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WATER COOLING SYSTEM
Hello, Everyone. Meet you any more after the last discussion about lubricants and lubrication. At this
moment, we are going to discuss about Water Cooling System. Since this article compiled from many
sources in English and I really do not have much time of transliterating, I hope that it still useful for us.
I. Introduction
The performance and efficiency of water-cooled spark-ignition or compression-ignition engines applied in
motor vehicles or for stand-by power use relies on effective heat exchange between the engine and the
surrounding medium. Performance here will require that there is proper carburetion, satisfactory oil viscosity
and by implication correct clearances of the engine’s static and moving parts. Water-Cooling Systems
consists of engine, (cooling jackets of the cylinder-block, cylinder head), radiator, fan, pump, engine
temperature control devices, water distribution pipes and ducts and other elements.
The engine parts of great concern are cylinder heads and wall liners, pistons, and valves. Carburetion
problems could arise due to poorly vaporised petrol leading to some of the combustion gases condensing
on the cylinder walls causing a possible dilution of the oil in the pump and likely corrosion. Proper moving
engine parts will require that, lubrication of the engine parts is adequate allowing for the oil to flow freely at
the right viscosity and temperature]. Engine combustion results in high temperature combustion gases.
These high temperatures produced in the cylinders are transferred through the cylinder wall liners, cylinder
heads, pistons and valves to the coolant by convection. In discussing such high temperatures, gives
estimates as high as 1270 K – 1770 K and thus, exposing engine metal parts to such high temperatures will
cause them to expand considerably, weaken them, result in high thermal stresses with reduced strength,
safety concerns in overheated cylinders attaining flash temperature of the fuel thereby likely leading to
preignition, cause lubricating oils to evaporate rapidly leading to sticking pistons, piston rings, cylinders and
eventual seizure and damage. A Cooling system is thus required to maintain stable operating temperature
for the engine and prevent failure.
The radiator used to get rid of this heat, is a heat exchanger, which transfers the heat from the coolant to
the air; the designs of which allow the coolant to flow through a bank of tubes exposed to the crossflow of
air, are of the two basic forms:
a. Cross-flow radiator, in which coolant flows from one side tank to the other, and
b. down-flow radiator, in which coolant flows from a top tank to a bottom tank
The number of tubes is an important factor in the design of the ideal radiator in terms of the adequate
surface cross-sectional area for effective cooling. The ideal radiator design should be compact, guided by
minimum weight considerations, but, able to offer a large and effective cooling surface area; with coolant
passages that should not be too small to avoid clogging by solid contaminants or scaling, with the attendant
likely blockage restricting or limiting coolant flow leading to overheat of the engine, fouling and thermal
corrosion, reduced endurance limit, and eventual stress corrosion cracking (SCC).
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II. Water Jacket
A water jacket is a water-filled casing surrounding a device, typically a metal sheath having intake and outlet
vents to allow water to be pumped through and circulated. The flow of water to an external heating or
cooling device allows precise temperature control of the device.
Water jackets are often used in watercooling. They are also used in laboratory glassware: Liebig, Graham,
and Allihn condensers. Water jackets were used to cool the barrels of machine guns at the time of the First
World War, but modern machine guns are air-cooled to conserve weight and hence increase portability.
In a reciprocating piston internal combustion engine the water jacket is a series of holes either cast or bored
through the main engine block and connected by inlet and outlet valves to a radiator.
Equipment such as tissue culture incubators may be enclosed in a water jacket kept at a constant
temperature
Fig. 1
Open block showing water jacket
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Fig. 2
Water jacket in an engine filled with coolant
Fig. 3
A Suzuki Aerio J20A Engine Block, showing its water jacket.
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III. Radiator
Radiators are heat exchangers used for cooling internal combustion engines, mainly in automobiles but also
in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plant or any similar use of
such an engine.
Internal combustion engines are often cooled by circulating a liquid called engine coolant through the
engine block, where it is heated, then through a radiator where it loses heat to the atmosphere, and then
returned to the engine. Engine coolant is usually water-based, but may also be oil. It is common to employ a
water pump to force the engine coolant to circulate, and also for an axial fan to force air through the
radiator.
In automobiles and motorcycles with a liquid-cooled internal combustion engine, a radiator is connected to
channels running through the engine and cylinder head, through which a liquid (coolant) is pumped. This
liquid may be water (in climates where water is unlikely to freeze), but is more commonly a mixture of water
and antifreeze in proportions appropriate to the climate. Antifreeze itself is usually ethylene glycol or
propylene glycol (with a small amount of corrosion inhibitor).
A typical automotive cooling system comprises:
1. a series of channels cast into the engine block and cylinder head, surrounding the combustion
chambers with circulating liquid to carry away heat;
2. a radiator, consisting of many small tubes equipped with a honeycomb of fins to convect heat rapidly,
that receives and cools hot liquid from the engine;
3. a water pump, usually of the centrifugal type, to circulate the liquid through the system;
4. a thermostat to control temperature by varying the amount of liquid going to the radiator;
5. a fan to draw fresh air through the radiator.
The radiator transfers the heat from the fluid inside to the air outside, thereby cooling the fluid, which in turn
cools the engine. Radiators are also often used to cool automatic transmission fluids, air conditioner
refrigerant, intake air, and sometimes to cool motor oil or power steering fluid. Radiators are typically
mounted in a position where they receive airflow from the forward movement of the vehicle, such as behind
a front grill. Where engines are mid- or rear-mounted, it is common to mount the radiator behind a front grill
to achieve sufficient airflow, even though this requires long coolant pipes. Alternatively, the radiator may
draw air from the flow over the top of the vehicle or from a side-mounted grill. For long vehicles, such as
buses, side airflow is most common for engine and transmission cooling and top airflow most common for
air conditioner cooling.
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Fig. 4
Typical Automotive Cooling System
Radiator construction
Automobile radiators are constructed of a pair of header tanks, linked by a core with many narrow
passageways, giving a high surface area relative to volume. This core is usually made of stacked layers of
metal sheet, pressed to form channels and soldered or brazed together. For many years radiators were
made from brass or copper cores soldered to brass headers. Modern radiators save money and weight by
using plastic headers and may use aluminium cores. This construction is less easily repaired than traditional
materials.
Honeycomb radiator tubes
An earlier construction method was the honeycomb radiator. Round tubes were swaged into hexagons at
their ends, then stacked together and soldered. As they only touched at their ends, this formed what
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became in effect a solid water tank with many air tubes through it. Some vintage cars use radiator cores
made from coiled tube, a less efficient but simpler construction.
Fig. 5
Honeycomb Radiator Tubes
Coolant pump
A sectioned view of the cylinder block, radiator and connecting hoses. The hoses link the tops and bottoms
of each, without any pump but with an engine-driven cooling fan
Fig. 7
Thermosyphon cooling system of 1937, without circulating pump
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Radiators first used downward vertical flow, driven solely by a thermosyphon effect. Coolant is heated in the
engine, becomes less dense, and so rises. As the radiator cools the fluid, the coolant becomes denser and
falls. This effect is sufficient for low-power stationary engines, but inadequate for all but the earliest
automobiles. All automobiles for many years have used centrifugal pumps to circulate the engine coolant
because natural circulation has very low flow rates.
Heater
A system of valves or baffles, or both, is usually incorporated to simultaneously operate a small radiator
inside the vehicle. This small radiator, and the associated blower fan, is called the heater core, and serves
to warm the cabin interior. Like the radiator, the heater core acts by removing heat from the engine. For this
reason, automotive technicians often advise operators to turn on the heater and set it to high if the engine is
overheating.
Temperature control
Waterflow control
Car engine thermostat
The engine temperature on modern cars is primarily controlled by a wax-pellet type of thermostat, a valve
which opens once the engine has reached its optimum operating temperature.
Fig. 8
Thermostat
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When the engine is cold, the thermostat is closed except for a small bypass flow so that the thermostat
experiences changes to the coolant temperature as the engine warms up. Engine coolant is directed by the
thermostat to the inlet of the circulating pump and is returned directly to the engine, bypassing the radiator.
Directing water to circulate only through the engine allows the temperature to reach optimum operating
temperature as quickly as possible whilst avoiding localised "hot spots." Once the coolant reaches the
thermostat's activation temperature, it opens, allowing water to flow through the radiator to prevent the
temperature rising higher.
Once at optimum temperature, the thermostat controls the flow of engine coolant to the radiator so that the
engine continues to operate at optimum temperature. Under peak load conditions, such as driving slowly up
a steep hill whilst heavily laden on a hot day, the thermostat will be approaching fully open because the
engine will be producing near to maximum power while the velocity of air flow across the radiator is low.
(The velocity of air flow across the radiator has a major effect on its ability to dissipate heat.) Conversely,
when cruising fast downhill on a motorway on a cold night on a light throttle, the thermostat will be nearly
closed because the engine is producing little power, and the radiator is able to dissipate much more heat
than the engine is producing. Allowing too much flow of coolant to the radiator would result in the engine
being over cooled and operating at lower than optimum temperature. A side effect of this would be that the
passenger compartment heater would not be able to put out enough heat to keep the passengers warm.
The fuel efficiency would also suffer.
The thermostat is therefore constantly moving throughout its range, responding to changes in vehicle
operating load, speed and external temperature, to keep the engine at its optimum operating temperature.
On vintage cars you may find a bellows type thermostat, which has a corrugated bellows containing a
volatile liquid such as alcohol or acetone. These types of thermostats do not work well at cooling system
pressures above about 7 psi. Modern motor vehicles typically run at around 15 psi, which precludes the use
of the bellows type thermostat. On direct air-cooled engines this is not a concern for the bellows thermostat
that controls a flap valve in the air passages.
Airflow control
Other factors influence the temperature of the engine, including radiator size and the type of radiator fan.
The size of the radiator (and thus its cooling capacity) is chosen such that it can keep the engine at the
design temperature under the most extreme conditions a vehicle is likely to encounter (such as climbing a
mountain whilst fully loaded on a hot day).
Airflow speed through a radiator is a major influence on the heat it loses. Vehicle speed affects this, in
rough proportion to the engine effort, thus giving crude self-regulatory feedback. Where an additional
cooling fan is driven by the engine, this also tracks engine speed similarly.
Engine-driven fans are often regulated by a viscous-drive clutch from the drivebelt, which slips and reduces
the fan speed at low temperatures. This improves fuel efficiency by not wasting power on driving the fan
unnecessarily. On modern vehicles, further regulation of cooling rate is provided by either variable speed or
cycling radiator fans. Electric fans are controlled by a thermostatic switch or the engine control unit. Electric
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fans also have the advantage of giving good airflow and cooling at low engine revs or when stationary, such
as in slow-moving traffic.
Before the development of viscous-drive and electric fans, engines were fitted with simple fixed fans that
drew air through the radiator at all times. Vehicles whose design required the installation of a large radiator
to cope with heavy work at high temperatures, such as commercial vehicles and tractors would often run
cool in cold weather under light loads, even with the presence of a thermostat, as the large radiator and
fixed fan caused a rapid and significant drop in coolant temperature as soon as the thermostat opened. This
problem can be solved by fitting a radiator blind to the radiator which can be adjusted to partially or fully
block the airflow through the radiator. At its simplest the blind is a roll of material such as canvas or rubber
that is unfurled along the length of the radiator to cover the desired portion. Some older vehicles, like the
World War I-era S.E.5 and SPAD S.XIII single-engined fighters, have a series of shutters that can be
adjusted from the driver's or pilot's seat to provide a degree of control. Some modern cars have a series of
shutters that are automatically opened and closed by the engine control unit to provide a balance of cooling
and aerodynamics as needed.
Coolant pressure
Because the thermal efficiency of internal combustion engines increases with internal temperature, the
coolant is kept at higher-than-atmospheric pressure to increase its boiling point. A calibrated pressure-relief
valve is usually incorporated in the radiator's fill cap. This pressure varies between models, but typically
ranges from 9 to 15 psi (0.621 to 1.03 bar; 62.1 to 103 kPa).
As the coolant expands with increasing temperature, its pressure in the closed system must increase.
Ultimately, the pressure relief valve opens, and excess fluid is dumped into an overflow container. Fluid
overflow ceases when the thermostat modulates the rate of cooling to keep the temperature of the coolant
at optimum. When the engine coolant cools and contracts (as conditions change or when the engine is
switched off), the fluid is returned to the radiator through additional valving in the cap.
Engine coolant
Before World War II, engine coolant was usually plain water. Antifreeze was used solely to control freezing,
and this was often only done in cold weather.
Development in high-performance aircraft engines required improved coolants with higher boiling points,
leading to the adoption of glycol or water-glycol mixtures. These led to the adoption of glycols for their
antifreeze properties.
Since the development of aluminium or mixed-metal engines, corrosion inhibition has become even more
important than antifreeze, and in all regions and seasons.
Boiling or overheating
On this type of system, if the coolant in the overflow container gets too low, fluid transfer to overflow will
cause an increased loss by vaporizing the engine coolant.
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Severe engine damage can be caused by overheating, by overloading or system defect, when the coolant is
evaporated to a level below the water pump. This can happen without warning, because at that point, the
sending units are not exposed to the coolant to indicate the excessive temperature.
Opening a hot radiator drops the system pressure immediately and may cause a sudden ebullition of super-
heated coolant. Therefore, since opening the cap on a hot radiator can result in steam burns to the unwary
person, radiator caps often contains a mechanism that attempts to relieve the internal pressure before the
cap can be fully opened.
In our Suzuki Aerio, several signals show that overheating is in progress, i.e. hot air conditioner; engine
stalled when stuck in traffic, and may more. Instead of using plain water as a coolant, use radiator coolant
which has better typical characteristics than water.
Figures are courtesy of Wikipedia & Lubrication Engineer
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