It is seen that the quantity of heat given to the cylinder walls is considerable and if this heat is not removed from the cylinders it would result in the preignition of the charge. In addition, the lubricant would also burn away, thereby, causing the seizing of piston. Excess heating will also damage the cylinder material.
Keeping the above factors in view, it is observed that suitable means must be provided to dissipate the excess heat from the cylinder walls, so as to maintain the temperature below certain limits. The engine gives best performance at temperature from 70 to 85 degree Celsius, which is called the optimum temperature limit. In optimum temperature limit, the engine overhaul efficiency is highest.
Air Cooling: The basic principle involved in this method is to have the current of air flowing continuously over the heated metal surface from where the heat is to be removed.
The heat dissipation depends upon following factors:
1. Surface area of metal in contact with air.
2. Mass flow rate of air.
3. Temperature difference between the heated surface and air.
4. Conductivity of metal used for engine.
Use of copper, aluminum and steel alloys has been made to improve heat transfer
because of their better thermal conductivity.
Water Cooling: In water cooling, the engine cylinders are surrounded by water jackets through which the cooling water flows. Heat flows from the cylinder walls into water which goes to the radiator where it loses its heat to the air.
Water cooling systems are of two types:
(a) Thermosiphon System
(b) Pump Circulation System
In the modern automobiles, pump circulation system is employed because in this system the required size of the radiator is much smaller than the thermosiphon system and the cooling is also faster.
Other Methods of Cylinder Cooling
Apart from the two widely used methods, i.e. air cooling and water cooling as discussed above, various other methods have been used to increase the cooling efficiency according to the need of the circumstances. The methods are:
1.Liquid cooling: Sometimes instead of water, other liquids having higher boiling points are used for engine cooling. The examples are glycerin (boiling point 290°C) and ethylene glycol (b.p. 195°C). Higher boiling point increases the capacity of the liquid to carry heat and consequently weight of the coolant and hence that of the radiator is decreased.
2.Pressure sealed cooling: The boiling point of water is raised at higher pressures, with consequent reduction in coolant and the radiator weights and greater thermal efficiency of the engine due to higher operating temperatures. For approximately 10 kN/m2 increase in pressure, the boiling point is raised by 2.5°C. To achieve this special pressure, a sealed radiator cap is used. The pressure blow off valve opens when the pressure in the system exceeds a certain predetermined value, say 50 kN/m2. A vacuum valve is provided to
safeguard against the radiator collapse due to vacuum if the steam inside suddenly condenses to form a vacuum.
In some engines, instead of an overflow pipe, an expansion reservoir is provided. This is so connected with the radiator that it receives the excess cooling water as the engine temperature and hence the cooling water temperature increases. When the cooling water cools down, its volume decreases and the cooling water in the reservoir returns to the radiator. The reservoir is usually made of translucent plastic so that it can indicate the level of the coolant anytime.
Advantages of Pressure sealed cooling system
- There is no loss of coolant.
- Cooling efficiency of the system is not decreased at high altitudes, where due to low temperature pressure, the boiling point is otherwise low.
Components of Cooling System
The cooling system is made up of the passages inside the engine block and heads, a water
pump to circulate the coolant, a thermostat to control the temperature of the coolant, a radiator
to cool the coolant, a radiator cap to control the pressure in the system, and some plumbing,
consisting of interconnecting hoses to transfer the coolant from the engine to radiator.
- Pressure Cap & Reserve Tank
- Head Gaskets & Intake Manifold Gaskets
cooling system work?
A cooling system works by sending a liquid coolant through passages in the engine block and heads. As the coolant flows through these passages, it picks up heat from the engine. The heated fluid then makes its way through a rubber hose to the radiator in front of the car. As it flows through the thin tubes in the radiator, the hot liquid is cooled by the air stream entering the engine compartment from the grill in front of the car. Once the fluid is cooled, it returns to the engine to absorb more heat. The water pump has the job of keeping the fluid moving through this system of plumbing and hidden passages.
A thermostat is placed between the engine and the radiator to make sure that the coolant stays above a certain preset temperature. If the coolant temperature falls below this temperature, the thermostat blocks the coolant flow to the radiator, forcing the fluid instead through a bypass directly back to the engine. The coolant will continue to circulate like this until it reaches the design temperature, at which point, the thermostat will open a valve and allow the coolant back
through the radiator.
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