What is car engine oil?

Engine oils

Motor oils operate under extreme conditions. Other lubricants used in automobiles, gear oils and lubricants are incomparably easier to carry out their functions. Without losing the necessary properties. Because they work in a relatively homogeneous environment, with more or less constant temperature, pressure and loads. Engine mode "ragged". The same portion of oil is subjected to thermal and mechanical stress every second. Because the lubrication conditions of different engine components are far from the same. In addition, engine oil is exposed to chemicals. Oxygen, other gases, products of incomplete combustion of the fuel, as well as the fuel itself, which inevitably enters the oil, although in very small quantities.

Functions of motor oils.

Reduce friction between contact parts, reduce wear and prevent abrasion of rubbing parts. Seal the gaps, first of all, between the parts of the cylinder-piston group, preventing or minimizing the penetration of gases from the combustion chamber. Protects parts from corrosion. To remove heat from friction surfaces. Remove wear parts from the friction zone, thereby slowing down the formation of deposits on the surface of engine parts. Some basic characteristics of oils. Viscosity is one of the most important characteristics of oils. Motor oils, like most lubricants, change viscosity depending on their temperature. The lower the temperature, the higher the viscosity and vice versa.

Engine oils and cold start

To ensure a cold engine start, run the crankshaft with a starter and pump oil through the lubrication system. At low temperatures, the viscosity should not be too high. At high temperatures, the oil should not have a very low viscosity to create a strong oil film between the friction parts and the required system pressure. Viscosity index. An indicator characterizing the dependence of oil viscosity on temperature changes. This is a dimensionless quantity, i.e. it is not measured in any units, it is just a number. The higher the viscosity index of the engine oil, the wider the temperature range in which the oil provides the engine. For mineral oils without viscous additives, the viscosity index is 85-100. Oils with viscous additives and synthetic components can have a viscosity index of 120-150. For deeply refined oils with low viscosity, the viscosity index can reach 200.

Motor oils. Flash point

Flash point. This indicator characterizes the presence of boiling fractions in the oil and, accordingly, is associated with the evaporation of the oil during operation. For good oils, the flash point should be above 225 ° C. In the case of poor quality oils, low viscosity fractions evaporate and burn quickly. This leads to high oil consumption and deterioration of its low-temperature properties. Base number, tbn. Indicates the total alkalinity of an oil, including that used by alkaline detergents and dispersants. TBN characterizes the ability of an oil to neutralize harmful acids that enter it during engine operation and resist deposits. The lower the TBN, the less active additives remain in the oil. Most gasoline engine oils typically have a TBN of 8 to 9, while diesel engine oils typically range from 11 to 14.

Alkaline number of engine oil

When the engine oil is running, the total base number inevitably decreases, and the neutralizing additives are activated. A significant reduction in TBN leads to acid corrosion, as well as to contamination of the internal parts of the engine. Acid number, tan. Acid number is an indicator characterizing the presence of oxidizing products in motor oils. The lower its absolute value, the better the operating conditions of the engine oil. And the more his remaining life. An increase in TAN indicates oil oxidation caused by long life and operating temperature. The total acid number is determined for the analysis of the condition of motor oils, as an indicator of the degree of oxidation of the oil and the accumulation of acidic products of fuel combustion.

Molecules of mineral and synthetic oils from motor oils

Oils are hydrocarbons with a certain number of carbon atoms. These atoms can be connected both long and straight chains or branched, for example, the crown of a tree. The straighter the chains, the better the properties of the oil. According to the classification of the American Petroleum Institute, base oils are divided into five categories. Group I, base oils, which are obtained by selective purification and deworming using solvents, conventional mineral. Group II, highly refined base oils with a low content of aromatic compounds and paraffins, with increased oxidative stability. Hydrotreated, improved mineral oils.
Group III, high viscosity index base oils obtained by catalytic hydrocracking, HC technology.

Manufacturing Motor Oils

In a special treatment, the molecular structure of the oil improves. Thus, the properties of group III base oils are close to synthetic group IV base oils. It is no coincidence that this group of oils is classified as semi-synthetic. And some companies even refer to synthetic base oils. Group IV, synthetic base oils based on polyalphaolefins, PAO. Polyalphaolefins obtained as a result of a chemical process have characteristics of a homogeneous composition. Very high oxidative stability, high viscosity index and the absence of paraffin molecules in their composition. Group V, other base oils not included in previous groups. This group includes other synthetic base oils and vegetable base oils. The chemical composition of mineral bases depends on the quality of the oil, the boiling range of the selected oil fractions, as well as on the methods and degree of purification.

Mineral engine oils

The mineral base is the cheapest. This is a product for direct distillation of oil, consisting of molecules of different lengths and different structures. Due to this heterogeneity, viscosity instability, temperature properties, high volatility, low oxidation stability. Mineral base, the most common motor oil in the world. A semi-synthetic mixture of mineral and synthetic base oils may contain from 20 to 40 percent “synthetics”. Manufacturers of semi-synthetic lubricants do not have special requirements regarding the amount of synthetic base oil in the finished engine oil. There is also no indication as to which synthetic component, group III or group IV base oil should be used in the production of semi-synthetic lubricants. By their characteristics, these oils occupy an intermediate position between mineral and synthetic oils, that is, their properties are better than conventional mineral oils, but worse than synthetic ones. At a price these oils are much cheaper than synthetic.

Synthetic Motor Oils

Synthetic oils have very good viscosity-temperature characteristics. First of all, it is a much lower pour point, -50 ° C -60 ° C than the mineral, and a very high viscosity index. Which greatly facilitates starting the engine in frosty weather. Secondly, they have a higher viscosity at operating temperatures above 100 ° C. Therefore, the oil film that separates the friction surfaces does not break under extreme thermal conditions. Other benefits of synthetic oils include increased shear resistance. Due to the homogeneity of the structure, high thermo-oxidative stability. That is, a low tendency to form deposits and varnishes. Transparent, very durable, practically insoluble films deposited on hot surfaces are called oxidative varnishes. As well as low evaporation and waste consumption compared to mineral oils.

Motor Oil Additives

It is also important that synthetics require the introduction of a minimum amount of thickening additives. And especially its high-quality varieties do not require such additives at all. Therefore, these oils are very stable, because additives are destroyed in the first place. All these properties of synthetic oils help reduce overall mechanical loss in the engine and reduce wear on parts. In addition, their resource exceeds the mineral resource by 5 or more times. The main factor limiting the use of synthetic oils is their high cost. They are 3-5 times more expensive than mineral ones. And especially its high-quality varieties do not require such additives at all, so these oils are very stable.

Motor Oil Additives

Antiwear additives. The main function is to prevent wear of the friction parts of the engine in places where the formation of an oil film of the required thickness is impossible. They work by absorbing a metal surface and then chemically reacting with it during metal-metal contact. The more active, the more heat is released during this contact, creating a special metal film with “sliding” properties. What prevents abrasive wear. Oxidation inhibitors, antioxidant additives. During operation, engine oil is constantly exposed to high temperatures, air, oxygen and nitrogen oxides. What causes its oxidation, the destruction of additives and thickening. Antioxidant additives slow down the oxidation of oils and the inevitable formation of aggressive deposits after it.

Engine oils - principle of operation

The principle of their action is a chemical reaction at high temperatures with products that cause oil oxidation. They are divided into inhibitor additives that work according to the total oil volume. And thermal-oxidative additives that perform their functions in the working layer on heated surfaces. Corrosion inhibitors are designed to protect the surface of engine parts from corrosion caused by organic and mineral acids formed during the oxidation of oils and additives. The mechanism of their action is the formation of a protective film on the surface of parts and the neutralization of acids. Rust inhibitors are intended primarily for the protection of steel and cast iron cylinder walls, pistons and rings. The mechanism of action is similar. Corrosion inhibitors are often confused with antioxidants.

Motor Oils and Antioxidants

Antioxidants, as mentioned above, protect the oil itself from oxidation. The surface of the metal parts is anticorrosive. They contribute to the formation of a strong oil film on the metal. What protects it from contact with acids and water, which are always present in the volume of oil. Friction modifiers. For modern engines, they are increasingly trying to use oils with friction modifiers. Which can reduce the coefficient of friction between friction parts to obtain energy-saving oils. The most well-known friction modifiers are graphite and molybdenum disulfide. In modern oils they are very difficult to use. Because these substances are insoluble in oil and can be dispersed in it only in the form of small particles. This requires the introduction of additional dispersants and dispersed stabilizers in the oil, but it still does not allow the use of such oils for a long time.

Motor oil qualification

Therefore, oil soluble fatty acid esters are currently commonly used as friction modifiers. Which have very good adhesion to metal surfaces and form a layer of molecules that reduce friction. To facilitate the selection of oil of the required quality for a particular type of engine and its operating conditions, classification systems exist. Currently, there are several classification systems for motor oils: API, ILSAC, ACEA and GOST. In each system, engine oils are divided into series and categories depending on quality and purpose. These series and categories were created at the initiative of national and international organizations of oil refineries and automakers. Purpose and quality level are the basis of the range of oils. Along with generally accepted classification systems, there are also the requirements and specifications of automobile manufacturers. In addition to the classification of oils by quality, the SAE viscosity classification system is also used.