Port a, full open for the exhaust.
Suppose the port a to be at the head end of the cylinder and open to the amount of the lead with the crank on the corresponding dead centre, and if the valve travel be made equal to twice the lap and the lead, the various positions of the valve will be as marked in Figs. from [3298] to [3302]; the event corresponding to each valve position being stated in the figures.
DOUBLE PORTED VALVES.
The term port applies strictly to the area of opening of the steam passage where it emerges upon the valve seat. The term steam passage includes the full length of the opening from the cylinder bore to the face upon which the valve is seated.
A double ported steam port is one in which there are two openings or steam ports, leading into one steam passage.
A double ported valve is one in which there are two ports at each end of the valve. These two ports in some cases admit steam to a single cylinder port, and in others to two steam ports, terminating in one steam passage.
A griddle valve is one that has two or more ports at each end upon a seat that has two or more ports for each steam passage.
Double ported valves are employed in some cases to increase the admission of live steam to the cylinder, and in others to increase the exhaust openings also. The effectiveness of a double ported valve is mainly valuable at the beginning of the stroke, and is especially valuable in cases when the travel of the valve is diminished to hasten the point of cut off, because in such cases the outer edges of the valve do not open the steam port to its full width, and a single port is apt to wire draw the steam. By the employment of more than one port, or several ports, a sufficient admission may be obtained with less valve travel.
