Stirling Engine Model

(Educational Model)

Team 10

Markis Loveless

Frank Foshee

Aaron Poche

William Mwangi

Barry Bosnyak

Abstract

The NREL has asked for a proof of concept model demonstrating a simple and low cost Stirling engine kit for young students to assemble and learn from. This proposal of a Stirling engine design was intended for educational purposes. Low cost, safety, reliability, portability, and educational potential were the primary considerations for the engine kit.

This design allows viewing of the internal components for educational learning via a simple, easy to understand setup and transparent components. It is compact and robust while replicating an inexpensive model that allows students of all ages to work and learn as a team through hands-on interaction. The total cost of the prototype, including the materials and the purchase of some special tools, was approximately $70.00. A mass produced kit version of this Stirling engine should cost $14.30 per unit when manufactured in a more numerous quantity. This prototype design required an approximate 75 man-hours to produce. This time included the total hours spent on research, development and construction. The best attraction to this Stirling engine design is that it is relatively easy to assemble for school children and once assembled serves as a robust working scientific model allowing for easy observation of the moving parts.

After the design and construction phases were complete, a working prototype of a gamma-configuration Stirling engine was accomplished, topping out at a speed of approximately 90 RPM. The total cost of the prototype was over the original estimated budget, due to special tooling that needed to be purchased. The model boasts a simple design sequence with readily and easily available parts and is within the assembly and operational capabilities of the desired audience of middle-school-aged children and above. Needing only a votive candle as an energy source, the model is safe for the desired audience to operate.

Table of Contents

Introduction and Project Description 1

Requirements of the Stirling Engine Model 1

Limitations of the Stirling Engine Model 1

Background 2

Thermodynamics 2

Advantages of the Stirling Engine 3

Disadvantages of the Stirling Engine 3

Engine Design 4

Mechanical Description 4

Manufacturing 5

Primary Fabrication Procedures 5

Secondary Fabrication Procedures 5

Parts List 6

Engine Schematic 7

Required Tools 8

Project Schedule 9

Project Budget 9

Results and Conclusions 10

Evaluation 10

Recomendations 11

References 12

Appendix 1 – Timesheets

Appendix 2 – Instruction Sheets

Introduction and Project Description

The purpose of the project was to design and assemble a working prototype of a Stirling engine. The engine was to be designed with the ultimate goal of serving as the basis for a kit that will be made available to middle and high school science classes to promote student interest in engineering. Given this goal, there were various requirements and limitations of the design.

Requirements of the Stirling Engine Model

· Must function, after manual start-up, by the Stirling cycle of heat transfer alone.

· Must allow viewing of moving parts.

Limitations of the Stirling Engine Model

· Must be within the assembly skills of middle school-aged children.

· Must not require intricate machining techniques to fabricate.

· Must be affordable.

· Ideally, parts should be available locally.

Background

The Reverend Robert Stirling, a 26-year-old Scottish Minister, originally patented the Stirling-Cycle Engine in 1816. He first proposed the idea of the regenerator or economizer as a way of using less coal in industrial heating processes and went on to develop the Stirling engine as a safe and efficient alternative to the steam engine. Once popular as a small source of mechanical power, with no potentially explosive boiler like a steam engine, Stirling engines were used to pump water and run machinery, often in remote rural areas. Stirling engines would not explode because the pressures were not elevated to that level. [1] The machine simply stopped if the heater section failed. [2] Today, Stirling engines are used in some very specialized applications, like in submarines or auxiliary power generators, where quiet operation is important. The engine can run on a variety of fuel sources and has a work output far closer to the theoretical ideal efficiency than most engines.[3] In California, Stirling Energy Systems is planning to build two solar power plants made up of arrays of mirrored dishes. Each dish will follow the path of the sun and focus sunlight onto the hot side of an electricity producing Stirling engine mounted to the dish. [4]

Thermodynamics

A Stirling “Air” engine is a mechanical device, which operates on a closed regenerative thermodynamic cycle with cyclic compression and expansion of the working fluid (air) at different temperature levels. The ideal Stirling pressure/volume cycle is represented in Figure 1. Stirling engines are unique heat engines because their theoretical efficiency is nearly equal to their theoretical maximum efficiency, known as the Carnot Cycle efficiency.[2] Stirling engines are powered by the expansion of a gas when heated, followed by the compression of the gas when cooled. The Stirling engine contains a fixed amount of gas that is transferred back and forth between a cold end and a hot end. The displacer piston moves the gas between the two ends and the power piston is driven due to the change in the internal volume as the gas expands and contracts. When one end is heated and the other kept cool, useful work can be obtained through a rotating shaft. It is a closed machine with no intake, or exhausts that result in very quiet operation. Anything that gives off heat can be used to run a Stirling engine. [3]

Advantages of the Stirling Engine

· The heat is external and the burning of a fuel-air mixture is accurately controlled.

· A continuous combustion process is used to supply heat, so emission of unburned fuel can be greatly reduced.

· Most types of Stirling engines have the bearing and seals on the cool side; consequently, they require less lubricant and last significantly longer between overhauls than other reciprocating engine types.

· The engine as a whole is much less complex than other reciprocating engine types. No valves needed. Fuel and intake systems are very simple.

· They operate at relatively low pressure and thus are much safer than typical steam engines.

· Low operating pressure allows the usage of less robust cylinders.

· They can be built to run very quietly and without air, for use in submarines.

· They hold promise as aircraft engines. They are quieter, less polluting, gain efficiency with altitude are more reliable due to fewer parts and the absence of an ignition system, produce much less vibration (airframes last longer) and safer, less explosive fuels may be used. [5]

Disadvantages of the Stirling Engine

· Stirling engines, especially the type that run on small temperature differentials, are quite large for power that they produce, due to the heat exchangers.

· A "pure" Stirling engine cannot start instantly; it literally needs to "warm up". This is true of all external combustion engines, but the warm up time may be shorter for Stirling.

· Power output of a Stirling is constant and hard to change rapidly from one level to another. [5]

Engine design

The gamma-style was chosen because of the team’s relatively low level of engineering knowledge and the considerable amount of information available on gamma-style Stirling engines. It was also hypothesized that this design would allow for relatively affordable components compared to other Stirling engine designs.

Mechanical Description

The main chamber is constructed of 3” clear acrylic pipe to allow viewing of the displacer piston. The operation of the power piston is also visible because, at the height of its stroke, it protrudes out of the top of the power cylinder. The heat transfer plates are lids from a food can (Inside of lids face outward on engine.) Plumber’s putty is used to seal the connection of the acrylic pipe with the lids. Aluminum “mud ring” electrical junction box covers are used along with 3” machine screws and wing nuts to clamp the main chamber shut. The mud rings also serve as additional area for the heat transfer plates. The displacer piston is a disc of 3/16” foam board (the kind used for presentations). The displacer piston has four 0.5” holes drilled through it. Aluminum screen window mesh (cut in a circle with the same diameter as the displacer piston) is placed on the top and bottom of the foam disc. This mesh, in conjunction with the holes in the foam, functions as a regenerator. The screen mesh also holds the foam in place on the displacer piston extension shaft. The extension shaft and the power piston are made of aluminum rod, 0.125” and 0.375” respectively, with eye-screws inserted into the tops of each. The bushing for the extension rod is 0.125” ID copper tubing, while the power cylinder is 0.375” ID brass tubing; both are connected to holes in the top plate with rubber grommets. The base and towers are made from plumbing hanger strap and bent to shape. The crankshaft is bent to shape from a stout wire coat hanger. The flywheel a compact disc with a hub of a champaign cork whittled to size. Pennies, taped to the edge of the flywheel, serve as extra mass as well as counterbalance. Connecting rods are lengths of music wire with electrical terminal connectors crimped and soldered to the ends. The original holes in the connectors needed to be widened to fit around the crankshaft and eye-screws.

With the ends of the connecting rods to the music wire being the only permanent connections, the engine can be assembled and disassembled, and adjustments made, repeatedly. The engine assembly requires very little or no tools, depending on the amount of fabrication done prior to kit component packaging.

The parts, including their materials and costs, are listed in the parts list (Table 1). The ID designations in table 3 are used to label parts in the engine schematic (Figure 3).

Manufacturing

The prototype is composed of various materials that are available at a variety of local suppliers. While no precision machining is necessary in the manufacturing of the engine components, some pre-fabrication procedures will be necessary to prepare the kit components for easy assembly by students. Pre-fabrication procedures are divided into two categories: primary and secondary. Primary pre-fabrication procedures consist of those not possible in, or not recommended for, the classroom setting. These procedures will need to be completed prior to kit packaging. Secondary pre-fabrication procedures consist of those that could be performed by students in a classroom setting but may also be performed prior to kit packaging to further improve the ease of assembly in the classroom setting. Even if all secondary procedures were performed in the classroom, there would be no need for students to use powered tools to assemble the engine.

Primary Fabrication Procedures

· Cutting of acrylic pipe to length using a miter saw equipped with an appropriate plastic cutting blade (80-tooth carbide tipped)

· Cutting of aluminum rods to length using a chop saw equipped with a grinding wheel

· Drilling of holes to receive eye screws in aluminum rods using either a powered drill and a steady hand or a drill press (preferred)

· Drilling of holes in upper heat transfer plate to receive power cylinder and extension rod bushing using either a powered drill or a drill press (preferred)

Secondary Fabrication Procedures

· Cutting of foam sheets into discs using either scissors or a yet to be determined circular cutting device (possibly a cookie-cutter)

· Cutting of copper and brass tubes to length using a pipe cutter

· Removal of food can lid using can opener (safety model that does not leave sharp edges is preferred)

· Bending of hanger strap to form support towers using either two pairs of pliers or a jig

· Cutting of music wire to form connecting rods using wire-cutters

· Cutting and bending of coat hanger to form crankshaft using wire-cutters and needle-nose pliers and/or a jig

Parts List

Table 1 – Parts List

Figure 3 - Engine Schematic

Required Tools

Some tools, powered tools and hand tools, will be required to fabricate and assemble the prototype (Table 2). Tools with no listed price are already in the possession of team members and will not add to the cost of building the prototype, though they may need to be acquired for future manufacturing if performed by a party other than Team 10.

Project Schedule

After the proposal for the Team 10 Stirling engine was approved on the third of March, fabrication and assembly of the model took place over the next two months. The team started by collecting the parts needed for assembly then began the first production of the model. Once the first tests of the model were complete the results were unsuccessful. Team 10 did further research and fabrication. After many trial runs and adjustments, a successful Stirling engine was produced. On the 23rd of April the team demonstrated the prototype for the class. For exact start to finish dates refer to Figure 4.