BACKGROUND AND PRIOR ART

The present invention concerns a valve arrangement to maintain a pre-determined pressure in a cooling system in a vehicle according to the introduction to claim 1.

Cooling fluid that circulates in a cooling system for the cooling of a combustion engine generally has an operating temperature of approximately 80°C-100°C. During a cold start of the combustion engine, the cooling fluid has a significantly lower temperature. The cooling fluid, however, occupies a larger volume in the cooling system when it is warm than when it is cold. In order to make possible a change of volume of the cooling fluid during operation, the cooling system comprises an expansion tank. The expansion tank is normally connected to other parts of the cooling system through a vertical line known as a "static line". The expansion tank is thus located at a certain height above the cooling fluid pump that circulates the cooling fluid in the cooling system. A column of cooling fluid that extends from the cooling fluid pump up to an expansion tank is obtained with such a design. There is in this way created an excess pressure in association with the inlet to the cooling fluid pump, such that cavitation does not arise during such occasions as, for example, the start of the cooling fluid pump. An excess pressure is created in the cooling system when the cooling fluid becomes heated and expands. The volume of the expansion tank, which is occupied by air and cooling fluid, is dimensioned such that an excess pressure of a suitable magnitude arises in the cooling system when the cooling fluid expands. The tendency of the cooling fluid pump to experience cavitation increases with the temperature of the cooling fluid. The excess pressure that is created when the cooling fluid is warm and the column of fluid at the static line together form an excess pressure at the inlet to the cooling fluid pump that ensures that the cooling fluid pump does not experience cavitation when the cooling fluid is warm.

A cooling system, however, is not fully sealed: it is inevitable that a small leakage of both air and cooling fluid from the cooling system takes place during operation of the combustion engine. The leakage reduces the pressure in the cooling system during operation of the combustion engine. The leakage, however, is generally so small that the pressure is reduced only by a negligible amount during normal operation of the vehicle. The pressure in the system may fall also if the excess pressure valve has opened and released air during an extreme operating condition when the cooling fluid has expanded. When the cooling fluid is cooled after a period of operation, it regains its original volume. A negative pressure in the cooling system is in this way created that corresponds to the leakage in the cooling system during the period of operation. The expansion tank comprises a non-return valve that opens and eliminates the negative pressure in the cooling system when the cooling fluid cools. The non-return valve in this way adjusts for the leakage that took place under operation after its occurrence. Transport vehicles, however, can be driven essentially 24 hours a day without intermediate periods in which the cooling fluid is cooled. The non-return valve thus cannot supply air that corrects for leakage in the cooling system. Even if the leakage of air and cooling fluid is small, the leakage during a long period of continuous operation may reduce the excess pressure to such a low level that there is a risk for cavitation at the cooling fluid pump.

US 2011/0308484 reveals an arrangement that is adapted to maintain a pre-determined pressure in a cooling system. The arrangement comprises a line with a one-way valve, a constriction and a pressure regulator that join an expansion tank in the cooling system to a charge air conduit. When the pressure regulator determines that the pressure in the expansion tank is too low, it is placed into an open condition such that pressurised air is led from the charge air conduit to the expansion tank. As soon as the pressure regulator determines that the pre-determined pressure has been achieved in the expansion tank, it is placed into a closed condition such that the flow of charge air to the expansion tanks ceases.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a valve arrangement with few components with which a pre-determined pressure can be maintained in a cooling system in a simple and reliable manner.

This object is achieved with a cooling system of the type described in the introduction that is characterised by the distinctive features that are specified in the characterising part of claim 1. The valve arrangement comprises a displaceably arranged piston that is displaced to an open condition as soon as the pressure in the cooling system falls below a pre-determined value. The valve arrangement delivers pressurised air to the cooling system when the piston is in the open condition. The supply of pressurised air rapidly raises the pressure in the cooling system to the pre-determined value. The piston is displaced to a closed condition in which the supply of pressurised air is interrupted when the pressure has reached the pre-determined value. The forces that produce the displacement of the piston are related to the pressure of the pressurised air, the pressure in the cooling system, and the magnitude of the piston areas on which the said pressure acts. With knowledge of the pressure of the pressurised air and of the pre-determined pressure in the cooling system, the said piston areas can be dimensioned relative to each other such that the piston opens when the pressure in the cooling system reaches a pre-determined value and is closed when the pressure in the cooling system exceeds this value. A valve arrangement with a displaceable piston can be given a simple design with few components, while at the same time it has a reliable function. According to one embodiment of the invention, the piston comprises a third piston area that is in contact with air at the pressure of the surroundings. Such a piston can comprise a side that is equipped with the first piston area, which is in contact with the pressure in the cooling system. The piston comprises on the second side the second piston area, which is in contact with the pressurised air, and the third piston area, which is in contact with air at the pressure of the surroundings. This remaining third piston area constitutes the difference between the first piston area and the second piston area. Such a piston can comprise a first part with a first diameter and a second part with a second diameter that is smaller than the first diameter, whereby the first part comprises an end surface that forms the first piston area, the second part comprises an end surface that forms the third piston area and that the surface at the transition between the first part and the second part forms the second piston area. The piston in this way has a simple design.

According to one embodiment of the invention, the valve arrangement comprises a first component that is adapted to define the position of the piston in the open condition. The piston is thus influenced by forces that strive to drive it to the closed condition, and opposing forces that strive to drive it to the open condition. The piston undergoes motion from the closed condition when the force that strives to drive the piston towards the open condition is larger than the force that strives to drive the piston to the closed condition. The motion of the piston must, for obvious reasons, be stopped at an appropriate open condition. The said first component can constitute a stop area or similar that comes into contact with a surface at the piston and stops the piston when it has reached an appropriate open condition. It is an advantage if the said first component has a first end that is united with a stationary unit, and a second end that is united with the piston. The first component can in this way stop the piston in the open condition when the piston has obtained a position at a determined distance from the stationary component. It is an advantage if the said first component has spring-loaded properties. The first component can, with such properties, brake the piston in a relatively gentle manner when it reaches the open condition. The first component may provide also a positioning of the piston in other conditions relative to the stationary unit. The first component can in this way, for example, position the piston in a radial position inside a hollow compartment in which the piston is arranged such that it can be displaced.

According to one embodiment of the invention, the valve arrangement comprises a second component that constitutes an air-tight wall between a region of the pressurised air passage and surrounding air. Since the pressurised air in the pressurised air passage and air at the pressure of the surroundings act on different pistons on the same side of the piston, the pressurised air passage must be separated from the region with air at the pressure of the surroundings. This can be achieved with an air-tight wall element that is fixed attached between the piston and a wall surface that defines the hollow compartment in which the piston is arranged such that it can be displaced. The said first and second components can constitute one and the same component. The component in this case thus constitutes both an air-tight wall while at the same time having the spring-loaded properties that stop the piston in an appropriate open condition.

According to one embodiment of the invention, the pressurised air passage comprises a part that has a transverse extent relative to the direction of piston motion, and that the piston is adapted to block this part of the pressurised air passage in the closed condition. The piston may have a corresponding transverse part that adjusts the width of this part of the pressurised air passage, depending on the position of the piston. The piston in the closed condition can in this way block this part of the pressurised air passage such that the flow of air through the pressurised air passage ceases. The transverse part of the piston can constitute the second piston area. The second piston area achieves in this way a double function. The pressurised air passage can comprise a seal that, together with the said part of the piston, blocks the pressurised air passage in the closed condition. A fully sealed contact between the said part of the piston and the seal can be obtained with the aid of the seal, such that the flow of pressurised air in the pressurised air passage ceases completely when the piston is in the closed condition. According to one embodiment of the invention, the pressurised air passage is arranged radially external to the piston and radially internal to a wall surface that defines a hollow compartment in which the piston is arranged such that it can be displaced. The pressurised air passage may have two parts with an axial extent external to the parts of the piston with different diameters and a part with a radial extent that unites the two axial parts.

According to one embodiment of the invention, the valve arrangement is adapted to lead pressurised air to an internal compartment in an expansion tank in the cooling system. Since an expansion tank already contains air in an upper region, it is appropriate to supply the pressurised air to this region of the expansion tank. The pressurised air that is supplied increases the air pressure in the region above the cooling fluid in the expansion tank. The air pressure in this way exerts a pressure force onto the cooling fluid in the expansion tank such that it acquires a corresponding pressure. The pressure of the cooling fluid in the expansion tank is transferred to the cooling fluid in other parts of the cooling system. Alternatively, the air may be supplied to the static line or to another suitable location in the cooling system.

According to one embodiment of the invention, the valve arrangement is adapted to receive pressurised air from a source of pressurised air in the form of an accumulator tank that stores pressurised air for an existing pressurised air system in the vehicle. Access to pressurised air that can be advantageously used for this purpose is essentially always available in heavy vehicles. A pre-determined relatively high air pressure is generally maintained during operation of a vehicle in an accumulator tank of a compressor that is driven by the combustion engine. Such accumulator tanks are relatively well-sealed such that pressurised air can be stored at a relatively large excess pressure for long periods of time even when the vehicle is not in operation. If the pressure in the accumulator tank is very high, the pressurised air line may comprise a constriction device with a fixed constriction that defines the pressure that the pressurised air has in the pressurised air passage. According to one embodiment of the invention, the cooling system comprises an excess pressure valve. When the cooling system is placed under pressure with cold cooling fluid, an increase in pressure in the cooling system is obtained as the cooling fluid is heated to its operating temperature. In the cases in which pressurised air is supplied to a cold cooling system, the pressure in the cooling system can become far too high when the cooling fluid reaches its operating temperature. Air is released from the cooling system with the aid of the excess pressure valve when the pressure exceeds too high a level. BRIEF DESCRIPTION OF DRAWINGS

One preferred embodiment of the invention will be described as an example below with reference to attached drawings, of which: Figure 1 shows a cooling system in a vehicle with a valve arrangement according to the invention,

Figure 2 shows the valve arrangement in a closed condition, and

Figure 3 shows the valve arrangement in an open condition. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Figure 1 shows schematically a vehicle 1. It is an advantage if the vehicle 1 is a heavy vehicle. The combustion engine 2 may be a supercharged diesel engine. The combustion engine 2 is cooled by cooling fluid that circulates in a cooling system. A cooling fluid pump 3 circulates the cooling fluid in the cooling system and through the combustion engine 2. After the cooling fluid has cooled the combustion engine 2 it is led to a line 4 that comprises a heat exchanger 4a for the cooling of a retarder.

Alternatively, the cooling fluid can be used as working medium in the retarder. The cooling fluid is led from the line 4 to a thermostat 5 in the cooling system. Before the cooling fluid achieves a normal operating temperature, the thermostat 5 is adapted to lead cooling fluid, through a line 6, to the cooling fluid pump 3 that is arranged in a line 7. When the thermostat 5 leads the cooling fluid to the cooling fluid pump 3, the cooling fluid is circulated in the cooling system without being cooled. As soon as the cooling fluid achieves a temperature that exceeds a pre-determined operating temperature, the thermostat 5 leads the cooling fluid, through a line 8, to an air-cooled cooling fluid cooler 9 that is mounted at a front part of the vehicle 1. The cooling fluid is cooled by a cooling flow of air in the cooling fluid cooler 9. The cooling flow of air is achieved by a cooler fan 10 and by the headwind caused by the motion of the vehicle. After the cooling in the cooling fluid cooler 9, the cooling fluid is led, through a line 11, back to the cooling fluid pump 3 in the line 7.

The volume of the cooling fluid in the cooling system varies with the temperature of the cooling fluid. The cooling system comprises an expansion tank 12 with an internal compartment that absorbs the varying volume of the cooling fluid. The expansion tank 12 in this case is connected, through a line 13, to the line 7 that is arranged on the suction side of the cooling fluid pump 3. The expansion tank 12 comprises, at an upper part, a cover 14 that can be removed to make it possible to fill the cooling system with cooling fluid. The cover 14 comprises an excess pressure valve 15, shown

schematically. The excess pressure valve 15 opens when the pressure in the expansion tank 12 exceeds a highest acceptable pressure in the cooling system. The excess pressure valve 15 can, for example, open at an excess pressure of 0.9 bar. The expansion tank 12 comprises also a non-return valve 16. The non-return valve 16 ensures that the pressure in the expansion tank 12 corresponds at least to the pressure of the surrounding air. Thus, it opens and allows air to enter if a negative pressure relative to the surroundings arises in the expansion tank 12.

The vehicle 1 in this case is equipped with a source of pressurised air in the form of an accumulator tank 17. The accumulator tank 17 contains pressurised air that is used in a pressurised air system to activate various components in the vehicle. A compressor maintains during operation of the combustion engine 2 a pre-determined relatively high air pressure in the accumulator tank 17. Since an accumulator tank 17 has a very well-sealed construction, the air pressure in the accumulator tank can be maintained relatively constant during a long period after the combustion engine 2 of the vehicle has been switched off. This means that the components driven by pressurised air can be used as soon as the vehicle 1 is to be used. The accumulator tank 17 is connected to the expansion tank 12 through a pressurised air line 18. The pressurised air line 18 comprises a valve arrangement 19 that can be adjusted to a closed condition in which it prevents pressurised air from being led from the accumulator tank 17 to the expansion tank 12, and adjusted to an open condition in which it allows pressurised air to be led from the accumulator tank 17 to the expansion tank 12. The pressurised air line 18 comprises a constriction device 20 that provides a fixed constriction of the pressurised air that is led from the accumulator tank 17 to the expansion tank 12. The air that reaches the valve arrangement 19 demonstrates in this way a lower pressure than that of the air in the accumulator tank 17. The presence of the constriction device 20 results in that the flow of air to the valve arrangement 19 does not become too high. It is in this way ensured that the expansion tank 12 is topped up with air at a suitable rate when the valve arrangement 19 is in its open condition. It is important also from the point of view of safety that the flow of air to the expansion tank does not become too high if the cover 14 should be opened by mistake. In order to constrict the air, the constriction device 20 comprises a flow channel that has a small cross-section. Given knowledge about the pressure in the accumulator tank 17, it is possible to dimension the constriction device 20 such that the pressurised air that is led to the valve arrangement 19 and the expansion tank 12 demonstrates a suitable value. The pressurised air line 18 comprises also a valve 21 with which the pressurised air connection between the accumulator tank 17 and the valve arrangement 19 can be broken. A control unit 22 is adapted to open the valve 21 when the combustion engine 2 is started and to close the valve 21 when the combustion engine is switched off. In this way, the valve arrangement 19 is not placed under load from pressurised air when the vehicle 1 is not in operation.

Figure 2 shows the valve arrangement 19 in greater detail. The valve arrangement 19 comprises a valve body in the form of a piston 23 that is arranged such that it can be displaced in an axial direction in a hollow compartment that extends through the walls of the expansion tank 12. The hollow compartment is defined by a wall surface 24. The piston 23 comprises a first part 23a with a first diameter dj and a second part 23b with a second diameter d2 that is smaller than the first diameter dj. The hollow compartment 24 is defined by a first wall surface 24a that encloses the first part 23a of the piston. The first wall surface 24a forms a hollow compartment with a somewhat larger diameter than the first part 23a of the piston. The hollow compartment 24 is defined by a second wall surface 24b that has an extent radially inwards relative to the first wall surface 24a. The hollow compartment 24 is defined by a third wall surface 24c that encloses the second part 23b of the piston. The third wall surface 24c forms a hollow compartment with a somewhat larger diameter than the second part 23b of the piston. The valve arrangement comprises a pressurised air passage 25 that is arranged in the gap compartment between the piston 23 and the wall surface 24 that defines the hollow compartment. The pressurised air passage 25 comprises a first part 25a that has an axial extent in a position radially external to the first part 23 a of the piston. The first part of the pressurised air passage 25a has an outlet 25aj that leads pressurised air out into the internal compartment 12a of the expansion tank. The first part 23 a of the piston has an end surface that constitutes a first piston area aj that is in contact with air in the internal compartment 12a of the expansion tank.

The pressurised air passage 25 comprises a second part 25b that has a radial extent in association with the second wall surface 24b. The piston 23 comprises a second piston area &2 that is in contact with pressurised air in the second part 25b of the pressurised air passage 25. The second part 25b of the pressurised air passage 25 comprises a seal 26 that is fixed attached at the second wall surface 24b. The seal 26 is adapted to come into contact with the second piston area &2 when the piston 23 has been displaced to a closed condition. The piston 23 can, in the closed condition, eliminate the flow of air through the pressurised air passage 25. Figure 2 shows the piston 23 in the closed condition. The pressurised air passage 25 comprises a third part 25c at which it receives pressurised air from the pressurised air line 18 through an inlet 25cj. The third part 25c of the pressurised air line has an axial extent in a position radially external to the second part 23b of the piston. The second part 23b of the piston comprises an end surface that forms a third piston area that is in contact with air at the pressure of the surroundings 28.

The piston 23 is connected at a peripheral part to a component 27, in association with the third piston area a^. The component 27 has a circular cross-section and an extent between a first end and a second end. The component 27 is connected to the piston 23 that is arranged such that it can be displaced at a first end and with the third wall surface 24a at a second end. The component 27 constitutes in this way a connection between the piston 23 arranged such that it can be displaced and the stationary wall surface 24. The component 27 has also spring-loaded properties such that it brakes the motion of the piston 23 with a spring force F _s when it is displaced from the closed condition. The component 27 is dimensioned such that it stops the piston 23 in a predetermined open condition. The component 27 constitutes also an air-tight wall between the pressurised air passage 25 that contains pressurised air and the surrounding air 28 at atmospheric pressure. The piston is influenced by a first force Fj that is determined by the first piston area aj multiplied by the pressure pj that is prevalent inside the expansion tank 12. This first force Fj and the spring force Fs of the component 27 strive to displace the piston 23 towards the closed condition. The piston 23 is influenced also by a second force F2 that is determined by the second piston area a2 multiplied by the pressure P2 of the pressurised air, and a third force F3 that is determined by the third piston area multiplied by the pressure P ₃ of the surroundings. The second force F2 and the third force F3 strive to displace the piston 23 towards the open condition. The pressure P2 of the pressurised air and the pressure P ₃ of the surroundings are essentially constant and thus also the forces F _2, F3 that strive to displace the piston to the open condition are essentially constant. Also the spring force F _s of the component 27 is known. The pressure inside the expansion tank 12 and thus also the pressure in the cooling system may, however, vary during operation, and thus also the first force Fj that strives to displace the piston 23 to the closed condition may vary during operation.

According to the invention, the first piston area aj and the second piston area a ₂ are dimension relative to each other such that the piston 23 is displaced towards the open condition when the pressure pj in the compartment inside the expansion tank 12a falls below a pre-determined pressure po, and towards the closed condition when the pressure pj in the compartment inside the expansion tank 12a exceeds the predetermined pressure po. The pre-determined pressure po constitutes a pressure that is appropriate to be maintained in the expansion tank 12 and in the cooling system.

As soon as the pressure pj in the cooling system falls below the pre-determined value po, the first force Fj that strives to displace the piston to the closed condition is reduced. When the first force Fj is reduced to a value such that it, together with the spring force Fs becomes smaller than the constant forces F2 and F3, this results in the piston 23 being displaced to the open condition. Pressurised air at pressure P2 flows in this way through the pressurised air passage 25 and into the expansion tank 12. The pressure pj in the expansion tank 12 consequently rises. The increasing pressure pj in the expansion tank 12 acts on the first area aj of the piston, which leads to the first force Fj increasing. When the sum of the first force Fj and the spring force Fs becomes larger than the constant forces F2 and F3, this results in the piston 23 being displaced to the closed condition. The supply of pressurised air to the expansion tank 12 ceases. The valve arrangement 19 does not require any active control. As soon as pressurised air is available in the pressurised air passage 25, it maintains the pre-determined pressure in the cooling system. A pre-determined pressure can be maintained in the cooling system with the aid of the valve arrangement 19 in essentially all operating conditions of the vehicle. In correlation with the activation of the ignition in a vehicle, the control unit 22 can open the valve 21 such that the valve arrangement 19 provides access to pressurised air in the pressurised air passage 25. Pressurised air can in this way be led into the expansion tank 12 and it can place the cooling system under pressure as soon as the cooling fluid pump 3 starts. This results in the need to arrange the expansion tank 12 at a suitable height above the cooling fluid pump in order to prevent cavitation being reduced. When the temperature of the cooling fluid rises after a cold start, the pressure in the cooling system increases. The excess pressure valve 15, however, prevents the pressure rising above too high a level. The invention is not in any way limited to the embodiment that has been described in the drawing: it can be freely varied within the scope of the patent claims.