The hot air engine, although theoretically recognized for some time
past as the most economical means of converting heat into motive
power, has up to the present met with little success. This is due to
the fact that the arrangement of the motors of this class that have
hitherto been constructed has been such as to render them but slightly
practical. In the Benier hot air engine (illustrated herewith),
however, obstacles that were once considered insurmountable have been
overcome, and the motor presents many advantages over all the types
that have preceded it. Among such advantages we shall cite the
possibility of utilizing air at a high temperature (1,200 or 1,500
degrees), while the rubbing surfaces remain at a moderate temperature
(60 or 80 degrees). The fire grate is placed in the interior of the
cylinder, and is traversed by the cold air forced by a pump. The
expanded hot gases fill the cylinder and act against the piston
directly above the grate.
The type herewith illustrated is of 6 horse power. The motive
cylinder, CC', is bolted to the extremity of the frame, A. Upon this
latter is fixed a column, B, which carries a working beam, E. This
latter transmits the motion of the piston, P, to the shaft, D. A pump,
G, placed within the frame, forces a certain quantity of cold air at
every revolution into the driving cylinder. The piston of this pump is
actuated by the connecting rod, G', jointed to the lever, F', which
receives its motion from the rod, F. A slide valve, _b'_, actuated by
a cam, regulates the entrance of the cold air into the pump during
suction, as well as its introduction into the cylinder. There is a
thrust upon the piston during its upward travel, and an escape of hot
gas through the eduction valve, _h_, during the downward travel.
The cylinder is in two parts, C and C'. The piston, which is very
long, rubs at its upper end against the sides of the cylinder, C. The
lower end is of smaller diameter, and leaves an annular space between
it and the cylinder. The grate is at the bottom of the cylinder, C'.
The sides of the cylinder at the level of the fire box are protected
with a lining of plumbago. When the piston is at the bottom of its
travel, the eduction valve closes. The slide valve, _b'_, establishes
a communication between the pump chamber and the cylinder. The air
contained in the pump is already compressed in the latter to a
pressure of nearly a kilogramme at the moment of the communication.
This air enters the cylinder, and the communication between the latter
and the pump continues until all the air is forced into the driving
cylinder, the piston of the pump being at the bottom of its travel,
and that of the cylinder about midway.
[Illustration: BENIER'S HOT AIR ENGINE.]
The air forced by the pump piston enters the cylinder through two
conduits, one of which leads a portion of it toward the top of the
cylinder, and the other toward the bottom. The lower conduit debouches
under the grate, and the air that passes through it traverses the fire
box, and the hot gas fills the cylinder. The conduit that runs to the
top debouches in the cylinder, C, at the lower limit of the surface
rubbed by the piston. The air that traverses this conduit is
distributed through the annular space between the piston and cylinder.
The hot gas derived from combustion can therefore never introduce
itself into this annular space, and consequently cannot come into
contact with the rubbing surfaces of the cylinder and piston.
As the quantity of air introduced at every stroke is constant, the
work developed at every stroke is varied by regulating the temperature
of the gas that fills the cylinder. When the temperature falls, the
pressure, and consequently the work developed, diminishes. This result
is obtained by varying the respective quantities of air that pass
through the fire box and around the piston. In measure as less air
passes through the fire box, the quantity that passes around the
piston augments by just so much, and the pressure diminishes. A valve,
_n'_, in the conduit that runs to the fire box is controlled by the
regulator, L', in the interior of the column. When the work to be
transmitted diminishes, the regulator closes the valve more or less,
and the work developed diminishes.
The coke is put by shovelfuls into a hopper, I. Four buckets mounted
upon the periphery of a wheel, I', traverse the coke, and, taking up a
piece of it, let it fall upon the cover, J, of the slide valve, _j_,
whence it falls into the cavity of the latter when it is uncovered,
and from thence into the conduit, _c'_, of the box, _j'_, when the
cavity of the valve is opposite the conduit. From the conduit, _c'_,
the coke falls upon the grate.
A small sight hole covered with glass, in the cover, J, permits the
grate to be seen when the cavity of the valve is opposite _c'_.
As in gas engines, a current of water is made to flow around the
cylinder, C', in order to keep the sides from getting too hot.
In order to set the engine in motion, we begin by opening the bottom,
C, of the cylinder, C', to clean the grate. This done, we close C and
introduce lighted charcoal through the conduit, _c'_ (the valve being
open). The valve is put in place, two or three revolutions are given
to the fly wheel, and the motor starts. The feeding is afterward done
with coke.
The parts that transmit motion operate under conditions analogous to
those under which the same parts of a steam engine do. The air pump
sucks and forces nothing but cold air, and nothing but cold air passes
through the distributing slide valve. The pump and valve are therefore
rendered very durable. The piston and cylinder, at the points where
friction exists, are at a temperature of 60 or 80 degrees. These
surfaces are protected against hot gas charged with dust.
The hot gas, which escapes from the cylinder through a valve, has
previously been cooled by contact with the sides of the cylinder and
by expansion. The eduction valve just mentioned works about like that
of a steam engine, and it is only necessary to polish it now and then
in order to keep it in good condition.--_Annales Industrielles._
* * * * *