Stirling Engine Model(Educational Model)

Team 10

Markis Loveless

Frank Foshee

Aaron Poche

William Mwangi

Barry Bosnyak

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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 Contentsii

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

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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.

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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 Engine2

Figure 1 - Ideal Stirling Cycle [2]

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

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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).

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Figure 2 - Stirling Engine Prototype

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

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Figure 3 - Engine Schematic

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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

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Table 1 - Required Tools

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.

Figure 4 – Project Timeline

Project Budget

The original budget of the project was $50 to produce the prototype with the goal a final per-manufactured-kit price of $12.00. On both accounts the budget was not met. While the total materials used in the engine prototype was $49.04 (tax not included), a special miter saw blade was purchased at a cost of $20 to cut the acrylic pipe to length. This pushed the total money spent on the prototype to just below $70.00. All other tools used were already owned by various team members. The final per-kit price is estimated to be $14.30 (tax not included). It was found that the smaller hardware components of the engine, not the larger - seemingly more important parts, increased the overall price more than originally anticipated.

The man-hours for the engine research, design, assembly, testing and project proposal (final report not included) totaled approximately 140 hours.

Results and Conclusions

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This new design of a low-cost concept model of the Stirling engine is an example of the gamma-style engine. It operates with a low heat source applied to the bottom plate of the engine, while using an ice cube on the top plate of the engine as a cooling feature. The main benefit of the engine is that it is a low cost design with materials that can be obtained from local suppliers (most from retail hardware stores) for a minimal cost, and by items that can be found in most households. Another great feature to this design is that there is little room for assembly errors.

The total estimated cost of this prototype is approximately $70.04. To mass-produce this design in a numerable quantity, the estimated cost is approximately $14.30 per unit to construct. The estimated time of research, construction and testing has totaled to be 140 man-hours.

This design operates in an effective and reliable manner. All of the components have been arranged in order to maneuver with a steady stream of motion. The cylinder piston rods and tubes provide the perfect variance for little friction. The regenerator that is placed inside the chamber allows the hot and cold air to flow freely from the top to the bottom of the chamber.

The Stirling engine model operates with little complications. However, further adjustments and tests are recommended to achieve more efficiency and increased engine performance and reliability before a final kit version is constructed. The most critical of these adjustments is the re-evaluation of the fly- wheel weights to allow the wheel to turn with a more consistent motion.

This new design of a gamma-style Stirling engine should allow middle and high school aged students to pursue an interest in scientific learning. This design allows students to work together in a teamwork atmosphere while applying some simple rules for the application of engineering and science. This design is a low cost design, while being reliable and robust. The design allows viewing of the internal components, and is easy to assemble.

Evaluation

The engine does work. It will run continuously with the power of 1 votive candle alone. Note that not all candles are created equal and better results were achieved with a thicker wick. With a one candle-no ice setup, the engine runs, but just barely so. There was significant slowing at one point in the rotation. This was corrected by placing 2 votive candles under the engine as well as ice on the top. This setup allowed smooth operation with no visible slowing in the cycle.

Due to time constraints, the team did not test the measurable work of the engine. Once further adjustments have been made to increase the efficiency and speed of the engine, tests will be conducted to measure the work that the engine can perform. The engine RMP was measured with an optical tester and a reflective material attached to the edge of the flywheel. The engine achieved approximately 90 RPMs.

It is believed that were 3 key design features that lead to successful operation beyond the obvious design of the engine:

1. Good fitMuch care was taken to find material for parts that fit very well together. For instance, the power

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piston and its cylinder fit close enough to create a seal but without causing friction during movement.

2. CounterbalanceThe engine would just not run without weighting the flywheel in such a way as to offset the weight of the power and displacer pistons. This weighting was done by taping pennies towards the outer edge of the flywheel. It should be noted that before trying to offset the weight, the team attempted to add a great deal of mass (proportionately) to the flywheel in an attempt to “push through” the bottom of the cycle. This was not the least bit successful. So it is determined that proper placement of flywheel mass in relation to piston weight is key, as opposed to amount of flywheel mass.

3. RegeneratorWhile it was not enough to overcome the lack of a counterbalanced flywheel, the addition of a regenerator to the displacer piston did produce better results than had previously been observed. Thus it is believed that a regenerator is a key design element for facilitating proper heat-transfer within the main chamber.

Recommendations

After testing the workability of the model, the team recommends the following:

1. For the flywheel to serve the intended purpose, proper counterbalance of the weight is necessary to help to keep the motion in progress without stopping. While the engine is functional, we believe that further adjustment is needed to find the optimal total mass of the flywheel and counterbalance of the pistons’ mass to produce smoother motion.

2. There is only one permanent connection necessary for the engine: the soldering of the connecting rod ends to the rods themselves. It is very important that these connections are made as precisely as possible for the engine to operate properly. As such, it is recommended that a better connection method be established so that adjustments can be made in the classroom setting. One possible solution is to use the same rod ends, but with a thicker rod material, so that the ends can be crimped on without the need for a permanent bonding agent.

3. To further reduce piston weight and increase engine efficiency, it is recommended that the power piston should be made hollow on the bottom.

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4. Rubber grommets were used as bushings for the crankshaft. As rubber is not a preferred material for a low-friction connection, it is recommended that another material be used for these bushings. Nylon or Teflon is recommended.

5. Some of the methods to fabricate the parts of engine were not very precise; the displacer piston was cut in a way that caused it to have a rough edge and the placement of the displacer piston’s extension rod was not perfectly centered in the upper (cold) heat transfer plate. These two imperfections resulted in the displacer piston needing to be smaller than would have otherwise been necessary in order to ensure that it did not make contact with the interior wall of the main chamber; this causes extra dead space in the chamber. It is recommended that a better cutting method be developed for the displacer piston; and that the hole for the displacer piston’s extension rob be more precisely drilled in the upper (cold) heat transfer plate.

6. To increase the efficiency of the engine, the introduction of a regenerator was necessary. On the prototype, the regenerator material covered the entire top and bottom surface of the displacer piston. It is recommended to test individual regenerators which cover only the 4 individual holes to reduce the total weight of the displacer piston.

References

[1]. Tech thermal energy cooperation; home page. History of Stirling engine, www.thermalengines.com/tech.html, (02/18/2009)

[2]. Wikipedia free encyclopedia. Stirling engine, www.wikipedia.org/wiki/stirlin-engine, (02/18/2009)

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[3]. Solar navigator; history of Stirling engine home page. www.solarnavigator.net/stirling_engine, (02/20/2009)

[4] Stirling Energy Systems, Stirling engine. http://www.stirlingenergy.com/default.aspe, (03/20/2009)

[5] Pure energy systems, Stirling engine. www.pewiski.com/index.plp/stilring-engine, (02/20/2009)

General knowledge recourses (not referenced in paper)

http://www.sensi.org/~svo/stirling/

http://www.solarheatengines.com/2008/06/09/simple-stirling-1-performance-with-and-without-regenerator/

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