Patent Description:
As known, some internal combustion engines are provided with a turbocharger supercharging system, which can increase the power generated by the engine using the enthalpy of the exhaust gases to compress the intake air from the engine and therefore increase the volumetric efficiency of the intake.

A turbocharger supercharging system comprises a turbocharger provided with a turbine, which is arranged along an exhaust conduit to rotate at high speed under the pressure of exhaust gases expelled from the engine, and a compressor, which is put in rotation by the turbine and is arranged along the air supply conduit compressing the intake air from the engine.

In particular, the useful area of the operating range is limited, on the left side of a plane reduced mass flow rate / compression ratio, by the pumping line and, at the right side of the same plane, by the so-called "saturation line". The pumping line therefore defines a first "forbidden" zone and consists of the location of points where the internal aerodynamic balance of the compressor is offset and there is a periodical, noisy and violent flow rate refusal to the mouth, with effects that can be destructive for the blading.

In a turbocharger supercharging system it is necessary to maintain the operating range of the turbocharger within a useful area dependent on the engine point both for functional reasons (i.e. to avoid malfunctions or else low performance), and for structural reasons (i.e. to avoid damage to the turbocharger).

For example, the patent application <CIT> describes a control method of an internal combustion engine supercharged by means of a turbocharger comprising a turbine and e compressor. The control method envisages determining the current mass flow rate of the compressor, determining a lower security threshold of the mass flow rate, and requiring that the current mass flow rate of the compressor is greater than the security threshold of the mass flow rate. The document <CIT> therefore describes a method that allows preventing damage to the turbocharger but does not allow optimizing the performance of the turbocharger itself.

The patent application <CIT> describes a control method of an internal combustion engine supercharged by means of a turbocharger comprising a compressor, a turbine adapted to drive into rotation the compressor under the action of the exhaust gases of the engine, and a wastegate valve adapted to regulate the flow of exhaust gases supplied as input to the turbine for controlling the speed of rotation of the turbine itself according to an output supercharged target pressure required by the compressor.

The control method described in the patent application <CIT> comprises the steps of measuring the pressure of intake air entering the compressor; determining the mass flow rate of the compressor, calculating, through a predetermined map that characterizes the operation of the compressor, and according to the preset limit speed of rotation, of the measured air pressure and of the mass flow rate, a supercharging limit pressure, which is related to the obtainable output air pressure from the compressor when the turbine rotates at a speed substantially equal to the preset limit speed; verifying if a requested supercharged target pressure satisfies a preset relation with a supercharged limit pressure calculated in the case where the relationship is met, actuating the wastegate valve for controlling the speed of rotation of the turbine according to the supercharged limit pressure thus reducing the rotational speed of the turbocharger at a value substantially equal to the preset limit speed.

The patent application <CIT> while indicating that in supercharging systems of the above described type it is necessary to be able to limit, at the varying of operating conditions of the engine operation, the maximum rotation speed of the turbocharger both for functional reasons and for structural reasons so as to avoid critical operating conditions that may cause damage to the turbocharger, it gives no indication on how to implement the limitation of the maximum rotation speed of the turbocharger.

The patent application <CIT> describes instead a control method of an internal combustion engine supercharged by means of a turbocharger provided with a turbine and a compressor that envisages providing in a plane Reduced Mass Flow rate / Compression Ratio at least one operating limit curve, at least one interaction curve of a wastegate valve regulating a bypass conduit of the turbine and at least one intervening curve of a Poff valve regulating a bypass conduit of the compressor. The control method according to the patent application <CIT> envisages the use of the operating limit curve for limiting the pressure target downstream of the compressor used by the motor control. The method further envisages controlling the opening of the wastegate valve if the intervening curve of the wastegate valve is exceeded, and control the opening of the Poff valve if the intervening curve of the Poff valve is exceeded. The control method described by <CIT> is able to ensure that the operating range of the turbocharger remains within the useful area in any working condition of the internal combustion engine.

The so-called "saturation line", which defines a second "forbidden" zone and corresponds to the reaching of sonic conditions (and consequent blocking of the flow) entering the turbine, defines the maximum possible flow that the compressor can provide in the given conditions of the intake environment. Close to the saturation line, the turbocharger reaches therefore very high speeds and is able to develop the maximum power for compressing air intake from the engine and thus increasing the volumetric efficiency of the aspiration. Unfortunately, however, close to the saturation line, due to the high speeds involved, it may occur that the turbocharger accelerates out of control until reaching the sonic block with destructive effects upon the turbocharger itself.

The aim of the present invention is to provide a control method of an internal combustion engine supercharged by means of a turbocharger, said control method is of inexpensive and simple implementation, and allows to ensure that the operating range of the turbocharger remains within the useful area close to the saturation line but without reaching the sonic block. According to the present invention a control method is provided of an internal combustion engine supercharged by means of a turbocharger as claimed by the attached claims.

The present invention will now be described with reference to the annexed drawings, which illustrate a not limiting embodiment, in which:.

In <FIG>, with number <NUM> is indicated as a whole an internal combustion engine supercharged by means of a turbocharger supercharging system <NUM>.

The internal combustion engine <NUM> has four cylinders <NUM>, each of which is connected to an intake manifold <NUM> by way of at least one respective intake valve (not shown) and to an exhaust manifold <NUM> by way of at least one respective exhaust valve (not shown). The intake manifold <NUM> receives fresh air (i.e. air coming from the external environment) through a suction conduit <NUM>, which is provided with an air filter <NUM> and is regulated by a throttle valve <NUM>. Along the suction conduit <NUM> an intercooler <NUM> is provided whose function is to cool the intake air. To the exhaust manifold <NUM> is connected an exhaust conduit <NUM> that feeds the exhaust gases produced by the combustion to an exhaust system, which emits the gases produced from burning into the atmosphere and usually comprises at least one catalyst <NUM> and at least a silencer (not shown) disposed downstream of the catalyst <NUM>.

The supercharged system <NUM> of the internal combustion engine <NUM> comprises a turbocharger <NUM> provided with a turbine <NUM>, which is arranged along the exhaust conduit <NUM> to rotate at high speed under the action of exhaust gases expelled from the cylinders <NUM>, and a compressor <NUM> which is arranged along the suction conduit <NUM> and is mechanically connected to the turbine <NUM> to be driven into rotation by the turbine <NUM> itself so as to increase the air pressure fed into the supply conduit <NUM>.

Along the exhaust conduit <NUM> a bypass conduit <NUM> is provided, which is connected in parallel to the turbine <NUM> so as to present its ends connected upstream and downstream of the turbine <NUM> itself. Along the bypass conduit <NUM> a wastegate valve <NUM> is arranged, which is adapted to regulate the flow of exhaust gases flowing through the bypass conduit <NUM> and is driven by an actuator <NUM>. Along the exhaust conduit <NUM> a bypass conduit <NUM> is provided, which is connected in parallel with the compressor <NUM> so as to present its ends connected upstream and downstream of the compressor <NUM> itself. Along the bypass conduit <NUM> a Poff valve <NUM> is arranged, which is adapted to regulate the flow of exhaust gases flowing through the bypass conduit <NUM> and is driven by an actuator <NUM>.

The internal combustion engine <NUM> is controlled by an electronic control unit <NUM>, which directs the operation of all components of the internal combustion engine <NUM> including the supercharging system <NUM>. In particular, the electronic control unit <NUM> drives the control actuators <NUM> and <NUM> of the wastegate valve <NUM> and of the Poff valve <NUM>. The electronic control unit <NUM> is connected to sensors <NUM> that measure the temperature To and pressure Po along the intake conduit <NUM> upstream of the compressor <NUM>, to sensors <NUM> that measure temperature and pressure along the suction conduit <NUM> upstream of the throttle valve <NUM>, and to sensors <NUM> that measure temperature and pressure inside the suction manifold <NUM>. In addition, the electronic control unit <NUM> is connected to a sensor <NUM> that measures the angular position (and hence the rotation speed) of a crankshaft of the internal combustion engine <NUM> and a sensor <NUM> that measures the phase of the intake and / or discharge valves. It is also important to highlight that there are no provided sensors adapted for measuring the rotation speed of the turbocharger <NUM>.

Among other things, the electronic control unit <NUM> maintains the operating range of the turbocharger <NUM> within a useful area. The following shows the control mode used by the electronic control unit <NUM> to keep the operating range of the turbocharger <NUM> in a useful area and to prevent the turbocharger <NUM> from reaching sonic conditions in the vicinity of a saturation line <NUM> (illustrated in <FIG> and <FIG>).

During a phase of design and development of the internal combustion engine <NUM>, the characteristic curves of the compressor <NUM> are analyzed (provided by the manufacturer of turbocharger <NUM>) in a plane Reduced Mass Flow rate / Compression Ratio. An example of the characteristic curves of a commercial compressor <NUM> is illustrated in <FIG>.

The characteristic curves shown in <FIG> are normalized to an absolute reference temperature To_rif and a absolute reference pressure Po_rif. On the left side of the plane Reduced Mass Flow rate / Compression Ratio is a first forbidden zone delimited by the pumping line, consisting of the location of points wherein the inside aerodynamic balance of the compressor <NUM> is offset and there is a periodical, noisy and violent flow refusal to the mouth, with effects that can be destructive for the blading.

Instead, in the right side of the plane Reduced Mass Flow rate / Compression Ratio there is a second forbidden zone delimited by the so-called saturation line <NUM> (shown in <FIG> and <FIG>), which corresponds to the attainment of sonic conditions (and thus blocking the flow) at the entrance of the turbine <NUM> and defines the maximum possible flow that the compressor <NUM> can provide in the given conditions of the suction environment.

According to that shown in <FIG>, through an analysis of the characteristic curves of the compressor <NUM> a curve <NUM> is determined limiting the rotation speed of the turbocharger <NUM> and a curve <NUM> which delimits the pumping of the turbocharger <NUM>. According the curves <NUM> and <NUM> two operating limit curves <NUM> and <NUM> are established that are used to restrict the target of pressure downstream of the compressor <NUM> used by the engine control. For determining the operating limit curve <NUM> a threshold S1 (constant or variable) is determined establishing the distance between the operating limit curve <NUM> and the curve <NUM> which limits the rotation speed of the turbocharger <NUM>; similarly, to determine the operating limit curve <NUM> a threshold S2 (constant or variable) is determined establishing the distance between the operating limit curve <NUM> and the curve <NUM>, which delimits the pumping of the turbocharger <NUM>.

Moreover, according to the curves <NUM> and <NUM> two intervening curves <NUM> and <NUM> are established of the wastegate valve <NUM> regulating the bypass conduit <NUM> of the turbine <NUM> and two intervening curves <NUM> and <NUM> of the Poff valve <NUM> that regulates the bypass conduit <NUM> of the compressor <NUM>. To determine the intervening curve <NUM> of the wastegate valve <NUM> a threshold S<NUM> (constant or variable) is determined establishing the distance between the operating limit curve <NUM> and the intervening curve <NUM> of the wastegate valve <NUM>; similarly, to determine the intervening curve <NUM> of the wastegate valve <NUM> a threshold S<NUM> (constant or variable) is determined establishing the distance between the intervening curve <NUM> of the wastegate valve <NUM>, and the curve <NUM>, which delimits the pumping of the turbocharger <NUM>. To determine the intervening curve <NUM> of the Poff valve <NUM> a threshold S<NUM> (constant or variable) is determined establishing the distance between the operating limit curve <NUM> and the intervening curve <NUM> of the Poff valve <NUM>; similarly, to determine the intervening curve <NUM> of the Poff valve <NUM> a threshold S<NUM> (constant or variable) is determined establishing the distance between the intervening curve <NUM> of the Poff valve <NUM> and the curve <NUM>, which delimits the pumping of the turbocharger <NUM>.

During operation of the internal combustion engine <NUM>, the electronic control unit <NUM> uses the operating limit curves <NUM> and <NUM> to limit the pressure target downstream of the compressor <NUM> used by the engine control. In other words, the engine control unit implemented in the electronic control unit <NUM> determines in a known way and as a function of the engine point, a target pressure downstream of the compressor <NUM>, which represents a desired and optimal pressure value downstream of the compressor <NUM>. If the target pressure downstream of the compressor <NUM> is compatible with the operating limit curves <NUM> and <NUM> then the target pressure downstream of the compressor <NUM> is maintained, otherwise if the target pressure downstream of the compressor <NUM> is not compatible with operating limit curves <NUM> and <NUM> then the target pressure downstream of the compressor <NUM> is limited to a maximum value compatible with the operating limit curves <NUM> and <NUM>.

In particular, to limit the target pressure downstream of the compressor <NUM> the reduced mass flow rate QAH of the compressor <NUM> is determined, according to the current reduced mass flow rate QAH of the compressor <NUM> the maximum possible compression ratio RC is determined by using the operating limit curves <NUM> and <NUM>, the maximum possible pressure is determined downstream of the compressor <NUM> by multiplying the absolute pressure Po, upstream of the compressor <NUM> for the maximum possible compression ratio RC, and the target pressure downstream of compressor <NUM> is limited to the maximum possible pressure downstream of the compressor <NUM> if the target pressure downstream of the compressor <NUM> is greater than the possible maximum pressure downstream of the compressor <NUM>.

The reduced mass flow rate QAHR of the compressor <NUM> is determined using the following equation: <MAT>.

The absolute reference temperature To_rif and the absolute reference pressure Po_rif are the conditions in which were derived characteristic curves of the compressor <NUM> and therefore of the curves <NUM>-<NUM> and are project data known in advance. The absolute temperature To upstream of the compressor <NUM> and the absolute pressure Po upstream of the compressor <NUM> are measured by sensors <NUM>. The mass flow rate QAH of the compressor <NUM> can be measured by a specifically dedicated flow rate sensor or can be estimated in a known way by the electronic control unit <NUM>.

According to a different embodiment not illustrated, the measure of the absolute temperature To upstream of the compressor <NUM> (i.e. substantially room temperature) may not be provided; in this case the reduced mass flow rate QAHR can be "partially" normalized on the basis of the relation between the pressure Po / Po_rif without taking into account the relation between the temperatures To and To_rif.

It is important to underline that the curves <NUM>, <NUM>, <NUM> and <NUM> are independent from the reduced limit speed NtcR of the turbocharger <NUM>, while the curves <NUM>, <NUM>, <NUM> and <NUM> are dependent on the reduced limit speed NtcR of the turbocharger <NUM> (i.e. vary at the varying of the reduced limit speed NtcR of the turbocharger <NUM>). In other words, for the turbocharger <NUM> is set a limit speed Ntc preset by the turbocharger <NUM> above which the turbocharger <NUM> is brought in a critical condition; using the preset limit speed NtcR of the turbocharger <NUM> the current reduced limit speed NtcR of the turbocharger <NUM> is calculated on the basis of the absolute temperature To upstream of the compressor <NUM> using the following equation: <MAT>.

At the varying of the absolute temperature To upstream of the compressor <NUM> and at the same preset limit speed Ntc of the turbocharger <NUM> the current reduced limit speed NtcR of the turbocharger <NUM> varies; therefore the electronic control unit <NUM> cyclically determines according to the absolute temperature To upstream of the compressor <NUM> and according to the preset limit speed Ntc of the turbocharger <NUM> (which always remains constant) the current reduced limit speed NtcR of the turbocharger <NUM> and according to the current reduced limit speed NtcR of the turbocharger <NUM> is able to determine the curves <NUM>, <NUM>, <NUM> and <NUM> to be used. Alternatively, since the preset limit speed Ntc of the turbocharger <NUM> is constant to simplify the management of the curves <NUM>, <NUM>, <NUM> and <NUM>, the curves <NUM>, <NUM>, <NUM> and <NUM> themselves may be stored in the electronic control unit <NUM> and parameterized according to the absolute temperature To upstream of the compressor <NUM>; in this way, the electronic control unit <NUM> does not need to calculate the current reduced limit speed NtcR of the turbocharger <NUM> and therefore choose the curves <NUM>, <NUM>, <NUM> and <NUM> to be used, but simply needs to update the curves <NUM>, <NUM>, <NUM> and <NUM> as a function of the absolute temperature To upstream of the compressor <NUM>.

According to another simplified embodiment (and therefore less accurate), instead of using the current reduced mass flow rate QAHR the current mass flow rate QAH (not reduced) or the target mass flow rate QAHP (reduced or not reduced) could be used.

Once determined the current reduced limit speed NtcR, the electronic control unit <NUM> is prepared to determine a critical threshold Mcritica of the reduced mass flow rate QAHR. As best illustrated in <FIG>, said critical threshold Mcritica delimits in the plane reduced mass flow rate / compression ratio a portion of the useful area of the operating range of the turbocharger <NUM> which will be hereinafter referred to as the critical area, as while remaining within the useful area it represents the area close to the attainment of sonic conditions (i.e. close to the saturation line <NUM>). The critical zone is characterized by the collapse of the efficiency of the compressor <NUM> and by a high instability of the speed of the turbocharger <NUM>, which may dangerously accelerate.

The critical threshold Mcritica is variable depending on the reduced limit speed NtcR (as best illustrated in <FIG>).

To reduce the instability characterizing the critical zone, the electronic control unit <NUM> is arranged to filter the current reduced mass flow rate QAHR used to limit the pressure target downstream of the compressor <NUM>. Similarly, the electronic control unit <NUM> is arranged to filter the pressure target downstream of the compressor <NUM>. The filtering capacity of the current reduced mass flow rate QAHR used to limit the pressure target downstream of the compressor <NUM> and the target pressure downstream of the compressor <NUM> is capable of reducing the dynamics of the abovementioned variables. According to a preferred variant, the filtering is achieved through a low pass first order filter.

In the case of reduced mass flow rate QAHR exceeding the critical threshold Mcritica, the control unit <NUM> is then prepared for filtering with a low pass first order type filter both the current reduced mass flow rate QAHR, and the target pressure downstream of the compressor <NUM>.

According to a preferred variant, the control unit is configured to determine a security threshold Mmax_turbo of the reduced mass flow rate QAHR. As best illustrated in <FIG>, the security threshold Mmax_turbo delimits a portion to avoid of the critical area, as it is the closest to achieve sonic conditions (i.e., closer to the saturation line <NUM>) and represents a reduced mass flow rate QAHR beyond which the turbocharger <NUM> should not go.

The security threshold Mmax_turbo is greater than the critical threshold Mcritica. Furthermore, the security threshold Mmax_turbo varies depending on the reduced limit speed NtcR (as best illustrated in <FIG>).

The electronic control unit <NUM> is configured to impose that the reduced mass flow rate QAHR of the compressor <NUM> is lower than the security threshold Mmax_turbo of the reduced mass flow rate QAHR.

The electronic control unit <NUM> is arranged to determine a security threshold Nmax_turbo of the speed of the supercharged internal combustion engine <NUM>. The security threshold Nmax_turbo of the speed of the supercharged internal combustion engine <NUM> is in turn determined as a function of the security threshold Mmax__turbo of the reduced mass flow rate QAHR.

In particular, the security threshold Nmax_turbo of the speed of the supercharged internal combustion engine <NUM> is calculated using the following equation: <MAT> Wherein:.

The security threshold Nmax_turbo is used to limit the speed of the supercharged internal combustion engine <NUM>, so that the current reduced mass flow rate QAHR is lower than the threshold Mmax_turbo.

According to a preferred variant, for the turbocharger <NUM> a preset speed limit Ntc of the turbocharger <NUM> is set above which the turbocharger <NUM> is brought into a critical condition; using the preset speed limit Ntc of the turbocharger <NUM> the current reduced speed limit NtcR of the turbocharger <NUM> is calculated based on the absolute temperature To upstream of the compressor <NUM> using the following equation: <MAT>.

At the varying of absolute temperature To upstream of the compressor <NUM> and at the same preset limit speed Ntc of the turbocharger <NUM> the current reduced limit speed NtcR of the turbocharger <NUM> varies; therefore the electronic control unit <NUM> cyclically determines according to the absolute temperature To upstream of the compressor <NUM> and according to the preset limit speed Ntc of the turbocharger <NUM> (which always remains constant) the current reduced limit speed NtcR of the turbocharger <NUM> and according to the current reduced limit speed NtcR of the turbocharger <NUM> is able to determine the curves <NUM>, <NUM>, <NUM> and <NUM> to be used. Alternatively, since the preset limit speed Ntc of the turbocharger <NUM> is constant to simplify the management of the curves <NUM>, <NUM>, <NUM> and <NUM>, the curves <NUM>, <NUM>, <NUM> and <NUM> themselves may be stored in the electronic control unit <NUM> and parameterized according to the absolute temperature To upstream of the compressor <NUM>; in this way, the electronic control unit <NUM> does not need to calculate the current reduced limit speed NtcR of the turbocharger <NUM> and therefore choose the curves <NUM>, <NUM>, <NUM> and <NUM> to be used, but simply needs to update the curves <NUM>, <NUM>, <NUM> and <NUM> as a function of absolute temperature To upstream of the compressor <NUM>.

According to what has been described insofar and as shown in <FIG>, the current reduced speed limit NtcR varies depending on several factors, in particular on the absolute temperature To upstream of the compressor <NUM>.

According to a preferred variant, in a preliminary stage of setting and tuning a lower limit speed of the turbocharger <NUM> and a higher limit speed of the turbocharger <NUM> are preset (which represents a limit of the turbocharger <NUM> beyond which is best to not exceed in order to not contract serious breakage or damage to the turbocharger <NUM> itself). In use, these two values are used to calculate a reduced lower limit speed Nrid_inf of the turbocharger <NUM> (which is calculated using the formula described above, and varies according to the preset lower limit speed of the turbocharger <NUM> and the absolute temperature To upstream of the compressor <NUM>) and a reduced higher limit speed Nrid_sup of the turbocharger <NUM> (which is also calculated using the formula described above, is greater than the reduced lower speed limit of the turbocharger <NUM> and varies according to the preset limit speed of the turbocharger <NUM> and of the absolute temperature To upstream of the compressor <NUM>). The reduced lower limit speed Nrid_inf of the turbocharger <NUM> and the reduced higher limit speed Nrid_sup of the turbocharger <NUM> delimit an overspeed area in the plane reduced mass flow rate / compression ratio. During the life-span of the turbocharger <NUM> it is often the case that the overspeed area moves in the plane reduced mass flow rate / compression ratio (for example, due to the influence of the absolute temperature To upstream of the compressor <NUM>). In use, once calculated the current reduced speed limit, the electronic control unit <NUM> is arranged to control the turbocharger <NUM> to bring the reduced speed limit to a value lower than the reduced lower limit speed Nrid_inf every time in which a value of the current reduced limit speed comprised within the overspeed interval is detected.

In particular, it is established at a preliminary stage of setting and tuning a first threshold value SOV_1 and the turbocharger <NUM> is controlled to bring the reduced speed limit to a value less than the reduced lower limit speed Nria_inf once a time interval has passed equal to the first threshold value SOV_1 from the moment a value of the current reduced speed limit comprised within the overspeed interval (as shown in <FIG>) is detected. In other words, when the electronic control unit <NUM> detects a current reduced limit speed within the overspeed interval, a timer is initialized to report the reduced limit speed to a value lower than the reduced lower limit speed Nrid_inf once a time interval has passed equal to the first threshold value SOV_1 (preferably by a soft fitting). According to a preferred variant, at a preliminary stage of setting and tuning a second threshold value SOV_2 is established. The electronic control unit <NUM> is arranged to initialize a timer every time that the reduced limit speed descends below the reduced lower limit speed Nrid_inf and inhibits the operation of the turbocharger <NUM> within the overspeed region for a time interval of duration value equal to the second threshold SOV_2.

The first threshold value SOV_1 and the threshold value SOV_2 are variable according to the state of ageing and wear on the supercharger <NUM>.

According to a preferred variant, the control unit <NUM> is capable of storing the total time spent within the overspeed area and inhibit the operation of the turbocharger <NUM> in the overspeed area for the remaining useful life of the turbocharger <NUM> itself once the total time is equal to a safe limit value (determined in a preliminary stage of setting and tuning).

Moreover, according to a preferred variant, the control unit <NUM> is adapted to inhibit the operation of the turbocharger <NUM> in the overspeed area when the reduced mass flow rate QAHR is above the critical threshold Mcritica.

According to a preferred embodiment, the threshold SOV_2 can vary according to the frequency of the most recent overspeed. In other words, the threshold SCV_2 is greater the more frequently a current reduced limit speed is detected within the overspeed range. The threshold SOV_2 may for example be calculated as follows: <MAT>.

Wherein SOV_3 is an operator to decrease (e.g. K*timer with K that represents a preset coefficient), while the summation of time spent within the overspeed interval and the timer are initialized at each trip of the supercharged internal combustion engine <NUM> (i.e. typically for each start/stop cycle of the supercharged internal combustion engine <NUM>) and the timer is started at the first overspeed. The function is preferably in increase.

According to a further variation, for each trip of the supercharged internal combustion engine <NUM> (i.e. for each start/stop cycle the supercharged internal combustion engine <NUM> itself). According to another embodiment, as soon as the control unit <NUM> verifies the operation condition within the overspeed area, a counter C of the time spent in overspeed is initialized. The counter C can be calculated using the following formula <MAT> wherein K1 and K2 are predetermined coefficients in a preliminary phase, while the summation of time spent within the overspeed interval and the summation of time spent outside the area of overspeed are initialized at each trip of the supercharged internal combustion engine <NUM>.

According to a preferred variant, in a preliminary phase of setting and tuning a fourth threshold value SOV_4 is established, which is compared with the counter C of the time spent in overspeed. In the case of counter C greater than or equal to the fourth threshold value SOV_4, the control unit <NUM> is adapted to inhibit the operation of the turbocharger <NUM> within the overspeed area. On the contrary, in case of counter C lower than the fourth threshold value SOV_4, the control unit <NUM> is set to allow the operation of the turbocharger <NUM> within the overspeed area (preferably with the assistance of a hysteresis operator).

According to a variant, the control unit <NUM> utilizes the current pressure of the turbocharger <NUM> instead of the current reduced limit speed to recognize the current operation in the overspeed area.

Claim 1:
Method for controlling an internal combustion engine (<NUM>) supercharged by means of a turbocharger (<NUM>) provided with a turbine (<NUM>) and a compressor (<NUM>); the control method comprises the steps of:
determining the current reduced mass flow rate (QAHR) of the compressor (<NUM>);
determining a critical threshold (Mcritica) of the reduced mass flow rate (QAHR) which delimits in the plane reduced mass flow rate / compression ratio a portion of the useful area of the operating range of the turbocharger (<NUM>) - critical area -;
determining a safety threshold (Mmax_turbo) of the reduced mass flow rate (QAHR), said safety threshold (Mmax_turbo) delimiting, in the plane reduced mass flow rate / compression ratio, an area close to the achievement of sonic conditions, wherein said safety threshold (Mmax_turbo) is greater than the critical threshold (Mcritica);
determining a safety threshold (Nmax_turbo) of the speed of the supercharged internal combustion engine (<NUM>) calculated using the following equation: <MAT> wherein:
Nmax_turbo safety threshold of the speed;
Mmax_turbo safety threshold of the of the reduced mass flow rate (QAHR);
To absolute temperature upstream of the compressor (<NUM>);
Po absolute pressure upstream of the compressor (<NUM>);
To_rif absolute reference temperature;
Po_rif absolute reference pressure;
Ncil number of cylinders (<NUM>) of the internal combustion engine (<NUM>); and
m mass of air drawn for each cylinder (<NUM>); and
imposing the fact that the reduced mass flow rate (QAHR) of the compressor (<NUM>) has to be lower than the safety threshold (Mmax_turbo) of the reduced mass flow rate (QAHR); and imposing the fact that the speed of the supercharged internal combustion engine (<NUM>) has to be lower than the safety threshold (Nmax_turbo) of the speed; using the safety threshold (Nmax_turbo) of the speed of the supercharged internal combustion engine (<NUM>) to limit the speed of the supercharged internal combustion engine (<NUM>), so that the current reduced mass flow rate (QAHR) is lower than safety threshold (Mmax_turbo) of the reduced mass flow rate (QAHR).