What are the main functions of engine oil?

The chemical characteristics of the motor oil

The properties of engine oils are heavily influenced by the type of base oil used, the manufacturing technology used, and the type and nature of additives that the base oil has.

• The lubrication and flow characteristics of the oils are unctuous and viscous.
• Greasiness refers to the ability of oil to adhere to metal surfaces and form an oil-resistant film on them, preventing direct contact between moving parts. This eliminates dry friction, ensures maximum lubrication, and prevents wear and grip. Mineral oils are greasy enough to allow an internal combustion engine to run. To increase the oiliness of the base oil, additives are added.
• Viscosity is the ability of oil to resist flow. The viscosity of the oil has a significant impact on the lubrication capacity of the moving parts, depending on temperature and speed, as well as the friction coefficient and power loss due to friction.
• The oiliness and viscosity of the oil both have a significant impact on engine wear during startup. Because of the unctuousness, the moving parts are covered with an oil film, and the viscosity ensures that the oil can reach the moving parts quickly.
• 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 oil might freeze, resulting in a 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 10W30 oil, the freezing point is at -30 °C.

The decrease in oil viscosity leads to:

• reducing losses due to friction
• reducing fuel consumption
• the engine starts easier
• a good 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 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 the presence of oxygen, the more pronounced the oxidation of the oil.

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.

The additives 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.

• 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.

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

• monograde
• multigrade

The oils are classified into 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

• winter: 0W, 5W, 10W, 15W, 20W, 25W
• 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
• Cx
• Ex

The characteristics of car engine oil with examples:

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. Best for engines running under heavy operating conditions.

A3 / B4. Recommended for direct injection diesel engines.

A5 / B5. 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)

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 advantages 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. The engine starts better.
• better performance at high temperatures
• solid deposits are harder to form