Chapter XL.—The Indicator

The indicator is an instrument which marks or draws a figure, or diagram as it is called, which shows the pressure there is in the cylinder at every point in the piston stroke, while it also shows the resistance offered by the same body of steam to the piston on its return stroke. From the form of this figure or diagram, the engineer is enabled to discover whether those parts of the engine whose operation regulates the admission of the steam to and its exhaust from the cylinder are correctly adjusted.

From the diagram the engineer may find the average or mean effective pressure of steam on the piston throughout the stroke, for use in calculating the power of the engine.

He may also locate the point of cut off, of release, the amount of back pressure, the degree of perfection of the vacuum in a condensing engine, and the amount of compression.

From the area of the diagram the engineer may also estimate the quantity of steam that is used, and supposing it to be dry steam, he may calculate the amount of water used to make the steam, and assuming one pound of coal to evaporate so much water, he may calculate the amount of coal used to produce the steam.

The indicators commonly used upon steam cylinders contain two principal mechanical movements; first, a drum revolving the piece of paper upon which the diagram is to be marked, and second, a piston and parallel motion for moving the pencil to mark the diagram upon the revolving paper.

The drum is given a motion that, to insure a correct diagram, is exactly timed with the piston motion.

The pencil is given a vertical movement; this movement must bear a constant and uniform relation to the pressure of the steam in the engine cylinder.

An indicator may be attached to each end of the cylinder or in the middle, with a pipe passing to each end of the cylinder, as in Fig. 3358, but an indicator of the usual construction and such as here referred to, can take a diagram, or card as it is sometimes called, from but one end of the cylinder at a time. The stop valves a and b are used, so that the communication between the indicator and one end of the cylinder may be shut off while a diagram is being taken from the other end, while both ends may be shut off when the indicator is not being used.

In the figure a piece of paper (or card, as it is commonly called) is shown in place upon the drum with a diagram upon it.

The Thompson Indicator is shown in Fig. 3359, and in section in Fig. 3360.

The Tabor Indicator is shown in Fig. 3361, and in section in Fig. 3362.

Both are made with the piston and parallel motion as light as possible, in order to enable the taking of diagrams at as high a speed of engine revolution as possible.

Each consists of a cylinder and piston, the bottom surface of the latter being in communication with the bore of the engine cylinder, so as to receive whatever steam pressure there may be in the cylinder.

This indicator piston receives, on its upper surface, the pressure of a spiral spring, which acts to resist the steam pressure.

The indicator piston rod actuates an arm or line on the end of which is a pencil, which, by means of a parallel motion, is caused to move in a straight line.

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 2⁄2 inches high and 4⁄4 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.

It also forms a line wherefrom the line of perfect vacuum, or that of full boiler pressure, may be marked.

The steam pressure at any point in the stroke is denoted by the height of the diagram above the atmospheric line, but the steam pressure thus taken is obviously above atmospheric, and is thus the same as the pressure of a steam gauge, which is also above the atmospheric pressure, and therefore represents the pressure that produces useful effect in a non-condensing engine.

This is what may be called a theoretical diagram, because, first, it supposes the steam not to be admitted to the cylinder until the piston was at the end of its stroke, and to attain its full pressure in the cylinder before the piston lead begins to move, whereas, in order to attain a full steam pressure at the beginning of the stroke, the valve must have lead.

Second, it supposes the cut off to be effected simultaneously, whereas the valve must have time to move and close the port, and during this time the steam pressure will fall, and the curve c of the diagram will therefore be rounded more or less according to the rapidity with which the valve closed.

Third, it supposes the steam to have exhausted down to atmospheric pressure by the time the piston had reached the end of the stroke, whereas the piston will have moved some part of the back or return stroke before the steam will have had time to exhaust down to atmospheric pressure; and,

Fourth, it supposes the steam to remain at atmospheric pressure until the piston arrives at the end of its return stroke, whereas the valve will begin to close the port and cause the steam remaining in the cylinder to compress before the piston has completed its return stroke.

In practice the diagram will, under favorable conditions, accord nearer to the shape shown in the lower part of Fig. 3363, in which the closure of the port for the cut off is shown by the curve at f. At the point denoted by g the valve began to close, and at the point denoted by h the cut off was completely effected, and the expansion curve began.

The curve beginning at d is caused by the gradual opening of the exhaust port.

The height of the line of back pressure above the atmospheric line shows the amount of back pressure.

At the point m, where the back pressure line rises into a curve, the valve had closed, shutting in the cylinder a portion of the exhaust steam, which is afterwards compressed by the piston.

This curve is therefore called the compression line or compression curve. The point at which it begins cannot be clearly seen when the exhaust port is closed slowly.

The compression curve ends at p, where it merges into the admission line, but the exact point where the compression ends and the admission begins cannot always be located, this being the case when the port is opened slowly or the compression extends through a large portion of the stroke.

The admission line is, however, in most cases nearly vertical when the valve has lead, because the valve opens the port quickly while the engine piston is moving at its slowest.

A diagram as drawn by the indicator does not account for all the steam that is used in the cylinder, however, as will be seen from Fig. 3363, because, as the paper drum of the indicator receives its motion from the engine cross head, its length represents the length of the piston stroke, whereas, there is a part of the cylinder bore between the piston (when it is at the end of the stroke) and the cylinder cover that is filled with steam as is also the steam passage.

This steam performs no useful work during the live steam period, but obviously expands during the expansion period, and therefore affects the expansion curve, and must be taken account of in calculating the consumption of steam, of water, or of coal from the diagram, or in marking in the true expansion curve.

In calculating the horse power, however, it may be neglected, as it does not enter into that subject.

But in any calculation involving the amount of steam used, the clearance must be marked in by a line at a right angle to the admission line and distant from the nearest point of the admission line to an amount that bears the same proportion to the whole length of the diagram as the clearance does to the whole contents of the cylinder.

The clearance line is shown at l, l′, in Fig. 3363, its distance from the admission line representing the amount of clearance which includes the contents of the steam port and passage, as well as that of the cylinder bore that is between the cylinder cover and the piston, when the latter is at the end of the stroke.

A method of measuring the amount of clearance has already been given with reference to stationary steam engines.

A diagram for a condensing engine is shown in Fig. 3364, which corresponds to Fig. 3363, except that the line of perfect vacuum or no pressure is marked in.

It represents a perfect vacuum, and must be marked on all diagrams from which the consumption of steam is to be calculated, because the quantity of steam used obviously includes that which is used in counter balancing the pressure of the atmosphere.

Learners often get confused on this point, hence it may be more fully explained as follows:

Suppose the engine piston to be blocked, in the middle of the cylinder, and has on one side of it a pressure of 20 lbs. of steam by steam gauge, and on the other the pressure of the atmosphere, and we might pump out the steam, thus leaving the cylinder empty on that side of the piston.

The atmosphere would then exert a pressure of about 14⁄2 lbs. per square inch on one side of the piston, and if we slowly admitted steam again, it would have to get up a pressure of 14⁄2 lbs. per square inch before the atmospheric pressure would be counterbalanced and the piston be in equilibrium.

But the steam gauge would at this time stand at zero, and not show that there was any steam in the cylinder, because the zero of the steam gauge is atmospheric pressure.

When, therefore, the steam gauge showed a pressure of 20 lbs. of steam in the cylinder, there would actually be a pressure of 34⁄2 lbs. of steam per square inch.

The clearance line and the vacuum line must both, therefore, be marked on the diagram when the quantity of steam used is to be computed from the diagram, and also when the proper or theoretical expansion curve is to be marked on the diagram.

This is clear, because in finding the expansion curve for a given volume of steam the whole of its volume must be taken into account, and this whole volume is represented by the area inclosed within the clearance line, the steam line, the expansion curve, the exhaust line, and the line of perfect vacuum, or line of no pressure.

The atmospheric line should be drawn after the diagram has been taken, and while the indicator is hot, as the expansion of the indicator affects the position of this line. It is drawn with the steam shut entirely off from the indicator, whose piston therefore has atmospheric pressure on both sides of it.

Whether the engine is condensing or non-condensing, the same amount of steam (all other things being equal) is used, the only difference being that in a condensing engine a greater portion of the steam is available for driving the piston.

If the condenser produced a perfect vacuum, the whole of the steam would be utilized in propelling the piston.

The “line of no pressure,” or of perfect vacuum, is marked as far below the atmospheric line as will represent the pressure of the atmosphere, which is, at the sea level, about 14.7 lbs. per square inch when the barometer stands at 29.99 inches.