If it was not for
the reversal of motion of the cut off valve through the rocker arm
this eccentric would be about in line with the crank, but on the other
end. The movement of the cut off valve, therefore, at the time of port
opening is very little, being about on its dead center, passing which,
it immediately commences to close.
The object of the peculiar construction of the rocker arm, and the
pivot for the cut off rocker being placed thereon, is to provide equal
travel on the back of the main valve, no matter what the cut off. I
have already explained, in connection with the slide valve, that
advancing the eccentric does not change the movement of the valve on
its seat, but simply its relation to the movement of the piston. You
will see that this is unchanged as using the main valve as a seat or
any other seat. If the main valve was to remain stationary, and only
the cut off valve to be operated by its eccentric, the movement of
this cut off valve on a certain plane would be the same for all
positions of the eccentric.
Moving the main slide does not affect the matter in any way, for it
moves at the same time the pivot of the cut off, and while the cut off
seat has assumed a different position with reference to the engine, it
is still as though stationary so far as the cut off valve is
concerned.
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