All About Engines
Forfatter: Edward Cressy
År: 1918
Forlag: Cassell and Company, LTD
Sted: London, New York, Toronto and Melbourne
Sider: 352
UDK: 621 1
With a coloured Frontispiece, and 182 halftone Illustrations and Diagrams.
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How a Modern Engine Works 9
the flywheel carries the crank over the dead centre,
so that at two points in its revolution the flywheel
alone keeps the engine working. For ninety-nine per
cent, of the time the engine drives the flywheel, and
for one per cent, the flywheel drives the engine.
But it does more than this. Not only are there
variations of pressure inside the cylinder, but the
turning effort of the connecting rod alters with its
position. Everyone who has turned the handle of a
grindstone knows that the greatest effect is obtained
when the push or pull is exerted at right angles to
the crank. When this greatest effort occurs in the
steam engine the flywheel is forced to turn more
rapidly, and when the effort falls off the speed de-
creases very slowly, because of the flywheel. The
flywheel, therefore, smooths out the inequalities of
motion and gives steadiness. It is like a policeman
keeping a crowd always on the move. But the
policeman does not work for nothing. His services
have to be paid for. Some of the force exerted by
the steam is employed in turning the heavy wheel.
There is no increase, but a decrease, in the work done
by the engine. The only thing that can be said in its
favour is that the engine would not work without it.
Let us now inquire how the steam is admitted to
each end of the cylinder at the right moment to
operate the piston. By referring to Fig. 6 it will
be observed that the ports are partly covered by a
sort of shallow box with the face downwards. This
is the slide valve, and from its shape it is often called
a “ D ” slide valve. The hollow in the under side