THERMOSTAT
It is already discussed in Article I that only optimum cooling of the engine is desirable and overcooling results in deterioration of engine efficiency. To keep a rigid control over the cooling, therefore, a thermostat is used, which automatically keeps the cooling water temperature at a predetermined value. Moreover, it also helps the engine to reach the operating temperature as soon as possible after starting as the engines are designed to operate most efficiently over a small temperature range of 80°to 100°c.
Two types of thermostats are used in automobiles:
l. Bellows or aneroid type.
2. Wax or hydrostatic type.
Bellows type thermostat
It consists of metallic bellows particularly filled with some volatile liquid like acetone, alcohol or ether which boils between 70-85°c. A valve is attached to one end of the bellows, while to the other end is attached a frame which fits into the cooling passage. The thermostat is fitted in the coolant hose pipe at the engine outlet. When the engine after start is warming up, it is desired that the cooling system should not operate so that the engine warms up early. During this period, the thermostat valve remains closed, because the liquid inside as yet has not changed its state and, therefore, does not exert any pressure on the valve. As the thermostat valve is closed with the coolant pump running, to avoid excessive pressure build-up, a part of the
held-up coolant is made to circulate back through a by-pass to the pump inlet. But as the coolant temperature reaches a predetermined value, (about 80°C) the liquid inside the thermostat is converted into vapour which exerts a pressure on the valve, which begins to open, so that the water circulation through the radiator starts. The valve then opens gradually further as the water temperature rises, until it is fully open at about 90-95°C. Thus the thermostat controls the flow of water through the radiator according to the engine cooling requirements. In this type of the thermostat, the valve movement depends upon the difference between the vapour pressure in the bellows at any given temperature and the cooling system pressure.
Wax Thermostat
A wax-element thermostat manufactured by the Dole valve Co., Illinois, U.S.A. As the coolant is heated, it transmits its heat to the copper-loaded wax having high coefficient of volumetric thermal expansion (0.28% per °C) which expands so that the rubber plug contracts against the plunger and exerts a force on it upwards so that it moves vertically. This movement of the plunger opens a valve in the thermostat (not shown) to allow coolant to flow through the radiator. This type of thermostat, in contrast to the bellows type, is not sensitive to the pressure variations, as discussed above. Thus it is more reliable to operate within the specified temperature range, due to which reason, it is being increasingly used.
Testing the Thermostat
There are several ways to test the opening temperature of a thermostat. One method does not require that the thermostat be removed from the engine. Remove the radiator pressure cap from a cool radiator and insert a thermometer into the coolant. Start the engine and let it warm up. Watch the thermometer and the surface of the coolant. When the coolant begins to flow, this indicates the thermostat has started to open. The reading on the thermometer indicates the opening temperature of the thermostat. If the engine is cold and coolant circulates, this indicates the thermostat is stuck open and must be replaced. The other way to test a thermostat is to remove it. Suspend the thermostat completely submerged in a small container of water so it does not touch the bottom. Place a thermometer in the water so it does not touch the container and only measures water temperature. Heat the water. When the thermostat valve barely begins to open, read the thermometer. This is the opening temperature of this particular thermostat. If the valve stays open after the thermostat is removed from the water, the thermostat is defective and must be replaced. Several types of commercial testers are available. When using such a tester be sure to follow the manufacturer’s instructions. Markings on the thermostat normally indicate which end should face toward the radiator. Regardless of the markings, the sensored end must always be installed toward the engine. When replacing the thermostat, also replace the gasket that seals the thermostat in place and is positioned between the water outlet casting and the engine block. Generally, these gaskets are made of a composition fiber material and are die-cut to match the thermostat opening and mounting bolt configuration of the water outlet. Thermostat gaskets generally come with or without an adhesive backing. The adhesive backing of gaskets holds the thermostat securely centered in the mounting flange, leaving both hands of the technician free to align and bolt the thermostat securely in place.
Two-stage thermostat
The construction of a two stage thermostat is similar to ordinary single stage thermostats described above, except that in this there is an additional subvalve, which starts opening at lower temperature (say 79°C), thereby allowing limited circulation of the coolant during warm-up period, thus preventing peak temperature hot spots. The main valve starts opening at some higher temperature, say 88°C and both valves are fully open at 100°C. Both the valves are fully closed below 79°C, providing quicker warm-up of the engine. Thus a two-stage thermostat provides more precise control of engine temperature by reducing peak temperature in critical areas.
Checking and Replacing Hoses
Carefully check all cooling hoses for leakage, swelling and chafing. Also change any hose that feels mushy or extremely brittle when squeezed firmly. Be especially watchful for signs of splits when hoses are squeezed. These splits have a habit of bursting wide open under pressure. Also look for rust stains around the clamps. Rust stains indicate the hose is leaking, possibly because the clamp has eaten into the hose. Loosen the clamp, slide it back, and check for cuts. Do not over look the small by-pass hose on some models. It is located between the water pump and engine block. Also, check the lower radiator hose very carefully. This hose contains a coiled wire lining to keep it from collapsing during operation. If the wire loses tension, the hose can partially collapse at high speed and restrict coolant flow. This results in a very elusive overheating problem. When replacing a hose, drain the coolant system below the level that is being worked on. Use a knife to cut off the old hose and loosen or cut the old clamp. Slide the old hose off the fitting. If the hose is stuck, do not pry it off. You could possibly damage the inlet/outlet nipple or the attachment between the end of the hose and the bead. Carefully cut the stuck hose off its fitting.
Clean off any remaining hose particles with a wire brush or emery cloth. The fitting should be clean when installing the new hose. Burrs or sharp edges could cut into the hose tube and lead to premature failure. Coat the surface with a sealing compound. Place the new clamps on each end of the hose before positioning the hose. Do not reuse old spring-type clamps, even if they look good. Slide the clamps to about ¼ inch (6.35 mm) from the end of the hose after it is properly positioned on the fitting. Tighten the clamp securely. Do not over tighten.
It is already discussed in Article I that only optimum cooling of the engine is desirable and overcooling results in deterioration of engine efficiency. To keep a rigid control over the cooling, therefore, a thermostat is used, which automatically keeps the cooling water temperature at a predetermined value. Moreover, it also helps the engine to reach the operating temperature as soon as possible after starting as the engines are designed to operate most efficiently over a small temperature range of 80°to 100°c.
Two types of thermostats are used in automobiles:
l. Bellows or aneroid type.
2. Wax or hydrostatic type.
Bellows type thermostat
It consists of metallic bellows particularly filled with some volatile liquid like acetone, alcohol or ether which boils between 70-85°c. A valve is attached to one end of the bellows, while to the other end is attached a frame which fits into the cooling passage. The thermostat is fitted in the coolant hose pipe at the engine outlet. When the engine after start is warming up, it is desired that the cooling system should not operate so that the engine warms up early. During this period, the thermostat valve remains closed, because the liquid inside as yet has not changed its state and, therefore, does not exert any pressure on the valve. As the thermostat valve is closed with the coolant pump running, to avoid excessive pressure build-up, a part of the
held-up coolant is made to circulate back through a by-pass to the pump inlet. But as the coolant temperature reaches a predetermined value, (about 80°C) the liquid inside the thermostat is converted into vapour which exerts a pressure on the valve, which begins to open, so that the water circulation through the radiator starts. The valve then opens gradually further as the water temperature rises, until it is fully open at about 90-95°C. Thus the thermostat controls the flow of water through the radiator according to the engine cooling requirements. In this type of the thermostat, the valve movement depends upon the difference between the vapour pressure in the bellows at any given temperature and the cooling system pressure.
Wax Thermostat
A wax-element thermostat manufactured by the Dole valve Co., Illinois, U.S.A. As the coolant is heated, it transmits its heat to the copper-loaded wax having high coefficient of volumetric thermal expansion (0.28% per °C) which expands so that the rubber plug contracts against the plunger and exerts a force on it upwards so that it moves vertically. This movement of the plunger opens a valve in the thermostat (not shown) to allow coolant to flow through the radiator. This type of thermostat, in contrast to the bellows type, is not sensitive to the pressure variations, as discussed above. Thus it is more reliable to operate within the specified temperature range, due to which reason, it is being increasingly used.
Testing the Thermostat
There are several ways to test the opening temperature of a thermostat. One method does not require that the thermostat be removed from the engine. Remove the radiator pressure cap from a cool radiator and insert a thermometer into the coolant. Start the engine and let it warm up. Watch the thermometer and the surface of the coolant. When the coolant begins to flow, this indicates the thermostat has started to open. The reading on the thermometer indicates the opening temperature of the thermostat. If the engine is cold and coolant circulates, this indicates the thermostat is stuck open and must be replaced. The other way to test a thermostat is to remove it. Suspend the thermostat completely submerged in a small container of water so it does not touch the bottom. Place a thermometer in the water so it does not touch the container and only measures water temperature. Heat the water. When the thermostat valve barely begins to open, read the thermometer. This is the opening temperature of this particular thermostat. If the valve stays open after the thermostat is removed from the water, the thermostat is defective and must be replaced. Several types of commercial testers are available. When using such a tester be sure to follow the manufacturer’s instructions. Markings on the thermostat normally indicate which end should face toward the radiator. Regardless of the markings, the sensored end must always be installed toward the engine. When replacing the thermostat, also replace the gasket that seals the thermostat in place and is positioned between the water outlet casting and the engine block. Generally, these gaskets are made of a composition fiber material and are die-cut to match the thermostat opening and mounting bolt configuration of the water outlet. Thermostat gaskets generally come with or without an adhesive backing. The adhesive backing of gaskets holds the thermostat securely centered in the mounting flange, leaving both hands of the technician free to align and bolt the thermostat securely in place.
Two-stage thermostat
The construction of a two stage thermostat is similar to ordinary single stage thermostats described above, except that in this there is an additional subvalve, which starts opening at lower temperature (say 79°C), thereby allowing limited circulation of the coolant during warm-up period, thus preventing peak temperature hot spots. The main valve starts opening at some higher temperature, say 88°C and both valves are fully open at 100°C. Both the valves are fully closed below 79°C, providing quicker warm-up of the engine. Thus a two-stage thermostat provides more precise control of engine temperature by reducing peak temperature in critical areas.
Checking and Replacing Hoses
Carefully check all cooling hoses for leakage, swelling and chafing. Also change any hose that feels mushy or extremely brittle when squeezed firmly. Be especially watchful for signs of splits when hoses are squeezed. These splits have a habit of bursting wide open under pressure. Also look for rust stains around the clamps. Rust stains indicate the hose is leaking, possibly because the clamp has eaten into the hose. Loosen the clamp, slide it back, and check for cuts. Do not over look the small by-pass hose on some models. It is located between the water pump and engine block. Also, check the lower radiator hose very carefully. This hose contains a coiled wire lining to keep it from collapsing during operation. If the wire loses tension, the hose can partially collapse at high speed and restrict coolant flow. This results in a very elusive overheating problem. When replacing a hose, drain the coolant system below the level that is being worked on. Use a knife to cut off the old hose and loosen or cut the old clamp. Slide the old hose off the fitting. If the hose is stuck, do not pry it off. You could possibly damage the inlet/outlet nipple or the attachment between the end of the hose and the bead. Carefully cut the stuck hose off its fitting.
Clean off any remaining hose particles with a wire brush or emery cloth. The fitting should be clean when installing the new hose. Burrs or sharp edges could cut into the hose tube and lead to premature failure. Coat the surface with a sealing compound. Place the new clamps on each end of the hose before positioning the hose. Do not reuse old spring-type clamps, even if they look good. Slide the clamps to about ¼ inch (6.35 mm) from the end of the hose after it is properly positioned on the fitting. Tighten the clamp securely. Do not over tighten.
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