[Footnote 1: A paper read before the Gas Institute, Manchester,
June, 1885.]
By DENNY LANE, of Cork.
Among the most useful inventions of the latter half of the nineteenth
century the gas-engine holds a prominent place. While its development
has not been so brilliant or so startling as that which we can note in
the employment of electricity, it holds, among the applications of
heat, the most important place of any invention made within that
period. Even amid the contrivances by which, in recent times, the
other forces of nature have been subdued to the uses of man, there are
only a few which rival the gas-engine in practical importance. With
regard to the steam-engine itself, it is remarkable how little that is
new has really been invented since the time of Watt and Woulfe. In the
specifications of the former can be shown completely delineated, or
fully foreshadowed, nearly every essential condition of the economy
and efficiency attained in our own days; and it is only by a gradual
"survival of the fittest" of the many contrivances which were made to
carry out his broad ideas that the steam-engine of the present has
attained its great economy.
It is but within the last fifty years that the laws of the relation
between the different physical forces were first enunciated by Justice
Grove, and confirmed by the classical researches of Dr. Joule--the one
a lawyer, working hard at his profession, the other a man of business
engaged in manufacture. Both are still living among us; the latter
having withdrawn from business, while the former is a Judge of the
High Court of Justice. I always regret that the claims of his
profession have weaned Justice Grove from science; for, while it may
be possible to find in the ranks of the Bar many who might worthily
occupy his place on the Bench, it would be hard to find among men of
science any with as wide-reaching and practical philosophy as that
which he owns. The chemist demonstrated long since that it was
impossible for man to create or destroy a single particle of
ponderable matter; but it remained for our own time to prove that it
was equally impossible to create or destroy any of the energy which
existed in nature as heat, mechanical power, electricity, or chemical
affinity. All that it is in the power of man to do is to convert one
of these forms into another. This, perhaps the greatest of all
scientific discoveries since the time of Newton, was first, I believe,
enunciated in 1842 by Grove, in a lecture given at the London
Institution; and it was experimentally proved by the researches of
Joule, described in a paper which he read at the meeting of the
British Association which was held at Cork--my native city--in 1843.
My friend Dr. Sullivan, now President of Queen's College, Cork, and I
myself had the privilege of being two of a select audience of half a
dozen people, who alone took sufficient interest in the subject to
hear for the first time developed the experimental proof of the theory
which welds into one coherent system the whole physical forces of the
universe, and enables one of these to be measured by another. One
branch of the "correlation of physical forces," as it was termed by
Grove, was the relation between mechanical power and heat, and the
convertibility of each into the other, which, under the name of
"Thermodynamics," has become one of the most important branches of
practical science.
Joule's first experiments clearly proved that each of these forms of
energy was convertible into the other; but some discrepancies arose in
determining the exact equivalent of each. His subsequent researches,
however, clearly demonstrated the true relation between both. Taking
as the unit of heat the amount which would be necessary to raise 1 lb.
of water 1° of Fahrenheit's scale (now called "the English thermal
unit"), he proved that this unit was equivalent to the mechanical
power which would be required to raise 772 lb. 1 foot, or to raise 1
lb. 772 ft. perpendicularly against the force of gravity. The
heat-unit--the pound-degree--which I will distinguish by the Greek
letter [theta], is a compound unit of mass and temperature; the
second--the foot-pound = f.p.--a compound unit of mass and space. This
equation, called "Joule's equivalent," or 1 thermal unit = 772
foot-pounds, is the foundation and the corner-stone of thermodynamics.
It is essential to understand the meaning of this equation. It
expresses the maximum effect of the given cause, viz., that if _all_
the heat were converted into power, or _all_ the power were converted
into heat, 1 thermal unit would produce 772 foot-pounds, or 772
foot-pounds would raise 1 lb. of water 1° Fahr. But there is never a
complete conversion of any form of energy. Common solid coal may be
partly converted into gases in a retort; but some of the carbon
remains unchanged, and more is dissipated but not lost. In the same
way, if I take five sovereigns to Paris and convert them into francs,
and return to London and convert the francs into shillings, I shall
not have 100 shillings, but only perhaps 95 shillings. But the five
shillings have not been lost; three of them remain in the French
_change de monnaies_, and two of them in the English exchange office.
I may have forfeited something, but the world has forfeited nothing.
There remains in it exactly the same number of sovereigns, francs, and
shillings as there was before I set out on my travels. Nothing has
been lost, but some of my money has been "dissipated;" and the
analogous case, "the dissipation of energy," has formed the subject of
more than one learned essay.
Before the invention of the steam-engine, the only powers employed in
mechanics were those of wind and water mills, and animal power. In the
first two, no conversion of one force into another took place; they
were mere kinematic devices for employing the mechanical force already
existing in the gale of wind and the head of water. With regard to the
power developed by man and other animals, we had in them examples of
most efficient heat-engines, converting into power a large percentage
of the fuel burnt in the lungs. But animal power is small in amount,
and it is expensive for two reasons--first, because the agents require
long intervals of rest, during which they still burn fuel; and next,
because the fuel they require is very expensive. A pound of bread or
beef, or oats or beans, costs a great deal more than a pound of coal;
while it does not, by its combustion, generate nearly so much heat.
The steam-engine, therefore, took the place of animal power, and for a
long time stood alone; and nearly all the motive power derived from
heat is still produced by the mechanism which Watt brought to such
great efficiency in so short a time.
Now the practical question for all designers and employers of
heat-engines is to determine how the greatest quantity of motive force
can be developed from the heat evolved from a given kind of fuel; and
coal being the cheapest of all, we will see what are the results
obtainable from it by the steam-engine. In this we have three
efficiencies to consider--those of the furnace, the boiler, and the
cylinder.
First, with respect to the furnace. The object is to combine the
carbon and the hydrogen of the coal with a sufficient quantity of the
oxygen of the air to effect complete combustion into carbonic acid and
water. In order to do this, we have to use a quantity of air much
larger than is theoretically necessary, and also to heat an amount of
inert nitrogen five times greater than the necessary oxygen; and we
are therefore obliged to create a draught which carries away to the
chimney a considerable portion of the heat developed. The combustion,
moreover, is never perfect; and some heat is lost by conduction and
radiation. The principal loss is by hot gases escaping from the flues
to the chimney. Even with well-set boilers, the temperature in the
chimney varies from 400° to 600° Fahr. Taking the mean of 500°, this
would represent a large proportion of the total heat, even if the
combustion were perfect; for, as a general rule, the supply of air to
a furnace is double that which is theoretically necessary. For our
present purpose, it will be sufficient to see how much the whole loss
is, without dividing it under the several heads of "imperfect
combustion," "radiation," and "convection," by the heated gases
passing to the chimney.
With a very good boiler and furnace each pound of coal evaporates 10
pounds of water from 62° Fahr., changing it into steam of 65 lb.
pressure at a temperature of 312°, or 250° above that of the water
from which it is generated. Besides these 250°, each pound of steam
contains 894 units of latent heat, or 1,144 units in all. A very good
condensing engine will work with 2.2 lb. of coal and 22 lb. of steam
per horse power per hour. Now. 1 lb. of good coal will, by its
combustion, produce 14,000 heat-units; and the 2.2 lb. of coal
multiplied by 14,000 represent 30,800[theta]. Of these we find in the
boiler 22 × 1,144, or 25,168 units, or about 81½ per cent., of the
whole heat of combustion; so that the difference (5,632 units, or 18½
per cent.) has been lost by imperfect combustion, radiation, or
convection. The water required for condensing this quantity of steam
is 550 lb.; and, taking the temperature in the hot well as 102°, 550
lb. have been raised 40° from 62°. Thus we account for 550 × 40 =
22,000, or (say) 71½ per cent. still remaining as heat. If we add this
71½ per cent. to 18½ per cent. we have 90 per cent., and there remain
only 10 per cent. of the heat that can possibly have been converted
into power. But some of this has been lost by radiation from
steam-pipes, cylinder, etc. Allowing but 1 per cent. for this, we have
only 9 per cent. as the efficiency of a really good condensing engine.
This estimate agrees very closely with the actual result; for the 2.2
lb. of coal would develop 30,800[theta]; and this, multiplied by
Joule's equivalent, amounts to nearly 24 millions of foot-pounds. As 1
horse power is a little less than 2 million foot-pounds per hour, only
one-twelfth, or a little more than 8 per cent. of the total heat is
converted; so that whether we look at the total quantity of heat which
we show unconverted, or the total heat converted, we find that each
supplements and corroborates the other. If we take the efficiency of
the engine alone, without considering the loss caused by the boiler,
we find that the 25,168[theta] which entered the boiler should have
given 19,429,696 foot-pounds; so that the 2 millions given by the
engine represent about 10 per cent. of the heat which has left the
boiler. The foregoing figures refer to large stationary or marine
engines, with first-rate boilers. When, however, we come to
high-pressure engines of the best type, the consumption of coal is
twice as much; and for those of any ordinary type it is usual to
calculate 1 cubic foot, or 62½ lb., of water evaporated per horse
power. This would reduce the efficiency to about 6 per cent. for the
best, and 3 per cent. for the ordinary non-condensing engines; and if
to this we add the inefficiency of some boilers, it is certain that
many small engines do not convert into power more than 2 per cent. of
the potential energy contained in the coal.
At one time the steam-engine was threatened with serious rivalry by
the hot-air engine. About the year 1816 the Rev. Mr. Stirling, a
Scotch clergyman, invented one which a member of this Institute (Mr.
George Anderson) remembers to have seen still at work at Dundee. The
principle of it was that a quantity of air under pressure was moved by
a mass, called a "displacer," from the cold to the hot end of a large
vessel which was heated by a fire beneath and cooled by a current of
water above. The same air was alternately heated and cooled, expanded
and contracted; and by the difference of pressure moved the piston in
a working cylinder. In this arrangement the furnace was inefficient.
As only a small portion of heat reached the compressed air, the loss
by radiation was very great, and the wear and tear exceedingly heavy.
This system, with some modifications, was revived by Rankine,
Ericsson, Laubereau, Ryder, Buckett, and Bailey. Siemens employed a
similar system, only substituting steam for air. Another system,
originally proposed by Sir George Cayley, consisted in compressing by
a pump cold air which was subsequently passed partly through a
furnace, and, expanding, moved a larger piston at the same pressure;
and the difference of the areas of the pistons multiplied by the
pressure common to both represented the indicated power. This
principle was subsequently developed by a very able mechanic, Mr.
Wenham; but his engine never came much into favor. The only hot-air
engines at present in use are Ryder's, Buckett's, and Bailey's,
employed to a limited extent for small powers. I have not said
anything of the thermal principles involved in the construction of
these engines, as they are precisely the same as those affecting the
subject of the present paper.
Before explaining the principle upon which the gas-engine and every
other hot-air engine depends, I shall remind you of a few data with
which most of you are already familiar. The volume of every gas
increases with the temperature; and this increase was the basis of the
air thermometer--the first ever used. It is to be regretted that it
was not the foundation of all others; for it is based on a physical
principle universally applicable. Although the volume increases with
the temperature, it does not increase in proportion to the degrees of
any ordinary scale, but much more slowly. Now, if to each of the terms
of an arithmetical series we add the same number, the new series so
formed increases or decreases more slowly than the original; and it
was discovered that, by adding 461 to the degrees of Fahrenheit's
scale, the new scale so formed represented exactly the increment of
volume caused by increase of temperature. This scale, proposed by Sir
W. Thomson in 1848, is called the "scale of absolute temperature." Its
zero, called the "absolute zero," is 461° below the zero of
Fahrenheit, or 493° below the freezing point of water; and the degree
of heat measured by it is termed the "absolute temperature." It is
often convenient to refer to 39° Fahr. (which happens to be the point
at which water attains its maximum density), as this is the same as
500° absolute; for, counting from this datum level, a volume of air
expands exactly 1 per cent. for 5°, and would be doubled at 1,000°
absolute, or 539° Fahr.
Whenever any body is compressed, its specific heat is diminished; and
the surplus portion is, as it were, pushed out of the body--appearing
as sensible heat. And whenever any body is expanded, its specific heat
is increased; and the additional quantity of heat requisite is, as it
were, sucked in from surrounding bodies--so producing cold. This
action may be compared to that of a wet sponge from which, when
compressed, a portion of the water is forced out, and when the sponge
is allowed to expand, the water is drawn back. This effect is
manifested by the increase of temperature in air-compressing machines,
and the cold produced by allowing or forcing air to expand in
air-cooling machines. At 39° Fahr., 1 lb. of air measures 12½ cubic
feet. Let us suppose that 1 lb. of air at 39° Fahr. = 500° absolute,
is contained in a non-conducting cylinder of 1 foot area and 12½ feet
deep under a counterpoised piston. The pressure of the atmosphere on
the piston = 144 square inches × 14.7 lb., or 2,116 lb. If the air be
now heated up to 539° Fahr. = 1,000° absolute, and at the same time
the piston is not allowed to move, the pressure is doubled; and when
the piston is released, it would rise 12½ feet, provided that the
temperature remained constant, and the indicator would describe a
hyperbolic curve (called an "isothermal") because the temperature
would have remained equal throughout. But, in fact, the temperature is
lowered, because expansion has taken place, and the indicator curve
which would then be described is called an "adiabatic curve," which is
more inclined to the horizontal line when the volumes are represented
by horizontal and the pressures by vertical co-ordinates. In this case
it is supposed that there is no conduction or transmission (diabasis)
of heat through the sides of the containing vessel. If, however, an
_additional_ quantity of heat be communicated to the air, so as to
maintain the temperature at 1,000° absolute, the piston will rise
until it is 12½ feet above its original position, and the indicator
will describe an isothermal curve. Now mark the difference. When the
piston was fixed, only a heating effect resulted; but when the piston
moved up 12½ feet, not only a heating but a mechanical, in fact, a
thermodynamic, effect was produced, for the weight of the atmosphere
(2,116 lb.) was lifted 12½ feet = 26,450 foot-pounds.
The specific heat of air at constant pressure has been proved by the
experiments of Regnault to be 0.2378, or something less than
one-fourth of that of water--a result arrived at by Rankine from
totally different data. In the case we have taken, there have been
expended 500 × 0.2378, or (say) 118.9[theta] to produce 26,450 f.p.
Each unit has therefore produced (26,450 / 118.0) = 222.5 f.p.,
instead of 772 f.p., which would have been rendered if every unit had
been converted into power. We therefore conclude that (222.5 / 772) = 29
per cent. of the total heat has been converted. The residue, or 71 per
cent., remains unchanged as heat, and may be partly saved by a
regenerator, or applied to other purposes for which a moderate heat is
required.
The quantity of heat necessary to raise the heat of air at a constant
volume is only 71 per cent. of that required to raise to the same
temperature the same weight of air under constant pressure. This is
exactly the result which Laplace arrived at from observations on the
velocity of sound, and may be stated thus--
Specific Foot- Per
heat. pounds. cent.
Kp = 1 lb. of air at constant pressure 0.2378 × 772 = 183.5 = 100
Kv = 1 lb. of air at constant volume 0.1688 × 772 = 130.3 = 71
------ --- ----- ---
Difference, being heat converted into power 0.0690 × 772 = 53.2 = 29
Or, in a hot-air engine without regeneration, the maximum effect of 1
lb. of air heated 1° Fahr. would be 53.2 f.p. The quantity of heat
Ky necessary to heat air under constant volume is to Kv, or that
necessary to heat it under constant pressure, as 71:100, or as
1:1.408, or very nearly as 1:SQRT(2)--a result which was arrived
at by Masson from theoretical considerations. The 71 per cent.
escaping as heat may be utilized in place of other fuel; and with the
first hot-air engine I ever saw, it was employed for drying blocks of
wood. In the same way, the unconverted heat of the exhaust steam from
a high-pressure engine, or the heated gases and water passing away
from a gas-engine, may be employed.
[Illustration]
We are now in a position to judge what is the practical efficiency of
the gas-engine. Some years since, in a letter which I addressed to
_Engineering_, and which also appeared in the _Journal of Gas
Lighting_,[2] I showed (I believe for the first time) that, in the
Otto-Crossley engine, 18 per cent. of the total heat was converted
into power, as against the 8 per cent. given by a very good
steam-engine. About the end of 1883 a very elaborate essay, by M.
Witz, appeared in the _Annales de Chimie et de Physique_, reporting
experiments on a similar engine, which gave an efficiency somewhat
lower. Early in 1884 there appeared in _Van Nostrand's Engineering
Magazine_ a most valuable paper, by Messrs. Brooks and Steward, with a
preface by Professor Thurston,[3] in which the efficiency was estimated
at 17 to 18 per cent. of the total heat of combustion. Both these
papers show what I had no opportunity of ascertaining, that is, what
becomes of the 82 per cent. of heat which is not utilized--information
of the greatest importance, as it indicates in what direction
improvement may be sought for, and how loss may be avoided. But, short
as is the time that has elapsed since the appearance of these papers,
you will find that progress has been made, and that a still higher
efficiency is now claimed.
[Footnote 2: See _Journal_, vol. xxxv, pp. 91, 133.]
[Footnote 3: Ibid., vol. xliii., pp. 703, 744.]
When I first wrote on this subject, I relied upon some data which led
me to suppose that the heating power of ordinary coal gas was higher
than it really is. At our last meeting, Mr. Hartley proved, by
experiments with his calorimeter, that gas of 16 or 17 candles gave
only about 630 units of heat per cubic foot. Now, if all this heat
could be converted into power, it would yield 630 × 772, or 486,360
f.p.; and it would require only 1,980,000 / 486,360 = 4.07 cubic feet to
produce 1 indicated horse power. Some recent tests have shown that,
with gas of similar heating power, 18 cubic feet have given 1
indicated horse power, and therefore 4.07 / 18 = 22.6 of the whole heat
has been converted--a truly wonderful proportion when compared with
steam-engines of a similar power, showing only an efficiency of 2 to 4
per cent.
The first gas-engine which came into practical use was Lenoir's,
invented about 1866, in which the mixture of gas and air drawn in for
part of the stroke at atmospheric pressure was inflamed by the spark
from an induction coil. This required a couple of cells of a strong
Bunsen battery, was apt to miss fire, and used about 90 cubic feet of
gas per horse power. This was succeeded by Hugon's engine, in which
the ignition was caused by a small gas flame, and the consumption was
reduced to 80 cubic feet. In 1864 Otto's atmospheric engine was
invented, in which a heavily-loaded piston was forced upward by an
explosion of gas and air drawn in at atmospheric pressure. In its
upward stroke the piston was free to move; but in its downward stroke
it was connected with a ratchet, and the partial vacuum formed after
the explosion beneath the piston, together with its own weight in
falling, operated through a rack, and caused rotation of the flywheel.
This engine (which, in an improved form, uses only about 20 cubic feet
of gas) is still largely employed, some 1,600 having been constructed.
The great objection to it was the noise it produced, and the wear and
tear of the ratchet and rack arrangements. In 1876 the Otto-Crossley
silent engine was introduced. As you are aware, it is a single-acting
engine, in which the gas and air are drawn in by the first outward,
and compressed by the first inward stroke. The compressed mixture is
then ignited; and, being expanded by heat, drives the piston outward
by the second outward stroke. Near the end of this stroke the
exhaust-valve is opened, the products of combustion partly escape, and
are partly driven out by the second inward stroke. I say partly, for a
considerable clearance space, equal to 38 per cent. of the whole
cylinder volume, remains unexhausted at the inner end of the cylinder.
When working to full power, only one stroke out of every four is
effective; but this engine works with only 18 to 22 cubic feet of gas
per horse power. Up to the present time I am informed that about
18,000 of these engines have been manufactured. Several other
compression engines have been introduced, of which the best known is
Mr. Dugald Clerk's, using about 20 feet of Glasgow cannel gas. It
gives one effective stroke for every revolution; the mixture being
compressed in a separate air-pump. But this arrangement leads to
additional friction; and the power measured by the brake is a smaller
percentage of the indicated horse power than in the Otto-Crossley
engine. A number of gas engines--such as Bisschop's (much used for
very small powers), Robson's (at present undergoing transformation in
the able hands of Messrs. Tangye), Korting's, and others--are in use;
but, so far as I can learn, all require a larger quantity of gas than
those previously referred to.
[Illustration: OTTO ATMOSPHERIC GAS ENGINE.]
[Illustration: CLERCK'S GAS ENGINE, 6 HORSE POWER.]
[Illustration: OTTO-CROSSLEY GAS ENGINE, 16 H.P.
Consumption 17.6 cubic feet of 16-candle gas per
theoretical horse power per hour.
Average pressure, 90.4 × constant, .568 theoretical
horse power per pound = 50.8 theoretical horse power.]
[Illustration: ATKINSON'S DIFFERENTIAL GAS ENGINE, 8 H.P.]
I have all along spoken of efficiency as a percentage of the total
quantity of heat evolved by the fuel; and this is, in the eyes of a
manufacturer, the essential question. Other things being equal, that
engine is the most economical which requires the smallest quantity of
coal or of gas. But men of science often employ the term efficiency in
another sense, which I will explain. If I wind a clock, I have spent a
certain amount of energy lifting the weight. This is called "energy of
position;" and it is returned by the fall of the weight to its
original level. In the same way if I heat air or water, I communicate
to it energy of heat, which remains potential as long as the
temperature does not fall, but which can be spent again by a decrease
of temperature. In every heat-engine, therefore, there must be a fall
from a higher to a lower temperature; otherwise no work would be done.
If the water in the condenser of a steam-engine were as hot as that in
the boiler, there would be equal pressure on both sides of the piston,
and consequently the engine would remain at rest. Now, the greater the
fall, the greater the power developed; for a smaller proportion of the
heat remains as heat. If we call the higher temperature T and the
lower T' on the absolute scale, T - T' is the difference; and the
ratio of this to the higher temperature is called the "efficiency."
This is the foundation of the formula we meet so often: E = (T - T')/T.
A perfect heat-engine would, therefore, be one in which the
temperature of the absolute zero would be attained, for (T - O)/T = 1.
This low temperature, however, has never been reached, and in all
practical cases we are confined within much narrower limits. Taking
the case of the condensing engine, the limits were 312° to 102°, or
773° and 563° absolute, respectively. The equation then becomes
(773 - 563)/773 = 210/773 or (say) 27 per cent. With non-condensing
engines, the temperatures may be taken as 312° and 212°, or 773° and
673° absolute respectively. The equation then becomes (773 - 673)/773
= 100/773, or nearly 13 per cent. The practical efficiencies are not
nearly this, but they are in about the same ratio--27/13. If, then, we
multiply the theoretical efficiencies by 0.37, we get the practical
efficiencies, say 10 per cent. and 5 per cent.; and it is in the
former sense that M. Witz calculated the efficiency of the
steam-engine at 35 per cent.--a statement which, I own, puzzled me a
little when I first met it. These efficiencies do not take any account
of loss of heat before the boiler. In the case of the gas-engine, the
question is much more complicated on account of the large clearance
space and the early opening of the exhaust. The highest temperature
has been calculated by the American observers at 3,443° absolute, and
the observed temperature of the exhaust gases was 1,229°. The fraction
then becomes (3443 - 1229)/3443 = 64 per cent. If we multiply this by
0.37, as we did in the case of the steam-engine, we get 23.7 per
cent., or approximately the same as that arrived at by direct
experience. Indeed, if the consumption is, as sometimes stated, less
than 18 feet, the two percentages would be exactly the same. I do not
put this forward as scientifically true; but the coincidence is at
least striking.
I have spoken of the illuminating power of the gas as of importance;
for the richer gases have also more calorific power, and an engine
would, of course, require a smaller quantity of them. The heat-giving
power does not, however, vary as the illuminating power, but at a much
slower rate; and, adopting the same contrivance as that on which the
absolute scale of temperature is formed, I would suggest a formula of
the following type: H = C (I + K), in which H represents the number of
heat-units given out by the combustion of 1 cubic foot of gas, I is
the illuminating power in candles, and C and K two constants to be
determined by experiment. If we take the value for motive power of the
different qualities of gas as given in Mr. Charles Hunt's interesting
paper in our Transactions for 1882, C might without any great error be
taken as 22 and K as 7.5. With Pintsch's oil gas, however, as compared
with coal gas, this formula does not hold; and C should be taken much
lower, and K much higher than the figures given above. That is to say,
the heating power increases in a slower progression. The data
available, however, are few; but I trust that Mr. Hartley will on
this, as he has done on so many other scientific subjects, come to our
aid.
I will now refer to the valuable experiments of Messrs. Brooks and
Steward, which were most carefully made. Everything was measured--the
gas by a 60 light, and the air by a 300 light meter; the indicated
horse power, by a steam-engine indicator; the useful work, by a Prony
brake; the temperature of the water, by a standard thermometer; and
that of the escaping gases, by a pyrometer. The gas itself was
analyzed; and its heating power calculated, from its composition, as
617.5[theta]. Its specific gravity was .464; and the volume of air was
about seven times that of the gas used (or one-eighth of the mixture),
and was only 11½ per cent. by weight more than was needed for perfect
combustion. The results arrived at were as follows:
Per cent.
Converted into indicated horse power,
including friction, etc. 17.0
Escaped with the exhaust gas. 15.5
Escaped in radiation. 15.5
Communicated to water in the jacket. 52.0
It will thus be seen that more than half of the heat is communicated
to the water in the jacket. Now, this is the opposite of the
steam-engine, where the jacket is used to transmit heat _to_ the
cylinder, and not _from_ it. This cooling is rendered necessary,
because without it the oil would be carbonized, and lubrication of the
cylinder rendered impossible. Indeed, a similar difficulty has
occurred with all hot-air engines, and is, I think, the reason they
have not been more generally adopted. I felt this so strongly that,
for some time after the introduction of the gas-engine, I was very
cautious in recommending those who consulted me to adopt it. I was
afraid that the wear and tear would be excessive. I have, however, for
some time past been thoroughly satisfied that this fear was needless;
as I am satisfied that a well-made gas-engine is as durable as a
steam-engine, and the parts subject to wear can be replaced at
moderate cost. We have no boiler, no feed pump, no stuffing-boxes to
attend to--no water-gauges, pressure-gauges, safety-valve, or
throttle-valve to be looked after; the governor is of a very simple
construction; and the slide-valves may be removed and replaced in a
few minutes. An occasional cleaning out of the cylinder at
considerable intervals is all the supervision that the engine
requires.
The very large percentage of heat absorbed by the water-jacket should
point out to the ingenuity of inventors the first problem to be
attacked, viz., how to save this heat without wasting the lubricant or
making it inoperative; and in the solution of this problem, I look for
the most important improvement to be expected in the engine. The most
obvious contrivance would be some sort of intercepting shield, which
would save the walls of the cylinder and the rings of the piston from
the heat of the ignited gases. I have just learned that something of
the kind is under trial. Another solution may possibly be found in the
employment of a fluid piston; but here we are placed in a dilemma
between the liquids that are decomposed and the metals that are
oxidized at high temperatures. Next, the loss by radiation--15 per
cent.--seems large; but this is to be attributed to the fact that the
inside surface of the cylinder is at each inward stroke exposed to the
atmosphere--an influence which contributes to the cooling necessary
for lubrication. The remaining 15 per cent., which is carried away by
the exhaust, is small compared with the proportion passing away with
the exhaust steam of a high-pressure or the water of a condensing
engine. As the water in the jacket can be safely raised to 212° Fahr.,
the whole of the jacket heat can be utilized where hot water is
required for other purposes; and this, with the exhaust gases, has
been used for drying and heating purposes.
With such advantages, it may be asked: Why does not the gas-engine
everywhere supersede the steam-engine? My answer is a simple one: The
gas we manufacture is a dear fuel compared with coal. Ordinary coal
gas measures 30 cubic feet to the pound; and 1,000 cubic feet,
therefore, weigh 33 lb. Taking the price at 2s. 9d. per 1,000 cubic
feet, it costs 1d. per lb. The 30 cubic feet at 630[theta] give
19,000[theta] all available heat. Although good coal may yield 14,000
units by its combustion, only about 11,000 of these reach the boiler;
so that the ratio of the useful heat is 11/19. The thermal efficiency
of the best non-condensing engine to that of the gas-engine is in the
ratio 4/22. Multiplying together these two ratios, we get
11 4 44
-- x ------- = ----
19 22_{1/2} 4.28
That is, speaking roughly, 1 lb. of gas gives about ten times as much
power as 1 lb. of coal does in a good non-condensing engine. But at
18s. 8d. a ton we get 10 lb. of coal for 1d.; so that with these
figures the cheapness of the coal would just compensate for the
efficiency of the gas. As to the waste heat passing away from the
engine being utilized, here the gas-engine has no advantage; and, so
far as this is concerned, the gas is about eight times dearer than
coal. The prices of gas and coal vary so much in different places that
it is hard to determine in what cases gas or coal will be the dearer
fuel, considering this point alone.
But there are other kinds of non-illuminating gases--such as Wilson's,
Strong's, and Dowson's--which are now coming into use; and at Messrs.
Crossley's works you will have an opportunity of seeing a large
engineering factory employing several hundred mechanics, and without a
chimney, in which every shaft and tool is driven by gas-engines
supplied by Dowson's gas, and in which the consumption of coal is only
1.2 lb. per indicated horse power. The greatest economy ever claimed
for the steam-engine was a consumption of 1.6 lb.; and this with steam
of very high pressure, expanded in three cylinders successively. Thus
in a quarter of a century the gas-engine has beaten in the race the
steam-engine; although from Watt's first idea of improvement, nearly a
century and a quarter have elapsed.
As regards the steam-engine, it is the opinion of competent
authorities that the limits of temperature between which it works are
so restricted, and so much of the heat is expended in producing a
change of state from liquid to vapor, that little further improvement
can be made. With respect to gas-engines, the limits of temperature
are much further apart. A change of state is not required, and so very
great improvement may still be looked for. It is not impossible even
that some of the younger members of our body may live to see that
period foretold by one of the greatest of our civil engineers--that
happy time when boiler explosions will only be matters of history;
that period, not a millennium removed by a thousand years, but an era
deferred perhaps by only half a dozen decades, when the use of the
gas-engine will be universal, and "a steam-engine can be found only in
a cabinet of antiquities."
_Discussion._
The President said this was a very delightful paper; and nothing could
be finer than Mr. Lane's description of the conversion of heat into
power, and the gradual growth of theory into practical work.
Mr. W. Foulis (Glasgow) agreed that it was admirable; but it required
to be read to be thoroughly appreciated. When members were able to
read it, they would find Mr. Lane had given a very clear description
of the elementary principles of thermo-dynamics in their relation to
the gas-engine and the steam-engine. There was very little in the
paper to raise discussion; but Mr. Lane had made exceedingly clear how
the present loss in a gas-engine was occasioned, and had also shown
how, in the future development of the engine, the loss might be saved,
and the engine rendered more efficient.
Mr. H.P. Holt (of Messrs. Crossley Bros., Limited) said he could
indorse everything Mr. Lane had said. He had found the paper most
interesting and instructive even to himself, though he had some little
practical experience of gas-engines, and was supposed to know a little
about them. He did not pretend to be able to teach other people; but
if he could say anything as to indicator cards, or answer any
questions, he should be happy to do so. (He then described the
indicator diagram of the atmospheric gas-engine.) In this engine the
proportion of the charging stroke to the whole sweep of the piston was
about 10 per cent.; and as the charge drawn in consisted of about 10
per cent. of gas, about 1-100 of the total sweep of the piston was
composed of the gas.
Mr. Foulis asked what proportion the power indicated on the diagram
bore to the power indicated on the brake in the atmospheric engine.
Mr. Holt said unfortunately he had not any figures with him which
would give this information; and it was so long since he had anything
practically to do with this form of engine, that he should not like to
speak from memory. He might add that the largest size of gas-engine
made (of about 100 horse power indicated) was at work at Messrs. Edwin
Butterworth and Co.'s, of Manchester. It was now driven by ordinary
coal gas; but Dowson plant was to be put up very shortly in order to
reduce the cost of working, which, though not excessive, would be
still more economical with the Dowson gas--probably only about 30s.
per week. The present cost was about £4 per week, though it was not
working always at full power.
Mr. T. Holgate (Batley) said he thought it was generally understood,
by those who had studied the subject, that the adoption of compression
of the gaseous mixture before ignition had, so far, more than anything
else, contributed to the improved working of gas-engines. This fact
had not been sufficiently brought out in the paper, although Mr. Lane
had clearly indicated some of the directions in which further
improvements were likely to obtain. Gas engineers were largely
indebted to Mr. Dugald Clerk for the statement he had made of the
theory of the gas-engine.[4] Mr. Lane had given some figures, arrived
at by Messrs. Brooks and Steward, from experiments made in America;
but, prior to these Mr. Clerk had given others which were in the main
in accordance with them. Professor Kennedy had also made experiments,
the results of which agreed with them.[5] The extent of the loss by
the cooling water was thus well ascertained; and it was no doubt by a
reduction of this loss that further improvement in the working of
gas-engines would eventually be obtained.
[Footnote 4: See Journal, vol. xxxix., p. 648.]
[Footnote 5: Ibid., vol. xl., p. 955.]
Mr. J. Paterson (Warrington) expressed his appreciation of the paper,
as one of exceptional interest and value. He said he did not rise with
a view to make any observations thereon. The analysis of first
principles required more matured consideration and thought than could
be given to it here. The opinion, however, he had formed of the paper
placed it beyond the reach of criticism. It was now many years since
his attention had been drawn to the name of Denny Lane; and everything
that had come from his facile pen conveyed sound scientific
conclusions. The paper to which they had just listened was no
exception. It was invested with great interest, and would be regarded
as a valuable contribution to the Transactions of the Institute.
Mr. Lane, in reply, thanked the members for the kind expressions used
with respect to his paper. His object in writing it was that any one
who had not paid any attention to the subject before should be able to
understand thoroughly the principles on which gas and hot-air engines
operated; and he believed any one who read it with moderate care would
perfectly understand all the essential conditions of the gas-engine.
He might mention that not long after the thermo-dynamic theory was so
far developed as to determine the amount of heat converted into power,
a very eminent French Engineer--M. Hirn--conducted some experiments on
steam-engines at a large factory, and thought he could account for the
whole heat of combustion in the condensed water and the heat which
passed away; so much so that he actually doubted altogether the theory
of thermo-dynamics. However, being open to conviction, he made further
experiments, and discovered that he had been in error, and ultimately
became one of the most energetic supporters of the theory. This showed
how necessary it was to be careful before arriving at a conclusion on
such a subject. He had endeavored, as far as the nature of the case
allowed, to avoid any scientific abstractions, because he knew that
when practical men came to theory--_x's_ and _y's_, differentials,
integrals, and other mathematical formulæ--they were apt to be
terrified.
The President said it was like coming down to every day life to say
that it was important that gas managers should be familiar with the
appliances used in the consumption of gas, and should be able, when
called upon, to give an intelligent description of their method of
working. A study of Mr. Lane's paper would reveal many matters of
interest with regard to this wonderful motor, which was coming daily
more and more into use, not only to the advantage of gas
manufacturers, but of those who employed them.
* * * * *