Internal Combustion Engines Faculty - Er. Ashis Saxena

Internal Combustion Engines

Faculty - Er. Ashis Saxena

IndexUnit 1

Introduction to I.C EnginesFuels

Unit 2SI Engines

Unit 3CI Engines

Unit 4Engine CoolingLubricationSuperchargingTesting and Performance

Unit 5Compressors

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%.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.


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.


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 To reduce the friction and

wear of parts subjected to wear of parts subjected to

relative motion (leading to relative motion (leading to

reduction of energy loss & reduction of energy loss &

increase the life of the increase the life of the

engine).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)


Sealing

The oil helps form a gastight seal

between piston rings and cylinder

walls (Reduces Blow-By).


Cleans

As it circulates through the engine, the oil picks up metal

particles and carbon, and brings them back down to the pan.


CoolsPicks up heat when moving through the engine and then drops into the cooler oil pan, giving up some of this heat.


Absorbs shockWhen heavy loads are imposed on the bearings, the oil helps to cushion the load.

Absorbs ContaminantsThe additives in oil helps in absorbing the contaminants that enter the lubrication system.

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 ratio of the tangential force to the normal force is known as

the dynamic coefficient of friction or thethe dynamic coefficient of friction or the

coefficient of friction, f.coefficient of friction, f.

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 This frictional resistance arises because the moving

surfaces are rough, and hence small irregularities will surfaces are rough, and hence small irregularities will

fit together at the contact area (interface) to give a fit together at the contact area (interface) to give a

mechanical lock to the motion.mechanical lock to the motion.


The resistance between moving surfaces can be reduced by the The resistance between moving surfaces can be reduced by the

introduction of a small film of lubricant between the moving introduction of a small film of lubricant between the moving

surfaces so that the two surfaces are not in physical contact.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.


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

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

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

The subsequent layers below keep moving at gradually reducing

velocities.


u

V = u

V = 0

The layer clinging to the surface of the stationary plate will have

zero velocity.


u

V = u

V = 0

The reason is that each layer of the oil is subjected to a The reason is that each layer of the oil is subjected to a

shearing stress and the force required to overcome this stress is shearing stress and the force required to overcome this stress is

the fluid friction. 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

The fluid or internal friction arose because of The fluid or internal friction arose because of

the resistance of the lubricant to shearing the resistance of the lubricant to shearing

stress. stress.

A measure of the resistance to shear a property

called dynamic viscosity or coefficient of

viscosity.


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 lubricationHydrodynamic lubrication or thick film lubrication

Hydrostatic lubricationHydrostatic lubrication

Boundary lubrication or thin film lubricationBoundary lubrication or thin film lubrication

Extreme pressure lubricationExtreme pressure lubrication

Types of 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.




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

Hydrostatic 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.

Boundary 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).

Extreme pressure lubrication

Before knowing about crankcase

ventilation it is important to know

about blowby.

BlowbyDuring 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.


The removal of blowby can be The removal of blowby can be

achieved efficiently by passing a achieved efficiently by passing a

constant stream of fresh air constant stream of fresh air

through the crankcase known as through the crankcase known as

crankcase ventilation.crankcase ventilation.


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Internal Combustion Engines Faculty - Er. Ashis Saxena