into place like ordinary piston rings. It is practically, in working, two D valves the two ends of which control admission and exhaust to and from the high-pressure cylinder, the inner rings doing same for the low. Fig. 162 shows the valve; Fig. 163 the ported seat or bushing in which it plays.

Figs. 164 and 165. Pilot Ports.

Q. How can we keep the cut-off the same when we give or increase lead?

A. By shortening the steam lap.

Q. How can compression or cushion be kept the same when we give or increase lead?

A. By shortening the exhaust lap.

Q. What is an auxiliary starting port?

A. As seen in Figs. 164 and 165, it is an extra port, the valve cage, in advance of the main steam port, to do the same as an excessive steam lead. In the example shown, it is 134 in. in advance of the main port; is 1% by 11⁄2 in., with 1/4 in. steam lap; it has 1 15/16 in. lead as against 3/16 in. for the main port. It is in action at all times when the engine is running.

CHAPTER L

VALVE DIAGRAMS*

Q. How may the relative positions of piston and valve, and the occurrences in cylinder and steam chest, be represented by a diagram?

A. According to Bilgram, by drawing a semicircle the diameter AB of which (Fig. 166) represents the piston stroke on one scale, the valve travel on another; drawing

parallel to this diagram a line DE, the distance of which from the latter represents the lineal valve lead; parallel to this again another line FG at a distance corresponding to the outside lap. Where this cuts the semicircle at G, strike one circle with the steam lap as radius (this will be tangent to the lead line) and another with either the exhaust lap or the inside clearance, as the case may be, as radius. From the center C of the semicircle draw two radii tangent to each of these lap circles, cutting the semicircle at points H, K, L. M. Where there is exhaust lap these cut the semicircle at points corresponding to crank*Not to be confused with indicator diagram or cards.

pin positions at cut-off, compression, release, and preadmission respectively. (Where there is exhaust clearance then compression and release will simply exchange places.) Perpendiculars Hh, Kk, Ll, Mm from these four cutting points to the semicircle diameter AB will cut the latter at points h, k, l, m, indicating the corresponding piston positions. The angle between the radius CG and the semicircle diameter is the amount in excess of 90° that the eccentric belly must be set ahead of the crank.

Q. Does the diagram thus made take any account of the irregularities caused by the angularity of the connecting rod?

A. No; it shows the occurrences as taking place in both ends of the cylinder exactly alike.

Q. How may the angularity of the connecting rod be taken account of?

A. By taking as radius the length of the connecting rod on the same scale as the piston stroke, and with centers on the semicircle produced in both directions drawing circular axes from H, K, L, M (instead of perpendiculars) to that diameter; the eight new cutting points will indicate the earlier and later piston positions due to the rod.

Q. Suppose there is so-called negative lead, how is the diagram made?

A. As before, only with the lead line DE below instead of above the semicircle diameter AB; the lap to be measured as before from the lead line and not from the diameter.

Q. How can this diagram be used to determine the desired amount of lap to cut-off, etc., at any desired piston position?

A. By working it backwards; assuming the points h, k,

Exhaus

Fig. 168. Zeuner Valve Diagram.

1, and m, erecting the perpendiculars (or striking the arcs) and finding the middle positions where these last cut the semicircle.

Q. Show the Sweet slide valve diagram, as slightly modified by Grimshaw.

A. Referring to Fig. 167, the diameter AB of the large circle represents the valve travel; that of the small central one the exhaust lap, and of the larger central one the steam lap, that of the small one at the end of the diameter the lineal lead. Then tangents to the lead and steam-lap circles will cut the valve travel circle at a point corre