The Ignition of Gases

Abstract

MANY of the difficulties connected with the ignition of gases are still unfathomed, although the subject has been investigated in one way or another from the time of Davy's well-known researches connected with safety in mines. Measurements of the rate of combustion were first attempted by Bunsen, but it was left to Berthelot, to Le Chatelier, and to Dixon to lay down the methods by which the propagation of combustion in gases could be satisfactorily studied. A further impetus to such investigations came with the development of the internal combustion engine. The recent work of Bone, which indicates that nitrogen plays a considerable part in the process of combustion, is particularly interesting and links the subject with the fixation of nitrogen. It was apt, therefore, that a discussion on the subject should have taken place at the meeting of the British Association at Southampton between the Chemistry and the Engineering Sections. Prof. H. B. Dixon opened the discussion with a historical survey of the subject. He described Berthelot's discovery of the detonation wave and Le Chatelier's experiments on the same subject at the same date. Le Chatelier discovered three stages of combustion; the stage of uniform propagation, developing into a vibratory type of combustion which precedes the last stage, the initiation of the detonation wave. Prof. Dixon illustrated his remarks by some of his beautiful photographs of the propagation of explosions in mixtures of oxygen with cyanogen, acetylene, and other gases. The explosion wave travels with uniform velocity at the speed at which sound would travel in the gas, taking into account its high temperature and state of compression due to the combustion. Experiments on the measurement of ignition temperatures, which he has recently carried out by two different methods, were described. Tizard and Pye have shown that the delay which may occur before ignition when a combustible mixture is suddenly compressed adiabatically, depends on the temperature coefficient of the gaseous reaction. Ignition occurs when the heat evolved by the ignition of a gas just exceeds that lost to the surroundings. For the purpose of their experiments, Tizard and Pye used a variable compression machine with a piston which could be stopped by a toggle joint as soon as the gas had been compressed to the desired amount. Prof. Dixon described a similar arrangement by means of which the ignition point of electrolytic gas has been found to be continuously lowered as oxygen is added, in agreement with Le Chatelier's earlier experiments. With methane-air mixtures there is first a lowering, then a rise of the ignition point. It may be mentioned that Tizard and Pye, on the other hand, have found that for heptane-air mixtures the ignition temperature is only changed some 8° C. when the heptane-to-air ratio is increased from i to 10. Prof. Dixon showed some interesting photographs of explosions occurring in gases compressed by his falling weight method; the combustion does not proceed from a point, but spreads out from an undefined region of luminosity.