The diagram is divided into ro equal parts, and the pressures are obtained by measuring each way outwards from the atmospheric line (No. 2°), in the middle between every two divisions with the scale accompanying the indicator.

Ex. 4. Work out an indicator card, and show the point where the steam is cut off, whether the slide valve is correct or requires alteration, and if the latter, what?

The double diagram is from the top and bottom of a sea-going coasting screw steamer.

The cylinders, two in number, are each 36 inches diameter; the piston travelling at a speed of 280 to 301 feet per minute.

The diagrams are taken from top and bottom without shifting the indicator, by having

the stop-cocks fitted into the top and bottom of the cylinders, connected by a tube, into the top of which is fitted the indicator. By this means a diagram can be taken from top and bottom by the same paper-both figures having the same atmospheric line. In oscillating engines, or those of the direct-acting kind, where it is inconvenient to erect a radius bar, it is usual to use a large and small pulley to reduce the stroke of the piston to that of the indicator. It will be convenient to make the small pulley in the form of a grooved cone, so as to be enabled, with facility, to adjust the stroke of the indicator to the proper length.

The foregoing diagram is taken from one of the engines of a well-known steam ship. The scale of the indicator was eight pounds to the inch.

There are some points in this diagram to which we wish to direct the reader's attention.

The line c g is the theoretical expansion curve drawn from the point c. The line CD is the line of perfect vacuum. The line b to c is the steam line, which does not in fact end at e, but at some unknown point beyond c.

From an examination of this diagram we learn that the exhaust port was covered at the point ƒ of the return stroke, and the vapour remaining in the cylinder was then compressed by the advance of the piston to a density at the commencement of the forward stroke, of about five pounds above the atmosphere. The port was then opened for admission, and the pressure instantly rose to fourteen and a half pounds above the atmosphere. The port being opened wider and wider, this pressure was maintained behind the advancing piston to the point c, at which it began to fall, at first very slowly, from the gradual closing of the port by the cut off valve. The point at which the port was covered cannot be identified. It was certainly, however, far beyond the point c, and strictly the steam line continues to the point of cut off; however, the pressure may fall before that point is reached. At the point d, the pressure had fallen by expansion to two pounds above the atmosphere. Here the valve began to open communication with the condenser, and before the piston commenced its return stroke the pressure on this side of it fell to nearly ten pounds below the atmosphere, and almost immediately after a vacuum of twelve pounds was formed; and when the return stroke was two-thirds accomplished, the counter pressure suddenly fell half a pound lower, and this vacuum was maintained until the exhaust port was closed at the port f.

It will be observed that the line of perfect vacuum is drawn at the bottom of the diagram, and the line of boiler pressure as shown by the gauge at the top. The line of perfect vacuum in diagrams varies in its distance from the atmospheric line; or, more correctly, the latter varies in its distance from the former, according to the pressure of the atmosphere, as shown by the barometer from 13.71 lbs. on the square inch, when the mercury stands at 28 inches, to 15:19 lbs. when it stands at 31 inches (vide Table I at end of work), and it should be drawn according as the fact of this can be ascertained. The engineer should always have a good aneroid in his pocket. The pressure of the atmosphere is usually reckoned at 15 lbs., which as a general rule is too high, being correct only when the barometer stands at 30·6 inches; but the error is unimportant. The principal object of knowing the exact pressure of the atmosphere is to ascertain the duty performed by the condenser and air pump. The temperature of the discharge being known, the pressure of vapour inseparable from that temperature is also known (vide Table at end of work), and this being deducted from the actual pressure of the atmosphere, the remainder is the total attainable vacuum at that pressure.

с

The space between the steam line b to c and the line of boiler pressure shows how much the pressure is reduced in the cylinder by throttling, or by the insufficient area of the ports, proper allowance being made for the difference of pressure necessary to give the required motion to the steam, and that between the line of counter pressure e to f, and the line of perfect vacuum, shows the amount of resistance to the motion of the piston.

When we wish to ascertain how effectually the resistance of the atmosphere is removed from the non-acting side of the piston, by those parts of the engine whose function is is, we must calculate separately the areas of the diagrams above and below the atmospheric line. In case of engines working very expansively, however, the expansion curve crosses the atmospheric line, and sometimes at an early part of the stroke, as in Fig. 52. In such cases, the whole space between the atmospheric line and the line of counter pressure should be credited to the condenser and air pump; not, of course, to be considered in estimating the power exerted, but for ascertaining the degree of economy in the consumption of steam, which depends greatly on the amount of vacuum maintained.

It will also be noted that the expansion curve c d does not coincide with the theoretical expansion curve c g.

8.75

Forward Stroke

the stop-cocks fitted into the top and bottom of the cylinders, connected by a tube, into the top of which is fitted the indicator. By this means a diagram can be taken from top and bottom by the same paper-both figures having the same atmospheric line. In oscillating engines, or those of the direct-acting kind, where it is inconvenient to erect a radius bar, it is usual to use a large and small pulley to reduce the stroke of the piston to that of the indicator. It will be convenient to make the small pulley in the form of a grooved cone, so as to be enabled, with facility, to adjust the stroke of the indicator to the proper length.

54.15

98.2

114.6

Return Stroke

The foregoing diagram is taken from one of the engines of a well-known steam ship. The scale of the indicator was eight pounds to the inch.

There are some points in this diagram to which we wish to direct the reader's attention.

The line e g is the theoretical expansion curve drawn from the point c.

с

с

The line CD is

the line of perfect vacuum. The line b to c is the steam line, which does not in fact end at e, but at some unknown point beyond c.

From an examination of this diagram we learn that the exhaust port was covered at the point ƒ of the return stroke, and the vapour remaining in the cylinder was then compressed by the advance of the piston to a density at the commencement of the forward stroke, of about five pounds above the atmosphere. The port was then opened for admission, and the pressure instantly rose to fourteen and a half pounds above the atmosphere. The port being opened wider and wider, this pressure was maintained behind the advancing piston to the point c, at which it began to fall, at first very slowly, from the gradual closing of the port by the cut off valve. The point at which the port was covered cannot be identified. It was certainly, however, far beyond the point c, and strictly the steam line continues to the point of cut off; however, the pressure may fall before that point is reached. At the point d, the pressure had fallen by expansion to two pounds above the atmosphere. Here the valve began to open communication with the condenser, and before the piston commenced its return stroke the pressure on this side of it fell to nearly ten pounds below the atmosphere, and aîmost immediately after a vacuum of twelve pounds was formed; and when the return stroke was two-thirds accomplished, the counter pressure suddenly fell half a pound lower, and this vacuum was maintained until the exhaust port was closed at the port f.

It will be observed that the line of perfect vacuum is drawn at the bottom of the diagram, and the line of boiler pressure as shown by the gauge at the top. The line of perfect vacuum in diagrams varies in its distance from the atmospheric line; or, more correctly, the latter varies in its distance from the former, according to the pressure of the atmosphere, as shown by the barometer from 13.71 lbs. on the square inch, when the mercury stands at 28 inches, to 15°19 lbs. when it stands at 31 inches (vide Table I at end of work), and it should be drawn according as the fact of this can be ascertained. The engineer should always have a good aneroid in his pocket. The pressure of the atmosphere is usually reckoned at 15 lbs., which as a general rule is too high, being correct only when the barometer stands at 30'6 inches; but the error is unimportant. The principal object of knowing the exact pressure of the atmosphere is to ascertain the duty performed by the condenser and air pump. The temperature of the discharge being known, the pressure of vapour inseparable from that temperature is also known (vide Table at end of work), and this being deducted from the actual pressure of the atmosphere, the remainder is the total attainable vacuum at that pressure.

The space between the steam line b to c and the line of boiler pressure shows how much the pressure is reduced in the cylinder by throttling, or by the insufficient area of the ports, proper allowance being made for the difference of pressure necessary to give the required motion to the steam, and that between the line of counter pressure e to f, and the line of perfect vacuum, shows the amount of resistance to the motion of the piston.

When we wish to ascertain how effectually the resistance of the atmosphere is removed from the non-acting side of the piston, by those parts of the engine whose function is is, we must calculate separately the areas of the diagrams above and below the atmospheric line. In case of engines working very expansively, however, the expansion curve crosses the atmospheric line, and sometimes at an early part of the stroke, as in Fig. 52. In such cases, the whole space between the atmospheric line and the line of counter pressure should be credited to the condenser and air pump; not, of course, to be considered in estimating the power exerted, but for ascertaining the degree of economy in the consumption of steam, which depends greatly on the amount of vacuum maintained.

It will also be noted that the expansion curve c d does not coincide with the theoretical expansion curve c g.