Bellows or aneroid type thermostat.

--Technical Videos--Thermostat is the most significant part of the cooling system of an automobile. It is located on the engine and acts as a gate (valve) between the engine cooling galleries (for coolant) and the radiator.

What is the purpose of cooling the engine?

The engine is made of metals or alloys, so as the engine temperature rises while working metals or alloys tend to expand beyond a particular limit of temperature (this result in the change in dimension of the engine resulting in failure). To avoid this cooling is required.
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Why a thermostat is used?

There is an optimum temperature at which engine can produce maximum efficiency (ranges between 75-85 degree Celsius). So only after engine reaching this particulate temperature cooling need to occur, to ensure this the thermostat is used. Thermostat allows the coolant to pass through the engine galleries only after the engine has attained the temperature range of (75-85 degree Celsius) which varies for engine to engine.

Two types of thermostats are used in automobiles:--Technical Videos--

1. Bellows or aneroid type.
2. Wax or hydrostatic type.

Fig: 1.

Main components of a Bellows type thermostat:

1. Casing.
2. Metallic bellows.
3. Valve

Casing: Body of the thermostat.

Metallic bellows: as shown in fig: this is the contained in the thermostat casing in which the volatile liquid is stored

Valve: this is the thermostat valve as shown on fig: 1 , as shown in fig:2 this fits to coolant (water) hose.

Working of a “Bellows type thermostat”:

The fig: 1 shows a Bellows type thermostat which is filled with some type of volatile liquid like acetone or alcohol etc which boils at 75-85 degree Celsius. A valve is at the top of the thermostat which is connected to the radiator cooling hose and the bellows to the frame which fits to the cooling passage of engine as shown in fig: 2.

Fig: 2.

So as when the engine starts the thermostat valve remains closed, so the coolant from the radiator will not pass through the engine galleries, which helps the engine to warm up faster. As engine attains a temperature range of 75-85 degree Celsius, the volatile liquid in the bellows vaporises and exerts a pressure on the valve. This results in the valve opening. As the thermostat valve opens the engine cooling galleries get connected to the radiator and the coolant (water) starts to circulate and the cooling occurs. --Technical Videos--