Functions of Engine Oil

An oil’s primary function is reducing friction. It does this by creating a film between surfaces to prevent contact, thereby reducing friction. However, lubricants are frequently needed to do more than just provide a slippery film between moving surfaces in contact. They are tasked to carry out several other functions, some of which might not immediately spring to mind when you think about engine oils or other lubricants. Let’s take a closer look at not-so-obvious lubricant functions.

Lubrication

The primary function of the engine oil is to lubricate moving parts. The oil forms a hydrodynamic film between metal surfaces, preventing metal-to-metal contact and reducing friction. When the oil film is not sufficient to prevent metal-to-metal contact, the following occurs:

• Heat is generated through friction
• Local welding occurs
• Metal transfer results in scuffing or seizing

Extreme Pressure Wear Control

Modern lubricants contain Extreme Pressure (EP) anti-wear additives. These additives form a chemically bonded molecular film on the metal surfaces at high pressures to prevent direct contact and wear when the load on the parts is high enough to eliminate the hydrodynamic oil film.

Cleaning

Oil acts as a cleaning agent in the engine by suspending contaminants within the fluid or by preventing the contaminants from adhering to components. Sludge, varnish, and oxidation buildup on the pistons, rings, valve stems, and seals will lead to severe engine damage if not controlled by the oil. Base oils possess a varying degree of solvency that assists in maintaining internal cleanliness. Solvency is the ability of a fluid to dissolve a solid, liquid or gas. While the solvency of the oil is important, detergents and dispersants play a key role. Detergents are additives that prevent contaminants from adhering to components, especially hot components such as pistons or piston rings. Dispersants are additives that keep contaminants suspended in the fluid. Dispersants act as a solvent, helping the oil maintain cleanliness and prevent sludge formation. Oil formulated with the optimal additives will hold these contaminants in suspension until they are removed by the oil filtration system or during the course of an oil change.

Protection

A lubricant must have the ability to prevent or minimize internal component corrosion. Oil provides a protective barrier, isolating non-like metals to prevent corrosion. Lubricants accomplish this either by chemically neutralizing corrosive products or by creating a barrier between the components and the corrosive material. Corrosion, like wear, results in the removal of metal from engine parts. Corrosion works like a slow acting wear mechanism.

Cooling

Lubricants are used to cool the parts of a component or machine while in operation – like a fan or air conditioner is used to cool the inside of a house. Engines require the cooling of internal components that the primary cooling system can not provide. Reducing friction minimizes heat in moving parts, which lowers the overall operating temperature of the equipment. Lubricants also absorb heat from contact surface areas and transport it to a location to be safely dispersed, such as the oil sump. Heat transfer ability tends to be a trait of the base oil’s thickness – lighter oils tend to transfer heat more readily.

Sealing

Lubricants are used to seal components from outside contamination, like windows in a house or automobile. They can act as a dynamic seal in locations such as valve stems, piston rings and cylinder contact areas to prevent contamination.

Shock-Damping

A lubricant can cushion the blow of mechanical shock, just as a shock absorber in a car dampens road vibrations and imperfections. A highly functional lubricant film can resist rupture and absorb and disperse these energy spikes over a broad contact area. When the mechanical shock to components is dampened, wear and damaging forces are minimized, extending the component’s overall life. The damping effect is essential to highly loaded areas such as the bearings, pistons, connecting rods, and the gear train.

Hydraulic Action / Transfer Energy

Because fluid lubricants are not readily compressible, they can act as an energy-transfer medium, such as in hydraulic equipment or valve lifters in an automotive engine. This key property allows lubricants to be used in heavy equipment to transfer energy from a hydraulic motor to pistons, which provide the means to actuate shovels, forklifts, and so on. Automatic transmissions are another good example; the torque converter uses the fluid to transfer rotating power from the engine to the transmission.

Oil Additives

Lubricating oil is formulated with additives designed to combat specific contaminants throughout its usable life. The additives used are more important to overall engine performance than the oil itself. Without additives, even the highest quality oil will not be able to satisfy engine requirements.

Review: The Role of Additives in Motor Oil Performance

In Conclusion

The important takeaway is that lubricant quality is not about excelling in any one performance area. It’s about the entire set of performance properties. In the key areas of quality, reliability and service life. Lubricants need to excel in reducing friction, and provide across-the-board performance and protection in all crucial aspects of lubrication.