Functions of Lubricating Oil

Functions of Lubricating Oil

Lubricating oils are classified by source into four main categories: animal oils, vegetable oils, petroleum lubricants, and synthetic lubricants. Petroleum-based lubricants account for over 90% of total consumption, and thus the term "lubricating oil" commonly refers to petroleum lubricants.

These oils are primarily derived from lubricant distillates and residual oil fractions obtained during crude oil distillation. Their key functions include reducing friction between moving components, as well as cooling, sealing, corrosion prevention, rust inhibition, insulation, power transmission, and contaminant removal in machinery and equipment.

The most critical properties of lubricating oil are viscosity, oxidation stability, and lubricity—all closely related to the composition of the lubricant fractions:

• Viscosity reflects the fluidity of the oil and is a vital quality indicator. Different operating conditions require different viscosity grades; high-viscosity oils are typically used for heavy loads and low-speed machinery.

• Oxidation stability refers to the oil’s resistance to oxidation under operating conditions influenced by temperature, atmospheric oxygen, and metal catalysis. Oxidation can lead to the formation of asphaltene-based deposits, sticky varnish, films, or viscous water-containing substances, degrading performance.

• Lubricity indicates the oil’s friction-reducing capability.

Additives are compounds added to lubricants to introduce new properties or enhance existing ones. They include:

• Antioxidants and metal deactivators

• Extreme pressure and anti-wear agents

• Friction modifiers

• Detergents and dispersants

• Foam inhibitors

• Rust and corrosion inhibitors

• Pour point depressants

• Viscosity index improvers

• Demulsifiers

Commercial additives are typically blends of several single-component additives, differing in composition and ratio according to specific performance needs.

Core Functions in Engines

1. Lubrication

Without proper lubrication, moving parts in an engine experience dry friction, generating extreme heat that can melt metal and cause severe damage or seizure. Lubricating oil forms a protective film on friction surfaces, reducing resistance. The strength and resilience of this oil film are crucial for effective lubrication. However, excessive oil can create disproportionate resistance, significantly affecting engine speed—making correct oil volume essential.

2. Cooling

Only a portion of the heat generated by fuel combustion is used for power output, overcoming friction, and driving auxiliary components. The remaining heat is dissipated partly through exhaust gases and partly via the engine cooling system. Lubricating oil also absorbs heat from cylinders, pistons, crankshafts, and other surfaces, carrying it to the oil pan for dissipation—preventing engine overheating and damage.

3. Cleaning

Engines produce various contaminants: dust and sand from intake air, carbon deposits from combustion, oxidized oil residues, and metal wear particles. These impurities can adhere to surfaces, accelerating wear. Oil circulation helps suspend and remove such deposits, preventing sludge buildup and maintaining smooth engine operation.

4. Sealing

Critical gaps exist between cylinders and pistons, piston rings and grooves, and valves and seats. While necessary to prevent jamming, these gaps can compromise sealing, reduce cylinder pressure, and lower engine output. The oil film formed in these spaces helps maintain an effective seal, sustains cylinder pressure, and prevents exhaust gases from leaking into the crankcase.

5. Rust and Corrosion Protection

Moisture from air, fuel, lubricants, and acidic combustion byproducts can corrode engine components. The oil film acts as a barrier, protecting metal surfaces from direct contact with water and corrosive gases.

6. Cushioning Impact Loads

During combustion, cylinder pressure rises sharply. The oil film in bearing clearances helps cushion impact loads on pistons, piston pins, connecting rods, and crankshafts. This results in smoother engine operation, reduced metal-to-metal contact, and minimized wear.