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.
From these tests the author concludes that (a) changes in jacket temperature definitely affect the relative antiknock value of fuels, (b) when comparing benzene and tetraethyl-lead blends with a fixed spark-advance the results obtained at different compression-ratios will differ, (c) adjusting the spark-advance for maximum power minimizes compression-ratio variations and (d), although all results may be functions of the cylinder-head temperature alone, the data are not sufficiently complete to be regarded as conclusive.