The exhaust valve is shaped so as to give a uniform passage for
the exhaust without any unnecessary space to increase the
clearance volume. When wide open with X at Xi, the edge 11
is at 10 and the edge 3 at 1. The distance between the points 2
and 1 is the maximum exhaust opening and point 2 is the point of
opening and closing when release and compression occur.
Only one end of the cylinder has been discussed, for simplicity
and because the solution for both ends would necessitate con-
siderable reduction in the size of the drawing; furthermore, the
solution for the other end is so similar that additional explanation
seems unnecessary.
The Dashpot. — When the trip mechanism of a Corliss engine
disconnects the hook from the valve the dashpot closes the
valve. The dashpot has two functions to perform:



CORLISS VALVE GEARS



153



1. It must exert a downward' pull on the valve.
2. It must be self cushioning.
Several different forms of dashpota are in common use;
they are all similar in action. As the wrist plate mechanism
draws the valve open, the dashpot rod (Figs. 145 and 150), with
the plunger at its lower end, is raised. A partial vacuum is thus
formed under the plunger and the atmosphere then exerts a




FiQ. 158. — Typical forma of dashpots.
downward pressure which is ready to force the valve closed as
soon as it is released by the trip. To reduce shock and pre-
vent the dashpot from being noisy air is used as a cushion to
bring the parts quietly to rest.
