A lubricating oil with the necessary properties and characteristics will (1) provide a film of proper thickness between the bearing surfaces under all conditions of operation, (2) remain stable under changing temperature conditions, and (3) not corrode the metal surfaces. If the lubricating oil is to meet these requirements, the engine operating temperature must NOT exceed a specified limit.

In internal-combustion engines, lubricating oil serves six functions:

1. Controls friction between load-bearing surfaces

2. Reduces wear by preventing metal-to-metal contact between moving parts

3. Limits the temperature by carrying away heat from fluid friction and combustion of fuel

4. Reduces corrosion by coating metal parts and by flushing debris from between moving parts

5. Dampers mechanical shock in gears

6. Forms a seal on the walls of the cylinders

Some of these functions and characteristics are discussed in the sections that follow.

Protective Film

Direct metal-to-metal contact of load-bearing surfaces is similar to the action of a file as it wears away metal. The filing action is a result of very small irregularities in the metal surfaces. The severity of the filing action depends on the finish of the surfaces, the force with which the surfaces are brought into contact, and the relative hard-ness of the materials. Lubricating oil fills the tiny cavities in bearing surfaces and forms a film between the sliding surfaces to prevent high friction losses and rapid wear of engine parts. The lack of a proper oil film will result in a seized (frozen) piston, wiped bearings, and stuck piston rings.

Cooling

Lubricating oil assists in cooling the engine because the constant flow of oil carries heat away from localized hot spots. The principal parts from which oil absorbs heat are the bearings, the journal surfaces, and the pistons. In some engines, the oil carries the heat to the sump where the heat dissipates in the mass of oil. However, most modern internal-combustion engines use a centralized pressure-feed lubrication system. This type of system has an oil cooler (heat exchanger) where the heat in the oil is transferred to the water circulating in the jacket-water cooling system.