Imágenes de páginas
PDF
EPUB

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.

[ocr errors]

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

[graphic][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

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

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

a special hard cast iron, easily drawn when damage or wear necessitates its removal. This liner is secured to the cylinder by bolts E, fig. 5, drawn through the back end of the compression chamber, and the front end, where it enters the cylinder proper, is fitted with a rolling rubber ring F to allow the liner in expanding or contracting to maintain a water-tight joint. The front end of the liner at D1 and D2 is fitted into the bed G, so that in disconnecting the cylinder from the base no water joints are broken.

[graphic][subsumed][merged small]

The piston H is arranged with bosses in the centre through. which the pin I is fitted, and by which the connecting rod J is coupled direct to the crank pin K. The base L, to which the bed G is firmly bolted, is provided with an arrangement of baffle plates, to silence the inrush of air through the pipe M, which forms the connection between the base and the box containing the air valve N, arranged below the direct line of fire, with a gas channel O to ensure the gas mingling with the inrushing air. P is the igniting port, in direct communication with the timing valve and hot tube Q. The exhaust port R, fig. 5, is shown

leading to the valve-box S, fig. 6. The cold water is admitted at the extreme bottom end of the combustion chamber at T, and the hot water outlet is arranged at U. V is the cylinder lubricator, from which the oil runs down a small pipe and enters the cylinder at W; it is worked by an endless leather belt from the cross shaft Y, which is driven by the worm wheels at Z.

The cam and lever arrangements are clearly shown. A1 represents the air cam, B' the normal working gas cam, C1 a gas cam used at starting only, D3 the gas roller controlled by the governor E1; F1 is a cranked rocking shaft, at one end carrying the pin upon which the gas roller works, and at the other transmitting the cam motion to the gas valve through the connecting link G1. The position of the lever H', which has control of the cams B', C', I', and K', is the normal position when the engine is running, but at starting it is moved in the direction indicated by the arrow until the cam I', which is a relieving cam for the exhaust, acts as well as the ordinary cam K' upon the exhausting lever J', and the cam C1 takes the place of the normal gas cam B1; L' is the timing valve cam and M1 a plug for the indicator.

The weak point in this engine is the position of the exhaust valve box. This being placed at the side and in direct communication with the main water jacket C, whilst the main valve opens to the cylinder, the vibration of the engine and the high temperature of the gases tend to break the joint, as the exhaust pipe is at one end rigid and does not respond to any oscillation set up by the engine.

Fig. 8 is an external elevation of a 30 H.P. NOM. engine capable of indicating 100 H.P. with ordinary gas when running at 160 revolutions per minute, and has a cylinder 17 inches diameter with a stroke of 24 inches.

A girder frame type of engine admits of the strains being taken in a direct line between the centres of the crank shaft and the cylinder, and prevents all possibility of the engine' panting.' In this engine there are two main girders, one from each bearing to the cylinder, the crank running between them. The combustion chamber is a separate casting bolted to the end of the cylinder, and the liner of the cylinder is held in position by bolts drawn through it. The air and gas valves are on the extreme

« AnteriorContinuar »