Patent Publication Number: US-2023138661-A1

Title: Cooling system and vehicle comprising such a cooling system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage Patent Application (filed under 35 § U.S.C. 371) of PCT/SE2021/050420, filed May 6, 2021, of the same title, which, in turn claims priority to Swedish Patent Application No. 2050583-0 filed May 19, 2020, of the same title; the contents of each of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a cooling system comprising capabilities for separation of air bubbles from coolant in the cooling system and facilitate migration of the air bubbles to an expansion tank. 
     BACKGROUND 
     A motor vehicle normally has vehicle components with mutually different cooling requirements, and it is often necessary to use separate cooling circuits for cooling these vehicle components. As an example, a hybrid or fully electric vehicle may comprise an electric energy storing device in the form of an electric battery or a set of electric batteries for supplying electric energy to an electric traction motor of the vehicle and one or more power electronic devices, such as for instance an inverter and a DC/DC converter, for controlling the flow of electric power between the electric energy storing device and the electric traction motor. These types of vehicle components are heated when in use and have to be cooled in order to operate satisfactory. The temperature of the electric batteries of a hybrid or fully electric vehicle should be kept rather low, normally below approximately 40° C., in order to prevent a too rapid aging thereof, whereas an inverter and a DC converter may withstand higher temperatures, normally up to approximately 60-75° C. Therefore, two separate cooling circuits are often used for cooling such vehicle components, wherein a cooling circuit with circulating coolant of lower temperature is used for cooling the electric batteries and another cooling circuit with circulating coolant of higher temperature is used for cooling the inverter and DC converter and other possible power electronic devices which require cooling. 
     When a vehicle component is cooled by coolant circulating in a cooling circuit, the vehicle component will give off heat to the coolant, which is thereby heated and expanded. The resulting total volume increase of the coolant in the cooling circuit depends on the original coolant volume and the temperature increase. In order to prevent the pressure from increasing too much in the cooling circuit, the cooling circuit is provided with an expansion tank which can accommodate the surplus coolant volume generated in connection with the expansion of the coolant. 
     Another important function of a conventional expansion tank in a cooling system of the above-mentioned type is that it should be possible for the coolant received in the expansion tank to be deaerated in the expansion tank before leaving the expansion tank. In a conventional cooling system of the type where a cooling circuit is connected to an expansion tank in a motor vehicle, there is a small continuous flow of coolant from the cooling circuit to the expansion tank and from the expansion tank back to the cooling circuit. The air which accompanies the coolant to the expansion tank is intended to rise to the surface of the coolant volume received in the expansion tank in order to accumulate in an air-filled space at an upper part of the expansion tank. Hereby, the coolant in the expansion tank is deaerated. As an alternative, the cooling system may comprise a separate deaeration device which is arranged in the cooling circuit for separation of air bubbles from the coolant circulating in the cooling circuit, wherein the deaeration device is connected to an expansion tank via a static line in order to allow air bubbles separated from the coolant in the deaeration device to migrate upwards in the static line to the expansion tank. A cooling system of the latter type is for instance previously known from U.S. Pat. No. 7,395,787 B1. 
     In a cooling system of a motor vehicle with two different cooling circuits, it is previously known to use a dual chamber expansion tank with a first expansion chamber for accumulating coolant which circulates in a first cooling circuit of the cooling system and an adjacent second expansion chamber for accumulating coolant which circulates in a second cooling circuit of the cooling system, to thereby save space in the vehicle and reduce the installation costs. Dual chamber expansion tanks of this type are for instance previously known from US 2011/0284107 A1 and WO 2019/113232 A1. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to achieve a further development of a cooling system of the above-mentioned type with two or more separate cooling circuits so as to provide a cooling system that is improved in at least some aspect. 
     According to the present invention, the above-mentioned object is achieved by means of a cooling system having the features defined in the claims. 
     The cooling system of the present invention comprises:
         a first cooling circuit for cooling at least one first component by means of coolant circulating in the first cooling circuit;   a second cooling circuit for cooling at least one second component by means of coolant circulating in the second cooling circuit;   a first coolant pump for circulating coolant in the first cooling circuit;   a second coolant pump for circulating coolant in the second cooling circuit;   an expansion tank provided with an expansion chamber for accumulation of coolant, wherein this expansion chamber is connected to the second cooling circuit in order to allow the expansion chamber to receive coolant from the second cooling circuit, and   a deaeration device, in the following referred to as the first deaeration device, arranged in the first cooling circuit for separation of air bubbles from the coolant circulating in the first cooling circuit, wherein the first deaeration device is located at a lower position than the expansion tank and connected to said expansion chamber via a static line, in the following referred to as the first static line, in order to allow air bubbles separated from the coolant in the first deaeration device to migrate upwards in the first static line towards the expansion chamber.       

     In the cooling system of the present invention, two separate cooling circuits of the cooling system are connected to one and the same expansion chamber in the expansion tank included in the cooling system. By using a separate deaeration device in the first cooling circuit and allowing the air bubbles separated from the coolant in this deaeration device to migrate upwards in a static line towards the expansion chamber of the expansion tank, the expansion chamber may receive air from the coolant in the first cooling circuit as well as from the coolant in the second cooling circuit without requiring any flow of coolant from the first cooling circuit to the expansion chamber, which in its turn implies that the first and second cooling circuits may be connected to one and the same expansion chamber in the expansion tank without the occurrence of any mixture in the expansion chamber between coolant from the first cooling circuit and coolant from the second cooling circuit. Thus, it will be possible to maintain different temperature levels in the two cooling circuits despite the fact that they are connected to the same expansion chamber in the expansion tank. When the cooling system is used in a hybrid or fully electric vehicle, the first cooling circuit may be a low temperature cooling circuit for cooling of an electric energy storing device of the vehicle and the second cooling circuit may be a high temperature cooling circuit for cooling of power electronics of the vehicle. The possibility to have the two cooling circuits connected to the same expansion chamber implies that it will be possible to use an expansion tank of a conventional and simple design as a common expansion tank for the cooling circuits, also when the coolant is to be at different temperature levels in the different cooling circuits. Furthermore, the possibility to achieve deaeration of the coolant in the first cooling circuit by means of the first deaeration device without requiring any flow of coolant from the first cooling circuit to the expansion tank implies that it will be possible to dispense with conventional deaeration lines for feeding of coolant from the first cooling circuit to the expansion tank. Such deaeration lines may be rather costly and may also take up a lot of space, and it is consequently of advantage to be able to dispense with the deaeration lines. 
     The first deaeration device comprises a deaeration chamber, the chamber comprising:
         a coolant inlet connected to a feed pipe of the first cooling circuit in order to allow coolant circulating in the first cooling circuit to flow from the feed pipe into the deaeration chamber via this coolant inlet;   a first coolant outlet connected to the first coolant pump in order to allow coolant to flow from the deaeration chamber to the first coolant pump via this first coolant outlet, wherein the first coolant pump is arranged immediately downstream of the first coolant outlet;   a second coolant outlet connected to said expansion chamber via the first static line, wherein the second coolant outlet is located at a higher position than the first coolant outlet.       

     With a first object being “located at a higher position” than a second object is herein meant that the first object is located at a higher position than the second object relative to a local gravity vector gv when the cooling system is mounted to a vehicle and the vehicle is positioned in an upright use position on a horizontal surface. 
     With the first coolant pump being arranged “immediately downstream” the first coolant outlet is herein meant that the first coolant pump is the first major component arranged downstream of the first coolant outlet. The first coolant pump need not however be located physically close to the first coolant outlet, even if this is one possibility. With first major component is herein meant that no other component which may produce such an amount of air bubbles that they may have a detrimental effect on the first coolant pump is arranged between the first coolant outlet and the first coolant pump. Another way of putting it is therefore that the first coolant pump is arranged downstream of the first coolant outlet in such a manner that the amount of air bubbles produced in the coolant between the first coolant outlet and the first coolant pump is negligible for the long-term operation of the pump. It is therefore evident that a component such as e.g. a temperature sensor may be arranged between the first coolant outlet and the first coolant pump, since such a component would not affect the coolant flow in such a way as to cause a large amount of air bubbles to be produced. On the other hand, a component such as a heat exchanger may not be arranged between the first coolant outlet and the first coolant pump since a heat exchanger could conceivable cause such a large amount of air bubbles being produced in the coolant that it may be detrimental to the operation of the first coolant pump. 
     The first coolant pump may be the first component arranged downstream of the first coolant outlet. 
     A first coolant pipe may be arranged between the first coolant outlet and the first coolant pump. In this case, a first end of the first coolant pipe may be connected to the first coolant outlet and a second end of the first coolant pipe may be connected to an inlet of the first coolant pump. 
     By having this type of deaeration device the coolant in the first coolant circuit may be deaerated in a simple manner and by placing the first coolant pump immediately downstream of the first coolant outlet it is ensured that no or a minimal amount of air bubbles will reach the first coolant pump. It is well known that air or air bubbles in the coolant is detrimental for pumps, e.g. since it may lead to cavitation which may damage the pump during long-term operation. Therefore, this solution increases the longevity of the system in a simple and cost-efficient manner. 
     The coolant inlet and the first coolant outlet may e.g. be spaced apart from each other in a longitudinal direction of the deaeration chamber. 
     According to an embodiment of the invention, the cross-sectional dimension of the deaeration chamber is larger than the cross-sectional dimension of the feed pipe to thereby allow air bubbles carried along with the coolant flowing through the feed pipe to enter the deaeration chamber via the coolant inlet and thereafter migrate in the deaeration chamber to the second coolant outlet. 
     With this type of deaeration device, the coolant in the first cooling circuit may be deaerated in a simple and efficient manner with the use of a component of simple construction which can be produced at low cost. 
     The cross-sectional dimension of the deaeration chamber is preferably so much larger than the cross-sectional dimension of the feed pipe that the relationship between the flow velocity of the coolant flowing through the deaeration chamber between the coolant inlet and the first coolant outlet and the flow velocity of the coolant flowing through the feed pipe is 1:2 or lower, preferably 1:3 or lower. Hereby, it will be possible, by a suitable control of the first coolant pump, to adapt the coolant flow through the first cooling circuit in such a manner that the flow velocity of the coolant in the feed pipe is sufficiently high to allow this coolant to move air bubbles forward along the feed pipe and into the deaeration chamber at the same time as the flow velocity of the coolant in the deaeration chamber is sufficiently low to allow said air bubbles to migrate in the deaeration chamber to the second coolant outlet of the deaeration chamber. If the flow velocity of the coolant is too low in the feed pipe, air bubbles may get stuck in the feed pipe. If the flow velocity of the coolant is too high in the deaeration chamber, the coolant may carry along air bubbles out of the deaeration chamber via the first coolant outlet. The first coolant pump may be controlled in such a manner that the coolant circulating in the first cooling circuit is continuously deaerated. However, the first coolant pump may as an alternative be configured to adapt the coolant flow in the first cooling circuit in such a manner that deaeration is effected intermittently or only at specific occasions. 
     According to another embodiment of the invention, the second coolant outlet is located in such a position in relation to the coolant inlet and the first coolant outlet that the coolant flow in the deaeration chamber between the coolant inlet and the first coolant outlet will move said migrating air bubbles in the longitudinal direction of the deaeration chamber towards the second coolant outlet. Hereby, the air bubbles migrating in the deaeration chamber towards the second coolant outlet do not have to move against the flow direction of the coolant in the deaeration chamber and the coolant flow in the deaeration chamber will consequently not counteract the migration of the air bubbles towards the second coolant outlet. On the contrary, the coolant flow in the deaeration chamber will promote the migration of air bubbles in the deaeration chamber towards the second coolant outlet and an efficient separation of air bubbles from the circulating coolant may thereby be achieved in the deaeration chamber. 
     According to another embodiment of the invention, the deaeration chamber has an elongated shape and is arranged with its longitudinal axis vertical or inclined in relation to a horizontal plane by an angle of 0-90°, preferably by an angle which is &gt;0° and &lt;=90°, more preferably by an angle of 10-90°, most preferably 15-90°. A deaeration device with such a deaeration chamber has a rather simple construction and can be produced in a simple and cost-efficient manner. By horizontal plane is herein meant the plane whose normal is parallel to the prevailing gravitational vector. When the deaeration chamber is arranged with its longitudinal axis inclined at an angle greater than 0° in relation to the horizontal plane, air bubbles may rise in the deaeration chamber and hit an inclined upper wall surface in the deaeration chamber, whereupon the air bubbles are conveyed along this wall surface towards the second coolant outlet under the effect of the coolant flow in the deaeration chamber between the coolant inlet and the first coolant outlet. Additionally, even at very low flow, air bubbles which have separated from the coolant will rise towards the static line due to the buoyancy of the air bubbles. Thus, an improved deaeration will be achieved. By having the inclination between 10-90° the chamber will, in most cases, be inclined towards the horizontal plane even if the vehicle is travelling on a downwards slope. Thereby the above described effect is still maintained even if the vehicle is moving in hilly terrain. By having the inclination between 15-90° it is ensured that the chamber will be inclined in relation to the horizontal plane for a great majority of a standard trip. This ensures that the deaeration of the cooling system is functioning optimally at practically all times, thereby minimizing the risk of detrimental effects due to air bubbles in the system. The inclination may also be 10-80° or 15-75° so as to take both upwards and downwards slope into consideration. 
     According to another embodiment of the invention, the first static line has a lower end and an upper end, wherein the first static line is connected to the first deaeration device at its lower end and wherein:
         the first static line slopes upwards along its entire length from its lower end to its upper end, or   the first static line is formed by several interconnected length sections which are arranged in series with each other and which consist of one or more first length sections, each of which sloping upwards as seen in a direction along the first static line from its lower end towards its upper end, and one or more horizontal second length sections.       

     Hereby, the first static line lacks downwardly sloping sections that could prevent air bubbles from migrating from the first deaeration device towards the expansion chamber of the expansion tank. 
     According to another embodiment of the invention, the cooling system also comprises a second deaeration device arranged in the second cooling circuit for separation of air bubbles from the coolant circulating in the second cooling circuit, wherein the second deaeration device is located at a lower position than the expansion tank and connected to said expansion chamber via a second static line in order to allow air bubbles separated from the coolant in the second deaeration device to migrate upwards in the second static line towards the expansion chamber. By using a separate deaeration device also in the second cooling circuit and allowing the air bubbles separated from the coolant in this deaeration device to migrate upwards in the second static line towards the expansion chamber of the expansion tank, the expansion chamber may receive air from the coolant in the second cooling circuit without requiring any flow of coolant from the second cooling circuit to the expansion chamber, which in its turn implies that it will be possible to dispense with conventional deaeration lines for feeding of coolant from the second cooling circuit to the expansion tank. 
     According to another embodiment of the invention, the second deaeration device comprises a deaeration chamber having:
         a coolant inlet connected to a feed pipe of the second cooling circuit in order to allow coolant circulating in the second cooling circuit to flow from this feed pipe into the deaeration chamber of the second deaeration device via this coolant inlet;   a first coolant outlet connected to the second coolant pump in order to allow coolant to flow from the deaeration chamber of the second deaeration device to the second coolant pump via this first coolant outlet, wherein the second coolant pump is arranged immediately downstream of the first coolant outlet; and   a second coolant outlet connected to said expansion chamber via the second static line and located at a higher position than the first coolant outlet of the deaeration chamber of the second deaeration device,   wherein the cross-sectional dimension of the deaeration chamber of the second deaeration device is larger than the cross-sectional dimension of the feed pipe of the second cooling circuit to thereby allow air bubbles carried along with the coolant flowing through the feed pipe of the second cooling circuit to enter the deaeration chamber of the second deaeration device via the coolant inlet and thereafter migrate in this deaeration chamber to the second coolant outlet.       

     With this type of deaeration device, the coolant in the second cooling circuit may be deaerated in a simple and efficient manner with the use of a component of simple construction which can be produced at low cost. 
     With the second coolant pump being arranged “immediately downstream” the first coolant outlet is herein meant that the second coolant pump is the first major component arranged downstream of the first coolant outlet. The first coolant pump need not however be located physically close to the first coolant outlet, even if this is one possibility. With first major component is herein meant that no other component which may produce such an amount of air bubbles that they may have a detrimental effect on the second coolant pump is arranged between the first coolant outlet and the second coolant pump. Another way of putting it is therefore that the second coolant pump is arranged downstream of the first coolant outlet in such a manner that the amount of air bubbles produced in the coolant between the first coolant outlet and the second coolant pump is negligible for the long-term operation of the second coolant pump. It is therefore evident that a component such as e.g. a temperature sensor may be arranged between the first coolant outlet and the second coolant pump, since such a component would not affect the coolant flow in such a way as to cause a large amount of air bubbles to be produced. On the other hand, a component such as a heat exchanger may not be arranged between the first coolant outlet and the second coolant pump since a heat exchanger could conceivable cause such a large amount of air bubbles being produced in the coolant that it may be detrimental to the operation of the second coolant pump. 
     The second coolant pump may be the first component arranged downstream of the first coolant outlet. 
     A second coolant pipe may be arranged between the first coolant outlet and the second coolant pump. In this case, a first end of the second coolant pipe may be connected to the first coolant outlet and a second end of the second coolant pipe may be connected to an inlet of the second coolant pump. 
     By having this type of deaeration device the coolant in the second coolant circuit may be deaerated in a simple manner and by placing the second coolant pump immediately downstream of the first coolant outlet it is ensured that no or a minimal amount of air bubbles will reach the second coolant pump. It is well known that air or air bubbles in the coolant is detrimental for pumps, e.g. since it may lead to cavitation which may damage the pump during long-term operation. Therefore, this solution increases the longevity of the system in a simple and cost-efficient manner. 
     The coolant inlet and the first coolant outlet of the deaeration chamber of the second deaeration device may be spaced apart from each other in a longitudinal direction of this deaeration chamber. 
     According to another embodiment on the invention, the cross-sectional dimension of the deaeration chamber of the second deaeration device is larger than the cross-sectional dimension of the feed pipe of the second cooling circuit such that the relationship between the flow velocity of the coolant flowing through this deaeration chamber between the coolant inlet and the first coolant outlet and the flow velocity of the coolant flowing through this feed pipe is 1:2 or lower, preferably 1:3 or lower. Hereby, it will be possible, by a suitable control of the second coolant pump, to adapt the coolant flow through the second cooling circuit in such a manner that the flow velocity of the coolant in the feed pipe of the second cooling circuit is sufficiently high to allow this coolant to move air bubbles forward along the feed pipe and into the deaeration chamber of the second deaeration device at the same time as the flow velocity of the coolant in this deaeration chamber is sufficiently low to allow said air bubbles to migrate in the deaeration chamber to the second coolant outlet of the deaeration chamber. The second coolant pump may be controlled in such a manner that the coolant circulating in the second cooling circuit is continuously deaerated. However, the second coolant pump may as an alternative be configured to adapt the coolant flow in the second cooling circuit in such a manner that deaeration is effected intermittently or only at specific occasions. 
     According to another embodiment on the invention, the second coolant outlet of the deaeration chamber of the second deaeration device is located in such a position in relation to the coolant inlet and the first coolant outlet of this deaeration chamber that the coolant flow in this deaeration chamber between the coolant inlet and the first coolant outlet will move said migrating air bubbles in the longitudinal direction of this deaeration chamber towards the second coolant outlet. Hereby, the air bubbles migrating in the deaeration chamber of the second deaeration device towards the second coolant outlet do not have to move against the flow direction of the coolant in this deaeration chamber and the coolant flow in this deaeration chamber will consequently not counteract the migration of the air bubbles towards the second coolant outlet. On the contrary, the coolant flow in the deaeration chamber of the second deaeration device will promote the migration of air bubbles in this deaeration chamber towards the second coolant outlet and an efficient separation of air bubbles from the circulating coolant may thereby be achieved in this deaeration chamber. 
     According to another embodiment on the invention, the deaeration chamber of the second deaeration device has an elongated shape and is arranged with its longitudinal axis vertical or inclined in relation to a horizontal plane by an angle of 0-90°, preferably by an angle which is &gt;0° and &lt;=90°, more preferably by an angle of 10-90°, most preferably 15-90° . The advantages associated with having the deaeration device inclined in such a manner have been described above and will not be repeated here for sake of brevity. Similarly, the inclination may also be 10-80° or 15-75° so as to take both upwards and downwards slope into consideration. 
     According to another embodiment on the invention, the second static line has a lower end and an upper end, wherein the second static line is connected to the second deaeration device at its lower end and wherein:
         the second static line slopes upwards along its entire length from its lower end to its upper end, or   the second static line is formed by several interconnected length sections which are arranged in series with each other and which consist of one or more first length sections, each of which sloping upwards as seen in a direction along the second static line from its lower end towards its upper end, and one or more horizontal second length sections.       

     The cooling system may comprise a radiator for cooling of coolant circulating in the first cooling circuit and/or a radiator for cooling of coolant circulating in the second cooling circuit. 
     Further advantageous features of the cooling system according to the present invention will appear from the description following below. 
     The invention also relates to a vehicle having the features defined in the claims. 
     Further advantageous features of the vehicle according to the present invention will appear from the description following below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       With reference to the appended drawings, a specific description of embodiments of the invention cited as examples follows below. In the drawings: 
         FIG.  1    is an outline diagram of a cooling system according to a first embodiment of the present invention, 
         FIG.  2    is a schematic vertical section through a deaeration device included in the cooling system of  FIG.  1   , 
         FIG.  3    is an outline diagram of a cooling system according to a second embodiment of the invention, 
         FIG.  4    is an outline diagram of a cooling system according to a third embodiment of the invention, 
         FIG.  5    is an outline diagram of a cooling system according to a fourth embodiment of the invention, and 
         FIG.  6    is an outline diagram of a cooling system according to a fifth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     A cooling system  1  according to an embodiment of the present invention is very schematically illustrated in  FIG.  1   . The cooling system  1  comprises a first cooling circuit  10  for cooling at least one first component  11  by means of coolant circulating in the first cooling circuit and a second cooling circuit  20  for cooling at least one second component  21   a ,  21   b  by means of coolant circulating in the second cooling circuit. The coolant flowing through the first and second cooling circuits  10 ,  20  is preferably water, possibly with anti-freezing additives such as for instance glycol. A first coolant pump  12  is provided in the first cooling circuit  10  in order to circulate the coolant in the first cooling circuit and a second coolant pump  22  is provided in the second cooling circuit  20  in order to circulate the coolant in the second cooling circuit. The first and second coolant pumps  12 ,  22  are preferably electrically driven pumps. The cooling system  1  may further comprise an electronic control unit  2  which is connected to the first and second coolant pumps  12 ,  22  and configured to control the operation thereof so as to thereby control the flow velocities of the coolant circulating in the first and second cooling circuits  10 ,  20 . 
     Furthermore, a cooling device  13 ,  23 , for instance in the form of a heat exchanger, is provided in each cooling circuit  10 ,  20  in order to remove heat from the coolant circulating therein. The cooling system  1  may for instance be used in a motor vehicle  3 , for instance in the form of a hybrid or fully electric vehicle, wherein each cooling device  13 ,  23  may have the form of a radiator, for instance a conventional coolant radiator. In this case, the first cooling circuit  10  may be a low temperature cooling circuit for cooling a vehicle component in the form of an electric energy storing device  11 , such as for instance an electric battery or a set of electric batteries, for supplying electric energy to an electric traction motor of the vehicle, whereas the second cooling circuit  20  may be a high temperature cooling circuit for cooling vehicle components in the form of power electronic devices, such as for instance an inverter  21   a  and a DC converter  21   b , for controlling the flow of electric power between the electric energy storing device  11  and the electric traction motor. 
     In the illustrated embodiment, the second cooling circuit  20  comprises a radiator bypass line  24  and a valve device  25  for controlling the coolant flow through the radiator  23  and the bypass line  24 . The coolant flowing through any of the radiators  13 ,  23  is cooled by means of ambient air which is blown towards the radiators when the vehicle  3  is in motion. The vehicle  3  may also be provided with a fan  4 , which, when so needed, may be operated in order to generate an air flow through the radiators  13 ,  23 . Even though the radiators  13 ,  23  are depicted in  FIG.  1    as being arranged in-line and being fed by air from the common fan  4  it is of course also possible to arrange the radiators  13 ,  23  distanced from each other and being fed by air from separate individual fans. 
     The cooling system  1  comprises an expansion tank  30  provided with an expansion chamber  31  for accumulation of coolant, wherein this expansion chamber  31  is surrounded by an external casing  32  of the expansion tank. The expansion tank  30  is provided with a closable refill opening  33  which is arranged in the casing  32  at an upper part thereof. Coolant may be introduced into the expansion chamber  31  via this refill opening  33  in order to provide for replenishment of the cooling system. The refill opening  33  is closed by means of a removable lid  34 . Furthermore, the expansion tank  31  is provided with a valve device (not shown) which comprises a pressure relief valve for limiting the pressure in the expansion chamber  31  and a return valve. This valve device may be arranged in the lid  34  or in the casing  32 . The pressure relief valve allows air and coolant to flow out from the upper part of the expansion chamber  31  when the pressure in the expansion chamber, due to an increase of the coolant volume, exceeds a pressure level given by the pressure relief valve. Thus, the pressure relief valve ensures that the pressure in the expansion chamber  31  cannot exceed a predetermined pressure level. The return valve allows air to flow into the upper part of the expansion chamber  31  from the surroundings when the pressure in the expansion chamber, due to a reduction of the coolant volume, becomes lower than a pressure level given by the return valve. 
     The expansion chamber  31  is connected to the second cooling circuit  20  in order to allow the expansion chamber  31  to receive coolant from the second cooling circuit  20 . 
     The cooling system  1  comprises a deaeration device  40  arranged in the first cooling circuit  10  for separation of air bubbles from the coolant circulating in the first cooling circuit, wherein this deaeration device  40  is located at a lower position than the expansion tank  30  and connected to the expansion chamber  31  of the expansion tank  30  via a first static line  5  in order to allow air bubbles separated from the coolant in the deaeration device  40  to migrate upwards in the first static line  5  towards the expansion chamber  31 . In the embodiment illustrated in  FIG.  1   , the first static line  5  is connected directly to the expansion chamber  31  via a first opening  35   a  provided in the casing  32  of the expansion tank. 
     The first static line  5  has a lower end  5   a  and an upper end  5   b , wherein the first static line  5  is connected to the deaeration device  40  at its lower end  5   a . In the embodiment illustrated in  FIG.  1   , the first static line  5  slopes upwards along its entire length from its lower end  5   a  to its upper end  5   b . However, as illustrated in  FIG.  4   , the first static line  5  may as an alternative consist of a combination of one or more first length sections  8   a , each of which sloping upwards as seen in a direction along the first static line from its lower end towards its upper end, and one or more horizontal second length sections  8   b , wherein these first and second length sections  8   a ,  8   b  are interconnected and arranged in series with each other. 
     In the embodiment illustrated in  FIG.  1   , the expansion chamber  31  is connected to the second cooling circuit  20  via a deaeration line  6  and a second static line  7 . When the second coolant pump  22  is in operation and circulates coolant in the second cooling circuit  20 , a small flow of coolant is in this case continuously directed from the second cooling circuit  20  to the expansion chamber  31  via the deaeration line  6  and returned from the expansion chamber  31  to the second cooling circuit  20  via the second static line  7 . Air bubbles that are accompanying the coolant into the expansion chamber  31  via the deaeration line  6  are allowed to migrate towards the surface  36  of the coolant in the expansion chamber and the air included in these air bubbles is accumulated in an air-filled space  37  at the upper part of the expansion chamber  31 . Thus, in this case, the deaeration of the coolant in the second cooling circuit  20  is effected in the expansion tank  30 , whereas the deaeration of the coolant in the first cooling circuit  10  is effected in the deaeration device  40 . However, the air separated from the coolant in the first cooling circuit  10  is accumulated in the expansion chamber  31  of the expansion tank together with the air separated from the coolant in the second cooling circuit  20 . In the illustrated example, the second static line  7  is connected directly to the expansion chamber  31  via a second opening  35   b  provided in the casing  32  of the expansion tank and the deaeration line  6  is connected directly to the expansion chamber  31  via a third opening  35   c  provided in the casing  32 . As an alternative, the deaeration line  6  may instead be connected to the second static line  7  and thereby indirectly connected to the expansion chamber  31  via the second static line. In the latter case, the air bubbles accompanying coolant from the deaeration line  6  into the second static line  7  are allowed to migrate upwards in the second static line  7  towards the expansion chamber  31 . 
     In the embodiment illustrated in  FIG.  1   , the deaeration device  40  comprises a deaeration chamber  41  (see  FIG.  2   ), which is enclosed by a casing  42  and which has:
         a coolant inlet  43  connected to a feed pipe  15  of the first cooling circuit  10  in order to allow coolant circulating in the first cooling circuit to flow from the feed pipe  15  into the deaeration chamber  41  via this coolant inlet  43 ;   a first coolant outlet  44  connected to the first coolant pump  12  in order to allow coolant to flow from the deaeration chamber  41  to the first coolant pump  12  via this first coolant outlet  44 , wherein the first coolant pump  12  is arranged immediately downstream of the first coolant outlet; and   a second coolant outlet  45  connected to the expansion chamber  31  via the first static line  5  and located at a higher position than the first coolant outlet  44 .       

     The coolant inlet  43  and the first coolant outlet  44  are spaced apart from each other in a longitudinal direction of the deaeration chamber  41 . 
     In the embodiment illustrated in  FIG.  1   , the first coolant outlet  44  is connected to a first end of a first coolant pipe  16  and an inlet of the first coolant pump  12  is connected to a second end of the first coolant pipe  16 . 
     The second coolant outlet  45  is located in an upper part of the deaeration chamber  41  in order to allow air bubbles that have migrated to the upper part of the deaeration chamber to leave the deaeration chamber and enter the first static line  5  via the second coolant outlet  45 . Air bubbles may hereby be separated from the coolant in the first cooling circuit  10 . The second coolant outlet  45  is preferably located at the highest point in the deaeration chamber  41 , but it may as an alternative be located slightly below the highest point in the deaeration chamber  41 . 
     The cross-sectional dimension of the deaeration chamber  41  is larger than the cross-sectional dimension of the feed pipe  15  such that the flow velocity of the coolant in the deaeration chamber  41  is lower than the flow velocity of the coolant in the feed pipe  15  leading to the deaeration chamber, to thereby allow air bubbles to be carried along with the rather rapid coolant flow in the feed pipe  15  and enter the deaeration chamber  41  via the coolant inlet  43  and thereafter migrate in the deaeration chamber  41  to the second coolant outlet  45 . The slower coolant flow in the deaeration chamber  41  will give the coolant an increased dwell time in the deaeration chamber, which in its turn will give air bubbles in the coolant a chance to migrate in the deaeration chamber  41  to the second coolant outlet  45 . 
     The cross-sectional dimension of the deaeration chamber  41  is preferably so much larger than the cross-sectional dimension of the feed pipe  15  that the relationship between the flow velocity v 1  of the coolant flowing through the deaeration chamber  41  between the coolant inlet  43  and the first coolant outlet  44  and the flow velocity v 2  of the coolant flowing through the feed pipe  15  is 1:2 or lower, preferably 1:3 or lower. 
     The deaeration chamber  41  may have a cylindrical shape, for instance a circular cylindrical shape, but it may as an alternative have any other suitable shape. 
     The electronic control unit  2  may be configured to control the first coolant pump  12  in such a manner that the flow velocity of the coolant circulating through the first cooling circuit  10  is maintained at such a value during normal operating conditions that the first cooling circuit is continuously deaerated during the normal operating conditions. However, the electronic control unit  2  may as an alternative be configured to control the first coolant pump  12  in such a manner that the flow velocity intermittently or only at specific occasions is set to a value adapted for an efficient deaeration of the coolant in the first cooling circuit  10 . 
     The second coolant outlet  45  is preferably located in such a position in relation to the coolant inlet  43  and the first coolant outlet  44  that the coolant flow in the deaeration chamber  41  between the coolant inlet  43  and the first coolant outlet  44  will move migrating air bubbles in the deaeration chamber  41  in the longitudinal direction of the deaeration chamber  41  towards the second coolant outlet  45 . Thus, in this case, the migration direction of the air bubbles in the deaeration chamber  41  corresponds to the flow direction of the coolant in the deaeration chamber  41  and the coolant flow in the deaeration chamber  41  will thereby promote the movement of the air bubbles towards the second coolant outlet  45 . 
     In the embodiment illustrated in  FIGS.  1  and  2   , the deaeration chamber  41  has an elongated shape and is arranged with its longitudinal axis  46  extending in vertical direction. In this case, the above-mentioned flow direction of the coolant in the deaeration chamber  41  is achieved by having the coolant inlet  43  located at a lower position than the first coolant outlet  44 , which gives an upwardly directed coolant flow in the deaeration chamber  41 . However, the deaeration chamber  41  may as an alternative be arranged with its longitudinal axis  46  inclined in relation to a horizontal plane by an angle a of 0-90°, e.g. by an angle which is &gt;0° and &lt;=90°, or an angle between 10-90° or 15-90°. When the deaeration chamber  41  is arranged with its longitudinal axis  46  inclined, for instance in the manner illustrated in  FIG.  3   , air bubbles may rise in the deaeration chamber  41  and hit an inclined upper wall surface  47  in the deaeration chamber, whereupon the air bubbles are conveyed along this wall surface  47  towards the second coolant outlet  45  under the effect of the coolant flow in the deaeration chamber  41  between the coolant inlet  43  and the first coolant outlet  44 . 
     In the embodiment illustrated in  FIG.  3   , the coolant in the second cooling circuit  20  circulates through the expansion chamber  31  of the expansion tank  30 , wherein the circulating coolant is directed to the expansion chamber  31  by a first coolant line  26  included in the second cooling circuit and directed from the expansion chamber  31  towards the second coolant pump  22  by a second coolant line  27  included in the second cooling circuit. Air bubbles that are accompanying the coolant into the expansion chamber  31  via said first coolant line  26  are allowed to migrate towards the surface  36  of the coolant in the expansion chamber and the air included in these air bubbles is accumulated in an air-filled space  37  at the upper part of the expansion chamber  31 . Thus, in this case, the deaeration of the coolant in the second cooling circuit  20  is effected in the expansion tank  30 . 
     In the embodiment illustrated in  FIG.  3   , the first static line  5  is indirectly connected to the expansion chamber  31  by having its upper end  5   b  connected to said second coolant line  27  of the second cooling circuit  20 . Air bubbles entering the second coolant line  27  from the first static line  5  may migrate upwards in the second coolant line  27  towards the expansion chamber  31  if the flow velocity of the coolant circulating in the second cooling circuit  20  is low enough. If the flow velocity of the coolant circulating in the second cooling circuit  20  is higher, said air bubbles will move along with the coolant circulating in the second cooling circuit and enter the expansion chamber  31  via the above-mentioned first coolant line  26 . 
     In the embodiments illustrated in  FIGS.  4 - 6   , the cooling system  1  comprises first and second cooling circuits  10 ,  20  essentially corresponding to the first and second cooling circuits described above with reference to  FIG.  1   . In these embodiments, the cooling system  1  comprises a first deaeration device  40  of the type described above arranged in the first cooling circuit  10  for separation of air bubbles from the coolant circulating in the first cooling circuit. Furthermore, the cooling system  1  also comprises a second deaeration device  50  arranged in the second cooling circuit  20  for separation of air bubbles from the coolant circulating in the second cooling circuit, wherein this deaeration device  50  is located at a lower position than the expansion tank  30  and connected to the expansion chamber  31  of the expansion tank  30  via a second static line  7  in order to allow air bubbles separated from the coolant in the second deaeration device  50  to migrate upwards in the second static line  7  towards the expansion chamber  31 . In the illustrated examples, the second static line  7  is connected directly to the expansion chamber  31  via an opening  35   b  provided in the casing  32  of the expansion tank. 
     The second static line  7  has a lower end  7   a  and an upper end  7   b , wherein the second static line  7  is connected to the second deaeration device  50  at its lower end  7   a . In the embodiments illustrated in  FIGS.  4 - 6   , the second static line  7  slopes upwards along its entire length from its lower end  7   a  to its upper end  7   b . However, the second static line  7  may as an alternative consist of a combination of one or more first length sections, each of which sloping upwards as seen in a direction along the second static line from its lower end towards its upper end, and one or more horizontal second length sections, wherein these first and second length sections are interconnected and arranged in series with each other. 
     In the embodiments illustrated in  FIGS.  4 - 6   , the second deaeration device  50  comprises a deaeration chamber  51 , which is enclosed by a casing  52  and which has:
         a coolant inlet  53  connected to a feed pipe  25  of the second cooling circuit  20  in order to allow coolant circulating in the second cooling circuit to flow from the feed pipe  25  into the deaeration chamber  51  via this coolant inlet  53 ;   a first coolant outlet  54  connected to the second coolant pump  22  in order to allow coolant to flow from the deaeration chamber  51  to the second coolant pump  22  via this first coolant outlet  54 , wherein the second coolant pump  22  is arranged immediately downstream of the first coolant outlet; and   a second coolant outlet  55  connected to the expansion chamber  31  via the second static line  7  and located at a higher position than the first coolant outlet  54 .       

     The coolant inlet  53  and the first coolant outlet  54  are spaced apart from each other in a longitudinal direction of the deaeration chamber  51 . 
     In the embodiment illustrated in  FIGS.  4 - 6   , the first coolant outlet  54  is connected to a first end of a second coolant pipe  17  and an inlet of the second coolant pump  22  is connected to a second end of the second coolant pipe  17 . 
     The second coolant outlet  55  is located in an upper part of the deaeration chamber  51  in order to allow air bubbles that have migrated to the upper part of the deaeration chamber to leave the deaeration chamber and enter the second static line  7  via the second coolant outlet  55 . Air bubbles may hereby be separated from the coolant in the second cooling circuit  20 . The second coolant outlet  55  is preferably located at the highest point in the deaeration chamber  51 , but it may as an alternative be located slightly below the highest point in the deaeration chamber  51 . 
     The cross-sectional dimension of the deaeration chamber  51  is larger than the cross-sectional dimension of the feed pipe  25  such that the flow velocity of the coolant in the deaeration chamber  51  is lower than the flow velocity of the coolant in the feed pipe  25  leading to the deaeration chamber, to thereby allow air bubbles to be carried along with the rather rapid coolant flow in the feed pipe  25  and enter the deaeration chamber  51  via the coolant inlet  53  and thereafter migrate in the deaeration chamber  51  to the second coolant outlet  55 . The slower coolant flow in the deaeration chamber  51  will give the coolant an increased dwell time in the deaeration chamber, which in its turn will give air bubbles in the coolant a chance to migrate in the deaeration chamber  51  to the second coolant outlet  55 . 
     The cross-sectional dimension of the deaeration chamber  51  is preferably so much larger than the cross-sectional dimension of the feed pipe  25  that the relationship between the flow velocity v 3  of the coolant flowing through the deaeration chamber  51  between the coolant inlet  53  and the first coolant outlet  54  and the flow velocity v 4  of the coolant flowing through the feed pipe  25  is 1:2 or lower, preferably 1:3 or lower. 
     The deaeration chamber  51  may have a cylindrical shape, for instance a circular cylindrical shape, but it may as an alternative have any other suitable shape. 
     The electronic control unit  2  may be configured to control the second coolant pump  22  in such a manner that the flow velocity of the coolant circulating through the second cooling circuit  20  is maintained at such a value during normal operating conditions that the second cooling circuit is continuously deaerated during the normal operating conditions. However, the electronic control unit  2  may as an alternative be configured to control the second coolant pump  22  in such a manner that the flow velocity intermittently or only at specific occasions is set to a value adapted for an efficient deaeration of the coolant in the second cooling circuit  20 . 
     In the embodiments illustrated in  FIGS.  4 - 6   , the second coolant outlet  55  is located in such a position in relation to the coolant inlet  53  and the first coolant outlet  54  that the coolant flow in the deaeration chamber  51  between the coolant inlet  53  and the first coolant outlet  54  will move migrating air bubbles in the deaeration chamber  51  in the longitudinal direction of the deaeration chamber  51  towards the second coolant outlet  55 . Thus, in this case, the migration direction of the air bubbles in the deaeration chamber  51  corresponds to the flow direction of the coolant in the deaeration chamber and the coolant flow in the deaeration chamber  51  will thereby promote the movement of the air bubbles towards the second coolant outlet  55 . 
     In the embodiments illustrated in  FIGS.  4 - 6   , the deaeration chamber  51  has an elongated shape and is arranged with its longitudinal axis  56  extending in vertical direction. However, the deaeration chamber  51  may as an alternative be arranged with its longitudinal axis  56  inclined in relation to a horizontal plane by an angle of 0-90°, e.g. by an angle which is &gt;0° and &lt;=90°, or by an angle of 10-90°, or 15-90°. 
     In the embodiments illustrated in  FIGS.  5  and  6   , the cooling system  1  also comprises a cab heating circuit  60 . A third coolant pump  62  is provided in the cab heating circuit  60  in order to circulate the coolant in the cab heating circuit. Furthermore, a heat exchanger  63  configured to heat air to be supplied to the vehicle cab is provided in the cab heating circuit  60 . 
     In the embodiment illustrated in  FIG.  5   , the cab heating circuit  60  is connected to the second deaeration device  50 . In this case, the coolant circulating in the cab heating circuit  60  is deaerated in the second deaeration device  50  and this coolant is also configured to absorb heat in the second deaeration device  50  from hotter coolant circulating in the second cooling circuit  20 . 
     In the embodiment illustrated in  FIG.  6   , the cooling system  1  comprises a third deaeration device  64  arranged in the cab heating circuit  60  for separation of air bubbles from the coolant circulating in the cab heating circuit, wherein this deaeration device  64  is located at a lower position than the expansion tank  30  and connected to the expansion chamber  31  of the expansion tank  30  via a third static line  9  in order to allow air bubbles separated from the coolant in the third deaeration device  64  to migrate upwards in the third static line  9  towards the expansion chamber  31 . The third deaeration device  64  may be designed in the same manner as the first and/or second deaeration device  40 ,  50 . In the embodiment illustrated in  FIG.  6   , a heating device  90  is provided in the cab heating circuit  60  in order to heat the coolant circulating in the cab heating circuit. 
     The invention is of course not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.