FR2953253A1 - METHOD FOR CONTROLLING TWO STAGE SUPERIORING OF FIXED GEOMETRY TURBOCHARGERS WITH DYNAMIC ESTIMATOR AND LIMITING THE PRESSURE BEFORE TURBINE - Google Patents

Abstract

This method for controlling the air supercharging of an internal combustion engine (1) of a motor vehicle equipped with a stepped turbocharger (6, 7) comprises two turbines (10, 12) driven in rotation by the exhaust gases of the engine (1), two superchargers (11, 13) driven by each of the turbines (10, 12), and two valve actuators (17, 18) high pressure and low pressure. This method comprises regulating the compression ratio of the compressor (PRc) around a compression ratio setpoint value (PRc, cons), and regulating the expansion ratio (PRt) of the turbine (12) to limit the value of the pressure (Pavt, t) upstream of the turbine (12), said regulation being implemented as soon as the pressure (Pavt, t) upstream of the high-pressure turbine (12) exceeds a threshold value (CONSPavt) .

Description

B08-4797EN - ODE / PL

Simplified joint-stock company known as: RENAULT s.a.s. Method of controlling a two-stage turbocharger supercharging with fixed geometry with dynamic estimator and pressure limitation before turbine Invention of: Laurent FONTVIEILLE

The invention relates to the control of internal combustion engines of motor vehicles. The invention relates to the control of a two-stage turbocharger supercharger with fixed geometry with dynamic estimator and pressure limitation. More particularly, the invention relates to the control of the two-stage air supercharging of such engines. A particularly interesting application of the invention relates to the control of the supercharging of a turbocharged supercharged diesel type engine, and more particularly with a two-stage turbocharger architecture. The control of the engine is the technique of adjusting the performance of an internal combustion engine by controlling all of its sensors and actuators. All the control laws and engine control parameters are contained in a computer called ECU or Electronic Control Unit. Supercharged engines comprise a turbocharger comprising a turbine driven in rotation by the exhaust gas and a compressor driven by the turbine and serving to increase the amount of air admitted into the cylinders. For this purpose, the turbine is placed at the outlet of the exhaust manifold while the compressor is mounted on the same axis as the turbine and is disposed upstream of the intake manifold. The turbochargers are connected in series, so that the low pressure compressor supplies the high pressure compressor with air and the high pressure turbine supplies the low pressure turbine with gas.

The power provided by the exhaust gases to the high and low pressure turbines can be adjusted by installing relief valves or vanes which affect the flow rate of gas passing through the turbine or the passage section provided for these gases.

This type of turbocharger is called a fixed geometry turbocharger. With the increased performance of supercharged engines, the boost pressure level increases so that turbochargers are more and more stressed. It is therefore important to control turbochargers as finely as possible in order to avoid their deterioration and to improve the behavior of the vehicle during acceleration, and in particular to increase the dynamics of the engine, that is to say its capacity to climb rapidly in diet. Pollution standards being more and more severe, the amount of particles released by an engine, especially a diesel engine, must be increasingly low. This is why the engine exhaust line is equipped with a particulate filter that reduces the amount of particles released into the environment. The introduction of such a device produces an increase in the exhaust back pressure. This against pressure is all the more important that the filter is loaded with particles. It is translated, vis-à-vis the turbocharger, by a reduction of the rate of expansion, and by a consequent reduction of the power provided by the exhaust gases to the turbine and a decrease in the performance of the engine. To achieve the same level of performance, it is necessary to maintain the expansion ratio by increasing the pressure upstream of the turbine. This increase is usually achieved by closing the relief valves or by acting on the fins. The regulation of the pressure in the engine intake manifold around the pressure reference value is conventionally carried out by means of PID regulators (Proportional, Integral, Differential) according to the evolution of the difference between the setpoint of pressure and the actual pressure measured. However, this control strategy is difficult to implement insofar as it must allow to enslave the pressure in the collector on the pressure setpoint both in steady state and transient. We have already tried, in the state of the art, to achieve this goal.

In this respect, reference may be made to document US 2003/00 100 19 which uses two regulators in cascade or in document FR 2 829 530 which proposes to regulate the pressure value upstream of the turbine of the turbocharger around a value of pressure set point corresponding to a maximum permissible pressure value upstream of the turbine of the turbocharger. Reference may also be made to the document WO 2004/00 99 84 which proposes to control the supercharging using a position setpoint which is a function of the engine speed or to the document WO 2004/027 238 which proposes to regulate in a sequential manner either the pressure the position of an actuator for adjusting the power of the exhaust gases. However, the solutions proposed in the state of the art do not make it possible to implement servo-control of the supercharging pressure in order to precisely control the pressure in the intake manifold of the engine both in steady state and in transient, while limiting the pressure upstream of the turbocharger to protect the engine and turbocharger. The object of the invention is therefore to overcome these disadvantages and to provide a method and a device for controlling the supercharging of a supercharged internal combustion engine to achieve this triple objective, namely control of the boost pressure in transient regime, control of the boost pressure in steady state and limitation of the pressure upstream of the turbine.

In the field of piloting double supercharging devices, the applicant has developed a dual-loop control method simultaneously driving the two turbochargers and described in the patent application FR 08 53686 filed June 4, 2008. Such a system provides outstanding performance but requires an exhaust pressure sensor upstream of the high pressure turbine. The implantation of such a sensor is expensive. The present invention proposes to delete this sensor. For this, the control method advantageously uses a compression ratio variable compressor to drive a turbocharger in the case where it is unique. In addition, in the case of a twin turbocharger, the method replaces a dual-loop control by a control at a given moment of one or other of the turbochargers, combined with a manager that selects the turbocharger piloted. The object of the invention is therefore, according to a first aspect, a method for controlling the supercharging of an internal combustion engine of a motor vehicle equipped with a stepped twin-turbocharger comprising two turbines driven in rotation by the combustion gases. engine exhaust, two superchargers driven by each of the turbines, and two high pressure and low pressure valve actuators to adjust the power of the exhaust gases. This method furthermore comprises the regulation of the compression ratio of the compressor around a setpoint value of the supercharging pressure ratio, as well as a regulation of the expansion ratio to limit the pressure upstream of the turbine, the regulation being set when the pressure upstream of the turbine exceeds a threshold value.

According to another characteristic of the invention, an activation and deactivation signal is produced for regulating the expansion ratio of the turbine as a function of the value of the pressure in the intake manifold and the pressure upstream of the turbine. the turbine of the turbocharger and according to the first and second threshold values. Preferably, the regulation of the expansion ratio of the turbine is deactivated when the value of the pressure in the intake manifold is greater than or equal to the first threshold value.

Advantageously, the regulation of the compression ratio of the compressor is deactivated when the value of the pressure in the exhaust manifold is lower than the second threshold value. According to another characteristic of the invention, the limitation of the pressure upstream of the turbine is effected by means of a regulation loop receiving as input the setpoint value and an estimate of the compression ratio of the compressor and delivering an output control signal for high pressure and low pressure valve actuators. Switching means arranged at the input of the regulation loop can be controlled according to the value of the activation and deactivation signal, so as to present at the input of the regulation loop, on the one hand, or the estimation the expansion ratio of the turbine, that is to say the estimation of the compression ratio of the compressor, and on the other hand, one or the other of the set values. The compression compressor compression setpoint can be developed from a mapping and the expansion ratio setpoint can be derived from the ratio of the pressure in the exhaust manifold to one or the other. other pressure values among the pressure measurement downstream of the low pressure turbine and the estimation of the pressure downstream of the high pressure turbine. Advantageously, the value of estimating the pressure downstream of the high-pressure turbine is derived from a dynamic model of the turbocharger at low pressure. According to a second aspect, the invention relates to a device for controlling the air supercharging of an internal combustion engine of a motor vehicle equipped with a stepped supercharger twin turbocharger provided with two turbines driven by the gases of engine exhaust, two superchargers driven by each turbine. In addition, the device comprises a control unit comprising the regulation of the compression ratio of the compressor around a set value of the boost pressure ratio and the regulation of the expansion ratio so as to limit the value of the pressure. upstream of the turbine, said regulation being implemented as soon as the pressure upstream of the turbine exceeds the threshold value.

According to yet another characteristic, a map is used in which prepositioning values of the high pressure and low pressure valve actuators are stored making it possible to adjust the power of the exhaust gases as a function of engine operating parameters and means for prepositioning said actuators from a value extracted from the map. Other objects, features and advantages of the invention will become apparent on reading the following description, given solely by way of nonlimiting example, and with reference to the appended drawings, in which: FIG. 1 schematically illustrates the structure of an internal combustion engine of the diesel type, of an automobile engine provided with a supercharging control device according to the invention; FIG. 2 represents a control stage of a circuit; - Figure 3 illustrates a dynamic estimator of a low pressure turbocharger based on a physical model. FIG. 1 diagrammatically shows the general structure of an internal combustion engine 1 of a motor vehicle, of the Diesel type, as well as its fresh air intake 2 and exhaust manifolds. 3. As seen in this figure, the fresh air intake circuit 4 in the engine 1 essentially comprises an air filter 5 supplying, via a first stage turbocharger 6 low pressure and a second high pressure turbocharger stage 7, the intake manifold 2 of the engine 1. With regard to the exhaust manifold 3, the latter collects the exhaust gases resulting from the combustion and evacuates the latter to the outside, through the turbochargers 6 and 7 and a particulate filter 8 to reduce the amount of particles, including soot, released into the environment. A heat exchanger 9, 9 'can be placed after each output of the compressors equipping the supply line 4 of the intake manifold 2 fresh air. The turbocharger 6 essentially comprises a turbine 10 driven by the exhaust gas and a compressor 11 mounted on the same axis as the turbine 10 and providing a compression of the air distributed by the air filter 5, in order to increase the amount of air admitted into the cylinders of the engine 1. The turbocharger 7 essentially comprises a turbine 12 driven by the exhaust gas and a compressor 13 mounted on the same axis as the turbine 12 and providing compression of the distributed air by the compressor 11, in order to increase the amount of air admitted into the cylinders of the engine 1. Furthermore, the engine 1 is further associated with a circuit 14, recirculation of the exhaust gas, for reinjecting a portion of these gases in the intake manifold 2 so as, in particular, limit the amount of nitrogen oxide produced while avoiding the formation of smoke in the exhaust gas. This circuit 14 essentially comprises a solenoid valve 15 which makes it possible to control the flow rate of recirculated exhaust gas. The engine 1 is associated with a circuit 16 for exhausting gases.

This circuit 16 essentially comprises high-pressure discharge valves 17 as in the English terminology under the term 'by-pass' and low-pressure valve 18, generally designated by the English terminology under the term `wastegate' valve, so as to modulate the power provided by the exhaust gases to the high and low pressure turbines 10 and 12. One of the principles of the invention is to drive only one turbocharger 6 or 7 at a time by acting on the high pressure valve actuator 17 or low pressure 18 corresponding.

Furthermore, an electronic control unit UCE, designated by the reference numeral 20, retrieves signals Peoll and Paät, t measuring the pressure respectively in the intake manifold 2 and upstream of the turbine 12 high pressure, delivered by appropriate measuring sensors provided for this purpose (not shown). It acts on a device for adjusting the power of the exhaust gases, for example discharge valves, so as to regulate the value of the pressure prevailing in the intake manifold 2 and the pressure value Pavt, t upstream turbocharger turbine 12 around respective setpoints CONSPeoll and CONSpavt. The unit UCE 20 also provides the operational control of the engine 1, in a manner known per se. It acts in particular on the solenoid valve 15 to adjust the amount of recirculated gas and adjusts the operating point of the engine 1. The present description relates to the limitation of the pressure Pavt, t upstream of the turbine 12 high pressure turbocharger 7. Also, the following description of the ECU unit 20 will relate to the essential means for implementing this limitation.

As can be seen in FIG. 1, the electronic control unit UCE 20 essentially comprises regulating means making it possible to limit the pressure Pavt, t upstream of the turbine 12 of the turbocharger 7. More particularly, the central unit 20 incorporates a regulation stage 21 of the expansion ratio PRt receiving as input the estimated expansion ratio PRt, esti and the estimated compression ratio PRc, esti, a first stage 22 for producing a first pressure threshold value CONSpcoll corresponding to a maximum permissible pressure value for the intake manifold 2 and a second stage 23 for producing a second pressure threshold value CONSpavt corresponding to a maximum permissible pressure value for the exhaust manifold 3, corresponding to the authorized pressure upstream of the turbine 12 of the turbocharger 7 high pressure.

The central unit 20 also incorporates a third stage 24 and 25 for developing the estimation of the expansion ratio PRt, esti of the turbine, respectively at high pressure and at low pressure, as well as a fourth stage 26 and 27 development of the setpoint value PRt, eOns of the expansion ratio of the turbine, respectively at high pressure and at low pressure. Referring to FIG. 2, the limitation of the pressure Pa.sub.t, t prevailing upstream of the turbine 12, is only active when the pressure Pavt, t upstream of the turbine 12 is greater than the first threshold value CONSpavt. In this case, the regulation of the expansion ratio PRt is implemented by the third and fourth stages 24, 25 and 26, 27 of the control unit UCE 20. With regard to the first stage 22 for producing the first pressure threshold value CONSpcoll, it will be noted that, for example, this stage is constituted by a memory zone with two inputs in which are loaded data, obtained by prior learning, constituted by a set of pressure threshold value CONSpcoll corresponding each engine operating speed and fuel consumption level.

With regard to the stage 23 for producing the second threshold value CONSpavt, the latter also consists of a memory area in which a threshold value CONSpavt is loaded corresponding to a maximum pressure value from which malfunctions occur. are likely to appear in turbochargers 6 and 7. This threshold value CONSpavt is preferably completed by a setpoint value CONS2t corresponding to a transient setpoint value and a setpoint value CONS2p corresponding to a value of setpoint in steady state.

As can be seen in FIG. 2, the regulation stage of the expansion ratio essentially comprises a circuit 25 essentially incorporating a regulator 26. The circuit 25 receives, as input, an error signal E delivered by a comparator 30 ensuring either the comparison between the setpoint value of the expansion ratio PRt, eOns and the estimate of the expansion ratio PRt, esti, or the comparison between the setpoint value of the compression ratio PRe, eons and the estimation of the compression ratio PRc , esti. The circuit 25 also incorporates a stage 31 for generating a signal CPAVT for activating and deactivating the regulation of the expansion ratio PRt. This signal CPAVT is elaborated in a logic circuit 32 as a function of the pressure value Peoil in the intake manifold 2, the pressure Paat, t upstream of the turbine 12 of the turbocharger 7 and as a function of the setpoint values CONSeoll and CONSpavt. For this purpose, the stage 31 for generating the signal CPAVT comprises first comparator means 33 which provide a comparison between the pressure value Peoil measured in the intake manifold 2 and the corresponding reference value CONSeoll, as well as means 34 comparing the pressure value Pavt, t measured in the exhaust manifold 3 and the threshold value CONSpavt corresponding, the result of these comparisons being provided to the logic circuit 32 suitable for the development of the activation signal and disable CPAVT. When the pressure value Pavt, t is greater than the corresponding threshold value CONSpavt, the signal CPAVT makes it possible to actuate switches 35 and 36 so as to position them on the setpoint of the expansion ratio PRt, eOns of the turbine 12 and on the estimation of the relaxation rate PRt, esti. In this way, the comparator 30 makes a comparison between the estimate of the expansion ratio PRt, esti and the setpoint of the expansion ratio PRt, aons. Similarly, when the pressure value Peoil is greater than the corresponding threshold value CONSPeoll, the signal CPAVT makes it possible to actuate switches 35 and 36 so as to position them on the threshold value of the compression ratio PRe, eons of the compressor 13 and on the estimation of the compression ratio PRc, esti. In this way, the comparator 30 makes a comparison between the estimation of the compression ratio PRc_estj and the threshold value of the compression ratio PRc, cons. In other words, as indicated above, when the measurement value Pa 1, of the pressure upstream of the turbine 12 is greater than the first threshold value CONSpavt, the regulation of the compression ratio PR is deactivated and the regulation of the PR relaxation rate, is activated in order to limit the pressure Part t. On the other hand, when the value of the pressure Pco11 of the collector 2 is greater than or equal to the threshold value CONSpcnli, the regulation of the expansion ratio PR1 is deactivated and the regulation of the compression ratio PR is activated. The error signal E delivered by the comparator 30 is transmitted to a regulator 37, so as to regulate either the expansion ratio PR, of the turbine 12, or the compression ratio PR, of the compressor 13.

Furthermore, to improve the response time of the control loop, a prepositioning value of the high pressure ('by-pass') 17 and low pressure ('wastegate') valves 18 is added to the output value of the regulator 37. This prepositioning value is extracted from a map C as a function of the engine speed N or the fuel flow W. This pre-positioning map C of the turbine 12 is incorporated in the ECU 20 and makes it possible to obtain a first estimated value of the adjustments of the turbocharger 7 as a function of the speed N and of the speed W and thus facilitate the adjustment. In addition, by correcting the value extracted from the map C depending in particular on atmospheric pressure and temperature. it is possible to refine the prepositioning value of the turbine 12 as a function, for example, of the altitude, or of the ambient temperature. It will be noted that this prepositioning value of the turbine 12 makes it possible to position the turbocharger 7 in a valid initial state during stable speeds and which thus makes it possible to approach transient conditions with a good initial setting.

The outputs of the regulator 37 and the map C are summed by means of an adder 38 in order to determine the position of the low-pressure and high-pressure discharge valves WG 17. As can be seen in FIG. 3, the dynamic estimator The behavior of the low-pressure turbocharger 6 is based on a physical model and makes it possible to determine the pressure estimate Pa, t upstream of the low-pressure turbine 10, as well as the pressure Pavai, c downstream of the low-pressure compressor 11. This dynamic estimate of the low-pressure turbocharger 6 makes it possible to eliminate the risks of overspeed on the high-pressure turbocharger under transient conditions without the addition of a specific sensor. The dynamic estimator has as input the airflow Wavt, e of the compressor 11 measured by a flowmeter (not shown), the pressure values Pavt, e and temperature Tavt, e upstream of the compressor 11, the flow Wavt, t of the low pressure turbine 10 and the value of the temperature Tavt, t upstream of the turbine 10 low pressure. The air flow values Wavt, e and Wavt, t and the pressure values Pavt, e and Pavt, t are obtained by sensors (not shown). The calculation of the power Pe at the low-pressure compressor 11 is calculated according to the following equation in the block 40: For this, the flow rate Wavt, e and the temperature Tavt e are measured upstream of the compressor 11. Cp is the capacity heat pump at constant pressure of the fluid flowing in the turbocharger 6 and y the ratio of the heat capacity at constant pressure on the heat capacity at constant volume of the same fluid.

The compression ratio PRc, esti and the efficiency re are obtained from static C maps provided by the 1 (, P = Wavt, cCp ùT vt, c PRS e ~~ -1 (1) ~ c builder of the turbocharger. These quantities are calculated according to variables determined by the following formulas: (VT {avt, c 1 PRc, esti ù / i Vavt, cp, Nt T (2) avt, c avt, c V1 avt, c Wavt, c Pavt , C The pressure Pavt, c upstream of the low pressure compressor 11 is measured The turbocharger Nt is supplied by the dynamic estimator, the functions fi and f2 for their data from the turbocharger manufacturer and contained in the block 41.

The air flow Wavt, c may for example be measured by means of a hot wire sensor (not shown) placed at the outlet of the air filter 5. The pressure Pavt, c can for its part be measured by a piezoelectric sensor. electric (not shown). The power Pt supplied by the turbine 10 to the motor shaft is calculated according to equation (4) in block 42: Y 1 \ 1 Y Pt = Wavt, tCp '1tTavt, t 1 (4) PRt, esti ~ i Here, the temperature Tavt, t upstream turbine 10 is not measured, and it is assumed that there is a static dependence between this temperature Tavt, t, the engine speed Nm and the load M: Tavt, t = f3 (M, Nm) (5) where M is the amount of fuel injected and Nm is the engine speed. Similar to what is done for the low pressure compressor 11, there is a static relationship between the flow rate Wavt, t of the turbine 10, the expansion ratio PRt, esti of the turbine 10, and the speed Nt of the turbine 10. There is also a relation between the efficiency rt of the turbine 10, the flow rate Wavt, t of the turbine 10 and the speed Nt of the turbine 10. Tic = f 1 (3) 20 (/ T vt, t 1 (6) w avt, tp _ 14 PRt, esti, Nt T avt, tt, tt, tt (7) 'tt f5 Wavtt PT avt, t avt, The functions f3, f4 are provided by the manufacturer and contained in block 43.

The flow rate Wavt, t of the turbine 10 is estimated from the air flow rate measured at the air inlet of the engine 1, using a first-order low-pass filter (not shown). . This simplification is justified by the fact that the low pressure estimator is used only when the valve (wastegate) is closed.

The efficiency Olt of the turbine 10 is thus determined from known or estimated quantities.

On the other hand, equation (4) requires to know the expansion ratio PRt, esti of the low pressure turbine, since this is not measured. For this, we use the pressure measurement PdtBP downstream of the

turbine 10 low pressure and we reverse the equation (6), so as to

obtain the following equation i PdtBPPRt, is 1 6, Wavt = f4 PRt, esti, Nt () Tavt, t Tavt We can consider a new function f5 contained in block 43, forming an inverse function, making it possible to obtain the relation next (1 VTavtt / PRt, esti 'f4 PRt, esti 1 ~ Tavt, t / (8) From the powers Pe and Pt, respectively downstream of the compressor 11 low pressure and upstream of the turbine 10 low pressure 25 uses the equation of the dynamics on the turbocharger 6 low pressure shaft according to the following equation: J.Nt dt = Pt P (9) where: J is the inertia of the low pressure turbocharger 6 and Nt is the regime of the low pressure turbine.

This is implemented in the estimator via a recursion equation according to equation (10) contained in block 44: Nk = Nk_1 + (P ùPJNK-1 / (10) where k is the The recurrence index represents the time, thus obtaining an estimate of the Nk regime of the low pressure turbocharger 6, but also of the compression ratio PRc, estimated in the compressor 11 and the expansion ratio PRt, of the turbine 10. of the expansion ratio PRt, esti turbine 10, the pressure PdtHP is deduced downstream in the high-pressure turbine 12. With a dynamic estimator of the turbocharger 6 low pressure, it avoids the risk of overspeed turbocharger 7 high pressure transient The setpoint PRt, eOns is calculated from the ratio between the pressure measurement Pa, t upstream of the turbine and the pressure value. Pdt be in ava 1 of the high pressure turbine 12, downstream of the low pressure turbine 10.

In the high-pressure control zone, the set point value of the expansion ratio PRt is calculated, eOns according to the following equation: CONS PttHP PR t, cons with: PdtHP the estimation of the pressure downstream of the high-pressure turbine 12 resulting from the dynamic model of the low pressure turbocharger 6, CONSPavt, the threshold value of the pressure upstream of the high pressure turbine 12 corresponding to the Peoli pressure in the exhaust manifold 3. In the low pressure regulation zone, the setpoint of the PRt, eOns is calculated according to the following equation: PR = t, cons CONSpavt PdtBP with: PdtBp the measurement of the pressure downstream of the turbine 10 low pressure, CONSPavt, the threshold value of the pressure upstream of the turbine 15 low pressure. The value of the estimation of the expansion ratio PRt, esti is calculated according to the following equations: In high pressure control zone, the estimate of the expansion ratio PRt, esti is calculated according to the following equation: Pavt, t 20 PRt , esti =

With the measurement of the pressure upstream of the high pressure turbine 12, PdtHp, the estimation of the pressure downstream of the high pressure turbine 12 is derived from the dynamic model of the low pressure turbocharger. In low pressure regulation zone, the estimation of the expansion ratio is calculated according to the following equation: ## EQU1 ## with measurement of the pressure upstream of the low pressure turbine, PdtBP , the measurement of the pressure downstream of the low pressure turbine 10.

Thanks to the invention which has just been described, it is possible to control the supercharging of an internal combustion engine and to limit the pressure upstream of the turbine.

Claims (10)

REVENDICATIONS1. A method for controlling the supercharging of an internal combustion engine (1) of a motor vehicle equipped with a stepped turbocharger (6, 7) comprising two turbines (10, 12) driven in rotation by the exhaust gases from the engine (1), two supercharging compressors (11, 13) driven by each of the turbines (10, 12), and two high pressure and low pressure valve actuators (17, 18), characterized in that it includes the regulation of the compression ratio of the compressor (PR) around a setpoint value of compression ratio (PRe, cons), and the regulation of the expansion ratio (PR () of the turbine (12) to limit the value of the pressure (Pavt, t) upstream of the turbine (12), said regulation being implemented as soon as the pressure (Pa ,, tt) upstream of the high-pressure turbine (12) exceeds a value threshold (CONSpayt). 2. Method according to claim 1, characterized in that an activation and deactivation signal (CPAVT) of the regulation of the expansion ratio (PRt) of the turbine (10, 12) is produced as a function of the value pressure (Pco11) in the intake manifold (2) and a corresponding first threshold value (CONSpcoii) and pressure (Pavt, t) upstream of the turbine (12) of the turbocharger (7) and a second corresponding threshold value (CONSpa, t). 3. Method according to claim 2, characterized in that the regulation of the expansion ratio (PRt) of the turbine is deactivated when the value of the pressure (Pcoii) in the intake manifold (2) is greater than or equal to at the first threshold value (CONSpconn). 4. Method according to any one of claims 1 to 3, characterized in that the regulation of the compression ratio (PRc) of the compressor (11, 13) is deactivated when the value of the pressure (Pavt.t) in the exhaust manifold (3) is greater than the second threshold value (CONSpa, -t) • 5. Method according to any one of claims 2 to 4, characterized in that the pressure limitation (Pavt.t) upstream of the laturbine (10, 12) is effected by means of a control loop (25) receiving as input the setpoint value (PRe, eons) and an estimate of the compression ratio of the compressor (PRe, eSti) and outputting a control signal (WG) of the high and low pressure valve actuators (17, 18) pressure, and in that switching means (35, 36) arranged at the input of the regulation loop (25) are controlled so as to have, at the input of the latter, on the one hand, or the estimation of the rate of expansion (PRt, esti) of the turbine, ie the estimation of the compression ratio (PRc, eSti) of the compressor and, on the other hand, one or the other of the set values (PRt, cons) or (PR ,, COnS), depending on the value of the enable and disable signal (CPAVT). 6. Method according to claim 5, characterized in that the set value (PRe, eons) is developed from a map C. 7. Method according to any one of claims 1 to 6, characterized in that the set value (PRt, e0ns) is developed from the ratio between the measurement of the pressure (Pavt, t) in the exhaust manifold (3) and any one of the pressure values among the pressure measurement (PdtBp) downstream of the low pressure turbine (10) and the pressure estimate (PdtHp) downstream of the high pressure turbine (12), depending on whether the control zone is at low pressure or at high pressure. 8. The method of claim 7, characterized in that the estimate of the pressure (PdtHp) downstream of the high pressure turbine (12) is derived from a dynamic model of the turbocharger 6 at low pressure. 9. Device for controlling the air supercharging of an internal combustion engine (1) of a motor vehicle equipped with a stepped turbocharger (6, 7) provided with two driven turbines (10, 12) in rotation by the exhaust gases of the engine (1) and two supercharging compressors (11, 13) driven by each of the turbines (10, 12), said device comprising a control unit (20) characterized in that the control unit (20) comprises regulating the compression ratio of the compressor (PRo) about a compression ratio setpoint (PRe, coaS), and comprising regulating the expansion ratio (PRt) ) so as to limit the value of the pressure (Pavt, t) upstream of the turbine (12), said regulation being implemented as soon as the pressure (Pa, t, t) upstream of the turbine (12) high pressure exceeds a threshold value (CONSpavt). 10. Device according to claim 9, characterized in that it comprises a map C in which are stored position values of the valve actuators (17, 18) high pressure and low pressure to adjust the power of the exhaust gas. according to the operating parameters of the engine, and in that it comprises means (37, 38) for prepositioning said actuators (17, 18) from a value extracted from the map C.