Engine Oil Functions

Modern engine oil has been carefully developed by engineers and chemists to perform several important functions. The efficient operation of an engine depends on the oil to:

The ease of starting an engine depends not only on the condition of the battery, ignition and fuel quality, but also on the flow properties of the engine oil. If the oil is too viscous or heavy at starting temperatures, it will impose enough drag on the moving parts that the engine cannot be cranked fast enough to start promptly and keep running.

Since cold temperatures thicken all oils, an oil for winter use must be thin enough to permit adequate cranking speeds at the lowest anticipated temperature. It must also be fluid enough to quickly flow to the bearings to prevent wear. In addition, the oil must be thick enough, when the engine reaches normal operating temperatures, to provide adequate protection.

Viscosity is an important characteristic of an oil which is defined as the oil’s resistance to flow. It can be measured in many ways, but a critical one for engine oil is referred to as the cold cranking temperature. It evaluates the ease with which the engine crankshaft can rotate at the specified temperature. This resistance, or fluid friction, keeps the oil from being squeezed out from between engine surfaces when they are moving under load or pressure. Resistance to motion or flow is a function of the molecular structure of the oil. Since it is this internal fluid friction that is responsible for most of the drag put on the starter during cranking, it is important to use an oil with viscosity characteristics that ensure ease of cranking, proper oil circulation, and high temperature protection.

The effect of temperature on viscosity varies widely with different types of oil. For this reason, a calculation has been developed to quantify the amount of viscosity change with temperature, which is known as the Viscosity Index (VI). An oil with a high viscosity index is one that shows less change in viscosity with temperature variation. Today, through the use of enhanced refining methods and special chemical additives, many high viscosity index engine oils are light enough to enable easy cranking at low temperatures and yet will still be viscous enough to protect satisfactorily at high temperatures.

These oils with high viscosity indexes are known as “multi-grade” oils. Often they are also called by names that imply all-season usage since they perform satisfactorily in both winter and summer. Multi-grade oils are most often recommended by vehicle manufacturers.

Soot is a by-product of diesel and some direct injection gasoline engine combustion. It is black carbonaceous particulate matter, which does not dissolve in lubricating oil, but can be suspended by the oil and removed during an oil change. If soot is not well dispersed it will cause the oil to thicken up from its original SAE viscosity grade. In addition, soot can agglomerate to form particles large enough to initiate abrasive wear and when the soot load of an oil gets too high, it settles out and forms sludge. Agglomerated soot and/or highly thickened oil can result in high pressure at the oil filter inlet. This can cause the filter by-pass to open and allow unfiltered oil into the engine.

Engine oils formulated to fight soot are able to disperse large amounts of soot without thickening up. Good soot dispersal stops large particles agglomerating, reduces abrasive wear and inhibits the formation of sludge.

For many years the U.S. Environmental Protection Agency (EPA) has set stringent limits on nitrogen oxide (NOx) and particulate matter (PM) emissions from on-road trucks and buses. Through a combination of engine redesign, ultra-low-sulfur diesel (ULSD) fuel and new engine oil technology, these new vehicles cut harmful emissions by 98 percent. In addition, the regulation required a 97 percent reduction in the sulfur content of on-road diesel fuel – from 500 parts per million (ppm) to 15 ppm – so the fuel won’t damage the new exhaust aftertreatment devices, specifically Diesel Particulate Filters (DPFs) that trap and further reduce soot emissions.

Engine manufacturers have been developing engines that not only utilize DPFs but also run on pollution-reducing ULSD fuel and utilize cooled exhaust gas recirculation (EGR) devices to redirect some of the exhaust gases normally emitted by the vehicle back into the engine, thus lowering NOx production but creating more internal soot.

Effective with the 2010 model year, limits for nitrogen oxide (NOx) emissions were further reduced. Most original equipment manufacturers (OEMs) implemented the use of Selective Catalytic Reduction (SCR) devices in order to meet the 2010 emission levels. The 2010 implementation of lower emissions limits did not require any changes to the lubricants specifications.

Once an engine is started, the oil must circulate promptly and lubricate all moving surfaces to prevent the metal-to-metal contact that would result in wear, scoring, or seizure of engine parts. Oil films on bearings and cylinder walls are sensitive to movement, pressure and oil supply. These films must be continually replenished by adequate flow and proper oil distribution.

As mentioned earlier, the viscosity of an oil must be low enough at the starting temperature to permit rapid cranking and starting, and high enough at peak operating temperatures to ensure adequate separation of moving parts for guaranteed engine protection.

Once the oil reaches the moving parts its function is to lubricate and prevent wear of the surfaces. Lubrication specialists describe several classes of lubrication.

Full-film or elasto-hydrodynamic lubrication occurs when the moving surfaces are continuously separated by a film of oil. The determining factor in keeping these parts separated is the viscosity of the oil at its operating temperature. The viscosity must remain high enough to prevent metal-to-metal contact. Since the metals do not make contact in full-film lubrication, wear is negligible unless the separated parts are scratched by particles of equal size or larger than the thickness of the oil film itself. Bearings on crankshafts, connecting rods, and camshafts normally operate with full-film lubrication.

Under some conditions, it is impossible to maintain a continuous oil film between moving parts and there is intermittent metal-to-metal contact between the high spots (asperities) on sliding surfaces. Lubrication specialists call this mixed film lubrication. Under these circumstances, the load is only partially supported by the oil film. The oil film is ruptured resulting in significant metal-to-metal contact. When this occurs, the friction generated between the surfaces can produce enough heat to cause one or both of the metals in contact to melt and weld together. Unless counteracted by proper additive treatment, the result is either immediate seizure or the tearing apart and roughening of the surfaces.

Boundary lubrication conditions exist during engine start-up and shutdown and often during the operation of a new or rebuilt engine. Boundary lubrication is also found around the top piston ring where oil supply is limited, temperatures are high, and a reversal of piston motion occurs. Without additive protection, the result would be excessive wear or seizure of the two surfaces.

Under full-film lubrication conditions, a thick film of oil prevents metal-to-metal contact between moving engine parts. Relative movement of these lubricated parts requires enough force to overcome the fluid friction of the lubricant. The viscosity of the oil should be high enough to maintain an unbroken film, but should not be higher than necessary, since this increases the amount of force required to overcome this fluid friction.

Vehicle manufacturers specify proper oil viscosity ranges according to expected ambient temperatures. This is to ensure that the lubricant will provide adequate, but not excessive, viscosity at normal operating conditions. When oil becomes contaminated, its viscosity changes. With soot, dirt, oxidation, or sludge, viscosity increases; with fuel dilution it decreases. Both directions of viscosity change are potentially harmful to the engine. For this reason, contaminant levels in engine oil must be kept low. This can be best accomplished by changing the oil and filter at proper intervals. If an engine oil does not disperse contaminants properly, the oil filter will plug and go into bypass allowing the contaminants to cause damage to the internal parts of the engine.

The amount and type of chemical additives is important for reducing friction under the extreme pressure conditions of boundary lubrication. The proper balance of the total additive system in a modern engine oil is critical if all lubrication conditions of an engine are to be satisfied. The oil formulator can achieve this balance of motor oil compounding only through much research, with emphasis on proof-testing in actual engines, both in the laboratory and in field service.

For each gallon of fuel burned in an engine, more than one gallon of water is formed. Although most of this water is in vapour form and goes out the exhaust, some condenses on the cylinder walls or escapes past the piston rings and is trapped, at least temporarily, in the crankcase. This occurs most frequently in cold weather before the engine has warmed up.

In addition to water and the by-products from incomplete combustion of the fuel, other corrosive combustion gases also get past the rings and are condensed or dissolved in the engine oil. Add to this the acids formed by the normal oxidation of oil and the potential for rust and corrosive engine deposits becomes significant.

The life of engine parts depends in part on the ability of the motor oil to neutralize these corrosive substances. Thanks to extensive research, effective oil-soluble chemical compounds have been developed. These are added to engine oils during manufacture to provide vital protection to engine parts.

In formulating today’s high quality motor oils, a basic objective is not only to keep engine parts clean, but also to prevent sludge and varnish deposits from interfering with proper engine operation.

Engine sludge formation is generally a problem of low engine temperature operation. Engine sludge deposits are formed by combinations of water from condensation, dirt and the products of oil deterioration and incomplete combustion. Sludge-forming materials are often so small initially that no oil filter can remove them. They are much smaller than the thickness of the oil film on engine parts and therefore cause no wear or damage so long as they remain small and well-dispersed. However, as their levels increase in the oil during use, they tend to join together to form larger masses and oil flow can be restricted.

Sludge formation is aggravated by water vapour which condenses in the crankcase in cold engine operation. The rate at which sludge-forming materials accumulate in the crankcase oil is related to several factors of engine operation. Factors such as, rich air-fuel mixtures which occur during starting or when a choke is sticking; operating with dirty air cleaners; or cases of ignition misfiring, increase the rate of sludge accumulation in the oil.

Straight mineral oils have only a very limited ability to keep these contaminants from coagulating and forming masses of sludge within the engine. This is the job of the detergent/dispersant additives that are blended into modern engine oils. These additives keep vital engine parts clean and oil contaminants suspended in such a fine form that they can be removed by regular oil and filter changes.

Detergent/dispersants are also very effective in preventing varnish deposits within an engine. Varnish-forming materials react chemically or combine with oxygen in the crankcase to form complex chemical compounds. These compounds continue to react with each other and with oxygen on the hotter parts of the engine, especially exhaust gas recirculation (ERG) valves and oxygen sensors and are baked by engine heat into a hard coating on the hotter parts of the engine. The hydraulic lifters, piston rings, and bearings are particularly sensitive to varnish deposits. If varnish-forming materials are allowed to accumulate in these areas, engine operation is impaired.

Engines cannot tolerate excessive amounts of sludge and varnish on sensitive parts. Sludge deposits collect on oil pump screens, limiting the flow of oil to vital engine parts and resulting in rapid and destructive wear. Piston rings which are stuck or sluggish because of varnish accumulation prevent the engine from developing full power. Sludged or plugged oil-control rings prevent removal of excess lubricant from the cylinder walls and result in excessive oil consumption.

In performing its lubrication function, some oil must reach the area of the top piston ring in order to lubricate and seal the rings and the cylinder walls. This oil is then exposed to the heat and flame of burning fuel and part of it actually burns off.

Modern refining techniques have produced oils that burn cleanly under these conditions, leaving little or no carbon residue. The detergent/dispersant additives in modern motor oils keep the piston rings free in their grooves, thereby maintaining compression pressures and minimizing the amount of oil reaching the combustion chamber. This not only reduces oil consumption, but more importantly, keeps combustion chamber deposits to a minimum.

Excessive combustion chamber deposits adversely affect engine operation. Deposits that form on spark plugs may cause the plugs to foul. Excessive deposit build-up causes pinging, knocking, or other combustion irregularities that reduce the efficiency and economy of the engine. Since these deposits also act as heat barriers, pistons, rings, spark plugs, and valves are not properly cooled. This can result in damage or even failure of the parts necessitating premature replacement/overhaul.

In preventing excessive combustion chamber deposits, it is important that a motor oil accomplish two things:

  • The oil must keep the piston rings free so that they can minimize the amount of oil reaching the combustion chamber.

  • That portion of the oil reaching the combustion chamber should burn as cleanly as possible.

Many people assume that engine cooling is accomplished only through the action of the fluid in the cooling system. This in fact does only about 60 percent of the cooling job. It cools the upper part of the engine only – the cylinder heads, cylinder walls, and the valves. The crankshaft, the main and connecting rod bearings, the camshaft and its bearings, the timing gears, the pistons, and many other components in the lower part of the engine are directly dependent on the motor oil for necessary cooling. All these parts have defined operating temperature limits which must not be exceeded. Some can tolerate fairly high temperatures while others, such as the main and connecting rod bearings, must run relatively cool to avoid failure. Circulating oil picks up heat and carries it to the crankcase or oil cooler. Afterwards, cooler fluid or surrounding air removes the excess heat.

To keep this cooling process working, large volumes of oil must be constantly circulated to the bearings and other engine parts before eventually returning to the oil pan to cool and be recirculated again. If the oil supply is interrupted, these parts heat up rapidly from increased friction and combustion temperatures. A bearing failure is often referred to as a “burned-out bearing” because temperatures rose high enough to actually melt the bearing metal.

While only a small quantity of oil is required at any one time and place to provide lubrication, the oil pump must circulate many litres/gallons of oil per minute. Chemical additives and the physical properties of the oil have little effect on its ability to provide adequate cooling. What is critical is the continuous circulation of large quantities of oil throughout the engine and over hot engine parts. This is made possible through the use of large-capacity oil pumps and oil passages adequate to handle the required volume of oil. These oil passages cannot do the job properly if they are allowed to become partially or completely clogged with deposits. When this happens, the oil cannot circulate or cool properly and early engine failure may result. This is another reason for changing the oil and filter before the contaminant level becomes too high. Proper cooling also requires that the oil level in the crankcase never be permitted to remain below the “add oil” line on the dipstick. This is to ensure sufficient retention time of the oil in the crankcase.

The surfaces of the piston rings, ring grooves, and cylinder walls are not completely smooth. If examined under a microscope, these surfaces would show minute hills and valleys. For this reason, the rings by themselves can never completely prevent high combustion and compression pressures from escaping into the low pressure area of the crankcase, which results in a reduction in engine power and efficiency. Engine oil fills in these hills and valleys on ring surfaces and cylinder walls and helps to seal in compression and combustion pressures. Since the oil film at these points is rather thin – generally less than 0.025 mm thick – it cannot compensate for existing excessive wear of rings, ring grooves, or cylinder walls. Where such conditions already exist, oil consumption may be high. It may also be high in a new or rebuilt engine until the hills and valleys on these surfaces have smoothed out enough to allow the oil to form the right seal.

With the many rapidly moving parts in an engine, air in the crankcase is constantly being whipped into the oil. This produces foam, which is simply a lot of air bubbles which may or may not readily collapse. These air bubbles normally rise to the surface and break, but water and certain other contaminants slow down the rate at which this occurs, and the result is foam.

Foam is not a good conductor of heat, so if the amount of foam is excessive, engine cooling will be impaired because the heat will not be dissipated. Foam also does not have much ability to carry a load and has an adverse effect on the operation of hydraulic valve lifters and bearings. This is because it contains air and air is easily compressible. On the other hand, oil which is free of air is virtually incompressible.

Many engines have variable timing units, fuel injectors, valve control solenoids, and many other units that require high pressure oil to make them operate properly. Foam or air entrainment in the oil causes failure modes and shuts the engine down.

As the lubricating oil performs the combination of functions described above, the collective outcome is fuel efficiency. Providing low frictional resistance among moving and reciprocating parts, the engines mechanical efficiency is optimized. Energy loss in the engine components is reduced and this results in less drain on the fuel system.

Low Speed Pre-Ignition (LSPI) is a phenomenon associated with gasoline direct injection (GDI) and turbocharged gasoline direct injection (TGDI) engines. The proper additive balance in a lubricating oil will help mitigate LSPI events. Unprotected, in severe cases, catastrophic engine failure can occur.


  1. Agriculture
  2. Consumer Automobile
  3. Construction
  4. Food & Beverage
  5. Forestry
  6. Heavy-Duty Transportation
  7. General Manufacturing
  8. Medium-Duty Transportation
  9. Mining
  10. Corrugated Packaging
  11. Gas Plants, Pipelines & Power Generation
  12. Rail
  13. Transit
  14. Waste Operations