It must be noticed here—(1) that steam is admitted at full pressure all through the stroke; (2) that admission begins and ends simultaneously with the stroke. Now, in actual practice it is necessary to admit steam before the piston has ended its travel, so as to cushion the violence of the sudden change of direction of the piston, its rod, and other moving parts. To effect this, the eccentric is set more than 90° in advance—that is, more than what the engineers call square. Fig. 26 shows such an arrangement. The angle between E and E1 is called the angle of advance. Referring to the valve, you will see that it has opened an appreciable amount, though the piston has not yet started on its rightwards journey.
"LAP" OF THE VALVE—EXPANSION OF STEAM.
In the simple form of valve that appears in Fig. 24, the valve faces are just wide enough to cover the steam ports. If the eccentric is not square with the crank, the admission of steam lasts until the very end of the stroke; if set a little in advance—that is, given lead—the steam is cut off before the piston has travelled quite along the cylinder, and readmitted before the back stroke is accomplished. Even with this lead the working is very uneconomical, as the steam goes to the exhaust at practically the same pressure as that at which it entered the cylinder. Its property of expansion has been neglected. But supposing that steam at 100 lbs. pressure were admitted till half-stroke, and then suddenly cut off, the expansive nature of the steam would then continue to push the piston out until the pressure had decreased to 50 lbs. per square inch, at which pressure it would go to the exhaust. Now, observe that all the work done by the steam after the cut-off is so much power saved. The average pressure on the piston is not so high as in the first case; still, from a given volume of 100 lbs. pressure steam we get much more work.
HOW THE CUT-OFF IS MANAGED.
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