In this cooling system water is used to remove the heat from the engine to atmosphere.

Two types of Water cooling system are adopted.

  1. Thermo – Syphon system and
  2. Pump assisted water cooling system

1) Thermo – Syphon system

Thermo – Syphon system is as shown in Figure. This system uses the natural circulation of water through the water passage around the engine block. When the engine cooling water gets heat from the engine body, due to heat, water expands and occupies large volume. So the density is reduced and it flows to the top of the radiator. The hot water from the top of the radiator passes down the radiator tubes where it is cooled by air passing through the radiator grill. As the water is cooled the density of water increases. This helps the water to flow down easily. On reaching the bottom tank its temperature

is less and it is ready to enter the cylinder and cylinder head again. So the cooler denser water from the bottom of the radiator moves into the regions vacated by the hot rising water.

Advantages & disadvantages of Thermo Syphon cooling system

  1. The speed of circulation depends on water temperature. The hotter the water, the faster it circulates.
  2. It is cheap and reliable and has no moving parts. The size of connection becomes large if engine size is large. So this system is no longer used
  3. Warm up is fairly quick.
  4. Circulation is proportional to engine load.

Thermo-Syphon System

2) Pump assisted water cooling system

In this cooling system, water is circulated by a pump driven by the crankshaft The forced circulation gives faster cooling. It consists of a radiator, cooling fan, water pump and a thermostat. Hoses are used to connect the radiator with engine.

Pump assisted water cooling system


The radiator transfers the heat absorbed by the water to the surrounding air. The radiator is flat and rectangular in shape. It consists of tubes which are made of copper or aluminum alloy. These tubes expose large surface area to the air stream. Fins are attached to these tubes. These fins improve the effectiveness of air-convection heat dissipation.

Cooling Fan

A fan is positioned between the radiator and the engine driven by engine crank shaft.

Fan is to increase the normal air movement across the radiator core tubes. The resultant air movement will provide a continuous air stream over the tubes and fins to dissipate the heat from the hot water which is circulated.


The function of the thermostat is to regulate heat dissipation by controlling the rate of water flow through the radiator. Normally engines are designed to operate most efficiently over a temperature range between 80°C and 100°C. Thermostat is placed in the outlet of cooling water from the engine. The thermostat has a valve, which opens only when the normal operating temperature of about 60 C is reached by the water. The water flows from engine to upper part of the radiator through thermostat.

Water pump

A centrifugal pump is used to speed up the rate of water circulation. Pump cooling is the most commonly used in almost all cars for positive flow of water.

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