Download File - Vigyan Pariyojana

Internal Combustion Engines
Faculty - Er. Ashis Saxena
Index
Unit 1
Introduction to I.C Engines
Fuels
Unit 2
SI Engines
Unit 3
CI Engines
Unit 4
Engine Cooling
Lubrication
Supercharging
Testing and Performance
Unit 5
Compressors
Unit - 4
Chapter – 4(b)
Lubrication
Prerequisites of Engine Lubrication
Friction

In engine frictional losses are mainly due to sliding as well as
rotating parts.

Engine friction is expressed in terms of frictional power (fp).

fp = ip – bp.

A good engine shall limit the total frictional losses below 30%.

To reduce friction an engineer shall do something.
Friction inside an IC engine
Friction inside a engine is in the following forms:

Direct Frictional losses

Pumping Losses

Power loss to drive components to charge and scavenge

Power loss to drive other auxiliary components
Direct Frictional losses

The power absorbed in an engine due to the relative motion of
different bearing surfaces such as piston rings, main bearings,
cam shaft bearings etc.

Direct frictional losses attribute to a higher value in
reciprocating engines due to a large number of moving parts.
Pumping Loss

The net power spent by the engine (piston) on the working
medium (gases) during intake and exhaust strokes is known as
pumping loss.

For two stroke engines it is negligible since the incoming fresh
mixture is used to scavenge the exhaust gases.
Power loss to drive components to
charge and scavenge

In certain type of four-stroke engines the intake charge is
supplied at a higher pressure than the naturally aspirated
engines.

For this purpose a compressor or turbine is used.

The devices used (turbine or compressor) take away a part of
the engine output considered as negative frictional loss.
Power loss to drive the auxiliaries

In an engine the accessories like water pump, lubricating oil
pump, fuel pump, cooling fan etc. takes a good percentage of
generated power output.

This is considered a loss since the power used to drive these
components reduces the net output of the engine.
Factors affecting Friction in IC Engines
Engine Design: The design parameters affecting friction losses are:
i.
Stroke-Bore ratio:

Lower stroke-Bore ratio reduces friction due to less frictional area.
ii.
Engine size:

Larger engines have more frictional surfaces, hence larger friction.
iii.
Piston rings:

Reducing the number of piston rings results in reduction of contact
surface from cylinder walls and hence reduces friction.
Factors affecting Friction in IC Engines
Engine Speed:
Friction increases rapidly with increasing speed & at higher speeds
mechanical efficiency starts deteriorating considerably.
Engine Load:
Increasing the load increases the maximum pressure in the cylinder
hence increase in friction values.
Oil viscosity:
Viscosity is directly proportional to friction loss. Increase in viscosity
increase in friction, decrease in viscosity, decrease in friction.
Factors affecting Friction in IC Engines
Cooling water Temperature:
Rise in cooling water temperature slightly reduces engine friction by
reducing oil viscosity (but to a limit only).
Why do we need Lubrication?
To reduce the friction and wear of
parts subjected to relative motion
(leading to reduction of energy loss
& increase the life of the engine).
Lubrication
Introduction of a substance (lubricant such
as oil, grease, etc.) between the contact
surfaces of moving parts is known as
lubrication.
And obviously it serves some
purposes.
What purpose does a lubrication system serves?
Lubricate

Reduces Friction by creating a thin film (Clearance) between
moving parts (Bearings and journals)
What purpose does a lubrication system serves?
Sealing

The oil helps form a gastight seal
between
piston
rings
and
cylinder walls (Reduces BlowBy).
What purpose does a lubrication system serves?
Cleans

As it circulates through the engine, the oil picks up metal
particles and carbon, and brings them back down to the pan.
What purpose does a lubrication system serves?
Cools

Picks up heat when moving through the engine and then
drops into the cooler oil pan, giving up some of this heat.
What purpose does a lubrication system serves?
Absorbs shock

When heavy loads are imposed on the bearings, the oil helps
to cushion the load.
Absorbs Contaminants

The additives in oil helps in absorbing the contaminants that
enter the lubrication system.
Theory of Lubrication

Consider two solid blocks in contact with each other.

In order to move the upper block over the surface of the lower
block a constant tangential force must be applied.

The force due to the weight of the upper block acting
perpendicular to the surface is called the normal force.

The ratio of the tangential force to the normal force is known as
the dynamic coefficient of friction or the
coefficient of friction, f.
Theory of Lubrication
Theory of Lubrication

To keep the block in motion, a constant tangential force
is required to overcome the frictional resistance
between the two surfaces.

This frictional resistance arises because the moving
surfaces are rough, and hence small irregularities will
fit together at the contact area (interface) to give a
mechanical lock to the motion.
Theory of Lubrication

The resistance between moving surfaces can be reduced by the
introduction of a small film of lubricant between the moving
surfaces so that the two surfaces are not in physical contact.

The lubricant film layer provides lesser resistance than that of
the solid surface and hence less force is required to accomplish a
relative motion.

The friction due to surface irregularities is also reduced.

The solid friction is replaced with a definitely lesser fluid friction.
Theory of Lubrication

Consider two parallel plates filled with viscous oil in between
them, of which one is stationary and other is in motion with a
constant velocity as shown in figure.
u
V=u
V=0
Theory of Lubrication

Let us imagine the film as composed of a series of horizontal
layers and the force, F causing layers to deform or slide over
another, just like a deck of cards.
u
V=u
V=0
Theory of Lubrication

The first layer clinging to the moving surface will move with the
plate because of the adhesive force between the plate and the
oil layer while the next layer is moving by at a slower pace.
u
V=u
V=0
Theory of Lubrication

The subsequent layers below keep moving at gradually reducing
velocities.
u
V=u
V=0
Theory of Lubrication

The layer clinging to the surface of the stationary plate will have
zero velocity.
u
V=u
V=0
Theory of Lubrication

The reason is that each layer of the oil is subjected to a
shearing stress and the force required to overcome this stress is
the fluid friction.

Hence the velocity profile across the oil film varies from zero at
the stationary surface to the velocity of the plate at the moving
surface.
V=u
V=0
Theory of Lubrication
 The
fluid or internal friction arose because of
the resistance of the lubricant to shearing
stress.
A
measure of the resistance to shear a property
called dynamic viscosity or coefficient of
viscosity.
Types of Lubrication
Considering the nature of motion between moving or sliding
surfaces, there are different types of mechanisms by which the
lubrication is done.

Hydrodynamic lubrication or thick film lubrication

Hydrostatic lubrication

Boundary lubrication or thin film lubrication

Extreme pressure lubrication
Hydrodynamic Lubrication
 Hydrodynamic
lubrication is said to exist when
the moving surfaces are separated by the
pressure of a continuous unbroken film or layer
of lubrication.
 In
this type of lubrication, the load is taken
completely by the oil film.
Hydrodynamic Lubrication
 The
basis of hydrodynamic lubrication is the
formation of an oil wedge.
 When
the journal rotates, it creates an oil taper
or wedge between the two surfaces, and the
pressure build up with the oil film supports the
load.
Hydrodynamic Lubrication
Hydrodynamic Lubrication
Hydrostatic Lubrication
 Hydrostatic
lubrication is essentially a form of
hydrodynamic lubrication in which the metal
surfaces are separated by a complete film of oil,
but instead of being self-generated, the
separating pressure is supplied by an external oil
pump.
Hydrostatic Lubrication
Boundary Lubrication

Boundary lubrication exists when the operating condition are
such that it is not possible to establish a full fluid condition,
particularly at low relative speeds between the moving or sliding
surfaces.

The oil film thickness may be reduced to such a degree that
metal to metal contact occurs between the moving surfaces.

The oil film thickness is so small that oiliness becomes
predominant for boundary lubrication.
Boundary Lubrication
Boundary lubrication happens when
A
shaft starts moving from rest.
 The
speed is very low.
 The
load is very high.
 Viscosity
of the lubricant is too low.
Extreme pressure lubrication

When the moving or sliding surfaces are under very high
pressure and speed, a high local temperature is attained.

Under such condition, liquid lubricant fails to stick to the
moving parts and may decompose and even vaporize.

To meet this extreme pressure condition, special
additives are added to the minerals oils.

These are called “extreme pressure lubrication.”
Extreme pressure lubrication

These additives form on the metal surfaces more
durable films capable of withstanding high loads and
high temperature.

Additives are organic compounds like chlorine (as in
chlorinated esters), sulphur (as in sulphurized oils), and
phosphorus (as in tricresyl phosphate).
Crankcase Ventilation
Before knowing about crankcase
ventilation it is important to know
about blowby.
Blowby

During the compression and
expansion strokes the gas inside
the cylinder gets past the piston
rings and enters the crankcase
which is called the blowby.
Crankcase Ventilation

It contains water vapor and sulphuric acid, if either the oil
or the fuel contains appreciable amount of sulphur which
might cause corrosion of steel parts in the crankcase.

When the amount of water vapor condensed becomes
considerable, in cold weather this may freeze and may
cause considerable damage to the lubricating oil pump.

Hence blowby shall be removed from the crankcase.
Crankcase Ventilation
 The
removal of blowby can be
achieved efficiently by passing a
constant stream of fresh air
through the crankcase known as
crankcase ventilation.