A. Release is the point in the travel of the piston when the port is
opened. Compression is the distance the piston travels after exhaust
port closes before the live steam port opens. During this travel of the
piston the exhaust port is closed so the moving piston compresses the
steam left in the cylinder.
40. Q. With an indirect valve motion and outside admission valve, what
would be the position of the eccentric relative to the crank pin on that
side? What with a direct valve gear? What difference between outside
admission valve and inside admission valve as to this position?
A. With an indirect valve motion and an outside admission valve, the
go-ahead eccentric follows the crank pin with engine running ahead.
Without any lap or lead it would be a quarter of a turn or 90 degrees
behind the pin, but as all valves have lap and lead, the eccentric is
advanced or placed toward the pin enough to move the valve the amount of
the lap and lead. With a direct valve gear and an outside admission
valve, the eccentric will be a quarter of a turn or 90 degrees ahead of
the crank pin and advanced enough to move the valve the amount of the
lap and lead. With an inside admission valve and an indirect valve
motion, the eccentric will come the same as for an outside admission
valve and direct motion, or more than a quarter of a turn ahead of the
pin. With an inside admission valve and direct motion, as piston valves
are usually put up, the eccentric will follow the pin less than a
quarter of a turn.
41. Q. What effect would be produced upon the lap and lead by changing
the length of the eccentric rod?
A. Lap depends on the construction of the valve. A change of the
eccentric rod would not effect it, but would widen the port opening at
one end of the travel and reduce it at the other. It should be equal at
both ends. Lead is controlled by the position of the eccentric on the
axle and it must be equal at both ends. Changing the length of the
eccentric rod from the proper one does not really affect the lead,
because no proper measurement can be made until lead is equal at both
ends. Therefore improper length of eccentric rods varies the port
opening at the beginning of the stroke of the piston at both ends.