The motor vehicle has many parts in contact that move against each other under varying speed and load conditions. When two surfaces are in contact and move against each other, three things happen:
- Energy is used to cause movement.
- The surfaces resist motion.
- The surfaces become warm then hot
- The surfaces wear away or erode.
The resistance to movement is friction.
Friction is the force that resist the movement of one surface over another surface. It is present at all times between all types of surfaces that are in contact, yet it is both of the surfaces or when movement occurs, only noticeable when an attempt is made to movement occurs:
- Friction increases with surfaces irregularities. A smooth surface has less friction than a rough surface.
- Friction is dependent on the surface material.
- Friction increase as the load increases.
- Friction is independent of speed. Although on a lubricated surface, friction increases as speed increases.
- Friction generates heat.
- The greater the friction, the greater the heat
Excessive heat :
- Causes lubrication failure;
- Changes the set clearance;
- Changes the surface
- Characteristic of the bearing
It is impossible to eliminate friction, but it can be reduced or minimized. Manufacturers minimized friction by incorporating in the component the most suitable type of bearing;
- Bearing surface material;
- Method of lubrication;
- Type of lubrication.
The above rarely require modification and should not be changed unless authorized by the manufacturer. However, you can in part ensure that the engine is maintained in a serviceable and reliable condition by understanding the function and requirements of the lubrication system and its component.
If excessive friction is present in an engine, part of the engine’s power is lost in overcoming this friction. To ensure that energy is not wasted through excess friction and that bearings and other related parts have a normal life, learn a correct method of servicing, inspection and adjustment to the lubrication system and components.
The proper function of the lubrication system relies on the correct lubrication or engine oil. All lubricating oils suitable for use the medium and high speed diesel engines contain most, if not all of the following additives. Indeed, many contain more than the major ones listed below:
- Oxidation inhibitors (anti-oxidants).
- Corrosion inhibitors (alkaline additives).
- Viscosity index improvers.
- Pour point depressants.
- Anti scuff (extreme pressure) additives.
- Foam inhibitors (anti foam additives).
They must be the correct viscosity or thickness to :
- Prevent metal to metal contact between moving parts reducing:
- Wear or erosion
- Assist in dissipating the heat generated by friction and combustion.
- Resist change due to high temperature.
- Retain viscosity and not become too thin.
- Resist deterioration from oxidation from oxidation, i.e. The formation of vanish like deposits due to the combination of oil and oxygen.
- Assist in sealing gas leakage between the piston, rings and cylinder walls.
- Prevent harmful deposits from forming on all internal engine parts.
- Remove particles of foreign and abrasives material from between moving parts.
- Retain these particles in suspension to prevent sludge forming.
To enable the oil to carry out the function required, chemicals are added, and these will give the oil some or all of the following properties:
- Ability to flow at varying temperatures.
- Prevention of oxidation when the oil is heated and stirred.
- Prevention of corrosion and rust.
- Prevention of foaming.
- Prevention of dirt deposits.
Service rating of lubricants
To identify the oil according to the chemicals added, the oil is given a ’Service Rating’ . This rating determines the condition of service for which the oil is best suited. There are several different ratings for engine lubricating oils:
- SA – Contains no additives and is for use in engines operating under ideal conditions.
- SB – Contains minimum additives to protect against corrosion, oil oxidation and scuffing (parts scrapping against each other).
- SC – Primarily used in 196-1967 model passenger cars. It has additives that provide protection against dirt deposits, wear, rust and corrosion.
- SD – Used in engines of models from 1968 onwards. It provides better protection than SC oil.
- SE – In addition to having the qualities of SC and SD oils. It is also a high detergent oil.
- SF – Similar to SE.
Diesel engine lubricating oils are given different service ratings. These ratings denote the severity engine operations for which the oil is most suitable. The ratings in order are CA, CB, CC and CD, the CD-rated oil being the one used for the most severe conditions.
The viscosity of an oil is its ability to flow at a certain temperature. An oil which flows slowly at the same temperature as a high viscosity.
In addition to the service rating, the lubricating oil is given a viscosity rating. The rating figures are based on the oil’s capacity to flow at both high and low temperatures and are determined by a test devised by the society by the Society of Automotive Engineers (SAE).
The oil is allocated an SAE number: SAE5, SAE10, SAE20, SAE30 and so on. The oil that has the lowest number has the lowest viscosity and therefore the thinnest.
The colder the oil becomes, the thicker it is and therefore it offers greater resistance to movement. This resistance to movement will affect the ability of the starter motor to turn over the engine on cold days, resulting in hard starting. To help overcome this problem, oils have been produced for use during cold weather; these are identified by the letter ‘W’ affixed to the grading number SAE 5W, SAE 10W, etc.Modern methods have produced oils suitable for use over a range of temperatures and these are multiple-viscosity oils or multi-grade oils.
Multiple-viscosity oil rated as SAE 10W-30 indicates that the viscosity of the oil is the same as SAE10W when it is cold and the same as SAE 30 when hot.
Contamination refers to the presence of unwanted material or contaminants in the engine oil. As oil circulate through the lubrication system, it becomes contaminated and unfit for use after a given time. Manufacturers specify vehicle distance or time periods at which the oil should be replaced. These recommendations are based on average conditions and the condition of the oil should be checked regularly. Contaminant of the oil is caused by:
- Contamination of the oil are caused by:
- Carbon deposits from engine combustion
- Dust and dirt brought into the engine with the air and fuel mixture;
- Fine pieces of metal, as a result of engine wear, becoming mixed with the oil;
- Raw fuel and combustion products escaping past the piston rings into the crankcase Water condensation from the air passing through the engine.
As this contamination builds up, m the oil becomes ineffective as a lubricant. If not changed, excessive engine wear Will take place.