1. Engine Friction
Dr Jehad Yamin

2. What is Friction?
If we have a closer look at the material under the microscope, we find that however polished it is, it always contains some peaks and bottoms (uneven surface).
If we bring two of these surfaces in contact with one another and allow some degree of relative motion between them the interlocking between these peaks and bottoms exerts some opposing force which tries to oppose this relative motion. This force is called Friction.
Engine friction is the difference between the indicated and brake power, i.e. power consumed in overcoming the frictional forces.

3. Effect of friction
Studies have shown that as the CR increases, the FMEP, peak cylinder pressure and thermal efficiency all do increase. It is, however, shown that for further increase in CR the gain achieved in thermal efficiency is lesser than the rise in the FMEP.
This causes power, thermal efficiency loss and increase in SFC, it also means that the amount of power converted to heat energy and is been rejected to the coolant also increases adding extra load on the cooling system.

4. Effect of some variables on frictional losses
Compression ratio (Higher the CR greater is the frictional losses).
Engine speed (Higher the RPM greater is the frictional losses).
Oil viscosity (Higher the viscosity greater is the frictional losses).
Coolant temperature (because of its effect on reducing oil viscosity, Higher the coolant temperature lesser is the frictional losses).

5. Methods of determining the frictional losses
From IP (by the indicator diagram plot) and BP (from dynamometer test).
Willans’ lines.
Morse test.
Motoring method.

6. Function of the lubricating system
Lubrication (i.e. to keep the moving parts sliding freely past each other and, thus, reduces the engine friction and wear).
Cooling (i.e. to keep the surfaces cool by taking away a part of their heat through oil passing over them). This cooling action takes part simultaneously with lubrication.
Cleaning (i.e. to keep the bearings and piston rings clean of the products of wear and products of combustion, especially carbon by taking them away and not allowing them to agglomerate and form sludge).
Sealing (i.e. it must form a good seal between piston rings and cylinder wall. It should be able to fill the smallest leakage paths and surface irregularities of the mechanical sealing element).
Noise reduction.

7. Properties of lubricant
Viscosity.
Viscosity index (It is an index that measures the variation of oil viscosity with temperature). A high viscosity index means lesser changes with temperature). The lubricating oil should maintain sufficient viscosity at higher temperatures and still not to be too viscous for easy starting of the car at lower temperatures.
Cloud and pour point. Is the temperature at which the oil will start to solidify if cooled. Pour point is the temperature just above which the oil sample will not flow under certain prescribed conditions.
Flash point. Is the temperature at which the vapor of the oil will flash when subjected to flame.
Fire point. Is the temperature at which the oil vapor will burn steadily for at least 5 seconds when lit by a flame.
Specific gravity.

8. Carbon residue.
Oiliness. Is the property of an oil to cling to the metal surface by molecular action and then provide a very thin layer of lubricant under certain boundary lubrication conditions.
Oxidation stability. It is the resistance to oxidation.
Cleanliness. Absence of water (causes corrosion) and sediments (causes wear).
Color. Just to indicate the degree of refining of the oil.
Acidity and neutralization number. Is a measure of its acidic or alkaline contents.

9. Types of additives
Detergents. Control of high temperature deposits and acts as effective acid neutralizer if overheated.
Dispersant. Control low temperature sludge and varnish deposits.
Anti-wear. Reduce wear and prevent scoring, galling and seizure.
V.I. Improver.
Pour point depressant. Reduce pour point of oil by interfering with wax crystallization.
Anti-foam. Reduce oil foaming by causing collapse of bubbles due to air entertainment.
Anti oxidant. Reduce oil oxidation to protect alloy bearings against corrosive attack.