702 TRANSACTIONS OP SECTION G. 



constant. I have recently drawn a series of characteristic energy-curves for par- 

 ticular engines, and these are published in Engineering, August 19 and 26, 1910. 

 A typical set is shown in fig. 6. 



The horizontal scale represents the number of British thermal units trans- 

 ferred across the boiler-heating surface per minute. This quantity is used as an 

 independent variable. Plotted vertically are corresponding horse-powers, each 

 experiment being shown by a black dot on the diagram. The small figures 

 against the dots denote the speed in revolutions of the crank-axle per minute. 

 Experiments at the same speed are linked by a faint chain dotted line. A glance 

 at the diagram will show at once how nearly all the experiments fall on a straight 

 line, notwithstanding the wide range of speed and power. 



The ordinates of the dotted curve just below the i.h.p. curve represent the 

 heat energy in the coal shovelled per minute into the fire-box — that is, the rate at 

 which energy is supplied to the locomotive. The thick line immediately beneath 

 it represents the energy produced by combustion. The vertical distance between 

 these two curves represents energy unproduced, but energy which might have 

 been produced under more favourable conditions of combustion. Some of the 

 unproduced energy passes out of the chimney-top in carbon monoxide gas, but. 

 the greater proportion is found in the partially consumed particles of fuel thrown 

 out at the chimney-top in consequence of the fierce draught which must be used 

 to burn the coal in sufficient quantity to produce energy at the rate required. 

 The rate of combustion is measured by the number of pounds of fuel burnt per 

 square foot of grate per hour. In land practice with natural draft 20 lb. of 

 coal per square foot of grate per hour is a maximum rate. In a locomotive the 

 rate sometimes reaches 150 lb. per square foot per hour. In the diagram shown 

 the maximum rate is about 120 lb. per square foot, and the dotted curve begins 

 to turn upwards at about 70 lb. per square foot per hour. The vertical distance 

 between the curves shows what has to be paid for high rates of combustion. 



I found that in almost every case the curve representing the energy actually 

 produced by combustion differed very little from a straight line, passing through 

 the origin, showing that at all rates of working the efficiency of transmission is 

 approximately constant. That is to say, the proportion of the heat energy actually 

 produced by combustion in the fire-box which passes across the boiler-heating 

 surface per minute is nearly constant and is therefore independent of the rate of 

 working. 



The lowest curve on the diagram represents (he rate at which heat energy 'is 

 transformed into mechanical energy in the cylinders of the locomotive. It seems 

 a small rate in proportion to the rate at which heat energy is supplied to the 

 fire-box, but it is not really so bad as it looks, because the engine actually trans- 

 formed 60 per cent, of the energy which would have been transformed by a 

 perfect engine working on the Rankine cycle between the same limits of pressure. 

 The engine efficiency is represented in a familiar way by a curve labelled 'B.T.H. 

 per i.h.p. minute.' It will be seen that the change of efficiency is small, notwith- 

 standing large changes in the indicated horse-power. 



The diagram indicates that the indicated horse-power is practically propor- 

 tional to the rate at which heat is transferred across the boiler heating-surface, 

 and as this is again proportional to the extent of the heating-surface, the limit of 

 economical power is reached when the dimensions of the boiler have reached the 

 limits of the construction-gauge, the boiler being provided with a fire-grate of 

 such size that, at maximum rate of working, the rate of combustion falls between 

 70 and 100 lb. of coal per square foot of grate per hour. A boiler of large heat- 

 ing-surface may be made with a small grate necessitating a high rate of com- 

 bustion to obtain the required rate of heat-production. Then, although a large 

 power may be obtained, it will not be obtained economically. 



Returning now to the consideration of the type of locomotive required for a 

 local service with frequent stops, the problem is to provide an engine which will 

 get into its stride in the least time consistent with the comfort of the passengers. 

 The average speed of a locomotive on local service is low. The greater part of 

 the time is occupied in reaching the journey speed, and the brake must then often 

 be applied for a stop a few moments after the speed has been attained. In 

 some cases the stations are so close together that there is no period between 

 acceleration iiid retardation, Without going into the details of the calculation I 



