Improvements in Hot Air Engines.
Year: 1891

page_text: Date of Application, 2nd Sept., 1991-Accepted, 2nd Sept., 1892
COMPLETE SPECIFICATION.
Improvements in Hot Air Engines.
I, ROBERT HOFFMANN of the firm of Jahn & Company, of Boitsfort near
Brussels, in the Kingdom of Belgium, Machine Builders, do hereby declare the
nature of this invention and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement :-
The engine constructed according to this invention is operated by the power
resulting from the increase and decrease of the volume of air alternately heated and
cooled.
The object of the present improvements is to considerably increase the motive
power yielded by hot-air engines while simplifying their construction and making
them serviceable for a long time.
In the accompanying drawings
Figure 1 is a vertical section through the axes of the cylinder and the driving
shaft. The heating takes place as in an ordinary stove. The fuel is introduced
at C and falls down upon the grate G. This arrangement enables the supply of
fuel to be automatic and to last a whole day.
D is the fire-proof lining of the iron casing or fire-box E the top of which is
closed-in by a plate F. For ordinary stoking there is also provided an opening or
door H. The whole structure rests upon a cast base I. The products of com-
bustion escape through the opening K to which is connected the chimney L. To
the plate is bolted the motor cylinder consisting of three cylindrical parts B B¹
and B. The manner in which these three parts are fitted together is shown in
Figure 2.
The cylinder is thus formed in parts for the purpose of preventing the walls of
the cylinder from conducting the heat of the lower part, constantly exposed to the
fire, to the upper part which is constantly cooled.
The insertion of an intermediate part B between the lower cylinder B',
constantly heated, and the upper cylinder B, constantly cooled, enables washers m
and of non-conducting material to be interposed between the flanges of the
cylinder sections, and thereby to check conduction of heat.
The bottom M of the cylinder B¹ is provided with a number of projections N for
securing to it the fire-proof material with which this cylinder is externally covered,
so as to prevent the bottom surface M from being unduly heated. To increase the
heating surface of the cylinder wall, moreover, suitable wings may be provided.
The construction of the cylinder wall B₁ as shown in Figure 2 gives very
favourable results. The lower portion O is considerably thicker than the upper
portion u which latter, however, is strengthened by longitudinal ribs which
increase the heating effect.
The upper cylinder B which is constantly cooled is surrounded by a water-
jacket O (Figure 1) in which water is circulated by means of any suitable
device.
The working piston A Figure 1 is provided with a cylindrical rod R' passing
through the guide Q. Throughout this rod is a perforation for the passage of
another piston rod R.
The construction of the piston A is shown in detail in Figures 3 and 4, the
former of which is a cross section and the latter a plan thereof.
A ring S the periphery of which is provided with inclined recesses r is screwed
upon the body of the piston to which it at the same time secures the leather ring T.
Underneath this ring and the piston there is a space m from which openings or
ports 1 extend to the lower surface of the piston.

page_text: The object of this arrangement is to enable the leather ring to move away from
the cylinder wall when the piston rises and thereby to allow the air freely to pass
through the recesses or passages r, the space m and the inclined passages / so that
the air passes into the space below the piston. As the piston descends, the air
which previously passed through the ports during the piston's up stroke has a
tendency to flow in the opposite direction and consequently presses the leather
collar T tightly against the cylinder wall. Upon the upper surface of the piston A
is fitted a bearing u (Figure 4) in which is pivotted a connecting rod V (Figure 1)
directly connected with the crank W and also connected in any convenient manner
with the displacing piston p.
The engine may also actuate a small feed pump whereby the circulation of the
water in the jacket O is produced.
The bracket support of Figure 1, is preferably integral with the said water-jacket
or reservoir O and carries the driving shaft upon which is mounted a fly-wheel, a
pulley, and bevel gearing for the operation of the governor, by which the motion
of the engine is regulated either by allowing part of the heated air to escape or by
diminishing the air supply to the fire.
The construction of the piston p is shown in detail in Figures 5 and 6.
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It consists of a cylindrical vessel divided into two parts or chambers 1 (Figure 5)
between which is interposed a sheet or layer of non-conducting material to prevent
conduction of heat from the lower to the upper part. Each chamber consists of
a cylindrical tube of wrought iron or other material and the lower end of part r is
closed by two iron plates i and c secured together by a screw 1. Between these
plates i and c is also interposed a layer or sheet of non-conducting material K.
The upper end of chamber r and lower end of chamber ¹ are closed by the iron
plates Between these two plates is also placed a layer of non-conducting
material. The two chambers r r are connected by a tube a screwed between them.
The upper chamber at its upper part is provided with a ring or crown c¹ to
which is fitted a cover or lid d carrying the bearing of the rod R of the piston p
as shown in Figure 6. The end f of the rod R made in the shape of a ball is
located within a bearing e on to which is screwed a cap g. At d and his arranged
the packing for ensuring a tight joint. A facuum is produced in the two chambers
by any suitable means, the screw being first unscrewed and afterwards put in
place again, so as to form an air-tight closing. By this means the shape of the wall
will not be affected by increase or decrease of the pressure exercised by air contained
in the chambers r r¹.
7°
The external diameter of the chamber is less than that of the other chamber
so that when the chamber forming the displacing piston reaches the lowest point of
its downward stroke, there remains a small annular space between the internal
surface of the cylinder B₁ and the external surface of the chamber or piston r in
which space is located the sleeve q, (Fig. 1).
Figures 7 and 8 show details of the construction of this sleeve which consists of
a solid wall from which project longitudinal wings b about two millimetres deep
and set at about two millimetres distance apart. The circular surface at the lower
end of sleeve q is bevelled so as to allow the air to pass freely from within the
sleeve to the outside and to ascend along the wings through the narrow air-channels
thus formed, the heating surface being thereby considerably increased.
The displacing piston p may be covered externally with a perforated cast plate
(Figure 1) so as to increase the heating surface.
This engine may be readily set in operation by turning the fly-wheel of a
revolution in the required direction. The crank is thus carried beyond the dead
point in its downward movement and causes the piston A to take up its lowermost
position. This operation, in any convenient manner causes the displacing piston in
moving upwards to come slightly nearer to the working piston. The cold air
contained between the two pistons is consequently compressed and forced into the
space between the walls of the cylinder and the displacing piston. This air
necessarily fills up the whole of the open space left by the rising of the displacing

page_text: piston, and when this piston reaches the dead point, it remains almost stationary
for a certain time and thereby allows the fresh supply of air to be heated and
to expand again.
Owing to this expansion the air rises again and acts upon the working piston
which also rises and causes the displacing piston rapidly to descend.
In descending the displacing piston displaces the hot air which as it is forced
upwards cools and decreases in volume thereby allowing the working piston to
descend in its turn and then to start a fresh stroke. For the purpose of rendering
the cooling of the air as effective as possible a special device is employed which will
be best seen in Figures 9 to 11.
In order to increase the cooling surface a number of tubes d are arranged around
the external wall of the cylinder B. These tubes receive the heated air contained
between the collars of the cylinder B and B and cause this air when cooled to
enter the upper portion of the cylinder between the displacing and working pistons.
These tubes d are screw-threaded and screwed into the shoulder or collar b in
which is formed the annular recess f of the cylinder B which constitutes an air
passage.
The lower portions of these tubes are loosely passed into corresponding openings
provided in the collar c. Figure 9 shows this connection of one of the pipes
with the collar c on an enlarged scale.
The lower end h of each of the pipes d is tapered and fits into a correspondingly
tapered brass plug i screwed into the collar so that the joint is formed between
the tapering end of the tube and the screw-thread of the collar.
Perfectly tight joints are thus obtained at both ends of each of the tubes d
thereby ensuring a complete separation of the inner space filled with hot air from
the external space containing cooling water which is one of the most important
features calculated to ensure the proper working of the engine and to increase the
amount of motive power yielded by hot air engines.
To admit air into the lower ends of the tubes d there is provided a circular
groove K forming an air passage. This groove may be of any suitable shape.
In Fig. 11 two modifications are shown the upper portion showing the solid parts
surrounding the fastening bolts while below is shown a perfectly circular groove.
To improve the operation of the displacing piston the lower portion of the
cylinder B (Fig. 9) is of somewhat larger internal diameter than the upper
portion X.
The cooling process which takes place with the assistance of the apparatus herein
described is as follows;-
The hot-air displaced by the displacing piston instead of passing as usual between
the walls of this piston and the cylinder upwardly, is forced by the said piston into
the pipes d which is facilitated by the flaring shape of the lower end of the
cylinder B.
When the displacing cylinder rises, the cooled air enclosed between the working
and the displacing pistons passes through the circular channel f into the pipes d
and hence through the circular channel K into the cylinder B₁ where it is heated
again and further expanded.
When thereupon the displacing piston descends the air confined between the
walls of the displacing piston and of the cylinder B" passes into the circular
conduit K and hence through the tubes d into the passage f when it again enters
the space between the displacing and the working pistons in which space it is
cooled and consequently decreases in volume thereby allowing the piston to
descend.
Having now particularly described and ascertained the nature of the said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is ;-
1. In a hot-air engine the employment of a casing such as E lined with fireproof
material D and covered with a plate F to which is secured the vertical motor

page_text: cylinder and of controlling and charging devices such as H and C substantially as
described and illustrated in Figure 1 of the accompanying drawings.
2. In a hot air engine a vertical cylinder formed in three parts such as B B B"
consisting of the lowermost part B which is exposed to the action of the fire an
intermediate part B" and the uppermost part B which is exposed to the action of
the cooling agent a washer or layer of non-conducting material being inserted
between the collars or flanges of B and B", and B₁ and B" respectively, for the
purpose of preventing the conduction of heat substantially as described and
illustrated in Figures 1 and 2 of the accompanying drawings.
3. In a hot air engine a cylinder formed of a number of parts the wall of the
lowest of which at its upper part u is made thinner than at its lower part and is
provided with longitudinal ribs such as v for the purpose of better utilizing the
heat substantially as described and illustrated in Figure 2 of the accompanying
drawings.
4. In a hot air engine the combination with a cylinder such as B of a water-
jacket surrounding it and fitted with a device mainly consisting of a number of
tubes such as d a widened portion or annular recess such as fa collar such as c and
annular recess or groove such as K the tubes d connecting f to K substantially as
described and illustrated in Figures 9, 94, 10 and 11 of the accompanying
drawings.
5. In a hot air engine a piston such as p mainly characterized by the division of
the said piston into two compartments such as r r separated from each other by a
layer or sheet of non-conducting material so as to prevent conduction of heat
substantially as described and illustrated in the accompanying drawings.
6. In a hot air engine a piston such as p the external diameter of the lower
part of which is smaller than that of the upper part and the arrangement of a
ribbed sleeve such as q 6 between the inner wall of the cylinder B and the external
wall of the compartment r substantially as described and illustrated in Figures 1,
5, 7 and 8 of the accompanying drawings.
7. In a hot air engine a working piston such as A enabling air to pass freely
between the cylinder wall and the outer periphery of the said piston A during its
upward stroke and preventing the air from escaping through the same apertures
during its downward stroke this construction being characterized by air channels
provided in a ring or crown such as S whereby a leather ring or collar T is secured
to the body of the piston and by an annular chamber or space such as m provided
in the body of the piston and air-channels such as also formed in the body of the
piston underneath the leather collar substantially as described and illustrated in
Figures 3 and 4 of the accompanying drawings.
8. The combination and arrangement of parts constituting the complete hot air
engine substantially as described and illustrated in the accompanying drawings.
Dated this 2nd day of September 1891.
W. P. THOMPSON & BOULT,
Agents for the Applicant.