198 REPORTS ON THE STATE OF SCIENCE.—1914. 
These considerations show how important the suction temperature is 
in combination with the indicator diagram, as from this temperature 
and the pressure and volume given by the diagram the state of the 
working agent all through the cycle can be determined, at least 
approximately. 
The values of the suction temperature for a particular engine are 
exhibited in fig. 12 above, and a diagram of the kind would be useful 
in connexion with any internal-combustion motor. 
To resume, it can now be assumed that it is possible to fix a 
temperature for one particular position of the state-point A, and then 
the temperature at the end of the change of state B, if not observed, 
can be calculated. With a knowledge of those temperatures the 
internal energy of the working agent can be read off from the curve 
(fig. 15), and then the first term on the right side of the equation, viz. 
Hs — Ea = change of internal energy 
is determined. 
The value of the second term on the right side of equation (1) is 
merely the value of the shaded area under the path AB expressed in 
foot-pounds. Consequently, from a pressure-volume diagram giving the 
initial and final conditions of the working agent and the path of the 
state-point in between, together with the temperature corresponding to 
one position of the state-point, the right side of the equation can be 
determined and the heat gained or lost by the working agent during 
the change can therefore be computed. If there is no gain or loss 
of heat the work done is done at the expense of the internal energy 
of the working agent itself. One of the main objects of the Committee 
has been to extend our knowledge of the physical constants of the 
gases by the careful examination of methods, apparatus, and results of 
various investigators, including members of the Committee, and change 
of state of the working charge in a gas-engine can now be followed 
with a degree of accuracy which hitherto has been impossible. 
A diagram from an actual gas-engine shows the PV changes during 
the whole of the four-stroke cycle, but the method explained above 
can only be applied to determine the heat exchanges during that part 
of the cycle when the weight of charge enclosed in the cylinder is 
constant—t.e. during the period between the closing of the suction- 
valve and the opening of the exhaust-valve. There is no difficulty 
in applying the method practically to a change of state along the com- 
pression-curve because the conditions 1 and 2 above are fulfilled. 
There is no chemical change and the weight of charge is constant. 
Applying the method to the analysis of the expansion-curve, however, 
there is difficulty. The left side of equation (1), Q, gives the heat 
gained or lost by the gas during a change of state. Q includes the heat 
gained by combustion as well as the heat gained or lost from outside, so 
that it must be written 
Q=04C 
where O represents the heat gained or lost to the outside, and C repre- 
