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.
The thermostat automatically adjusts the amount of water entering the 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 refined paraffin in the thermostat temperature sensing body is solid, and the thermostat valve is closed 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 through the radiator and the thermostat valve, and then flows back to the engine through the 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 is often opened and closed during operation, resulting in oscillation.
State judgment
When the engine starts to run cold, if there is cooling water flowing out from the 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 cooling water of the engine 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 the water temperature at the radiator water inlet is low, and the radiator is filled 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.
Regular inspection
Thermostat switch status
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, fully open temperature and lift of the main valve of the thermostat in the temperature adjustable constant temperature heating equipment. 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
Generally, the coolant of the water-cooling 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 air bubbles in the water cooling system; the disadvantage is that oscillation occurs when the thermostat works.
For example, when starting a cold engine in winter, the thermostat valve is closed 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 is stable, the thermostat valve will become stable and will 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 can precisely control the temperature of the coolant, 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. [2]
Improvements to the Wax Thermostat
Improvements in Temperature Controlled Drive Components
Shanghai University of Engineering and Technology has developed a new type of thermostat with paraffin thermostat as the parent body and a cylindrical coil spring-shaped copper-based shape memory alloy as the temperature control drive element. The thermostat biases the spring when the temperature of the starting cylinder of the car is low, and the compression alloy spring makes the main valve close and the auxiliary valve open for a small cycle. When the coolant temperature rises to a certain value, the memory alloy spring expands and compresses the bias. The spring makes the main valve of the thermostat open, and as the coolant temperature increases, the opening of the main valve gradually increases, and the auxiliary valve gradually closes to perform a large cycle.
As a temperature control unit, the memory alloy makes the valve opening action change relatively smoothly with temperature, which is beneficial to reduce the thermal stress impact of the low temperature cooling water in the water tank on the cylinder block when the internal combustion engine starts, and at the same time improves the service life of the thermostat. However, the thermostat is modified on the basis of the wax thermostat, and the structural design of the temperature control drive element is limited to a certain extent.
Valve Improvements
The thermostat has a throttling effect on the cooling liquid. The loss of the cooling liquid flowing through the thermostat leads to the power loss of the internal combustion engine, which cannot be ignored. The valve is designed as a thin cylinder with holes on the side wall, and the liquid flow channel is formed by the side hole and the middle hole, and brass or aluminum is used as the valve material to make the valve surface smooth, so as to reduce the resistance and improve the temperature. efficiency of the device.
Flow circuit optimization of cooling medium
The ideal thermal working state of the internal combustion engine is that the temperature of the cylinder head is relatively low and the temperature of the cylinder block is relatively high. For this reason, the split-flow cooling system iai appears, and the structure and installation position of the thermostat play an important role in it. The installation structure of the joint work of the thermostats, two thermostats are installed on the same bracket, the temperature sensor is installed at the second thermostat, 1/3 of the coolant flow is used to cool the cylinder block, 2/3 The coolant flow is used to cool the cylinder head.
