mm); stack section: holds the temperature gradient (stack) and rejects heat (ambient exchanger, jacket) (68.65 mm); thermal break (optional): insulates the alternator from the stack section; costs power (25.4 mm); piston (in the module): the module's Ø48 clearance-seal piston cup in its 42 mm bored spool is the engine piston (42 mm); linear alternator module: mass-spring of the free piston (flexures, bounce gas) and the generator (147 mm)

**How much power does it make?**

Model: 10 W electric at 13 % on helium at 10 bar with 550 °C hot gas and ±5 mm stroke; 1.7 W on air at 10 bar; 0.10 W on air at 1 bar. It is a modest engine: the free-piston Stirling generator on this site makes several times more from the same alternator.

**Where does the thermal break go?**

At the cold joint, J2, and only as a spacer with its own two clamps: the hot joint runs at about 550 °C, too hot for an insulating ring rated to about 260 °C, and a ring in a single joint is bridged by the clamp anyway. The hot joint gets a flat ring rated for at least 600 °C.

**What changes when I swap or extend a section?**

Hot section cup-2in-60 (the build): 47 Hz, 10.2 W, 12.6 %; hot section cup-2in-90 (liner 47): 46 Hz, 11.3 W, 11.5 %; hot section cup-2in-120 (liner 77): 45 Hz, 11.6 W, 10.2 %; hot section closed-spool-2in-152.4 (liner 109.4): 44 Hz, 11.3 W, 8.9 %; stack section 30 (48.65 spool): 49 Hz, 14.2 W, 8.9 %; stack section 40 (58.65 spool): 48 Hz, 12.0 W, 10.9 %; thermal-break spacer added (J2 → 25.4 + one joint): 45 Hz, 4.3 W, 6.3 %; 3 in spool added on the cold side: 42 Hz, 0.0 W, 0.0 %; 6 in spool added on the cold side: 40 Hz, 0.0 W, 0.0 % (helium 10 bar, model).

**Is the module's piston the engine piston?**

Yes. The gas column is much shorter than a wavelength, so the piston can be anywhere along it; a separate piston and rod would only add a seal, mass and dead volume.

**Why is the efficiency low, and what is better?**

A standing-wave stack works through imperfect thermal contact and cannot reach the efficiency of a regenerator with traveling-wave phasing. A regenerator plus a feedback path (a thermoacoustic-Stirling engine, as the twin-piston and ring engines on this site) or a free-piston Stirling engine does much better from the same heat.

## Fits with

- [Build list: Linear alternator module, 2 in, closed (sealed bounce space, chargeable)](https://xemstudio.com/objects/list-linear-alternator-module-2in-closed/)
- [Thermoacoustic stack-section spools: 2 in × 48.65, 58.65 and 68.65 mm (fabrication spec)](https://xemstudio.com/objects/thermoacoustic-stack-section-spools/)
- [Thermoacoustic hot-cavity liners: Ø47.5 / Ø44 tubes that set a 40 mm hot exchanger flush with a closed hot end's face](https://xemstudio.com/objects/thermoacoustic-hot-cavity-liner/)
- [Tri-clamp thermal-break spacer, 2 in × 25.4 mm: an insulating section with its own two clamp faces](https://xemstudio.com/objects/tri-clamp-thermal-break-spacer/)
- [Thermoacoustic fin sleeve for the flame-shroud band: 2 in tube, 34 mm, fins to Ø72, ears inside Ø73](https://xemstudio.com/objects/thermoacoustic-shroud-fin-sleeve/)
- [Linear alternator module, 2 in tri-clamp: datasheet, closed and open frame (mass, stiffness, frequency, Bl, R, L, stroke, pressure) and test procedure](https://xemstudio.com/objects/linear-alternator-module-2in-datasheet/)
- [Build list: Thermoacoustic standing-wave engine, half-wave, propane, 2 in tri-clamp](https://xemstudio.com/objects/list-ta-standing-half-wave-2in/)
- [Build list: Thermoacoustic-Stirling engine with twin free-piston alternators, 2 in, pressurised](https://xemstudio.com/objects/list-ta-twin-piston-2in/)
- [Build list: Free-piston Stirling generator, 2 in tri-clamp (pressurised, linear alternator)](https://xemstudio.com/objects/list-fpse-generator-2in/)