Motor oils contribute to combustion-chamber deposits and therefore to octane-requirement increase (ORI) and surface ignition. An investigation of the separate components of motor oils in laboratory engines showed that base oils add the largest increment to ORI, viscosity-index improvers add less, and detergents and inhibitors add the least. Surface ignition is reduced by phosphorus, barium, calcium, and zinc ─ elements commonly found in motor-oil additives. Phosphorus is especially effective; all tested additives containing it were equally effective at the same concentration of phosphorus.
The components chosen for a motor oil determine how much it will contribute to ORI and surface ignition. A good choice of base stock, viscosity-index improver, and detergent for an SAE 10W-30 oil gives an ORI 6 octane numbers lower than a poor choice. Similarly, choosing phosphorus-containing detergents and inhibitors reduces octanes needed to suppress surface ignition as much as l4 units. Good formulation of motor oils can do much to save costly octane numbers.
