Engine oil is obviously another area of great importance for the automotive chemist, and the design and analysis of these oils play a tremendous role in the operation of a motor vehicle. Engine oils lubricate engine parts, inhibit corrosion of these parts, assist with vehicle cooling, and help seal some of the components in the vehicle. Traditional engine oil is derived from base petroleum stock and refined to meet the requirements of the American Petroleum Institute (API). The API sets minimum performance standards for motor oil. The base stock will contain additives to improve the oil’s detergency, extreme pressure performance, and corrosion inhibition. The API groups oils according to base stock, some basic terms should be defined for engine oils:

API base stock oil groups

Group oil type

I fractionally distilled petroleum followed by solvent extraction

II Fractionally distilled petroleum followed by hydro cracking

III Group II oils further hydrocracker to produce higher viscosity index values

IV Base stock oil of polyalphaolefins (AOs)

V Polyolesters / polyalkylene glycols (PAGs)/others

Cracking: Heating of higher molecular weight petroleum fractions in the presence of a catalyst to give a lower molecular weight fraction.

Dew axing process: Lubrication feed stocks typically have an increased wax content from other processing and refining (de-asphalting). These waxes can be removed catalytically or by solvent dew axing. A reactor contains a dew axing catalyst followed by a second reactor with a hydrogen finishing catalyst to saturate olefins from the dew axing reaction.

Fractional distillation: Separation of a mixture of components or fractions by their boiling points by heating them to that point. This is a continuous process in the petroleum industry and the most common separation technique used in refineries.

Hydro treating: Generally, the first process before cracking. Petroleum fractions are reacted with hydrogen at 400°C with a cobalt oxide/molybdenum oxide catalyst . This decreases the amount of nitrogen and sulfur compounds and prevents poisoning.

Kinematic viscosity: Viscosity is a fluid’s internal resistance to flow, or its thickness. Kinematic viscosity is a measure of the ratio of the viscous force to the inertial force of the fluid or v = µ/ρ,

where µ is the dynamic viscosity (in centipoises [cP]), ρ is the density (in grams per cubic centimeter), and v is the kinematic viscosity (in centistokes [cSt]).

Reforming: Catalytic reforming is the process of increasing the number of double bonds on a petroleum product but maintaining the same number of carbon atoms. This process is done at high temperatures in the presence of a platinum or rhenium catalyst on alumina.

Solvent extraction: Solvent extraction is a method of separating compounds based on their solubility in different immiscible liquids. In industrial processes, solvents are typically transferred from an aqueous phase to an organic phase.

Viscosity index: Viscosity index is a measure of the change of kinematic viscosity with temperature and indicates an oil’s ability to lubricate with a change in temperature.

The VI scale set by the Society of Automotive Engineers (SAE) is 0 (worst) to 100 (best). VI = 0 is naphthenic oil and VI = 100 is paraffinic oil.

Engine oil Function

Chemists play a role in the addition of additives to improve the properties of motor oil. As steel and aluminum parts move closely against one another, friction between the two parts creates heat and wears away the metallic surfaces. This will lead to decreased efficiency, increased fuel consumption, decreased power output, and eventually engine failure. Engine additives are devised to combat these consequences. A list of engine additives follows; however, rather than discussing each one in depth, we will discuss only some of the more important ones:

Extreme pressure additives

Detergent additives

Metal deactivators

Corrosion inhibitors

Antifoam agents

Friction modifiers

Viscosity index improvers

Antioxidants anti wear additives; and• emulsifying agents

Motor oil will create a film between the moving metallic parts, thereby lessening the contact between them, decreasing the amount of friction and wear, and removing heat. Toward the top of the piston around the compression rings, temperatures can reach up to 160°C, while in diesel engines the temperatures can raise by another 150°C. Secondary heat exchangers called engine oil coolers (EOCs) are a design feature added to minimize these effects. They are basically plate coolers made of four or five thin plates located on an external loop in the airflow of the vehicle.

Another function of the motor oil is to coat the parts to minimize exposure to oxygen. Corrosion inhibitors within the oil assist in this function. Sludge buildup is a result of degradation of the oil after exposure to the heat and extreme pressure conditions of the engine. Many motor oils have detergents and dispersants to combat this problem. However, microscopic particles will invariably be produced from wear and cause erosion. The oil filter is another design feature that will help with the overall process. Engine oil lubricates rotating parts in the crankcase of a vehicle as it is applied to crankshaft journals, rods, and bearings. During testing and design of oil coolers, thermocouples are placed at the engine sump at the bottom of the crank to measure temperature of the oil. In many engines, tabulators are placed in the oil pan to spread the oil on the reciprocating members of the engine. The oil pump will send oil through the filter into the galleries and across critical parts such as the camshaft bearings. Oil from holes in the main journals will move through passageways inside the crankshaft to exit holes in the rod journals to lubricate the rod bearings and connecting rods. Passageways through the rods carry oil from the rod bearings to the rod–piston connections and lubricate the contacting surfaces between the piston rings. Interior surfaces of the cylinders are lubricated from oil coming from the rod bearings. There it will be a barrier between the piston rings and cylinder walls before oil returns to the sump.

Engine oil grades

Engine oil is graded based on an SAE coding system based on kinematic viscosity. The grades range from 0 to 60 in increments of 10. An oil grade with a “W” behind it designates a winter or cold-start grade; this means that the oil will have a better flow viscosity at lower temperatures. For single-grade oils, viscosity is measured at 100°C and the range that the viscosity falls in is called its weight. Single-grade oil’s viscosity will increase on a logarithmic scale as the temperature increases. Specific motor oil will have a high viscosity when cold and a lower viscosity when hot. The viscosity difference in single-grade oils will be too large to provide adequate protection for today’s vehicles. Viscosity improvers such as poly butane are added to decrease the difference between the two extremes. It is these additives that designate a multi grade oil, which will have the viscosity of the base number when cold and the viscosity of the second number when hot. Multi grade oils have two grade designations. For instance, the “30” in the 5W30 grade means that the viscosity of this oil at 100°C corresponds to the viscosity of a single-grade 30 oil at the same temperature. The first number, “5,” is associated with the winter measurement and is not rated at a single temperature. A multi grade with a first number of 5 can be utilized as well as a single-grade 5. SAE procedures test a 5W oil at –30°C; a 0W oil is tested at –35°C and a 10W oil is tested at –25°C on a cold crank simulator.