WHY DECREASING THE VALVE-TRAVEL INCREASES THE PERIOD OF EXPANSION.
Increase of expansion follows reduced valve-travel, from a similar cause to that which produces expansion when lap is added to the edge of a slide-valve. When the valve is made with the face merely long enough to cover the steam-ports, there can be no expansion of the steam; for, so soon as the valve ceases to admit steam, it opens the steam-port to the exhaust. When lap is added, however, the steam is inclosed in the cylinder, without egress for the time that it takes the lap to travel over the steam-port. An arrangement of motion which will make the valve travel quickly over the port, has a tendency to shorten the period for expansion; while making the valve travel slowly over the port, has the opposite effect, and protracts expansion. A valve with, say, five inches travel, has a comparatively long journey to make during the stroke of the piston; and the lap-edges will pass quickly over the steam-ports,—much more quickly than they will when the travel is reduced to three inches. In a case of this kind, there is more than the mere reduction of travel to be considered. Suppose the valve has one inch lap at each end. When it stands on the middle of the seat, it has a reciprocating motion of two and one-half inches at each side of that point to make. At the beginning of the stroke, it has been drawn aside one inch (we will ignore the lead), but still has one and one-half inch to travel before it begins to return. On the other hand, when the travel is reduced to three inches, the valve has only one and one-half inch to travel away from the center; and, one inch being moved to draw the lap over the port, there only remains one-half inch for the valve to move before it must begin returning. This entails an early cut-off; for the valve must pass over the ports with its slow motion, and be ready to open the port on the other end, before the return stroke. Thus a travel of five inches draws the outside edge of the valve one and one-half inch away from the outside of the steam-ports, three inches travel only draws it one-half inch away, and a greater reduction of travel decreases the opening in like proportion.