﻿Ignition of Gases by Sudden Compression. 93 



three times as great when the fan was running as when the 

 gas was " stagnant." This will be referred to later when 

 discussing the results of the experiments of ignition. Raising 

 the compression ratio also increases the cooling factor for 

 air. This is also to be expected since the distance between 

 the top of the piston and the head of the cylinder is lessened. 

 The cooling factor for " stagnant " air in the apparatus was 

 found to increase from about 0*76 at a compression ratio of 

 6 to 1 to about 1*0 at a compression ratio of 10 to 1, the 

 distance between the piston and cylinder head being approxi- 

 mately 4 cm. (1*6 in.) at the lower and 2*3 cm. ('9 in.) at the 

 higher compression ratio. The experimental error in the 

 cooling factors, obtained by experiments similar to that 

 quoted above, is probably about 5 per cent. It should be noted 

 that the rate of loss of heat under the above conditions is 

 considerable. For instance, in the experiment quoted the 

 maximum difference in temperature between the gas and the 

 walls after compression is 276° ; and since the specific heat 

 c v is about 0*18, the air is losing heat at the maximum 

 temperature at the rate of 



0'18 x '85 x 276 = 42 calories per second per gram. 



It follows that if an explosive mixture of gases is suddenly 

 compressed to its ignition temperature, in such an apparatus 

 as that described above, the initial rate of the chemical 

 reaction at the lowest temperature at which ignition is ob- 

 served must be considerable, for the evolution of heat due to 

 the reaction must equal approximately the rate at which heat 

 is lost to the walls. For example, the total heat of com- 

 bustion of a mixture of a paraffin hydrocarbon and air, in the 

 correct proportions to burn to C0 2 and water, is approximately 

 700 calories per gram of mixture. If it ignites when suddenly 

 compressed to such a temperature that the rate of loss of heat 

 is 35 calories per gram, the reaction, if it continued uniformly 

 at the initial rate, would be complete in 20 seconds. This 

 illustration may serve to show the general nature of the 

 reactions that occur on sudden compression ; what occurs in 

 practice is that the gas, or part of the gas, reacts so that the 

 evolution of heat takes place at a somewhat higher rate than 

 the loss of heat by conduction etc. ; hence the temperature 

 of the reacting gases must automatically increase, and with 

 it the rate of reaction, until the gas " explodes." The interval 

 between the end of compression and the explosion must clearly 

 depend mainly on three factors : (a) the compression tempera- 

 ture, (b) the temperature coefficient of the reaction, (c) the 

 rate of loss of heat to the walls. 



