In 1921 Thomas Midgley, Jr. and T.A. Boyd in the United States discovered the effectiveness of tetraethyl lead (TEL) as an
"antiknock additive." The addition of TEL to the fuel reduced the knock, permitted higher compression ratios, and resulted in higher efficiencies.
Anti-Knock Index (AKI) or Pump Octane Number (PON), which is equal to the average of the Research Octane Number and the Motor Octane Number. The octane in the gasoline increases the activation energy needed for auto-ignition of the gasoline. The higher activation energy means less knocking of the engine.
RON (Research Octane Number) is determined in a single cylinder variable compression ratio engine that operates at 600 rpm with a 65.6 ºC inlet air temperature at standard barometric pressure.
MON (Motor Octane Number) is determined at engine speed of 900 rpm and 148.9 ºC inlet air temperature.
Anti Knock Index (AKI) = (RON+MON)/2
Anti Knock Index (AKI) also referred to as Road Octane Number (RdON) or Pump Octane Number (PON)