How The Thermostat Works

A thermostat is a valve that controls the coolant flow path. It is an automatic temperature adjustment device, usually containing a temperature sensing component, which turns on and off the flow of air, gas or liquid by thermal expansion or cold contraction.

Function of thermostat

The thermostat automatically adjusts the amount of water entering the engine radiator according to the temperature of the cooling water, and changes the circulation range of the water to adjust the heat dissipation capacity of the cooling system and ensure that the engine works within a suitable temperature range. The thermostat must be kept in good technical condition, otherwise it will seriously affect the normal operation of the engine. If the main valve of the thermostat is opened too late, it will cause the engine to overheat; if the main valve is opened too early, the engine warm-up time will be prolonged and the engine temperature will be too low.

All in all, the role of the thermostat is to keep the engine from getting too cold. For example, after the engine is working normally, the temperature of the engine may be too low if there is no thermostat when driving in winter. At this time, the engine needs to temporarily stop the water non-circulation to ensure that the engine temperature is not too low.

How the wax thermostat works

The main thermostat used is a wax type thermostat. When the cooling temperature is lower than the specified value, the delicate paraffin in the thermostat is solid, and the thermostat valve closes the gap between the engine and the radiator under the action of the spring. The coolant is returned to the engine through the water pump for a small circulation in the engine. When the temperature of the coolant reaches the specified value, the paraffin begins to melt and gradually becomes a liquid, and the volume increases and the rubber tube is compressed to shrink. When the rubber tube shrinks, an upward thrust is applied to the push rod, and the push rod has a downward reverse thrust on the valve to open the valve. At this time, the coolant flows back to the engine through the radiator and thermostat valve, and then flows back to the engine through the engine water pump for a large cycle. Most of the thermostats are arranged in the water outlet pipeline of the cylinder head. The advantage of this is that the structure is simple, and it is easy to remove air bubbles in the cooling system; the disadvantage is that the thermostat often opens and closes during operation, resulting in oscillation. 

When the engine starts to run cold, if there is cooling water flowing out from the water inlet pipe of the upper water chamber of the water tank, it means that the main valve of the thermostat cannot be closed; when the temperature of the engine cooling water exceeds 70 ℃, the upper water chamber of the water tank enters If there is no cooling water flowing out of the water pipe, it means that the main valve of the thermostat cannot be opened normally, and repairs are required at this time. The inspection of the thermostat can be carried out on the vehicle as follows:

Inspection after the engine is started: Open the radiator water inlet cover, if the cooling level in the radiator is static, it means that the thermostat is working normally; otherwise, it means that the thermostat is not working properly. This is because when the water temperature is lower than 70°C, the expansion cylinder of the thermostat is in a contracted state and the main valve is closed; when the water temperature is higher than 80°C, the expansion cylinder expands, the main valve gradually opens, and the circulating water in the radiator begins to flow. When the water temperature gauge indicates below 70°C, if there is water flowing at the inlet pipe of the radiator and the water temperature is warm, it means that the main valve of the thermostat is not closed tightly, causing the cooling water to circulate prematurely.

Check after the water temperature rises: In the early stage of the engine operation, the water temperature rises rapidly; when the water temperature gauge indicates 80, the heating rate slows down, indicating that the thermostat works normally. On the contrary, if the water temperature has been rising rapidly, when the internal pressure reaches a certain level, the boiling water suddenly overflows, which means that the main valve is stuck and suddenly opened.

When the water temperature gauge indicates 70°C-80°C, open the radiator cover and the radiator drain switch, and feel the water temperature by hand. If both are hot, it means that the thermostat is working normally; If there is no water flowing out or little flowing water at the water inlet pipe of the chamber, it means that the main valve of the thermostat cannot be opened.

The thermostat that is stuck or not closed tightly should be removed for cleaning or repair, and should not be used immediately.

Regularly check, edit and broadcast

Thermostat switch status

According to the information, the safe life of the wax thermostat is generally 50,000km, so it is required to be replaced regularly according to its safe life.

Thermostat location

The inspection method of the thermostat is to check the opening temperature of the main valve of the thermostat, the fully open temperature and the lift. If one of them does not meet the specified value, the thermostat should be replaced. For example, for the thermostat of the Santana JV engine, the opening temperature of the main valve is 87°C plus or minus 2°C, the fully open temperature is 102°C plus or minus 3°C, and the fully open lift is >7mm.

 

Thermostat arrangement

In general, the coolant of the water-cooled system flows in from the body and flows out from the cylinder head. Most thermostats are located in the cylinder head outlet line. The advantage of this arrangement is that the structure is simple, and it is easy to remove the air bubbles in the water cooling system; the disadvantage is that oscillation will occur when the thermostat works.

For example, when starting a cold engine in winter, the thermostat valve closes due to low coolant temperature. When the coolant is in a small cycle, the temperature rises quickly and the thermostat valve opens. At the same time, the low-temperature coolant in the radiator flows into the body, so that the coolant cools down again, and the thermostat valve is closed again. When the coolant temperature rises again, the thermostat valve opens again. Until the temperature of all the coolant stabilizes, the thermostat valve tends to stabilize and does not open and close repeatedly. The phenomenon that the thermostat valve is repeatedly opened and closed in a short period of time is called thermostat oscillation. When this phenomenon occurs, it will increase the fuel consumption of the car.

The thermostat can also be arranged in the water outlet pipe of the radiator. This arrangement can reduce or eliminate the oscillation phenomenon of the thermostat and precisely control the coolant temperature, but its structure is complex and the cost is high, and it is mostly used in high-performance cars and cars that often drive at high speeds in winter.

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