shaft bearings have been decided. In the horizontal engine of Ex. B there is nothing to prevent the eccentrics being placed as near to the crank as possible, without fouling the guide bars, and, as will be seen by looking at the cylinder drawing, Fig. 181, this means that the port face may be as close in as is possible with other necessary proportions of the cylinder. (153) Vertical Cylinder with Piston Valve (Ex. A, Fig. 180). The steam chest extends for the whole length of the cylinder, and is provided with loose top and bottom covers, the bottom cover having a stuffing box for the valve-rod. The cross section inside, as seen in the plan, is rectangular with semicircular ends, the steam inlet being at the back, and the exhaust outlet at the front. The chest is divided horizontally by four plates forming a continuation of the walls of the steam passages, each plate having a central hole so that the liner may be tightly fitted in place, bearing against the edges of the holes. The liner has a small top flange, and contains openings at three different heights, which form the steam ports, the object of the liner being to allow of these ports being cut more conveniently and accurately than would be possible if the liner was cast with the cylinder. As seen in plan, there are three openings or ports at each height, the upper and lower ones communicating with the steam passages, and the centre ones with the space marked SS between the two middle division plates. The steam inlet leads into this space, so that the steam makes its way into the inside of the liner through the central openings, and then passes through the upper and lower openings into the top and bottom steam passages, as these are uncovered by the piston valve (see Fig. 185). The space marked SS thus serves as a steam reser voir. The exhaust steam passes out from the cylinder through the same openings, but above and below the valve, and then escapes through the front passage marked ESP, to the exhaust pipe. This passage extends the whole length of the steam chest, and, therefore, the elevation of the outside of the chest is of oblong outline, and similarly for the side elevation, a web of metal being cast on the back of the chest (marked w) and connecting the top and bottom ends. The shape of these ends is shown in plan by the outline marked a, b, c, d, e, f, which may also be taken to represent the plan of the top of the cylinder casting with the covers removed, the plan of the bottom end being similar. covers for the valve chest cover the circular openings in the top and bottom thickness of metal, and in outline are oblong with semicircular ends, as shown by the plan of the valve chest; the flange of the top cylinder cover is cut away next the valve chest, The to give room for the valve chest cover. It will be seen that the distance of the ports from the cylinder requires long steam passages, and, therefore, a considerable addition to the clearance, but this could only be avoided by using a large diameter of valve, which would necessitate a larger valve chest, or by arranging the valve and eccentric-rods, so that they are not in the same vertical line. The lower flange of the cylinder is for bolting to the top of the engine standard. (154) Horizontal Cylinder, with Expansion Valve (Ex. B, Fig. 181). As already pointed out, this cylinder is fitted with a liner, and is provided with steam jackets, the steam from the cylinder jacket passing to the cover jackets through an opening not shown in the drawing. The piston-rod passes through both covers, in order to drive the air pump, the front stuffing box being prolonged and extended in order to provide lugs or projections to which the engine guide bars may be bolted. The port face is brought as near the cylinder centre as possible, allowing for the liner, jacket space, steam passages, and walls, and, as shown in the elevation and plan, has a projecting flat face, which can be readily machined to provide a good working surface for the valve. The expansion valve-rod passes through the back of the chest, and its stuffing box is a separate casting, having a flange bolted to the chest, which also forms a support for the expansion gear (Fig. 186). The top and bottom flanges of the chest are outside, and the two end flanges inside, thus giving a smaller cover, the inside ribs on the cover serving both to strengthen it and to prevent the expansion valve from moving from its seat. A projection is cast above the valve chest, to which is bolted the steam and exhaust pipes. The steam enters the chest through the inlet, and makes its escape as exhaust through the exhaust port into the space marked EP, and thence to the exhaust pipe. The cylinder is bolted to the bed plate by the flanges FF, which extend for the whole length of the cylinder along each side. By looking at the general arrangement (Fig. 179) it will be seen that the distance from the cylinder centre to the bottom of the flanges may be as small as possible, consistent with the necessary clearance between the covers and the bed plate. The height of the cylinder centre above the bed plate should always be as small as possible, as the bed is recessed to give clearance for the crank pin. (155) Length of Cylinder. The length of the cylinder inside from cover to cover is evidently decided by the length of the piston stroke, the width of the piston, and the clearance between the piston and covers when at the end of the stroke. But as the covers are always let into the cylinder for a short distance, as shown in Figs. 180 and 181, the total length of cylinder from face to face, outside, and without covers, is equal to (Stroke of piston) + (width of piston) + (clearance × 2) + (distance the covers are let into cylinder × 2). If the cylinder has only one loose cover, as in Fig. 180, the length will be less by the distance for one cover. (156) Clearance. This is made as small as possible consistent with safety, as all clearance means a waste of steam. It should not be less than ", and for small sized engines is usually " or". For vertical engines the clearance is greater at the bottom, and should be greater for high than for low speeds. 16 (157) Size of Steam Ports.—The amount of steam which can enter the cylinder depends upon the size of the steam port, and as the steam required depends upon the piston speed, it is evident that the port areas must bear some relation to this speed. It is usual to proportion the ports so that the velocity at which the exhaust steam must pass through them in order to escape freely in front of the piston is not greater than 100 feet per second, but when convenient it is better to make the ports large so that the velocity need not exceed 80 or 90 feet per second, the former being a usual practice. Hence, if we know the piston speed and diameter, we can determine the port area, for we have Area of steam ports in square inches = Area of piston in square inches x piston speed in inches per second Velocity of steam in inches per second These proportions give ample size for the steam to get into the cylinder, since its pressure is then greater than at exhaust, although, as will be seen later, the whole of the port is not uncovered for the admission. The larger the port in a direction at right angles to the movement of the valve, the larger the area uncovered for the same movement, therefore the aim is to get the ports as long and as narrow as possible. Practical experience has adopted a port length varying from 0.5 to 0.75 of the cylinder diameter, the most common length being 0·6d. Hence the width of the port can be found by dividing the port area by the port length, taking dimensions for the length to the nearest", and for the width to the nearest". |