How does the engine Water Cooling System work?


In this engine Water Cooling System, water is circulated through water jackets around each of the combustion chambers, cylinders, valve seats, and valve stems. The circulating water then passes through the engine jackets in the block and cylinder head, takes the heat of the combustion. When it passes through the radiator, it is cooled by air drawn through the radiator by a fan and by airflow developed by the forward motion of the vehicle. After passing through the radiator, the water again goes in the engine jacket.


There are two systems of water cooling:

  1. Thermosiphon system. In this system of water cooling, the circulation of water is obtained due to the difference in densities of hot and cold regions of the cooling water. There is no pump to circulate the water. The hot water from the engine jacket being lighter rises up in the hose pipe and goes in the radiator from the top side. It is cooled there and hence goes down at the bottom side of the radiator, from where it goes again in the engine jackets. The system is quite simple and cheap, but the cooling is rather slow. To maintain continuity of the water flow, the water must be maintained up to a cert minimum level. If the water level falls down, the circulation will discontinue and the cooling system will fail.
  2. Pump circulation system. In this system of water cooling, the circulation of water is obtained by a pump. The pump is driven by means of a V-belt from a pulley on the engine crankshaft. The system is more effective. The circulation of water becomes faster as the engine speed increases. There is no necessity of maintaining the water up to the correct level.  
engine Water Cooling System
engine Water Cooling System


  1. Radiator.
  2. Thermostat.
  3. Pump.
  4. Fan.
  5. Water Jackets.


The radiator is a device for having a large amount of cooling surface to a large amount of air so that the water circulating through it is cooled efficiently in the engine water cooling system. It consists of an upper tank and a lower tank and between them a core. The upper tank is connected to the water outlet or outlets from the engine jacket by a hosepipe, and the lower tank is connected to the jacket inlet through the water pump. The core is a radiating element, which cools the water.

There are two basic types of radiator cores-tubular type and cellular type. In tubular type core, the upper and lower tanks are connected by a series of tubes through which water passes. Fins are placed around the tubes to improve heat transfer Air passes around the outside of the tubes, between the fins, absórbing heat from the water in passing. In cellular type core, air passes through the tubes and the water flows in the spaces between them. The core is composed of a large number of individual air cells that are surrounded by water. Because of its appearance, the cellular type is usually known as a honeycomb radiator, especially when the cells are hexagonal in form.


In a tubular radiator, because the water passes through all the tubes, if one tube becomes clogged, the cooling effect of the entire tube is lost. In a cellular radiator, the clogging of any passage results in a loss but of a small part of the total cooling surface.

Radiators are also classified according to the direction of the water flow through them. In some, the water flows from top to bottom-down flow type radiators. In other, the water flows horizontally from an input tank on one side to another tank on the other side-cross flow type radiator.

Radiators are usually made of copper and brass because of their high heat conductivity. The various sections of the radiator are almost completely joined together by soldering.


A thermostat valve is used in the water cooling system to regulate the circulation of water in the system to maintain the normal working temperature of the engine parts during the different operating conditions. The thermostat valve automatically works in the cooling system. When the engine is started from cold. the thermostat valve prevents the flow of water from the engine to the radiator so that the engine readily reaches its normal working temperature, after which it automatically comes into action. Generally, the thermostat valve does not permit the water below 70°C. Fig. 19.5 shows the water cooling system using the thermostat valve.

Two types of thermostats are used in automobile vehicles: Bellows type and Pellet type. All thermostats work on the same general principle-a heat unit operating a valve.

Thermostat valve
Thermostat valve

In bellows type thermostat, shown in Figure the heating unit consists of closed bellows filled with a volatile liquid under reduced pressure. When the bellows are heated, the liquid vaporizes and creates enough pressure to expand the bellows. The movement of the bellows operates a linkage that opens the valve to pass the water through it. When the bellows are cooled, the gas condenses, the pressure is reduced and the bellows collapse to close the valve, thus stopping the water circulation.

In the pellet thermostat, the heating unit consists of a sealed wax pallet, which expands on heating and contracts on cooling. The pellet is connected by piston and flange to a valve so that on the expansion of the pellet, it opens the valve. When the pellet contracts on cooling, a coil spring closes the valve.


A pump is used in the water cooling system to increase the velocity of the circulating water. Impeller type pump is mounted at the front end of the cylinder block between the block and the radiator. The pump consists of a housing with inlet and outlet; and an impeller. The impeller is a flat plate mounted on the pump shaft with a series of flat or curved blades or vanes. When the impeller rotates, the water between the blades is thrown outwards by centrifugal force; and is forced through the pump outlet and into the bottom of the radiator and the water from the radiator is drawn into the pump to replace the water forced through the outlet.

Water Pump
Water Pump

the engine crankshaft, The pump is driven by a belt to the drive pulley mounted on the front end of the engine crankshaft. The impeller shaft is supported on one or more bearings. A seal prevents water from leaking out around the bearing. The packless type pump is used in a modern engine. The packing gland type pump is found only in old models


A fan is mounted behind the radiator on the water pump shaft. It is driven by the same belt that drives the pump and the generator. The purpose of the fan is to draw air through the radiator. When the vehicle is going at high speed, the natural draft of air passing through the radiator is sufficient for cooling, but when the engine is going at low speed, or upside a hill, the natural draft is certainly insufficient to produce the desired cooling effect. Here the fan serves the purpose.

The fan requires a little power and usually runs at a speed greater than that of the crankshaft. Many engines use a variable speed fan drive which reduces fan speed to conserve power at high engine speed and also when cooling requirements are low. It contains a small fluid coupling partly filled with special silicon oil. When engine cooling requirements are severe, as during high temperature, high-speed operation, more oil is injected into the fluid coupling. This causes more power to pass through the coupling. Fan
speed, therefore, goes up. When cooling requirements are low, as, during cool weather, intermediate speed operation oil is withdrawn from the fluid coupling so that less power passes through and fan speed goes down.

Cooling fan
Cooling fan

Flex fan consists of flexible blades. The pitch of the blades decreases as the fan speed increases, owing to centrifugal force. With this result, each blade pushes less air and thus power needs and noise are less at higher speeds.

In some cooling systems, a thermostatically controlled power booster fan is used. It is driven through a liquid clutch that slips after reaching a set speed. It is designed to limit fan speed to the lower ranges ln order to reduce engine noise.


Water jackets are cast into the cylinder block and heads. Jackets are simply the passages through which water circulates around the cylinders, valve ports and seats, combustion chambers and any other hot parts that require cooling. The heat of combustion is conducted through the metal walls to the water in the jackets which removes the excess heat as it circulates through them. The figure shows water jackets in the L-head engine.

Watch the animation video below to know how the water Cooling System of a vehicle works?

Also, Read

  1. How does the Engine Cooling System work? Methods of Engine Cooling System.
  2. How Does An Engine Air Cooling System work?

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