WO2019166846A1

WO2019166846A1 - Lubrication system and method for a motor vehicle and motor vehicle comprising such lubrication system

Info

Publication number: WO2019166846A1
Application number: PCT/IB2018/000336
Authority: WO
Inventors: Yann BOETE
Original assignee: Volvo Truck Corporation
Priority date: 2018-03-01
Filing date: 2018-03-01
Publication date: 2019-09-06

Abstract

The invention relates to a lubrication system (2) for a motor vehicle (1), this system comprising an oil circuit (4), a first oil tank (6) for supplying the oil circuit, a pump (8) for pumping oil from the first tank to the oil circuit, a second oil tank (10), also for supplying the oil circuit, and a pneumatic circuit (12), which comprises a compressed air tank (120) for supplying the second oil tank (10) with air, so as to be able to expel the oil contained in the second oil tank (10) to the oil circuit (4). The first oil tank (6) and the second oil tank (10) are not one and the same tank.

Description

Lubrication system and method for a motor vehicle and motor vehicle comprising such lubrication system

The invention concerns a lubrication system and method for a motor vehicle.

Technological background

Motor vehicles include one or more lubrication systems to reduce friction between different mechanical parts. In particular, we often find a lubrication system for the engine, gearbox, differential, etc. The oil, used as a lubricant, reduces wear and tear on the parts, and also evacuates part of the thermal energy resulting from friction.

Traditionally, oil is pumped from a tank to the parts to be lubricated. In practice, a pump is used that is mechanically driven by the engine crankshaft. For lubrication to be effective, oil must be pumped at a certain pressure level. At start-up, the crankshaft runs at crankspeed, so the pump speed is not sufficient for the lubrication system to operate. Typically, at start-up, the crankshaft is driven in rotation by the starter, at a speed between 80 and 120 revolutions per minute, while at least 500 to 600 revolutions per minute are required to operate the pump.

It is therefore understandable that, at each start-up, there is a whole transitional period during which the lubrication system(s) is/are not functional. Until now, this has not been particularly troublesome, as the number of starts made during the life of the vehicle has been relatively limited. However, with the advent of Stop & Start standby systems, the number of engine starts has increased considerably, and the lack of lubrication at each start leads to premature wear and tear of mechanical parts in the long term.

In response to this problem, manufacturers have developed more robust mechanical parts, playing with the nature of the materials used. Another solution is to deactivate the Start & Stop system after a certain number of starts, typically around 100 000, to prevent the wear and tear of engine parts from becoming critical.

However, these solutions are not really satisfactory in terms of both feasibility and cost.

It is to these disadvantages that the invention aims in particular to remedy, by proposing a new automotive lubrication system, with which oil can be injected even when the vehicle is started up, i.e. when the pump does not provide sufficient oil flow and/or pressure.

Concerning automotive lubrication systems, JP 2005 090362 A discloses several embodiments of an engine arrangement, wherein lubricating oil can be supplied to a bearing portion of the crankshaft while the engine is stopped. Typically, the engine includes an auxiliary oil tank capable of supplying oil to the bearing portion of the crankshaft. A stop state detection means is provided for detecting when the engine stops. As a result, when the engine stops, the auxiliary oil tank supplies the bearing portion with oil. To this end, the engine includes a circulation pump arrangement that includes an oil supply pump for transferring oil from the tank to the bearing portion and an oil return pump for transferring oil from the main oil tank to the auxiliary oil tank.

JP 2005 090362 A tackles the problem of engine wear due to lack of lubrication when the engine stops, i.e. when the ignition key is switched from the ON position to the OFF position. To the contrary, the main goal of the invention is to lubricate when the engine is started, in particular during the transitory period during which the main oil pump does not function.

Besides, US 2017 0219085 A1 also discloses a lubricating device of a power transmission device for a vehicle. In the example of figure 5, the lubricating device includes a first oil pump and a second oil pump as suction devices. The first oil pump and the second oil pump are respectively connected to a first supply route and a second supply route that are different and independent of each other, thereby lubricating the respective portions of the power transmission device in a shared manner. The first oil pump is a mechanical oil pump that is mechanically rotationally driven via a pump drive gear, while the second oil pump is a mechanical oil pump that is coupled to the input shaft so as to be mechanically rotationally driven by the engine. The first oil pump and the second oil pump suck oil from an oil storage portion provided at the bottom of a case. The oil storage portion includes a first oil storage portion, a second oil storage portion and a third oil storage portion that are separated from one another by partition walls. A suction port of the first oil pump is disposed in the second oil storage portion. A suction port of the second oil pump is disposed in the third oil storage portion.

In a stationary state, in which the operations of the first oil pump and the second oil pump are stopped during the stop of the vehicle, the oil storage portion has a stationary oil level that exceeds the partition walls as shown by a two-dot chain line in FIGS. 2 and 5. Accordingly, the differential device is partially immersed in the lubricating oil in the stationary state where the oil level exceeds the partition walls. In this way, the lubricating oil is scraped up by the differential ring gear and so on at the time of start of the vehicle, so that the lubricating oil is scattered to the respective portions of the power transmission device. Therefore, it is possible to ensure a good lubrication state even at the time of start of the vehicle at which it is difficult to supply a sufficient amount of the lubricating oil by the first oil pump. Summary of the invention

For this purpose, the invention concerns a lubrication system for a motor vehicle, this system comprising an oil circuit, a first oil tank for supplying the oil circuit, a pump for pumping oil from the first tank to the oil circuit, a second oil tank, also for supplying the oil circuit, and a pneumatic circuit, which comprises a compressed air tank for supplying the second oil tank with air, so as to be able to expel the oil contained in the second oil tank to the oil circuit. The first oil tank and the second oil tank are not one and the same tank.

Thanks to the invention, the second oil tank enables supplying the oil circuit even at the start of the vehicle, when the mechanical pump is not capable of delivering a sufficient pressure. Accordingly, this system is particular suitable for vehicles equipped with“Stop and Go” feature, for which the number of engine ignitions is very high.

Advantageous, but optional features of the system are defined below:

- The oil circuit is the oil circuit of a combustion engine, a gearbox or of a differential.

- The compressed air tank is an air tank used to supply air to all pneumatic actuators of the vehicle, such as brake or suspension actuators.

- The second oil tank is arranged below the maximum oil level in the first tank when the pump is stopped.

- The second oil tank is located inside the first tank.

- The system comprises connecting means for filling the second tank from the first tank.

- Said connecting means comprises a non-return valve and the second tank is filled from the first tank by following the principle of communicating vessels. This means that there is a free flow from the first tank to the second tank.

- The pneumatic circuit comprises a controlled valve arranged between the second tank and the compressed air tank.

- The controlled valve is a solenoid valve, preferably a 3/2-way valve.

- The controlled valve comprises a first port connected to the compressed air tank, a second port connected to the second tank and a third port opened to the atmosphere. The controlled valve can take a first position where the first port and the second port are connected to each other and a second position where the second port and the third port are connected to each other.

- The system comprises a pressure regulating valve between the second tank and the oil circuit. - The system comprises a valve, preferably a check valve, which is arranged between the second tank and the oil circuit and which is configured to prevent oil from flowing back from the oil circuit to the second oil tank.

- The system comprises a closure valve automatically closing the connection between the second tank and the oil circuit when the oil level in the second tank reaches a minimum level.

- The second oil tank is arranged at the bottom of the first oil tank. An oil drain opening, closed for instance by a bleeder screw, is provided at the lowest location inside the second oil tank so as to perform a complete oil drain of the second oil tank and of the first oil tank in one operation.

- The first tank is an oil sump.

- The pump is mechanically driven by a vehicle engine, in particular by an internal combustion engine of the motor vehicle.

- The second oil tank contains a different volume of oil than the first oil tank and is preferably smaller than the first oil tank.

The invention also concerns a motor vehicle comprising a lubrication system as defined above.

Eventually, the invention concerns a lubrication method for a motor vehicle, wherein the motor vehicle includes an engine and a lubrication system comprising an oil circuit, a first oil tank for supplying the oil circuit, a pump for pumping oil from the first tank to the oil circuit, a second oil tank, also for supplying the oil circuit, and a pneumatic circuit, which comprises a compressed air tank for supplying the second oil tank with air, so as to be able to expel the oil contained in the second oil tank to the oil circuit. The first oil tank and the second oil tank are not one and the same tank. The method includes steps consisting, when the engine is started, in a) supplying the oil circuit first with oil from the second oil tank and then b) when the pump is capable of delivering sufficient pressure, supplying the oil circuit with oil from the first oil tank.

Preferably, the lubrication system includes a compressed air tank and, at step a), air is sent under pressure to the second oil tank so as to expel oil from the second tank in the oil circuit.

Advantageously, step b) also consists in switching off the supply of the oil circuit via the second oil tank when at least one of the following conditions is met:

- the time elapsed since the engine start or since the air under pressure has been started to be sent to the second oil tank reaches a determined time threshold,

- engine speed reaches a determined speed threshold, or

- oil pressure in the oil circuit is above a determined pressure threshold. Brief description of the drawings

Further advantages and advantageous features of the invention are disclosed in the following description, provided in reference to the appended drawings, solely for exemplary non-limitative purpose. In the drawings:

-figures 1 to 3 are schematic depictions of a lubricating system according to the invention, illustrated in three different configurations: and

-figure 4 is a motor vehicle, in particular a tractor truck, comprising such lubricating system.

Description of example embodiment(s) of the invention

Figures 1 to 3 show an example embodiment of a lubrication system 2 for a motor vehicle 1. The motor vehicle 1 , which is represented on figure 4, is a heavy-duty vehicle, in particular a tractor truck. Alternatively, the invention is obviously applicable to any other motor vehicle, provided that such vehicle includes a source of compressed air.

Preferably, the motor vehicle 1 includes a controller (not represented) for controlling the engine ignition. Typically, this controller includes the “Stop and Go” function that allows shutting down the engine when the vehicle is stopped, for example at a red light or in traffic jam. Such function is now well known in the automotive field, that is why it is not further described in this paper.

The lubrication system 2 comprises an oil circuit 4 which is, in the preferred embodiment of the figures, the oil circuit of a combustion engine 16.

In the depicted example, the combustion engine 16 includes three combustion cylinders 160 capable of driving a crankshaft 162 in rotation about its axis. Obviously, the number of combustion cylinders 160 is not limitative.

The lubrication system 2 further comprises a first oil tank 6 for supplying the oil circuit 4 and a pump 8 for pumping oil from the first tank 6 to the oil circuit 4.

In the example, the pump 8 is mechanically driven by the vehicle engine 16. Transmission means, typically including one or more gears (no represented), enable transmitting the movement of the crankshaft 160 or any other rotating part of the engine to the pump 8.

In the preferred embodiment of the figures, the first tank 6 is an oil sump.

The oil sump 6 is preferably a plastic tank screwed under the engine block, with a sealing gasket (not represented). Alternatively, the oil sump 6 may be made of sheet steel or cast in aluminum. The oil sump 6 contains the oil needed to lubricate the moving parts of the engine 16. When engine rated speed is set, oil is drawn from it by a filter screen (not represented) of the oil pump 8, which distributes it under pressure, via an oil filter (not represented), to the various components (connecting rods, camshaft, bearings, etc.). It then descends by simple gravity.

More specifically, the internal combustion engine (ICE) 16 includes, in the example, four hydrostatic bearings 164 for supporting and guiding the crankshaft 162 in rotation about its axis. Obviously, the number of bearings 164 is not limitative. The bearings 164 are part of the oil circuit 4, meaning that they are each supplied with oil flowing in the oil circuit 4.

A hydrostatic bearing is known as such, that is why the hydrostatic bearings 164 are not further described. Each hydrostatic bearing 164 is supplied with oil under pressure, so that oil forms a cushion around the shaft 162. Typically, the oil pressure within each bearing 164 is about 2 bar. Hydrostatic bearings provide the advantage of having a very low friction coefficient.

The pump 8 is capable of delivering oil at 2 bar only after a certain period of time since the start of the engine. At start-up, the crankshaft 160 runs at crank speed, so the pump speed is not sufficient for the lubrication system 2 to operate.

Therefore, the lubrication system 2 includes a second oil tank 10, also for supplying the oil circuit 4 and a pneumatic circuit 12 which comprises a compressed air tank 120 for supplying the second oil tank 10 with compressed air, so as to be able to expel the oil contained in the second oil tank 10 to the oil circuit 4.

In order to avoid mechanical failure(s) and/or other malfunctioning of the system 2, too much oil should not be sent too quickly into the oil circuit 4. However, a characteristic of compressed air discharge is that it allows a relatively high flow rate to be achieved. Accordingly, the lubrication system 2 preferably includes a pressure reducing valve 18 between the second tank 10 and the oil circuit 4 for limiting the pressure of the oil expelled from the second oil tank 10 to the oil circuit 4.

Also, the lubrication system 2 advantageously includes a valve 20, in particular a check valve, which is arranged between the second tank 10 and the oil circuit 4 and which is configured to prevent oil from flowing back from the oil circuit 4 to the second oil tank 10. The second oil tank 10 contains preferably a different volume of oil than the first oil tank 6 and is preferably smaller than the first oil tank 6. The first oil tank 6 and the second oil tank 10 are not one and the same tank. In the example, the second oil tank 10 is located inside the first tank 6. Accordingly, the second oil tank 10 does not add any clutter to the engine. In the example, the volume of the second oil tank is comprised between 1L and 10L. In a variant, the second oil tank 10 can be separate and outside from the first oil tank 6.

In the example, the second oil tank 10 is fixed at the bottom of the first oil tank 6.

Advantageously, the second oil tank 10 is below the maximum oil level in the first tank 6. In other words, the top surface inside the second oil tank 10 is below the maximum oil level in the first tank 6 that is reached when the engine and the pump 8 are stopped for some time.

Preferably, the lubrication system 2 includes a connecting means 14 for filling the second tank 10 from the first tank 6.

According to a preferred embodiment, said connecting means comprises a nonreturn valve 14. The connecting means 14 is configured so as to prevent oil contained in the second tank 10 from flowing in the first oil tank 6. In other words, oil may only flow from the first tank 6 to the second tank 10.

The oil sump 6 (or crankcase) is equipped at its lowest point with a drain opening (not represented), dedicated to the periodic draining of the engine 16. Typically, a bleed screw (not represented) with a sealing washer is provided for closing the drain opening/

Advantageously, the drain opening is provided at the bottom of the second tank 10, so that both tanks 6 and 10 can be emptied at the same time. Indeed, during draining of the engine, the tank 10 fills from the oil sump 6 at the same time as it empties, for as long as oil is contained in the first oil tank 6. This eases maintenance operations.

Advantageously, the compressed air tank 120 is an air tank used to supply air to all pneumatic actuators of the vehicle 1 , such as the brake or suspension actuators. Accordingly, the compressed air already on board the vehicle is used, which means that there is no additional space requirement.

In the example, the pneumatic circuit 12 comprises a controlled valve 122 arranged between the second tank 10 and the compressed air tank 120. Typically, the controlled valve 122 is a solenoid valve, preferably a 3/2-way valve. Basically, electric energy stored in the vehicle battery is used to selectively power the solenoid of the valve 122.

Advantageously, the controlled valve 122 comprises a first port 122.1 connected to the compressed air tank 120, a second port 122.2 connected to the second tank 10 and a third port 122.3 opened to the atmosphere.

In the example, the controlled valve 122 can take a first position, represented on figures 1 and 3, where the second port 122.2 and the third port 122.3 are connected to each other and a second position, represented on figure 2, where the first port 122.1 and the second port 122.2 are connected to each other. In known manner, the valve 122 is switched from the first position to the second position by the action of a solenoid. In the example, the 3/2-way valve 122 is a monostable valve, meaning that, when the solenoid is not supplied, the valve 122 is returned to the first position by a spring.

A lubrication method for the motor vehicle 1 is described below.

At a first step a), when the engine 16 is started, i.e. when the ignition switch of the engine is turned on, the oil circuit 4 is first supplied with oil from the second oil tank 10.

In the example, at step a), air is sent under pressure to the second oil tank 10 so as to expel oil from the second tank 10 in the oil circuit 4. Typically, the air is drawn from the compressed air tank 120 described above. With this respect, the solenoid of the valve 122 is electrically supplied and the valve 122 is switched from its first position to its second position, meaning that the compressed air tank 120 is put in communication with the second oil tank 10, as represented by arrows F1 on figure 2. Arrows F1 show the air flow between the air tank 120 and the oil tank 10.

When entering in the tank 10, compressed air expels oil from the second tank 10 in the oil circuit 4, as represented by arrows F2 on figure 2.

Then, at a step b), when the pump 8 is capable of delivering sufficient pressure, the oil circuit 4 is supplied with oil from the first oil tank 6, meaning that the pump 8 feeds the oil circuit 4 with oil, as represented by arrows F3 on figure 3.

Typically, the pump 8 is capable of delivering sufficient pressure when the engine rated speed is set, i.e. when combustion occurs in the cylinders 160 and when the crankshaft 162 is driven in rotation by the alternating motion of the cylinders 160. At this time, the rotation speed of the crankshaft 160 is superior to 600 revolutions per minute, which is enough to enable the pump 8 to deliver oil at a sufficient pressure (of 2 bar in the example). ’

Preferably, step b) also consists in switching off the supply of the oil circuit 4 via the second oil tank 10. This means that, when engine rated speed is set, oil circuit 4 is exclusively supplied with oil from the first tank 6. Accordingly, the solenoid electric power supply is cut off, and the valve 122 is switched from its second position to its first position by the action of the return spring of the valve, meaning that the second oil tank 10 is opened to the atmosphere. Then, the remaining air inside the tank 10 advantageously escapes through the outlet port 122.3 of the valve 122.

In the example, the system includes a timer (not represented) for controlling the period of time during which the solenoid valve 122 is in the second position. This period of time is predetermined in function of the pressure inside the air tank 120, of the capacity of the oil tank 10, etc. After such period of time has elapsed, the solenoid electric power supply is cut off, and the valve 122 is switched from its second position to its first position by the action of the return spring of the valve. Using a timer enables to avoid sending compressed air inside the oil circuit 4. It also enables controlling the quantity of oil expelled in the oil circuit 4 from the second tank 10.

Alternatively, the system 2 could be provided with a pressure sensor (not represented) for measuring the pressure inside the oil circuit 4. Accordingly, the solenoid valve 122 could be switched back in its first position when the pressure inside the oil circuit 4 is above the regulated oil pressure delivered by the second tank 10, indicating that main oil pump 8 is delivering sufficient oil pressure. Therefore, the air supply to the second oil tank 10 is cut off to avoid sending air in the oil circuit 4. Obviously, the predetermined threshold mentioned above is chosen with a margin of safety, meaning that during step a), the oil tank 10 is preferably not completely emptied.

Step b) then also consists in switching off the supply of the oil circuit 4 via the second oil tank 10 when at least one of the following conditions is met:

- the time elapsed since the engine start or since the air under pressure has been started to be sent to the second oil tank 10 reaches a determined time threshold,

- engine speed reaches a determined speed threshold, or

- oil pressure in the oil circuit 4 is above a determined pressure threshold.

In an another embodiment, the system 2 could be provided with a closure valve automatically closing the connection between the second tank 10 and the oil circuit 4 when the oil level in tank 10 reaches a minimum level.

More specifically, the system 2 could be provided with a float system that could be used instead or in complement of the timer or of the pressure sensor. In this particular embodiment, the float would block the outlet of the oil tank 10 connected to the oil circuit 4 when the oil tank 10 is empty or almost empty. The float would then prevent compressed air contained in the tank 10 from entering the oil circuit 4.

After step a) is complete, i.e. after the second oil tank 10 has been partially emptied, the second oil tank 10 automatically fills up with oil form the first tank through the check valve 14, as represented by the arrow F4 on figure 3. Indeed, the second oil tank 10 is arranged below the maximum oil level in the first tank 6 and the engine assembly is designed such that the first tank remains approximately at the atmosphere pressure, that’s why when the second tank level is below the first tank level, and when the pressure inside the second oil tank 10 is at the atmospheric pressure, i.e. when the valve 122 is in the first position, a differential pressure exists between both tanks 6 and 10. So, oil will flow through the check valve 14 and refill the second tank 10. It doesn't matter if valve 14 stays open while the vehicle is moving. What is important is that valve 14 closes when tank 10 is pressurized. It is possible to use a valve with very low differential pressure, or even with inverse differential pressure (which closes only if the pressure in tank 10 is higher than the pressure in tank 6 of several dozen kPa). In other words, the second tank 10 is refilled from the first tank 6 by following the principle of communicating vessels.

In a non-represented alternative embodiment, the oil circuit 4 may also be the oil circuit of a gearbox or of a differential.

In another non-represented alternative embodiment, the second oil tank 10 could be outside the first tank 6. Preferably, the second oil tank 10 could be positioned downstream of the pump 8. This would allow the pump 8 to be used to fill the second oil tank 10 from the first oil tank 6, in particular when engine speed is established. Accordingly, the second oil tank 10 could be located almost anywhere relative to the first tank 6.

The features of the described embodiment and of the alternatives not shown can be combined together to generate new embodiments of the invention.

Claims

1. Lubrication system (2) for a motor vehicle (1 ), this system comprising:

- an oil circuit (4),

- a first oil tank (6) for supplying the oil circuit,

- a pump (8) for pumping oil from the first tank to the oil circuit,

- a second oil tank (10), also for supplying the oil circuit, and

- a pneumatic circuit (12), which comprises a compressed air tank (120) for supplying the second oil tank (10) with air, so as to be able to expel the oil contained in the second oil tank (10) to the oil circuit (4), wherein the first oil tank (6) and the second oil tank (10) are not one and the same tank.

2. The system according to claim 1 , wherein the oil circuit (4) is the oil circuit of a combustion engine (16), a gearbox or of a differential.

3. The system according to claim 1 or 2, wherein the compressed air tank (120) is an air tank used to supply air to all pneumatic actuators of the vehicle, such as brake or suspension actuators.

4. The system according to one of the preceding claims, wherein the second oil tank (10) is arranged below the maximum oil level in the first tank (6) when the pump (8) is stopped.

5. The system according to one of the preceding claims, wherein the second oil tank (10) is located inside the first tank (6).

6. The system according to one of the preceding claims, comprising connecting means for filling the second tank (10) from the first tank (6).

7. The system according to claims 4, 5 and 6, wherein said connecting means comprises a non-return valve (14) and the second tank (10) is filled from the first tank (6) by following the principle of communicating vessels.

8. The system according to one of the preceding claims, wherein the pneumatic circuit (12) comprises a controlled valve (122) arranged between the second tank (10) and the compressed air tank (120).

9. The system according to the preceding claim, wherein the controlled valve (122) is a solenoid valve, preferably a 3/2-way valve.

10. The system according to the preceding claim, wherein the controlled valve (122) comprises a first port (122.1) connected to the compressed air tank (120), a second port (122.2) connected to the second tank and a third port (122.3) opened to the atmosphere and wherein the controlled valve can take a first position where the first port (122.1 ) and the second port (122.2) are connected to each other and a second position where the second port (122.2) and the third port (122.3) are connected to each other.

11. The system according to one of the preceding claims, comprising a pressure regulating valve (18) between the second tank (10) and the oil circuit (4).

12. The system according to one of the preceding claims, comprising a valve (20), preferably a check valve, which is arranged between the second tank (10) and the oil circuit (4) and which is configured to prevent oil from flowing back from the oil circuit (4) to the second oil tank (10).

13. The system according to one of the preceding claims, comprising a closure valve automatically closing the connection between the second tank (10) and the oil circuit (4) when the oil level in the second tank (10) reaches a minimum level.

14. The system according to one of the preceding claims, wherein the second oil tank (10) is arranged at the bottom of the first oil tank (6) and wherein an oil drain opening, closed for instance by a bleeder screw, is provided at the lowest location inside the second oil tank (10) so as to perform a complete oil drain of second oil tank (10) and first oil tank (6) in one operation.

15. The system according to one of the preceding claims, wherein the first tank (6) is an oil sump.

16. The system according to one of the preceding claims, wherein the pump (8) is mechanically driven by a vehicle engine (16), in particular by an internal combustion engine of the motor vehicle.

17. Motor vehicle (1 ), comprising a lubrication system (2) according to any previous claim.

18. Lubrication method for a motor vehicle (1 ), wherein the motor vehicle includes an engine (16) and a lubrication system (2) comprising:

- an oil circuit (4),

- a first oil tank (6) for supplying the oil circuit,

- a pump (8) for pumping oil from the first tank to the oil circuit, and

- a second oil tank (10), also for supplying the oil circuit,

wherein the first oil tank (6) and the second oil tank (10) are not one and the same tank,

the method including steps consisting, when the engine is started, in: a) supplying the oil circuit (4) first with oil from the second oil tank (10) and then

b) when the pump is capable of delivering sufficient pressure, supplying the oil circuit (4) with oil from the first oil tank (6).

19. Method according to previous claim, wherein the lubrication system includes a compressed air tank (120) and wherein, at step a), air is sent under pressure to the second oil tank (10) so as to expel oil from the second tank in the oil circuit.

20. Method according to claim 19, wherein step b) also consists in switching off the supply of the oil circuit (4) via the second oil tank (10) when at least one of the following conditions is met:

- the time elapsed since the engine start or since the air under pressure has been started to be sent to the second oil tank (10) reaches a determined time threshold,

- engine speed reaches a determined speed threshold,

- oil pressure in the oil circuit (4) is above a determined pressure threshold.