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It is generally accepted that the modern gas engine, as we have it at present, has been developed by experiment, and so far most engineers, after lamentable errors and waste of time, have reluctantly adopted the Beau de Rochas cycle, as being the simplest system on which an engine can be constructed that will give satisfactory results.
It is not in the scope of this treatise to deal with any other cycle, but it may safely be taken that engines which give an impulse every revolution when working at full power will ultimately take the place of the simple form of ‘Four' cycle engine.
GENERAL DESCRIPTION OF • OTTO CYCLE GAS ENGINE
THE present-day form of the commercial gas engine has a single cylinder, and gives an impulse every two revolutions when working at full power, and closely resembles a single-acting steam engine, with the working parts of excessive strength. The cylinder is open-ended, and has a trunk piston having sufficient bearing surface to dispense with the use of crosshead slides, the connecting rod joining directly the piston and the crank. The crank shaft is heavy and flywheels large, as considerable energy has to be stored to take the piston through the negative parts of the cycle (see fig. 1). The cylinder is considerably longer than the piston stroke, leaving a space at the back end, into which the piston does not enter, called the combustion chamber. Below the centre line of the engine is placed a shaft, rotating at half the speed of the crank shaft, from which it receives its motion by worm gearing, actuating at suitable times the air, gas, exhaust, timing valve, governor, and cylinder lubricator. The cylinder serves alternately the purpose of motor and pump, as during the forward stroke of the piston the air and gas valves are opened by means of cams and levers, and the gas and air enters from about the beginning to the end of the stroke, the return stroke being utilised in compressing the mixture into the compression space; and when the piston is full in the pressure has risen to
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. 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
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; ID 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.
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.