CHAPTER XI. PRELIMINARY CONSIDERATIONS ON THE APPLICABILITY OF VARIOUS KINDS OF STEAM-ENGINES TO VARIOUS PURPOSES. THE first step to be taken when it is proposed to construct a steam-engine, consists in determining the general form of the main features of the engine, without regard to the minor details; wherefore, it is proper to enter upon this subject before following up complete descriptions of steam machinery. This chapter will be devoted to a comparison of the different arrangements already mentioned, regard being had only to the main features; such as cylinder, connecting rod, beam, crank, &c.,' as applied to a variety of purposes. Steam-engines will be first divided into condensing and noncondensing, the former being the most costly in construction, and taking up a great deal of room, but making full amends for this in the economy with which they work. The latter kind of engine is exceedingly compact, and capable of working at very high speeds, simple in construction, and cheap, occupying a small space, but very far inferior to the condensing engine in point of economy of fuel. Engines may also be divided into three classes: stationary, marine, and locomotive. In the stationary class, we have subdivisions according to form, as follows: beam engines, vertical engines, table engines, horizontal engines, inclined engines, oscillating engines, pendulous or inverted oscillating engines, grasshopper engines, rotatory engines, and disc engines. Marine engines may be divided into the following classes: first, paddle engines and screw engines, according to the method of propulsion; secondly, according to the form, into side-lever engines, upright engines, inclined engines, oscillating engines, horizontal engines, rotatory engines, disc engines, &c. Locomotive engines include those for railway purposes and those which run on common roads. They may be divided into two classes: one with the engines above the boiler, and the other with the engine beneath it. They are invariably high-pressure, as it is necessary that they should occupy but small space, and be as light as possible. Let it be required to design an engine for manufacturing purposes, then it will be necessary to consider which class of engine will be most suitable to the purpose, a rotatory motion being supposed to be required. If the neighborhood in which the engine is to be erected be plentifully supplied with water, then it will be advisable to construct an engine on the condensing principle, for the sake of economy of working. If, however, there be not room for the bulky machinery required, then a high-pressure éngine must be employed. The beam-engine will be found to work very steadily, and is perhaps the most convenient form that can be adopted when a high velocity is not required; but if the speed must be considerable, then an engine of lighter parts will be preferable. It may be desirable to examine the action of reciprocating masses thus employed. Examining the action of the engine during one stroke, we find that we have the piston, piston rod, &c., in a state of rest at the commencement of the stroke. These bodies are then set in motion, their velocity gradually increasing, but before the next stroke can be made, the work accumulated in these parts must be absorbed, and the manner in which this absorption is effected is one of vital importance. If the steam be simply cut off, then this work accumulating in the above-mentioned masses of metal will evidently be expended upon the bearings and joints; but if the method usually known as cushioning be adopted, the greater part of the accumulated work will be economized. By cushioning the piston is meant the introduction of the steam for the following stroke, before the termination of the previous one; then the greater portion of the work accumulated will be expended in compressing the steam, and so soon as the crank has passed the centre, or dead point, the piston will change the direction of its movement, and the compressed steam will expand, and give up the work which it had absorbed; hence the reciprocating engine may by careful management be caused to work with great smoothness and regu larity. In the more compact class of steam-engines, such as table APPLICABILITY OF STEAM-ENGINES TO VARIOUS PURPOSES. 111 engines, horizontal, inclined, and vertical-engines, a less degree of cushioning is required than in beam-engines: because in the former case the moving masses do not possess so much inertia as do those of the latter class of engines. It has been proposed to use masses of metal to counterbalance the effects of the reciprocating parts of steam-engines, being of equal weight with the latter, and always moving in an opposite direction. This, however, is perfectly superfluous for fixed engines. With regard to rotatory-engines, it may be desirable to offer a few remarks in this place. In these machines there are no reciprocating parts having sufficient inertia to render them worthy of serious consideration, wherefore, high velocities may safely be used. There is also the advantage on their side, in point of uniformity of movement, as the moment of power about the main shaft remains constant for any position of the same, the variations entailed by the crank being thus avoided. It is a great mistake to imagine that any saving is effected by having an exceedingly heavy fly-wheel, to render less evident the jerks and reactions produced by the reciprocation of the machinery of the ordinary engine; for although the velocity may be thereby rendered more uniform, yet the jerks and vibrations will still exist, although they be less evident. Hitherto rotatory engines have not been attended by results sufficiently satisfactory to induce their employment by the generality of mechanical engineers: this being due, in a great many engines, to the impossibility of keeping the valves and packings in a steam-tight condition, which defect destroys the economy of the machine, a large quantity of steam being lost by leakage, the difference between the surfaces of contact of the moving parts of the two classes of steam-engine being as follows: In the reciprocating engine surfaces of contact of any desired extent may be obtained, and these surfaces may be scraped so as to work upon each other almost perfectly steamtight; whereas with rotatory engines it is but seldom that good steam-joints can be obtained, the engineer being, in the greater number of cases, obliged to substitute these broad surfaces of contact by others so narrow, that, practically speaking, they may be regarded as simple lines. From these remarks it may be concluded that for the purposes above mentioned, where great steadiness is required, beam-engines may be used with advantage. Next to these, with regard to uni formity of movement, oscillating engines may be ranked, more especially when constructed with the cylinders above the main shaft, which form is known as the pendulous engine; and if this be made with a proper regard to the principles which regulate the motion of vibrating masses it will be found to work with great smoothness. With regard to marine engines, the following remarks will embody most of the considerations by which the engineer is directed to a conclusion as to the class of machine to be employed for any particular vessel. The first consideration is the available space, which is generally rather in defect as regards height, of that which is most convenient; hence from time to time various means of overcoming this inconvenience have been devised. The first consisted in the employment of beams, placed beneath the cylinders, with connecting rods attaching their extremities to the piston rod and to the crank; oscillating engines have, however, been found, on the whole, most convenient for paddle-wheel engines. The introduction of the screw-propeller has given rise to a great variety of designs, the main objects being to shorten the screw shaft as much as possible, and to let the engines act upon it direct; that is to say, without the use of tooth wheels, or spur gearing, as it is generally called. With regard to locomotives, there is but little to be said, the circumstances of the case requiring always a compact design, and admitting only, in the present state of science, of the use of hori zontal or inclined engines, with fixed cylinders. These cylinders are sometimes placed within the framing of the engine, and below the boiler, and at other times on the outside of the framing. In concluding these general remarks, it is thought desirable to direct the reader's attention to the accompanying Plate, No. XI., illustrative of the main features of various descriptions of engines; the lettering is the same on all the figures:-a is the cylinder, c the piston rod, d the beam, e the connecting rod, f the crank, g the main shaft, and h the eccentric, by which the valves which admit the steam to the main cylinder are worked. CHAPTER XII. ON THE DETAILS OF STEAM-ENGINES. Cylinders and Valves. PRELIMINARY theoretical and general practical considerations having been discussed, the next step to be taken will consist in an account, principally of a descriptive character, of the various details or elementary parts of steam-engines as they exist, without regard to the purposes to which they are applied; and in following out this course care will be taken to describe fully every peculiarity of form, and to indicate the end intended to be gained by such peculiarity, without, however, entering into any considerations of a purely theoretical character. One of the most general details of all classes of steam-engines is the steam-cylinder, which will therefore first require attention. Cylinders may be divided into two classes. fixed and oscillating. Fig. 44. α Fig. 44 represents a section of an ordinary fixed cylinder; that is to say, it exhibits the form of a cylinder which has been cut through the centre, the cut parts being exposed to view in plan or vertical d section. It is of course circular; a represents the body of the cylinder, b and c are covers, of which however the description will be postponed for the present, the body or central part of the cylinder being first considered. It is necessary to provide means for the entrance and exit of steam to and from the upper and lower д parts of the cylinder; the extremities of the passages through which the steam påsses from the steam chest to the interior of the cylinder are shown at d and e, f indicating the entrance to the pipe through which the steam, having done its д |