Explain why expansion of components must be controlled.
Explain why expansion of components must be controlled.
Look back over the previous section and write out a list of the key bullet points here:
COMPONENTS AND OPERATION
Coolant The coolant is a mixture of water and antifreeze. The antifreeze is needed to prevent water expanding as it freezes. The force from that expansion would be sufficient to cause engine cylinder blocks and radiators to burst apart.
Antifreeze Sufficient antifreeze is needed for the climate in which the vehicle is operated. Modern antifreeze formulae are also designed to give year- round protection by increasing the boiling point of the Antifree
ze
coolant for hot-weather use.
concentr ate
Heat Transfer All three forms of heat transfer are used in the cooling system:
Convection occurs in the water jacket, …by
Heat can be
transferred… convection
creating flows of internal coolant from the cylinder block to the cylinder head.
Conduction occurs through the cylinder …conduction
…and
and combustion-chamber surfaces as
radiation
heat passes to the coolant. Radiation of heat occurs from the
radiator and cooling fins when heat is dissipated to the atmosphere.
Rate of Heat Transfer The amount of heat transfer is dependent on four main factors:
The temperature difference between the engine and coolant; The temperature difference between the coolant and the air
stream passing through the radiator; The surface area of the radiator tubes and fins; The rate of flow of air and coolant through the radiator.
Thermostat Liquid-cooling systems traditionally used a thermostat in the
outlet to the top hose to control engine temperature.
A thermostat is a temperature-sensing Cooling system and valve that opens when the coolant is hot Thermostat
and closes as the coolant cools down. This allows hot coolant to flow from the engine to the radiator where it cools down and returns to the engine. The cooled coolant in the engine acts on the thermostat and it closes.
Coolant Flow The coolant re-heats in the engine and the thermostat opens and the cycle of hot coolant flow to the radiator and
cool coolant returning to the engine repeats itself. Although this system provides a reasonably effective method of engine- temperature control, it does produce a fluctuating temperature. However, a steady temperature is required for very clean and efficient combustion.
Bypass-Mixing Cooling System Modern engine design is moving towards a system with the
thermostat in the radiator bypass channel. When the thermostat opens, it allows cold
water from the radiator to mix with the hot- Coolant flow water flow in the bypass as it enters the
water pump. This system provides a steady engine temperature and prevents the fluctuating-temperature cycle of the earlier system. The modern system is shown here with arrows indicating the coolant flow.
Heat Distribution The heat distribution within the engine needs to be controlled. The temperature around all cylinders and combustion chambers should be identical. The heat removed by the cooling system has, therefore, to be consistent for all areas of the engine. All modern engines have
a fairly rapid coolant circulation within the engine so that an
even temperature distribution is achieved.
Water (Coolant) Pump The water (or coolant) pump draws the coolant through a radiator bypass channel when the engine is cold and from the radiator when the engine is hot. The impeller on the water pump
drives the coolant into the engine coolant Water-pump action passages or water jacket. Water-jacket
passages are carefully designed to direct the coolant around the cylinders and upwards over and around the combustion chambers.
Coolant Density The density of coolant falls as it heats up and, as the temperature approaches boiling point, bubbles begin to form. These bubbles can create areas in the water jacket where the coolant is at a lower density and the actual mass of coolant in those areas is reduced. The reduced mass of coolant therefore cannot effectively absorb heat efficiently in order to cool the engine.
Cavitation Another problem of poor heat transfer and lowered coolant density occurs when the rapid flow of coolant into and out of restrictions in the water jacket induces a phenomenon known as ‘cavitation’. This results in localized drops in pressure and density in the coolant.
Heat Distribution The two causes of localized coolant-density change - bubble formation and cavitation - can seriously affect the performance of the cooling system. This is because an even heat distribution around the cylinders and combustion chambers is not maintained.
Pressurized Cooling Systems To overcome these problems, all liquid cooling systems are pressurized. When hot, most modern systems have an operating pressure equivalent to about one atmosphere (1 bar, or 100 kPa).
Expansion The pressure is obtained by restricting the loss of air above the coolant in a radiator header tank or an expansion tank. As coolant heats Expansion
up it expands. If the air above the coolant has less tank space to occupy, and it cannot immediately escape,
it increases in pressure.
Radiator Pressure Cap A pressure- sensing valve in the radiator cap allows
this higher pressure to escape but
retains the operating pressure. Radiator pressure-cap
details
Pressure-Cap Vacuum Valve As the engine cools down, the coolant contracts and the pressure drops. A vacuum valve in the pressure cap allows air to return to the system. This prevents depressurization below atmospheric pressure and also the risk of the inward collapse of components. An early sign of the failure of this valve to open is a top hose that has collapsed.
Increased Coolant Density The pressure in the system acts on the coolant to increase the density, which would otherwise have fallen without the increase in pressure. This helps to reduce the risk of cavitation and to increase the boiling point of the coolant under pressure. The advantages are a more efficient cooling system with a higher safe operating temperature. It can also be used at high altitudes without the need for modification.
Summary A cooling system is needed to prevent engine damage caused by overheating. It also helps to reduce emissions by shortening the engine warm-up time. Heat is used from the cooling system to operate the heater.
Name and state the purpose of FIVE main cooling system components.
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COOLING & HEATING
Cooling System In a liquid-cooling system, the coolant carries heat from the engine to the radiator. Airflow through the radiator dissipates the heat into the atmosphere. Air is forced through the radiator by the forward movement of the vehicle, or is assisted by a fan fitted behind the radiator.
Cooling Fan The fan can be driven by an electric motor, or by
a belt from the crankshaft. Traditional engines had the fan mounted on the front of the water pump with a "V" belt driving the fan and pump.
Fan Design A number of energy- saving fan designs have been used such as variable-pitch and viscous- hub types. Vehicles that are regularly used for carrying loads, or for towing, can be fitted with
Twin electric fans
secondary fans to improve cooling efficiency and prevent engine overheating.
In-Car Heating Some of the surplus heat from the cooling system is used for in-car heating. Pipes and hoses from the water jacket carry hot coolant to a heater radiator or matrix fitted into the heater housing.
Heater Controls Two methods of heat control are used. One uses a
water valve to control coolant flow through the heater. The other, which has a continuous coolant flow, uses control flaps to mix hot
Heat control methods
and cold air in the heater housing.
Fresh-Air Ducts Ducts into and out of the heater direct air to the screen, side screens and passenger Heater air
Fresh air control flaps fascia vents compartment. This is for demisting,
defrosting and warming the passenger compartment. Control flaps in the heater direct the airflow to the ducts. Fresh-air vents in the fascia can direct either hot or cold air into the vehicle interior.
Component Design Some of the cooling system and heater components have different designs. Many of these have been
developed to improve the efficiency of the system, or because of changes in vehicle design.
Describe the operation of a water-cooled system car heater AND how the temperature is controlled.
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ANTIFREEZE
Coolant The coolant is a mixture of water, antifreeze and inhibitors. The antifreeze is usually ethylene glycol, which
needs inhibitors to prevent corrosion and foaming. These inhibitors have a life span of about two years, which means that the coolant should be changed at biennial intervals. Selection of the correct coolant mixture must be made to meet the manufacturer’s specifications. Aluminium-alloy engines are more prone to corrosion than cast-iron engines.
Antifreeze Antifreeze is mixed to a specified ratio with water. Many manufacturers specify a 50/50 mixture of water and antifreeze, which allows higher engine temperatures before the coolant boils and prevents freezing.
Ethylene Glycol An ethylene glycol antifreeze solution has an added advantage. It forms a semi-solid wax solution prior to solidification and this enables any expanding ice crystals to move within the water passages.
Frost Protection A 50/50 coolant mixture will increase the boiling point to 106°C (223°F) and provide protection down to - 34°C (-30°F). For colder temperatures down to -65°C (-90°F),
a maximum mixture of 65% ethylene glycol can be used. Higher concentrations begin to freeze at higher temperatures and therefore no more than 65% ethylene glycol should be used.
Hard-Water Areas Many areas have ‘hard’ water that contains calcium or chalk. This separates from the water when it is heated. Deposits can
Radiator
form inside the water jacket or radiator where water
passages
they can block small water passages. Frequent topping up with mains water in hard-water areas should be avoided. In these areas, it may be recommended to use distilled water, or water from outside the area.
State TWO purposes of antifreeze.
State how the percentage of antifreeze in a vehicle is determined.
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ROUTINE MAINTENANCE
Servicing Scheduled service requirements for the engine cooling system consist of checks on its performance and operation. All of the quick checks described in the Checking System Performance section should be completed. Obtain data from the vehicle manufacturer’s service schedules for the work to be carried out at any particular mileage or time interval.
Routine Replacement The main cooling-system items for routine replacement are the antifreeze and coolant. Rubber components such as hoses and drive belts are replaced if they begin to show signs of deterioration. A preventative maintenance programme would include replacement of hoses and drive belts at, say, three or four yearly intervals. Report any faults found during service operations to the owner or driver of the vehicle.
Worksheet Routine maintenance inspections,
lubrication and replacement of parts. The routine maintenance items are mainly quick checks to
ensure that the system is operating correctly, and is likely to do so until the next scheduled service. The detail of the checks is covered in the Checking System Performance section. Any additional items that are specified in the service schedule should be carried out in accordance with the
manufacturer’s instructions.
Pollen Filters On vehicles fitted with pollen filters, the paper element should be replaced at the specified mileage/kilometres, or more
Use good
frequently in very dusty conditions. Replacing the quality filters coolant with a new water and antifreeze solution is
covered by the next worksheet.
Worksheet Drain and top up coolant for Radiator
being drained
winter/all-year-round usage. There are two types of service schedule. Most
types are based on mileage and time but some older schedules were based on seasonal requirements. The reasons for seasonal maintenance are still valid and can be used on top of mileage and time service schedules.
Seasonal Checks Cooling-system maintenance should match the season. In the summer when hot weather is expected, it is important that the system is working efficiently and that checks for leakage and coolant and airflow through the radiator are carried out. Any faults found should be reported to the customer for permission to fit replacement parts.
Winter Conditions During winter months, the risk from coolant freezing is high. Adequate antifreeze strength is necessary to prevent damage to the engine and radiator because water expands during freezing.
Draining Coolant Drain the coolant into a clean drain tray and transfer to a clean can or tank for disposal at an authorized site. The container
Coolant being
should be marked to show the contents as drained antifreeze - ethylene glycol. Never use food
containers for this purpose. Follow the vehicle manufacturer’s instructions for the method of draining.
Antifreeze Observe the manufacturer’s recommendation for antifreeze type and quantity. Antifreeze solutions for year-round use have
Check
additives and inhibitors to make them suitable for antifreeze
regularly
this type of application.
Filling the Cooling System Some engines will fill without incurring the problem of air bubbles forming in the water jacket or heater. However, if Bleed valves
problems are encountered, bleed the system in in cooling
system
accordance with the vehicle manufacturer’s instructions. Where bleed valves are fitted, open these before filling and close them when coolant flows freely. When an engine has to be run to force coolant through the heater, take care to keep clear of rotating components and hot coolant.
Safety First Show the customers what to look for and explain how important it is for safety reasons that they do not
remove the pressure cap until the engine has cooled down. Overheating and Freezing For drivers, there are two main
concerns related to the cooling system. These are overheating and freezing. Overheating is more common during the summer months and frequently occurs on long journeys and in traffic queues. A reminder during the Spring for the need to check the cooling system for coolant level, water pump drive-belt condition and for hose condition is appreciated by customers.
Pre-Winter Checks During the autumn, remind them of the need for a pre-winter check of the coolant antifreeze content. Most manufacturers now recommend a 50% ethylene-glycol solution. Explain why it important that this coolant mixture should be used for topping up that system.
Frozen Coolant If a customer reports a frozen cooling system, advise them to make sure that the engine is gently warmed until the coolant thaws. They must then have the antifreeze content checked and topped up as soon as possible. If necessary, explain how the coolant in the engine can be liquid but, if the radiator is frozen, the engine will still overheat. This is because the coolant cannot circulate.
Coolant Leaks Coolant leaks can wreck engines. A driver should watch the temperature gauge and make the daily, or weekly, checks needed for early detection of cooling-system problems and particularly before any long journeys. Advise the customer, if necessary, that ethylene glycol is a skin irritant and can damage or discolour some types of paintwork.
Customer Care All faults should be reported to the vehicle owner or driver, together with recommendations for further diagnostic investigations or repair work. Always ensure that the vehicle is clean before returning it to the customer.
Summary Remember, regular checks of the cooling system will keep the vehicle reliable – and the customer happy!
Look back over the previous section and write out a list of the key bullet points here:
AIR SUPPLY, EXHAUST AND EMISSIONS AIR POLLUTION FROM MOTOR VEHICLES
Introduction Atmospheric pollution has become a serious problem to the health of people and to the environment. Many urban areas are now heavily polluted, with people
Composition of
exhaust suffering medically from the effects of
vehicle exhaust pollution. Fossil Fuels There have been many changes in
climatic conditions in the world. Many of these have occurred over a long period and animals and plants have adapted to the
Ford Prodigy - Hybrid
changes naturally. However, the rapid burning vehicle of fossil fuels during this century has
increased carbon-dioxide levels in the atmosphere.
Vehicle designs are concentrating on weight reduction, aerodynamics, reducing rolling resistance, and on fuel-efficient engines. Alternative fuel sources to reduce fossil-fuel usage and to conserve the world’s stock of these fuels have also been developed.
Carbon Dioxide Carbon dioxide allows the sun's heat in, but reduces the ability of the heat to radiate outward, causing the Earth to warm up. Many studies of the warming process indicate that the rate of Earth warming is increasing too quickly and preventing animals and plants from adapting. During the history of the Earth, rapid changes like this have caused the extinction of some species of animals and plants.
Weather Patterns As a result of warming, weather patterns change. Arid areas become wet and wet areas become dry. Drought conditions become common in heavily populated areas and other areas suffer severe flooding. Because the distribution of populations and agricultural production are linked, they end up in the wrong climatic conditions. The consequences are severe shortages of water and poor agricultural production.
Ozone Layer A layer of ozone in the stratosphere filters harmful radiation. Ozone, or trioxygen (O 3 ), is a form of oxygen with 3 oxygen atoms. Vehicle emissions and other industrial chemicals, such as the CFCs used in refrigeration, air-conditioning and aerosols, rise up into the stratosphere and chemically combine with the ozone. This causes it to break down into less beneficial substances. The deterioration of the ozone layer allows an increase in the harmful radiation that
reaches the Earth’s surface, which can cause skin and other cancers.
Environmental Regulations Environmental regulations are now in place to find safer alternatives, or to reduce the production and use of the most harmful pollutants. Other regulations and agreements are seeking to reduce the production of carbon dioxide by improving the efficiency of fossil-fuel burners. For retaining the energy produced, improvements will also be introduced, such as the use of insulation and other methods.
Lead Lead has, until recently, been used as an additive in petrol in order to slow down the combustion process. This was to eliminate knocking or pinking in the engine. It made engines more efficient but the lead did not burn and was, instead, passed into the atmosphere from the exhaust and produced airborne concentrations that were capable of causing many physical disabilities, including brain damage.
For this reason, lead additives are no longer used and modern engines are now designed to run on lead-free fuel. There may
be a small portion of naturally occurring lead in some fuels but, because this is very low, the description ‘lead- free’ is more precisely a statement that lead additives have not been used.
Sulphur Another naturally occurring substance in fossil fuels, particularly diesel, is sulphur. This does not burn but, during combustion, chemically reacts with oxygen in the air to
form sulphur dioxide (SO 2 ). This passes from the engine exhaust into the atmosphere where it combines with water to form sulphuric acid (H 2 SO 4 ) and falls back to earth as acid rain, which destroys trees, plants, other vegetation and aquatic life in streams, rivers and lakes. Fuel suppliers remove, or reduce, the amount of sulphur during the refining process.
Nitrogen Oxides Air consists of approximately 80% nitrogen which, under normal circumstances, is an inert gas. An inert substance is one that has very little chemical reaction and does not burn, or mix easily, with other chemicals. Nitrogen, however, will mix with oxygen in high temperatures to form nitrogen oxides (NO x ). These combine in exceptional geographical and meteorological conditions to form smog, acids and increases in low-level ozone. This serves to make a very unpleasant atmosphere in which to live. Many respiratory and asthmatic fatalities occur under these conditions.
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ENGINE COMBUSTION
Combustion The combustion of fuel inside the engine is a chemical process that combines the carbon and hydrogen in the fuel with oxygen in order to release energy. Slightly less than 20% of air is made up from oxygen. Complete combustion
produces carbon dioxide (CO 2 ) and water (H 2 O). Neither of these is directly harmful. Both are naturally occurring substances in large concentrations in the atmosphere. However, carbon-dioxide concentrations are increasing and contributing to the greenhouse effect.
Incomplete Combustion Incomplete combustion leaves some of the carbon and oxygen not fully combined. The product of this is carbon monoxide (CO), which is toxic. Small quantities of carbon monoxide molecules are dangerous because they attach themselves to red blood cells. This reduces the oxygen that the cells normally carry around the body. The result is oxygen deprivation, brain damage and fatality.
Unburnt Fuel Another product of incomplete combustion is particles of fuel that have not been burnt. These are carried, with the exhaust gases, into the atmosphere and are called unburnt hydrocarbons (HC). Very small amounts of hydrocarbons in the atmosphere can cause respiratory problems.
Engine Oil Engine oil drawn into the combustion chamber, either from the inlet valve stem, or by bypassing the pistons, can also be sources of hydrocarbon pollution. Oil vapours form in the engine crankcase and can escape into the atmosphere. A positive crankcase ventilation system is now used to draw the vapours into the engine so that they are burnt to form water and carbon dioxide.
Evaporative Emissions Previously, vapour in the tank was directly vented to the atmosphere. This is no longer the case, but the fuel tank must still be vented to the atmosphere to allow air to flow into
Emission-control system the tank as fuel is used. A charcoal
filter is now used to prevent the loss of fuel vapour and for the expansion of the fuel when the weather is hot. The fuel vapour in the charcoal canister is drawn into the engine and burnt.
Air-Fuel Ratio Good fuel economy is obtained with a lean air-to-fuel mixture. However, this mixture produces higher combustion temperatures and greater risks of nitrogen oxides being formed. In order to prevent, or reduce to a minimum, the formation of nitrogen oxides, the combustion temperature has to be kept as cool as possible and the amount of oxygen limited to match the quantity of fuel delivered.
Exhaust Gas In order to reduce the amount of oxygen in the air charge, a gas that is low in oxygen can be introduced. This maintains the total air-charge mass to give good compression pressures and efficient operation of the engine. The available gas is the exhaust gas that has already used up its oxygen content during combustion. The addition of a regulated charge of exhaust gas reduces the oxygen content of the new charge to suit the amount of fuel delivered. This in turn reduces the combustion temperature and limits the formation of nitrogen oxides. The catalytic conversion of any remaining harmful gases can give a clean exhaust gas.
Explain what is meant by ‘incomplete combustion’.
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SYSTEMS DEVELOPMENT FOR ENVIRONMENTAL PROTECTION
Reducing Pollution Vehicle engine and component manufacturers have put a great deal of effort into reducing pollution. Lead is no longer needed in
Valves
petrol because other less-damaging substitutes have been found. The changes in the fuel have necessitated the use of hardened valves and valve seats and changes to the ignition timing and fuel- delivery systems.
Air-Intake Systems Air-intake systems have been developed from a simple ducting to a complex air- flow design adapting to the changing speed and load Air filter conditions of the engine. Filtration is also an important aspect.
Electronic Control Electronic control of the combustion process has achieved reductions in CO, NO x and HC emissions. Exhaust gases are
Lambda sensor
monitored in an electronic engine control module from signals sent from a lambda, or oxygen, sensor in the exhaust. This then allows fuel and air supplies to be accurately merged for near-perfect combustion.
Pollutant Control The remaining pollutants in the exhaust gases, which
cannot be controlled by the electronic Air injection Catalytic systems, can be converted into less- converter
harmful substances. This is achieved by using air injection into the exhaust and/or a catalytic converter.
Atomization Developments to improve the atomization and the mixing of the fuel in the incoming air stream include heating the inlet
Air
manifold, or heating the air as it enters the inlet temperature
control valve
manifold. This can be achieved with a heater element below the carburettor, or by preheating the air by ducting the air supply over the exhaust manifold.
Oil and Fuel Vapours Oil and fuel vapours are trapped and routed through the engine to be burnt. Positive crankcase ventilation and a charcoal filter EVAP canister in an evaporative canister are used for this purpose. Nitrogen-oxide formation is reduced with exhaust-gas recirculation (EGR).