qip ice 25 cooling systems

8/10/2019 Qip Ice 25 Cooling Systems

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Internal Combustion Engines

Lecture-25

Ujjwal K Saha, Ph.D.Department of Mechanical EngineeringIndian Institute of Technology Guwahati

Prepared underQIP-CD Cell Project


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SignificanceIn a spark ignition engine, cooling must

be satisfactory to avoid pre-ignition andknock. In a compression ignition engine,since a normal combustion is aided,cooling must be sufficient to allow the

parts to operate properly. In short, coolingis a matter of equalization of internaltemperature to prevent local overheatingas well as to remove sufficient heat energyto maintain a practical overall workingtemperature.


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Reasons for Coolingto promote a high volumetric efficiencyto ensure proper combustion, and

to ensure mechanical operation & reliability.

Effect of Over-coolingthe thermal efficiency is decreased due to

more loss of heat carried by the coolant

the vapourization of the fuel is less resultingin lower combustion efficiencylow temperature increases the viscosity of

lubricant causing more loss due to friction.


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Types of Cooling System

Air cooling (o r d ire c t c o o lin g ) system

Liquid cooling (o r in d ire c t c o o lin g ) system

Remark: Aviation engines, motor cycleengines and scooter engines are aircooled; while the stationery andautomobile engines are liquid cooled.


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Air cooling system

Air cooled engines depend on airflowacross their external surfaces of the enginecylinders to remove the necessary heat. Theamount of heat dissipated depends upon:

the area of cooling surface incontact with the air

mass flow rate of airtemperature difference between

cylinder and air andconductivity of metal.


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Cooling fins in air cooled system

The area of cooling surface is increased byforming thin fins, either integrally by machiningthem on the outer walls of the engine cylinderand cylinder head or by attaching separate finsto them.


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Air cooling system - Advantages

The absence of radiator, cooling jackets, coolant and pumps make the

engine lighter.The engine can be operated in cold

climate where liquid may freeze.

In places where water is scarce, aircooled engine is an advantage.

Handling of liquid coolant requirespiping and pumping auxiliaries.

Air cooled engines have no coolantleakage or freezing problems.


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Air cooling system - Disadvantages

Relatively large amount of power is

used to drive the cooling fan.Engines give low power output.

Cooling fins under certain conditionsmay vibrate and amplify the noise level.

Cooling is not uniform.

Engines are subjected to highworking temperature.


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Liquid cooling systems - Types

Direct or non-return systemThermosyphon system

Forced circulation cooling system

Evaporative cooling system


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R a d i a

t i o n

Combustion

of air-fuelcharge

Conduction

Conduction

Conduction

L i q u

i d c o n v e c

t i o n m o v e m e n

t

Radiation through gasGas convection current

Gas stagnant filmOil filmMetal cylinder wallScale filmLiquid stagnant filmLiquid convection current

Heat Transfer through aliquid cooled cylinder wall

Direct or non-return system

The heat released fromthe combustion of air-fuelmixture is transferred in all

directions to the walls ofthe combustion chambers,cylinders and pistons bydirect radiation, byconvection currents of gasrubbing against astationary gas film, andthen by conductionthrough this stagnantboundary layer of gas andan oil film to the metalwall.


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Schematic diagram of a

thermosyphon system

Thermo-syphon system

In this system, a fanrotated by the crankshaftdraws cold air from outsidethrough the radiator. Theradiator is connected tothe engine block bymeans of two pipes. Thehot water passes throughsome thin pipes built in theradiator, where it gets

cooled. Thus, the fluidcirculates through thesystem in the form ofconvective currents.

Radiator

Cylinder

Piston


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Schematic diagram of a

forced circulation system

Forced circulation cooling system

This system is used in a large number of vehicleslike cars, buses, trucks and other heavy vehicles.Here, circulation of water takes place with

convection currents helped by a pump.The water or coolant is

circulated through

jackets around the partsof the engine to becooled, and is kept inmotion by a centrifugal

pump, driven from theengine. A thermostat isused to control the watertemperature required forcooing.


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Cooling is independent of temp. =>Engine is overcooled(range of temp.=75-90 0C)

Pump-cooling or forced cooling

Pump is introduced between radiator and engine block

Rotated by crankshaft by means of a belt

Water is circulated with force => heat is removed quickly

Limitation

Can be overcomeby using thermostat.

Pump cooling system


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Schematic diagram of an

evaporative cooling system

Evaporative cooling system

In this system, the engine will be cooledbecause of the evaporation of the water in thecylinder jackets into steams.

The advantage isbeing taken from thehigh latent heat ofvaporization of waterby allowing it toevaporate in thecylinder jackets. Thissystem is used forcooling of manytypes of industrialengines.

R a d i a t o r

C y l i n d e r b l o c k

P u m p

W a te r S t e a m


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Rejects the coolant heat to the surrounding air Disperses the heated coolant into fine streams

so that small quantities of coolant are brought incontact with large metal surface areas, which inturn are cooled by air stream Two types of radiators

(1) Down-flow type(2) Cross-flow type

Availability of space dictates the choice ofradiator, both are equally efficient. Various design of radiator cores are used forcooling the water

Parts of cooling systems - Radiator


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Honeycomb block type core Film type of radiator core

Tube and fin type core Tube and corrugated type core


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Materials for radiator

Good corrosion resistance

Good thermal conductivity Must possess the required strength

Must be easily formable Yellow brass, copper are used

(soldered easily => easy repair) Aluminium is used where weight is critical


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Can operate reliablywithin the specifiedtemperature range. Heat is transmitted towax, which has highcoefficient of thermalexpansion. Upon being heated,wax expands and therubber plug presses theplunger forcing it tomove vertically

upwards.

Wax Thermostat

Wax Thermostat


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Cooling fan

Maintain an adequate air flow across theradiator matrix. Serves the purpose when natural draft is notsufficient to cool e.g., at low speed but heavyload, when vehicle ascends uphill etc. Driven by a belt run by crankshaft.

Limitations

Rising level of noise. Increasing power

consumption with enginespeed. Tendency to overcool.Cooling fan


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Cooling fan


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Liquid cooling system - Advantages

Because of even cooling of cylinder barreland head (due to jacketing) makes itpossible to reduce the cylinder head and

valve seat temperatures.The volumetric efficiency of water cooled

engines is higher than that of aircooled

engines.Compact design of engines with

appreciably smaller frontal area is possible.

In case of water cooled engines,installation is not necessarily at the front ofthe mobile vehicles, aircraft etc. as thecooing system can be conveniently located.


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Liquid cooling system - Disadvantages

The system requires moremaintenance.

The engine performance becomessensitive to climatic conditions.

The power absorbed by the pump isconsiderable and affects the poweroutput of the engine.

In the event of failure of the coolingsystem serious damage may be causedto the engine.


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Use of Anti-freezers

During winter or when the engine is kept outof operation in cold places, the cooling water inthe cylinder jackets, radiator tanks and leadingpipes will freeze, expand and lead to theirfracture. To prevent damage to the engine andradiator during winter weather, suitable liquids or

compound substances ( ( k n o w n a s a n ti k n o w n a s a n ti - - freezers) freezers) which go into solution are added to the water tolower the freezing temperature of the coolant.

Ethylene glycol is the most widely usedautomotive cooling-system antifreeze,although methanol, ethanol, isopropyl alcohol,

and propylene glycol are also used.


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Requirements of anti- freezers

They should thoroughly mix with water.

They should not corrode the surfaces withwhich they are in contact.

Their boiling point should be high so thatthe loss due to evaporation is minimum.

They should not deposit any foreign matterin the jackets, hose, pipes or radiator.

It should be chemically stable, a goodconductor of heat, and a poor conductor ofelectricity.


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Lubricating Oil as Coolant

The lubricating oil used in an engine

also helps to cool the engine. The hotterparts like piston face and back surfaceof piston crown is subjected to oil flow,usually done by spraying the oil bypressurized systems or by splash in non-pressurized system. Other componentslike camshaft and connecting rods are

also cooled by oil circulation through oilpassages.


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Conclusions

A cooling system must be provided notonly to prevent damage to the vital parts ofthe engine (due to high temperature), butthe temperature of these components mustbe maintained within certain limits in orderto obtain maximum performance from theengine. To keep the cylinders fromoverheating, they are surrounded with awater jacket on liquid cooled engines, orwith a finned surface in air cooled engines.

Most small and medium sized engines usedin two-wheelers, automobiles, aircraft areair cooled, while the stationery and someautomobile engines are liquid cooled.


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Conclusions

A ty p ic a l a u to m o tiv e c o o lin g sy ste m c o m p rise s:

a series of channels cast into the engine block andcylinder head, surrounding the combustion chamberswith circulating water or other coolant to carry awayexcessive heat

a radiator consisting of many small tubes equipped

with a honeycomb of fins to radiate heat rapidly, thatreceives and cools hot liquid from the engine

a centrifugal-type water pump to circulate coolant

a thermostat that maintains constant temperature byautomatically varying the amount of coolant passinginto the radiator, and

a fan, which draws fresh air through the radiator.


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Technologists , Addison Wisley.3.3. Fergusan CR,Fergusan CR, andand Kirkpatrick ATKirkpatrick AT ,, (2001), Internal Combustion Engines , John

Wiley & Sons.4.4. Ganesan V Ganesan V ,, (2003), Internal Combustion Engines , Tata McGraw Hill.5.5. Gill PW, Smith JH,Gill PW, Smith JH, andand Ziurys EJZiurys EJ ,, (1959), Fundamentals of I. C. Engines , Oxford

and IBH Pub Ltd.6.6. Heisler H,Heisler H, (1999), Vehicle and Engine Technology, Arnold Publishers.7.7. Heywood JB,Heywood JB, (1989), Internal Combustion Engine Fundamentals , McGraw Hill.

8.8. Heywood JB,Heywood JB, andand Sher E,Sher E, (1999), The Two-Stroke Cycle Engine , Taylor & Francis.9.9. Joel R, Joel R, (1996),(1996), Basic Engineering Thermodynamics, Addison-Wesley.10.10. Mathur ML, and Sharma RP,Mathur ML, and Sharma RP, (1994), A Course in Internal Combustion Engines,

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The MIT Press, Cambridge, Massachusetts.

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