Stirling Engine Model with Low-Temperature Differential Drive

Heat engines are usually associated with flames, boilers and hundreds of degrees — but the Stirling cycle has a quieter cousin. A low-temperature-differential (LTD) Stirling engine runs on a temperature difference of only a few degrees Celsius: published demonstrators are documented running on differentials of about 7–10 °C, even from a mug of hot water or a warm hand. That makes the LTD engine a perfect desktop demonstrator of thermodynamics: no combustion, no pressure vessel, no electricity — just air being shuttled between a warm plate and a cool one, and a flywheel that keeps turning while the difference persists. This project builds that demonstrator in gamma configuration — the large displacer cylinder and the small power piston in separate cylinders, coupled through one crankshaft — from acrylic, brass and steel parts cut and fitted with ordinary workshop tools, following the published hardware-store LTD build methodology.

1. The engine is placed so its aluminium bottom plate sits over a mug of hot water at approximately 90 °C (recommended heat source), while the acrylic top plate stays near room temperature — this temperature difference is the engine's only fuel. Allow approximately 30–60 s of preheat (expected) for the bottom plate to warm through.
2. Give the flywheel a gentle flick by hand to start — LTD models rarely self-start, and this is documented behaviour, not a defect. The flywheel's inertia carries the mechanism through the points of the cycle where the gas produces no useful force.
3. Expansion stroke: with the displacer rising, the trapped air is shoved down against the heated bottom plate. The air warms and expands, pushing the small power piston outward — this is the working stroke that feeds energy into the crankshaft.
4. Transfer: the crank, with the displacer leading the power piston by approximately 90° (design target), now drives the displacer downward, shifting the same trapped air up to the cool top plate region. The air cools and its pressure falls.
5. Compression stroke: with gas pressure low, the flywheel's momentum drives the power piston back inward, re-compressing the cooled air — this costs less work than the expansion stroke returned, so every revolution adds net work to the flywheel.
6. The cycle repeats as long as the temperature difference persists; as the water cools toward room temperature the engine slows and stops, which itself demonstrates that a heat engine needs a ΔT, not heat alone.

Tech stack:

• Aluminium sheet and acrylic sheet fabrication
• Brass cylinder boring and honing
• Graphite piston fitting (oil-free, low-friction)
• Steel-wire crank linkages
• Flywheel machining and balancing
• Stirling thermodynamic cycle theory
• LTD engine tuning (clearance, phase angle)
• Drill-press and hand-tool fitting

• [Large transparent displacer chamber] A wide, shallow displacer cylinder — approximately 200 mm diameter — with a clear acrylic top plate, so the thin displacer disc shuttling air between the hot bottom plate and the cool top plate is visible while the engine runs.
• [Gamma layout with separate power cylinder] Displacer and power piston sit in separate cylinders on one crankshaft, the defining gamma arrangement — mechanically simpler to build than beta, since the power piston never shares its bore with the displacer and no hot sliding seals are needed.
• [Low-friction brass power cylinder] A close-fitting graphite piston runs in a honed brass bore with no oil and no rings; at low ΔT the pressure force on the piston is tiny, so this near-frictionless fit is what makes the whole engine possible.
• [Crank and flywheel drive] Steel-wire linkages connect the displacer rod and power piston to a crank with the displacer leading by approximately 90° (design target), and an aluminium flywheel carries the rotation through the cycle's weak points.
• [Runs on a cup of hot water] The signature LTD behaviour: no flame, no burner, no electricity — set over a mug of approximately 90 °C water, it preheats in about 30–60 s and keeps turning while the temperature difference persists.
• [Tunable at build time] Displacer clearance, linkage lengths and crank phase angle are adjustable in the build, so the student can experiment with what makes an LTD engine start and keep running (configurable at build).
• [Future scope — not built] A micro-generator or tachometer output is deliberately not included; it is listed as future scope in the report, not part of the delivered build.

• Working LTD Stirling engine model, assembled and tuned
• Dimensioned 2D drawings of the displacer chamber, crank layout and flywheel
• Complete material list with hardware-store sourcing notes
• Assembly and tuning guide (displacer clearance setting, phase-angle adjustment, friction debugging)
• Theory notes: Stirling cycle derivation, gamma layout, regenerator concept, why LTD works
• Project report PDF (background, methodology, cycle analysis, results discussion)
• PPT presentation for final review
• Viva Q&A preparation document (thermodynamics, Stirling cycle, gamma vs alpha vs beta, LTD operation)