- Radiator
- Coolant hoses
- Water pump
- Cooling system thermostat
- Cooling fans
- Temperature indicators
- Radiator pressure cap
- Recovery system
- Boiling point & pressure
- Electrolysis
- Centrifugal force
- Thermo-switch
Radiator
Many radiators are mounted at the front of the vehicle in the path of greatest airflow. The air carries heat away, cooling the liquid before it returns to absorb more heat from the engine.
Where a radiator is mounted also depends on space - how the engine is mounted. A header tank can be mounted away from the radiator, where it provides a coolant supply, stored above the engine. It can be made of sheet metal, or hardened plastic.
The radiator has 2 tanks and a core.
The materials used in the radiator must be good heat conductors like brass or copper. Brass and copper are often used for tanks, combined with a copper core.
Modern vehicles often use plastic tanks combined with an aluminium core. This saves weight but still provides good heat transfer.
The core consists of a number of tubes that carry coolant between the 2 tanks. The tubes can be in a vertical downflow pattern, or a horizontal crossflow pattern.
A crossflow radiator fits more easily under a steeply sloped bonnet.
In the core, small, thin, cooling fins are in contact with the tubes. The shape of the fins increases the surface area exposed to the air.
Where coolant touches tube walls, and where the tubes touch the fins, heat is removed from the coolant by conduction, then by radiation and convection at the surface of the fins. Air rushing by carries the heat away.
Liquid emerges cooler at the bottom of the radiator. It travels through the lower radiator hose to the water pump inlet, then through the engine again.
Coolant hoses
Coolant is transferred throughout the cooling system by hoses.
Most vehicles have the engine mounted on flexible mountings to reduce noise and vibration. Since the radiator is mounted to the vehicle body, flexible hoses are needed. Coolant is also carried to the heating system which is usually inside the cabin of the vehicle.
Coolant hoses vary in diameter depending on the volume of coolant that passes through them. Heater hoses carry a smaller volume.
Most hoses are made of rubber, and since they are subject to pressure, they are reinforced with a layer of fabric. They are moulded to a special shape to suit the model and make of vehicle. Some heater coolant hoses also have special shapes. All hoses are subject to hot coolant and high under-bonnet temperatures, and they can deteriorate and fail.
Water pump
The water pump is usually in front of the cylinder block, belt-driven from a pulley, on the front of the crankshaft. A hose connects it to the bottom of the radiator where the cooler liquid emerges.
It has fan-like blades on a rotor or impeller. Coolant enters the center of the pump. The rotor spins, and centrifugal force moves the liquid outward. It is driven through the outlet into the cooling passages called waterjackets.
Waterjackets are passages in the engine block and cylinder head that surround the cylinders, valves and ports.
Coolant can be also directed to hot spots such as the exhaust ports in the cylinder head, to stop local overheating.
Cooling system thermostat
The thermostat helps an engine to warm up. It’s found in different positions on different engines.
It is a valve that operates according to coolant temperature. When coolant is cold, a spring holds the valve closed.
When a cold engine starts, coolant circulates within the engine block and cylinder head and through a coolant bypass to the water pump inlet. It can’t get to the radiator.
As the engine warms up, the coolant in the engine gets hotter and hotter.
This thermostat has a wax-like substance that expands as the engine nears its operating temperature. This starts to open the valve. Coolant starts to flow to the radiator.
Thermostats have a small hole or valve to let out air that was trapped in the engine block.
Heated coolant is pumped from an outlet in the cylinder head. It goes into the upper radiator hose, then to the radiator.
Cooling fans
In a vehicle moving at high speed, airflow through the radiator cools the coolant, but at low speed or when the engine is idling, extra airflow comes from a fan.
Fans can be driven in different ways. More and more modern vehicles now use an electric fan. Air-conditioned cars often have extra fans.
Electric fans can be behind the radiator, in front, or both. This arrangement would be difficult with a belt-driven fan. Some fans can be driven from the crankshaft.
When an engine is mounted longitudinally, its fan is usually mounted on the water pump shaft. The drive belt then turns the water pump and fan. Some use a hydraulic link from the power steering system.
Fan blades can be rigid or flexible. Rigid blades tend to be noisy and use more energy. This noise can be reduced by using irregular spacing of the fan blades.
Some vehicles use a shroud to direct all of the air that the fan moves, through the radiator core. At high speeds, plenty of air is already flowing through the radiator. If the fan is always working at full speed, it’s a waste of energy. And since the engine drives the fan, it’s a waste of fuel too. What’s needed is some way to control the fan. A heat-sensitive switch in contact with the coolant can work like a thermostat, and turn the fan on and off according to coolant temperature.
Another way to alter the speed of the fan is with a viscous hub. This type of fan slips when it is cold, but as the engine heats up, it grips more and more.
Temperature indicators
Overheating can seriously damage an engine, so having warning of trouble is obviously useful.
A device that’s sensitive to engine temperature sends readings to a temperature gauge or a warning lamp. To give an accurate reading this sensor must always be immersed in liquid.
Indicators that measure coolant levels can give warning if the level falls too low.
Radiator pressure cap
If a coolant boils, it can be as serious for an engine as having it freeze.
Boiling coolant in the waterjacket becomes a vapor. No liquid is left in contact with the cylinder walls or head. Heat transfer by conduction stops. Heat builds up,
And that can cause serious damage.
One way to prevent this is with a radiator-pressure cap that uses pressure to change the temperature at which water boils.
As coolant temperature rises, the coolant expands and pressure in the radiator rises, and that lifts the boiling point of the water.
Engine temperature keeps rising, and the coolant expands further. Pressure builds against a spring-loaded valve in the radiator cap until at a preset pressure, the valve opens.
In a recovery system, the hot coolant flows out into an overflow container.
As the engine cools, coolant contracts and pressure in the radiator drops. Atmospheric pressure in the overflow container then opens a second valve, a vacuum vent valve, and overflow coolant flows back into the radiator.
This system stops low pressure developing in the radiator, and that stops atmospheric pressure collapsing the radiator hoses.
Recovery system
A recovery system maintains coolant in the system at all times.
As engine temperature rises, coolant expands. Pressure builds against a valve in the radiator cap until, at a preset pressure, the valve opens. Hot coolant flows out into an overflow container.
As the engine cools, coolant contracts and pressure in the radiator drops. Atmospheric pressure in the overflow container opens a second valve, and overflow coolant flows back into the radiator.
No coolant is lost and excess air is kept out of the system. Like water, air contains oxygen which reacts with metals to form corrosion.
Boiling point & pressure
Water at atmospheric pressure at sea level boils at 100 degrees Celsius or 212 degrees Fahrneheit. That is it's 'boiling point'.
If the water is put under pressure, higher than atmospheric pressure, it boils at a higher temperature.
If the pressure is decreased below sea level atmospheric pressure, it boils at a lower temperature.
Therefore, raising pressure above atmospheric pressure increases the boiling point. Lowering it below atmospheric pressure lowers the boiling point.
Changing water pressure changes the temperature at which it boils.
Electrolysis
Electrolysis is also a method of epilation. In chemistry and manufacturing, electrolysis is a method of separating bonded elements and compounds by passing an electric current through them.
Electrolysis is a chemical and electrical process. It occurs when two different metals are in contact, in the presence of a moist agent such as water. One of the metals is corroded away.
This can occur even in pure water. Immersed in this water are 2 plates, one of aluminium alloy, the other of cast iron. The atomic structure of aluminium means it loses electrons easily, leaving behind aluminium ions which are positively charged. Negative oxygen ions in the water are then attracted to the aluminium ions, and they join, to form deposits of aluminium oxide. As a result the aluminium alloy is eaten away, or corroded.
Overview
The source material is dissolved in an appropriate solvent, or melted, so that constituent ions are available in the solution. An electrical potential is applied across a pair of conductors immersed in the liquid. The negatively charged conductor is called the cathode, and the positively charged conductor is called the anode. Each conductor attracts the ions of the opposite charge. Therefore, positively charged ions (cations) move towards the cathode while negatively charged ions (anions) move to the anode. The energy required to separate the ions, and increase their concentration at the electrodes, is provided by an electrical power supply that maintains the potential difference across the electrodes. At the electrodes, electrons are absorbed or released by the ions, forming concentrations of the desired element or compound. For example, when water is electrolyzed, hydrogen gas (H2) will form at the cathode, and oxygen gas (O2) at the anode. This was first discovered by William Nicholson, an English chemist, in 1800.
The amount of electric energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. The losses can (theoretically) be arbitrarily close to zero, so the maximum thermodynamic efficiency equals the enthalpy change divided by the free energy change of the reaction. In most cases the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance in the electrolysis of steam into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the heating value of the produced hydrogen is higher than the electric input. In this case the efficiency can be said to be greater than 100%. (It is worth noting that the maximum theoretic efficiency of a fuel cell is the inverse of that of electrolysis. It is thus impossible to create a perpetual motion machine by combining the two processes.)
The following technologies are related to electrolysis:
- Electrochemical cells, including the hydrogen fuel cell, use the reverse of this process.
- Gel electrophoresis is an electrolysis where the solvent is a gel: it is used to separate substances, such as DNA strands, based on their electrical charge.
Centrifugal force
Centrifugal force is a force pulling outward on a rotating body.
A vehicle turning a curve is a similar system to this rotating body, so it is subject to centrifugal force too. Centrifugal force resists turning, and tries to keep the vehicle moving in a straight line. Centrifugal force is also the force that causes an out-of-balance wheel to vibrate.
Centrifugal force can also be useful. When coolant enters the center of this pump, and the rotor spins, centrifugal force moves the liquid outward.
Thermo-switch
A thermo-switch opens and closes according to pre-set temperature levels. Some are mechanical, others are electrical. It may be designed to switch off when temperature rises above a certain level, or it can be made to switch on, when the temperature reaches a certain level.
Heat switches can operate on the bimetallic strip principle. It consists of two different metals or alloys attached back-to-back. As different metals and alloys heat and cool, they expand, and contract, differently. That means that if they are joined, and then heated, the faster expansion of one will force the whole strip into a curved shape.
As the strip changes shape, it can be designed to complete a circuit, and a resulting electrical signal can then do a range of tasks, or it might have a mechanical effect, simply opening a passageway.
Cooling then produces the opposite effect. Breaking the circuit, and closing the passage.
