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A system, which controls the engine temperature, is known as a cooling system.
NECESSITY OF COOLING SYSTEM
The cooling system is provided in the IC engine for the following reasons:
• The temperature of the burning gases in the engine cylinder reaches up to 1500 to 2000°C, which
is above the melting point of the material of the cylinder body and head of the engine. (Platinum, a
metal which has one of the highest melting points, melts at 1750 °C, iron at 1530°C and
aluminium at 657°C.) Therefore, if the heat is not dissipated, it would result in the failure of the
cylinder material.
• Due to very high temperatures, the film of the lubricating oil will get oxidized, thus producing
carbon deposits on the surface. This will result in piston seizure.
• Due to overheating, large temperature differences may lead to a distortion of the engine
components due to the thermal stresses set up. This makes it necessary for, the temperature
variation to be kept to a minimum.
• Higher temperatures also lower the volumetric efficiency of the engine.
REQUIREMENTS OF EFFICIENT COOLING SYSTEM
The two main requirements of an efficient cooling system are:
1. It must be capable of removing only about 30% of the heat generated in the combustion chamber. Too
much removal of heat lowers the thermal efficiency of the engine.
2. It should remove heat at a fast rate when the engine is hot. During the starting of the engine, the
cooling should be very slow so that the different working parts reach their operating temperatures in a short
time.
TYPES OF COOLING SYSTEM
There are two types of cooling systems:
(i) Air cooling system and
(ii) Water-cooling system.
AIR COOLING SYSTEM
In this type of cooling system, the heat, which is conducted to the outer parts of the engine, is radiated and
conducted away by the stream of air, which is obtained from the atmosphere. In order to have efficient
cooling by means of air, providing fins around the cylinder and cylinder head increases the contact area.
The fins are metallic ridges, which are formed during the casting of the cylinder and cylinder head
The amount of heat carried off by the air-cooling depends upon the following factors:
(i) The total area of the fin surfaces,
(ii) The velocity and amount of the cooling air and
(iii) The temperature of the fins and of the cooling air.
Air-cooling is mostly tractors of less horsepower, motorcycles, scooters, small cars and small aircraft
engines where the forward motion of the machine gives good velocity to cool the engine. Air-cooling is
also provided in some small industrial engines. In this system, individual cylinders are generally employed
to provide ample cooling area by providing fins. A blower is used to provide air.
Advantages of Air Cooled Engines
Air cooled engines have the following advantages:
1. Its design of air-cooled engine is simple.
2. It is lighter in weight than water-cooled engines due to the absence of water jackets, radiator,
circulating pump and the weight of the cooling water.
3. It is cheaper to manufacture.
4. It needs less care and maintenance.
5. This system of cooling is particularly advantageous where there are extreme climatic
conditions in the arctic or where there is scarcity of water as in deserts.
6. No risk of damage from frost, such as cracking of cylinder jackets or radiator water tubes.
WATER COOLING SYSTEM
It serves two purposes in the working of an engine:
a) It takes away the excessive heat generated in the engine and saves it from over heating.
b) It keeps the engine at working temperature for efficient and economical working.
This cooling system has four types of systems:
(i) Direct or non-return system,
(ii) Thermo-Syphone system,
(iii) Hopper system and
(iv) Pump/forced circulation system.
Though the present tractor has a forced circulation system, it is still worthwhile to get acquainted with the
other three systems.
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 engine cylinder. The hot water is not cooled for reuse but simply discharges. The
low H.P. engine, coupled with the irrigation pump is an example.
Thermo-Syphone Water Cooling System
This system works on the principle that hot water being lighter rises up and the cold water being heavier
goes down. In this system the radiator is placed at a higher level than the engine for the easy flow of water
towards the engine. Heat is conducted to the water jackets from where it is taken away due to convection
by the circulating water. 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 m the
system. This helps in keeping the engine at working temperature.
Disadvantages of Thermo-Syphone System
1 Rate of circulation is too slow.
2. Circulation commences only when there is a marked difference in temperature.
3. Circulation stops as the level of water falls below the top of the delivery pipe of the radiator. For these
reasons this system has become obsolete and is no more in use.
Hopper Water Cooling System
This also works on the same principle as the thermo-syphone system. In this there is a hopper on a jacket
containing water, which surrounds the engine cylinder. In this system, as soon as water starts boiling, it is
replaced by cold water. An engine fitted with this system cannot run for several hours without it being
refilled with water.
Force Circulation Water Cooling System
This system is similar in construction to the thermo-syphone system except that it makes use of a
centrifugal pump to circulate the water throughout the water jackets and radiator
The water flows from the lower portion of the radiator to the water jacket of the engine through the
centrifugal pump. After the circulation water comes back to the radiator, it loses its heat by the process of
radiation. This system is employed in cars, trucks, tractors, etc.
Parts of Liquid Cooling System
The main parts in the water-cooling system are: (i) water pump, (ii) fan, (iii) radiator and pressure cap, (iv)
fan belt (v) water jacket, (vi) thermostat valve, (vii) temperature gauge and (viii) hose pipes.
Water Pump
This is a centrifugal type pump. It is centrally mounted at the front of the cylinder block and is usually
driven by means of a belt. This type of pump consists of the following parts: (i) body or casing, (ii)
impeller (rotor), (iii) shaft, (iv) bearings, or bush, (v) water pump seal and (vi) pulley.
The bottom of the radiator is connected to the suction side of the pump. The power is transmitted to the
pump spindle from a pulley mounted at the end of the crankshaft.
Seals of various designs are incorporated in the pump to prevent loss of coolant from the system.
Fan
The fan is generally mounted on the water pump pulley, although on some engines it is attached directly to
the crankshaft. It serves two purposes in the cooling system of a engine.
(a) It draws atmospheric air through the radiator and thus increases the efficiency of the radiator in
cooling hot water.
(b) It throws fresh air over the outer surface of the engine, which takes away the heat conducted by
the engine parts and thus increases the efficiency of the entire cooling system.
Radiator
The purpose of the radiator is to cool down the water received from the engine. The radiator consists of
three main parts: (i) upper tank, (ii) lower tank and (iii) tubes.
Hot water from the upper tank, which comes from the engine, flows downwards through the tubes. The
heat contained in the hot water is conducted to the copper fins provided around the tubes.
An overflow pipe, connected to the upper1 tank, permits excess water or steam to escape. There are three
types of radiators: (i) gilled tube radiator, (ii) tubular radiator (Fig. b)
and (iii) honey comb or cellular radiator (
Gilled tube radiator:
This is perhaps the oldest type of radiator, although it is still in use. In this, water flows inside the tubes.
Each tube has a large number of annular rings or fins pressed firmly over its outside surface.
Tubular radiator: The only difference between a gilled tubes radiator and a tubular one is that in this case
there are no separate fins for individual tubes. The radiator vertical tubes pass through thin fine copper
sheets which run horizontally.
Honey comb or cellular radiator: The cellular radiator consists of a large number of individual air cells
which are surrounded by water. In this, the clogging of any passage affects only a small parts of the cooling
surface. However, in the tubular radiator, if one tube becomes clogged, the cooling effect of the entire tube
is lost.
Thermostat Valve
It is a kind of check valve which opens and closes with the effect of temperature. It is fitted in the water
outlet of the engine. During the warm-up period, the thermostat is closed and the water pump circulates the
water only throughout the cylinder block and cylinder head. When the normal operating temperature is reached, the thermostat valve opens and allows hot water to flow towards the radiator (Fig. 8.5a).
Standard thermostats are designed to start opening at 70 to 75°C and they fully open at 82°C. High
temperature thermostats, with permanent anti-freeze solutions (Prestine, Zerex, etc.), start opening at 80 to
90°C and fully open at 92°C.