Effects of under cooling and over cooling:
Engine should be cooled within a particular temperature limits. It should not be too cooled or too heated up. Getting the engine too much cooled is called overcooling. And if the engine is over heated it is to under cool. Both under cooling and over cooling have individual disadvantages. Over cooling results in the increase of viscosity of the lubricating oil, which in turn result in the increase of friction between the moving parts.
If the engine gets warmed up excessively, it should be cooled so as to keep the correct alignment of the engine. Under cooling also keeps the engine in correct position and increase the life of the engine. Evaporation of lubricating oil that lubricates the piston and cylinder wall is also another reason of under cooling. This will result in metal to metal contact of the piston and cylinder wall leading to piston crown. Burning of and warping of exhaust valves setting up of thermal stresses in the cylinder, cylinder head and piston. This may lead to cracking of them.
Evaporative cooling system:
In this system, the engine will be cooled because of the evaporation of the water in the cylinder jackets into steams.
The principle of this system is being taken from the high latent heat of vaporization of water by allowing it to evaporate in the cylinder jackets. This system is used for cooling of many types of industrial engines.
Working: the water coming out of engine jacket is allowed to be heated to 100oC. this method of cooling utilizes the latent heat of water to obtain cooling with minimum water. The cooling circuit is such that coolant always maintained in liquid form but the steam formed is flashed off in a separate vessel as shown in Fig. 4.1 this system used for big-capacity stationary engines used for power generator.
Advantages of evaporative cooling system:
i) The specific fuel consumption of water-cooled engine is less than that of the air-cooled engine.
ii) Uniform cooling of valve, cylinder head and cylinder is possible with water cooling.
iii) It is necessary to keep the engine at the front for the mobile vehicles and air craft engines for air-cooling but this is not at all necessary with water-cooled engines. The cooling system can be located conveniently at required place.
iv) As far as design of cooling system is concerned, the engine size does not involve any serious problem.
v) Compact engine design with minimum frontal area as possible.
Disadvantages of evaporative cooling system:
i) It is dependent on supply of water.
ii) Serious damage may be caused to the engine within a short period of time in the case of failure of the cooling system.
iii) The pump requires considerable power compared with the power required for the fan.
iv) Initial and maintenance costs are higher.
v) Freeze of water in very cold areas becomes a problem.