132

MODERN STEAM PRACTICE.

atmosphere acting on the top of the piston. This is all the indicatorcan give off, except showing at what part of the stroke the steam iscut off, and the behaviour of the valve in admitting the steam into the

Fig. 76.—Indicator Diagram, from Eccentric Valve Motion.

engine cylinder. Such a diagram is delineated: A B is the atmos-pheric line, C denotes the pressure above it at the commencementof the stroke, D is the point of cut-off, E is the point where the steamin the cylinder falls to the atmospheric line, F G is the vacuum line,and GH is in compression. The valve has shut the opening fromthe condenser, and the compressed vapour and steam admitted bythe lead of the valve causes the pencil to rise rapidly to the point Con the commencement of the stroke. Then from C to E denotes thesteam pressure on the engine piston, and from F to G the vacuum,while G H is the volume of cushioning required to check the motionof the piston at the end of the stroke, in a gradual manner. Theamount of compression being greater for a heavy piston having ahigh velocity than for a lighter piston having the same velocity,bearing in mind that lighter pistons of exceeding high velocity mayrequire more cushioning or opening by valve, technically termed“ lead,” than heavy ones moving slowly. It must be noted that thepoint D in the diagram only approximately shows that part of thestroke where the steam ports are entirely shut, or the communicationfrom the boiler cut off by the valve. This defect in the diagram isinherent in all when the valve is actuated on by an eccentric, as themotion of the eccentric is very slow when shutting the ports, whilethat of the piston is rapid. Thus, to a certain extent, the steam iswire-drawn, so that the pressure in the cylinder is gradually reduced,