Improvements in Hot-air Engines.
Year: 1890

page_text: Date of Application, 23rd Sept., 1890-Accepted, 1st Nov., 1890
COMPLETE SPECIFICATION.
Improvements in Hot-air Engines.
I, GEORGE FREDERICK REDFERN, of the firm of G. F. Redfern & Co., the General
Patent Office, No. 4, South Street, Finsbury, in the County of Middlesex, Patent
Agents, do hereby declare the nature of this invention (a communication from
abroad by A. Ventzki, of Graudenz, in the Empire of Germany, Engineer, and in
what manner the same is to be performed to be particularly described and ascertained
in and by the following statement
This invention relates to a high-pressure hot-air engine in which the working
piston moves in a chamber filled with air under high pressure and shut off from
communication with the atmosphere. By this means not only is the air underneath
the piston compressed, but the air situated above the piston is at high pressure, the
piston thus working in an atmosphere of its own of special density.
The coefficient of expansion of air remains constant no matter what may be its
tension or pressure, the result being that highly compressed air only requires to be
slightly warmed in order to produce a relatively high pressure.
This allows a
relatively great difference of temperature to exist between the fire-chamber and
the interior of the air heating chamber. As, however, the average coefficient of
expansion bears a direct relation to the difference of temperature between the firing-
chamber and the interior of the air heating chamber, and as the conductivity of air
at high pressure increases with the density, it follows that air at high pressure may
be employed in spite of its high specific heat, enabling a much higher degree of
mechanical effect to be obtained than when ordinary atmospheric air is employed
without rendering larger heating surfaces necessary.
In the accompanying drawing-
Figures 1, 2 and 3 are respectively a vertical section, a side elevation, and a
sectional plan through the cylinder of a hot-air engine constructed according to this
invention and arranged vertically.
Figure 4 is a longitudinal section, shewing the engine arranged horizontally.
The usual air-heating chamber B is suspended in the fire-chamber A, the cylinder D
surrounding the regenerator C, being mounted on it. The cylinder E which extends
downwards for a considerable distance into the air-heating chamber serves as an inner
jacket for the regenerator C which is formed of perforated copper. The expeller or
plunger F moves up and down in the cylinder E. The cylinder G with the working
piston H rests upon the cylinder D, the latter being closed air-tight by the cover 1,
the result of which is that there is an equal pressure of air both above and below the
working-piston when the engine is at rest, because the piston never fits so tight as to
prevent this equality of pressure taking place. To effect the equalization a movable
connecting tube a, indicated by dotted lines in Figure 1, may be introduced, if
necessary.
When it
If now the space in the cylinder G above the working-piston be filled with
compressed air an equalization takes place underneath the piston and in the
air-heating chamber B by means of the tube a. After firing has been commenced
the air in the air-heating chamber expands and raises the piston H.
approaches its highest point e the plunger or compressor F descends and forces the
hot air through the regenerator C to the working cylinder G surrounded by a cooling
jacket K. In passing through the regenerator the air gives up to it the greater
portion of its heat and is further cooled by the walls of the working cylinder, thereby
lowering the pressure. The vis viva of the fly-wheel and the periodically higher
pressure in the upper chamber O cause the piston H to descend.
The air which escapes during the ascent of the plunger or expeller F, and during
the descent of the piston H, passes in the opposite direction to that which it
traversed before, namely, from above through the regenerator (back to the lowest

page_text: portion of the air-heating chamber, and so takes up again or re-absorbs the beat
which had been given up to the copper regenerator, the result being that only an
equivalent amount of heat to that which has been expended in performing the work,
or lost by radiation or conduction has to be supplied by the fire.
If the working cylinder G be really air-tight the engine will continue to work
with the same body of compressed air, and but a very small consumption of fuel is
necessary, no new force being required for the introduction of fresh compressed air.
In order to cause an equal air pressure in the working cylinder, the shafts d, di
which carry the arms b, c work in an oil-bath formed by a side extension G¹ of the
working cylinder G. The compressed air which is enclosed in the chamber or space O
of the working cylinder G cannot escape through the stuffing-box of the shaft d,
and can at most only press the oil through it. Since, however, oil requiries a much
greater degree of pressure to cause its escape than air does, the escape of the oil can
only occur in very small quantities which are received by the funnel and conducted
by the tube i to the air-pump L. The pump L drives the oil that has been forced
out back again into the oil-bath of the shafts d, d¹, so that a constant level of the oil
in the bath is maintained.
This function of the pump L is, however, of secondary importance, its principal
duty being to bring the air pressure in the chamber of the working cylinder G
up to the desired point when the machine is started and to maintain it at that point.
For this purpose the rocking motion of the shaft d is converted, in the ordinary
way, into an up and down motion of the pump-cylinder, and the pump Lis
provided with an air-chest k from which a tube passes into the chamber 0 of the
working cylinder G. Owing to the fact that the upright tube projects into the
middle of the air-chest which is filled with oil up to the orifice of the said tube,
the pressure valve of the air-pump is permanently under pressure of oil, and so in
case of leakage prevents the escape of air.
When the engine is first started it works with ordinary atmospheric air, its power
being then very limited. The air-pump, however, gradually introduces air into the
closed upper chamber and compresses it until a certain pressure has been attained.
The maximum pressure in the engine then corresponds to the proportion of the
dead space in the air-pump to the contents of the pump-cylinder or its working
space.
For instance the pressure in the chamber must amount to 10 atmospheres when
the dead space in the air-pump amounts to 10 of its contents. When this degree
of pressure in the engine has been reached, the pump L acts merely as an air-buffer
without introducing any more air into the chamber O
The air pressure in the chamber O becomes equalized with that below the piston as
already stated, the result being that the engine has a double action, the heated air
raising the piston, until it has expanded to such an extent that it has in its heated
and expanded condition the same pressure as the air periodically compressed in the
upper chamber O, that is to say, at this moment (the dead point) the pressure is the
same on both sides of the piston. The expeller or plunger F then forces the heated
air through the regenerator C into the working cylinder G, and as it is by this means
cooled, a relative vacuum is formed underneath the working piston H, or in other
words there is an excess of pressure above the piston, the result being that it is forced
downwards.
The advantages of the herein before described engine consist in the employment of
compressed air, a much more advantageous medium than ordinary atmospheric air,
and the arrangement by means of which, even when the packing of the pistons is
imperfect, the compressed air remains completely shut off from the external
atmosphere.
Having now particularly described and ascertained the nature of the said invention
and in what manner the same is to be performed, I declare that what I claim is :-
1. A high-pressure hot-air engine in which the air used for working it is
employed at high pressure, and is absolutely shut off from communication with the

page_text: atmosphere, this being effected by exposing only imperforate walls or tight packings
to the direct action of the high-pressure air, those parts of the engine which transmit
power externally (the revolving shafts d, d¹) together with their packings as also
the air-pump valve being placed in a bath of dense liquid and being thereby only
indirectly exposed to the pressure of the air, thus allowing only the said liquid, and
not the air under pressure, to pass through the intermediate spaces, substantially
as hereinbefore described with reference to the accompanying drawing.
2. The improved hot-air engine hereinbefore described and illustrated in the
accompanying drawing.
Dated the 23rd day of Sentember 1890
G. F. REDFERN & Co.,
4, South Street, Finsbury, London, Agents for the Applicant.