An examination of the indicator diagrams taken from these engines shows
that no wire-drawing takes place, and that, though the expansion is
carried to a point beyond the ordinary requirements, the compression is
but slightly increased. In all the diagrams taken from this valve motion
there is seen the clear, full upper line showing an abundant admission of
steam without any wire-drawing, and also the distinctly marked points
where "cut-off" or "suppression" and where "release" takes place, showing
the rapid action of the valves at those points.
It is well known to engineers that to obtain the maximum advantage out of
compounding, it is necessary to cut off in the low pressure cylinder at a
point corresponding to the relation between the low and the high, and that
point should be unaltered, whereas the point of cut-off in the high may at
the same time be varied to suit the work to be done.
In an ordinary link motion engine (where both links are connected to the
same weigh shaft), when linking up the high pressure cylinder to cut-off
short, the same change is necessarily made in the low. By the use of the
Joy gear, cut-off valves may be fitted to both cylinders, that for the low
pressure being fixed at the constant position required by the proportion
of the cylinders, while that on the high is adjustable; of course, in this
case, the position of the quadrants must be only changed for reversing.
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