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the predetermined amount relative to the capacity of the space to the cylinder volume. Meanwhile the timing valve has opened communication with an incandescent tube, and the compressed charge igniting, the pressure rises so rapidly that the maximum is reached before the piston has moved appreciably on its second forward stroke. The whole stroke is used for expansion as the piston is under its highest pressure at the commencement. This is called the power stroke; near the end of it the exhaust valve opens, and the return stroke is occupied in driving out the burnt gases, except that portion remaining in the compression space untraversed by the piston. These operations form a complete cycle, and the piston is again in the position to take in the charge required for the next impulse.

The regulation of the speed is controlled by a governor arranged to throttle the gas or effect a complete cut-out.

Although the idle strokes of the Otto' cycle are far from theoretically correct, experience has proved that the use of one cylinder and piston, serving alternately the purpose of motor and pump, has undoubted compensating advantages.

This cycle of operations, both for single cylinder and multicylinder engines, can be better seen by reference to figs. 1, 2, 3, and 4.

FIG. 3

Fig. 1, A, represents the conditions at the commencement of the charging stroke of a single cylinder engine. The air and gas valves are shown open to allow the free inlet and admixture

FIG. 4

of air and gas whilst the piston travels to the end of its outward journey. B shows the valves closed and piston ready to

perform its second operation-viz. that of compressing the charge already drawn in. C represents the power stroke; D the exhausting stroke, showing the piston ready to expel the products of combustion.

Fig. 2 shows in another form the cycle of operations of various types of engines, and the cycles of figs. 3 and 4 correspond to section B of fig. 2.

CHAPTER III

DEFINITION OF HORSE-POWERS

AT present much confusion is caused by the way in which various makers give the power of their engines, Nominal, Indicated, Brake, Effective, and Actual horse-power being often used to describe the same engine.

The term 'nominal' is an arbitrary term, without a satisfactory basis to justify its use, as it is not common to any two makers, and even with the same makers the given indicated horse-power has varied from twice to four times the given nominal power.

Indicated horse-power until quite recently was based upon a good-sized diagram and the maximum number of ignitions. obtainable-viz. half the number of revolutions of the crank shaft and was supposed to represent the maximum indicated horse-power; whereas, as a matter of fact, the maximum working load would be at least 20 per cent. less.

Brake, effective, or actual horse-power are only different expressions for the same thing-viz. the net available horsepower that can be taken from the engine. These latter, it will be seen, are the only definitions common to all engines, and the standard by which they should be judged for continuous runs of at least six hours.

The expressions used throughout this Treatise will be:
Nominal horse-power, NOM. H.P.

Indicated horse-power, I.H.P.

Brake, effective, or actual horse-power, B.H.P.

Many people have had their faith in the value of gas engines very much shaken because of want of practical advice in laying them down. Experience teaches that in the first place a considerable margin of power must be provided over and above that specified by the makers. Gas engines will not work regularly and satisfactorily over a run of an indefinite length of time with the full specified load, and there is no doubt that the makers are mainly responsible for the disfavour with which they are often viewed, as they repeatedly-no doubt driven to it by competition-recommend engines which will barely do the work required of them. There is a constitutional difference between the systems of obtaining power from a steam and gas engine that places them on an entirely different basis. A steam engine may be worked up to its test load for almost any length of time, but not so a gas engine. What is given off over a few hours' test must not be considered the working power of the engine, and if gas engine makers will only acknowledge this practical fact and fix a working load' for each engine, instead of the maximum B.H.P. or I.H.P. as at present, they will soon rise higher in popular favour.

The author's experience is that with a load two-thirds of that specified by the makers, gas engines work smoothly and well, but with anything over this, trouble is to be expected,

CHAPTER IV

TYPES OF ENGINES BY VARIOUS MAKERS

Crossley Bros. (Limited), Manchester

FIG. 5 is a longitudinal section, fig. 6 a plan, and fig. 7 an external elevation of a 7 H.P. NOM. engine, having a cylinder 8 inches in diameter, and a stroke of 18 inches, and may be taken as well representing this firm's present practice.

The cylinder A and combustion chamber B are arranged with a continuous water jacket C, and are fitted with a liner D of

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FIG. 5.-7 H.P. NOM. CROSSLEY ENGINE (LONGITUDINAL SECTION)

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