ITUB20159294A1 - METHOD FOR DETERMINING THE INSTANTANEOUS ROTATION SPEED OF A TURBOCHARGER IN AN OVERALLLY OCCURRED INTERNAL COMBUSTION ENGINE - Google Patents

Description

"METHOD FOR DETERMINING THE INSTANT ROTATION SPEED OF A TURBOCHARGER IN A BOOSTED INTERNAL COMBUSTION ENGINE"

TECHNIQUE SECTOR

The present invention relates to a method for determining the instantaneous speed of rotation of a turbocharger in a supercharged internal combustion engine.

ANTERIOR ART

A supercharged internal combustion engine is equipped with a turbocharger (turbocharger driven by the exhaust gases or volumetric turbocharger driven by the crankshaft) which at some moments compresses the intake air to increase volumetric efficiency. As a result of the action of the turbocharger, in a supercharged internal combustion engine in the plenum of the intake manifold there may be a slight depression determined by the intake action generated by the cylinders (turbocharger not active) or there may be an overpressure determined by the compression action of the turbocharger (active turbocharger). Consequently, in a supercharged internal combustion engine it is important to be able to precisely control the rotation speed of a turbocharger compressor which causes the overpressure in the intake manifold plenum.

The best known and most used solution to determine the rotation speed of the compressor provides for a sensor (preferably with a hall effect or eddy currents) which detects the passage of the compressor blades and determines the speed of the turbocharger itself as a function of the frequency of passage of the blades. The sensor is normally fixed to a wall of a compressor diffuser in a position facing and near the blades.

This solution, however, has the disadvantage of guaranteeing unsatisfactory performance in terms of reliability and furthermore requires mechanical machining on the compressor which could be expensive for processing times and costs.

Furthermore, in a supercharged internal combustion engine it is extremely important to be able to precisely control the power delivered by the turbocharger turbine. For example, the lubrication (i.e. the quantity of control fluid circulated by a lubrication pump) and the conditioning of the turbocharger are managed by an open-loop control unit through an estimate of the power dissipated by the turbocharger in the most operating conditions. burdensome; this solution is clearly inefficient and involves an evident waste of the control fluid used for the lubrication of the turbocharger.

To solve this problem, document EP-A-2392804 describes a method for determining the rotation speed of a turbocharger which provides, during normal operation of the internal combustion engine, to detect the intensity of a detected sound signal by means of a microphone. by the rotation of the turbocharger; and determining the evolution over time of the frequency content of the sound signal detected by the rotation of the compressor. More specifically, the method provides for determining, completely independently of the sound signal detected by the rotation of the turbocharger, a first estimate of the rotation speed of the turbocharger with which to determine a plausible frequency range for the rotation speed of the turbocharger; determining a second estimate of the speed of rotation of the turbocharger as a function of the frequency content of the sound signal detected by the rotation of the turbocharger within the range of plausible frequencies; and validating the second estimate of the rotational speed of the turbocharger by the first estimate of the rotational speed of the turbocharger. Typically, the first and second estimates of the turbocharger rotation speed are obtained through parameters provided by the engine control, such as the compression ratio and the flow rate of air passing through the compressor. It is evident that a determination method of the type just described is not sufficiently robust since information provided by the engine control is required and any errors on this information would make the estimation of the turbocharger rotation speed completely unreliable.

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a method for determining the instantaneous speed of rotation of a turbocharger in a supercharged internal combustion engine, which method of determination is free from the drawbacks of the state of the art and, in particular, is easy and economical. implementation.

According to the present invention there is provided a method for determining the instantaneous speed of rotation of a turbocharger in a supercharged internal combustion engine according to what is claimed by the attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the attached drawings, which illustrate a non-limiting example of embodiment, in which:

Figure 1 is a schematic view of a supercharged internal combustion engine provided with a control unit which implements the method of determining the rotation speed of a turbocharger object of the present invention;

figure 2 is a schematic view of a turbocharger of the internal combustion engine of figure i;

Figure 3 illustrates the intensity of the signal generated by the internal combustion engine by operating the discrete Fourier transform;

Figure 4 illustrates the signal of Figure 3 after the removal of the ambient noise;

Figure 5 illustrates the spectrum of an index calculated starting from the signal of Figure 4;

Figure 6 is a block diagram which schematically illustrates the method for determining the instantaneous speed of rotation of a turbocharger object of the present invention; And

Figure 7 illustrates the comparison between the speed of the turbocharger of Figure 2 detected by means of a suitable sensor and the speed of the turbocharger of Figure 2 determined by the method of the present invention,

PREFERRED EMBODIMENTS OF THE INVENTION

In Figure 1, the number 1 indicates as a whole an internal combustion engine supercharged by means of a turbocharger supercharging system 2.

The internal combustion engine 1 comprises four cylinders 3, each of which is connected to an intake manifold 4 via at least one respective intake valve (not shown) and to an exhaust manifold 5 via at least one respective exhaust valve (not shown ). The intake manifold 4 receives fresh air (ie air from the external environment) through an intake duct 6, which is provided with an air filter 7 and is regulated by a butterfly valve 8. An intercooler 9 is arranged along the intake duct 6 and has the function of cooling the intake air. Connected to the exhaust manifold 5 is an exhaust duct 10 which supplies the exhaust gases produced by combustion to an exhaust system, which emits the gases produced by combustion into the atmosphere and normally comprises at least one catalyst 11 and at least one silencer ( not shown) arranged downstream of the catalyst 11.

The supercharging system 2 of the internal combustion engine 1 comprises a turbocharger 12 provided with a turbine 13, which is arranged along the exhaust duct 10 to rotate at high speed under the action of the exhaust gases expelled from the cylinders 3, and a compressor 14, which is arranged along the intake duct 6 and is mechanically connected to the turbine 13 to be driven into rotation by the turbine 13 itself so as to increase the pressure of the air fed into the intake duct 6.

Along the discharge duct 10 there is a bypass duct 15, which is connected in parallel to the turbine 13 so as to have its ends connected upstream and downstream of the turbine 13 itself; a wastegate valve 16 is arranged along the bypass duct 15, which is adapted to regulate the flow rate of the exhaust gases flowing through the bypass duct 15 and is piloted by an actuator 17. Along the intake duct 6 is provided a bypass duct 18, which is connected in parallel to the compressor 14 so as to have its ends connected upstream and downstream of the compressor 14 itself; a Poff valve 19 is arranged along the bypass duct 18, which is adapted to regulate the flow of air flowing through the bypass duct 18 and is piloted by an actuator 20.

The internal combustion engine 1 is controlled by an electronic control unit 21, which supervises the operation of all the components of the internal combustion engine 1.

The internal combustion engine 1 also comprises a canister circuit 22, which has the function of recovering the fuel vapors that develop in a fuel tank 23 and of introducing such fuel vapors into the cylinders 3 so that they are burned; in this way, it is avoided that the fuel vapors which develop in the fuel tank 23 can escape from the fuel tank 23 (in particular when the fuel cap is opened during a refueling) and disperse freely in the atmosphere.

According to what is illustrated in Figure 1, a control system 24 is provided which, in addition to the control unit 21, comprises at least one sensor 25 connected to the control unit 21.

In particular, according to a first variant, the sensor 25 is a sensor of the acoustic pressure level, i.e. a microphone 25, which is connected to the control unit 21 and is able to detect the intensity S of the sound signal detected by the movement of the turbocharger 12. According to what is better illustrated in Figure 2, the compressor 14 comprises a bladed disc rotatable about a rotation axis X and a fixed diffuser 27. The microphone 25 is arranged in such a way as to detect the intensity S of the sound signal emitted by the rotation of the blades 28. The microphone is preferably arranged in a position facing and near the turbocharger 12 and sheltered from environmental noise such as not to be affected in a way excessive environmental noise (produced for example by the horn, by the onset of knocking phenomena, etc.).

According to a second variant, not shown, the sensor 25 is an accelerometer, which is connected to the control unit 21, is preferably installed on a scroll 26 of the compressor 14 and is able to detect the acceleration of the compressor 14.

According to a further variant not shown, the control system 24, in addition to the control unit 21, comprises both an accelerometer, which is connected to the control unit 21, is preferably installed on a scroll 26 of the compressor 14 and is able to detect the acceleration of the compressor 14; both the sensor 25 of the acoustic pressure level, i.e. the microphone 25, which is connected to the control unit 21 and is able to detect the intensity S of the sound signal which detects the movement of the compressor 14.

Finally, the control system 24 comprises a further sensor 29 connected to the control unit 21 and arranged in a position such as to detect only the ambient noise and such as not to be affected by the noise generated by the turbocharger 12.

The sensor 29 can alternatively be an acoustic pressure level sensor, i.e. a microphone 29, or an accelerometer 29.

The strategy for determining the instantaneous rotation speed wT of the turbocharger 12 implemented by the control unit 21 as a function of the signals acquired by the sensors 25 and 29 is described below.

The control unit 21 is first of all arranged to acquire the signal coming from both the sensor 25 and the sensor 29.

The control unit 21 receives at its input the sound signals detected both by the sensor 25 and by the sensor 29, which are sampled and processed to give rise to a finite sequence of L samples, ie a discrete-time signal. This finite sequence of L samples, that is the discrete time signal, is then processed by operating a discrete Fourier transform (DFT). The discrete Fourier DFT transforms provide a sampled version of the spectrum of the sound signals detected by both sensor 25 and sensor 29, i.e. it returns the values in terms of modulus (i.e. amplitude) and phase (i.e. position) and the spectrum of the detected sound signals both from the sensor 25 and from the sensor 29 it assumes at certain equally spaced frequencies. In other words, the discrete Fourier DFT transform represents a frequency sampling of the spectrum of sound signals detected by both sensor 25 and sensor 29.

Two vectors are therefore stored inside the control unit 21, each of which comprises a number L of elements (also called bins) which represent the frequency sampling of the spectrum of sound signals detected by sensor 25 and sensor 29 respectively. other words, a bin represents the discretization step of the discrete Fourier DFT transform. The number L of bins is suitably chosen in order to satisfy both the need for speed and precision in the response and represents the time discretization step of the discrete Fourier DFT transform. The information content of each of the bins L is equal to the ratio fs / L where L represents the number of bins and fs represents the sampling frequency. The sampling frequency fs is at least double the frequency of the phenomenon to be observed (for the Nyquist-Shannon theorem) but, at the same time, high values would excessively aggravate the computational burden of the control unit 21; in this case, plausible values for the sampling frequency fs are between 100 and 200 kHz.

According to a preferred variant, the discrete Fourier DFT transforms of the sound signals detected by both the sensor 25 and the sensor 29 are calculated synchronously for the two sensors indicated with 25 and 29.

Figure 3 illustrates the modulus of the discrete Fourier DFT transform of the sound signal detected by the sensor 25 which is normally referred to as the "spectrum" of the signal. It has been possible to experimentally note that for medium-low frequency values there is a considerable environmental noise, which depends on the speed of the internal combustion engine 1 as well as on the frequency.

The control unit 21 is therefore arranged to remove the environmental noise from the discrete Fourier DFT transform of the sound signal detected by the sensor 25 through the discrete Fourier DFT transform of the sound signal detected by the sensor 29.

In particular, according to a preferred variant, the control unit 21 is configured in such a way as to calculate the difference between each module of the discrete Fourier DFT transform of the sound signal detected by the sensor 25 and the corresponding module of the discrete Fourier DFT transform of the signal. sound detected by the sensor 29.

According to a further variant, the control unit 21 is configured in such a way as to calculate the difference between each module of the discrete Fourier DFT transform of the sound signal detected by the sensor 25 and a noise index INOISE obtained from the discrete Fourier DFT transform of the signal. sound detected by the sensor 29. According to a preferred variant, the noise index INOISE is calculated as an average of the modules of the discrete Fourier transform DFT of the sound signal detected by the sensor 29 for a number L 'of bins of interest. In other words, for the medium-low values of interest of the frequencies, the mean value of the modules is calculated from the discrete Fourier DFT transform of the sound signal detected by the sensor 29 in order to obtain the noise INOISE index. This noise index INOISE is then subtracted from the modules of the discrete Fourier DFT transform of the sound signal detected by the sensor 25 to remove the ambient noise.

Figure 4 illustrates the modulus of the discrete Fourier DFT transform of the sound signal detected by the sensor 25 from which the environmental noise has been eliminated as described in the preceding discussion.

After eliminating the ambient noise, the control unit 21 is configured to calculate an index PRj of relevance for each bin L * of interest which will subsequently be used to determine the instantaneous speed of rotation wTC of the turbocharger 12.

The PRj of relevance is calculated for each bin L * of the vector (i.e. for each frequency) as follows:

PSD

<PR> j = k = j ± LPR [1]

∑ <p> SDk

k = j-LPR> k ≠ j

PRj index of relevance for the j-th bin of the vector;

PSDjmodule of the discrete Fourier transform of the sound signal detected by the sensor 25 for the j-th bin of the vector; And

LRP number of bins of the vector that are used in a right neighborhood and a left neighborhood of the j-th bin L * of the vector.

In other words, the index PRj of relevance establishes to what extent the modulus of each bin L * of the vector (i.e. each frequency) is predominant with respect to the modules of the bins in an interval of adjacent frequencies (of amplitude equal to 2 * LRP). The interval of adjacent frequencies is centered on the j-th bin of the vector, is symmetrical with respect to the j-th bin of the vector and does not include the PSDj module of the discrete Fourier transform for the j-th bin of the vector under examination.

Once the relevance index PRj has been calculated for each bin j of the vector, the control unit 21 is set up to calculate a further index MPRj which is obtained through the product of the index PRj of relevance for the j-th bin of the vector and the PSDj module of the discrete Fourier transform for the j-th bin of the vector.

The MPRj index is calculated for each bin j of the vector (i.e. for each frequency) as follows:

MPRj = PRj * PSDj [2]

PRj index of relevance for the j-th bin of the vector; And

PSDjmodule of the discrete Fourier transform of the sound signal detected by the sensor 25 for the j-th bin of the vector.

Figure 5 illustrates the spectrum of the MPRj index calculated for each bin j of the vector by means of the discrete Fourier DFT transform of the sound signal detected by the sensor 25 (i.e. the values assumed by the modulus / intensity of the discrete Fourier DFT transform of the sound signal detected by the sensor 25). It has been possible to experimentally note that the instantaneous speed of rotation of the turbocharger 12 is associated with frequency values characterized by a peak in the spectrum of the MPR index illustrated in Figure 5; that is a bin characterized by a high value of the MPRj index contains information relating to the instantaneous speed wTc of rotation of the turbocharger 12. More in detail, a bin characterized by both a high value of the PR relevance index and a high value of the PSD module of the discrete Fourier transform of the sound signal detected by the sensor 25 contains information relating to the instantaneous speed of rotation of the turbocharger 12. Therefore, even more so, the MPR index which is nothing more than the product of the PR relevant index for the module PSD will be high for those bins which contain information relating to the instantaneous speed of rotation of the turbocharger 12 in order to define a rather robust and at the same time simple estimate of this property.

The strategy implemented by the control unit 21 to recognize the peak in the spectrum of the MPRj index illustrated in Figure 5 and, consequently, to determine the instantaneous speed of rotation of the turbocharger 12 is schematically illustrated in Figure 6.

Firstly, in a tuning phase, a lower RPML0W limit value is identified, below which the strategy does not allow to reliably recognize the instantaneous speed wTc of rotation of the turbocharger 12. It has been experimentally verified that a value RPMLOW lower limit equal to 1.8 * 10 <5> rpm allows to optimize the performance of the speed estimation strategy wTC Snapshot of rotation of the turbocharger 12.

The strategy receives in input (INPUT block in Figure 6) the spectrum of the MPR index calculated for each bin j of the vector by means of the discrete Fourier DFT transform of the sound signal detected by the sensor 25 cleaned of environmental noise as described in the preceding treatment and shown in Figure 5.

First, the local MPR 'maximum of the MPR index is sought in a narrow range in the vicinity of the bin which corresponds to the lower limit RPMLOW value and represents a threshold below which the strategy does not work.

The local MPR 'maximum is then compared with a lower threshold MPRLOw value; if the local maximum MPR 'is lower than the lower threshold MPRLow value, then the instantaneous speed wTc of rotation of the turbocharger 12 is estimated to be equal to the lower limit RPML0W and, at the next iteration, the search for the maximum MPR is performed again' local just described.

If, on the other hand, the local maximum MPR 'is higher than the lower threshold value MPRLOw, then the strategy for estimating the instantaneous speed wTc of rotation of the turbocharger 12 is activated. In the CALCULATION block, the control unit 21 proceeds to obtain the maximum MPRMAX value of the MPR index contained within a narrow band defined in a preliminary setting and calibration phase. At the same time, the control unit 21 proceeds to recognize the maximum PSDMAX value of the PSD index contained within a further narrow band defined in a preliminary setting and calibratable step.

The MPR ^ maximum value of the MPR index just obtained is then compared with an upper threshold MPRHIGH value. If the maximum MPRMAX value is higher than the upper threshold MPRHIGH value, then this maximum MPRMAX value is considered acceptable and the corresponding number of revolutions is provided which represents the instantaneous speed wTc of rotation of the turbocharger 12.

If the maximum MPRMAX value is lower than the upper threshold MPRHIGH value, then a further check is carried out. The maximum PSDMAX value of the newly obtained PSD module is compared with a respective upper threshold PSDHIGH value. If the maximum PSDMAX value is higher than the upper threshold PSDHIGH value, then this maximum PSDMAX value is considered acceptable and the corresponding number of revolutions is provided which represents the speed wTC Snapshot of rotation of the turbocharger 12. In this way, it is possible obtain information on the instantaneous speed wTc of rotation of the turbocharger 12 even in those cases in which the index PR of relevance is not particularly high (for example due to a particularly "dirty" area in terms of environmental noise) but a significant value of the PSD intensity of the modulus of the discrete Fourier transform.

If the maximum PSDMAX value is lower than the upper threshold PSDHIGH value, then a COUNT counter is incremented,

In the event that the COUNT counter is less than or equal to 2, then one returns, to the next iteration, in the OUTPUT block providing the value of the speed wTC Snapshot of rotation of the turbocharger 12 determined at the previous iteration and then one returns to the CALCULATION block in which the search for the maximum MPRMAX value of the MPR index and the PSDMAX maximum value of the PSD index just described has been performed again.

If the COUNT counter is greater than 2, then you go back to the INPUT block and the strategy is substantially deactivated.

The limit value of 2 for the COUNT counter is variable and can be calibrated during setup.

Figure 7 compares the trend of the instantaneous rotation speed wTc of the turbocharger 12 estimated by means of the strategy described in the preceding discussion (indicated with MPRA) and the trend of the instantaneous rotation speed wTc of the turbocharger 12 determined by means of a laboratory reference sensor housed in a seat obtained in the scroll 26 of the compressor 14 (indicated by Sref).

In the above discussion, explicit reference was made to the case in which there is a single sensor 25 and a single sensor 29, which can however be advantageously replaced by a number of sensors 25 'defining a "virtual sensor 25" and a number of sensors 29 'defining a "virtual sensor 29".

The method for determining the instantaneous speed of rotation of the turbocharger 12 described up to now has several advantages. In particular, although it is advantageous in terms of costs, easy and economical to implement and does not involve an increase in the computational burden for the control unit 21, it allows to reliably estimate the speed wTC Snapshot of rotation of the turbocharger 12 using only the content of the signal coming from the sensors 25 and 29 and avoiding the use of information provided by the engine control which, if compromised, would also risk making the estimate of the instantaneous speed of rotation wTc of the turbocharger 12 unreliable.

Claims (1)

CLAIMS 1, - Method for determining the instantaneous speed (wTC) of rotation of a turbocharger (12) equipped with a turbine (13) and a compressor (14) which compresses the intake air in an internal combustion engine (1) supercharged; and comprising a first sensor (25) adapted to detect the intensity (S) of the signal generated by the movement of the turbocharger (12) and arranged in a position facing the turbocharger (12); the method involves; detecting by means of the first sensor (25) the intensity of the signal generated by the rotation of the turbocharger (12); processing the signal detected by said first sensor (25) by operating a fast Fourier transform (FFT) so as to obtain a discrete Fourier transform (DFT) which represents a sampling in frequency of the spectrum of said signal; calculating a first index (MPR) for each frequency of the discrete Fourier transform (DFT) of the signal detected by the rotation of the turbocharger (12); And determine the instantaneous speed (wTC) of rotation of the turbocharger (12) as a function exclusively of the spectrum of the first index (MPR) calculated for each frequency of the discrete Fourier transform (DFT) of the signal generated by the rotation of the turbocharger (12). 2. A method according to Claim 1, in which the step of determining the instantaneous speed (wTC) of rotation of the turbocharger (12) does not involve using information provided by the engine control. 3.- Method according to Claim 1 or 2, wherein the supercharged internal combustion engine (1) comprises a second sensor (29) which is arranged in a position such as to detect only the ambient noise and not to be affected by the movement of the turbocharger (12); the method involves detecting the intensity of the ambient noise by means of the second sensor (29); processing the signal detected by said second sensor (29) by operating a fast Fourier transform (FFT) so as to obtain a discrete Fourier transform (DFT) which represents a sampling in frequency of the spectrum of said signal; And determining the instantaneous speed (wTC) of rotation of the turbocharger (12) as a function of the discrete Fourier transform (DFT) of the signal detected by said second sensor (29). 4.- Method according to claim 3 and comprising the further step of removing the ambient noise from the spectrum of the discrete Fourier transform (DFT) of the signal generated by the rotation of the turbocharger (12) by means of the spectrum of the discrete Fourier transform (DFT) of the signal detected by said second sensor (29). 5.- Method according to Claim 4 and comprising, for each frequency of interest of the discrete Fourier transform (DFT) of the signal generated by the rotation of the turbocharger (12), the further step of calculating the difference between the modulus of said discrete transform of Fourier (DFT) and the corresponding modulus of the discrete Fourier transform (DFT) of the signal detected by said second sensor (29). 6.- Method according to Claim 4 and comprising the further steps of: calculating a noise index (INOISE) for the discrete Fourier transform (DFT) of the signal detected by the second sensor (29); And calculate, for each frequency of the discrete Fourier transform (DFT) of the signal generated by the rotation of the turbocharger (12), the difference between the modulus of the said discrete Fourier transform (DFT) of the signal generated by the rotation of the turbocharger (12) and l noise index (INOISE). 7. A method according to claim 6, wherein the index (INOISE) of noise is calculated by averaging the modules of the discrete Fourier transform (DFT) of the signal detected by the second sensor (29) for a frequency range of interest. 8.- Method according to one of the preceding claims, in which for each frequency of the discrete Fourier transform (DFT) of the signal generated by the rotation of the turbocharger (12), the first index (MPR) is calculated through the product between a second (PR ) relevance index of said frequency and the modulus (PSD) of the discrete Fourier transform of the signal generated by the rotation of the turbocharger (12) for said frequency. 9. A method according to Claim 8 and comprising the further step of determining the instantaneous speed (wTc) of rotation of the turbocharger (12) as a function of the peak value of the spectrum of the first index (MPR). 10.- Method according to claim 9 and comprising the further steps of: search for the maximum value (MPR ^) in the spectrum of the first index (MPR); compare the maximum (MPRMSX) value with a value (MPRHIGH) of upper threshold; And determine the instantaneous speed (wTC) of rotation of the turbocharger (12) only if the maximum value (MPRMSX) is greater than the upper threshold value (MPRHIGH). 11. A method according to claim 10 and comprising the further steps of: search for the local maximum value (MPR ') in the spectrum of the first index (MPR) in a narrow range in the vicinity of a lower limit value; comparing the local maximum value (MPR ') with a lower threshold value (MPRLQW); And search for the maximum value (MPRMAX) in the spectrum of the first index (MPR) only if the local maximum value (MPR ') is higher than the lower threshold value (MPR ^). 12, - Method according to claim 10 or 11 and, in case the maximum (MPRMAX) value is less than the value (MPRHIGH) of upper threshold, comprising the further steps of determining the instantaneous speed (wTC) of rotation of the turbocharger (12) as a function of the peak value of the module (PSD) of the discrete Fourier transform of the signal generated by the rotation of the turbocharger ( 12). 13.- Method according to claim 12 and comprising the further steps of: finding the maximum modulus (PSDMAX) of the discrete Fourier transform of the signal generated by the rotation of the turbocharger (12); compare the maximum module (PSDMAX) with a respective upper threshold value (PSDHIGH); And determine the instantaneous speed (wTC) of rotation of the turbocharger (12) only if the maximum module (PSDMAX) is greater than the respective upper threshold value (PSDHIGH). 14.- Method according to one of claims 6 to 13, wherein the second (PR) index for each frequency of the discrete Fourier transform (DFT) of the signal generated by the rotation of the turbocharger (12) is calculated by: PSD pRjk = j + LPR ∑ PSDt k = j-LPR, k ≠ j PRj second relevance index for the j-th frequency of the discrete Fourier transform (DFT) of the signal generated by the rotation of the turbocharger (12); PSDjmodule of the discrete Fourier transform (DFT) of the signal generated by the rotation of the turbocharger (12) for the j-th frequency; And LRP number of frequencies of interest that defines the width of a frequency range centered on the jth frequency. 15. A method according to one of the preceding claims, in which the first sensor (25) and the second sensor (29) are alternatively microphones (25, 29) or accelerometers (25, 29).