Understanding Engine Lubrication: Types of Friction and Motor Oil

Posted on Jan 10, 2025 in Geology

Types of Friction

The effects of friction are reduced by using materials with good sliding qualities and appropriate finishes. The existence of an oil film results in three kinds of friction:

  • Fluid: Occurs when an oil film is interposed between two surfaces, keeping them completely separate. This occurs in the pressure greasing of the engine (supports and pads of the crankshaft, camshaft supports, rocker’s shaft).
  • Semifluid: Occurs when the oil film adhering to surfaces is not sufficiently abundant and uniform, resulting in friction between metals and, therefore, wear (segment and cylinder bearings in cold start).
  • Dry: Occurs when there is no grease, and the metals of both pieces are in direct contact. This generates high temperatures that produce excessive expansion. This happens if there is a failure of lubrication.

Lube Oil Service

  • Reduces friction and mechanical losses.
  • Refrigerates lubrication areas.
  • Increases the seal between the segments and improves cylinder compression.
  • Cushions and smooths bearing efforts.
  • Cleans and carries away dust from abrasive wear (soot and other combustion products).

Motor Oil

Subjected to high temperatures and pressures, motor oil forms a thin film that clings to surfaces to reduce friction.

Features

  • Maintains viscosity when hot and sufficient fluidity to facilitate cold starts.
  • Protects from corrosion of the cylinder walls.
  • The oil that passes into the combustion chamber must burn without leaving residue on spark plugs, valves, and segments.
  • Maintains lubricating properties for extended periods of engine operation.

There are factors that limit their lifespan, but the use of additives improves the performance characteristics, lengthening it.

Types of Motor Oil

  • Mineral: Composed of mineral oil. It is obtained by refining crude oil.
  • Synthetic: A chemical that changes the molecular structure and removes certain unwanted mineral particles.

Advantages of Synthetic Oil

  • Good low-temperature fluidity.
  • Stable viscosity.
  • High resistance to oxidation.
  • Is detergent, dispersing, and does not tend to form deposits.
  • Semisynthetic: Base mineral oil to which synthetic oil is added at a rate to improve its qualities.

Oil Additives

  • Antioxidant: Reduces oxidation in contact with air.
  • Anticorrosive: Neutralizes the acids that form inside the engine.
  • Detergents: Clean lines and surfaces to avoid the formation of deposits.
  • Dispersants: Prevent the accumulation of waste that creeps into the crankcase, leaving them in suspension until the oil change.

Classification of Oil

By Conditions of Service

They are classified in laboratories.

By Viscosity

The SAE classification relates viscosity to temperature. It consists of 10 degrees, the first six from 0 to 25 with the letter W for Winter, and 4 units, ranging from 20 to 50, indicating the behavior of the viscosity of hot oil (100 degrees). These grades indicate the minimum temperature at which the oil can be used.

  • Monograde: It is marked with a single degree of viscosity. It is used where the ambient temperature does not undergo major changes.
  • Multigrade: They have a wider scope of use. Multigrade oils are identified with two different viscosity grades that correspond to the behavior in cold and hot temperatures.

API Classification

S for Otto engines, C for diesel engines (current: SJ’97, SL’01, SM’04 / CF, CF2, CF4, CG4, CH4, CI4). Each category acts better than the previous one; the most recent is a substitute for any of the above.

ACEA Classification

Classified into three groups: A for gasoline engines, B for light diesel passenger cars, and E for heavy diesel engines.