Steam Engines

by Anonymous

STEAM ENGINE TESTING

The principal information sought in the usual test of a steam engine is:

1. The indicated horsepower developed under certain standard conditions.

2. The friction of the engine, from which is determined the mechanical efficiency.

3. The steam consumption per indicated horsepower.

4. The general action of the valves.

5. The pressure conditions in the cylinder at different periods of the stroke.

The ultimate object of an efficiency test is to determine the foot-pounds of work delivered by the engine per pound of coal burned in the boiler furnaces. The general method of finding the pounds of dry steam evaporated per pound of coal has been treated in Machinery’s Reference Series No. 67, “Boilers,” under the head of “Boiler Testing.” In the present case it is, therefore, only necessary to carry the process a step further and determine the foot-pounds of work developed per pound of steam.

The apparatus used in engine testing, in addition to that used in boiler testing, consists of a steam engine indicator and reducing device for taking diagrams, and a planimeter for measuring them afterwards. If the test is made independently of the boiler test, a calorimeter for measuring the amount of moisture in the steam should be added to the outfit.

It has already been shown how a diagram may be made to represent graphically the work done in a steam engine cylinder during one stroke of the piston. The diagrams shown thus far have been theoretical or ideal cards constructed from assumed relations of the pressure acting and the distance moved through by the piston. An indicator is a device for making a diagram of what actually takes place in an engine cylinder under working conditions. Such a diagram shows the points of admission, cut-off, and release, and indicates accurately the pressures acting upon both sides of the piston at all points of the stroke.

Fig. 49. Steam Engine Indicator

A common form of steam engine indicator is shown in [Fig. 49]. It consists of a cylinder C which is placed in communication at E with one end of the engine cylinder by a proper pipe connection, provided with a quick opening and closing cock or valve. The cylinder C contains a piston, above which is placed a coil spring of such strength that a given pressure per square inch acting upon the lower side of the piston will compress the spring a definite and known amount. Extending through the cap or head of cylinder C is a stem attached to the piston below, and connected by suitable levers with a pencil point P. The arrangement of the levers is such that a certain rise of the piston causes the point P to move upward in a vertical line a proportional amount.

The springs used above the piston vary in strength, and are designated as 20-pound, 40-pound, 60-pound, etc. A 20-pound spring is of such strength that a pressure of 20 pounds per square inch, acting beneath the piston in cylinder C, will raise the pencil point 1 inch. With a 40-pound spring, a pressure of 40 pounds per square inch will be required to raise the pencil 1 inch, and so on for the other strengths of spring.

The hollow drum D rotates back and forth upon a vertical stem at its center, its motion being produced by the string H, which is attached by means of a suitable reducing motion to the cross-head of the engine. The return motion to the drum is obtained from a coil spring contained within it and not shown. The paper upon which the diagram is to be drawn is wound around the drum D, and held in place by the spring clip F.

In taking an indicator card, the length of stroke must be reduced to come within the limits of the drum, that is, it must be somewhat less than the circumference of drum D. In practice, the diagram is commonly made from 3 to 4 inches in length. There are a number of devices in use for reproducing the stroke of the engine on a smaller scale. The most accurate consists of a series of pulleys over which the cord passes on its way from the cross-head to the indicator drum.

The indicator is connected with the engine cylinder by means of special openings tapped close to the heads and either plugged or closed by means of stop-cocks when not in use. In some cases two indicators are used, one being connected to each end of the cylinder, while in others a single indicator is made to answer the purpose by being so piped that it can be connected with either end by means of a three-way cock. After the indicator is connected and the cord adjusted to give the proper motion to the drum, a card is attached, after which the three-way cock is opened and steam allowed to blow through the indicator to warm it up. The cock is now closed and the pencil pressed against the drum to get the so-called atmospheric line. The cock is again opened, and the pencil pressed lightly against the drum during one complete revolution of the engine. The cock is then thrown over to connect the indicator with the other end of the cylinder and the operation is repeated.

Fig. 50. A Typical Indicator Diagram

The indicator card obtained in this way is shown in [Fig. 50]. It is sometimes preferred to take the diagrams of the two ends on separate cards, but it is simpler to take them both on the same one, and also easier to compare the working of the two ends of the cylinder.

Fig. 51. Diagram for Illustrating Method of Computation

The analysis of a card for practical purposes is shown in [Fig. 51]. Suppose, for example, that the length of the diagram measures 3.6 inches; the distance to the point of cut-off is 1.2 inch; and the distance to the point of release is 3.3 inches. Then, by dividing 1.2 by 3.6, the cut-off is found to occur at 1.2 ÷ 3.6 = 1⁄3 of the stroke. Release occurs at 3.3 ÷ 3.6 = 0.92 of the stroke. Compression begins at (3.6 - 0.5) ÷ 3.6 = 0.86 of the stroke. The diagrams shown in [Figs. 50] and [51] are from non-condensing engines, and the back-pressure line is therefore above the atmospheric line, as indicated.

The indicator diagram gives a means of determining the mean effective pressure, from which the power of the engine can be found from the previously given equation

	APLN
I. H. P. =	———.
	33,000

The method of determining the mean effective pressure is as follows: First measure the area of the card in square inches, by means of a planimeter (an instrument described later), and divide this area by the length in inches. This gives the mean ordinate; the mean ordinate, in turn, multiplied by the strength of spring used, will give the mean effective pressure in pounds per square inch. For example, suppose that the card shown in [Fig. 51] is taken with a 60-pound spring, and that the area, as measured by a planimeter, is found to be 2.6 square inches. Dividing the area by the length gives 2.6 ÷ 3.6 = 0.722 inch as the mean ordinate, and this multiplied by the strength of spring gives a mean effective pressure of 0.722 × 60 = 43.3 pounds per square inch.

In practice, diagrams taken from the two ends of the cylinder usually vary more or less, due to inequalities in the valve action. Again, the effective area of the piston on the crank end is less than that on the head end, by an amount equal to the area of the piston rod. For these reasons it is customary to compute the mean effective pressure of all the cards separately, and take, for use in the formula, the average of the various computations. The corrected value of the piston area is, as already stated, equal to (2A - a)⁄2, in which A is the area of the piston, and a the area of the piston rod. Substituting these values for A and P in the formula, together with the length of stroke and average number of revolutions per minute, the indicated horsepower is easily computed.

In making an ordinary test, diagrams are taken from both ends of the cylinder at 10-minute intervals for several hours, depending upon the accuracy required. The revolutions of the engine are counted for two or three-minute periods each time a pair of cards are taken, or still better, an automatic counter is used for the run, from which the average number of revolutions per minute may be determined.

The friction of the engine is determined by taking a pair of cards while “running light,” that is, with the belt thrown off, or the engine uncoupled, from the dynamo, if part of a direct-connected outfit. The friction load is then computed in horsepower from the indicator cards, and subtracted from the indicated horsepower when loaded. Thus we obtain the delivered or brake horsepower. The delivered horsepower divided by the indicated horsepower gives the mechanical efficiency. This may be expressed in the form of an equation as follows:

I. H. P. - friction loss
————————	= mechanical efficiency.
I. H. P.