To draw the cards shown in Fig. 8 any scales may be chosen,
but it is convenient to have the height of the card appxQxunately
half the length. Starting, then, with an initial pressure at B, the
line from B to C should first be nearly horizontal and then slope
down to represent the throttling or wire-drawing when the valve
closes. From C to R the curve is drawn as a rectangular hyper-
bola (with as an origin) because the actual diagram from an
engine shows this curve to be very nearly a rectangular hyperbola,
and it is a very simple curve to draw. To construct the curve, a
radial line is drawn fromO, and horizontal and vertical lines from
some point, such as C, through which the curve is to pass. The
radial line cuts the horizontal and vertical lines and is used as the
diagonal of a rectangle; the corner of the rectangle opposite C is
another point on the curve. The compression curve from KtoA
is also drawn as a rectangular hyperbola through the known point
K. Fig. 8 shows cards for a non-condensing engine, since the back
pressure is above atmospheric pressure. Due to friction in the
exhaust pipes and passages, the pressure in the cylinder does not
fall quite so low as at the discharge end of the exhaust pipe.
Depending on the freedom of the exhaust this back pressure varies



VALVE GEARS




Extreme Rjairlon.
INTRODUCTION 9

in practice from a fraction of a pound to about two pounds or
even more in some cases.
