The earlier in the stroke this is accomplished the more benefit you get
of the expansive force of the steam.
The reverse on traction engines is usually arranged to cut off at I/4,
I/2 or 3/4. To illustrate what is meant by "cutting off" at I/4, I/2 or
3/4, we will suppose the engine has a I2 inch stroke. The piston begins
its stroke at the end of cylinder, and is driven by live steam through
an open port, 3 inches or one quarter of the stroke, when the port is
closed by the valve shutting the steam from the cylinder, and the piston
is driven the remaining 9 inches of its stroke by the expansive force of
the steam. By cutting off at I/2 we mean that the piston is driven half
its stroke or 6 inches by live steam, and by the expansion of the steam
the remaining 6 inches; by 3/4 we mean that live steam is used 9 inches
before cutting off, and expansively the remaining 3 inches of stroke.
Here is something for you to remember: "The earlier in the stroke you
cut off the greater the economy, but less the power; the later you cut
off the less the economy and greater the power.
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