Cooling System - IC Engines : Unit-III

Cooling System

1. Engine Coo ling System 1 Dr. S. VIJAYA BHASKAR PROFESSOR IN MECHANICAL ENGG. Sreenidhi Institute of Science and Technology Yamnampet , Ghatkesar , Hyderabad, Telangana 501301

2. 10 R a d i a ti o n C o m bu s ti o n o f a i r- f u e l c h a r g e C on d u c ti o n C ond u c t i on C ond u c t i on L i q ui d c o n v e c t i o n m o v e m e nt G a s c o n v e c t i o n c u rr e nt R a d i a t i o n t h r o ug h g a s O i l f ilm G a s s t a g nan t f i l m M e t a l c y li nd e r w a ll S c a l e fi l m L i q u i d s t a gna n t f il m L i qu i d c o n v e c ti o n c u r r ent Hea t Transfer through a liquid cooled cylinder wall Heat Transfer to Walls  The hea t released from the combustio n o f air-fuel mixtur e is transferred in all direction s to the walls of the combustio n chambers , cylinder s and piston s by direc t radiation , by convectio n current s o f gas rubbing agains t a stationar y ga s film , and then by conduction through this stagnant boundar y laye r o f ga s and a n oi l fil m to th e metal wall.

3. NEED FOR COOLING SYSTEM During the process of converting thermal energy to mechanical energy, high temperatures are produced in the cylinders of the engine as a result of the combustion process. A large portion of the heat from the gases of combustion is transferred to the cylinder head and walls, piston and valves . Unless this excess heat is carried away and these parts are adequately cooled , the engine will be damaged . A cooling system must be provided not only to prevent damage to the vital parts of the engine, but the temperature of these components must be maintained with in certain limits So the duty of the cooling system is to keep the engine not too hot or not too cold in order to attain max. η

4. Significance of Cooling 4 In a spark ignition engine , coolin g must be satisfactory to avoid pre-ignition and knock . In a compressio n ignition engine, since a normal combustion is aided, cooling must be sufficient to allow the part s to operat e properly . In short , cooling is a matter of equalization of internal temperature to prevent local overheating a s well a s to remove sufficient hea t energy to maintain a practical overall working temperature. Rapidly Auto-igniting

5. Reasons for Cooling 5 to ensur e prope r combustion to promote a high volumetric efficiency , and to ensur e smooth mechanical operatio n & reliability.

6. Effec t o f Over-cooling 6 the thermal efficiency is decreased due to more loss of heat carried by the coolant the vapourization o f the fuel is less resulting in lower combustio n efficiency low temperature increases the viscosit y of lubricant causin g more loss du e to friction .

7. CHARACTERISTICS OF AN EFFICIENT COOLING SYSTEM The following are the two main characteristics desired of an efficient coolie system: ( i ) It should be capable of removing about 30% of heat generated in the combustion chamber while maintaining the optimum temperature of the engine under all operating conditions. (ii) It should remove heat at a faster rate when engine is hot . However during starting of the engine the cooling should be minimum , so that the working parts of the engine reach their operating temperatures in a short time.

8. Type s o f Cooling System 8 Air coolin g (or direct cooling) system Liquid coolin g (or indirect cooling) system FYI: Aviation engines, motor cycle engine s, scooter engines ar e air cooled; while the stationery and automobil e engine s ar e liquid cooled

9. Air C oolin g S ystem 9 Air coole d engine s depen d o n airflow acros s their externa l surfaces o f the engine cylinder s to remove the necessar y heat . The amount of heat dissipated depends upon: the area of cooling surface in contact with the air mass flow rate of air temperature difference between cylinde r an d atmospheric ai r and conductivity of metal.

10. Cooling fins in ai r coole d system 10  The are a o f coolin g surface is increased by forming thin fins , eithe r integrally b y machining them o n the oute r walls o f the engin e cylinder and cylinder head or by attaching separate fins to them.

11. Air Cooling System ….

12. A dvantages - Air coolin g system The jackets, 12 absence coolant of radiator, an d pumps cooling make the engin e lighter . The engin e ca n b e operated in cold weather conditions where liquid may freeze in water cooling system . In place s where water is scarce , air cooled engine is an advantage. Handling of liquid coolant requires pipin g an d pumpin g auxiliaries . So, ai r cooled engines have no coolant leakage o r freezing problems.

13. Air cooling system - D isadvantages Cooling fins unde r certai n conditions may vibrate and amplify the noise level. Cooling is not uniform. Engines are subjected to high working temperature . Used in small engines i.e., Engines with low power output. 13

14. Liquid coolin g systems - T ypes Direc t o r N on- R etur n system Thermosypho n system Evaporative cooling system Forced circulatio n coolin g system 14

15. Non-Return Water Cooling System This is suitable for large installations and where plenty of water is available. The water from a storage tank is directly supplied to the walls of engine cylinder. The hot water is not cooled for reuse but simply discharges.

16. Thermo-sypho n system Because of the low density at Tank A , the hot fluid travels up and reaches to Tank B. When the fluid travels from pipe p1 to pipe p2, the hot fluid gets cooled. Here the fluid circulates through the system in the form of convection currents In this system the circulation of water is due to difference in temperature of water . So in this system pump is not required but water is circulated because of density difference only.

17. Thermo-sypho n system I n this system Tank A represents the cylinder jackets and Tank B represents the Radiator. In this system the radiator is placed at a higher level than the engine for the easy flow of water towards the engine. As the water jacket becomes hot , it rises to the top of the radiator . Cold water from the radiator takes the place of the rising hot water and in this way a circulation of water is set up the system. So in this system pump is not required but water is circulated because of density difference only.

18. Evaporative cooling system This is predominately used in stationary engine and in many types of industrial engines. . In this system , the engine will be cooled because of the evaporation of the water in the cylinder jackets into steams. In this system, the water evaporates and becomes steam due high temperatures of the cylinder jackets. This steams is naturally Circulated into Radiator without pump

19. In fig. evaporative cooling with air cooled condenser. In this case water is circulated by the pump A and when delivered to the overhead tank B part of it boils out. The tank has portion C. The vapors rise above the portion C and because of the condensing action of the radiator tube D, condensate flow into the lower tank E from which it is picked up and return to the tank B by the small pump F. The vertical pipe G is in communication with the outside atmosphere to prevent the collapsing of the tank B and E when the pressure inside them due to condensation fall below the atmosphere. Evaporative cooling with air cooled condenser

20. Schematic diagram of a forced circulatio n system Force d circulatio n coolin g system 20 This sys t e m is used in a larg e numbe r o f vehicl e s like cars , buses , trucks and othe r heav y vehicles. Here , circulatio n o f water takes plac e with convectio n current s helped b y a pump .

21. Schematic diagram of a forced circulatio n system Force d circulatio n coolin g system 21 Th e water o r coolan t is circulated through water jacket s around th e parts o f the engine to be cooled , and is kep t in mot i o n b y a centrifugal pump , drive n fro m the engine . A thermosta t is used to contro l th e water temperature required for cooing.

22. Force d circulatio n coolin g system (3D)

23. Force d circulatio n coolin g system

24. Rejects the coolant heat to the surrounding air Disperse s the heate d coolan t into fine streams so that small quantities of coolant are brought in contact with large metal surface areas, which in turn ar e coole d b y ai r stream Two types of radiators Down-flow type Cross-flow type Availability of space dictates the choice of radiator, both are equally efficient. Variou s desig n o f radiator core s ar e use d for coolin g the water Parts o f cooling system s - Radiator 26

25. Honeycomb block type core Film type of radiator core Tube and fin type core Tube and corrugated type core 27

26. 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 28

27. Liquid cooling system - A dvantages 29 Becaus e o f eve n coolin g o f cylinde r barrel and head (due to jacketing) makes it possible to reduce the cylinde r hea d and valve seat temperatures. The volumetric efficiency of water cooled engines is higher than that of air- cooled engines. Compact desig n o f engine s with appreciabl y smaller frontal are a is possible. In case of water cooled engines, installation is no t necessaril y a t the front of the mobile vehicles, aircraft etc. as the cooing system can be conveniently located.

28. Liqui d coolin g syste m - Disadvantages The system requires more maintenance. The engine performance becomes sensitive to climatic conditions. The powe r absorbed b y the pum p is considerable an d affect s the power outpu t o f the engine. In the even t o f failure o f the cooling system serious damage may be caused to the engine. 30

29. Us e o f Anti-freezers During winter or when the engine is kept out of operation in cold places, the cooling water in the cylinde r jackets, radiator tanks an d leading pipes will freeze, expand and lead to their fracture. To prevent damage to the engine and radiator during winter weather, suitable liquids or compoun d substance s ( know n a s anti - freezers) which go into solution are added to the water to lower the freezing temperature o f the coolant. Ethylene glyco l is the most widely used automotiv e cooling-syste m antifreeze , althoug h methanol, ethanol , isopropyl alcohol, an d propylen e glyco l ar e also used. 31

30. Requirements of anti- freezers The y shoul d thoroughly mi x with water. The y shoul d no t corrod e th e surface s with which they ar e in contact. Thei r boilin g poin t shoul d b e hig h s o that the loss du e to evaporatio n is minimum. The y shoul d no t deposi t an y foreig n matter in the jackets, hose , pipe s o r radiator. I t shoul d b e chemicall y stable , a good conducto r o f heat , an d a poo r conducto r of electricity. 32

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34. Lubricating Oil as Coolant The lubricating oi l used in a n engine also help s to coo l the engine . The hotter part s like piston face an d bac k surface o f piston crow n is s u bjected to oi l flow, usually done by spraying the oil by pressurized systems or by splash in non- pressurized system. Other components like camshaf t an d connecting rods are also cooled by oil circulation through oil passages. 36

35. Anti-Freeze Solution… Ethylene glycol Ethylene Glycol based water solutions are common in heat-transfer applications where the temperature in the heat transfer fluid can be below 32 o F (0 o C) . Ethylene glycol is also commonly used in heating applications that temporarily may not be operated (cold) in surroundings with freezing conditions - such as cars and machines with water cooled engines. Propylene glycol

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