Radiation, although the subject of study for many years, is not yet thoroughly understood. The investigations of von Helmholtz 30 years ago showed that from 10 to 20 per cent of the total heat of combustion is due to radiation; but flames burning in the atmosphere show different characteristics from those subjected to a change of density in a combustion-chamber and the same conclusions do not apply. The possibility of a non-luminous flame's causing loss of heat during and after combustion was first noted by Professor Callendar in 1907. The principal theory as to the source of radiation is that it is due to the vigorous vibration of the gas molecules formed on combustion, and that, like the high-frequency radiations producing light, it is caused by chemical rather than thermal action. It has been shown that radiation emanates almost wholly from the carbon dioxide and the water molecules.
As the total heat given off when a mixture burns in a gas engine is divided into two forms, (a) that due to conduction, and (b) that due to radiation, special instrumentation was necessary for properly observing them. This consisted of a stroboscope by which the duration of the luminous flame, the characteristic differences in color and brightness at different phases of combustion and the variations of the flame with changes in the quality of the mixture could be noted, and a thermopile for measuring the changes in the radiation of the flame. The investigation covered four fuels, namely, kerosene, kerosene plus 3 cc. of lead per gallon, a high-test gasoline that was free from detonation and a new Navy gasoline that contained 3 cc. of tetra-ethyl lead per gallon. Varying the mixture-ratio, detonation and the distribution of the radiant energy was found to affect the amount of radiant energy produced. It decreases as the mixture-ratio is leaned, or enriched to a comparatively low value; but it increases if the mixture-ratio is made very rich and may even be greater than the maximum occurring at the theoretically correct mixture-ratio; and it varies with the pressure. The conclusion reached is that the radiation produced during internal combustion is a function of the chemical reaction involved to a much greater extent than are merely the temperatures of the gases.
An extensive study of radiation has been made by a number of investigators. In spite of this fact, the phenomenon is far from being thoroughly understood. This is especially true insofar as it applies to the internal-combustion engine. In presenting this paper no attempt will be made to explain the phenomena with which it deals nor to give an explanation of the results obtained. The object of the paper is to present data showing how radiation varies with the changes in the character of combustion.