What are the main functions of engine oil?

What is the role of car engine oil?

Combustion engines have an oil lubrication system. This system uses engine oil, which performs several functions:

Lubrication: to reduce friction between moving parts
Cooling: of the parts with which it comes in contact, especially of the piston
Cleaning: of the parts on which the residues of the combustion process are deposited (eg cylinders, segments, piston)
Protection against chemical corrosion of engine parts
Improving sealing like oil bath sealing, oil filter, etc.
Force agent, in the case of hydraulic drives, especially in the case of variable distribution.

What is the composition of engine oils?

The oils used in the lubrication circuits of heat engines have a complex composition. Depending on the type of engine oil, they are composed of:
base oils, 75-85%
additives, in the proportion of 25-15%
The basic oils fulfill the main function of the lubrication system, namely lubrication. In turn, depending on the raw material used and the manufacturing process, the base oils are:
Mineral basic oils are extracted through various processes the raw material being oil
Synthetic base oils are obtained by chemical processes (synthesis)
Semi-synthetic base oils are mixtures of mineral oils and synthetic oils in a proportion of 20-30%.
Additives are chemicals (organic compounds, based on metals or polymers) that, in combination with the base oil, have the role of improving certain characteristics of the oils used in the lubrication systems of heat engines.

The physicochemical characteristics of the motor oil

The physicochemical properties of engine oils depend to a large extent on the type of the base oil (petroleum or synthetic elements), on the technology used in the manufacture as well as on the type and nature of the additives introduced into the base oil.

  • The lubrication and flow characteristics of the oils are unctuous and viscous.
  • Greasiness is the ability of the oil to adhere to the metal surfaces and to form on them an oil-resistant film that prevents direct contact between the moving parts. This eliminates the dry friction, ensures the lubrication to the limit, and avoids wear and grip. Mineral oils are sufficiently greasy to allow the operation of an internal combustion engine. Additives are added to the base oil to increase the oiliness.
  • Viscosity is the property of the oil to oppose the flow (relative motion of the constituent particles). The viscosity level of the oil significantly influences the lubrication capacity of the moving parts, depending on the temperature and speed, the friction coefficient as well as the loss of power due to friction.
  • Both the oiliness and the viscosity of the oil have a great influence on the wear during the thermal engine start-up process. Due to the unctuousness, the moving parts are covered with an oil film (during the previous operation) and the viscosity ensures the quick access of the oil to the moving parts.
  • The temperature has a major influence on the viscosity of the oil, the higher the temperature, the less the oil becomes viscous and flows more easily. At low temperatures, however, the alkaline hydrocarbons form a crystalline network that causes the oil to freeze, ie the total loss of flow capacity.

Engine oil circuit in a car

The freezing point of the oil is the temperature at which the oil ceases to flow, approaching the solid-state. For example, for a Valvoline 15W-40 oil, the freezing point is at -30 ° C.

The decrease in oil viscosity leads to:

  • reducing losses due to friction
  • reducing fuel consumption
  • easier starting of the engine
  • engine operation at very low temperatures

The increase in viscosity leads to:

  • A decrease in oil consumption
  • Better sealing between the piston, the segments, and the cylinder
  • Kinematic viscosity of oil as a function of temperature

The engine oil should have a slight variation of the viscosity with the temperature. Also, the oil lubrication system must be adapted to the different viscosities of the oil and the engine tests to be made with oils with different viscosity characteristics.

Sources of degradation of engine oil
Oxidation stability is another characteristic of motor oil. This is the ability of the oil to maintain its unchanged characteristics throughout its use or storage.

Temperature negatively influences oil oxidation. The higher the temperature and in the presence of oxygen, the more pronounced the oxidation of the oil.

The oxidation process of mineral oils results in oxygenated compounds (acids, aldehydes, ketones) as well as resin products. Also, under certain conditions of temperature, pressure and in the presence of certain catalysts, by the reaction of the hydrocarbons in the oil with the oxygen, insoluble products in the oil form that lead to the increase of the viscosity.

In the same way, the resins, under certain temperature conditions, form hard deposits (lakes) on the surfaces of the segments and pistons. These deposits immobilize the segments in the piston channels which leads to the loss of the combustion chamber seal. The resins, due to the deposits, can also produce the channels and holes in the lubrication system.

The oxidation substances in the oil hurt its characteristics. The resulting deposits reduce the thermal conductivity of the part walls (piston, segments, cylinders) and favor the formation of mud in the oil bath at low temperatures.

Also, the acids resulting from the oil oxidation process have a corrosive effect on the metal parts as well as on the friction layers on the pistons, stains, etc.

Engine operation at high temperatures (high speeds), for a long time, accelerates the oil oxidation process.

Oil degradation also occurs due to its contamination with other substances. Engine oil can be contaminated with fuels, water or coolant, particles or impurities. Fuel contamination (gasoline or diesel) results in decreased viscosity, which involves poor lubrication of moving parts. Also, the flammable temperature of the oil decreases which can cause its self-ignition.

Contamination with water or coolant leads to oil emulsification and reduced lubrication capacity. Also, the viscosity of the oil increases and the additives can be separated from the oil which leads to the drastic reduction of the oil lubrication capacity.

Particle contamination is due to incomplete combustion of the fuel and increases the viscosity of the oil. Also, the oil can be contaminated with metal particles (from parts) or impurities from the air. The oil particles, if they reach the metal surfaces in contact, lead to an intensification of the wear process.

Additive characteristics of engine oil

The additives are complex products, of organic or metallic nature, soluble in the lubricant mass, which have the role of improving the characteristics of the motor oil. The additives are of several types, each having an impact on a certain characteristic of the motor oil. The name and mode of action of the main additives are summarized in the table below.

Additives and Mode of Action

  • Aliphatic Compounds – Improve the oiliness of motor oil
  • Polymers – Improves cold fluidity and a hot viscosity
  • Detergents-dispersants – Works on oil-insoluble materials preventing the formation of deposits and neutralizing acid residues
  • Antioxidants, anti-corrosion, and anti-wear – Reduce the oxidation process of the oil
  • Rustproof – Helps to form an oil film on metal surfaces
  • Anti-foam – Reduces the process of foaming the oil (dissolving the air in the oil mass)

Engine oil classification

Engine oils are classified according to several civil or military standards. The most commonly used standards are: SAE, ACEA, API, ILSAC, JAVA, EMA, car manufacturers standards and military standards (MIL-L).

1. The SAE standard

Heat engine oils are classified by several international standards. The most common form of classification is the one after viscosity, which is regulated by SAE standards.

The SAE J 300 standard classifies car oils according to their viscosity at different temperatures. Thus are classified oils:

  • monograde
  • multigrade

The oils are classified in 11 classes/degrees, starting from the lowest temperatures (-35 ºC) and up to the maximum operating temperature of the thermal engine (100 ºC).

The monograde oils

These are:

  • winter: 0W, 5W, 10W, 15W, 20W, 25W
  • in summer: 20, 30, 40, 50, 60

Winter oils must meet the requirements regarding maximum viscosity at a minimum temperature as well as minimum viscosity at maximum temperature. For summer oils only the minimum viscosity is required at the maximum temperature.

For example, in Europe, due to the variation of summer-winter temperature, multigrade oils are used. They can be used for a minimum and maximum range of ambient temperatures.

2. The ACEA standard

The Association of European Automobile Manufacturers (ACEA) classifies motor oils according to their destination: cars with gasoline or diesel engines and commercial vehicles.

The ACEA standard published in 2010 can be used for the certification of motor oils until December 22, 2014. After this date, the new motor oils will be certified according to the ACEA standard issued in 2012.

Engine oils are classified into 3 categories:

  • Ax / Bx – for petrol and diesel engines
  • Cx – for petrol and diesel engines with exhaust after-treatment systems
  • Ex – commercial vehicles

The characteristics of car engine oil are described below:

A1 / B1
Standard quality, normal oil change interval, low viscosity oil at high temperatures (used to reduce fuel consumption)

A3 / B3
Standard quality, extended oil change interval (used for engines under heavy operating conditions)

A3 / B4
Similar A3 / B3 Recommended for direct injection diesel engines

A5 / B5
Similar A1 / B1 Recommended for performance engines

C1 Standard quality, normal oil change interval, low viscosity oil at high temperatures, low sulfur content (used for particulate filter engines or three-way catalyst)

C2 Standard quality, normal oil exchange range, low viscosity oil at high temperatures (used for particulate filter engines or three-way catalyst)

C3 Standard quality, normal oil change interval, (used for particulate filter engines or three-way catalyst)

C4 Standard quality, normal oil exchange range, low sulfur content (used for particulate filter engines or three-way catalyst)

A1 / B1 oil is considered to have standard, basic performance. The best-performing oils are those rated A5 / B5.

The oils recommended for engines with exhaust after-treatment systems (Cx) are classified according to the minimum viscosity at high temperatures and the sulfur content.

Oils with low viscosity at high temperatures and those with low sulfur content are not compatible with all types of engines. For the optimum oil choice, consult the car maintenance manual!

3. API standard

The American Institute of Petroleum (API) classifies oils according to the type of engine: gasoline or diesel.

Category S applies to spark-ignition petrol engines. The grades in force for category S are: J, L, and M described below:

  • SJ For engines manufactured until 2001
  • SL For engines manufactured until 2004

SM For engines manufactured after 2004 with stable life-long characteristics (oils with good oxidation stability, limitation of solid deposits, improved wear protection and good performance at low temperatures)

The best-performing oils according to the API standard, for petrol engines, are those labeled SM.

Category C applies to diesel engines for cars and commercial vehicles. The grades in force for category C are: H4, I4, and J4.

H4 For engines with pollution standards since 1998

I4 For engines with 2004 pollution standards (with exhaust gas recirculation systems)

J4 For engines with 2007 pollution standards, low sulfur content, recommended for particulate filter engines

The best-performing oils according to the API standard, for diesel engines, are those labeled J4.

The advantage of synthetic oils

Synthetic motor oils are obtained by chemical processes, the components of which are carefully controlled. Synthetic motor oil, combined with a complete package of additives, provides optimum lubrication and engine protection. Compared to mineral oils, synthetic oils have:

  • lower viscosity at lower temperatures (easier engine start)
  • better stability at oxidation, at high temperatures
  • better strength in forming solid deposits
  • stability in terms of characteristics, throughout life.

Example of engine oil

Name: ELF Competition ST

Base oil type: Semi-synthetic

Classification Standard SAE: 10W-40

Standard classification ACEA: A3 / B4

Standard classification CF: API SL

Standard classification Renault: RN0700

Standard classificationVolkswagen: VW 501.01 / 505.00

Standard classification Mercedes-Benz: MB-Approval 229.1


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