An expansion curve that is lower than it should be may be caused by a leaky piston, by a valve that leaks on the exhaust side but not on the steam side, or if the exhaust valve is separate from the steam valve, it may leak while the steam valve is tight.
It may also be caused by the cylinder being unduly cooled, as from water accumulating in a steam jacket.
There are many defects in the adjustment of the valve gear, or of improper proportion in the parts, that may be clearly shown by a diagram, while there are defects which might exist and that would not be shown on the diagram.
It is possible, for example, that a steam valve and the engine piston may both leak to the same amount, and as a result the expansion curve may appear correct and not show the leak.
Fig. 3376.
Insufficient valve lead would be shown by the piston moving a certain portion of its stroke before the steam line attained its greatest height in [Fig. 3376], in which from a upwards, the admission line, instead of rising vertically, is at an angle to the right, showing that the piston had moved a certain portion of its stroke before full pressure of steam was admitted.
That too small a steam port or steam pipe did not cause this defect may be known from the following reasoning:
The port opened when the pencil was at a, which shows that the valve had lead. At this time the piston was near the dead centre and moving slower than it was when the pressure reached its highest point on the diagram, and since the steam line is fairly parallel with the atmospheric line, it shows that the port was large enough to maintain the pressure when the piston was travelling fast, and therefore ample when the piston was moving slow.