IN THIS PAPER the author discusses the significance of the various tests for motor fuels, particularly in the light of extensive research work along these lines in the past few years by various industrial laboratories and the United States Bureau of Standards. A bibliography of the literature on the subject supplements the paper.
Although a large part of the public still seems to assume that the principal difference to the car user between different grades of gasoline is in mileage per gallon, actually, if today's best and poorest commercial gasolines are compared, the difference in mileage is very small compared with the differences in engine-starting ability, antiknock quality, vapor-locking tendency and liability to injure the engine or the fuel-induction system.
Volatility is generally believed to be the most important property of a motor fuel, but recent research has demonstrated that the 10-per cent point on the American Society for Testing Materials distillation curve is the best measure of the ease of starting in cold weather and of the danger of vapor lock, while the 90-per cent point is the best measure of the ease of uniform distribution and lack of crankcase-oil dilution in an engine after it has warmed up. Other intermediate points are of some importance in determining the ease of acceleration, especially while the engine is being warmed up; but, in general, suitable limits on the 10, 50 and 90-per cent points of a fuel will, the author asserts, assure good performance under all conditions so far as fuel volatility is concerned. The 10-per cent point especially must be closely controlled to assure easy starting without too much danger from vapor lock.
Antiknock quality is today a property of almost equal importance, from the standpoint of performance, with volatility of the fuel in its various aspects. Actual engine tests under carefully controlled conditions are as yet the only reliable measure of antiknock value. Difficulties in developing a standard engine-test are discussed in detail.
Sulphur content, gum content, and freedom from corrosive substances are each of importance in determining whether a gasoline can be used steadily without corroding or clogging the fuel system or the engine. Other properties of gasoline have little or no influence on the performance of a motor fuel, except that variations in viscosity or gravity may necessitate the readjustment of the carbureter.
High volatility and antiknock value and low sulphur and gum content cost money to obtain, but frequently the automotive engineer, by correct design, can decrease the need for too-rigid specifications on all those points. Of special importance in this connection are (a) more care in designing to prevent overheating of fuel lines, vacuum tank and other parts of the fuel system, thus permitting the use of valuable fuels without danger of vapor lock; (b) prevention of crankcase corrosion, thus raising the permissible limits for sulphur by eliminating the condensation of water in engines and crankcases; and (c) more uniform cooling of engines, especially aircraft engines, to reduce the tendency of hydrocarbon fuels to detonate.