PLAIN SLIDE-VALVE. 17
mum steam port opering. He may then determine the proper
eccentricity by trial as just explained, or he may make a deter-
mination by the following method.
PROBLEM III. Given the maximum port-opening, the crunk- angle at cut-off, and the lead-angle, to find the eccentricity, the lap, und the angular advance.
Assume an eccentricity, preferably a little larger than that required, and solve the problem by the method of Problem 1, by drawing the crank-positions at OA and OC for admission and cut-off, and bicccting the angle AOC and drawing the valve-circle OP 1. Draw the lap-arc qg", thus determining the maximum port-opening "P₁. From this trial port-opening and eccen- tricity, and the desired port-opening, determine the proper excen- tricity by the following proportion:
Assumed Required Assumed Required
port-opening: port-opening::eccentricity:eccentricity; which is evident from the similarity of the figures. In the case represented by Fig. 2, Pl. IV, the desired port- opening is of an inch; an eccentricity of T of an inch gave a port-opening of of an inch, so that the proportion
grave
e=1 1 of an inch.
With this excentricity the second valve-circle OP is drawn, from which all the elements of the valve may be taken.
In general, it is preferable to solve the proportion numerically, but if desired a graphical solution may be made as follows: Lay off the line 0 g"Pi, in a convenient position, and join e₁₁; make or equal to the given maximum port-opening, and draw ce parallel to c₁e; then is the centre of the required valve- circle and OP is the required eccentricity.
Modifications. In general it is not of great importance that the cut-off shall occur exactly at the chosen crank-angle or piston- position, and it is seldom necessary to know the angular advance
