that is, away from the head end of the cylinder. This pressure is used in an engine to overcome resistance, and as the piston moves forwards work is done. The amount of work done is represented by the total average pressure multiplied by the distance through which the piston moves. When the piston is moving away from the head of the cylinder, work, therefore, is being done on the piston by the pressure of the gas within the cylinder, the pressure being represented by the curve no, and the average height of the ordinates a‘a, b‘b, c’c etc., represents the average pressure on the piston during its expansion stroke. If the piston did not return, the work would be represented by this average pressure during the expansion stroke multiplied by the length of the stroke. In returning, however, the piston is usually called on to compress a certain amount of gas and therefore does work on the gas. This work must therefore be subtracted from that done on the piston by the gas during expansion.
26. The pressure during the return, or compression, stroke is represented by the line om. This work performed by the piston is known as negative work. The average pressure during the performance of this negative work must therefore be subtracted from the average pressure during the performance of work on the piston, that is, during the performance of positive work. This subtraction is most easily accomplished by taking the average of the ordinate between the two curves om and no which is the same as subtracting the ordinates of those points on the lower curve from those directly above them on the upper curve. For example, the effective pressure at the point a is determined by either subtracting the ordinate a′a′′′ from the ordinate a′a, or by measuring directly the ordinate a′′′a. The ordinate representing the average height of the diagram is called the mean ordinate, and the pressure represented by the mean ordinate is known as the mean effective pressure, usually represented by the letters M. E. P.
27. The mean ordinate is obtained in practice by dividing the area enclosed by the diagram by the length L, Fig. 7.
