Numerous attempts have been made to supersede steam as a motive power,
with the view to avoid the loss of heat by its absorption in the steam
in a latent state. Mercury vapour and spirit vapour have been tried,
in the expectation that as they possess much less capacity for heat,
an equal pressure might be obtained, with a diminished loss of heating
power. Several gaseous agents have been applied to the same purpose, of
which carbonic acid gas seemed to present the best prospect of success,
because it becomes expanded with a comparatively small increase of
temperature. None of these attempts to produce a motive power superior
to steam have yet proved successful. They have all, after a short
season of promise, dropped out of notice; and the only one that is
still in the field, struggling for superiority, is the air engine.
The first known air engine was invented by Sir George Cayley, in 1803.
In his engine the air was heated by passing directly through the hot
coals of the furnace, which some engineers yet consider to be the best
mode of expansion; but its operation did not answer expectations. Mr.
D. Stirling, of Dundee, afterwards improved on Sir George Cayley's
plan, and introduced a method of regaining the heat from the expanded
air, after it had done its work in the cylinder, and of applying it to
expand the air again. Engines on this construction have been for some
years working in Scotland, and in 1850 Mr. Stirling took out a patent
for an improvement in the arrangement, which is stated to have been
very successful.
Though Sir George Cayley and Mr. Stirling were the first in the field
as inventors of air engines, the name of Mr. Ericsson, an American,
is more closely associated with the invention, as he has for many
years been conducting experiments on a large scale, and has tried his
"caloric engine" on land, and on a ship of large burthen, built for the
purpose.
The principle and the working of Mr. Ericsson's caloric engine is
nearly the same as Mr. Stirling's; but as it has been brought most
prominently into notice, we shall direct attention more particularly
to its construction and performances. Mr. Ericsson obtained a patent
for his caloric engine in this country in 1833, and a subsequent patent
for improvements on it was taken out in 1851. During those years, and
to a late period, he was indefatigably working out the principle, and
numerous highly favourable reports have from time to time been made of
the results of the experiments; but the advantages to be derived from
the air engine remain nevertheless very questionable.
The object attempted to be gained is to make the same heating power
do its work again and again. Atmospheric air, after being expanded by
passing over an extensive hot surface, exerts the force thus acquired
to raise the piston of a large cylinder, and it is then attempted
to abstract the heat as the air issues out, and to apply it to the
expansion of a further quantity.
The practicability of this plan has undergone much discussion; its
friends and foes being equally confident in their opinions. The
former pronounce it to be one of the most valuable inventions of
the age, being calculated to economize heat, and to give greatly
additional impulse to navigation; whilst its opponents declare that the
calculations are erroneous, the experiments fallacious, and that the
expanded air consumes more heating power than steam.
In one of the favourable notices of Mr. Ericsson's engine in an
American publication, it is thus described:--"Two caloric engines have
been constructed in New York, one of 5-horse power, the other of 60.
The latter has four cylinders; two of 6 feet diameter, placed side by
side, surmounted by two of much smaller size. Within are pistons, so
connected that those in the lower and upper cylinders move together.
A fire is placed under the bottom of the large cylinders, called the
working cylinders; those above are called the supply cylinders. As
the piston in the supply cylinder moves down, valves at the top admit
the air. As they rise, those valves close, and the air passes into
a receiver and regenerator, where it is heated to about 450°, and
entering the next working cylinder, it is further heated by a fire
underneath to 485°. The air is thus expanded to double its volume;
and supposing the supply cylinder to be half the size of the other,
the air, when expanded, will completely fill the larger cylinder. As
the area of the piston of the smaller cylinder will be only half that
of the larger, and as the air will be of the same pressure in both,
the total pressure on the piston of the large cylinder will be double
that on the small one. This surplus furnishes the working power of
the engine. After the air in the working cylinder has forced up the
piston within it, a valve opens; and as the air passes out, the piston
descends by gravity, and cold air rushes in, and fills the supply
cylinder.
"The most striking feature is the regenerator. It is composed of wire
net, placed together to a thickness of about 12 inches. The side
of the regenerator, near the working cylinder, is heated to a high
temperature. The air passes through it before entering the working
cylinder, and becomes heated to 450°. The additional heat of 30° is
communicated by the fire underneath to the large cylinder. The expanded
air forces the cylinder upwards, valves open, and it passes from the
cylinder, and again enters the regenerator. One side of the regenerator
is kept cool by the air on its entering in the opposite direction at
each stroke of the piston; consequently, as the air of the working
cylinder passes out, the wires abstract its heat so effectually, that
when it leaves the regenerator, it has been robbed of all except about
30°. In other words, as the air passes into the working cylinder, it
gradually receives from the regenerator about 450° of heat; and as it
passes out, this is returned to the wires, and it is thus used over and
over again; the only purpose of the fires beneath the cylinders being
to supply the 30° of heat which are lost by radiation and expansion.
"The regenerator in the 60-horse engine measures 26 inches in height
and width. Each disc of wire composing it contains 676 superficial
square inches, and the net has 10 meshes to the inch. Each superficial
inch, therefore, contains 100 meshes, and there are 67,600 in each
disc; and as 200 discs are employed, the regenerator contains
13,520,000 meshes, with an equal number of small spaces between the
discs as there are meshes; therefore, the air is distributed into
27,000,000 of minute cells. The wire in each disc is 1,140 feet long;
and the total length of wire in the regenerator is 41½ miles, or equal
to the surface of four steam boilers, each 40 feet long and 4 feet
diameter."
The accounts received from America of the great success that had
attended the working of Mr. Ericsson's air engine, on the ship
"Ericsson," attracted much attention in this country, and formed
the subject of two evenings' discussion in the Institution of Civil
Engineers. The most prevalent opinion was, that it is impossible to
regain the heating power without corresponding loss of mechanical force
or the addition of heat, and that there must have been some fallacy in
the reports of the work done and of the quantity of fuel consumed.
It is, indeed, evident that nothing approaching the amount of heat
said to have been recovered could be regained by passing through
the regenerator; for as the apparatus becomes heated by the first
portions of air passing through it, the temperature of the quantity
that afterwards passed must at least be equal to that of the heated
wires, and the last portions of air would consequently scarcely part
with any caloric to the regenerator, previously heated to nearly
its own temperature. Experience has since proved that the notion of
regaining the heat by the regenerator was fallacious, for in the
last improvements in Mr. Ericsson's engine, it is stated that the
regenerator has been abandoned, and the plan has been adopted of
cooling the air as it issues from the large cylinder, by passing it
through tubes surrounded by cold water, and then using the same air
over again.
One great practical inconvenience in the use of the air engine was the
necessity of having enormously large cylinders to attain the required
power, with the low amount of pressure that can be procured by the
expansion of the air. The consequent friction increased the loss of
power, and the difficulty of lubricating the pistons added to the
practical objections to the air engine. To overcome these objections,
the air in Mr. Stirling's engine is compressed before it is heated, by
which means an equal amount of pressure is obtained on a smaller piston.
The air engine would in many respects possess advantages over the steam
engine, if it could be worked economically. The space occupied by the
boilers would be saved, and the danger of explosions would be avoided;
for hot air does not scald, and the quantity at any time expanded would
be too small to do much injury.
A patent has since been obtained by Messrs. Napier and Rankine, for
improvements in the air engine, which they anticipated would remove
the objections that have been raised to the engines of Stirling and
Ericsson. The heating surface has been greatly increased by employing
tubes; and other defects in the former engines, to which their want
of complete success is attributed, have been remedied, so that Mr.
Rankine, in his description of the improvements at the meeting of the
British Association at Liverpool, confidently anticipated to effect a
great saving of heating power, combined with the other advantages of
the air engine. He estimated the consumption of fuel by a theoretically
perfect air engine on Mr. Stirling's principle at 0·37 lbs. per horse
power per hour; whilst a theoretically perfect steam engine would
consume 1·86 lbs. The actual average consumption of a steam engine
is, however, 4 lbs. of fuel per horse power per hour, and the actual
consumption of Stirling's engine is stated by Mr. Rankine to have
been 2·20 lbs, and that of Ericsson's 2·80 lbs. It appears from this
statement, therefore, that the air engines of Messrs. Stirling and
Ericsson are superior in point of economy of fuel to steam engines; and
if Mr. Rankine's anticipations of the superiority of his air engine be
realized, it will effect still greater economy. In Messrs. Napier and
Rankine's engine, the air is compressed before expansion, so that the
size of the cylinders may be reduced to even smaller dimensions than
the cylinders of steam engines of equal power.