Transcript Engine Friction

Engine Friction
Dr Jehad Yamin
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.
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.
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).
Methods of determining the frictional losses
• From IP (by the indicator diagram plot) and BP
(from dynamometer test).
• Willans’ lines.
• Morse test.
• Motoring method.
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.
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.
• 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.
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.