Stirling Engine Design & Manufacture

This project involved the complete design, manufacture, and functional validation of a low-differential Stirling engine, executed across two parallel courses. In one course, I developed a full CAD definition of the engine, including part modelling, assemblies, and design optimisation. In the other, I physically manufactured, fitted, and assembled the engine from raw stock using manual machine tools, culminating in a fully operational heat engine.

The project demanded the integration of thermodynamic principles, precision mechanical design, manufacturing process planning, and iterative fitting to achieve reliable engine operation under low temperature differentials.

Problem

To design, manufacture, and assemble a fully functional low-differential Stirling engine capable of sustained rotation, while adhering to strict design, manufacturing, and safety constraints.

Key requirements included:

• Complete CAD modelling of all custom components and assemblies
• Proper integration of off-the-shelf components via a full BOM
• Manufacture of critical components using manual lathes and mills
• Precision control of fits between pistons, cylinders, shafts, and bearings
• Demonstration of functional performance (engine start-up and sustained RPM)
• Professional shop conduct, inspection gates, and documentation compliance

The engine's low operating pressure and temperature differential placed exceptional emphasis on friction reduction, sealing quality, and dynamic balance.

Approach

The project was executed along two tightly coupled tracks: digital definition and physical realisation.

On the design side, I modelled the complete engine in CAD, beginning with individual components and progressing to subassemblies and a fully constrained final assembly. Particular attention was paid to assembly logic, mate strategy, crank geometry, and piston phase relationships. Multiple assembly configurations were created to analyse piston positions at top-dead-centre and bottom-dead-centre, ensuring correct timing between the displacer and power piston.

A dedicated design exercise focused on flywheel optimisation. Within fixed geometric constraints, I evaluated trade-offs between mass, cost, and rotational inertia using a cost-weighted analytical model that accounted for material usage, waste, cut length, and pierce operations. The final flywheel design balanced sufficient inertia for torque smoothing with manufacturability and cost efficiency.

In parallel, the manufacturing component translated nominal CAD geometry into a functional physical system. Components were produced through structured machining operations including facing, turning, grooving, drilling, reaming, and parting. Critical features were inspected at defined checkpoints before progression.

A key learning outcome was selective fitting: power piston diameters were calculated based on the measured bore of the finished power cylinder rather than nominal values. This ensured appropriate sealing while minimising friction, a decisive factor in low-differential Stirling engine performance.

Final assembly involved deburring, surface preparation, bearing installation, linkage alignment, and iterative tuning to reduce parasitic losses and achieve smooth rotation.

Methods & Tools

• Parametric CAD modelling and assemblies (SolidWorks)
• Assembly mate strategy and configuration management
• Bill of Materials (custom + vendor components)
• Flywheel inertia and cost optimisation modelling
• Manual lathe and mill operations
• Speed and feed calculations
• Precision measurement (calipers, micrometers, bore gauges)
• Reaming and selective fitting techniques
• Deburring and surface finishing for low-friction systems
• Mechanical assembly and functional testing
• Engineering documentation and drawing interpretation

Outcome

• Successfully designed and assembled a complete low-differential Stirling engine
• Achieved smooth mechanical operation with sustained rotation
• Demonstrated correct piston phasing and effective torque smoothing via flywheel inertia
• Produced a complete CAD assembly with fully defined part files and BOM
• Manufactured all critical components to functional fit rather than nominal assumption
• Met all safety, inspection, and documentation requirements

The finished engine demonstrated the direct relationship between design intent, manufacturing quality, and system-level performance.

Key Leverage

This project crystallised the distinction between nominal design and functional reality.

In CAD, components fit perfectly by definition. In the shop, performance is governed by microns, surface finish, friction paths, and cumulative error. By working both sides simultaneously, I developed a systems-level understanding of how thermodynamics, kinematics, tolerances, and manufacturing discipline converge in a real machine.

The Stirling engine served as a precise proving ground for professional mechanical engineering practice: if any single component was careless, the entire system failed to run. The final success was therefore not the result of one good part, but of disciplined execution across the whole system.