in the accompanying fig1 , reference numeral 1 indicates as a whole an internal combustion engine having four inline cylinders 2 . each cylinder 2 accommodates a respective piston 3 mechanically connected via a connecting rod to a driving shaft 4 to transmit the force generated by combustion within the cylinder 2 to the driving shaft 4 . the internal combustion engine 1 is controlled by an electronic control unit 5 ( normally called an “ ecu ”) which is arranged in the vicinity of the internal combustion engine 1 and is normally housed inside the engine compartment of the vehicle ( not shown ). the electronic control unit 5 includes a microphone 6 ( an acoustic - type pressure sensor 6 ), which is housed inside the control unit 5 and is adapted to detect the intensity of the noise generated by the internal combustion engine 1 ( i . e ., it is adapted to detect the intensity of the acoustic pressure waves generated by the internal combustion engine 1 ). the electronic control unit 5 detects , by microphone 6 , the intensity s of the noise generated by the internal combustion engine 1 ( i . e . of vibrations generated by the internal combustion engine 1 ) in a predetermined amplitude measurement time window ( normally of the order 1 - 5 tenths of a second ). in the electronic control unit 5 , the intensity s of the noise generated by the internal combustion engine 1 is digitized using a sampling at a relatively high frequency ( of the order of 50 khz ). thereafter , the electronic control unit 5 determines the value of at least one synthetic index i by elaborating the intensity s of the noise generated by the internal combustion engine 1 in the measurement time window . specifically , the value of the synthetic index i is calculated as a function of the intensity s of the noise generated by the internal combustion engine 1 in the measurement time window in such a way that the synthetic index i is a “ synthesis ” of the intensity s of the noise generated by the internal combustion engine 1 in the measurement time window . the synthetic index i is compared with at least one predetermined comparison quantity th and then the type of fuel actually used by the internal combustion engine 1 is recognised as a function of the comparison of the synthetic index i to the comparison quantity th . the comparison quantity th may be determined experimentally during a calibration step which is carried out by feeding different fuels having known features to the internal combustion engine 1 suitably provided with laboratory instruments . normally , the comparison quantity th is associated with a specific recognition operating point of the internal combustion engine 1 ; in other words , the comparison quantity th is determined in the recognition operating point and is therefore valid only at or in the vicinity of the recognition operating point . the operating point of engine 1 ( also called engine point ) is generally identified by a value of the engine speed and a load value . the load value is provided by the suction pressure or by the suction efficiency ( i . e . the ratio between the amount of air actually drawn and the maximum amount of air that can be drawn ). the comparison of the synthetic index i to the comparison quantity th is only made when the current operating point of the internal combustion engine 1 is in a neighbourhood of the recognition operating point , i . e . when the difference between the current parameters ( engine speed and load ) and the recognition operating point parameters is “ small ” ( i . e . lower , in absolute value , than a threshold ). during the system calibration , the recognition operating point is chosen in such a way as to optimise the differences between different fuels ; specifically , the differences that can be perceived in the noise generated by the internal combustion engine 1 according to the type of fuel used are less apparent in some operating points and more apparent in other operating points . in order to simplify the recognition of the type of fuel used , it is clear that it is convenient to choose the recognition operating point in an area where the differences between different fuels are maximum . in order to increase the possibility to carry out the recognition , it is possible to use multiple comparison quantities th , each of which is associated with its own recognition operating point different from recognition operating points of the other comparison quantities th . when the current operating point of the internal combustion engine 1 near the recognition operating point , and a recognition of the type of fuel actually used by the internal combustion engine 1 is to be made , the engine control is forcedly altered with respect to the normal standard engine control so as to amplify the behavioural differences of the different types of fuel that can be used by the internal combustion engine 1 . in other words , in order to perform the recognition of the type of fuel actually used by the internal combustion engine 1 with higher reliability , rather than using the normal standard engine control ( which is intended to generate the driving torque required by the driver , minimising the generation of pollutants and minimising fuel consumption ), a special engine control is used which is intended to enhance the behavioural differences of the different types of fuel that can be used by the internal combustion engine 1 without excessively affecting the operating regularity . according to one embodiment , in order to perform a recognition of the type of fuel actually used by the internal combustion engine 1 , the engine control is forcedly altered ( compared to the normal standard engine control ) to use , as a reference , an abnormal stoichiometric air / fuel ratio that is different from the stoichiometric air / fuel ratios of the fuels that can be used by the internal combustion engine 1 . for example , if the fuels that can be used by the internal combustion engine 1 are e22 ( mixture consisting of 22 % ethanol and 78 % petrol ) and e100 ( mixture consisting of 100 % ethanol , i . e . pure ethanol ), the stoichiometric air / fuel ratio of fuel e22 is equal to 13 . 5 , while the stoichiometric air / fuel ratio of fuel e100 is equal to 9 . accordingly , the engine control operates using , as a reference , a stoichiometric air / fuel ratio equal to 13 . 5 if fuel e22 is used , or using , as a reference , a stoichiometric air / fuel ratio equal to 9 if fuel e100 is used . in order to perform a recognition of the type of fuel actually used by the internal combustion engine 1 , the engine control uses , as a reference , an abnormal stoichiometric air / fuel ratio that is different from both the stoichiometric air / fuel ratio of fuel e22 and the stoichiometric air / fuel ratio of fuel e100 . for example , the engine control may use as a reference an abnormal stoichiometric air / fuel ratio from 10 to 12 for the short time during which the intensity s of the noise generated by the internal combustion engine 1 is acquired ( i . e . the measurement time window ). when the engine control uses , as a reference , the abnormal stoichiometric air / fuel ratio ( e . g . equal to 11 ), if the fuel that is actually used by the internal combustion engine 1 is e22 , then there would be a rich combustion , i . e . in excess of fuel ( the actual coefficient x , which indicates the relationship between the air / fuel ratio and the actual stoichiometric air / fuel ratio , would be about 0 . 81 ). similarly , if the fuel that is actually used by the internal combustion engine 1 is e100 , then there would be a lean combustion , i . e . in shortage of fuel ( the actual coefficient x would be about 1 . 2 ). in other words , when the engine control uses as a reference the abnormal stoichiometric air / fuel ratio , and the amount of fuel being injected remains the same , a higher driving torque is generated ( therefore , greater power and more energy involved which results in stronger noise ). if the fuel that is actually used by the internal combustion engine 1 is e22 , a lower driving torque is generated ( therefore , lower power and less energy involved which results in weaker noise ) when the fuel that is actually used by the internal combustion engine 1 is e100 . it is therefore clear that the use of the abnormal stoichiometric air / fuel ratio enhances the differences of noise determined by two types of fuel for the short time during which the intensity s of the noise generated by the internal combustion engine 1 is acquired ( i . e . the measurement time window ). to summarise , when the current operating point of the internal combustion engine 1 near the recognition operating point and a recognition of the type of fuel actually used by the internal combustion engine 1 is to be performed , the engine control is forcedly altered ( compared to the normal standard engine control ) to amplify the behavioural differences of the different types of fuel used by the internal combustion engine 1 by using an air / fuel ratio for the engine control that is different from the abnormal stoichiometric air / fuel ratios of the fuels that can be used by the internal combustion engine 1 . in one embodiment , the intensity s of the noise generated by the internal combustion engine 1 in the measurement time window is previously filtered with a band - pass filter or by using a filter with “ weighting a ” ( a type of equalisation that boosts the frequencies more perceived by the human being and cuts the less audible ones ). by way of example , the filtering band of the band - pass filter can be between 10 hz and 16 khz ( i . e ., the band - pass filter attenuates the frequencies below 10 hz and higher than 16 khz and enhances the frequencies between 10 hz and 16 khz ). in a first simplified ( and more robust ) recognition mode , the electronic control unit 5 recognises a first type of fuel if the synthetic index i is either higher or lower than the comparison quantity th , and recognises a second type of fuel if the synthetic index i is correspondingly lower or higher than the comparison quantity th . this first simplified mode is of the “ binary ” type , i . e . it only provides the choice between two different types of fuel as a function of the comparison of the synthetic index i to the comparison quantity th . in a second , more refined ( and potentially less robust ) recognition mode , the electronic control unit 5 recognises the type of fuel by an interpolation performed as a function of the comparison of the synthetic index i to the comparison quantity th . in this second , more refined recognition mode , at least two comparison quantities th are normally used , which delimit a window within which the synthetic index i is , and the fuel type is recognised by an interpolation between the types associated with the two comparison quantities th . in one embodiment , the electronic control unit 5 calculates the synthetic index i directly as a function of the variation in time of the intensity s of the noise generated by the internal combustion engine 1 , and then calculates the value of the synthetic index i in the time domain . in particular , after filtering , the absolute value of the intensity s of the noise generated by the internal combustion engine 1 is integrated in time within the measurement time window in order to determine the synthetic index i . in other words , the synthetic index i is equal to the integral over time within the measurement time window of the absolute value of the intensity s of the noise generated by the internal combustion engine 1 which has been previously filtered . the intensity s of the noise generated by the internal combustion engine 1 is a function of the power developed by the combustion in the cylinders 2 of the internal combustion engine 1 . accordingly , the synthetic index i is a function of the energy generated by the combustion in the cylinders 2 of the internal combustion engine 1 during the measurement time window . in another embodiment , the electronic control unit 5 calculates the fft ( fast fourier transform ) of the intensity s of the noise generated by the internal combustion engine 1 in the measurement time window , and then calculates the value of the synthetic index i in the frequency domain as a function of the amplitude of at least one harmonic of the fft . however , this embodiment requires a much higher computing power since the fft calculation is much more complex than the simple calculation of a time integral . in the embodiment described above , the sensor used by the electronic control unit 5 is a microphone 6 and it detects the intensity s of the noise generated by the internal combustion engine 1 . in an equivalent embodiment , the sensor used by the electronic control unit 5 is an accelerometer 7 which is directly mounted on the internal combustion engine 1 and detects the intensity s of the mechanical vibrations generated by the internal combustion engine 1 . in other words , in order to recognise the type of fuel actually used , the electronic control unit 5 uses the intensity s of vibrations generated by the internal combustion engine 1 , and such vibrations can may be acoustic ( sound ) and thus detected by microphone 6 , or mechanical and thus detected by accelerometer 7 . it should be noted that the mechanical vibrations generated by the internal combustion engine 1 are closely related with the noise generated by the internal combustion engine 1 , as they are both originated by the same physical phenomena created by the combustion of fuel in the cylinders 2 ; therefore , considering the mechanical vibrations generated by the internal combustion engine 1 is sufficiently equivalent to considering the noise generated by the internal combustion engine 1 . according to one embodiment , the intensity s of the mechanical vibrations measured by accelerometer 7 in the measurement time window is previously filtered by a band - pass filter which acts in the window 3 - 12 khz ( i . e ., the band - pass filter attenuates frequencies lower than 3 khz and higher than 12 khz and enhances frequencies between 3 - 12 khz ). this recognition method can be used when the lambda probe in the exhaust of the internal combustion engine 1 does not provide reliable information , or when the internal combustion engine 1 is cold is immediately following a cold engine start . in this way , it is possible to perform an initial recognition of the type of fuel actually used by the internal combustion engine 1 immediately after the cold start of the internal combustion engine 1 itself , and thus without waiting the time ( several seconds ) needed to bring the lambda probe to temperature . furthermore , the recognition method described above can be used in “ recovery ” mode when the lambda probe in the exhaust of the internal combustion engine 1 is not working properly ; in other words , the type of fuel actually used is normally recognised using the information provided by the lambda probe , and in case of malfunction of the lambda probe , the type of fuel actually used is recognised according to the recognition method described above , which does not provide for the use of the information provided by the lambda probe . finally , this recognition method can be used as a comparison sample with the same recognition performed using the information provided by the lambda probe so as to increase the recognition reliability . the recognition method described above has numerous advantages as it is also easily implemented in an already existing electronic control unit 5 and does not require a high additional computational burden ( particularly when the synthetic index i is calculated using an integration over time of the intensity s of the noise generated by engine 1 ). furthermore , the recognition method described above allows the type of fuel actually used by the internal combustion engine 1 to be estimated with and very high accuracy and reliability . finally , the recognition method described above is completely independent of the information provided by the lambda probe in the exhaust of the internal combustion engine 1 and therefore it can be used both when the lambda sensor is not working properly ( i . e ., when the lambda probe is cold or faulty ) and as a comparison sample for the same recognition performed using the information provided by the lambda sensor . the invention has been described in an illustrative manner . it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation . many modifications and variations of the invention are possible in light of the above teachings . therefore , within the scope of the appended claims , the invention may be practiced other than as specifically described .