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Octane number requirement studies were made with eight late-model passenger cars to evaluate effects of temperature, humidity, and altitude. Primary reference fuels and 1965 CRC full-boiling reference fuels were used. Temperature and humidity tests were made on a chassis dynamometer at sea level, while effect of barometric pressure was studied on the road at altitudes up to 6000 ft. Regression analysis was used to study the correlation between changes in octane number requirement and changes in temperature and humidity. These correlations were linear and varied among the cars. The average changes in ONR were somewhat smaller, but in fair agreement with those reported in the literature. The altitude effect also varied among cars tested and was generally nonlinear. On the average, the effect of barometric pressure from sea level to 3000 ft was smaller than concluded from earlier studies; from 3000-6000 ft, the pressure effect was larger.

SEVERAL factors are involved in the answer to the question, “How do atmospheric conditions affect the ability of a fuel to satisfy the antiknock requirement of automotive engines?” As is well known, an increase in atmospheric temperature increases the octane-number requirement of engines. This paper points out, however, that this causes little change in the road octane-number ratings of commercial fuels. Increasing the absolute humidity has the opposite effect to increasing the temperature and tends to counteract the undesirable effects of changing temperature throughout the various seasons of the year. Increasing the barometric pressure or decreasing the wind velocity both increase the tendency of commercial fuels to knock. Factors indirectly related to weather conditions, such as the coolant or thermostat used in an engine, also affect the knocking tendency of a fuel.