SE541765C2

SE541765C2 - An oil system for lubrication and cooling in a vehicle driven at least partly by an electrical machine

Info

Publication number: SE541765C2
Application number: SE1651052A
Authority: SE
Inventors: Jörgen Forsberg; Kym Schaube; Per Arnelöf
Original assignee: Scania Cv Ab
Priority date: 2016-07-13
Filing date: 2016-07-13
Publication date: 2019-12-10
Also published as: SE1651052A1; DE102017006366A1

Abstract

The present invention relates to oil system (8) in a vehicle (1) driven by a motor arrangement comprising an electrical machine (6). The oil system comprises a lubrication circuit (9) configured to direct oil to at least one component (7) to be lubricated, and a cooling circuit (20) configured to direct oil to the electric machine (6). The oil system comprises further a first pump (10) which is driven by the motor arrangement and configured to provide an oil flow rate in the lubrication circuit (9), a second pump (21) which is driven by an electrical motor and configured to provide an oil flow in the cooling circuit (20) and at least one connection line (16, 25) provided with a pressure control valve (17, 26) configured to open and to direct an excess oil flow rate in one of the circuits (16, 25) to the other circuit (16, 25).

Description

An oil system for lubrication and cooling in a vehicle driven at least partly by an electrical machine BACKGROUND TO THE INVENTION AND PRIOR ART The present invention relates to an oil system for lubrication and cooling in a vehicle driven at least partly by an electrical machine according to the preamble of claim 1.

A drive train of a hybrid vehicle may be driven by an electrical machine and a combustion engine. The electrical machine works as an engine and provides power during certain operating conditions such as during idling conditions, at low speeds and accelerations of the vehicle. During certain operating conditions, the electrical machine works as a generator where it supplies electrical energy to a battery. The combustion engine may drive the vehicle independently or together with the electrical machine. The electrical machine gets hot during operation. Conventional cooling of electrical machines may be performed by air or water led past an external surface of a housing enclosing the electrical machine. A more effective cooling method is to spray oil on the stator windings of the electrical machine. A cooling system for an electrical machine has to be able to provide a high oil flow rate to the electrical machine.

A hybrid vehicle may comprise a lubrication system for lubrication of components such as bearings and gear. The lubrication system may comprise a fixed displacement pump delivering a constant volume of lubricant during each pumping cycle. The pump may be driven by a component in the drive line such as a shaft in a gearbox. In this case, the pump provides a lubricant flow rate in the lubrication system related to the speed of the component in the drive line. The lubricant flow rate results in a pressure in the lubrication system. The viscosity of a lubricant such as an oil is significantly higher at low temperatures than at high temperatures. As a consequence, the pressure in the system will be high at low lubricant temperatures.

Conventional lubrication systems comprise a relief valve which opens at a regulating pressure and limits the pressure in the system. The existence of the relief valve prevents damage of pressure sensitive components in the system. A further object of the relief valve is to prevent excessive lubricant flow rate in the system. A lubricant flow rate which is too high results in a high pressure in the system and an excessive consumption of pumping energy. The relief valve is dimensioned to open at a pressure corresponding to an acceptable lubricant flow rate at a low lubricant temperature. Thus, the relief valve prevents an insufficient lubricant flow rate to the component at low lubricant temperatures.

US 2013/0213607 shows an oil system for supplying oil to an electrical machine and lubrication to a motive power transmission mechanism. The oil system includes an oil pump unit in the form of a mechanical pump and an electrical pump. The pumps are arranged in parallel in the oil system. The pumps can be used simultaneously or separately.

SUMMARY OF THE INVENTION The object of the invention is to provide an oil system providing an efficient lubrication in a vehicle at the same time as it provides an efficient cooling of an electrical machine in the vehicle.

This object is achieved by the oil system defined in the introduction which is characterized by the features defined in the characterising part of claim 1. The oil system comprises a lubrication circuit and a cooling circuit. The lubrication circuit must be able to provide a sufficient oil flow rate to the component to be lubricated even when the temperature of the oil is low and it has a high viscosity. A first pump driven by a component in the drive line of the vehicle fulfills this requirement. Such a first pump can be defined as a mechanical pump. However, it is difficult to control the oil flow rate provided by a mechanical pump since it is related to the speed of the component in the drive line. There is sufficiently low cooling demand of an electrical machine when the temperature of the oil is low and it has a high viscosity such as after a cold start. During remaining operating conditions the electrical machine has a cooling demand. The cooling demand of the electrical machine may change quickly and it may be very high during certain operating conditions. In view of these facts, it is suitable to use a second pump in the cooling circuit in the form of a pump powered by an electric motor. Such an electrical pump is easy to control within a large speed range with a high precision.

The existence of the at least one connection line between the lubrication circuit and the cooling circuit in the oil system makes it possible to direct an excess oil flow rate in one of the circuits to the other circuit. Thus, in case the mechanical pump provides an excess oil flow rate in the lubrication circuit, it is possible to direct that excess oil into the cooling circuit. In this case, the electrical pump is controlled in a manner such that it provides a smaller oil flow rate in the cooling circuit. The oil flow rate provided by the electrical pump and the excess oil flow rate provided by the mechanical pump may provide together an oil flow rate to the electrical machine at which it receives sufficient cooling. On the other hand, in case the mechanical pump provides insufficient oil flow rate in the lubrication circuit, it is possible to control the electrical pump such that it provides an excess oil flow rate in the cooling circuit which is directed, via the connection line, to the lubrication circuit. The excess oil flow rate provided by the electrical pump and the oil flow rate provided by the mechanical pump may provide together an oil flow rate to the component at which it receives sufficient lubrication.

According to the present invention, the oil system comprises a first connection line comprising a first pressure control valve configured to sense the pressure in the lubrication circuit and to open at a first regulating pressure corresponding to a suitable oil flow rate in the lubrication circuit for lubrication of said component such that an excess oil flow rate is redirected from the lubrication circuit, via the connection line, to the cooling circuit. The pressure in the lubrication circuit is related to the oil flow rate in the lubrication circuit. Thus, it is possible to estimate a pressure in the lubrication circuit which corresponds to an oil flow rate at which said component provides an efficient lubrication. In case the oil flow rate and the pressure in the lubrication circuit exceeds the first regulating pressure, the pressure control valve is opened and an excess oil flow in the lubrication circuit is directed, via the connection line, to the cooling circuit.

According to the present invention, the oil system comprises a second connection line comprising a second pressure control valve configured to open when the pressure in the lubrication circuit is lower than a second regulating pressure corresponding to a suitable oil flow rate in the lubrication circuit such that an oil flow rate is directed from the cooling circuit, via the connection line, to the lubrication circuit. The pressure in the lubrication circuit is related to the oil flow rate in the circuit. Thus, it is possible to estimate a second regulating pressure in the lubrication circuit corresponding to an oil flow rate at which said components provides a sufficient lubrication. In case the oil flow rate and the pressure in the cooling circuit is lower than the second regulating pressure, the second pressure control valve is open and a part of the oil flow rate in the cooling circuit is directed, via the second connection line, to the lubrication circuit.

According to an embodiment of the present invention, the oil system comprises a first check valve configured to allow oil flow in one direction through the lubrication circuit and prevent oil flow in an opposite direction. In case the pressure in the cooling circuit is higher than in the lubrication circuit, there will be an oil flow from the cooling circuit to the lubrication circuit. The existence of the first check valve prevents an oil flow in the wrong direction towards the mechanical pump in the lubrication circuit.

According to an embodiment of the present invention, the oil system comprises a second check valve configured to allow oil flow in one direction through the cooling circuit and prevent oil flow in an opposite direction. In case the mechanical pump provides a too high oil flow in the lubrication circuit, a part of the oil flow is directed, via the first connection line, to the cooling circuit. The existence of the second check valve prevents an oil flow in the wrong direction towards the electrical pump in the cooling circuit.

According to an embodiment of the present invention, the oil system comprises a third check valve configured to allow oil flow in one direction through the second connection line and prevent oil flow in an opposite direction. In case the pressure in the lubrication circuit is lower than a regulation pressure of the second pressure controlled valve in the second connection line at the same time as the cooling circuit has an even lower pressure, the third check valve prevents an oil flow from the lubrication circuit, via the second connection line, to the cooling circuit.

According to an embodiment of the present invention, the oil system comprises a control unit configured to control the oil flow rate provided by the second pump by means of information about at least one operating parameter. The control unit estimates a required oil flow rate to be provided by the second pump by means of said operating parameter. Thereafter, it activates the electrical motor such that the second pump delivers the required oil flow rate. The control unit may be configured to control the oil flow rate provided by the second pump by means of information about a parameter related to the oil flow rate provided by the first pump and the temperature of the electrical machine. The temperature of the electrical machine indicates a required oil flow rate for cooling of the electrical machine. The required oil flow rate for lubrication of said component is usually known. In view of these facts, the control unit estimates the required total oil flow rate to be provided by the first pump and the second pump. Thereafter, the control unit estimates the oil flow rate to be provided by the second pump as the difference between the required total oil flow rate and the oil flow rate provided by the first pump. Alternatively or in combination, the control unit may control the oil system by means of information about other parameters such as, for example, surrounding temperature, the power output of the electric machine, location based prediction (GPS).

According to an embodiment of the present invention, the oil system comprises a locking device configured to lock the first pressure control valve in the closed position during operating conditions when the oil has a lower temperature than a regular operating temperature. During operating conditions when the oil has a low temperature, the pressure in the lubrication circuit will be higher even at relatively small oil flow rates. Furthermore, there is no cooling demand of the electrical machine when the oil temperature is low. The locking device ensures that the entire oil flow rate from the first pump is directed to the component to be lubricated even if the pressure in the lubrication circuit exceeds the first regulating pressure. Thus, the locking device ensures a sufficient lubrication of said component when the oil has a low temperature.

According to an embodiment of the present invention, the oil system comprises a temperature sensor configured to sense the temperature of the oil in the lubrication and to initiate a movement of the locking device to a locking position when the oil has a lower temperature than a regular oil temperature. The temperature sensor device may be arranged in contact with the oil in the lubrication circuit. In this case, the temperature sensor device detects temperature changes practically immediately. Alternatively, the temperature sensor device may be arranged in a wall or similar located close to the lubrication circuit. According to a further alternative, the temperature sensor device may be arranged in an oil sump. In the latter cases, the temperature sensor device detects temperature changes with a certain delay.

The temperature sensor device may comprise a casing enclosing a material configured to change volume at the predetermined temperature. The material may be a suitable wax material changing phase between a solid phase and a liquid phase at said predetermined temperature. Such a temperature sensor device is inexpensive and it has a very reliable function. Alternatively, the temperature sensor device may comprise components changing shape at different temperature such as bimetallic strips. The temperature sensor device may comprise a movement transmission mechanism configured to transfer a movement from the temperature sensor device to the locking device dependent on the temperature of the lubricant. Such a movement transmission mechanism may be used to switch the locking device between a locking position in which it locks the first pressure control valve and a non-locked position in which it does not lock the first pressure control valve.

According to an embodiment of the present invention, the lubrication circuit comprises a relief valve configured to limit the pressure in the lubrication circuit to a maximum pressure. During operating condition when the oil temperature is low, the locking device is activated such that it locks the first pressure control valve. As a consequence, the first pressure control valve is not able to reduce the pressure in the lubrication circuit during operating conditions when the oil has a low temperature. In this case, the relief valve will limit the pressure and the flow rate to the component to be lubricated. Furthermore, the relief valve prevents damage of pressure sensitive components in the lubrication circuit.

According to an embodiment of the present invention, the lubrication circuit and the cooling circuit receive and return oil from a common oil sump. Due to this fact, it is not necessary to arrange certain components in both the circuits. Since the oil is used interchangeably in the two circuits, it is usually enough to have one oil filter in one of said circuits. Furthermore, the oil system may comprise only one oil cooler which is arranged in the cooling circuit. Since a low oil temperature facilitates the cooling of the electrical machine, it is an advantage to arrange the oil cooler in the cooling circuit.

According to an embodiment of the present invention, the oil system is arranged in a vehicle driven by the electrical machine and a combustion engine. Such a vehicle may be a hybrid vehicle.

BRIEF DESCRIPTION OF THE DRAWING A preferred embodiment of the invention is described below by way of an example with reference to the attached drawing, on which: Fig. 1 shows a vehicle comprising an oil system for lubrication and cooling according to the present invention and Fig. 2 shows the oil system in Fig. 1 more in detail.

DETAIFED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Fig 1 shows a drive line for a vehicle in the form of a hybrid vehicle 1. The drive line comprises a combustion engine 2, a gear box 3, a number of drive shafts 4 and drive wheels 5. An electrical machine 6 is arranged in a housing between the combustion engine 2 and the gear box 3. During certain operating conditions, the electrical machine 6 supplies power to the drive line. During other operating conditions, the electrical machine 6 works as a generator where it supplies electrical energy to a battery in the vehicle 1. The combustion engine 2 may drive the vehicle independently or together with the electrical machine 6. At least the gearbox 3 comprises components 7 in the form of bearings, shafts and gears which need to be lubricated during operation of the vehicle. The electrical machine 6 is heated during operation and it needs to be cooled. A schematically indicated oil system 8 lubricates said components 7 and cools the electrically machine 6.

Fig. 2 shows the oil system 8 more in detail. The oil system 8 comprises a lubrication circuit 9 configured to lubricate said components 7. The lubrication circuit 9 comprises a mechanical pump 10 pumping oil from an oil sump 11. In this case, the mechanical pump 10 is a fixed positive-displacement pump. Thus, the mechanical pump 10 delivers a constant volume of oil to the lubrication circuit 9 during each pump cycle and at a given speed. The mechanical pump 10 is driven by a component in the drive line of the vehicle. In this case, the oil flow rate pumped in the lubrication circuit 9 by the mechanical pump 10 varies with the speed of the component in the drive line. The oil flow rate pumped in the lubrication circuit 9 by the mechanical pump 10 may vary within a relatively large range during different operating conditions of the vehicle 1. The lubrication circuit 9 comprises a return line 9a provided with a relief valve 12. The relief valve 12 is dimensioned to open at a maximum acceptable pressure in the lubrication circuit 9. In case the relief valve 12 opens, a part of the oil in the lubrication circuit 9 is directed back to the oil sump 11 via the return line 9a. As soon as the pressure in the lubrication circuit 9 has decreased to a lower pressure than the maximum pressure, the relief valve 12 is closed and the oil flow through the return line 9a ceases. The relief valve 12 has the task to limit the oil flow rate in the lubrication circuit 9 at low oil temperatures and to prevent damage of pressure sensible components.

A first check valve 13 is arranged in the lubrication circuit 9 in order to prevent oil flow in an incorrect direction. The oil leaving the first check valve 13 enters an oil filter 14. A bypass line 9b including a bypass valve 15 may provide an oil flow past the oil filter 14. Only in case the oil filter 14 is clogged, the bypass valve 15 is to be open. An inlet end of a first connection line 16 is connected to the lubrication circuit 9 in a position downstream of the oil filter 14. A first pressure control valve 17 is arranged in the first connection line 16. The first pressure control valve 17 is dimensioned to open at a first regulating pressure pi. The first regulating pressure pi is lower than the regulating pressure of the relief valve 12. In case a higher pressure prevails in lubrication circuit 9 than the first regulating pressure pi, the first pressure control valve 17 is open such that a part of the oil in lubrication circuit 9 is directed into the first connection line 16. As soon as the pressure in the lubrication circuit 9 has decreased to a lower pressure than the first regulating pressure pi, the first pressure control valve 17 is closed and the oil flow through the first connection line 16 ceases. A temperature sensor 18 is arranged in contact with the oil in the lubrication circuit 9. A locking device 19 is configured to lock the first pressure control valve 17 in a closed position when the oil has a lower temperature than a regulation temperature T of the temperature sensor 18. The oil flow in lubrication circuit 9 is directed from the oil filter 14 to the components 7 to be lubricated when the first pressure control valve 17 is closed. One of the components 7 to be lubricated may be a shaft supporting at least one gear wheel in the gearbox 3. The shaft may be a main shaft or a counter shaft in the gearbox 3. When the oil has lubricated said components 7a, it is directed back to the oil sump 11.

The oil system comprises a cooling circuit 20 directing oil from the oil sump 11 to the electrical machine 6. An electrical pump 21 pumps oil through the cooling circuit 20. The electrical pump 21 is driven by an electric motor 22. A second check valve 23 prevents an oil flow in an incorrect direction in the cooling circuit 20. An oil cooler 24 cools the oil before it enters the electrical machine 6. The oil can be cooled in the oil cooler 24 by coolant circulating in a cooling system cooling the combustion engine 2. An outlet end of the first connection line 16 is connected to the cooling circuit 20 in a position upstream of the cooler 24. Thus, the first connection line 16 makes it possible to direct oil from the lubrication circuit 9 to the cooling circuit 20. The oil leaving the oil cooler 24 is directed to the electrical machine 6. When the oil has cooled the electrical machine 6 it is directed back to the oil sump 11. The oil system 8 comprises a second connection line 25. The second connection line 25 has an inlet end connected to the cooling circuit 20 in a position downstream of the cooler 24 and an outlet end connected to the lubrication circuit 9 in a position upstream of said components 7 to be lubricated. The second connection line 25 comprises a second pressure control valve 26. The second pressure control valve 26 is controlled by the pressure in the lubrication circuit 9. The second pressure control valve 26 is set in a closed position when the pressure in the lubrication circuit 9 exceeds a second regulating pressure p2. The second pressure control valve 26 is set in a open position when the pressure in the lubrication circuit 9 is lower than the second regulating pressure p2. A third check valve 27 prevents an oil flow in an incorrect direction through the second connection line 25. Thus, the second connection line 25 makes it possible to direct oil from the cooling circuit 20 to the lubrication circuit 9.

The oil system 8 comprises a control unit 28 which controls the operation of the electrical motor 22 and thus the speed of the electrical pump 21. The control unit 28 controls the operation of the electrical pump 21 by means of information 29 about the speed of the mechanical pump 10 and thus the oil flow rate pumped by the mechanical pump 10 in the lubrication circuit 9. The control unit 28 also controls the operation of the electrical pump 21 by means of information 30 about the temperature of the electrical machine 6. The control unit 28 may receive information from a temperature sensor sensing the temperature of the stator windings of the electrical machine 6.

During a cold start of the vehicle 1, the oil in the oil sump 11 has a low temperature and a high viscosity. The oil has a lower temperature than the regulation temperature T of the temperature sensor 18. The mechanical pump 10 may start to pump oil through the lubrication line 9 at the same time as the combustion engine 2 starts. Since the oil has a high viscosity, it requires a relatively high pumping energy to circulate the oil through the lubrication circuit 9. As a consequence, the pressure in the lubrication circuit 9 is high. The oil has a lower temperature than the regulation temperature T of the temperature sensor 18 and the first pressure control valve 17 is locked by the locking device 19 in the closed position. Thus, the first pressure control valve 17 is prevented to be opened despite the fact that the pressure in the lubrication line 9 is higher than first regulating pressure p1. Consequently, the entire oil flow rate pumped by the mechanical pump 10 is directed to said components 7 to be lubricated. In case the oil flow rate to the component 7 and the pressure in the lubrication circuit will be too high, the relief valve 12 is open. The control unit 28 receives continuously information from the temperature sensor 30 about the temperature of the electrical machine 6. As long as the temperature of the electrical machine 6 is below a suitable operating temperature, there is no cooling demand of the electrical machine 6. Thus, the electrical pump 21 is not activated and it is no oil flow through the cooling circuit 20 to the electrical machine 6.

After a certain period of operation, the temperature of the oil has increased to a suitable operating temperature. The suitable operating temperature is higher than the regulation temperature T of the temperature sensor 18. As a consequence, the first pressure control valve 17 is not any longer locked in the closed position. The control unit 28 receives substantially continuously information 29 about the speed of the mechanical pump 10 and information 30 about the temperature of the electrical machine 6. The first pressure control valve 17 is open at a first regulating pressure pi corresponding to the pressure arising in the lubrication circuit when the mechanical pump 10 pumps a too high oil flow rate in the lubrication circuit 9 for providing an acceptable lubrication of said components 7.

During operating conditions when the mechanical pump 10 pumps a suitable oil flow rate to the components 7 to be lubricated, the first pressure control valve 17 is maintained in the closed position and the entire oil flow rate produced by the mechanical pump 10 is directed to said components 7. As a consequence, no oil flow will be directed from the lubrication circuit 9, via the first connection line 16, to the cooling circuit 20. As soon as the temperature of the electrical machine 6 exceeds a predetermined temperature, the control unit 28 activates the electrical pump 21 such that it provides an oil flow rate in the cooling circuit 20 which provides a suitable cooling of the electrical machine 6. Consequently, when the mechanical pump 10 provides an oil flow rate in the lubrication circuit suitable to lubricate said components 7, the electrical pump 21 is controlled to provide an oil flow rate in the cooling circuit suitable to cool the electrical machine 6. In this case, there is no oil flow, between the circuits 9, 20 via the connections lines 16, 25. As a result, the lubrication circuit 9 and the cooling circuit 20 operate as two separate circuits.

During operating conditions when the mechanical pump 10 provides a too high oil flow rate in the lubrication circuit 9 for lubrication of said components 7, the pressure in the lubrication circuit 9 will be higher than the first regulating pressure pi of the first pressure control valve 17. As a consequence, the first pressure control valve 17 is open and an excess oil flow rate in the lubrication circuit 9 is directed, via the first connection line 16, to the cooling circuit 20. The oil flow rate directed to the components 7 provides a desired lubrication of the components 7. The second check valve 23 prevents an oil flow towards the electrical pump 21. The control unit 28 receives information 29 indicating the oil flow rate pumped by the mechanical pump 10 in the lubrication circuit 9. The control unit 28 estimates the excess oil flow rate directed from the lubrication circuit 9, via the first connection line 16, to the cooling circuit 20. The control unit 28 receives information 30 about the temperature of the electrical machine 6 and estimates a required oil flow rate for cooling of the electrical machine 6. The control unit 28 estimates an oil flow rate from the electrical pump 21 which together with the excess oil flow rate from the connection line 16 create the required oil flow rate for cooling the electrical machine 6. Finally, the control unit 28 activates the electrical pump 21 such that it provides said estimated oil flow rate.

During operating conditions when the mechanical pump 10 provides a too low oil flow rate in the lubrication circuit 9 for lubrication of said components 7, the pressure in the lubrication circuit 9 will be lower than the second regulating pressure p2 of the second pressure control valve 26. As a consequence, the second pressure control valve 26 is opened. As a consequence, a part of the oil flow rate in the cooling circuit 20 will flow, via the second connection line 25, to the lubrication circuit 9. The first check valve 13 prevent an oil flow in a wrong direction in the lubrication circuit 9 towards the mechanical pump 10. The control unit 28 receives information 29 indicating that the oil flow rate pumped by the mechanical pump 10 in the lubrication circuit 9 is too poor for a sufficient lubrication of the components 7. The third check valve 27 prevents a flow in wrong direction through the second connection line 25. The control unit 28 has access to information about a required oil flow rate for lubrication of the components 7 during substantially all operating conditions. The control unit 28 estimates an excess oil flow rate to be directed from the cooling circuit 20, via the second connection line 16, to the lubrication circuit 9 in order to provide a desired lubrication of said components 7. The control unit 28 receives information 30 about the temperature of the electrical machine and it estimates a required oil flow rate to the electrical machine 6. The control unit 28 estimates the required oil flow rate from the electrical pump 21 for providing said excess oil flow rate to the lubrication circuit 9 and the oil flow rate for cooling of the electrical machine 6. Finally, the control unit 28 activates the electrical pump 21 such that it provides the required oil flow rate.

The invention is in no way limited to the embodiment to which the drawing refers but may be varied freely within the scopes of the claims. The vehicle can be solely driven by an electric machine. Thus, the vehicle does not need to be a hybrid vehicle. The lubrication circuit may be used to lubricate arbitrary components in the gearbox, the electric machine or in the combustion engine etc.

Claims

1. An oil system (8) in a vehicle (1) driven at least partly by an electrical machine (6), wherein the oil system comprises a lubrication circuit (9) configured to direct oil to at least one component (7) to be lubricated, a cooling circuit (20) configured to direct oil to the electric machine (6), and a first pump (10) which is driven by a drive line in the vehicle (1) and configured to provide an oil flow rate in the lubrication circuit (9), a second pump (21) which is driven by an electrical motor and configured to provide an oil flow in the cooling circuit (20), characterized in that the oil system comprises a first connection line (16) comprising a first pressure control valve (17) configured to sense the pressure in the lubrication circuit (9) and to open at a first regulating pressure (pi) corresponding to a suitable oil flow rate in the lubrication circuit (9) for lubrication of said component (7) such that an excess oil flow rate is directed from the lubrication circuit (9), via the connection line (16), to the cooling circuit (20) and a second connection line (25) comprising a second pressure control valve (26) configured to open when the pressure in the lubrication circuit (9) is lower than a a second regulating pressure (p2) corresponding to a suitable oil flow rate in the lubrication circuit (9) such that an oil flow rate is directed from the cooling circuit, via the second connection line (25), to the lubrication circuit (9).

2. An oil system according to claim 1, characterized in that it comprises a first check valve (13) configured to allow oil flow in one direction through the lubrication circuit (9) and prevent oil flow in an opposite direction.

3. An oil system according to claim 1 or 2, characterized in that it comprises a second check valve (23) configured to allow oil flow in one direction through the cooling circuit (20) and prevent oil flow in an opposite direction.

4. An oil system according to any one of the preceding claims, characterized in that it comprises a third check valve (27) configured to allow oil flow in one direction through the second connection line (25) and prevent oil flow in an opposite direction.

5. An oil system according to any one of the preceding claims, characterized in that it comprises a control unit (28) configured to control the speed of the second pump (21) by means of information about at least one operating parameter.

6. An oil system according to claim 5, characterized in that the control unit (28) is configured to control the speed of the second pump (21) by means of information about a parameter related to the oil flow rate provided by the first pump (10) and the temperature of the electrical machine (6).

7. An oil system according to any one of the preceding claims, characterized in that it comprises a locking device (19) configured to lock the first pressure control valve (17) in the closed position during operating conditions when the oil has a lower temperature than a regular operating temperature (T).

8. An oil system according to claim 7, characterized in that the oil system (8) comprises a temperature sensor (18) configured to sense the temperature of the oil in the lubrication circuit (9) and to initiate a movement of the locking device (19) to a locking position when the oil has a lower temperature than the regular operating temperature (T).

9. An oil system according any one of the preceding claims, characterized in that the lubrication circuit (9) comprises a relief valve (12) configured to limit the pressure in the lubrication circuit (9) to a maximum pressure.

10. An oil system according to any one of the preceding claims, characterized in that the lubrication circuit (9) and the cooling circuit (20) receive oil from and return oil to a common oil sump (11).

11. 1 1. An oil system according to any one of the preceding claims, characterized in that the oil system comprises only one oil filter (14) arranged in one of said circuits (9, 20).

12. An oil system according to any one of the preceding claims, characterized in that the oil system comprises only one oil cooler (24) which is arranged in the cooling circuit (20).

13. An oil system according any one of the preceding claims, characterized in that it is arranged in a vehicle driven by an electrical machine (6) and a combustion engine (2).