The paper or card being in place upon the drum, and steam let into the indicator, the pencil lever is moved until the pencil touches the paper as lightly as possible, and as a result of the combined movements of the pencil and drum, the diagram is marked, its form being illustrated in [Fig. 3363], which represents a diagram placed above a cylinder, and the engine piston in three positions; first at the beginning of the stroke; second, at the point of cut off (which is supposed to be at one-third of the stroke); and third, at the point of release where the valve first opens the port for the exhaust. For convenience, the diagram is shown as long as the cylinder, but the actual diagram usually measures about 21⁄2 inches high and 41⁄4 inches long.
Supposing the cylinder to be filled with air, and the engine piston in position 1, and the indicator piston would be at the corner a of the diagram; but if steam were admitted, the pencil would rise vertically, marking the line from a to b, which is therefore called the admission line, or by some, the induction line.
If on reaching b the pressure was enough to move the engine piston, that piston and the indicator drum would move simultaneously, and as long as live steam was admitted the line from b to c would be drawn, hence this is called the steam line, its length denoting the live steam period.
The cut off occurs when the engine piston is in position 2, and the indicator pencil at c.
From this point the pencil will fall, in proportion as the steam pressure falls from expansion until the exhaust begins, the piston then being in position 3, and the pencil at d.
The line from c to d is therefore called the expansion line or expansion curve, and the point d the point of release or point of exhaust.
We have now to explain that in reality the whole of the remainder of the line of the diagram is, in reality, the exhaust line, yet there is a difference between the part of the line from point d to the end e of the diagram, and that part from se to a, inasmuch as that during the period of exhaust from d to e, the pressure is helping to propel the piston, while after e is reached, whatever steam pressure there may remain in the cylinder acts to retard the piston.
The line from d to e is therefore the exhaust line, and that from e to a is the back pressure line or counter pressure line.
In this example it has been supposed that while the piston was moving from position 3 to the end of its stroke, and the pencil from d to e, the indicator piston would have a steam pressure on it equal to atmospheric pressure, hence the line from e to a, in this case, represents the atmospheric line, and also the back pressure line.
The atmospheric line is a line drawn when there is no steam admitted to the indicator, and represents a pressure above a perfect vacuum equal to the pressure of the atmosphere. Its use is to show the amount of back pressure, and in a condensing engine to show the degree of vacuum obtained.