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Assuming that the earth once formed part of the sun, the whole of the energy at our command for commercial purposes can be traced back to the sun as source. This energy we have received from it in the form of heat, and under certain circumstances the heat is stored up in a latent form in chemical compounds such as coal, petroleum, etc. With our present knowledge it is exceedingly difficult to extract the latent energy from coal and petroleum in any other form but heat, and in order to do so to our greater benefit, it is necessary to study the laws of heat and heat engines. The law which states the relation between heat and other forms of energy such as electricity, mechanical work, is called the principle of the conservation of energy, and forms the first law of thermodynamics. It is enunciated as follows. Whenever a body does work or has work done upon it, there is a disappearance or an appearance of heat, and the amount of heat thus produced or used up is always exactly proportional to the work which is done. The ratio of the amount of work which a certain quantity of heat can produce has been therefore termed the mechanical equivalent of heat.

It has been found by experiment, taking the calorie (C.G.S. unit) as the unit of heat and the kilogramme metre as the unit of work or energy, that the mechanical equivalent is 424. That is to say, the heat necessary to raise the temperature of one kilogramme of pure water at 0° Centigrade through 1° C. (the calorie) is equal to the work done in raising 424 kilogrammes to a height of one metre.

In nearly all commercial heat engines the heat is converted into the energy of movement (kinetic energy) by using some body such as water vapour, gas, or air as an intermediary agent. We do not, however, know at present how to transform heat into mechanical work without losing a greater part of it in the process. Even in the most perfect heat engines at least 70% of the heat is lost, only about 30% being converted into mechanical energy. This is as yet the most perfect result which engineers have obtained even with the most elaborate precautions. As a rule the loss is greater; for instance, many good machines which we consider efficient burn one kilogramme of coal, giving out 8000 calories, equivalent to 3,400,000 kilogramme-metres, and transform only about 400,000 kilogramme-metres into work, the rest, forming nearly 80%, is lost.

It has been the aim of engineers for many years past to reduce this extravagant waste by every means possible, and the very fact that such a waste exists, clearly shows that our vaunted engines are hopelessly wrong in their principle. There is reason, however, to hope that one day we may, by converting the chemical energy of coal direct into electricity, and thereby avoiding the wasteful heat altogether, reclaim at least 80% of the latent energy which nature has so bountifully supplied to us.

It can be shown mathematically that the ratio of the quantity of heat actually converted into work to the total heat used by an engine depends on the temperature at which the heat was absorbed and on the temperature at which the waste heat was discharged. For instance, in a gas engine the efficiency depends on the temperature of the gases directly after the explosion, and on the temperature of the exhaust gases after the work has been done. The exact relation is as follows: the above stated ratio, which is called the theoretical or thermal efficiency, is equal to the difference between the temperature of the hot gases immediately after explosion, and the temperature of the gases of the exhaust divided by the temperature of the hot gases after explosion. This somewhat cumbrous statement may be expressed more clearly in algebraic symbols—

W

T₂–T₁

—

=

———

H

T₂

where W is the amount of work done by an engine supplied with a quantity of heat, H, and T₂ is the temperature of the heated gases which expand doing work, and are thereby cooled to the temperature T₁, at which they are exhausted.

It is therefore evident, that to make an engine work perfectly efficiently we must obtain an amount of work from it exactly equivalent to the heat put in. That is to say, W must equal H in the above equation. We therefore have the efficiency of such a perfect engine

T₂–T₁

W

=

———

=

—

=

1.

T₂

H

It must not be forgotten that T₂ and T₁ are reckoned not in the ordinary scales of temperature such as Fahrenheit and Centigrade, but on the absolute scale, absolute zero being that temperature at which a body has no molecular motion.

Calculations based upon various considerations point to the fact that absolute zero corresponds to about −273° C.

We have just pointed out that in a perfectly efficient engine

T₂–T₁

———

=

1.

T₂

In order that this may be so, we must have T₁ = 0, the absolute zero.

In practice it is impossible to make the temperature of the exhausted gases as low as this, and so the only way to obtain more efficient engines is to make T₂ as large as possible, that is to say, the initial temperature of the gases must be high.

It is, however, just as possible to turn all the heat supplied to a heat engine into work as it is to use up all the energy of a waterfall in a turbine, because the level from which the zero of the potential of the energy water is measured is the centre of earth, which is as inaccessible as absolute zero of temperature.

It therefore behoves us to make the ratio of the initial and final temperatures of the gas which does work in a gas engine as large as possible, and it is for this reason that gas engines can be made more efficient than steam engines, for in the former a momentary initial temperature of 1500° C. may be obtained by the combustion, whilst steam at 200 lbs. on the square inch is at about ⅒th of that temperature. There are practical difficulties which prevent higher initial temperatures being used, residing chiefly in the fact that at 400° C. iron is red-hot, so that any lubricant coming into contact with it is decomposed and loses its lubricating properties. Even at 300° C. most lubricating oils in contact with the air become oxidized and destroyed.

This difficulty of lubrication, by limiting the temperature, at the same time limits the efficiency, and not till some new lubricant is discovered which defies heat will there be much improvement in this direction.

Even as it is, it is necessary to cool the sides of the vessel or cylinder in which the gases expand, and in doing so we lose a great deal of heat.

Hot-air engines using ordinary air as the expansible gas have been devised from time to time, but they have not met with much success owing to their weight and the large amount of space they take up, neither are they as efficient as a good modern gas engine. We will not, therefore, study the theory of hot-air engines, but further consider the details of gas engines, whose superiority over all other heat engines we think we have sufficiently pointed out.

It seems at the present date almost impossible to conceive anything fresh in the cycle of operation of a motor using explosive gases, so numerous and varied are the already existing types. All possible combinations appear to have been considered, and even repeated, for in many recent patents old ideas have once more been brought forward which date back to the early attempts of Lebon, Barnett, Beau de Rochas. The greater number of existing types are based in principle on two or three fundamental ideas, and their improvement is rather to be found in their mechanical design than in the conception of a new cycle.

This fact enables us to classify gas engines very much more easily, because, apart from some perfection of detail, they fall naturally into several groups, which will prevent the reader from losing his way in what otherwise might be chaos. We shall therefore, in describing individual engines later on in this book, follow a systematic course, and arrange the different systems into four classes, which we shall consider in turn.

Motors using

1 (1) coal gas.

2 (2) carburetted gas.

3 (3) petroleum.

4 (4) water gases.

And in order to classify them according to the principles of their cycle of operations, irrespective of their fuel, M. Witz places them in four groups—

1 (1) Explosion of the gases without compression.

2 (2) Explosion of the gases with compression.

3 (3) Combustion of the gases with compression.

4 (4) Atmospheric motors.

The first group of this second classification is formed by motors which have developed the idea conceived in 1860 by M. Lenoir. For the first half of the forward stroke the piston draws in a mixture of gas and air; the valves being then closed, and ignition taking place, the explosion drives the piston to the end of the stroke. The return stroke is made use of to expel the gases through the exhaust. Before igniting the gases which have been drawn in they may be compressed either by a separate pump, or in a chamber forming a continuation of the cylinder.

The arrangement is characteristic of the second group. This again can be modified to form the third group, by allowing the gases to burn under constant pressure throughout the stroke instead of violently exploding at the commencement. Engines using this sort of progressive combustion have been designed by Simon and Brayton.

Finally, in the fourth group the explosion is merely used for obtaining a partial vacuum under the piston, and the work is done by the excess of atmospheric pressure acting on its external surface. It is almost unnecessary to state that this method has been completely abandoned, and has been replaced by a sort of combination type, in which the explosion is used in the forward stroke and atmospheric pressure in the return stroke, such a motor as the Bisschop gas engine being therefore practically double-acting.

The table on page 20, which we have borrowed from M. Witz’s very complete work on gas engines, shows at a glance the cycle of operations in the cylinders of the different types: they are arranged in parallel columns, in order to make it more easy for the reader to compare the operations undergone by the gases before and after their combustion. It is necessary to subdivide the motors of the second group into three, according as the cycle of operations is completed in one, two, or three complete revolutions of the fly-wheel. Perhaps this subdivision is somewhat unnecessary, because the employment of a second cylinder for compressing the gases does not alter the character of the cycle, but we think that it will make the classification clearer if we proceed in this manner.

Group I. Without compression.	Group II. With compression.	Group III. Combustion and compression.	Group IV. Atmospheric.
1. Explosive mixture enters the cylinder at atmospheric pressure	1. Explosive gases enter the cylinder at atmospheric pressure	1. Explosive mixture enters the cylinder at atmospheric pressure	1. Explosive mixture enters the cylinder at atmospheric pressure
	2. Compression of the gaseous mixture	2. Compression of the gaseous mixture
2. Explosion at constant volume	3. Explosion at constant volume	3. Combustion at constant pressure	3. Explosion at constant volume
3. Expansion of gases in cylinder	3. Expansion of gases		3. Piston driven back by the pressure of the atmosphere
4. Products of combustion expelled from the cylinder	5. Products of combustion expelled from the cylinder	4. Products of combustion expelled from the cylinder	4. Products of combustion expelled from the cylinder

Group I.

Explosion without compression.

 Lenoir

 Kinder & Kinsey

 Hugon

 Ravel

 Turner

 Bénier

 Parker

 Hutchinson

 Forest

 Baker

 Economic motor

 Crown

 Laviornery

 Lentz

Group II.

Explosion with compression.

1 (1) Two-cycle type.Dugald-ClerkKoerting-LieckfeldWittig & HeesAndrews (Stockport)BenzRavelBaldwinTaylor (Midland)CampbellBénier

2 (2) Four-cycle type.MillonOttoLinfordCrossleyMaximMartiniLenoirSimplexKoerting-BouletLombardDurandDaimlerVarchalouskiAtkinsonTentingDiedrichsAdamRagotForestNoëlCharonNielLablinPoussantRogerLetombeLacoinCronanCadiotDürkoppBrouhotLevasseurFieldingDelahayeAcméCuinat

Group III.

Combustion with compression.

 Brayton

 Hoch

 Simon, et fils

 Livesay

 Crowe

 Gardie

 Overmand

Group IV.

Atmospheric motors.

 Otto & Langen

 Bisschop

 Gilles

 Hallevell

 Robson

 François

Carburetted Air Engines.

 Lenoir

 Forest

 Tenting

 Daimler

 Le Marcel (Cadiot)

 Durand

 De Dion-Bouton

 Bollée

 Pelloree

 Le Pygmée

 Klause

Oil Engines.

 Brayton

 Priestman

 Ragot

 Otto

 Crossley-Holt

 Niel (Atlas)

 Hornsby-Akroyd

 Grob-Capitaine

 Merlin

 Knight-Weyman

 Griffin

 Pinkney

 Levasseur

 Root

 Rationnel

 Dawson

 The “Gnome”

On page 21 we give a table embracing all the best known types of gas engines, which will also help to avoid the confusion arising from the fact that some motors exist which belong to neither one nor another, but are combinations of one or more groups. Such hybrid motors have been devised amongst others by Schweizer and Siemens. In the former the power of the explosion is used to compress a considerable volume of air, which is then used for working a compressed air engine. In the latter the gas heats a quantity of air which drives a hot-air motor. In this table we have also, specially grouped apart, engines using carburetted air (air which has been passed through a volatile spirit such as benzoline) and petroleum. The list may be found somewhat incomplete, as more than 250 gas engines have been devised and patented in the last twenty-five years; but on the other hand, many of these have been failures, and we have only included those motors which can undoubtedly be considered commercial successes. These we will now study.