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Engine Cooling System

Heat is a form of energy that can be sensed by a change in temperature. The engine uses
chemical energy in the fuel and converts it into heat and then into movement. The energy
conversion process in an engine is not very efficient and only about 30% is converted into
movement energy. Of the remaining heat, up to 50% goes out of the exhaust and the rest heats the
engine.
Combustion of fuel inside a cylinder develops a very high temperature (Appx. 2200℃). At
this temperature the engine parts will expand and tend to seize. Similarly the lubricating oil will
lose its property. This is done by the cooling system to prevent overheating and eventual failure of
the valves or piston.
Less severe, overheating may reduce volumetric efficiency owing to a lower charge density,
and may cause detonation, pre-ignition and lubrication difficulties.
Since excessive cooling also results in a lower thermal efficiency with fuel vaporization and
oil contamination problems, there is an optimum running temperature, and as the ambient air
temperature varies considerably the degree of cooling should be controllable. Therefore it is
necessary to keep the engine temperature to operating limits.
Heat is removed from the engine by cooling media (water or air) and is dissipated to the
atmosphere.

Types of cooling systems

There are two types of cooling systems used in engines.
• Direct cooling - air cooling.
• Indirect cooling - water cooling.
Air-cooled engines
In air-cooled (Fig 1) engines, cylinders are semi-
independent. They are not grouped in a block. Metal fins (1)
are provided on the head (2) and cylinder (3), to help dissipate
heat from the engine. In some engines fans are also used to
improve air circulation around the cylinders and heads. This
type of cooling system is employed in two-wheelers and small
stationary engines. These are used in both S.I. and C.I.
engines.

Water cooling
Two types of water cooling systems are used.
• Thermo-siphon system (Fig 2)
• Forced Feed system (Fig 3)

Thermo-siphon system
In this system no pump is used for water circulation. Water
circulation is obtained due to the difference in the densities of
hot and cold water. Water absorbs the heat and rises up in the
block (1) and goes to the radiator’s (2) top side. Water is
cooled in the radiator (2). It again goes to the water jackets in
the engine. To maintain a continuous flow of water the level
of water is maintained at certain minimum level. If the water
level falls down the circulation will discontinue. This system
is simple but the rate of cooling is very slow.

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Forced feed system (Pump circulation system)

In this system water is circulated by a pump (3). The pump is
driven by a belt (5) which is connected with the crankshaft
pulley. The circulation depends upon the engine speed. More
water is circulated at higher engine speed. The water absorbs
heat from the engine and flows to the radiator’s (2) top tank.
Water from the top tank of the radiator (2) flows down to the
bottom tank. The fan (4) draws the air through the radiator’s
fins and cools the hot water. Cold water from the bottom tank
is again pumped to the engine and the cycle is repeated.

Coolant or Liquid
Pure water has more heat-carrying ability, so water would be the best coolant to use. But
water presents other challenges. It forms rust on iron engine parts. The rust is then carried off to
other cooling areas. The resulting corrosion interferes with heat transfer even before the build-up
plugs the radiator and fills the cooling system with sediment.

Sometimes instead of water, other liquids having higher boiling points are used for engine
cooling. The examples are glycerine b.p. 290℃, and ethylene glycol b.p. 195℃. Higher boiling
point increases the capacity of the liquid to carry heat. Consequently weight of the coolant and
hence that of the radiator is decreased. Coolant, also known as antifreezes.
It protects engines from overheating also lubricates the moving parts it comes into contact
with, which protects damage to the water pump, head gasket, the cylinder and piston timing. It also
helps reduce corrosion and engine rust. Coolant also provides resistance to freezing. It won’t freeze
and expand in hyper-cool temperatures like water would. That protects your engine from cracking
and experiencing increased pressure.

Anti- Freeze mixtures

1 Wood alcohol
2 Denatured alcohol
3 Glycerine
4 Ethylene glycol
5 Propylene glycol
6 Mixture of alcohol and glycerine

Nowadays, automotive service providers use three basic types of engine coolant:
• IAT – Inorganic Additive Technology
For decades, this distinctive (special) green-colored coolant protected cooling systems, but it is
rarely used as factory fill in modern cars. One reason is the fast depletion rate of its additives, which
means it has to be changed more frequently, usually every two years or 24,000 miles.

• OAT – Organic Acid Technology

Commonly required for vehicles manufactured by General Motors, and some other automakers,
OAT coolants are not compatible with other types. Usually orange, yellow, red or purple, OAT
coolants are typically changed every five years or 50,000 miles.

• HOAT – Hybrid Organic Acid Technology

Providing the benefits of both IAT and OAT coolants, HOAT coolants are primarily orange and
yellow and are common in Chrysler and Ford vehicles. OAT coolants are typically changed every
five years or 50,000 miles, although some automakers specify intervals as long as 10 years or
150,000 miles.
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What percentage of coolant is water?

The optimum for most coolant systems is 50 percent coolant and 50 percent good-quality water, and
in general coolants tolerate dilution down to about 40 percent concentrate and 60 percent water.

Components of water cooling system

Radiator
Water pump
Thermostat Valve
Fan
Hose pipe

Radiator
The purpose of a radiator in the cooling system is to
cool hot water coming out of engine.
It has a large cooling surface area to allow enough of air to
pass through it. Water circulated through it is cooled by the
passing air.
The radiator (Fig 1) consists of an upper tank (1), a
lower tank (2) and in between the upper and lower tank
radiator cores (3) are provided. The upper tank (1) is
connected to the water outlet of the engine through a rubber
hose. The lower tank (2) is connected to the water pump
through rubber hoses.
Radiator cores are classified into two types.
• Tubular core (Fig 2)
• Cellular core (Fig 3)
Tubular core
In a tubular type the upper and lower tanks are connected by tubes. Water passes through these
tubes. Cooling fins are provided around the tubes, to absorb and radiate heat to the atmospheric air.
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Cellular cores
In the cellular type a large number of individual air cells are provided and surrounded by water.
Because of its appearance, the cellular type is known as a ‘honeycomb’ radiator. The material of the
core is of copper and brass. The parts are normally connected together by soldering.

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Pressure cap
In normal atmospheric conditions water boils at 100°C. In
higher altitude height the atmospheric pressure is low and
water boils at a temperature below 100°C. To increase the
boiling temperature of water the pressure of the cooling
system is increased. This is achieved by providing
pressure caps to seal the system. The coolant loss, due to
evaporation is also minimized, by using a pressure cap
(Fig 4). It also permits the engine to operate at a higher
temperature so that better efficiency of the engine is
achieved. The pressure cap is fitted in the filler neck
portion on the top of the radiator tank. If pressure is
increased by 15 P.S.I., the boiling temperature raises to
113°C.
The pressure cap has two valves.
- Pressure valve
- Vacuum valve
Pressure valve
If the pressure in the system rises it may damage
the components. To avoid this, a pressure relief valve (1)
is used to release the excess pressure. It is a spring- loaded
valve. The spring’s (2) tension depends on the system’s
pressure.
When the cooling water of the engine is heated up it expands
which results in high pressure in the system. If the force due to pressure
is more than the spring’s (2) tension the valve opens and water
vapour/steam escapes through the overflow pipe (3) until the pressure is
lowered to the pre-set value.
Vacuum valve
When the engine cools down the pressure in the system decreases due to
loss of the coolant and a vacuum is created. (This valve is also located in
the cap and fitted in the filler neck of the radiator) At this time the
vacuum valve (4) (Fig 5) opens and air flows into the system until the
vacuum is filled up in the system.
In some engines an overflow pipe is connected to an expansion tank (5).
The expansion tank (5) (Fig 6) collects the water vapour during the pressure valve operation, and
the same vapour, after condensing, goes to the radiator when the
vacuum valve is in operation.

Water pump
The centrifugal type water pump (Fig 4) is used in engines. It is
mounted on the front side of the cylinder block or head. The water
pump is driven by the crankshaft pulley through the fan belt. The
impeller (1) is mounted on one end of the water pump shaft (2).
The shaft (2) is fitted in the pump housing with bearings. A water
seal is provided in the pump to prevent leakage of water and to
prevent water entering into the bearings. When the impeller
rotates it draws water from the lower tank of radiator, and pumps
water to the engine block, by centrifugal force under pressure. The fan is mounted on the water
pump pulley.

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Thermostat
The thermostat (Fig.5) helps to bring the cold engine to the operating temperature quickly. It is
fitted in between the water outlet of the cylinder head (1) and the inlet (2) of the radiator in the
water cooling system. When the engine is cold, the thermostat (4) is closed. It does not permit water
to enter the radiator. Water recirculates in the engine through the bypass hole (2) and the engine
reaches the operating temperature quickly. Once the engine has reached the operating temperature
the thermostat (4) opens. It closes the bypass hole (2) and now permits water to enter the radiator
tank (3). Thermostats are rated to open at different temperatures. Two types of thermostats are used.
• Bellows type (Fig 6)
• Wax type (Fig 7)
• Bi-Metal Type

Bellows type
It has a flexible metal bag closed at both ends. The
metal bag is partially filled with ethyl which has a low
boiling temperature.
When the engine is cold the valve (1) closes its
outlet passage and does not allow water to reach the radiator top tank from the engine, but is
circulated through the bypass port to the engine.
When the water reaches the working temperature, ethyl in the closed bellow (2) expands and
opens the valve (1). Now the water reaches the radiator top tank from the engine. In the valve’s
opened position the bypass passage is closed.

Wax pellet type

In this type a wax pellet (3) (Fig 8) is used as a heating
element. When the circulating water’s temperature is lesser than the
operating temperature, the spring (1) keeps the valve (2) in the
closed position and the water does not reach the radiator top tank
from the engine.
As the water reaches the operating temperature the wax pellet
expands and forces the valve (2) to open against the spring tension.
Now the water reaches the radiator top tank, from the engine. At this
position the bypass port is closed by the valve.

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Engine Fan
A fan is used to ensure an adequate air flow through the radiator when this is not provided by the
forward speed of the vehicle. The fan was traditionally fitted to the front of the water pump shaft
and is driven by the same belt that drives the pump and the
generator (Fig. 13-8).
Some longitudinal engines still use this system, but the fan,
formerly a pressed-steel component, now incorporates a
thermostatic viscous hub and nylon fan blades. The viscous hub
is a fluid clutch using silicon oil. The operation of the clutch is
temperature controlled with a bimetallic valve. When the air
flow temperature over the viscous hub is cool, the valve remains
closed and the clutch is inoperative. When the air flow
temperature over the viscous hub increases, the valve in the hub
opens and the viscous fluid is driven outwards by centrifugal
force. The increased force in the fluid locks the plates in the hub together to engage the clutch drive
to the fan.

An improved temperature-sensing arrangement is for the fan to be

driven by an electric motor mounted on a cowl frame attached to the
radiator. A plastic fan is fitted to the motor spindle and operates when a
temperature-sensitive switch closes. The electrical supply to the motor
is connected through a relay.
Many vehicles, particularly those fitted with air conditioning, have
two-speed fan circuits. These have a control circuit to switch the motor
(or motors) to half speed at 95°C, and full speed at 100°C. This
arrangement can be also be operated by
the engine management system.

Cooling system hoses are manufactured from fabric-reinforced

rubber, and are moulded to suit the vehicle application. Connectors
are cast, or formed, with a raised lip on the pipes leading into, and
out of, other components. The hoses are held with round clips that
can be drawn tight to give a watertight seal (e.g. jubilee clips).

Fan belts
Most fan belts are V type (Fig. 13-11). Friction between the sides
of the belt and the sides of the grooves in the pulleys causes the driving power
to be transmitted through the belt from one pulley to the other. The V -type
belt provides a substantial (sufficient) area of contact, so that considerable
power may be transmitted; the wedging action of the belt as it curves into the
pulley grooves aids in preventing belt slippage. Figure 13-11 shows a V belt
in place on the generator, engine fan, and crankshaft pulley of an engine.

Temperature gauge
Purpose
It is used to know the temperature of water in the cooling system of engine at
all times. It cautions the driver against overheating of the engine.

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• It consists of an engine unit (1) immersed in the engine

coolant in the cylinder head or cylinder block in the form of
a pellet. (Fig 3)
• It is made of special material whose electrical resistance
increases when temperature is lowered and it reduces when
the temperature is increased.
• The resistance unit is provided with the dash unit (2) and it
is fitted on the panel board.
• The dash unit consists of a dial (3) pointer (4), a magnet
(5) and coil (6) and (7). (Fig 4)
• The two terminals of gauge are connected to the ignition switch (8) and the engine unit (1). The
operating current is supplied from the battery through the ignition switch.

Working
When the coolant temperature rises, the engine unit becomes hot. When the engine unit
temperature is high the resistance is less and more current passes to the right coil of the indicating
units.
The difference in the strength of the magnetic field between the two coils increases and the
armature and pointer move towards the right to indicate a high temperature.
When the engine coolant temperature falls down, the resistance becomes high. This results
in less current flowing through the left coil, and the magnetic field becomes less and causes the
armature and pointer to move towards the left to indicate lower temperature.

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