11.5.
Oil Additives
Additives are used in motor oils for three different reasons : (a) to replace some properties removed during refining, (fc) to reinforce some of the oil’s natural properties, and (c) to provide the oil with new properties it did not originally have. Oils from some petroleum oil fields require more and different additives than oils from other fields. Additives are usually classified according to the property they add to the oil. Some of the common additives are anti-oxidants, corrosion preventatives, detergents and dispersants, extreme pressure and anti-wear additives, viscosity index improver, pour point depressants, metal deactivators, water replants, emul-sifiers, dyes, colour stabilizers, odor control agents and foam inhibitors. With improper additive selection, the additives may oppose each other and lose their benefit to the oil. Each oil manufacturer balances the additives in his oil to provide oil with desirable properties, which meet the engine’s needs.
Pour-point Depressants.
The pour point of oil is the lowest temperature at which the oil pours or remains fluid. At extremely low temperature some oils become so thick that they do not pour at all. When this happens, excessive wear occurs on bearings, piston rings, and cylinders during initial operation after the engine is started in cold weather. Certain additives are used in many engine oils to depress the pour point.
Oxidation Inhibitors.
A considerable amount of oil agitation occurs in the oil pan because of the rotation of the crankshaft and connecting rods. When high-temperature oil is agitated, oxygen in the air combines with the oil, oxidising some of the oil into a sticky tar like substance and other corrosive compounds. The tar like substance can clog oil passages, which may cause some components to operate without proper lubrication. The corrosive compounds may erode bearings prematurely. Therefore, oil oxidation is undesirable. Oxidation inhibitors are added to engine oils to assist the oil in resisting oxidation.
Corrosion and Rust Inhibitors.
Acids can form in the oil at high temperatures and cause corrosion of engine bearings. Corrosion inhibitors are added to the oil to prevent this corrosion. Moisture may form on engine parts because of the extreme temperature changes encountered in colder climates. Rust inhibitors are added to the oil to disperse the water from the metal surface, allowing an oil film to form on the surfaces. Small amounts of acid may be formed from the combustion process. The rust inhibitors help to neutralise acids that get past the piston rings and collect in the oil.
Extreme Pressure Resistant.
In many engines, the oil pump delivers oil to the engine lubrication system at 207 to 414 kPa. However, the wedge-shaped oil film in engine bearings may be subjected to pressures up to 6900 kPa when the engine is operating under heavy load conditions. Extreme pressure additives prevent the oil from being squeezed out of the bearing surfaces when subjected to high pressures.
Detergents and Dispersants.
Since carbon is a by-product of the combustion process, carbon formation occurs in combustion chamber and around the piston ring grooves. Oxidation of the oil may result in thick tar like deposits in the oil or on engine parts. Detergents are added to the oil to slow down the formation of these deposits. The detergent loosens the deposits of carbon or tar like material and then carries them to the crankcase. Smaller particles are removed from the oil by the oil filter. Heavier particles drop to the bottom of the crankcase, and they are removed when the oil is changed. The dispersant in oil keeps the carbon particles divided in the oil. Without the dispersant, carbon particles would clot together in the oil and form large particles, which could plug the oil filter and reduce its effectiveness. These large particles could also restrict oil passages, which might cause major damage to engine components due to insufficient lubrication.
Antifoaming Additives.
The rotating movement of the crankshaft and connecting rods may foam the oil in the crankcase. Excessive foaming of the oil destroys the normal hydrodynamic oil film on bearing surfaces and other engine parts. Oil foaming can also produce noisy valve lifters. Engine parts wear prematurely when oil foaming causes inadequate lubrication of the bearing surfaces and other components. Antifoaming additives prevent excessive oil foaming. Overfilling the oil pan supports oil foaming. Therefore, the oil level must be at the specified mark on the dipstick.
Friction Modifiers (Oiliness Agents).
Some refiners add friction modifiers to their engine oil, which reduce the friction between the layers of oil in the hydrodynamic oil film on bearing or other surfaces. Also it is expected that fuel economy improves with the use of friction modifiers in the engine oil.
