CHAPTER II_Gas and Petroleum Engines

THE WORKING PRINCIPLES OF THE GAS ENGINE

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 T_{2} - T_{1}

— = —————————————

H T_{2}

where W is the amount of work done by an engine supplied with a

quantity of heat, H, and T_{2} is the temperature of the heated gases

which expand doing work, and are thereby cooled to the temperature

T_{1}, 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

T_{2} - T_{1} W

= ————————————— = — = 1.

T_{2} H

It must not be forgotten that T_{2} and T_{1} 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

T_{2} - T_{1}

= ————————————— = 1.

T_{2}

In order that this may be so, we must have T_{1} = 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 T_{2} 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 1/10th 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) coal gas.

(2) carburetted gas.

(3) petroleum.

(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) Explosion of the gases without compression.

(2) Explosion of the gases with compression.

(3) Combustion of the gases with compression.

(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.

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) _Two-cycle type._

Dugald-Clerk

Koerting-Lieckfeld

Wittig & Hees

Andrews (Stockport)

Benz

Ravel

Baldwin

Taylor (Midland)

Campbell

Bénier

(2) _Four-cycle type._

Millon

Otto

Linford

Crossley

Maxim

Martini

Lenoir

Simplex

Koerting-Boulet

Lombard

Durand

Daimler

Varchalouski

Atkinson

Tenting

Diedrichs

Adam

Ragot

Forest

Noël

Charon

Niel

Lablin

Poussant

Roger

Letombe

Lacoin

Cronan

Cadiot

Dürkopp

Brouhot

Levasseur

Fielding

Delahaye

Acmé

Cuinat

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.