Session 5 : Cooling System

All the heat produced by the combustion of fuel in the engine, is not converted into useful power at the crankshaft.
Only 20% of the heat energy is used to make the crankshaft rotate. About 35% of the heat is dissipated in the exhaust process.

If this heat is not removed from the cylinders, it would end up in the pre-ignition of the charge (fuel-air mixture).

Furthermore, the excess heat would burn away the Lubricating oil, and damage the cylinder material.

The purpose of the cooling system is, to dissipate the excess heat from the engine, and maintain an optimum temperature.
At an optimum temperature of 70°- 85°, the engine gives the best performance,and the overall efficiency will be high.

Methods of Cooling

Air cooling

Principle of Air cooling is, to allow the current of air, flowing consistently over the hot metal surface, from where the heat is to be removed.

The rate of 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.

Copper, aluminum and steel alloys are used, to improve heat transfer, because of their better thermal conductivity.

For better air-cooling, the outer surface of the engine is, provided with fins over the cylinder barrels.

Higher the temperature difference, between the heated surface and the air, higher will be the heat dissipation.
A metal having better conductivity dissipates more amount of heat.

Air cooled system is generally used in small engines, like motorcycles, scooters and small aeroplanes.

Water Cooling System

Engine block

1.Water cooling system works by sending a liquid coolant, through water-jackets in the engine block and heads.

2.As the coolant flows through these passages, it picks up heat from the engine.

3.Then heated coolant flows through a rubber hose, to the radiator in front of the car.

4.As the hot coolant flows through the thin tubes in the radiator, it is cooled by the air flowing through the grill in front of the car.

5.Once the fluid is cooled, it returns back to the engine to absorb more heat.

6.The water pump keeps the fluid flowing, throughout this water cooling system.

Radiator

Thermostat in the cooling system

A thermostat is placed between the engine and the radiator, to ensure the coolant stays above a preset temperature.
If the coolant temperature is low, the thermostat blocks the flow to the radiator, bypassing the liquid directly back to the engine.
The coolant will continue to circulate like this, until it reaches the preset temperature.
Once the preset temperature attained, the thermostat will open a valve and allow the coolant back through the radiator.

Pressure sealed cooling

With the increase in pressure, the boiling point of water is raised by 2.5°C.
To achieve this special pressure, a sealed radiator cap is used.
When the pressure exceeds a certain limit it blows off valve, and releases the water into an expansion reservoir.
When the radiator water cools down, its volume reduces and the water in the reservoir returns again to the radiator.
The reservoir is usually made of translucent plastic, so that the level of the coolant is visible anytime.
The advantage of Pressure sealed cooling is that, there is no loss of coolant.
Even at high altitudes where the pressure is low, the cooling efficiency is maintained.