ACTION OF RECIPROCATING ENGINES Most, though not all, reciprocating engines are double-acting: that is to say, steam from the boiler is alternately admitted to each side of the piston. In each double stroke, or revolution, there are f our events for each end of the cylinder :—(r) Admis sion, which begins by the opening of a steam-valve when the piston is at or very near the limit of its travel; (2) Cut-off, at which the steam-valve is closed and admission ceases. This may take place early in the stroke. The steam which is enclosed behind the piston then expands, with falling pressure, while the stroke continues, until (3) Release occurs ; that is to say, an exhaust valve opens, allowing the steam to escape from the cylinder. Its discharge generally continues for a large part of the back-stroke, until event (4), at which the exhaust-valve closes, and Compres sion begins : from there to the end of the back-stroke, the steam remaining in the cylinder is compressed into the clearance space behind the piston. This compression of the residue of steam, which is called cushioning, assists smoothness of working as the piston passes what is called the dead-point, at the limit of its stroke ; and the cushioning effect is often augmented by giving the steam-valve what is called "lead,"—that is, causing it to open a little before the piston reaches the dead point.
Distribution of the Steam : Indicator Diagram.—Together, these various events of the stroke constitute what is termed the distribution of the steam. They are conveniently exhibited by drawing an indicator diagram of the action, such as that illustrated in fig. 4. There, on a base PQ which represents the stroke, lines are drawn to show the continuous changes of pressure that go on within the cylinder during the whole action. Starting from the beginning of the stroke at a, for one side of the piston, steam is admitted up to the point b which shows the cut-off. At (or near) the end of the forward stroke, at c, the steam is allowed to escape. Compression, in the return stroke, begins at d. From b to c the steam confined in the cylinder is expanding, with falling pressure, doing work on the piston. The whole work done in the revolution, namely the integral of the force acting on the piston and the distance through which it moves, is represented by the enclosed area of the indicator diagram, abed.
The diagram is idealised in the sense that the events are shown as if they happened suddenly, giving sharply defined changes from admission to expansion and so on. In any real engine the events are necessarily gradual, for no valve—whatever its type —can close or open quite instantaneously. A real indicator dia gram accordingly has a smoother outline with rounded corners to mark the places of cut-off, release, compression and admission.
In practice release always occurs before the piston has quite com pleted its stroke.
The distance PQ which represents the stroke may (by the use of an appropriate scale) be interpreted as a volume, namely the volume swept through by the piston, and a point 0 behind P may be so taken that OP represents, on the same scale, the volume of the clearance. The distance measured horizontally from the line OY to any point of the curve represents the whole volume of steam behind the piston at that point, and the curve between admission and release exhibits the relation of the pressure to the total volume of the steam enclosed behind the piston throughout its process of expansion.
Actual indicator diagrams may be automatically drawn by attaching to the cylinder of the engine a device in which a pencil is made to rise or fall proportionally to the variations of steam pressure, while a paper, on which it inscribes a record, moves back and forth horizontally through distances proportional to the move ment of the engine piston. Many forms of indicator have been designed to give, in some such way, a diagram in which the co ordinates represent respectively the displacement of the piston and the pressure of the steam against it. From such diagrams it is easy to infer the mean effective pressure throughout the stroke, and also to observe how the valves are working. By connecting the instrument successively to each of the two ends of the cylin der, in a double-acting engine, a complete record is obtained for the two strokes which make up a revolution. The "indicated horse-power" is determined by the formula where and are the areas of the two sides of the piston, in square inches; p, and p, are the mean effective pressures on the two sides, in lb. per sq. inch, as determined from the diagrams; n is the number of revolutions per minute, and L is the stroke in feet. In trials of a steam engine such diagrams are taken during a considerable period. The amount and condition of the steam passing through the engine is observed, by measuring either the quantity of water discharged from the condenser (if of the surface type), or the quantity of feed-water required to be supplied to the boiler to keep the water level constant. From these data it becomes possible to compare the power developed with the heat supplied. Measurements of the heat rejected may also be made by observ ing the quantity of condensing water used and its rise in tempera ture ; and together these figures give material for drawing up a balance sheet of the disposal of thermal energy.