Gas-Engine Indicator-Diagram, 61 



cubic feet of air and coal-gas becomes, after complete combus- 

 tion, 6*4977 cubic feet of carbonic acid, water-vapour, and 

 nitrogen, reduced to the same pressure and temperature with- 

 out condensation of the water- vapour. Now as there is always 

 an excess of air, and as the mixture before combustion has 

 added to it nearly six cubic feet of the products of previous 

 combustions, we have something like 13*3 cubic feet before 

 combustion becoming 13*0377 cubic feet after combustion, at 

 equivalent pressures and temperatures. The contraction is 

 only about 2 per cent. We therefore conclude that we may 

 regard the fluid in a gas-engine as a fluid which, however it 

 may receive heat, obeys approximately the characteristic law, 



■— constant 



(where p is pressure, v volume, and T the absolute tempera- 

 ture), in so far as mechanical actions are concerned. That is, 

 we may speak of it as a perfect gas, which receives heat from 

 without, neglecting the fact that it is its own molecular energy 

 which appears as heat. It is also approximately true that the 

 ratio of the specific heats of the fluid is unchanged by com- 

 bustion taking place. 



In Table II. we give a similar comparison for Dowson 

 gas :— 



2*1325 cubic feet of combustible mixture become 1*9143 

 cubic feet ; 



or, taking into account the clearance-space and its products of 

 past combustions, we have, say, 



4 cubic feet becoming 3*7818 cubic feet ; 



or contracting by nearly 5-g- per cent, of its volume. 



In Tables III. and IV. we calculate the specific heat of a 

 mixture of 1 cubic foot of coal-gas, 5*76 cubic feet of air, and 

 4*5 cubic feet of products of a previous combustion, before 

 and after combustion takes place. The Centigrade scale of 

 temperature is employed. 



In Tables V. and VI. similar calculations are made for the 

 usual mixture of Dowson gas with air and products of previous 

 combustion. 



