Fan. 19, 1882] 
NATURE 
281 
engaged in considering the differences between the various kinds 
of these machines and the improvements that can be effected in 
them, the mechanical engineer should give his careful attention 
to the possible improvements that can be made in the engines 
that drive the electric generators. 
As long as the lighting of our large cities was performed by 
gas the cheap manufacture of illuminating gas was the important 
question, but now that electric lighting bids fair to displace other 
systems the question that has special interest is, not the extrac- 
tion of illuminating gas from coal, but the employment of the 
store of energy in the latter to set in rapid rotation dynamo- 
electric machines for producing the electric current used in 
lighting. 
At present steam-engines are chiefly used to drive the dynamo 
machines, but even with the best engines and boilers it is well 
known that the fuel consumption is excessive compared with the 
actual work done. So good an authority as Sir William Arm- 
strong has recently said that with a good condensing engine only 
one-tenth of the whole heat energy of the fuel is realised in 
useful work, and this is no exaggeration of facts. What there- 
fore must be said of a small engine and boiler of the ordinary 
type? The main reason why the efficiency of even the best 
steam-engines is so low is because in an ordinary engine steam 
can only be used at a comparatively low temperature ; for it can 
be proved that, with the temperatures which can be used in con- 
densing-engines, the efficiency of even an imaginary perfect 
engine, without friction and loss of heat, cannot exceed 4%, or 
only double the efficiency of a good modern steam-engine ; that 
is to say that a good engine of large size uses only ,4, of the total 
heat, and that it is not possible to use more than 3, with an 
engine of perfect mechanism. 
It may be assumed that in Jarge compound marine engines the 
fuel consumption is about 2 lbs. per indicated horse-power, but 
it cannot yet be said that engines of this class and of very high 
power will be used in central stations for electrical purposes ; at 
any rate it must be remembered that besides other considerations 
there is a great objection to the use of a single very large engine 
to electrically light a district, for the accidental stoppage of this 
engine would plunge the whole neighbourhood into ,darkness. 
Engines and boilers of the portable type are those generally 
used now for electrical purposes, and in a competition in Eng- 
land of several of the best engines of this class the fuel consump- 
tion was about 4 1b. per indicated horse-power per hour; but 
in daily practical work it may be assumed that 6 to 7 lb. more 
nearly represent their usual fuel consumption. This gives an 
efficiency of only about 34. 
With a hot-air engine there is this great disadvantage, that it 
is extremely difficult to prevent the lubricants from being burnt 
and the air vessel being injured by heat, since the latter vessel 
must be kept as hot or hotter than the air, because the tempera- 
ture of the air is raised by an external fire. The only other 
motor suitable for electrical purposes (apart from machines 
driven by water or wind power) is the gas-engine. In the latter 
the power is obtained by the admission of an explosive mixture 
of gas and air into the cylinder, and the piston is driven by the 
explosion produced on the ignition of this mixture. 
Now there is this great difference between a hot-air engine 
and a gas-engine, that in the latter the high temperature arising 
from the explosion is produced z#side the cylinder, and not out- 
side: so that, although the gas at the moment after explosion is 
extremely hot, the cylinder, piston, and lubricant may be kept 
cool by an external stream of water, which is of course impos- 
sible in a’ hot-air engine where the air is heated from the out- 
side. Again, the very high temperature developed in the 
cylinder after the explosion has taken place is rapidly reduced 
by the piston doing work before there is time for the gas to give 
up much of its heat to the cylinder and piston. Steam, how- 
ever, can only be used at a very high temperature, provided the 
apparatus is made exceedingly strong. 
With the present temperatures employed, the theoretical effi- 
ciency of a gas-engine might be raised from 56 to 75 per cent., 
if loss of heat by conduction, radiation, and convection, as well 
as friction, could be prevented; while in a condensing steam- 
engine the greatest efficiency that could be obtained with the 
present temperatures employed could never exceed about 20 per 
cent. 
It was thus shown that practically a gas-engine admits of being 
worked with much greater efficency than either a steam-engine 
or a hot-air engine—that is to say, the percentage of heat the 
former turns into mechanical work is much greater than with the 
lattertwo. It was, however, necessary to consider the economy of 
working, which depends on the relative price of the fuel em- 
ployed, and other items of working cost. Comparative esti- 
mates were therefore given of the working cost of a steam-engine 
of the portable type and of an Otto gas-engine, both indicating — 
30 horse-power, for 300 days of nine hours each (the horse-power 
about necessary to keep alight the 400 Swan incandescent lamps 
used to illuminate the Salle du Congres during this lecture), The 
cost of the coal-gas was taken at three shiliings per 1000 cubic 
feet (or about 13 centimes per cubic metre, only about half the 
actual price in Paris), and it was thus seen that, in spite of the 
very great relative efficiency of the gas-engine, the cost of work- 
ing with ordinary coal-gas is greater than in the case of the 
steam-engine, Ordinary coal-gas, however, has been prepared 
for producing not heat, but light, and has therefore been elabo- 
rately purified at a considerable cost, so that when used in a 
gas-engine it is used for a purpose quite different from that for 
which it was intended. 
A gas-engine burning illuminating gas is, in fact, in the same 
position as was a few years ago an electromotor, or machine 
for converting electric energy into mechanical power. An elec- 
tromotor is an extremely efficient machine, but the fuel burnt to 
produce the electricity was, until quite recently, zinc, and conse- 
quently was far too expensive to allow the use of electromotors 
to be commercially successful. So in the same way, if it is 
attempted to work gas-engines by burning illuminating gas at 
even 133 centimes the cubic metre, or half the actual price of 
the ordinary gas in Paris, they cannot even be worked as econo- 
mically as steam-engines, in spite of their superior efficiency and 
of the much smaller cost for superintendence. But if it be 
possible to manufacture for their use a cheap heating gas in the 
same way as it is now possible to produce electric energy econo- 
mically by burning coal, which is a much cheaper fuel than zinc, 
then the result, as you will see, becomes just the reverse, and 
small gas-engines driven with such gas not only greatly surpass 
in economy steam-engines of the same size, but produce energy 
at a cheaper rate per horse-power than the largest steam-engines 
ever made, 
The lecturer then described what had been done by Dr. 
Siemens and others who have made a heating gas for furnace 
work by means of passing air only, or air with a small admix- 
ture of steam, through a mass of burning fuel. Such gas, how- 
ever, contains too much nitrogen (60 to 70 per cent.) to be 
suitable for gas-engines and other purposes requiring it to be 
used in small quantities, and the plant is large and costly, 
Reference was then made to what had been done by Mr. Dow- 
son of London, who has perfected a gas-generating apparatus, 
into which he passes steam at pressure with a certain portion ot 
air. This he effects by an arrangement similar to a steam-engine 
injector or a jet pump. The air thus drawn into the generator 
serves to keep the column of fuel through which it passes at a 
high temperature, without an exterior fire, so that the decompo- 
sition of the steam and the other chemical reactions take place 
without interruption. The working of the generator is thus 
regular, and the gas is produced without fluctuations in quality. 
Experiments were made with a eudiometer, in which were 
three volumes of the Dowson gas and one of oxygen, and on 
exploding the mixture, 36 per cent. of the total disappeared. 
This corresponded with the following composition of the Dow- 
son gas, viz. hydrogen, 20 per cent. ; carbon monoxide, 30 per 
cent. ; carbon dioxide, 3 per cent. ; and nitrogen, 47 per cent. 
by volume. It was also shown that this gas burns without 
smoke or any deposit of soot on a piece of porcelain, whether 
placed above or in the middle of the flame. 
About 50 per cent. of this gas is combustible, and its calorific 
power, or the number of heat units produced by the combustion 
of a cubic metre, is 1,558,358. Its calorific intensity is 2268° C. 
To compare it with ordinary coal-gas we may take the calorific 
power of a cubic metre of the latter to be 5,590,399, and its 
calorific intensity as 25549? C, 
In the Otto gas-engines a large proportion of air is mixed with 
the coal-gas, so that the effect of the explosion may continue 
during the stroke of the piston by the air taking up some of the 
heat produced ; and as the Dowson gas requires less air for its 
combustion, it is found that in the same cylinder there is not 
more nitrogen and unused oxygen in the charge of Dowson gas 
with its mixture of air, than with coal-gas and the quantity of 
air which is given to the latter. That is to say that the same 
power can be developed in the engine with coal-gas or Dowson 
gas if the supply of gas and air be exactly proportioned. 
