How to Make a Model Hot-Air Engine By HENRY GREENLY
T
HE hot -air engine, except perhap s for for the water-motor, is the simplest of all prime movers to reproduce in miniature. No boiler is required, no danger attends its use, and within a very few seconds of lighting th e lamp it is ready to start . For these reasons reasons the writer has chosen the hot-air engine as the subject matter for the first article of the winter session. session. A hot-air engine is certainly a beginner's model. Of course, such an engine cannot be made to develop much power, but what model engine does ? Under ordin ary circumstances so long as the maker, when his
task task is finished, see sees s the the wheels wheels go round, he is suffic sufficien iently tly rewarded. If it will will also also drive some other light running machine so much the better. The hot-air engine, originally called the "caloric engine," is a comparatively ancient invention, invention, dating dating from from 180 1807. 7. Theoretically it should be a perfect heat engine (whic (which h the th e steam engine engine is certainly not), but there are so many practical difficulties in the design and construction that it is in reality one of the most inefficient of all machines used for providing power from the latent heat of coal. Although in the early fifties Ericsson built a large passengercarrying ship driven b y hot-air engines, readers must abandon any idea of using an engine of this type for the the purpo se of driving a locomotive or aeroplane. The weight per horse-power is excessive ; quite two hundred tim times more thanthat of a modern aeroplane petrolmotor. There are are two systems under which hot-air engines work: work: t h e
HOW
TO
MAKE
A
MODEL
"c los ed" cyc cycle le and the " o p e n " cyc cycle le.. The former deals with a given quantity of air which is locked up in the apparatus, and which is heated and cool cooled ed down alterna tely. This system is the one which the writer lias adopted for th e model illustra ted herewit h. As will will be seen by the explanatory diagram, Fig. 2, two cylinders are employed, the larger one (the displacer cylinder) being closed closed at both bo th ends, and the smaller (power) cylinder open at one end. The close closed d end of the th e power cylinder is connected by a pipe to the displacer cylinder. Inside the displacer cylinder is a large drum or piston which does not quite fit the cylinder. The function of this piston is to force the air from one end of the displacer cylinder to the other, the upper end being cooled by a water jacket, the lower end being placed in the furnace and subjected to the heat of the fire or lamp. This action, of course course,, causes th e air to expand and contract, and therefore to force out or relieve the pressure from the piston in the power cylinder. In Fig . 2 the displacer piston is at the top of its stroke, and
FI G . 3.— HO W a hot-air engine engine works—the works—the power stroke.
the air is in contact with the walls walls of the cylinder which are being heated by the lamp. The air expands and forces down the power piston. At the end of the power stro ke the displacer piston moves downward, the crank of this pistonjbeing placed at right angles to the power crank. The air passes to the upper end of the displacer cylinder through the space between piston and cylinder |walls |walls and is cooled. cooled. Fig. 3 shows shows this thi s action acti on at th e most mos t favourable point of the stroke, the air contracting and relieving the pressure on the power power piston. The process process is repeated with every revolution of the engine. The fire provides for the heating and expansion, and the cold water for the extraction of the heat and contraction of the working medium, viz.,
HOT-AIR
ENGINE
the given given amount of air trapped in the appa ratus . Both require constant replenishing, and what is just as important, the air in the cylinders mus t have no chance of gettin g out. The displacer cylinder and pis ton mus t be quite air-
FIG. FI G. 3.—Ho —How a hot-air hot-air engine works—t works—the he suction stroke. stro ke. tight, the gland and the power piston must be in the same condition, and at the same time work quite freely freely.. The variati on in pressure pressure in a hot-air engine is very small—one or two pounds per square inch—and therefore friction must be reduced to the minimum. But to come to the construction of a working model hot-air engine, which is the raison d'etre of thi s article. No expensive materia ls are required, and no castings, except one for the th e flywheel. flywheel. A collection of tinned tin ned iron cans, some strip and flat brass, steel wire for shafts and piston-rods, a short length of 3/16" copper pipe, and some drawn tube for the power piston and cylinder, should be made before commencin commencing g th e work. Wit h regard to the tins the sizes of the displacer, cylinder, and its furnace and water-jacket may have to be modified to suit the supplies, but a Lyle's 2lb. syrup tin would work well for furnace j and a stou t air-t ight canister 2-1/2" dia. by
FIG. 4.—A suggested implement for cutting round holes in end of tin canister.
4-1/4" high for th e displacer cylinder. The displacer piston should not be more than 2-1/4" diameter diam eter by 2-1 2-1/1 /16" 6" high. The joints joi nts of the th e cylinder should all be rolled joints. The
THE CAPTAIN bottom of the tin must be the bottom of the cylinder, and as this is subjected to the heat of the th e lamp, a plain soldered-i soldered-in n end will not do. Although solder solder is of no use alone, any tendency tendency of the rolled rolled-en -end d joint jo int or side seam seam to leak may may be stopped by solderi soldering ng fro from m the
I'iG.
5.—Clip for holding displacer cylinder to furnace casing.
inside inside of the tin. tin . For flux, any of the solderin soldering g pastes, resin, resin, or spirits of salts " kille killed d " with zinc, may be employed for this purpose. The furnace casing is a tin placed bottom upwards. The lower lower part is pierced with airairholes, and with a large "nick" through which the l a m p is inse inserte rted. d. The top also has holes to allow allow the th e products of combustion to escape, and, what is more difficult, a hole in the end of the tin must be cut to fit the displacer cylinder. To do this thi s well a special tool working from a centre point is to be advised, or the canister may be placed on a wooden wooden block held in the lathe and the hole in the end turned out. A su it ab le tool is shown in Fig. 4, which which sketch is self-explanatory. As the furnace casing cannot be soldered to the displacer cylinder, and any riveted riveted joint might cause the latter to leak, a clip arrangement is suggested. suggested. This is detailednn Fig. 5, and is [made [made from from strip brass or iron, and the ends are
FI G. 6.—Component'parts 6.—Component'parts of gland on displacer displacer cylinder.
riveted on to the th e furnace. furnace. Another Another satisfactory satisfactory method of supporting the th e displacer displacer cylinder in a fixed position from from the base-board would would be to sold solder er strips on to the water-ja water-jacket cket and to bring them right down to the base outside the furnace casing. casing. The water jacket is another tin with an open top, soldered (soldering is sufficient fixing owing Air f>if>e to the presence of the water) on to the outside of the displacer cylinder. The top end of cylinder is shown in the general draw drawin ing, g, F i g . i , placed placed inside the displacer cylinder. If a suitable lid which will fit inside cannot be obtained, the top of of the th e cycylinder may be fitted with an ordinary lid with an outside flange, and the cooling water allowe allowed d to flow all over the surface surface of the lid. In some some resp respec ects ts t h i s arrangement may improve the th e effi effi-ciency of the engine. The idea is illustrated in the diagrams Figs. 2 and 3. A packed gland
HOW TO MAKE A MODEL HOT-AIR ENGINE is necessary to allow the piston-rod, which may be of 3/32" steel wire, to pass through. The piston-rod should be soldered and nutted into both ends of the disp displa lace cerr dr um . Th Thee component parts of the gland are shown in Fig . 6. The stu ds should be soldered into the lid, and to ensure a better joint should be made of brass wire in preference to steel. The beam should be made of two strips of brass rod about 5/16" by 1/16" section, and should be supported from two brackets of similar material soldered on to the side of the water-jacket. water-jacket. A swiv swivel el link built up of of tube and plate (see Fig. 8) will be necessary to provide for the truly vertical movement of the piston-rod. To the other end of the beam a connecting rod of steel wire (an old cycle spoke) with brass blocks screwed on at each end is required to transmit the motion from the disc-crank on the end of the crank-shaft. The bearings for the crank-shaft can be made out of plate material (iron (iron or brass), the bottom., being flanged at right angles to provide a fixing to the base-board. The bearing plate next to the power cylinder may also be flanged at the th e top and bored for the. cylinder tube. This may be soldered in. The piston must be turned; and if a lathe is not no t available available this part of the work must be given out to a trade firm, or the assistance of a friend possessing this useful tool must be invoked. The piston is made in two parts ; the upper part
is turned turned from from a piece of 1" 1" rod brass, the t he inside being lightened as much as possible, as shown in the detail drawing, drawing, Fig. 9. Then the tipper part should be soldered to a piece of tube, and the whole turned down down to a tight tig ht working fit in the cylinder. cylinder. To render render it fairly fairly air-tight air-ti ght and at the same same time quite quite free, the t he piston piston should should be ground in with pumice powder and oil. The abrasive should then be washed out of both the cylinder and piston with paraffin. The connecting rod should be filed up out of strip brass to the shape and length shown in Fig. 9. It is attached to the piston by a gudgeon-pin, which fits the former tightly, in the th e usual usual gas or petrol petrol engine fashion. fashion. The connecting rod may be kept central on the pin by placing two washers made of tube, one on each side of the little end of the connecting rod. The cylinder should be lubricated with heavy oil. The stroke of the power power cylinder cylinder may be a little longer than that of the displacer piston. The cranks shown are of the disc variety. The connecting p i p e betw betwee een n the power and displacer cylinders should be connected to the latter quite close to the water - jacket, so that th at the t he sol sol-der will not be melted melted by the heat of the lamp. All leaks s ho ul d b e avoided. avoided. It is importan importantt that t hat t h e displa displace cerr crank should be set at 90 degs. in advanc vance e of t h e power crank. The position of displacer crank in advanc vance e of t h e power crank settles the direction of the rotation of the engine.
