COOLING SYSTEM OF AUTOMOBILE

COOLING SYSTEM OF AUTOMOBILE

INTRODUCTION
During the combustion of air-fuel mixture enormous amount of heat is produced inside the engine cylinder. The temp. as high as 2500C by the burning gases. The temp. is so high that it will break the lubricating film between the moving parts or may cause any mechanical breakage of the engine parts. So this temp. must reduce to such a value about 200C – 250C at which the engine work efficiently. The duty of cooling system is to keep the engine from getting too hot not to keep it cool.
PROPERTIES OF AN EFFICIENT COOLING SYSTEM
1. An efficient cooling system removes 30 to 35% of heat generated in the combustion chamber. Too much removal of heat decreases thermal efficiency of the engine.
2. It removes heat at a fast rate when the engine is hot & at a slow rate when the engine is started until the engine reaches at its normal operating temp.

METHODS OF COOLING
There are four methods of cooling
1. Air cooling
2. Water cooling
3. Liquid cooling
4. Steam cooling
Most automobile engine use air cooling & water cooling methods. Liquid & steam cooling are rarely used.

AIR COOLING
In this method of cooling, the heat is dissipated directly to the air after being conducted through the cylinder walls. First, flanges on the outer surfaces of the cylinders & heads serve to increase the area exposed to the cooling air & to raise the rate of cooling. The basic principle is to flowing of air continuously over heated surface of the engine from where heat is to be removed. The amount of heat dissipated depends upon the following factors:
1. Surface area of metal into contact with air.
2. Rate of air flow
3. Temp. difference between the heated surface & the air
4. Conductivity of the metal
For effective air cooling, the surface area of the metal which comes in contact with air is increased by providing fins over the cylinder barrels. More the surface area is contact with air more heat is dissipated. Higher the rate of air flow, higher the heat is dissipated.

Cooling fins
The surface area over the cylinder is increased by means of fins. These fins are either cast as an integral part of the cylinder or separate finned barrels are inserted over the cylinder barrel. The fins are made of about the cylinder wall thickness at their roots, tapering down to about one-half the root thickness. Experimental considerations are to allow 1400 to 2400cm2 of cooling fins area per horse power. This gives the correct cylinder temp. at 50-70km/h air speed.
Fan cooling
Fan cooling is used in larger air cooled engine, particularly on cars. A fan having two or four blades is driven either at engine speed or twice the engine speed & the air flow is directed on the cylinder heads. The cooling depends chiefly upon the engine speed & not upon the forward speed of the car. The fan usually absorbs about 1 H.P for every 15 to 20 H.P output.

ADVANTAGES OF AIR COOLED ENGINES
1. Lighter in weight due to the absence of radiator, cooling jackets and coolant.
2. No topping up the cooling system.
3. No leaks to guard against.
4. Anti freeze not required.
5. Engine warms up faster than with water cooled designs.
6. Can be operated in cold climates where water may freeze.
7. Can be used in areas where there is scarcity of cooling water.
DISADVANTAGES
1. Less efficient cooling, because the coefficient of heat transfer for air is less than that for water.
2. Not easy to maintain even cooling all around the cylinder, distortion of the cylinder may take place.
3. More noisy operation.
4. Limited use in motor cycles and scooters where the cylinders are exposed to air stream.
Modern air cooled engines
At present, air cooling is used on engines in scooters, motor cycles, aero planes, combat tanks small stationary installations & in one model in American rear engine car. In Germany air cooling is used in some petrol & C.I engines including 2, 4 and 8 cylinder models.
The volkswagon, Dutch D.A.F citrogen two cylinders opposed, chevolet corvair six cylinders horizontally opposed, fiat 500D, two cylinders in line & N.S.U two cylinders are the examples of modern air cooled engine.

WATER COOLING
In this method of cooling, water is circulated through water jackets around each of the combustion chambers, cylinders valve seat and valve stems. The circulating water, when passes through the engine jackets in the block and cylinder heat, takes heat of the combustion. When it passes through the radiator, it is cooled by air drawn through the radiator by a fan and by air flow developed by the forward motion of the vehicle. After passing through the radiator, the water again goes in the engine jackets.

Systems of water cooling
There are two types of water cooling
1. Thermosiphon system
In this water cooling, the circulating of water is obtained due to the difference in densities of hot and cold regions of the cooling water. There is no pump to circulate the water. The hot water from the engine jackets being lighter rises up in the hose pipe and goes in the radiator from the top side. It is cooled there and hence goes down at the bottom side of the radiator, from where it goes again in the engine jackets. The system is quite simple and cheap but the cooling is rather slow. To maintain continuously of the water flow the water must be maintained up to a certain minimum level. If the water level falls down, the circulation will be discontinued & the cooling system will be fail.
2. Pump circulation system
In this system of water cooling, the circulation of water is obtained by a pump. The pump is driven by means of a v-belt from a pulley on the engine crankshaft. The system is more effective. The circulation of water becomes faster as the engine speed increases. There is no necessity of maintaining the water up to a correct level.

Components of water cooling
1. Radiator
2. Thermostat
3. Pump
4. Fan
5. Water jackets

Radiator
Radiator is a device for having large amount of air so that the water circulating through it is cooled efficiently. It consists of an upper tank and a lower tank and between them a core. The upper tank is connected to the water outlet or outlets from the engine jacket by a hose pipe & the lower tank is connected to the jackets inlet through the water pump. The core is a radiating element, which cools the water.
There are two basic type of radiator cores-tubular type and cellular type. In tubular type core the upper & lower tanks are connected by a series of tubes through which water passes. Fins are placed around the tubes to improve heat transfer. Air passes around the outside of the tubes, between the fins, absorbing heat from the water in passing.
In cellular type core, air passes through the tubes and the water flows in the spaces between them. The core is composed of a large number of individual air cells which are surrounded by water.
Radiators are also classified according to the direction of flow of water through them. The water flows from top to bottom down flow type radiators. The water flows horizontally from an input tank on one side to another tank on the other side-cross flow type radiator.
Radiators are usually made of copper & brass because of their high heat conductivity.

Thermostat
A thermostat valve is used in the water cooling system to regulate the circulation of water in system to maintain the normal working temp. of the engine parts during the different operating conditions. The thermostat valve prevents the flow of water from the engine to radiators so that the engine readily reaches to its normal working temp. after which it automatically comes into action. The thermostat valve does not permit the water below 70C.
Two types of thermostats are used in automobile vehicle. Bellows type & pellet type
In bellows type thermostat, the heat unit consists of a closed bellows filled with a volatile liquid under reduced pressure. When the bellow is heated, the liquid vapourizes and creates enough pressure to expand the bellows. The movement of bellows operates a linkage which opens the valve to pass the water through it. When the bellows is cooled, the gas condenses the pressure is reduced and the bellows collapse to close the valve, thus stopping the water circulation.
In pellet thermostat, the heat unit consists of a sealed in wax pallet, which expands on heating and contracts on cooling. The pallet is connected by piston and flange to a valve, so that on expansion of the pallet it opens the valve. When the pallet contracts on cooling, a cool spring closes the valve.

Water pump
A pump is used in the water cooling system to increase the velocity of the circulating water. Impeller type pump is mounted at the front end of the cylinder block between the block and the radiator. The pump consists of a housing with inlet & outlet and a impeller. The impeller is a flat plate mounted on the pump shaft with a series of flat or curved blades or vanes. When the impeller rotates, the water between the blades is thrown outwards by centrifugal force and is forced through the pump outlet and into the bottom of radiator and the water from the radiator is drawn into the pump to replace the water forced through the outlet.
The pump is driven by a belt to the drive pulley mounted on the front end of the engine crank shaft. The impeller shaft is supported on one or more bearings. A seal prevents water from leaking out around the bearings.
Fan
A fan is mounted behind the radiator on the water pump shaft. It is driven by the same belt that drives the pump and generator. The purpose of the fan is to draw air through the radiator. When the vehicle is going at high speed, the natural draft of air passing through the radiator is sufficient for cooling. But when the engine is going at low speed or upside a hill, the natural draft is certainly insufficient to produce the desired cooling effect.
When the engine cooling requirements, as during high temp. high speed operation, more oil i.e. special silicon oil is injected into fluid coupling. This causes more power to pass through the coupling. Fan speed, therefore goes up. When cooling requirements are low, as during cooling weather intermediate speed operation oil is withdrawn from the fluid coupling so that less power passes through and fan speed goes down.

Water jackets
Water jackets are cast into the cylinder block & heads. Jackets are simply the passages through which water circulates around the cylinders, valve ports & seats, combustion chambers and any other hot parts require cooling. The heat of combustion is conducted through the metal walls to the water in the jackets which removes the excess heat as it circulates through them.

LIQUID COOLING
In this method of cooling, instead of water, other liquids having higher boiling points are used for cooling. Glycerine (boiling point 290C) and ethylene glycol (boiling point 195C) are examples of such liquids. Due to their higher boiling point these liquids have increased capacity to carry heat and hence the weight of coolant and radiator is decreased.

STREAM COOLING
In this method of cooling, steam is used for cooling. The cooling system consists of the same components as those for water cooling, except the radiator. The radiator is in the form of condenser in this system. The circulation of water is made by a pump. The water in the cylinder jackets is converted into steam, which flows out at the top of engine block and goes to the bottom of the radiator. The steam is condensed in the radiator and the water thus formed is again circulated by the pump. The latent heat of steam serves the purpose of cooling. When the hot water in the cylinder jacket converts into steam, it takes heat from the cylinder walls & cooling them.

MAINTENANCE OF COOLING SYSTEM
The proper operation of the cooling system gives satisfactory performance of an engine. The cylinder bores are completely surrounded by a water jacket which presents distortion of the cylinder walls. Direct cooling and large water holes, properly placed in the cylinder head gasket causes more water flow past the valve seat & carry the heat away from the valves giving positive cooling of valves and seats.
Flushing is accomplished through the system in a direct opposite to the normal coolant flow. The action causes the water to get behind the corrosion deposits and force them out. To do this, remove the upper and lower radiator hoses. Then attach a drain hose at the top of the radiator. Attach a new piece of hose to the radiator outlet at the bottom and insert the flushing gun. Connect the water hose of the flushing gun to a water line and air hose to an air line. Turn on the water and when the radiator is full, apply the air in short blasts. Continue this flushing operation until the water runs clear through the top hose.
With the thermostat removed attach a lead ways hose to the hose inlet. Also attach a length of new hose to the water outlet connection at the top of the engine. Turn the water on and fill the water jackets and then apply air in short blasts. Care must be taken to prevent damage to the core.

CONCLUSION
The cooling system of the automobile keeps the engine from getting too hot, not to keep it cool. It also increases the thermal efficiency of the engine.

BIBLIOGRAPHY
The seminar topic “Cooling System of Automobile” refereed from the Automobile Engineering book by R.B Gupta.