Cooling System of Engine

The purpose of cooling system is to remove the heat and keep the engine at an appropriate temperature, no matter under what conditions the automobile is running. A large amount of heat is generated with the burning of air-fuel mixture in the engine cylinder. It has been experimentally found that about 30% of the heat generated is converted into mechanical work. Out of the remaining heat (about 70%) about 40% is carried away by the exhaust gases into the atmosphere. The remaining part of the heat (about 30%), if not removed by any other means, can cause serious troubles to the engine. The following two types of cooling system are used in automobile engines:

1. Air cooling system

This type of cooling system is mostly employed in light engines such as in motor cycles and scooters. The air-cooled engines contain fins or ribs on the outer surfaces of the cylinders and cylinder heads. These fins provide more area for air contact, resulting in better radiation of heat.

2. Liquid or water cooling

This type of cooling system consists of a radiator, water pump, water jacket, thermostat, fan and other components. Each cylinder of the engine is surrounded by a water jacket. While the engine is running, the heat generated in each cylinder is conducted through the cylinder wall and into the coolant, which is circulated inside the water jacket by the water pump. The heated coolant is then fed to the radiator, where it is cooled by air that passes through an array of cooling fins. Then, the coolant is recirculated to the engine.

A thermostat is fitted between the engine and radiator. When the engine is started from cold, the thermostat closes to prevent coolant from entering the radiator until the engine has warmed up. It may be noted that the coolant circulates only around the engine when the thermostat is closed. In case of non-closure of the thermostat, the engine would take an excessively long time to reach its optimum temperature.

It may be noted that the water is generally used as coolant to absorb and transfer heat in order to prevent overheating of the engine. Though water is a satisfactory liquid to use for absorbing and transferring heat, yet it has several drawbacks. The following are the drawbacks of using water as a coolant medium.

• It has a relatively low boiling point and freezes rapidly, thus it cannot flow around the cooling system. Furthermore, freezing increases the volume of water and this may rupture the cylinder block or other cooling system parts.
• It causes rust and scale formation inside the cooling system.

For the above reasons, some chemicals known as anti-freeze solutions are mixed with water. Ethylene glycol is considered the best anti-freeze. When mixed with water, it lowers the freezing point of water below 0°C and increases the boiling point of water above 100°C. Some inhibitors and additives are also added to the coolant (mixture) to prevent the buildup of rust and scale.