# LTD (low-temperature differential, coffee-cup) Stirling kit: plates, chamber ring, displacer, top plate with syringe cylinder

> How do I build a coffee cup Stirling engine and what are the plate and displacer dimensions?

- Category: Stirling engines › Can & LTD engines
- Kind: Hardware
- Basis: Senft / Kolin LTD gamma proportions: displacer diameter ≫ stroke (D/stroke ≈ 15–25), displacer swept volume 10–20× the power piston swept volume, thin chamber (height ≈ 1.5 × displacer thickness)
- Page: https://xemstudio.com/objects/ltd-coffee-cup-engine-kit/ · JSON: https://xemstudio.com/api/geometry?object=ltd-coffee-cup-engine-kit · Plain text: https://xemstudio.com/objects/ltd-coffee-cup-engine-kit/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- An LTD runs on a cup of coffee or a warm hand: a wide thin chamber between two conductive plates (aluminium, 1–3 mm), a foam displacer filling it with a 2 mm gap, a tiny power piston (syringe) and a crank with the displacer 90° ahead. Everything is about low friction and large area: Ø100 chamber, 16 mm tall, Ø96 × 8 foam displacer, 6 mm displacer stroke, 10 mL syringe at 4–6 mm stroke.
- Plates: the printed bottom plate is a drilling template; the real plate must be metal (aluminium 2 mm) for heat to get in. The top plate can be printed (it is the cold side: PETG, with the syringe socket and rod guide) or metal with the printed boss glued on. Chamber ring: printed, 10 mm wall, sealed to the plates with silicone or a 1.5 mm O-ring in the Ø100 groove (a static seal between ring and plates; nothing moving touches it, the piston and rod run on clearance fits).
- Numbers: 10 mL syringe (Ø14.5) at 5 mm stroke = 0.83 mL; displacer Ø96 × 6 mm stroke = 43 mL → ratio 52 (high, fine for ΔT 20 K). Flywheel Ø100 CD or the crank kit. Runs at 1–3 Hz on 15–20 °C ΔT.
- LES: the warm-hand demonstrator that opens the thermodynamics video.

## Sizes

| part | Ø100 kit | Ø150 kit |
|---|---|---|
| chamber bore × height | 100 × 16 | 150 × 16 |
| displacer | Ø96 × 8 foam, 6 stroke | Ø146 × 8, 6 stroke |
| plates | Ø120 × 2 Al | Ø170 × 2 Al |
| power piston | 10 mL syringe, 5 stroke | 20 mL syringe, 5 stroke |
| displacer : piston volume | ~50 | ~60 |
| ΔT to run | 15–20 K | 10–15 K |

## Where the files are

- Wikipedia: [Stirling engine](https://en.wikipedia.org/wiki/Stirling_engine) — web — CC BY-SA 4.0 (text)
- James R. Senft: [An Introduction to Low Temperature Differential Stirling Engines](https://www.stirlingengine.com/) — book — Moriya Press (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (9 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/index.json

- ltd-100-bottom-plate-template: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-bottom-plate-template.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-bottom-plate-template.stl) — bottom plate template Ø120 × 3 (cut aluminium to this; the printed one is for layout), 6 × M3
- ltd-100-chamber-ring-h16: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-chamber-ring-h16.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-chamber-ring-h16.stl) — chamber ring Ø100 bore × 16 tall, 10 mm wall, 6 × M3; the displacer runs inside
- ltd-100-displacer-foam-template: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-displacer-foam-template.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-displacer-foam-template.stl) — displacer template Ø96 × 8 (cut from foam; 2 mm radial gap)
- ltd-100-top-plate-syringe: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-top-plate-syringe.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-top-plate-syringe.stl) — top plate Ø120 with 3/32 in rod guide boss on centre and a 10 mL syringe socket as the power cylinder
- ltd-150-bottom-plate-template: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-bottom-plate-template.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-bottom-plate-template.stl) — bottom plate template Ø170 × 3 (cut aluminium to this; the printed one is for layout), 6 × M3
- ltd-150-chamber-ring-h16: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-chamber-ring-h16.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-chamber-ring-h16.stl) — chamber ring Ø150 bore × 16 tall, 10 mm wall, 6 × M3; the displacer runs inside
- ltd-150-displacer-foam-template: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-displacer-foam-template.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-displacer-foam-template.stl) — displacer template Ø146 × 8 (cut from foam; 2 mm radial gap)
- ltd-150-top-plate-syringe: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-top-plate-syringe.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-top-plate-syringe.stl) — top plate Ø170 with 3/32 in rod guide boss on centre and a 10 mL syringe socket as the power cylinder
- ltd-crank-stand-pair: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-crank-stand-pair.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-crank-stand-pair.stl) — crank stand 45 mm tall with a 623 pocket (print two)

## How it fits

### Connects to
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/): crank disc, con-rods, flywheel
- [Syringe pistons: ring-less printed pistons with a con-rod fork for 10, 20 and 30 mL syringe barrels](https://xemstudio.com/objects/syringe-pistons/): power piston in the top plate's syringe

### Alternatives
- [Can-engine printed kit: can caps, diaphragm clamps, displacer guides, crank stands, CD/platter hubs, syringe cradles](https://xemstudio.com/objects/can-engine-printed-kit/): can engine

### Also needed
- [Plastic syringes 1–60 mL as pistons and cylinders](https://xemstudio.com/objects/plastic-syringes/): barrel
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/): 623 in the stands

## Questions

**What size displacer for a coffee cup Stirling engine?**

Ø96 × 8 mm foam in a Ø100 × 16 chamber with a 6 mm stroke; the displacer swept volume should be 20–60× the power piston's.

**Why does an LTD have such a big displacer?**

With only 15–20 K of temperature difference the gas changes pressure very little, so you need a lot of gas shuttled per cycle and almost no friction: big area, small stroke, tiny piston.

**Can the plates be 3D printed?**

The cold (top) plate yes; the hot plate must be metal (2 mm aluminium) or the heat never gets into the gas. Use the printed bottom plate as the drilling template.

## Fits with

- [Plastic syringes 1–60 mL as pistons and cylinders](https://xemstudio.com/objects/plastic-syringes/)
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/)
- [CDs, DVDs and hard-drive platters as flywheels](https://xemstudio.com/objects/cd-dvd-and-drive-platters/)
- [Hobby brass tube (K&S), 1/16–21/32 in and 1–12 mm, telescoping](https://xemstudio.com/objects/brass-tube-ks/)
- [Balloons, gloves, inner tubes and silicone sheet as diaphragms](https://xemstudio.com/objects/balloons-and-diaphragms/)
- [Stirling engine configurations: which type needs which parts on this shelf](https://xemstudio.com/objects/stirling-configurations-guide/)