FR3049311A3 - "METHOD FOR CONTROLLING A MOTOR POWERTRAIN ACCORDING TO A PARAMETER REPRESENTATIVE OF THE REDUCTION OF THE LUBRICATING FLUID LUBRICATION POWER" - Google Patents

Abstract

The invention proposes a method for controlling a power unit of a motor vehicle comprising a motor (12) equipped with a thermal circuit (22) lubrication in which circulates a fluid (24) lubricant, said method acting on a parameter of dynamic operation of the heat engine, the method comprising a step (E3) for generating a control setpoint so that said dynamic operating parameter of the motor (12) is less than its maximum limit after its application, characterized in that it comprises at least one preliminary restriction step (E2) during which the maximum limit of the operating parameter is modified as a function of at least one parameter representative of the reduction of the lubricating power of the lubricant fluid (24).

Description

"Method of controlling a power train according to a parameter representative of the reduction of the lubricating power of a lubricating fluid"

TECHNICAL FIELD OF THE INVENTION The invention relates to a method for controlling a power unit of a motor vehicle comprising a heat engine and in which the heat engine comprises at least one lubrication circuit in which a lubricating fluid circulates, the control method acting on at least one dynamic operating parameter of the heat engine, in particular among the engine torque, the engine load and the engine speed, the method comprising a step of generating a control setpoint determined so that said parameter of Dynamic engine operation is below a maximum limit.

BACKGROUND ART OF THE INVENTION

The dynamic operation of the thermal engines is generally characterized by at least two dynamic operating parameters.

The first dynamic operating parameter is the engine speed. This is the number of revolutions per unit of time achieved by the motor shaft. This operating parameter acts directly on the speed of the motor vehicle.

The second dynamic operating parameter is the motor torque. The engine torque is related to the pressure exerted on the engine pistons in the combustion chamber. The engine torque is also directly related to the engine load. This operating parameter acts on the power required to advance the motor vehicle at a given speed, for example in the event of a change of road gradient.

The maximum limit that can be reached by the engine torque depends on the engine speed. Thus, for a given engine speed, the torque that can be delivered by the engine is limited by a maximum torque. This maximum torque varies according to the engine speed.

These characteristics are generally recorded in the form of a map in an electronic control unit of the vehicle. An example of such a mapping has been shown in FIG. 4 in the form of a two-dimensional graph comprising an abscissa, here representing the scale of the engine speed, and an ordinate representing here the scale of the torque of the engine. engine. An operating zone is bounded horizontally by a minimum engine speed limit and a maximum engine speed limit. The operating zone is delimited upwards by a curve representing the maximum torque of the engine as a function of the engine speed.

When the vehicle engine is running, it can be represented by an operating point in the operating zone. The selected point thus indicates the engine speed and the torque produced by the engine.

The operating zone contains all the operating points for which the operating parameters of the heat engine are below their maximum limit. The heat engine is thus likely to operate in a secure manner. Outside this operating zone, the optimal operation of the motor is not guaranteed. Thus, the maximum engine speed is a speed beyond which the mechanical elements of the engine may be damaged. This maximum limit is determined either theoretically or by experimentation. Likewise, the minimum engine speed is the speed below which the engine is likely to stall.

Such mapping is used by different powertrain control methods.

Thus, according to a first example, it is known to use this mapping in the transmission ratio change control methods. It is known that a change in transmission ratio has the effect of acting on the engine speed and torque by changing the operating point of the engine. For this purpose, the motor vehicle generally comprises an electronic control unit which receives at every moment sensor measurements, such as the vehicle speed, the position of the accelerator pedal, the engine torque. The electronic control unit generates a transmission ratio change instruction according to the measured data and according to predetermined gearshift laws. The electronic control unit uses mapping to ensure that the transmission ratio change thus developed will operate the engine at an operating point in the determined operating zone.

In the case of an automatically controlled transmission, the electronic control unit thus automatically controls the transmission gearbox shift.

In the case of a manually controlled transmission, the electronic control unit communicates the gear ratio change instruction to the driver via an appropriate interface, for example via a screen that displays to the instruction on the dashboard.

It is known that the gearshift laws can be modified by the driver for the gearbox to be controlled according to a so-called "sporty" mode in which the gear changes allow the operation of the engine at operating points in the vicinity of the gears. maximum limits of the operating area. The motor is thus biased to the limits of its physical stops of rotation and / or torque. In such a mode of operation of the gearbox, it is permitted to "jump" a transmission ratio by controlling a rise or a downshift of several transmission ratios.

According to a second example, it is also known to use such a map to automatically control the operation of the engine, for example in the context of a method of automatic regulation of the speed of the vehicle. In this case, an electronic control unit generates an acceleration control command of the engine so that its operating point remains in the determined operating zone of the map.

Furthermore, it is also known to lubricate the engine with a lubricating fluid, usually oil. The lubricant fluid ensures a safe operation of the engine, especially in extreme conditions, when the operating point of the engine is close to the maximum limits of the operating area. Nevertheless, the fuel, generally a hydrocarbon, tends to be diluted in the lubricating fluid as the vehicle is used.

For example, in a compression-ignition engine, commonly known as a diesel engine, the principle of supplying fuel by direct injection into the combustion chamber is at the origin of a transfer of a portion of the fuel to the engine. low engine where it mixes with the lubricating oil.

This transfer of fuel to the engine lubrication system is further enhanced for engines equipped with exhaust aftertreatment systems, such as particulate filters or catalytic converters. Indeed, in engines equipped with such post-treatment systems, additional fuel injections can be made at the combustion chambers at a time when the fuel will not be burned but sent to the exhaust line where it will be used to regenerate the aftertreatment systems of the exhaust gases, for example for the combustion of soot accumulated in the particulate filters or to modify the oxidation state of the medium inside the catalyst system. The dilution rate of the lubricating oil by the fuel can thus reach values of 10% by volume and more. The introduction of fuel into the lubricating oil circuit results, on the one hand, in the alteration of the characteristics of the lubricant, for example a reduction in its viscosity, a dilution of the additives, and, on the other hand, an increase in the volume present in the sump. This results in a deterioration of the operation of the engine which is manifested by a reduced oil pressure and an abnormally high oil consumption and eventually leads to increased wear of the mechanical parts, or even the engine breakage.

Thus, the dilution of hydrocarbons in the lubricating fluid alters its lubricity. This is one of the reasons why it is advisable to regularly renew the lubricating fluid. The alteration of the lubricating power of the lubricating fluid can be aggravated in certain extreme operating contexts of the engine, in particular according to the temperature of the lubricant, according to the rotational speed of the engine components or according to the contact pressure exerted between the various elements. of the motor.

As a result, the operating points located in certain critical portions of the operating zone, especially near the maximum speed limits "Nmax" and / or torque "Tmax", no longer allow to guarantee a safe operation of the engine. In other words, with the alteration of the lubrication power, the mechanical strength of certain engine components is no longer guaranteed for the operating points in the critical portions.

For example, in the event of a significant alteration of the lubricating power of the lubricating fluid, the electronic control unit is likely to endanger the mechanical integrity of the engine by controlling a transmission ratio change that passes the operating point in a direction. critical portion of the operating area.

It has already been proposed to determine the dilution of hydrocarbons in the lubricating fluid to control the alteration of its lubricity. The document EP-B1-1.614.870 thus proposes a method for estimating the alteration of the lubricant. In the event of excessive alteration, the driver is warned of the need to perform a drain to renew the lubricating fluid. However, such a method does not allow the use of the engine to its optimum operating capabilities, except to perform very frequent emptying.

It is also possible to remedy this problem by decreasing the surface of the operating zone by a safety margin sufficient to prevent degradation of the engine in case of high dilution rate of hydrocarbons.

However, such a method also does not allow to use the engine at full capacity when the lubricating fluid is at full capacity of its lubricity.

BRIEF SUMMARY OF THE INVENTION

To solve these problems in particular, the invention proposes a method of the type described above, characterized in that it comprises at least one preliminary restriction step during which the maximum limit of the operating parameter is modified as a function of at least a parameter representative of the reduction of the lubricating power of the lubricating fluid.

According to other characteristics of the invention: the restriction step is initiated when the parameter representative of the reduction of the lubricating power of the lubricating fluid passes a determined threshold; the parameter representative of the reduction of the lubricating power of the lubricating fluid is the rate of dilution of hydrocarbons in the lubricating fluid; the dilution ratio is measured by a sensor fitted to the lubrication circuit; the dilution ratio is estimated as a function of the evolution of operating parameters of the heat engine; the maximum limit of the operating parameter also varies according to the operating temperature of the motor; - the operating parameter is the engine speed; - the operating parameter is the motor torque; - the maximum torque limit is also set according to the engine speed; the control method acts simultaneously on the engine torque and the engine speed, the step of generating a command setpoint implementing a mapping memorized in an electronic control unit, the mapping being a modeling in a two-dimensional space corresponding respectively to the speed and the torque, of an operating zone in which the heat engine is able to operate in a secure manner, the control setpoint being determined so that the dynamic operating parameters of the engine are included in said operating zone after its application; the powertrain comprises a gearbox that can be coupled to the heat engine, the electronic control unit being able to develop a transmission gear shift setpoint adapted so that, after application of said setpoint, the parameters of operation of the engine are less than their respective maximum limits determined according to the value of the parameter representative of the reduction of the lubricating power of the lubricating fluid.

BRIEF DESCRIPTION OF THE FIGURES Other characteristics and advantages of the invention will appear during the reading of the following detailed description for the understanding of which reference will be made to the appended drawings in which: FIG. 1 is a view from above which schematically represents a motor vehicle comprising a powertrain equipped with a heat engine and a gearbox; - Figure 2 is a front view which schematically shows the heat engine of Figure 1 comprising a lubrication circuit; FIG. 3 is a two-dimensional graph showing the evolution of the degree of hydrocarbon dilution in the lubricating fluid of the heat engine of FIG. 2 as a function of time; FIG. 4 is a two-dimensional graph which represents a map stored in an electronic control unit of the vehicle of FIG. 1 and in which the abscissa represents the scale of the engine speed while the ordinate represents the scale of the engine torque, an operating zone of the heat engine of Figure 2 being represented by hatching; FIG. 5 is a view similar to that of FIG. 4 which represents the cartography in which maximum torque and speed limits are shown which limit the operating zone and which vary according to the degree of hydrocarbon dilution in the lubricating fluid; FIG. 6 is a block diagram which represents a control method of the power unit of the vehicle of FIG. 1 made according to the teachings of the invention; FIG. 7 is a view similar to that of FIG. 4, which represents the operating zone when the hydrocarbon dilution rate in the lubricating fluid is very low, as well as operating points corresponding to changes in the ratio of transmission amounts and descendants; FIG. 8 is a view similar to that of FIG. 7 in which the operating zone has been restricted a first time as a function of an average rate of hydrocarbon dilution in the lubricating fluid, this restriction prohibiting two of the points; transmission shown in Figure 7; FIG. 9 is a view similar to that of FIG. 7 in which the operating zone has been restricted a second time as a function of a high degree of hydrocarbon dilution in the lubricating fluid, this restriction prohibiting four of the points; transmission shown in Figure 7.

DETAILED DESCRIPTION OF THE FIGURES

In the remainder of the description, elements having identical structure or similar functions will be designated by the same references.

FIG. 1 diagrammatically shows a road motor vehicle which is equipped with a power unit comprising a thermal motor 12 and a gearbox 16 which is capable of being coupled to the thermal engine 12. The thermal motor 12 is intended to drive the driving wheels 14 of the vehicle through the gearbox 16. The gearbox 16 transmits the engine torque to the wheels 14 with a gear ratio selected which will be called transmission ratio.

The engine 12 is for example a diesel engine or a gasoline engine.

Alternatively, it is a hybrid traction vehicle which comprises, in addition to the engine, an electric motor.

The gearbox 16 here comprises a plurality of predetermined discrete transmission ratios, for example six forward transmission ratios. The gearbox 16 is controlled manually and / or automatically.

The vehicle 10 also comprises an electronic control unit 18. The electronic control unit 18 is able to receive operating information from the motor 12 and the gearbox 16 through a set of measurement sensors. This is for example the "N" of the motor 12, the torque "T" of the motor 12, the transmission ratio engaged in the gearbox 16, etc. Such sensors are well known in the automotive field and will not be described in more detail.

As shown diagrammatically in FIG. 2, the thermal engine 12 comprises moving elements, for example a crankshaft, connecting rods, pistons, camshafts, etc. To ensure optimum operation of the thermal motor 12, the latter comprises at least one lubricating circuit 22 in which a lubricating fluid 24 circulates.

The lubricating fluid is generally lubricating oil. This oil makes it possible in particular to reduce the friction between the mobile elements and their fixed support, to improve the seal between different parts of the engine or to improve the thermal resistance of the engine.

The lubrication circuit 22 generally comprises a reservoir delimited by a casing 26 which is fixed at the bottom of the thermal engine 12. The lubricant 24 contained in the reservoir is sucked by a pump 28 and / or projected by gear trains towards the various moving elements. The lubricating fluid 24 then flows by gravity to the reservoir 24 before it is again fed to the moving elements. The dynamic operating state of the thermal engine 12 is generally characterized by its speed "N", that is to say the speed of rotation of the motor shaft, and by its torque "T", that is to say ie the pressure applied by the explosion of the air / fuel mixture in each combustion chamber on the pistons.

To make it possible to use the capacities of the thermal engine 12 optimally, it is known to produce what is commonly called a "mapping" 34 of the thermal engine 12. As illustrated in FIG. 4, the cartography is a modeling in a space of at least two dimensions, corresponding respectively to the "N" regime and to the "T" pair, of an operating zone 36 in which the thermal engine 12 is likely to work in a secure way. In the representation of FIG. 4, the abscissa represents a scale of the "N" regime of the thermal engine 12 while the ordinate axis represents a scale of the "T" torque of the thermal engine 12.

The operating zone 36 has been marked by hatching. It is bounded transversely by a first vertical straight line 38 corresponding to a minimum engine "N" speed and by a second vertical straight line 40 corresponding to a maximum engine speed "Nmax". It is delimited upwards by a curve 42 corresponding to the maximum torque "Tmax" that can be delivered as a function of the "N" engine speed. The dynamic operating state of the thermal engine 12 is thus likely to be materialized by an operating point in this space.

It is preferable to keep the operating point within the operating zone 36, and in particular within the maximum limits of the operating zone 36. For the following, the expression "maximum limit" refers to a maximum limit "Nmax" of engine speed "N", materialized by the vertical line 40, and a maximum limit "Tmax" of the torque "T" engine, materialized by the curve 42. If the maximum limits "Tmax" and "Nmax" are exceeded, the moving parts of the thermal motor 12 may in fact be subjected to excessive stresses that may lead to the deterioration of the motor 12 .

As shown in FIG. 4, the surface of the operating zone 36 is likely to vary as a function of the engine temperature, represented for example by the temperature of a cooling fluid of the engine. When the heat engine 12 heats up, the curve 42 actually moves upward, increasing the area of the operating zone 36, as indicated by the curves 42A and 42B. For example, when the thermal engine 12 is moderately hot, the operating zone 36 is delimited by the curve 42A, the area of the operating zone 36 is increased by the space portion 36A. Similarly, when the heat engine 12 is very hot, the operating zone 36 is delimited by the curve 42B, the operating zone 36 is thus increased by the portion 36B of space in addition to the portion 36A.

Furthermore, the lubrication circuit 22 is a closed circuit in which the lubricant 24 is reused for a long time. As explained in the preamble of the description, hydrocarbons are gradually diluted in the lubricant fluid. The graph of FIG. 3 illustrates the evolution over time of the level of hydrocarbon diluted in the lubricating fluid 24. The upright sections of the curve correspond to an increase in the dilution of the hydrocarbons in the lubricating fluid, while the descending sections 32 correspond to an evaporation of the hydrocarbons by temperature rise caused by the use of the vehicle. As time passes, it is found that the amount of diluted hydrocarbons is greater than the amount of evaporated hydrocarbons, thus gradually increasing the rate of hydrocarbon dilution in the lubricating fluid.

It has been found in use that the lubricating power of the lubricating fluid deteriorates as the amount of hydrocarbon diluted in the lubricating fluid increases. This alteration can become detrimental to the integrity of the thermal engine when operating at operating points near the maximum limits of the operating zone.

However, many vehicle control methods acting on the dynamic operating parameters of the engine, including the "T" torque of the engine and / or the "N" engine speed, use the mapping 34. Such a control method generally comprises a step of developing a command setpoint, by the electronic control unit 18, which is determined so that the dynamic operating parameters of the engine are included in said operating area after its application. For this purpose, the map 34 is stored in the electronic control unit 18,

If the control set point places the operating point of the thermal motor 12 near its maximum limits while the lubricating fluid has a highly altered lubrication power, the application of the control instruction may lead to the degradation of the 12 thermal engine.

To solve this problem while allowing optimal use of the engine capacity at any time, the invention proposes that said control method comprises at least one preliminary step of restricting the operating zone which is triggered according to at least one parameter representative of the reduction of the lubricating power of the lubricating fluid.

The restriction of the operating zone 36 makes it possible to exclude critical portions made dangerous for the integrity of the thermal engine 12 by the alteration of the lubricating power of the lubricating fluid.

The parameter representative of the reduction of the lubricating power of the lubricating fluid is here the rate of dilution of hydrocarbons in the lubricating fluid.

The dilution ratio is for example measured by a sensor fitted to the lubrication circuit. An example of such a measuring device is described in detail in WO-A1-2005 / 071403.

In a variant, the dilution ratio is estimated as a function of the revolution of operating parameters of the heat engine. An example of such an estimation method is described in detail in EP-B1-1.614.870.

The restriction of the operating zone 36 consists in subtracting one or more critical portions from the operating zone 36.

The restrictions are likely to be applied to the "N" mode and / or the "T" engine torque.

In the example of FIG. 5, the restriction applied during the restriction steps comprises a determined maximum limit 44A, 44B of the "N" motor speed. For a given dilution ratio, it is a constant value regardless of the torque "T" of the engine.

The restriction applied during the restriction steps also includes a determined maximum limit 46A, 46B of the engine torque "T". The maximum limit of the torque "T" engine here has a constant value regardless of the engine speed "N".

These limits are constant here whatever the temperature of the thermal engine 12.

In variant not shown of the invention, the limits are variable depending on the temperature of the engine.

According to another variant not shown of the invention, the restriction applied during the restriction steps comprises a maximum limit "Tmax" determined torque "T" engine. The maximum limit "Tmax" of the torque "T" engine here has a variable value at least as a function of the "N" engine.

Referring to Figure 6, prior to any development of a control instruction involving mapping 34, the control method comprises a step "El" for determining the dilution ratio.

If the dilution ratio is less than a first threshold value, as represented by line 48, the forming step is initiated directly by applying the limitations of the unrestricted operating zone 36.

On the other hand, if the dilution ratio is greater than the first threshold value 48, the "E2" restriction step of the operating zone is triggered. During this step "E2", the portions to be removed from the operating zone 36 are determined as a function of the dilution ratio.

In the example shown in FIGS. 3 and 5, when the value of the dilution ratio lies in a first interval between the first threshold value 48 and a second upper threshold value 50, the thermal motor 12 is considered to function according to a first degraded mode. During this first degraded mode, a first limit 44A of torque "T" and a first limit 46A of speed "N" are applied to the operating zone 36. Thus, the hatched portions 52A located beyond these limits 44A, 46A are removed from the operating zone 36.

When the value of the dilution ratio is greater than the second upper threshold value 50, it is considered that the thermal engine 12 must operate in a second degraded mode. During this second degraded mode, a second torque limit 44B "T" and a first speed limit 46B "N" are applied to the operating zone 36. Thus, the hatched portions 52B located beyond these limits 44B, 46B are removed from the operating zone 36.

In a non-represented variant of the invention, the restriction step may select another number of degraded modes, for example one or more than three. In case, one will define a number of threshold values of adapted dilution ratio.

According to yet another variant not shown of the invention, the maximum limits of said operating parameter are capable of continuously varying as a function of the value of the parameter representative of the alteration of the lubricating power of the lubricating fluid. At the end of this "E2" restriction step, the step "E3" for generating the command setpoint is triggered. This step will use the operating zone 36 determined in the previous step to develop a command setpoint able to keep the operating point of the engine in the operating zone after its application.

In the example shown in Figures 7 to 9, the method is designed to develop a gear ratio change instruction.

In a known manner, when climbing from a lower transmission ratio to a higher transmission ratio, the "N" engine speed decreases while the torque "T" engine increases. Conversely, when downshifting from an upper transmission ratio to a lower transmission ratio, the engine "T" torque decreases while the engine "N" engine speed increases. Thus, the gear ratio setpoint acts simultaneously on the two operating parameters of the thermal engine 12.

When the electronic control unit 18 develops a transmission ratio change instruction, these variations of the operating parameters of the thermal engine 12 are taken into account so that the application of the setpoint keeps the operating point of the motor in the transmission. zone 36 of operation. Thus, the transmission ratio change setpoint is adapted so that, after the transmission ratio has been changed, the operating parameters of the engine remain in the operating zone 36 determined as a function of the value of the parameter representative of the reduction of the lubrication power. lubricating fluid.

In FIG. 7, the operating zone 36 is shown in the case where the dilution ratio is lower than the first threshold value 48.

Operating point 54A represents the current operating point of the thermal motor 12 for a given transmission ratio. The operating point 54B represents the operating point provided for the thermal motor 12 by the electronic control unit 18 in the event of a downshift of a gear ratio. The operating point 54C represents the operating point provided for the thermal motor 12 by the electronic control unit 18 in the case of a downshift of two gear ratios.

As can be seen in FIG. 7, in the case of a dilution ratio lower than the first threshold value 48, the two operating points 54B and 54C provided are included in the operating zone 36. The electronic control unit 18 is therefore likely to keep the two operating point 54B and 54C as candidates that may be the object of the control instruction. In certain contexts, for example in the case of sporty driving, the electronic control unit 18 is capable of issuing a downshift instruction of two transmission ratios.

In FIG. 8, the operating zone 36 is represented in the case where the dilution ratio is between the first threshold value 48 and the second threshold value 50.

In the same situation as above, it is found that the operating point 54C is now outside the operating zone 36 because of the restriction imposed during the "E2" restriction step. The engine "N" regime would indeed be greater than the maximum limit 44A. The electronic control unit 18 can therefore only issue a downshift instruction of a single transmission ratio, the downshift of two transmission ratios being prohibited because of the excessively high dilution ratio.

In FIG. 9, the operating zone 36 is shown in the case where the dilution ratio is greater than the second threshold value 50.

In the same situation as above, it is found that the two operating points 54B and 54C provided are now outside the operating zone 36 because of the restriction imposed during the "E2" restriction step. The engine "N" regime would indeed be greater than the maximum limit 44B. The electronic control unit 18 will therefore not be able to issue any downshifting instruction, any downshifting being prohibited because of the excessive dilution rate.

The same situation applies to upshifts when the current operating point is too close to the maximum torque limit "T".

Thus, the operating point 56A represents the current operating point of the thermal engine 12 for a given transmission ratio. The operating point 56B represents the operating point provided for the thermal motor 12 by the electronic control unit 18 when a gear ratio is mounted. The operating point 56C represents the operating point provided for the thermal motor 12 by the electronic control unit 18 when two gear ratios are mounted.

As can be seen in FIG. 7, in the case of a dilution ratio lower than the first threshold value 48, the two operating points 56B and 56C provided are included in the operating zone 36. The electronic control unit 18 is therefore likely to keep the two operating points 56B and 56C as candidates that could be the object of the command setpoint. In certain contexts, for example in the case of sporty driving, the electronic control unit 18 is capable of issuing a rising instruction of two transmission ratios.

In FIG. 8, the operating zone 36 is represented in the case where the dilution ratio is between the first threshold value 48 and the second threshold value 50.

In the same situation as above, it is found that the operating point 56C is now outside the operating zone 36 because of the restriction imposed during the "E2" restriction step. The engine torque "T" would indeed be greater than the maximum limit 46A. The electronic control unit 18 will therefore be able to emit only a rising setpoint of a single transmission ratio, the rise of two transmission ratios being prohibited because of the excessively high dilution ratio.

In FIG. 9, the operating zone 36 is shown in the case where the dilution ratio is greater than the second threshold value 50.

In the same situation as above, it is found that the two operating points 56B and 56C provided are now outside the operating zone 36 because of the restriction imposed during the "E2" restriction step. The engine torque "T" would indeed be greater than the maximum limit 46B. The electronic control unit 18 will therefore not be able to issue any setpoint of climb, any climb being prohibited because of the excessive dilution rate.

The process carried out according to the teachings of the invention thus makes it possible to take full advantage of the engine's capacities by taking into account the gradual deterioration of the lubricating power of the lubricating fluid.

The control method has been described in application to the development of a transmission ratio change instruction.

It will be understood that the control method is also applicable to processes for generating a command setpoint for the thermal engine 12, for example in the context of an automatic speed control process of the vehicle.

Claims (11)

A method for controlling a power unit of a motor vehicle comprising a motor (12) thermal and wherein the engine (12) has at least one thermal circuit (22) lubrication in which circulates a fluid (24) lubricant, the control method acting on at least one dynamic operating parameter of the heat engine, in particular among the torque (T) of the engine, the engine load and the engine speed (N), the method comprising a step (E3) of elaboration a control setpoint determined so that said dynamic operating parameter of the motor (12) is less than a maximum limit, characterized in that it comprises at least one preliminary step (E2) of restriction during which the maximum limit of the operating parameter is modified according to at least one parameter representative of the reduction of the lubricating power of the lubricant fluid (24). 2. Method according to the preceding claim, characterized in that the step (E2) of restriction is engaged when the parameter representative of the reduction of the lubricating power of the lubricating fluid passes a determined threshold. 3. Method according to any one of the preceding claims, characterized in that the parameter representative of the reduction of the lubricating power of the fluid (24) lubricant is the hydrocarbon dilution rate in the lubricating fluid. 4. Method according to the preceding claim, characterized in that the dilution ratio is measured by a sensor fitted to the lubrication circuit. 5. Method according to claim 3, characterized in that the dilution ratio is estimated as a function of the evolution of operating parameters of the heat engine. 6. Method according to any one of the preceding claims, characterized in that the maximum limit of the operating parameter also varies as a function of the operating temperature of the motor (12). 7. Method according to any one of the preceding claims, characterized in that the operating parameter is the speed (N) of the engine. 8. Method according to any one of the preceding claims, characterized in that the operating parameter is the torque (T) of the motor. 9. Method according to the preceding claim, characterized in that the maximum limit (Tmax) of the torque (T) is also established according to the engine speed. 10. Method according to any one of the preceding claims, characterized in that the control method acts simultaneously on the torque (T) of the motor (12) and the speed (N) of the motor (12), the step (E3 ) of development of a command setpoint implementing a mapping (34) stored in an electronic control unit (18), the mapping (34) being a modeling in a two-dimensional space, respectively corresponding to the regime (N ) and the torque (T), an operating zone (36) in which the engine (12) is able to operate in a safe manner, the control setpoint being determined so that the dynamic operating parameters of the engine are included in said operating zone (36) after its application. 11. Method according to any one of the preceding claims, characterized in that the powertrain comprises a gearbox (16) capable of being coupled to the engine (12) thermal, the electronic control unit (18) being capable of to develop a transmission ratio change instruction adapted so that, after application of said setpoint, the operating parameters of the engine (12) are below their respective maximum limits (Tmax, Nmax) determined as a function of the value of the parameter representative of the reduction of the lubricating power of the lubricating fluid.