Inasmuch as the heat or power developed by any fuel or combustible compound depends on the rate of flame propagation through the mixture, to increase the power and efficiency of present types of internal-combustion engine, the rate of flame propagation must be increased. Improvements in production engines to date have resulted primarily from modifications of the engine. Although the burning characteristics of conventional and low-priced fuels have received attention, nothing, with the exception of the heating of the hot-spot, has changed the conditions of the delivering and mixing of the charge during the last 20 years.
Without giving consideration to the physical or chemical composition of the various fuel compounds in use and without attempting to reconcile the results obtained with some of the published results derived from the mathematical analyses of combustion chemistry, the author gives some observations and conclusions deduced from a series of experiments carried on under conditions under which practical results and careful study in an endeavor to correlate the phenomena were all that could be obtained.
The fuels studied were distillates, 32 to 36-deg. Baumé fuel oil, 40 to 41-deg. fuel oil, an unrefined kerosene, a 58-deg. gasoline with from 25 to 27 per cent of benzol added and a 68 to 70-deg. straight-run gasoline. In this study, two engines of special and similar design were used, in which first consideration was given to a thorough mechanical mixing of the fuel and the air previously to ignition, the fuel being handled by a liquid-fuel injection-system without preheating either the fuel or the air. These tests are said to be the first successful handling of high and low-gravity fuels without preheating in an engine having low compression and electrical ignition.
The general conclusions reached are that the rate of flame propagation through an optimum fuel-air mixture varies (a) probably directly with the physical mixture condition of the fuel and the air at the time of ignition, (b) very slowly within certain limits with different grades of fuel and (c) very slowly with the condition of the fuel at the moment of ignition, that is, little difference seems to exist between the fuel in a vaporized state and that in a foglike condition, provided the fuel and the air have been well mixed; (d) with different compression pressures, it varies in a manner that cannot definitely be stated; (e) very much higher rates of flame propagation than are being obtained in present engines seem to be an inherent property of the fuels now in common use; and (f) the weakest link in present engines is the fuel-distribution system.
Although carbureter development has reached a high degree of efficiency in measuring the fuel charge per intake-stroke, it is not known how much of the fuel delivered by the jets during a given intake-stroke reaches the cylinder nor what proportion of a given fuel-charge is thrown out of the air-stream at the manifold bends, the fuel being carried back and forth in the manifold by the reversing air-flow and portions of it being dragged into any cylinder by the friction of the air. If the conditions existing in a manifold are of this description, it would be easy in the opinion of the author, to account for the low efficiency of gasoline engines and to work out a simple theory as to the cause of detonation.