Engine Cooling | Air Cooling System – Advantages and Disadvantages
In a SI engine, cooling must be satisfactory to avoid pre-ignition and knock. In a compression ignition engine, since a normal combustion is aided, cooling must be sufficient to allow the parts to operate properly. In short, cooling is a matter of equalization of internal temperature to prevent local overheating as well as to remove sufficient heat energy to maintain a practical overall working temperature.
Requirements of cooling system in the IC engine
The cooling system is provided in the IC engine for the following reasons:
-The temperature of the burning gases in the engine cylinder reaches up to 1500 to 2000°C, which is above the melting point of the material of the cylinder body and head of the engine. (Platinum, a metal which has one of the highest melting points, melts at 1750 °C, iron at 1530°C and aluminium at 657°C.) Therefore, if the heat is not dissipated, it would result in the failure of the cylinder material.
-Due to very high temperatures, the film of the lubricating oil will get oxidized, thus producing carbon deposits on the surface. This will result in piston seizure.
-Due to overheating, large temperature differences may lead to a distortion of the engine components due to the thermal stresses set up. This makes it necessary for, the temperature variation to be kept to a minimum.
-Higher temperatures also lower the volumetric efficiency of the engine.
Effect of overcooling:
-Thermal efficiency is decreased due to more loss of heat carried by the coolant
-The vaporization of the fuel is less resulting in lower combustion efficiency
-Low temperature increases the viscosity of lubricant causing more loss due to friction
There are mainly two types of cooling systems:
(a) Air cooled system, and
(b) Water cooled system
Air Cooled System:
-Air cooled system is generally used in small engines say up to 15-20 kW and in aero plane engines.
-In this system fins or extended surfaces are provided on the cylinder walls, cylinder head, etc. Heat generated due to combustion in the engine cylinder will be conducted to the fins and when the air flows over the fins, heat will be dissipated to air.
-The amount of heat dissipated to air depends upon:
(a) Amount of air flowing through the fins
(b) Fin surface area
(c) Thermal conductivity of metal used for fins
-For efficient cooling the length of the fins and the spacing between them is quite important
-Larger inter spacing between the fins offers larger area for cooling air but the heating of the air is less, so more cooling air is required
-Smaller inter spacing between the fins results in smaller flow area of cooling air and hence input cooling air is less
-Usually fin height varies from 15 to 25 mm
Advantages of air cooled engines
Air cooled engines have the following advantages:
1. Its design of air-cooled engine is simple.
2. It is lighter in weight than water-cooled engines due to the absence of water jackets, radiator, circulating pump and the weight of the cooling water.
3. It is cheaper to manufacture.
4. It needs less care and maintenance.
5. This system of cooling is particularly advantageous where there are extreme climatic conditions in the arctic or where there is scarcity of water as in deserts.
6. No risk of damage from frost, such as cracking of cylinder jackets or radiator water tubes.
Disadvantages of air cooled engines
-Relatively large amount of power is used to drive the cooling fan.
-Engines give low power output.
-Cooling fins under certain conditions may vibrate and amplify the noise level.
-Cooling is not uniform.
-Engines are subjected to high working temperature.
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