Weather or Knock Road Ratings and Requirements, Year-Round 540247
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.
Conditions requiring maximum antiknock quality in a fuel are found when the atmospheric effects of low humidity and high temperature are combined with the engine-cooling effects of high-temperature thermostats and glycol antifreezes. Minimum requirements occur when the weather is cool and moist, water is used as a coolant, and a low-temperature thermostat is used. Actual effects vary widely from car to car but may be as great as 15 octane numbers.