THE DOUBLE-ACTION OSCILLATING ENGINE.
The cylinder of the double-action engine is of necessity made with ports very similar to those of the horizontal engine already described. There is a solid piece attached to the cylinder as before, which is drilled down to the upper and lower part respectively of a central boss, turned very flat upon the face, and which has to work against a similar flat surface as in the last engine. But the ports in the latter are four instead of two, and in an engine with upright cylinder would be cut as follows, and as shown in Fig. 70, C.
Fig. 70.
Those on the right marked st are steam-ports, which, being drilled into one behind, are connected with the boiler. The other two marked ex, are similarly exhaust-ports opening into the air. The spaces between a b and c d of fig. C must be wide enough to close the steam-ports in the cylinder, when the latter is perpendicular and the engine at rest. When the cylinder leans to the left, oscillating on the central pin between the ports in the middle of the circle, the lower port of it will evidently be in connection with the steam-port in C, while the upper port of the cylinder will be opposite to the exhaust. As the cylinder is carried over towards the right, the upper steam-ports will come into action in a similar way, while the lower exhaust-port is also carrying off in turn the waste steam. The impetus, therefore, of the fly-wheel has here only to carry the ports over the spaces a b, c d, and to prevent the crank stopping on the two dead points. This, therefore, is a genuine double-action engine, and will answer, even on a large scale, very satisfactorily. If you do not quite understand the action of these ports, cut out two pieces of card, E F. Let E represent the cylinder. Draw circles, and cut two ports. Cut another piece of card to represent the brass block, with ports, c d; pin them together through the centres of the circles, and they will easily turn on the pin. Mark the ports, so that you will see at a glance which are steam and which exhaust. Now cut out the ports with a penknife, and as you work the two cards together, swaying that which represents the cylinder to and fro upon the other, you will see when the ports in each card agree with one another, and which are opposite to which. This will teach you far better than any further written explanation. You will also see that, instead of making the steam and exhaust ports respectively with a division between, the two steam-ports may be in one curve united, and likewise the two exhausts; but take care not to unite the exhaust with the steam-ports. There is no way so easy as this of reversing the action of the steam; it is, in fact, a circular slide-valve, but wonderfully easy to make, because you have no steam-case to make, nor any attachments whatever.
The faces of the valve are kept in close contact in one of two ways—either the centre-pin is fixed into the cylinder face, and after passing through the brass boss with the ports, is screwed up with a nut at the back; or else there is fixed a small pillar or upright on the opposite side of the cylinder, and a little pointed screw passing through this presses against the cylinder, and makes a point of resistance, against which it centres, and on which it turns. This is shown at fig. A. A small indentation is made where the point comes in contact with the cylinder.
In a locomotive engine there are two such cylinders, working against opposite faces of the same brass block containing the ports. The cranks are also two, on the shaft of the driving-wheels, and are at right angles to each other; so that when one piston is at the middle of its stroke, the other is nearly or quite at the end of it. Thus, between the two there is always some force being exerted by the steam; and the dead points of one crank agree with the greatest leverage of the other. In locomotives, too, the cylinders generally are made as in the present drawing, viz., to oscillate on a point at the middle of their length; but it is just as easy to have the two ports meet at the bottom instead, so that the point of oscillation may be low down, like the single-acting cylinders of the last sketch, and this is generally done when the cylinder is to stand upright.
There is no occasion for me to draw an engine with double-acting oscillating cylinders, because in appearance it would be like the single-acting one; but whereas the latter is of absolutely no use, seeing that the greater part of its motion depends on the impetus of the fly-wheel, the former can be made to do real work, and is the form to be used for marine and locomotive engines. For the former, oscillating cylinders with slide-valves are used in practice; but for real locomotives fixed cylinders are always used. Of course either will answer in models, and it will be good practice to try both.
I have now given sufficient explanation of how engines work, and how they may be made, to enable my young mechanic to try his hand at such work. The double-action oscillating engines especially are well worthy of his attention, as he may with these fit up working models of steam-boats and railway trains, which are far more difficult to construct with fixed cylinders and slide-valves. I shall therefore close this part of my work with a description of one or two useful appliances to help him in the manipulative portion of his labour,—for here, as in most other matters, head and hand and heart must work together. The heart desires, the head plans, the hands execute. I think, indeed, I might without irreverence bring forward a quotation, written a very long time ago by a very clever and scientific man, in a very Holy Book: “Whatsoever thy hand findeth to do, do it with all thy might.” Depend upon it, success in life depends mainly upon carrying into practice this excellent advice. If you take up one piece of work, and carelessly and listlessly play at doing it, and then lay it down to begin with equal indifference something else, you will never become either a good mechanic or a useful man. If you read of those who have been great men—lights in their generation—you will find generally that they became such simply by their observance of that ancient precept of the wise man. They were not so marvellously clever—they seldom had any unusual worldly advantages; but they worked “with all their might,” and success crowned their efforts, as it will crown yours if you do the same.
