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REDLINING AT 20 RPMThe Stirling engine has long captivatedinventors and dreamers. Here are com-plete plans for building and operating atwo-cylinder model that runs on almostany high-temperature heat source.Stirling engines are external combustion engines, which

means no combustion takes place inside the engine andthere's no need for intake or exhaust valves. As a result,Stirling engines are smooth-running and exceptionally quiet.Because the Stirling cycle uses an external heat source, it

can be run on whatever is available that makes heat - anythingfrom hydrogen to solar energy to gasoline.Our Stirling engine consists of two pistons immersed in two

cans of water. One can contains hot water and the other cold.The temperature difference between the two sides causes theengine to run. The difference in the hot and cold side tempera-tures creates variations in air pressure and volume inside theengine. These pressure differences rotate a system of inertialweights and mechanical linkages, which in turn control thepressure and volume of the air cylinder.

Set up: p.94 Make it: p.96 Useit: p.IOl,

William Gurstelle serves en MAKE's l'echnlCal Aavlsery Saara ana IS the author of Backyard BallistIcs and Adventuresfrom the Technology Underground

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Every heat engineworks on a cycle. Whenheat is applied to aworking fluid, the fluidundergoes some sort ofchange - its pressure,volume, or temperatureis increased by theadded heat - and in sodoing, the fluid doesmeaningful work onits surroundings. Workcould mean makinga piston move, or aturbine, or some othermechanical object. TheStirling cycle is a four-step process, using hotair as its working fluid.

Four Steps of the Stirling Cycle

0 e e 0iI~ ij~+(}~+G ~i+G 1+0o-t (}+g 0+U ~ o-]"1< ~ ~ ~*COOLING EXPANSION COMPRESSION HEATINGCold piston (left) moves Hot air is forced to the As air in the cold With the cold pistonupward by flywheel iner- left cylinder, forcing the water contracts, fully down, most airtia, drawing hot air over cold piston up. This is the cold piston is on hot side andto cold side. the power cycle. moves down. getting reheated.

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THE STORY OF THESTIRLING ENGINEAll engines run on heat cycles. More properly calledthermodynamic cycles, each of these cycles hasa name. Cars run on the Otto cycle, trucks on theDiesel cycle. Power plants often run the Rankine,while gas turbines run the Brayton cycle.

One cycle in particular has long captivated inven-tors and dreamers - the Stirling cycle. The Stirlingcycle was among the first of the thermodynamiccycles to be exploited by engineers. Comparedto other engine types, it is ancient. When it waspatented as a new type of engine by a Scottish clericin 1816, scientists hadn't even come up with theidea of thermodynamic cycles.

Robert Stirling, a young Scottish Presbyterianassistant minister, had the idea for a new type ofheat engine that used hot air for its working fluid.Until then, the steam engines of Watt and Newcomenwere the only heat engines in use.Stirling Engines Go to Work ...and Are Laid to RestStirling's idea was to alternately heat and cool air ina cylinder using articulated mechanical arms anda flywheel to coax the machine to run in a smooth,endless cycle.

Although complex and expensive for its time,Reverend Stirling made it work. As early as 1818,his engine was in use pumping water from a stonequarry. By 1820, a 45-horsepower Stirling enginewas driving equipment in the Scottish foundrywhere his brother worked.

Auto manufacturers have experimented with theStirling for years. Its numerous good qualities makethe Stirling an attractive candidate to replace oraugment internal combustion engines.

Automakers worked closely with the federalgovernment from 1978 to 1987 on Stirling engineprograms. The goals were ambitious: low emissionlevels, smooth operation, a 30% improvementin fuel economy, and successful integration andoperation in a representative U.S. automobile.

General Motors placed one in a 1985 ChevroletCelebrity, and met all of the program's technicalgoals. But improvements in the efficiency of existingengine types, coupled with the status quo's far less

expensive cost structure, doomed the Stirling toautomotive irrelevance.

The External Combustion RevivalThe Stirling idea was dusted off in the mid-1990s.A prototype Stirling hybrid propulsion system wasintegrated into a 1995 Chevrolet Lumina. But thattest was not particularly successful, as the hybridvehicle failed to meet several key goals for fuelefficiency and reliability. The program was aban-doned. Still, Stirling engine advocates continueto research and apply the technology. The bigbreakthrough may yet arrive, possibly in a hybridelectric-Stirling engine.

While not terribly complex, the engineeringanalysis of the engine's thermodynamic cyclegoes beyond the scope of this article. Suffice it tosay that Stirling engines operate on a four-partcycle in which the air inside the engine is cyclicallycompressed, heated, expanded, and cooled, and asthis occurs, the engine produces useful work.

While most heat engines are fairly understand-able to interested amateurs, building one yourselfis an altogether different prospect. Most enginesrequire carefully machined metal parts, withclose tolerances and tightly sealing clearancesfor pistons and/or rotating parts. Robert Stirling'sheat engine is an exception. Or at the very least.making a working model can be done without anydifficult machining.About MAKE's Stirling EngineThis article provides step-by-step instructionsfor building a straightforward Stirling externalcombustion engine.

This engine is simple and cheap, and once youget it going, you really get a feel for how this sort ofengine works. It chugs along at a leisurely 20 to 30rpm, its power output is minuscule, and it makes adelightful squishing/chuffing noise as it operates.

But be forewarned: All engines, even the metal-can Stirling described here, are complex mechani-cal devices in which myriad mechanical movementsmust come together in precise fashion in order toattain cyclical operation.

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If. Locate the center of the oppositeend of the 3fs"-diameter dowel, anddrill a pilot hole and screw the #214screw eye into the center. Apply superglue and accelerator spray.

2 FABRICATE THE CRANKSHAFTThe crankshaft consists of a metal rod bent in a precise way that holds the piston connecting rods in alignment.2a. Layout bend lines on the rod asaccurately as possible using a perma-nent marker, as shown on the benddiagram.

2b. Using a hammer, vise-grips, andvise, bend the metal rod as shown.Use special care when bending therod to make the bend sizes andshapes correspond closely to thediagram. The 2 bends (the cranks)must be offset by exactly 90 degrees,and the distance from the end of thecrank to the centerline of the crank-shaft must be %".

2c. Insert the plastic spacers into thesheaves. Tighten the setscrew insidethe collar of the sheave to lock theplastic spacer in place. Do not putflywheels on the crankshaft yet.

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PROJECTS STIRLING ENGINE www.makezine.coml07Ist ir l ingengine

3 ASSEMBLE THE AIR CYLINDER3a. Before soldering or gluing, cutdown the 23,4" pipes if necessary,so that the overall distance of thefinished assembly will be 7ljz",center-to-center.

23/4 " (approx.)

3b. Solder or epoxy the copperpipes and fittings together as shown,making certain the connections areairtight and leak-free. Note the align-ment: the copper tee is rotated 90degrees from the plane formed by theother 2 holes in the assembly.3c. Place the rubber stopper into themiddle hole, in the tee. This is thesystem's water drain.

4 ASSEMBLE THE WATER RESERVOIRS4a. Remove the top from each steelcan, leaving the bottom intact. Sandedges smooth.

4b. Mark a 3,4"-diameter circle in thecenter of the bottom of each can.4c. With a utility knife, carefully make8 to 12 radial slits on the bottom of thecan, but within the 3,4"circle. The slitsshould form a star shape, radiatingout from the center.

4d. Push the 5" copper pipe intothe can's bottom, through the holeformed by the slits. Slide the pipe untiljust 1" of pipe still extends out thebottom.

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If you are soldering thepipe into the can, the bot-tom of the can should bevery heavily scored witha file to provide a toothysurface that the solder canstick to.

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4e. With the pipe concentric andparallel to the sides of the can, solderthe pipe in place. (Alternatively, youcan seal the pipe-to-can connectionwith slow-curing, waterproof epoxyglue, taking care to seal the pipecarefully so it will not leak. Allow to drycompletely.)

5 MAKETHE FRAME

Do not give up hope onthe soldering. It is verydifficult to do, but perse-verance will payoff.

Sa. Using deck screws or nails,assemble wooden pieces A-Oto form a frame, as shown.

6 ASSEMBLE THE. STIRLING ENGINEGa. Insert the water reservoir assem-blies into the air cylinder assembly.Fill the reservoir cans with water andcheck for leaks. Repair leaks withepoxy and let dry.

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PROJECTS STIRLING ENGINE

6b. Measure and then mark a spoton each 1"x2" frame piece, 3%" fromthe Back edge of the frame. Placethe combined water reservoir and aircylinder assembly on the 1"x2" framepieces at the marked spots. Now placethe %" copper pipe clamps over theassembly. Screw the pipe clamps intothe 1"x2" pieces. The clamps musthold the combined assembly firmlyin place.

6c. Slide the screw eyes on the con-necting rods onto the crankshaft, sothat 1 screw eye is on each of the 2cranks. Place the soda-can pistonsinside each of the water reservoirs sothat each soda can rests on copperpipes. Turn the crankshaft so that oneof the cranks is pointing downward.

Holding the crankshaft level, lift thecrankshaft until the can correspond-ing to the bottomed crank is about W'above the top of the copper pipe. Thisis the desired height for the crank-shaft. Mark this height on the upright2"x4" and attach the angle bracket atthis point, making sure that the holethrough which the crankshaft willpass is located 3%" from the back ofthe 2"x4".6d. Slide 1 flywheel onto each end ofthe crankshaft. Position the flywheelsso that they are as far inboard aspossible without interfering withthe cranks or piston rods. Glue theflywheels onto the crankshaft usingsuper glue and accelerator spray.You're done!

FINISHE)

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MAKER, STARTYOUR ENGINE

To start your Stirling engine, turn the crankshaft untilboth cranks are tilted upwards at 4S-degree anglesto the vertical. With the stopper removed from thedrain, fill each side with water, until a trickle runs outthe drain. Dry it and replace the stopper.

Designate one side as the hot side, then heat thewater on that side to boiling with a propane torch.This takes a while, depending on the heat output ofthe torch. Be patient.

When the water is ready, start the engine bygiving the flywheels a small push. The rotation isdetermined by this rule: the cold side is 90 degreesbehind the hot side.

If built properly, your engine will dip and lift, dipand lift. 20 to 30 times per minute to the chuff-chuffbeat of Robert Stirling's ancient idea.

TROUBLESHOOTING1. Make sure the engine is level. The crankshaftmust revolve freely, and the connecting rods shouldstay in the middle of each crank as it rotates. Useshims or cardboard to level the system. If the con-necting rods will not stay centered on the cranks,you can add a small wire loop or small nut to the rodon either side of the eye screw, fastening them intoplace with super glue.2. You may have to experiment to find the bestflywheel weight. If the flywheels are too heavy, themetal rod will bow, interfering with the crankshaft'srotation. But if the flywheels are too light, therewon't be enough inertia to carry the crankshaft pastthe volume compression phase and into the nextexpansion stroke. If this happens, the engine willpulse but not run cyclically. You can add weight tothe flywheel by simply taping bolts or other weightyobjects to its perimeter.3. Large steel cans full of water take time to heat. Bepatient, and let the water heat to 200F or more.4. Minimize friction and interference. Friction isyour engine's greatest enemy. Minimize rubbingbetween pistons and water cans, between connect-ing rods and cranks, and between the crankshaftand the metal support angles that attach it to thewooden frame.5. Add a regenerator. A regenerator consists of asmall piece of heat-conducting metal gauze placedin the air cylinder just behind the rubber stopper. Aregenerator will improve cycle efficiency and makethe machine turn faster. The copper gauze sold forcleaning kitchen pots ("Chore Boy") works well.