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.