Engine Thermostat


Thermostats regulate the temperature of the engine cooling water (jacket water) when the engine is in operation. The thermostat allows the engine to heat-up quickly when cold and maintain a desired temperature once the desired temperature has been reached. Thermostats are installed on many water cooled internal combustion (IC) engines, both four strokes and two strokes.

Engine Thermostat

Engine Thermostat


Thermostat Components

Main Valve / Primary Valve

The main valve controls the flow of engine cooling water to the radiator. As the cooling water temperature increases, the valve gradually opens and discharges more cooling water to the radiator, where it is cooled.

Engine Thermostat Components

Engine Thermostat Components

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By-Pass Valve / Secondary Valve

When the by-pass valve is open, cooling water by-passes the engine radiator and circulates within a closed circuit. As the engine cooling water temperature increases, the by-pass valve begins to close and cooling water is directed towards the radiator. The engine would quickly overheat if cooling water was not diverted to the radiator for cooling.

Charge Cylinder

The charge cylinder is full of wax which begins to melt (change from solid to liquid state) as the engine cooling water temperature increases. Melting of the wax causes the by-pass valve to close and the main valve to open; this causes cooling water to be directed towards the radiator. When the cooling water temperature is lower i.e. when the engine is cold, the wax is in a solid state and the radiator by-pass valve is open.

Seal / Gasket

A seal is used to prevent any unwanted flow past the thermostat main valve. The seat where the seal sits must be clean in order for the seal to function correctly.


A spring returns the main valve to the closed position once the cooling water temperature decreases below a certain point.

Secondary Spring

As the cooling water system temperature decreases, the charge cylinder wax changes state and a rod contracts into the charge cylinder. The secondary spring allows the rod to retract to a defined point, after which the spring's residual stress prevents the rod retracting further.

Air Bleed

The air bleed is used to remove air (bleed air) from the cooling water system. It should be ensured that the air bleed is rotated to the highest position possible when the thermostat is installed as this is where the air will gather. Not all thermostats have an air bleed.


How do engine thermostats work?

The below video is an extract from our Internal Combustion Engine Basics Online Video Course.

The thermostat consists of a primary valve, secondary valve (by-pass valve), seal, springs and charge cylinder. The charge cylinder is full of wax with a pre-defined melting point (the point at which the wax changes from solid to liquid). The wax changes state from solid to liquid at approximately 80°C in most engine cooling water systems.

After the change of state within the charge cylinder, the liquid wax occupies more space than when it was in a solid state. A rod inserted into the charge cylinder is pushed outwards linearly as there is now no longer space for the rod within the charge cylinder. The rod pushes the by-pass valve closed whilst at the same time opening the main valve. The temperature of the engine thus controls the movement of the rod and consequently the amount of cooling water sent to the radiator, or bypassing the radiator.

Car Radiator

Car Radiator

The thermostat has a proportional response design. The temperature of the cooling water dictates how much the thermostat opens or closes, and if cooling water is sent to the radiator, or, bypasses the radiator; this is effectively a feedback loop.

Hot cooling water temperature (>80°C) = Cooling water sent to radiator.

Cold cooling water temperature (<80°C) = Cooling water recirculated (radiator bypassed)


 Want to learn more? Click here to learn about the engine cooling water system.


3D Model Details

This 3D model shows all major components associated with a typical thermostat, these include:

  • Main Valve
  • Air Bleed
  • Seal
  • Charge Cylinder
  • Spring
  • Secondary Spring
  • Bypass Valve


Additional Resources