The goal of adding lead to gasoline was not to increase the octane rating, but rather to reduce engine knocking. Knocking is when an incomplete combustion occurs within the cylinder, imparting a sideways force on the piston. This causes the piston to slam into the side of the cylinder, causing a loud ping or a knock sound. By adding tetra-ethyl lead to gasoline, Midgley discovered that a uniform combustion could occur within the gasoline, resulting in minimal lateral force on the piston.
Octane rating is actually a measure of how much you can compress a fuel before it spontaneously explodes, so while the addition of the tetra-ethyl lead did alter the octane rating of the fuel, this was not the goal but only a means to accomplish the goal.
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