AUTOMOTIVE ENGINEERING

P.CHINNA RAO
ASSISTANT PROFESSOR
DEPARTMENT OF MECHANICAL ENGINEERING
RGUKT-AP
SRIKAKULAM
 NEED OF COOLING IN AUTOMOBILE

 When fuel is supplied to an engine for combustion then the heat

generated increases the engine temperature.

 It has been observed that the properties of metals change if the

temperature is raised beyond certain limits and that increased
temperature damages the engine.

 Hence it is essential to control the rise in temperature by providing

certain means for cooling the engine in automobiles.

 The cooling system serves three important functions.

 It removes excess heat from the engine.

 It maintains the engine operating temperature where it works most
efficiently.
 finally, It brings the engine up to the right operating temperature as
quickly as possible.
 COOLING SYSTEM

• The cooling systems control the engine temperature by

dissipating heat into the atmospheric.

• About 30% of heat is lost by the cooling system and only

10% of heat is lost by radiation.

• About 25% to 30% of heat is used to perform work. The rest

of the heat is lost through the exhaust gases and the cooling
medium.
• Lubricating oil is also affected by the heat of combustion. Its
viscosity decreases with the rise in temperature which
increases the friction losses.
• The heat of friction is carried away by the cooling medium
and the exhaust gases.
• There are two types of cooling systems:

1. Direct cooling, i.e. air cooling.

2. Indirect cooling, i.e. water cooling.
 AIR COOLING

Working:
• In case of the air cooling system or the direct cooling system,
air is brought in contact with the engine parts.
• However, the specific heat of air is low (0.24), and therefore
a large amount of air is needed to cool the engine parts.
• Also air is not a good conductor of heat and it carries heat by
the convection method only. It is for this reason that the air
contact area is increased by providing fins.
• In an automobile, such air-cooled engines are exposed to the
atmosphere for cooling by the draughts obtained due the
motion of the vehicle.
• The fins are made of aluminium which is a good conductor
of heat and the shape increases the surface area for cooling.

• Due to inertia a certain time elapses before the heat begins to

flow from the cylinder to the cooling fins. Since the heat is
not quickly dissipated due to the characteristics of air, the
cylinder temperature is higher in air-cooled engines than in
water-cooled engines.
 AIR COOLING

Advantages:

1. The air cooling system is lighter in weight, i.e. less weight-

power ratio.
2. It does not require a radiator and a water pump.
3. No antifreezes agents are required as in water-cooled
engines.
4. No salt and mud deposits are formed in water-cooled
engines.
5. Air-cooled engines are cheaper.
Disadvantages:
1. The engine is not cooled efficiently and so runs hot.
2. Due to non-uniform cooling, temperature stresses are
developed in the engine parts.
3. Engines are noisier.
4. It needs an impeller to blow air over the fins. This impeller
is noisy and it absorbs more power in case of scooters, in
which engines are covered.
 WATER COOLING
Working:
• The water cooling system is an indirect cooling system. Water is
cheap and easily available. Its specific heat being greater than that
of air, water can absorb more heat than air. Water absorbs heat and
dissipates it into the atmospheric air. Therefore, the same water can
be circulated by a pump and used again.

• Water is forced by a water pump in the water jacket of the engine.

Hot water comes out from the jacket of the engine and is passed
into the radiator. The vertical radiator tubes are brought in contact
with the atmospheric air. Water passing through these tubes gets
cooled and is collected at the bottom of the radiator. The water
pump recirculates water through the water jacket of the engine.
• There is also a fan which is driven by the engine or electrically as in
a modern car. The fan draws air and blows it over the engine parts.
Thus the partial vacuum created behind the radiator, increases the
pressure difference across the radiator and more air flows through
the narrow gaps between the radiator tubes. Thus the quantity of air
passing through the radiator is increased. This air drawn by the fan
is blown over the engine body and the engine temperature is
reduced.
Advantages of Forced Cooling:

1. Forced cooling reduces the size of the radiator. Therefore a smaller radiator is
needed.
2. Water under pressure is forced into every corner of the water jacket. Therefore no
hot spots are left in the combustion chamber.
3. This system gives a rapid cooling effect.
4. Scale formation does not take place.
5. Pressurized cooling has an advantage that its boiling temperature is increased and
its heat absorption capacity is also increased.
 PARTS OF COOLING SYSTEM

The cooling system consists of the following main parts:

 Water jacket

 Water pump

 Thermostat

 Fan and fan belt

 Radiator and radiator cap

 Water Jacket:

• The water jacket is the space inside the engine through which
water is circulated for cooling the hot parts of the engine.
Therefore the water jacket is a hollow space in the engine.

 Water Pump:
• The water pump is a centrifugal pump driven by the crank
through a belt. The water pump is shown in Fig. It takes water
from the bottom of the radiator and forces it into the water
jacket for circulating water in the engine.
• The pump is made watertight by a special rubber seal encased in
a metal shell, and provided with a spring which presses the seal
against the front face of the impeller shaft bush. The seal is
press fitted in the pump body so that no water can leak between
the seal and the pump body even after a long period and without
any adjustment being necessary.
 Thermostat:

• A thermostat valve is used in the water cooling system to

regulate the circulation of water in the system. The
continuous circulation of water is necessary for maintaining
the normal working temperature of the engine parts during
different operating conditions.

• Two types of thermostat are used in automobile vehicles:

• Bellows type

• Pellet type

• The working of the Bellows type thermostat. When water

temperature is low, the thermostat having air inside, contracts
and closes the valve. Thus cold water is not circulated and
the heat lost by cooling is reduced.
• When the engine runs and the water in the jacket is
sufficiently warmed then the hot water heats the thermostat.
The thermostat expands and pushes off the valve from its
seat. The water which is under pressure in the jacket starts
flowing through the radiator and the water pump.
• The most recent trend in the automotive industry is to use the
wax Pellet type of thermostat. A wax pellet type of
thermostat fitted in a Maruti engine is shown in Fig.

• The temperature sensitive material in the thermostat is placed

in a metal case and it expands when heated and contracts
when cooled.

• When the pellet is heated and it expands, the metal case

pushes down the valve and opens it. As the pellet is cooled,
its contraction allows the spring to close the valve.

• Thus the valve remains closed while the coolant is cold,

preventing the circulation of coolant through the radiator.
The Maruti thermostat is designed to open at a temperature
of 82°C.
 Fan and fan belt:

• A fan belt of V-section gives drive to a fan from the

crankshaft pulley.

• The fan is placed behind the radiator to force air over the
engine body. This creates a pressure difference between the
front and the rear of the radiator, and the rate of flow of air
through the radiator is increased.

• Such increase in the air flow provides effective cooling for

the water circulated through the radiator.
 Radiator:
• A radiator is a device which is used as a heat exchanger
between the hot water and atmospheric air. Radiators are
essentially used in heavy duty automobiles for cooling the
automobile engines.

• Water, which is forced by a pump into the water jacket of the

automobile engine, is sent to the radiator, from where water
is passed through the tubes of the radiator which dispel the
heat to the air which flows round the tubes.

• Thus a radiator is a device which permits the cooling of

water through the transfer of heat to the atmospheric air.

• Basically, radiators are of two types:

(i) Tubular type, and
(ii) Cellular type
(i) Tubular type:
• In the tubular type core, the upper and lower tanks are
connected by a number of tubes arranged in series.
• These vertical tubes carry hot water from the upper tank to
the lower tank. While water passing through these tubes, heat
is transferred to air which flows through the core.

• Between the water tubes or water channels there are spacers

which provide additional indirect radiation which helps in
the rapid cooling of hot water.
• The tubular type core increases resistance to air flow through
the core. Therefore a stronger fan is needed to overcome this
resistance caused by the water tubes.
• An advantage of this type of core is that it has fewer soldered
joints and hence is more robust than the cellular type core.
For this reason, the tubular type core is preferred for use in
heavy vehicles in which the radiator is subjected to very
severe stresses.
• The disadvantage of this type of core is that if one of the
tubes is clogged, the cooling effect of the entire tube is lost.
(ii) Cellular type:
• The cellular type of core is also known as the air tube cellular core. This type of radiator consists of a
number of horizontal tubes which carry atmospheric air.
• The space between the tubes is used for the flow of hot water which is to be cooled. Thus the cellular
core is composed of a large number of individual air cells which are surrounded by water.
• Due to its appearance, the cellular type of core is also known as the honey comb radiator.

• The chief advantages of cellular radiators are:

• The cellular radiator provides better cooling.
• It does not require stronger fans.
• Air resistance is minimized.
• The clogging of the passage for water flow does not give rise to serious problems.
• It is light in weight.
 Anti Freeze Solutions

• In excessively cold weather, the water in the radiator and in

the jacket must not freeze otherwise the radiator or other
engine parts may be broken or damaged.

• To avoid such damages, an antifreeze mixture is added to

water so that the water freezing point of water is lowered.

• The antifreeze mixtures commonly used are—wood alcohol,

denatured alcohol, glycerine, ethylene glycol, propylene
glycol, mixtures of alcohol and glycerin, etc. Out of all these
mixtures, alcohol must be added frequently because alcohol
evaporates quickly.
 Activity

Write Brief note on “ Evaporative Cooling and Pressure Sealed

Cooling” by referring Review and Research Journal Papers?