All About Engines

Power and Its Measurement 3T7 pressure throughout one complete revolution of the engine. The effective pressure at any point of the stroke is represented by the vertical distance between the upper and lower lines in the diagram, and the mean P L A.N effective pressure, which is P in the formula —-—:—-—’’ is represented by the mean distance between those lines. To ascertain this, divide the whole diagram into a number of vertical strips, say io, draw a line down the middle of each strip, measure the lengths of these “ mid-ordinates,” add them up, and divide by io to obtain the average. The pressure represented by one inch in height of the diagram depends, of course, upon the strength of the spring, and the value furnished by the makers for the spring employed. But the horse-power of the engine—the indicated horse-power—can be found directly from the diagram itself. The width of the diagram represents to scale a force, and the length represents also to scale the distance over which that force acts, so that their product represents the work done by the force. And, as the average width multiplied by the length gives the area of the figure, this area represents to scale the work done in one revolution of the engine. The engineer does not trouble to measure and calculate the average width, but uses a planimeter, an instru- ment which, when applied to the diagram, enables him to read off the area directly. Since this area represents to scale the work done in one revolution of the engine, he has only to multiply this work by