Cooling system and service 143 Basic cooling system 144 Heat and temperature 145 Heat transfer 146 Liquidcooling systems 146 Coolingsystem components 148 Radiator assembly 152 Radiator pressure cap and reservoir 153 Coolant 154 Coolingsystem service 155 Coolingsystem repairs 158 Water pump overhaul 160 Coolingsystem problems 161 Trouble diagnosis guide 162 Technical terms 162 Review questions 162
Trang 1Cooling system and service
Chapter 10
Basic cooling system
Heat and temperature
Heat transfer
Liquid-cooling systems
Cooling-system components
Radiator assembly
Radiator pressure cap and reservoir
Coolant
Cooling-system service
Cooling-system repairs
Water pump overhaul
Cooling-system problems
Trouble diagnosis guide
Technical terms Review questions
Trang 2The cooling system absorbs about one-third of the heat
produced by the engine It not only removes heat, but
is responsible for keeping the engine at its most
efficient operating temperature Only about a quarter
of the heat generated in the engine is actually used, the
rest of the heat has to be disposed of to prevent
damage to the engine.
There are two general types of cooling systems: air
cooling, in which the air is blown directly over the
engine, and liquid cooling, in which coolant is
circulated through the engine and radiator Most
automotive engines are liquid-cooled and this type of
system is covered here.
Basic cooling system
A basic liquid-cooling system is shown in Figure 10.1.
The main parts are:
1 radiator
2 water pump
3 water-jackets
4 radiator hoses
5 thermostat
6 fan
7 coolant.
The basic function of the cooling system is to
transfer heat from inside the engine to the outside air.
It does this by circulating the coolant (water with
additives) through the engine and the radiator The water pump takes coolant from the bottom of the radiator and pumps it through the engine to the top of the radiator.
Heat originates in the combustion chambers of the engine and is transferred (by conduction) through the cylinder walls and cylinder head to the water-jackets, where it is transferred to the coolant As it circulates, coolant carries the heat from the engine to the top of the radiator.
Hot coolant passing down through the radiator transfers heat to the radiator Heat is then dissipated by the air which passes over the fins and through the radiator core The airflow is assisted by the fan When the coolant reaches the lower radiator tank, it is cool enough to re-enter the engine to remove more heat Some heat is also dissipated by radiation Heat radiates from the outside of the engine and from the exhaust into the atmosphere, to other parts of the engine, and to the body.
The thermostat is located at the outlet from the cylinder head This is a heat-operated valve which prevents coolant from flowing through the radiator until the engine warms up.
Need for a cooling system
Combustion of the air–fuel mixture in the cylinders of the engine produces a considerable amount of heat and high temperatures Heat is absorbed by the cylinder
figure 10.1 The parts of a liquid cooling system
Trang 3walls, the cylinder head and the pistons They, in turn,
must be protected by the cooling system so that they
do not become overheated.
Cooling also prevents the oil on the engine parts
from breaking down and losing its lubrication
properties While the engine must be cooled, it still
needs to operate at a high temperature Removing too
much heat would lower the engine’s thermal
efficiency, and useful energy would be lost.
Heat and temperature
With engines, we are concerned with both heating and
cooling Heat is a form of energy, and it is the heat
from the burning fuel in the combustion chamber that
provides the energy that causes the engine to function.
Heat and temperature are not the same Heat is
energy, while temperature is the degree of hotness, or
coldness Something is referred to as being hot when it
is above normal atmospheric temperature, and cold
when it is below atmospheric temperature.
To understand the difference between heat and
temperature, consider two pieces of steel, a large piece
and a small piece Both can be at the same
temperature, but the large piece will contain more heat.
Effects of heat
When heat is applied or removed from any substance,
it can be affected in the following ways:
1 Change of temperature Heat applied causes the
temperature to rise, and heat removed causes the
temperature to fall.
2 Change of colour Heat applied to metals,
particularly steel, causes a change in colour If a
bright steel surface is heated, it will gradually
change colour, and, depending on the temperature,
different colours will be obtained An engine part
that has been overheated can usually be identified
because it will be discoloured.
3 Change of state Heat can change a solid to a liquid,
and a liquid to a gas For example, ice can change
to water and water to steam Metal heated during
welding will change from a solid to a liquid.
4 Change of volume Heat applied causes expansion,
and heat removed causes contraction This is
because the molecules of the substance that
is heated will move further apart and so increase the
volume, while molecules of the substance that is
cooled will move closer together and so decrease
the volume.
All substances expand when heated and contract when cooled Gas expands easily to many times its size, but liquids and solids expand only a small amount Their molecules are fixed and are not free to move like those of a gas.
Behaviour of water
The behaviour of water is different to all other liquids.
It contracts when cooled until it reaches 4°C, and from this temperature until it freezes to become ice, it expands When cooled below 0°C, ice contracts like any other solid.
Because of this, antifreeze chemicals are added to the coolant in the cooling system to prevent it from freezing Without this protection, the water or coolant could freeze, and expansion could damage the engine.
■ When servicing vehicles that operate in freezing conditions, special precautions must be taken to make sure that the coolant contains the correct proportion of antifreeze.
Thermometer
The thermometer is an instrument for measuring temperature (Figure 10.2) It consists of a glass tube, with a bulb at its base filled with mercury When heated, the mercury expands and rises up a narrow bore in the tube A scale on the thermometer is graduated in degrees On the Celsius scale, water boils
at 100°C and freezes at 0°C.
figure 10.2 Mercury thermometer with Celsius scale –
water boils at 100°C and freezes at 0°C
Trang 4Heat transfer
Heat can be transferred in three ways:
1 by conduction
2 by convection
3 by radiation.
All of these are used to remove heat from the engine.
Heat always moves from a hotter place to a colder one.
Figure 10.3 shows how heat from the coolant in a
radiator is transferred to the cooler air.
Radiation
With radiation, heat is transferred across space as rays These are transformed into heat when they strike a colder object, so that the temperature of the object is then increased.
Dark-coloured materials radiate heat better than light-coloured ones For this reason, cooling fins on cylinders and radiators are usually painted mat black
so that the heat will be more effectively radiated into the surrounding air Dark substances are also good absorbers of heat by radiation.
Liquid-cooling systems
There are a number of variations to liquid-cooling systems While they all function in a similar manner, the location of the components varies with the type of engine.
Figure 10.5 shows the arrangement for a transverse engine The engine has been sectioned to show the water-jackets around the cylinders The radiator is at the front of the vehicle and the water pump is at the front of the engine Arrows show that the coolant flows from the bottom of the radiator to the water pump, then through the cylinder block and cylinder head to the top
of the radiator.
Coolant flow
Figure 10.6 is a simplified arrangement of a cooling system viewed from above the engine Arrows show the coolant flow The thermostat, which controls the flow through the radiator, is at the rear of this engine, although in many engines, it is at the front.
When the engine is cold, the thermostat is closed This blocks off the flow to the top of the radiator, and the pump circulates the coolant within the engine only When operating temperature is reached, the thermostat
figure 10.3 Transfer of heat in a radiator
figure 10.4 Heat applied to the edge of the container
produces convection currents in the water – a spot of dye in a transparent container allows this effect to
be seen
Conduction
In the engine, heat is conducted from the combustion
chamber through the metal parts of the engine to the
coolant in the cooling system.
■ There are good conductors and bad conductors of
heat Metals are good conductors, but asbestos,
wood, paper and most non-metal materials are bad
conductors and so can be classed as heat
insulators.
Convection
This is heat transfer by the movement of the molecules
of the substance It relates to gases and liquids, but not
to solids When a liquid or a gas in a container is
heated, it expands and its density is reduced.
The heated particles become lighter and so they
float upwards, allowing the colder, denser particles to
sink towards the bottom of the container This sets up
convection currents – the principle is illustrated in
Figure 10.4.
Trang 5opens and coolant circulates through the radiator as
well as the engine.
This engine has a heated intake manifold The heat
is provided by coolant flowing through passages in the
manifold This type of manifold is only used with carburettors and throttle-body fuel injection Heating the manifold improves vaporisation of the air–fuel mixture before it enters the engine.
figure 10.5 Cooling system
1 radiator, 2 radiator cap, 3 hose to reservoir, 4 upper radiator hose, 5 lower radiator hose, 6 intake pipe,
7 thermostat, 8 water pump, 9 drive belt, 10 cylinder head, 11 heater inlet hose, 12 heater outlet hose HOLDEN LTD
figure 10.6 Diagram of the coolant flow for a carburettor engine FORD
Trang 6To provide heating inside the vehicle, small hoses
carry coolant to and from the heater core which is
located under the dash of the vehicle The flow of
coolant is controlled by the water valve.
Cooling-system components
Water-jackets
The water-jacket is the name for the spaces around the
cylinders and within the cylinder head that carry the
coolant These spaces are cast into the cylinder head
and block during manufacture Because the valve seats
and guides need cooling, the head is designed to allow
the coolant to reach these areas There are coolant
passages between the cylinder block and the cylinder
head which direct the flow of coolant.
Water pumps
Water pumps are usually mounted at the front end of
the cylinder block, between the block and the radiator.
The pump consists of a housing with a coolant inlet
and an impeller (Figure 10.7).
Coolant enters the pump at the front of the impeller.
When the impeller rotates, coolant between the blades
is thrown outwards by centrifugal force and is forced
through the pump body and into the cylinder block.
The pump inlet is connected by a hose to the
bottom of the radiator, and coolant from the radiator is drawn into the pump to replace the coolant that is pumped through the engine.
The impeller shaft is supported in one or more bearings A seal between the impeller and the housing prevents coolant from leaking out around the bearing The pump shaft is fitted with a flange for a pulley that
is driven by a belt from the crankshaft pulley.
Most water pumps are driven by an external belt, but Figure 10.8 illustrates a water pump that is driven
by the timing belt The pump is mounted to the front of the engine and fitted with a notched pulley, which is engaged with the teeth of the timing belt.
figure 10.7 A water pump located on the front of the
engine
figure 10.8 Parts of a water pump that is driven by the
timing belt MAZDA
Drive belts
Two types of belts are used for fans and water pumps: V-belts and ribbed belts.
1 V-belts With this type of drive belt, friction between the sides of the belt and the sides of the pulley grooves enables drive to be transmitted from one pulley to another V-belts have a wedging action in the pulley grooves that helps to prevent belt slip (see Figure 10.24).
2 Ribbed belts These are a combination of a flat belt and a V-belt (Figure 10.9) The pulleys used with the belts have a number of small grooves, and the belt has a number of small ribs to match.
Belt-driven fans
The purpose of the fan is to produce a large flow of air through the radiator core The fan has a number of blades They can be made of steel or of a plastic material.
Trang 7Belt-driven engine fans are usually mounted on the
end of the water pump shaft and driven by the same
belt that drives the water pump and the alternator.
Fans are mounted close to the radiator and many
fans are partly enclosed in a shroud This increases the
fan’s efficiency by ensuring that all the air that is
moved by the fan must first pass through the radiator
core.
Variable-speed fans
Some fans are fitted with a variable-speed coupling.
This is a type of fluid coupling filled with silicone,
which allows the fan to operate only when it is needed.
This is done to conserve energy.
A sectional view of a water pump that has a fluid
coupling for the fan is shown in Figure 10.10 The
water pump is operated continuously by the drive belt whenever the engine is running The driving member
of the coupling (3) is fixed to the hub and so it also operates continuously.
The fan is attached to the driven member of the coupling (4), so that it is operated through the fluid in the coupling, but at a variable speed.
Coupling operation
The diagrams in Figure 10.11(a) to (c) show the operation of a viscous fan coupling It consists basically of two chambers: the working chamber with the drive plate (or driving member) and the storage chamber The chambers are connected by a valve and contain silicone oil.
With the valve open, the oil circulates between the two chambers With the valve closed, the oil is held in the storage chamber The valve is controlled by a bimetal spring on the front of the coupling The spring
is sensitive to temperature changes.
The coupling operates as follows:
1 Engine cold The silicone oil has been pumped into the storage chamber by the coupling rotating (Figure 10.11(a)) The valve is closed by the bimetal spring to prevent oil from entering the working chamber The fan will operate, but at a very low speed.
2 Engine warm Heat in the engine compartment distorts the bimetal spring (Figure 10.11(b)) This pulls against the shift pin to open the spring valve and allow oil to flow into the working chamber As
a result, a hydraulic coupling is formed between the drive plate and the fan housing and this increases the fan speed.
3 Engine hot At higher temperatures, the bimetal spring distorts more and opens the spring valve further (Figure 10.11(c)) This allows more oil to enter the working chamber and further increase the fan speed A point is reached where the speed of the drive plate and the speed of the fan are the same.
Electric fans
Electric fans can be installed in front of the radiator or behind the radiator More than one electric fan is used when the vehicle is fitted with air conditioning, or when additional cooling is required.
The components of an electric fan assembly are shown in Figure 10.12 These include an electric motor, with a plastic fan fitted to its shaft, and a plastic shroud The motor is mounted on the shroud, which is
figure 10.9 Section through a V-ribbed belt and pulley
MAZDA
figure 10.10 Water pump with a fluid coupling for the fan
1 bimetal spring, 2 housing, 3 driving member,
4 driven member, 5 pulley, 6 water pump bearing, 7 impeller,
8 coolant inlet, 9 seal, 10 pulley mounting, 11 bearing,
12 fluid coupling TOYOTA
Trang 8located behind the radiator Two of these fan assemblies can be located side-by-side.
Electric fans are used extensively on vehicles with transverse engines This enables the fan (and the radiator) to be fitted to the front of the vehicle, some-thing that would be difficult to achieve with a belt drive.
Fan operation and control
Figure 10.13 shows the basic arrangement of an electric fan and its controls There are two parts to the electrical circuit: the fan operating circuit and the fan switch circuit.
1 Operating circuit The circuit includes the fan motor, the fan relay and the fan switch The circuit can be traced from the top fuse (6) and connector (7) to the fan motor and then to earth through the fan relay (4) When the relay points are closed, the fan will operate.
2 Switch circuit The switch circuit is from the bottom fuse (6) through the windings of the relay to
figure 10.11 Operation of a variable-speed (viscous) fan
coupling VOLKSWAGEN
figure 10.12 Electric fan and radiator assembly FORD
Trang 9the fan switch and then to earth The fan switch is a
thermo switch that has its end fitted into the cooling
system The switch is normally closed, but opens
when the coolant temperature reaches 100°C.
■ This is for a single-speed fan Dual-speed fans are
also used These are arranged to operate at a
slower speed when less air flow is needed.
Fan switch operation
When the coolant temperature is less than 100°C, the
fan switch is closed Current flows from the fuse,
through the relay windings and the fan switch to earth.
This energises the relay windings and holds the relay
points open so that the fan does not operate.
When the coolant reaches 100°C, the fan switch
opens, the relay windings are de-energised and the
relay points close This completes the fan circuit to
earth and the fan operates.
The fan will continue to operate until the coolant
temperature drops and the fan switch again closes.
When this occurs, the relay windings are energised and
the relay points open to stop the fan.
The fan will cut in and out as the temperature of the
coolant varies at the fan switch.
■ The fan relay is an electromagnetic switch When
magnetised, the relay points open and when
demagnetised, the relay points close.
Thermostat
The thermostat is usually located in a small housing
attached to the cylinder head (Figure 10.14) The
housing also includes the coolant outlet from the
cylinder head to the radiator, and the thermo switch for the electric fan.
The function of the thermostat is to close off the coolant outlet when the engine is cold This restricts circulation to within the engine until operating temperature is reached The thermostat then opens to allow coolant to flow through the radiator.
The thermostat consists of a temperature-sensitive device, which controls the opening and closing of a valve in the coolant passage.
Thermostat operation
Most thermostats are of the wax-pellet type (Figure 10.15) In the closed (cold) position, the valve
is held on its seat by the spring, so that coolant cannot pass through the thermostat This blocks off the passage to the radiator, except for a small bleed hole.
As the temperature of the coolant increases, the wax in the pellet expands and applies pressure to the rubber diaphragm This tries to force the pin out, but the pin is fixed and cannot move, so the pellet container moves downwards This moves the valve off
figure 10.13 Electric fan and associated components
1 shroud, 2 fan, 3 electric motor, 4 fan relay,
5 fan switch, 6 fuses, 7 connector FORD
figure 10.14 Thermostat and housing
1 air bleed, 2 thermo switch, 3 cover, 4 upper
radiator hose connection, 5 heater hose, 6 thermostat,
7 thermostat housing, 8 gasket HOLDEN LTD
Trang 10its seat, opening the valve and allowing coolant to flow
to the radiator.
When the engine temperature drops, the wax in the
pellet contracts and allows the spring to close the
valve, blocking the flow of coolant to the radiator.
■ Thermostats are designed to open at specific
temperatures For example, a thermostat
desig-nated as an 80°C unit will start to open between
78°C and 82°C and will be fully open at 95°C.
Temperature indicators
The cooling system has a temperature gauge and
sometimes a warning light Any unusual rise in
temperature is a warning to the driver The engine
should be stopped and checked before serious damage
occurs.
A thermo sensor in the radiator or cooling system is
used to operate the gauge or warning light on the
instrument panel.
Interior car heater
Vehicle interior heaters have a small radiator that
transfers heat from the cooling system to the passenger
compartment of the vehicle.
Hot coolant from the engine is circulated inside the
radiator and a small electric fan blows air through the
radiator (Figure 10.16) The air absorbs heat so that
warm air enters the passenger compartment of the
vehicle.
The coolant to the heater can be turned off, when it
is not in use, by a shut-off valve which is operated by
the heater controls.
The volume of air flow can be controlled by altering the motor speed The direction of air flow is controlled by opening and closing shutters in the ducting through which the air flows.
Radiator assembly
The radiator consists of two tanks and a core The core
is made up of a number of tubes which carry the coolant between the tanks.
Air from the fan, and also from vehicle movement, passes between the tubes and removes heat from the coolant in the tubes The tubes have fins which increase the surface area over which the air flows and this improves heat transfer.
There are two designs of core: cores with centre fins and cores with horizontal fins Each has a slightly different construction Passenger cars and light commercial vehicles usually have cores of the tube and centre-fin type, as shown in Figure 10.17 In this design, the fins are in the form of corrugated strips between the tubes There can be a single row of tubes,
or two or more rows of tubes.
figure 10.15 Construction of a wax-pellet thermostat
figure 10.16 Principle of a car interior heater FORD
figure 10.17 Radiator core with a double row of tubes
MAZDA