32 ENGINE TESTS. 



for by the indicator " consists in measuring the diagrams for 

 the necessary data, and using the formula 



(c + e)Wx-(h + e)Wh] 

 m.e.p. 



in which " m.e.p." is the mean effective pressure measured from 

 the diagrams as pointed out ; " c " the proportion of the forward 

 stroke completed either at cut-off or release, according as the 

 determination is made at one point or another ; " h " the pro- 

 portion of the return stroke uncompleted at compression ; " e " 

 the proportion of the clearance space ; " Wx " the weight of one 

 cubic foot of steam at the cut-off or release pressure ; and 

 " Wh " the weight of one cubic foot of steam at the compres- 

 sion pressure. 



The points on the diagram where these measurements are 

 taken are illustrated in the sample diagram Fig. 1 given below. 

 These points are located as follows : The point of cut-off is 

 marked at the beginning of the expansion line after the steam 

 valve has completely closed. It is at the point where the curve 

 changes its direction from that due to the gradually closing 

 steam valve to that of the expansion line. The point of re- 

 lease is marked at the end of the expansion line just before 

 the curve begins to drop, due to the opening of the exhaust 

 valve. Likewise the compression point is fixed at the begin- 

 ning of the true compression line, or at the end of the curve 

 formed by the gradually closing exhaust valve. The principle 

 followed is to locate the points of cut-off and release so as to 

 account for all the steam present in the cylinder at the instant 

 the steam valve is closed, and for all the expanded steam 

 present just before the exhaust valve opens. The compression 

 point is located with the idea of obtaining a measurement of 

 all the exhaust steam which is retained in the cylinder at the 

 moment the exhaust valve is closed. 



In all these tests the computations are made both at the cut- 

 off point and the release point. It is desirable to do this, be- 

 cause there is often a considerable difference between the two 

 quantities ; and where there is such a difference, much more can 



