Through the tops of these lines we may draw the theoretical compression curve, which is shown dotted in.
To find the amount of steam actually saved by the compression, we have to consider the compression curve only, beginning at the point of the diagram where it is considered that the compression actually began, and ending where the compression line joins the admission line, and the horizontal distance between these two points represents the length of the cylinder bore filled by the compression.
To find the average amount to which the steam is compressed, we must draw within this length of the diagram, and within the boundaries of the compression curve, and the line of no pressure ordinates corresponds to those given for finding the average shown pressure of a diagram, as explained with reference to that subject, taking care to have the end ordinates spaced half as wide as the intermediate ones, as explained with reference to [Fig. 3372].
Chapter XLI.—AUTOMATIC CUT OFF ENGINES.
An automatic cut off engine is one in which the valve gear is so acted upon by the governor as to keep the speed of the engine uniform under variations of the load the engine drives, and notwithstanding variations in the boiler pressure. This it accomplishes by varying the point in the piston stroke at which the live steam is cut off. This is economical because it enables the engine to use the steam more expansively than is possible with engines having throttling governors, which govern the engine speed by wire drawing the steam.
There are two principal forms of automatic cut off engines, first, those in which the steam valve spindle or rod is released from the parts that move it to open for admission, while dash pots, weights, or springs close the valve to effect the cut off; and second, those in which the travel of the valve is varied so as to alter the point of cut off.
The first usually employ fly ball governors which actuate cams or stops to trip the valves for the steam cylinders. The second usually employ wheel governors or speed regulators, as they are sometimes termed.
The distinctive features in the action of the first, of which the Corliss engine is the most important, is that as two admission and two exhaust valves are used, therefore the amount of the valve lead, the point of exhaust and amount of the compression remain the same at whatever point in the piston stroke the cut off may occur; whereas in the second, the lead increases, the cut off occurs earlier, and the compression increases in proportion as the cut off occurs earlier in the piston stroke. In this class of engine the steam valve travels as quickly when opening the steam port for a short and early period of cut off as it does for a late one, hence the amount of steam port opening is as full, with reference to the piston motion, for an early as it is for a late point of cut off. In other words, there is the same amount of steam port opening for the first, second, third, and fourth inch of piston motion, let the point of cut off occur at whatever point in the piston motion it may. In engines which vary the point of cut off by reducing the travel of the slide valve, this is accomplished by using double ported valves or griddle valves.