Laboratory test-methods of indicating the volatility characteristics and the starting capability of fuels used in internal-combustion engines are described, together with the testing apparatus and procedure, the objective having been the development of a simple method or methods of measuring the volatility of individual samples of motor fuel. The requirements are that the methods be practicable from the standpoint of routine laboratory, give directly the necessary information regarding characteristics of the fuel and be sufficiently precise to permit their use as specification tests.
Defining “volatility” as the property of a substance which causes it to be dispersed readily into the air, the author states that volatility as regards fuels is understood to indicate the quantity of fuel that will evaporate into air under definite conditions, so as to be enabled to speak of volatility as a definite quantitative property, and discusses both operating and starting volatility. Since the fuel must be evaporated into the air-fuel mixture before it can be burned, volatility is a very important characteristic of fuels and can be expressed conveniently as the percentage of the fuel which could be evaporated at a specified temperature into air at atmospheric pressure when the air and fuel are present in definite proportions by weight. Another and practically equivalent measurement would be the temperature at which the fuel in a given air-fuel mixture would be evaporated completely or to some definite percentage, this being indicative of the volatility characteristics that affect complete utilization, crankcase-oil dilution and the like. A fuel that might be very satisfactory from the standpoint of continuous operation where a large percentage of the fuel was vaporized might not permit starting even at moderately low temperatures; therefore, the starting volatility, or the volatility at low temperatures with only a small percentage of the fuel vaporized, must also be right if the fuel is to be wholly satisfactory as regards volatility. Starting volatility can be measured by the quantity of fuel that must be supplied to produce at some chosen low-temperature a given lean air-fuel mixture or, more simply stated, a mixture that will explode.
Unsatisfactory features of the Engler distillation-test are enumerated, a simple test for starting volatility that anyone can make is presented and the specification of fuels is treated. Of the last subject it is said that although complete distillation data on a fuel give valuable information to the refiner or to the laboratory worker, it seems that, for specification, two points on the equilibrium distillation-curve would be adequate to fix the suitability of the fuel.