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TENDENCY to detonate is probably the most important factor in determining the usefulness of fuels for internal-combustion engines. Although it is possible, by various means, to measure more or less accurately the relative knocking-characteristics of fuels, no way has heretofore been found of rating fuels that does not depend upon some arbitrary non-reproducible conditions and measurements. The general methods adopted have consisted in comparing one fuel with another, but no absolute standard has been available. Knocking is a function of several variables, the knocking characteristics of which have been found by keeping a certain number of them constant while certain others are varied, thus rating them in terms of load, the spark-advance necessary to produce knocking, the position of the throttle at which knocking begins, and the like.

DATA that were obtained while investigating some of the variables affecting the relative antiknock values of certain fuels are presented to show that if one condition of knock testing is varied, at least one other condition must also be varied. Increasing the jacket temperature necessitates increasing the knock intensity, decreasing the throttle opening or the compression ratio or retarding the spark. Two sets of tests were run. One consisted in adding tetraethyl lead or crude benzene to one of the six test fuels to make it equal in knock intensity to each of the other five. In the other series the quantities of tetraethyl lead that must be added to a straight-run Mid-Continent gasoline to give knock ratings equal to different percentages of chemically pure benzene in the same fuel were determined. The results of both series, which led to somewhat opposing conclusions, are presented in tables and charts, and a possible explanation of this conflict is given.