Automotive Lubricants - Engine Oil
Automotive Lubricants - Engine Oil
Automotive Lubricants is a material that is used to control (or, more commonly, to reduce) friction and wear on the surfaces of bodies in relative motion. Lubricants are also used to remove heat and wear debris, supply additives into the contact, transmit power, protect, and seal depending on their nature.  Lubricants can be liquid (oil, water), solid (graphite, graphene, molybdenum disulfide), gaseous (air), or semisolid (grease). Based on the base oil, liquid lubricants are classed as synthetic/semi-synthetic, mineral, or environmentally acceptable (biodegradable). The majority of lubricants (5-30%) contain additives to improve their performance. The application specifies which oil, also known as the base oil, should be used. Synthetic oils are often employed in harsh situations. When the environment is at stake, environmentally friendly lubricants must be utilised.  

Automotive Lubricants Evolution

Automotive Lubricants were invented thousands of years ago, as early as 2400 BC (and possibly earlier) when an Egyptian statue was brought to the cemetery of Tehuti-Hetep, El-Bersheh, using a liquid (oil/water) lubricant to minimise friction between sledge and ground/sand.  By 1400 BC, Egyptians were lubricating chariot axles with animal fat. However, a good theoretical explanation of lubricant action was only attainable with the discovery of Sir Isaac Newton's viscous flow principles in 1687.  Professor Osborne Reynolds derived the classical equation of thin film lubrication based on viscous flow rules. This equation is the foundation of the classical hydrodynamic lubrication theory, which is the most extensively used lubrication theory. Only mineral oils, vegetable oils or fats, and greases were commonly used as lubricants in the early twentieth century. However, solid lubricants, synthetic oils, water and gas-based lubricants have recently been added to the lubricant idea.  The concept's widening has been complemented with a better understanding of lubricant performance and limitations.  

Mechanisms of Lubrication

Nature's surfaces are all rough, at least microscopically (and even atomically). When two rough surfaces glide (or roll) against each other, the asperities of the surfaces interlock, attach, and cause friction.  The purpose of any lubrication strategy is to separate the rubbing surfaces by a lubricant layer, which prevents (or at least reduces) direct contact of the bodies, as seen in the image below. The friction and wear of the materials can be managed by selecting an appropriate lubricant.  

Lubricant Varieties

Automotive Lubricants are categorised into the following groups based on their state:
  • All liquid lubricants, including mineral, natural, and synthetic oils, emulsions, and so on.
  • Solid lubricants include powders, coatings, and composites (graphite, polytetrafluorethylene, Molybdenum disulfide, and so on).
  • Gaseous lubricants: typically air, but any other gas can be used.
  • Semisolid lubricants: greases are often made of a soap that has been emulsified with mineral or vegetable oil.

Automotive Lubricants can also be categorised according to their primary function:

  • Anti-wear additives (AW): decreases wear by promoting the formation of a protective layer on surfaces (zinc dialkyldithiophosphate).
  • Extreme pressure additives (EP): prevents seizure by forming a coating on the components' surfaces (graphite, molybdenum disulfide).
  • Friction modifiers: solid particles used to control friction (graphite, molybdenum disulfide, tungsten disulfide, etc.).
  • Corrosion inhibitors: provide a corrosion resistant layer to shield surfaces from the attack of chemically active substances such as oxygen.
  • Viscosity Index Improvers: used to avoid or minimise lubricant viscosity index deterioration at higher temperatures.

Mineral (crude oil) and synthetic (man-made or altered, with defined structure) oils are also commonly used to classify oil lubricants.


Automotive Lubricants in liquid form

The liquid lubricants are classified into the following categories:

  • Base hydrocarbon oils
  • Natural fats and oils
  • Synthetic oils that do not contain hydrocarbons
  • lubricant containing water

Liquid Lubricant Properties

Automotive Lubricants performance is determined by a number of essential features. The viscosity and density of a lubricant are the most essential characteristics in lubrication theory. The thickness of a lubricant film that separates the surfaces is determined by viscosity (greater viscosity, thicker film, see hydrodynamic lubrication for an example of an equation). The lubrication regime and hence the level of friction are normally established based on the layer thickness and the combined roughnesses of the surfaces.  It should also be noted that viscosity characterises the lubricant's internal friction; hence, if the viscosity is excessively high, the lubricant friction itself becomes high, and the benefit of employing a lubricant can be minimal. Even if it has a minor impact, the density of the lubricant determines the film thickness of the lubricant and hence influences the lubrication regime.