Engineering Portfolio

Stirling Engine

What is a Stirling Engine

A gamma Stirling engine is a closed-cycle, external-combustion heat engine where a displacer piston shuttles gas between a hot and cold region, while a separate power piston converts the resulting pressure changes into mechanical work.

Stirling engines in general (including alpha and beta types) are used in more demanding applications such as solar dish power systems, industrial waste-heat recovery, silent generators, submarine propulsion, cryogenic coolers, and residential combined heat-and-power units. Overall, gamma engines are ideal for demonstration and small-scale mechanical work, while more advanced Stirling designs serve specialized industrial, scientific, and energy-generation roles.

Thermodynamics of a Stirling Engine

The Stirling engine cycle is a repeating process where a sealed amount of gas is heated and cooled to make the engine run. First, the gas is moved to the hot side, where it heats up, expands, and pushes the power piston outward to produce work. Next, the gas is shifted to the cold side, where it cools down and its pressure drops. The piston then moves inward easily, compressing the cool gas with little effort. Finally, the gas is moved back toward the hot side, warming up again and preparing for the next expansion.

This continuous sequence of heating → expansion → cooling → compression creates a loop that drives the engine smoothly, using any external heat source.

My Goal

My goal was to build a gamma Stirling engine that successfully demonstrated the Stirling cycle. I designed and assembled the displacer cylinder, power piston, crankshaft, and regenerator, ensuring the correct phase difference so the gas moved between hot and cold sides properly. By minimizing dead volume and optimizing heat transfer, the engine ran smoothly, clearly showing the heating, expansion, cooling, and compression steps. Completing the project gave us hands-on experience in thermodynamics, and mechanical design, and it was rewarding to see outher engine operate successfully.

Subsystems

Drivetrain

Requirements and Specifications

The drive train of a Stirling engine MUST

Maintain continuous piston motion
Ensure proper 90-degree phase relationship between pistons
Minimize mechanical energy loses (i.e. friction)

Design Features

Flywheel and Drivetrain Design

The flywheel is a critical component for maintaining stable and continuous operation in the Stirling engine. Real engines deviate from ideal Stirling cycle assumptions due to finite heat transfer, thermal resistances, and temperature gradients. These effects produce fluctuations in gas pressure and piston force, which can lead to intermittent or unstable motion. The flywheel provides rotational inertia, storing energy during high-torque periods and releasing it during low-torque portions of the cycle, thereby smoothing torque fluctuations and preventing stalling.

Both the power and displacer pistons are mechanically coupled to a common drive shaft through the flywheel. The flywheel’s inertia maintains angular momentum through regions of low or negative torque, enabling continuous piston motion despite variations in thermodynamic forcing. This inertial coupling is essential for overcoming transient losses and ensuring consistent engine operation.

Shaft and Bearings

The drivetrain consists of a single shaft supported by two ball bearings mounted in an aluminum frame. The flywheel is rigidly attached to the drive shaft using clamping flanges, ensuring that the shaft and flywheel rotate as a single rigid body. Spacers were designed to seat against the inner races of the bearings, preventing excessive bearing preload and minimized frictional losses. The use of 3D-printed spacers also allowed precise control of spacer length for maintaining shaft alignment and preventing unintended side loads that could introduce parasitic forces into the linkage system.

Piston Phase Control and Linkages

Proper Stirling engine operation requires a 90-degree phase offset between the power and displacer pistons. To maintain this relationship:

A custom clamp indexes both piston linkages to the drive shaft at predetermined angular positions
Each linkage uses a 4-40 screw as a crank pin
Ball bearings and 3D-printed spacers are incorporated to reduce friction and allow smooth relative motion
Nylon lock nuts prevent loosening due to vibration

This arrangement ensures that the pistons move through their full stroke range without interference and maintains consistent kinematics.

Flywheel Geometry and Material

Diameter: 3.0 in
Material: Aluminum, chosen for low cost, manufacturability, and adequate stiffness

The flywheel design incorporates modular stainless-steel perimeter weights:

Attachment method: Screws and nylon lock nuts
Purpose: Adjust total rotational inertia without redesigning the flywheel

Design Rationale

One important consideration before discussing the overall design rationale of the system is a key specification that was known to the me early in the project. According to the manufacturer’s specification sheet, the selected power piston is capable of producing a maximum force of approximately 30 lbf before failure. Consequently, a large portion of the drivetrain and supporting components were designed and specified around this limiting force to ensure safe and reliable operation.

Flywheel Inertia

The flywheel was fabricated from aluminum so that it could be readily waterjet cut while still providing sufficient stiffness and dimensional stability for my application. Using SolidWorks, the area moment of inertia of the original flywheel geometry was calculated and found to be approximately 4.28 Kg*m^2. Increasing the rotational inertia of the flywheel would improve rotational smoothness and assist with engine startup by reducing cyclic speed fluctuations.

To increase the flywheel inertia while maintaining design flexibility, 1/8-inch steel plates were sourced and water-jet cut into removable weights that were fastened to the flywheel. SolidWorks analysis showed that each added weight increased the flywheel’s rotational inertia by approximately 11.01 Kg*m^2. Given enough space to add 4 sets of weights, this totals to a ~93% increase in rotational inertia (totaling 48.6 Kg*m^2) This modular approach allowed the total inertia to be adjusted experimentally, preventing the system from being over-initialized in the event that the engine output torque was lower than predicted. This precaution was especially important given that preliminary empirical testing of a smaller demonstration engine indicated that first-order thermodynamic models tended to overestimate actual performance.

Bearing Selection

Bearing selection was a critical aspect of the mechanical design, as the bearings directly influence frictional losses and overall system reliability. Each bearing was expected to experience a radial load on the order of the piston force, approximately 30 lb. Bearings with a dynamic radial load capacity of approximately 140 lbs were selected. This resulted in a safety factor of approximately 4.7, providing sufficient margin for continuous operation and accounting for uncertainties in loading, alignment, and transient forces. Open bearings were selected so they could be lubricated if needed.

SolidWorks Motion Analysis

Linkage dimensions were based on reference designs from similar Stirling engines and fabricated from aluminum to minimize mass while providing adequate strength.

Kinematic compatibility was the primary design concern. A motion study in SolidWorks defined the geometry of both the power and displacer linkages, ensuring that each piston achieved its required stroke and maintained the correct phase relationship. Linkage lengths were adjusted so that maximum piston travel was limited to 1/8 in short of the cylinder ends, preventing interference and mechanical damage. These constraints established the final linkage geometry and ensured reliable operation throughout the full range of motion.