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.
