By Harry Walton

light, or plugging away hard under anyload short of stalling it.

You'll fire it up to mystify friends orjust to see it go. Its rocker-arm actionis fascinating to watch.

Dating back to 1816, the hot-air engineis also one of the latest subjects of re-search. It could be fueled by nuclearheat, and a highly sophisticated design,to be powered by solar heat, is proposedfor generating electricity aboard spacesatellites.

This simple one, fueled by alcohol,will drive small models, pump water, spina display turntable, or turn a fan. It re-quires no castings, and involves some in-teresting but not difficult lathe work. Funto build and operate, it's a conversationpiece for your den or workshop, or a safebut exciting gift for any youngster whocan be trusted with matches.142 POPULAR SC IENCE A P R I L 1961

A FREE

PS Save-lt

BlueprintSave the following

four pages for yourpersonal project file.Pinch the blue sheetstogether, gently liftfrom the magazine,and tear them loosefrom the two staples.The blueprint willthen lie flat for use.

JUST 15 seconds after you light up,this little engine chugs into action.It will keep going as long as it hasfuel—at a lively 1,000 r.p.m. running

Steps in machining the parts for the air-cooled hot-air engine

ALTHOUGH not hard to build, thisengine won't forgive bad workman-

ship. Mechanical freedom and airtight-ness are essential. It should both turneasily and bounce back against compres-sion. Metallic auto-body putty, thinnedwith lacquer thinner, should be used onall joints to seal against pinhole leaks.

The frame. This is two pieces of 1/2"dural bolted together. Lay out the cyl-inder standard first. Transfer the centerof the upper (displacer) recess to the op-posite face by drilling it through No. 60.

In the four-jaw chuck, first center thelower (power-cylinder) recess. Bore it1/4" deep and .044" smaller than the out-side cylinder diameter; then thread it 40threads per inch. Reverse the work withthe displacer recess centered. Bore thisalso undersize by double the threaddepth, and 5/16" deep. It will overlap andbreak through the other recess, the holeforming the transfer port.

If your lathe doesn't cut threads, bothrecesses may be smooth-bored, the cyl-inders turned to push fits, and seatedwith auto-body putty.

With the standard still chucked, drilland countersink for the 1/4"-20 brass boltthat forms the gland. Insert the bolt witha smear of sealer under its head, andtighten a nut on the other side. Do notremove the piece from the chuck to dothis; unscrew the chuck from the latheif necessary.

Remount it to centerdrill the bolt head.Run a No. 43 drill through. Follow witha new No. 42 or a 3/32"' reamer to make

a free-sliding, absolutely shakeless fit forthe displacer-piston rod. A reamer canbe made from a piece of the rod stockby filing a long flat taper on it andstoning the flat smooth.

Bearing plate and housing. Clamp theplate to the standard with bottom andside flush. Spot holes to be tapped byrunning a drill through those in thestandard. In the lathe, turn out the 3/16"deep step or shoulder; then bore throughto 3/4" diameter. The 1" hole is for ap-pearance only, and optional. Saw andfile the top radius to shape and the slotin line with the bearing center.

For the bearing housing, chuck 1"aluminum and turn the shoulder to apress fit in the plate. Drill the throughhole; then bore for the inboard ball bear-ing. Reverse the piece, centering it care-fully, and bore the seat for the outboardbearing. If you prefer a plain brass orbronze sleeve bearing, drill an oil holeafter pressing it in. Press the housingcarefully into the bearing plate so thatthe shoulder seats squarely. Do not pushin the ball bearings as yet.

Displacer cylinder and piston. As airmust be alternately heated and cooled inthe cylinder at every revolution, it mustbe thin-walled. Twist 1" steel conduitonto a tight-fitting arbor centerdrilled fortailstock support. Turn the outside toleave a wall about .010" thick up to 1/4"of one end. Thread this end to fit therecess and cut to 3 1/2" length.

Silver-solder or braze a domed coppercap into the unthreaded end. Test for air

CONTINUED 143

leaks by blowing into the other end withthe joint under water.

The displacer piston must be light,airtight, and a loose fit to let air flowaround it. An aluminum cigar tube is theright size and has a domed end.

Chuck a piece of aluminum bar; turnto 25/32", centerdrill, and tap for the rod.Recess the end to lighten it; then turna 1/4" length to a close push fit for thetube with a shallow groove at the end ofit. Push the tube on to run true. Feed around-nose tool, mounted upside down,against the groove to spin the edge of thetubing in. Apply a coat of sealer aroundthe spun joint.

Thread the displacer rod in the latheto insure getting the threads square (adie held against a tailstock pad will do).Smear a little sealer on the threads be-fore screwing the rod in.

Power cylinder and piston. A bronzebearing bushing makes a good cylinder.Lap the inside smooth, if necessary, withfine abrasive cloth in a slotted dowel.Thread one end to fit the recess, andchamfer the bore at the other for in-sertion of the piston.

Bore out the inside of the power piston,then twist onto an arbor. Turn the out-side to a virtually airtight fit in the cyl-inder. Final fitting may be done byhoning with a fine stone, abrasive clothon a metal block, or a split lap. Lightlyoiled, the piston should strongly resistbeing pulled out of the cylinder when theother end is held shut. Pushed in, thepiston should act as if there is a springbehind it, rebounding vigorously againstcompression, and hold this air cushionfor several seconds.

Insert the displacer rod and mountboth cylinders. The displacer must slidefull length without rubbing. Submergethe assembly in water past the gland.Pushing in the power piston will causebubbles to reveal any air leaks. Only oneor two slow bubbles from around the dis-placer rod are permissible.

Crankshaft and crank assembly. Turna narrow thrust shoulder on the web anddrill to a press fit on the shaft. Make surethe main crankpin is absolutely parallelto the shaft. Drive the inboard ball bear-ing on the shaft, insert it in the housing,and press the outer bearing into place.

Always apply driving pressure to the racebeing seated, not through the balls.

Now you're ready for the crank as-sembly. Adjust displacer stroke by mov-ing the rocker pivot up or down in theslot; fix the rocker head to move thedisplacer full length without hitting ateither end. Screw on the crank disk, lock-ing it, by the jam nut, concentric withthe crank web. The engine shown runsclockwise as seen from the crank end;to reverse rotation, turn the crank diskover. Push the power crankpin throughthe con rod and tighten its nut at thecenter of the slot.

Cooling fins. These are necessary forsteady running. They can be cut fromsheet aluminum with a fly cutter, thenchucked and bored to an interference fiton the cylinder. Lay them on a plateheated by a torch or stove burner (nottoo high, or the aluminum will melt).Pick up a hot fin with pliers, quicklyslide it into position, and let it cool.

Base and lamp. Frame a metal platewith 3/4"-high strips of wood or metalchannel. Mount the engine with two 3/16"bolts screwed into the standard. A shal-low fuel tank may be soldered up of tin-plate. A tube carries alcohol to theburner, a piece of 1/8" pipe (3/8" o.d.)with wicking. Add a wire-screen gratefor burning matchsticks or charcoal.

Furnace. A taper-necked beverage canmakes a good one. Punch a hole in thedomed bottom for the wick tube. Cutan opening for the displacer cylinderand a firedoor as shown. Drill air holesaround the bottom.

Flywheel. This should be of moderateweight and of a size to clear the base,but is otherwise not critical. It may havea pulley for a spring belt.

Timing. The crankpin slot permitsshifting the power stroke slightly to bothsides of its normal 90-degree positionbehind the displacer stroke. To checktiming, run the engine and then blow outthe flame. Running should continue withan even beat on both in and out strokes,right down to a tick-over.

If the engine doesn't run, check forbinding and air leaks. Sometimes a leakydisplacer piston is at fault. Remove itfrom the engine and plunge it into veryhot water. Bubbles will show leaks.

146 POPULAR SCIENCE APRIL 1961

TURN STEEL CONDUIT on an arbor to make thethin-walled displacer cylinder. A short end sec-tion (left of toolpost) is left thick and threadedto screw into the cylinder standard.

HINGED FIREDOOR has a mica window. Alcoholis the best fuel, but a fly-screen grate makes itpossible to burn wood. Engine starts cold, runshalf a minute on three paper matches.

BORE CYLINDER STANDARD from both sides, thepower-cylinder recess first. The recess for thedisplacer cylinder breaks through as above,leaving an open port between the cylinders. Besure to drill and ream the displacer-rod gland(arrow) at the same chucking.