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.