Thermostat is a device that automatically supports need temperature in engine cooling system. Thermostat allows you to quickly warm up a cold engine at startup.

There are liquid and solid filled thermostats. In liquid thermostats used an easily evaporating liquid (a mixture of 70% ethyl alcohol and 30% water). As solid filler, ceresin with copper chips is used, which has a large coefficient of volume expansion.

Thermostat Construction

1 – Water pump housing; 2 -corrugated balloon; 3 – rod; 4 – gasket; 5 – thermostat valve; 6 – hot fluid outlet pipe; 7 – thermostat housing; 8 – housing bracket; 9 – thermostat balloon; 10 – solid filler (ceresin); 11 – rubber membrane; 12 – guide bush; 13 – return spring; 14 – rocker arm; 15 – buffer; 16 – inlet pipe.

The liquid thermostat consists of housing 7 with windows, a corrugated balloon 2 and a valve 5. The lower part of the corrugated balloon is rigidly connected to the housing bracket 8. A rod 3 with a valve is soldered to the upper part of the balloon. The stem can move in the housing guide. Sometimes a small hole is made on the thermostat valve at the edge for air to escape when pouring liquid into the engine cooling system. The sealed corrugated balloon contains liquid, which occupies approximately half the internal volume of the balloon.
The air from the balloon is evacuated, and under normal conditions the balloon is compressed and the valve is closed.

How a thermostat works

If the temperature of the liquid in the cooling system does not exceed 73 degrees Celsius, then the balloon is compressed and the valve is closed. The liquid enters the pump through the bypass channel, bypassing the radiator. As the engine warms up, the fluid in the cooling system heats up. When the temperature rises above 73-83 C, the liquid in the balloon begins to evaporate, the pressure in the cylinder rises and the valve opens. Coolant enters to the radiator. At 88-94 C, the thermostat valve is fully open.

Thermostat Operation Scheme

The solid-filled thermostat is located between the inlet pipe 16 and the outlet pipe 6. A valve 5 pivotally connected to the stem is constantly pressed against the housing 7 by a spring 13. The latter rests on a rubber membrane 11, which is sandwiched between the balloon 9 and the guide sleeve.
The inner space of the container is filled with filler 10.
Until the engine warms up, the filler is in a solid state and the thermostat valve is closed. With increasing temperature of the liquid in the cooling system to 70 C or more, the volume of the filler increases, since ceresin melts and presses on the membrane. The membrane bends upward, presses through the buffer 15 onto the stem, which turns the valve 5, as a result of which the coolant enters to the radiator. When the coolant temperature decreases, the volume of the filler decreases, and the thermostat valve closes under the action of the return spring.