Engine oil applications Engineer oil is applied to metal surfaces to prevent direct contact by the use of controlled lubrication films and chemical barriers against heat, pressure and movement. The operating conditions of moving components such as pistons, crankshaft bearings, camshafts, and valve lifters are demanding in terms of temperature (in some areas over 150 o C) and loading (hundreds of megapascals contact pressure), and the relative velocity (nearly zero at startup, thousands of rpm at operating speed).
Lubrication In the absence of proper lubrication, the metal engine parts would wear out quickly, heat up and sustain surface damage. Numerous individuals consider engine oil a way of reducing friction but in practice, engine oil forms intricate protective mechanisms of the moving metal surfaces. At the microscale, synthetic parts of even polished metals contain surface asperities, small peaks and saddles which would weld together, shear off, or abrade under load, unless spaced apart.
These challenges including high-performance engine oil products, address these challenges through a combination of base oil properties and carefully selected additives.
Why Metal-to-Metal Contact Is Destructive in Engines
Engineering Direct metal-to-metal contact contributes to the faster rate of surface deterioration and subsequent component breakage.
Apparently smooth machined surfaces are rough on the microscopic scale with asperities less than a few microns thickness. Under loading of two such surfaces, these peaks come together first when there is sliding or rolling of the two surfaces. The resultant friction also creates a lot of localized heat- usually to the extent that either a temporary welding (adhesive wear) or material transfer occurs between components. The hard particles on sheared asperities attach as plastic asperities and carve grooves into the less rigid surfaces which accumulate more particles, resulting in abrasive wear.
The process is compounded quickly in engines: a scuff of adhesive wear takes place on a cylinder wall or the surface of a bearing; abrasive particles spread and three-body abrasion takes place elsewhere. With time, unrestrained contact causes tolerances to be eroded, clearances to be increased, oil consumption to be raised and the compression and efficiency to degrade. The cumulative effect reduces the life of the engine remarkably.
The Three Lubrication Regimes in Engine Operation
The lubrication of the engines does not stay unchanged, it changes between the regimes which vary with the speed, the load, the temperature, and the condition of the oil.
This was exemplified in the Stribeck curve, which plotted coefficient of friction against a product of viscosity, speed and load. Practically, in normal operation engines go through all of the three regimes.
| Lubrication Regime | Operating Condition | Protection Mechanism |
| Boundary lubrication | Start-up / low speed / high load | Additive films |
| Mixed lubrication | Transitional operation | Oil + additives |
| Hydrodynamic lubrication | Steady operation / high speed | Full oil film |
At start up, shut down or heavy load at low relative motion, the dominant lubrication regime is the boundary lubrication. In this case, the oil film is thin enough not to completely separate asperities and the oil is therefore protected by chemical films of additive asperities adsorbing or reacting with metal surfaces.
Mixed lubrication comes as the middle ground between them where partial film separation and minimal asperity contact takes place. This is shared by both additive-derived and hydrodynamic pressure films.
The hydrodynamic lubrication dominates in steady cruising which provides enough speed and viscosity to form a complete film of fluid which completely keeps the surfaces apart and achieves minimal direct contact. On the issue of the degradation of such films over time, see our account upon the lubrication film.
How Lubrication Films Separate Metal Surfaces
Ideally positioned film of oil places weak physical barriers between metal surfaces avoiding destructive interaction.
The film thickness can generally vary between several microns in full hydrodynamic operation to nanometers in the extreme conditions at the boundaries. This separation is determined by the hydrodynamic pressure that will be produced in convergent geometry of moving parts, including journal bearings or piston rings.
The role of viscosity is center stage: the thicker the film the more easily it will form at a given speed and load whereas the lower the viscosity grade the more the formulation must be optimized to ensure protection. Speed causes an increase in the thickness of the films through convergent action, or the process of wedging: wedging forces the oil to move into the contact zone, where it accumulates pressure which causes surfaces to rise apart.
When situations with hydraulic conditions are reached and at least maintained in actual engines, friction is reduced drastically, however, the system should be able to pass through mixed and boundary states without disproportional wear.
The Role of Engine Oil Additives in Metal Protection
Additives give essential protection where physical films are inadequate as in the case of boundary as well as mixed regimes.
Anti-wear additives, including the zinc dialkyldithiophosphate (ZDDP), attach themselves on metal surfaces and reacts to form sacrificial phosphate-based films in the presence of heat during friction. These glassy layers cut across the base metal at a favorable rate cuts off the adhesive wear, and scuffing. Other chemistries such as organophosphates or sulfur compounds form the same barriers.
These films are recycled through contact as and when necessary, providing long duration required. To have more insight into the discussed mechanisms, have a look at our guide to engine oil additives.
Measuring Wear Protection Performance in Engine Oils
Wear is not an aspect that can be illustrated visually or through mere inspection; wear must be tested through a controlled way to measure the extent of protection.
The four-ball wear test (ASTM D4172) is a standard laboratory test in which a rotating ball of steel compresses three stationary balls under load measuring scar diameter after a designated period of time. The presence of smaller scars is an indicator of high quality anti-wear.
Engine sequence tests, including ASTM Sequence III series, are tests of actual valvetrain wear, over conditions of high temperature. Specific contacts are simulated by bench tests such as the Cameron-Plint rig or the SRV rig.
Such approaches are related to the performance in the field, which assists the formulators to prove protection to the engine deployment. A thorough description of such tests can be seen in the wear protection performance tests for engine oil.
How Oil Formulation Influences Surface Protection Over Time
The balanced interaction between additives and base oils is a successful method of protecting over a long-term.
High quality or synthetic base stocks of mineral are found to offer thermal and oxidative stability to the films, without affecting the viscosity or film strength. Additives should not be able to fade out quickly.
Oxidation makes oil sludgy and it becomes contaminated (fuel dilution, soot, water) films and wear. Adequately formulated ones counter such degradations thus maintaining protective potential during the drain interval. Learn more about these dynamics in our article on how engine oil protects metal surfaces.
Cleanliness and Wear Protection Are Closely Linked
The purity of the engine also directly contributes to good wear protection.
There can be deposits at critical portions of the flow, such as a varnish or carbon or sludge, or, when deposited in surfaces, they abrade the films along with their asperity contacts. Detergents neutralise acids and suspend contaminants and dispersants put the contaminants into suspension to be eliminated by filtration.
Uniform additive adsorption and formation of films on clean metal places less stress on localized wear. This balance is compromised by dirty conditions which hasten degradation. To elaborate further, see our explanation of engine oil wear protection science.
Common Misunderstandings About Engine Lubrication
There are still a number of myths regarding the way in which lubrication functions in practice.
It is a widespread notion that thicker oil is always defense in the best interest. Actually, higher viscosity may slow viscosity flow in cold start when up to 3/4 of the total wear actually occurs, and slower viscosity fuel consumption with no commensurate advantages in current day engines with lower viscosities.
There is one more myth, which supports the idea that additives are important at high mileage. The first operation of the anti-wear chemistry will yield immediate protection in the oil that is necessary under a critical situation of boundaries of the oil life.
Lastly, there are those that assume that wear happens at high RPM. High speeds generate heat; however, much of the wear is usually observed in case of low-speed, high-load events or startups when hydrodynamic films are the thinnest.
Conclusion — Lubrication Is a Controlled Engineering System
The process of lubrication in an engine is an engineered sophisticated system that combines the hydrodynamic film formation, viscosity dynamics, and interaction with chemical surfaces. Protection is not dependent on any of them, and is the result of their judicious combination, that is to keep apart in different circumstances, and to withstand degradation through time.
Engine durability, efficiency, and reliability are based on this premise. The knowledge of such principles would be useful to understand why formulation decisions and maintenance are such a big issue to long-term performance.