Engine oil serves many purposes in an engine. Its main job is to prevent wear and friction. These are caused when the various metal parts of the engine grind together causing heat, friction and then wear. As motor oil flows around your engine it coats and lubricates the metal surfaces acting as a barrier or film, stopping the metal parts from actually touching each other, thus reducing friction and wear as well as dispersing the heat generated during the process.
Engine oil also helps to keep your engine clean and free from build-ups by using intelligent detergents and dispersants that clean and hold in suspension the by-products of combustion (like silica and acids) that are not caught by the oil filter. These contaminants are removed during the oil change process.
Oil also disperses layers of extra protection such as Zinc, which acts as sacrificial agent between metal parts improving wear protection, especially in older push rod type engines.
Apart from protecting the engine, oil reduces rust and corrosion by limiting your engine’s exposure to oxygen and preventing attack on metal surfaces from acids created through the combustion process.
Engine oil also -
1) Permits easy starting
Most wear occurs in an engine at start up. Therefore, the oil must have the correct low temperature viscosity to flow quickly to the bearings and valve train to prevent wear. Some engines require a low viscosity oil to actually start at all, especially some diesel engines found in four wheel drives, where the oil is used to operate the pump to prime the fuel injectors.
2) Reduces friction
The oil film reduces friction simply because there is no metal-to-metal contact. Choosing the correct viscosity is important and depends very much on the engine type and recommendations based upon manufacturer requirements. If the film thickness is too low, frictional heat my be generated that causes engine wear. If the viscosity is too high, more energy is required by the engine reducing efficiency. Correct oil selection is therefore a balance between what is needed to protect the component without generating excessive drag and heat, thereby decreasing fuel economy, thus choosing the correct oil viscosity is important.
3) Protects against Rust & Corrosion
As oils degrade, they form corrosive by-products so the oil contains anti-corrosion and acid neutralising additives to protect components against rust and corrosion.
4) Be compatible with engine seals
Oils must lubricate and not cause deterioration, shrinking or hardening of commonly used seals.
5) Prevent Foam
Foam reduces the lubrication properties of the oil, therefore oils must be resistant to foaming or be able to “release” or “dissipate” any foam quickly.
6) Cool the engine
At least 40% of the engine is cooled by the oil, not the radiator system. This means the oil is always under heat stress (oxidation) as it transfers heat from hot spots back to the sump. This includes main and big end bearings, the crankshaft, connecting rods, other bearings, pistons and the timing gear.
7) Reduce combustion chamber deposits
Some oil will always reach the combustion chamber either via the cylinder walls or via the valves. It is then burned off with the fuel. It therefore must burn clean enough that it does not build up on valve seats or piston tops which can cause problems.
8) Keep engine components clean
Oils need to be very stable under heat and not cause system deposits. They also have the added responsibility of keeping internal surfaces clean and to prevent sludge. This is achieved by a combination of detergent and dispersant additives.
9) Lubricate parts and prevents wear
This is the basic function of all oils. Keeping the moving parts separated. In general, the thicker the oil film, the better the wear protection, but the oil additives also play an important role. Modern oil additives allow an oil of lower viscosity to be used and still provide the same level of protection.
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