_THE STEAM ENGINE INDICATOR._
PURPOSE OF THE INDICATOR.
In the course of these pages, reference has several times been made
to the indicator. This instrument is of great service to the steam
engineer in helping him to determine with accuracy questions of steam
distribution, that, without its aid, would remain in the realms of
speculation. Its diagram presents a reliable record of what is going on
inside the cylinders of a steam engine; and master mechanics who desire
to run their engines economically, can not afford to dispense with the
accurate information imparted by the indicator respecting individual
locomotives.
DESCRIPTION OF INSTRUMENT.
The indicator consists essentially of a small steam cylinder, whose
under side is connected by pipes to the main cylinder of the engine
under inspection. Inside the indicator cylinder is a nicely fitting
piston, whose upward movement is resisted by a spring of known
strength. The piston-rod passes up through the top of the indicator
cylinder; and its extremity is connected with mechanism for operating a
pencil, and marking on a card a diagram whose lines coincide with the
movement of the indicator piston.
OPERATION OF THE INDICATOR.
[Illustration: FIG. 30.]
[Illustration: FIG. 31.]
Fig. 30 is a perspective view of the Tabor indicator, an instrument
well adapted for application to high-speed engines, such as the
locomotive. In Fig. 31 the indicator is shown in section. The
construction of the instrument can be well understood from a careful
examination of these figures. In the sectional figure, the piston is
seen with its rod encircled by the spring passing up through the top
of the cylinder, and connecting with the lever which operates the
marking-pencil. By ingeniously designed mechanism, the lever is made
to move the pencil in a perfectly straight vertical line. The card to
be marked is fastened in the paper drum attached to the indicator.
This drum receives a circular motion from a cord which is operated by
the cross-head, and this connection is so arranged that the drum will
begin to move round just as the main piston begins its stroke. The
circular motion of the drum is continued till the piston reaches the
end of its stroke, when the drum reverses its movement, and returns
to the exact point from whence it started. Now the indicator cylinder
being in communication with the main cylinder, when the latter begins
to take steam, the pressure will be applied to the indicator piston,
which is pushed upward, at the same time transmitting its movement to
the pencil. The indicator piston will rise and fall in accordance with
the steam-pressure in the cylinder; and, the circular movement of the
drum coinciding with the cross-head movement, the pencil will describe
a diagram which represents the pressure inside the main cylinder at the
various points of the stroke.
LINES OF THE DIAGRAM.
[Illustration: FIG. 32.]
Fig. 32 represents an indicator diagram, such as might be taken from
a locomotive with well-constructed valve-motion cutting off steam at
about one-third of the stroke. In the figure, the line _IJ_ represents
the line that would be drawn by the pencil when no steam is admitted
to the indicator piston; so it is called the atmospheric line. This is
the neutral line of the diagram representing the position of the pencil
when both sides of the indicator piston are exposed to the atmosphere:
hence it is the base line from which pressures either above or below
are measured. The line _AB_ is the admission line, so called because
its beginning _A_ represents the point where the valve begins admitting
steam to the cylinder: _BC_ is the steam-line, usually considered as
beginning at the point of positive change in direction of the admission
line. Admission of steam does not cease till the point _D_ is reached,
but wire-drawing begins at _C_. The curve _DE_ is the expansion line,
and is traced while the valve is closed, and previous to the opening
of the exhaust; the pressure falling by expansion. At _E_, release
begins, and the exhaust line extends to _F_; from _F_ to _G_ is the
line of counter-pressure, drawn during the return stroke of the piston
and while the exhaust is open; and _G_ to _A_ is the compression line,
drawn during the return stroke after the exhaust-valve has closed.
DATA NECESSARY FOR ANALYZING THE DIAGRAM.
In proceeding to ascertain from the diagram calculations of the work
done by the engine, it will be remembered that vertical measurements
represent pressures in the cylinder, and horizontal measurements
positions of the piston. To understand the language of the diagram
properly, it is necessary that several things having reference to
permanent or accidental conditions be known. Those absolutely necessary
are, the scale of the indicator spring, the diameter of the engine
cylinder and stroke of the piston, the strokes made per minute, and
the boiler pressure. It is also desirable to know the dimensions of
steam-pipes, steam-ports, and exhaust nozzles.
ADVANTAGES OF INDICATING LOCOMOTIVES.
The purposes for which the indicator can be advantageously applied
to locomotives, are to show the condition of the valves and pistons,
and to prove whether or not the steam is used properly within the
cylinders. By its use the amount of power developed by the cylinders
can be computed.
Were the indicator used daily on locomotives, much waste of coal now
going on through steam being lost by leaky valves and pistons would be
avoided. The graphic tale of the diagram would force out of practice
the ordinary habit of running engines with the steam throttled, and
indubitable proofs of ruinous back pressure would make contracted
nozzles intolerable.
There is not room in a work of this kind to give the necessary
information for those who wish to learn the operating of the indicator
and the analysis of the diagram. Engineers who are ambitious to enter
upon this line of study, should secure one of the standard books
treating the subject.