Fig. 28.
Look at Fig. 27. Here we have a slide-valve, with faces much wider than the steam ports. The parts marked black, P P, are those corresponding to the faces of the valves shown in previous diagrams ([p. 54]). The shaded parts, L L, are called the lap. By increasing the length of the lap we increase the range of expansive working. Fig. 28 shows the piston full to the left; the valve is just on the point of opening to admit steam behind the piston. The eccentric has a throw equal to the breadth of a port + the lap of the valve. That this must be so is obvious from a consideration of Fig. 27, where the valve is at its central position. Hence the very simple formula:—Travel of valve = 2 × (lap + breadth of port). The path of the eccentric's centre round the centre of the shaft is indicated by the usual dotted line (Fig. 28). You will notice that the "angle of advance," denoted by the arrow A, is now very considerable. By the time that the crank C has assumed the position of the line S, the eccentric has passed its dead point, and the valve begins to travel backwards, eventually returning to the position shown in Fig. 28, and cutting off the steam supply while the piston has still a considerable part of its stroke to make. The steam then begins to work expansively, and continues to do so until the valve assumes the position shown in Fig. 27.
If the valve has to have "lead" to admit steam before the end of the stroke to the other side of the piston, the angle of advance must be increased, and the eccentric centre line would lie on the line E2. Therefore—total angle of advance = angle for lap and angle for lead.
LIMIT OF EXPANSIVE WORKING.
Theoretically, by increasing the lap and cutting off the steam earlier and earlier in the stroke, we should economize our power more and more. But in practice a great difficulty is met with—namely, that as the steam expands its temperature falls. If the cut-off occurs early, say at one-third stroke, the great expansion will reduce the temperature of the metal walls of the cylinder to such an extent, that when the next spirt of steam enters from the other end a considerable proportion of the steam's energy will be lost by cooling. In such a case, the difference in temperature between admitted steam and exhausted steam is too great for economy. Yet we want to utilize as much energy as possible. How are we to do it?
COMPOUND ENGINES.
In the year 1853, John Elder, founder of the shipping firm of Elder and Co., Glasgow, introduced the compound engine for use on ships. The steam, when exhausted from the high-pressure cylinder, passed into another cylinder of equal stroke but larger diameter, where the expansion continued. In modern engines the expansion is extended to three and even four stages, according to the boiler pressure; for it is a rule that the higher the initial pressure is, the larger is the number of stages of expansion consistent with economical working.