Liquid Cooled Systems

8/7/2019 Liquid Cooled Systems

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Types of Cooling Systems:

1. Liquid Cooled Systems

2. Air Cooled Systems

Liquid Cooled Systems:



In this system mainly water is used and made to circulate through the

jackets provided around the cylinder, cylinder-head, valve ports and seats

where it extracts most of the heat.

The heat is transferred from the cylinder walls and other parts

by convection and conduction. The liquid becomes heated in its passage

through the jackets and is in turn cooled by means of an air-cooled radiator

system. The heat from liquid in turn is transferred to air. Hence it is called

the indirect cooling system.

Water-cooling can be carried out by any one of the following fivemethods:

1) Direct or non-return system

2) Thermosyphon system

3) Forced circulation cooling system

4) Evaporative cooling system

5) Pressure cooling system

Direct or Non-return System:

This system is useful for large installations where plenty of water is

available. The water from a storage tank is directly supplied through an inlet

valve to the engine cooling water jacket. The hot water is not cooled for

reuse but simply discharged.



Thermosyphon System:

The basic principle of thermosyphon can be explained with respect to figure.

Heat is supplied to the fluid in the tank A. Because of the relatively lower

density, the hot fluid travels up, its place being taken up by comparatively

cold fluid from the tank B through the pipe p2.

The hot fluid flows through the pipe p1 to the tank B where it gets

cooled. Thus the fluid circulated through the system in the form of

convection currents.



For engine application, tank A represents the cylinder jackets while

tank B represents a radiator and water acts the circulating fluid. In order to

ensure that coolest water is always made available to cylinder jackets, the

water jackets are located at a lower level than the radiator.

Forced Circulation Cooling System:

This system is used in a large number of automobiles like cars, buses and

even heavy trucks. Here, flow of water from radiators to water jackets is by

convection assisted by a pump.

y The main principle of this system is explained with the help of a

block diagram shown in figure.

y The water or coolant is circulated through jackets around theparts of the engine to be cooled and kept in motion by a

centrifugal pump which is driven by the engine.



y The water is passed through the radiator where it is cooled by

air drawn through the radiator by a fan and by the air draft due

to the forward motion of the vehicle.

y A thermostat is used to control the water temperature requiredfor cooling. This system mainly consists of four components, viz.,

a radiator, fan, water pump and a thermostat. The details of

these components are shown in figure.

Radiator:

The basic requirement of a radiator is to provide a sufficiently large

cooling area for transmission of heat from the coolant to the air.



Fan:

The fan is mounted on the impeller spindle driven by a suitable belt

pulley arrangement as shown in figure draws air through the spaces

between the radiator tubes thus bringing down the temperature of the

water appreciable.

Thermostat:

y The thermostat is normally fitted either to the cylinder head

water-outlet passage below the top hose, or in a special hosing

near the water pump.

y The purpose of this device is primarily to restrict the circulation

of water to cylinder head and block during the warm-up period.

Then when a designed temperature is reached, the thermostat

opens and the circulation of water starts in the radiator.

Evaporative Cooling System:



In this system, also called steam or vapour cooling, the temperature of

the cooling water is allowed to reach a temperature of 100 degree C.

This method of cooling utilizes the high latent heat of vaporization of

water to obtain cooling with minimum of water. Figure shows such a

system. The cooling circuit is such that coolant is always liquid but the

steam formed is flashed off in the separate vessel. The makeup water so

formed is sent back for cooling.

This system is used for cooling of many types of industrial engines.

Pressure Cooling System:

As shown in the figure a cap is fitted on the radiator with two-valves, a

safety valve which is loaded by a compression spring and a vacuum valve.

When the coolant is cold both the valves are shut, but as the engine



warms up the coolant temperature rises until it reaches a certain preset

value corresponding to the desired pressure when the safety valve opens;

but if the coolant temperature falls during the engine operation the valve

will close again until the temperature again rises to the equivalent

pressure value.

When the engine is switched off and the coolant cools down vacuum

begins to form in the cooling system, but when the internal pressure falls

below atmospheric the vacuum valve is opened by the higher outside

pressure and the cooling system then attains atmospheric pressure.

A safety valve is incorporated in the filler cap so that if an attempt is

made to unscrew it while the system is under pressure, that first

movement of the cap at once relieves the pressure and thus prevents the

emission or the blowing off the cap due to higher internal pressure.

Advantages of Liquid-Cooling System:

1) Compact design of engines with appreciably smaller frontal area is

possible.

2) The fuel consumption of high compression liquid-cooled engines are

rather lower than for air-cooled ones.

3) Because of the even cooling of cylinder barrel and head due to jacketing

makes it possible to reduce the cylinder head and valve seat

temperatures.

4) Volumetric efficiency of water-cooled engines is higher than that of air-

cooled engines.

5) The size of the engine does not involve serious problem as far as design

of cooling system is concerned.



Disadvantages Liquid-Cooling System:

1) This is dependent system in which supply of water for circulation in the

jacket is required.

2) Power absorbed by the pump for water circulation is considerably higher

than that for cooling fans.

3) In the event of failure of cooling system serious damage may be caused

to the engine.

4) Cost of system is considerably high.

5) The performance is weather sensitive.

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Liquid Cooled Systems

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