Patent Publication Number: US-2022228536-A1

Title: Validation of a signal from a crankshaft sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase Application of PCT International Application No. PCT/EP2020/065139, filed Jun. 2, 2020, which claims priority to French Patent Application No. 1906054, filed Jun. 7, 2019, the contents of such applications being incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of fuel injection in an internal combustion engine and, more specifically, it relates to a method for managing the synchronization of an internal combustion engine. In particular, the aim of the invention is to enable a return to the normal operating mode of the engine after a transient failure of the signal originating from a crankshaft sensor has been detected. More specifically, the invention relates to a method for validating the elimination of a transient failure of the signal originating from a crankshaft sensor of an internal combustion engine of a vehicle. 
     BACKGROUND OF THE INVENTION 
     In a known manner, an internal combustion engine of a vehicle, for example, a motor vehicle, comprises hollow cylinders each demarcating a combustion chamber, into which a mixture of air and fuel is injected. This mixture is compressed in each cylinder by a piston and is ignited so as to cause the piston to translationally move inside the cylinder. The movement of the pistons in each cylinder of the engine rotates an engine shaft, called “crankshaft”, allowing, via a transmission system, the wheels of the vehicle to rotate. The air is admitted into the combustion chamber via one or more intake valve(s) that are regularly opened and closed. Similarly, the gases originating from the combustion of the fuel are discharged by one or more exhaust valve(s). In a known manner, the valves are connected to one or more camshaft(s) for controlling their movement in order to successively open and close them. 
     In a known solution, a crankshaft sensor and a camshaft sensor are mounted in the vehicle and detect, during their rotation, the teeth of targets respectively mounted on the crankshaft and on the camshaft. The target of the crankshaft comprises a predetermined number of evenly spaced apart teeth, as well as a free space of teeth for finding a reference position of the crankshaft. The target of the camshaft comprises a small number of teeth, for example, three or four, that are of different lengths and/or are unevenly distributed so that they can be easily identified. Each sensor generates a signal from the teeth that it detects in order to respectively measure the angular position of the crankshaft and the angular position of the camshaft. During an engine cycle, the crankshaft rotates twice, whereas the camshaft rotates only once. 
     Engine synchronization can be achieved by combining the two items of information coming from a crankshaft sensor and from one or more camshaft sensor(s). The crankshaft sensor thus allows the position of the one or more piston(s) in the cylinders, and therefore the position of the engine, to be estimated. This position can be estimated with an asymmetry of approximately 360 degrees. This means that the position of the piston in the cylinder is known, but the phase of the combustion cycle in which it is located is not known. This asymmetry is generally called the signature or gap. Thus, the cooperation of the crankshaft signal with the camshaft signal allows the cam edge number viewed by the camshaft sensor to be determined. The cam edge number is determined by associating the number of crankshaft edges received after the signature with the cam edge immediately viewed by the camshaft sensor. Ultimately, this allows the correct phasing to be determined, i.e. in a certain manner, the position of each piston in the combustion cycle. 
     However, the crankshaft sensor can generate a faulty signal, in particular by detecting a higher or lower number of teeth of the crankshaft target between the two signatures, for example, one tooth more or one tooth less. Metal particles also can be placed at the tooth free space, thus preventing the sensor from detecting the free space. 
     When a failure of the signal received from the crankshaft sensor is detected, then operating the engine computer in a mode called “degraded” mode is known. In this degraded mode, the position of the engine is only determined from the signal of the camshaft sensor, which on average proves to be more accurate than the signal from the crankshaft sensor when said sensor is defective. However, the target mounted on the camshaft comprises a small number of teeth; the position determined thus is not very precise. The operation of the engine is then no longer optimal, which in particular increases the discharges of polluting gases from the engine. 
     In a known manner, the signal from the crankshaft sensor is monitored. When the failure is eliminated, the computation returns to a normal operating mode, if, during a certain number of revolutions of the engine, preferably of the order of 1,000 revolutions, no new failure is detected during this number of revolutions. However, such a solution is time consuming. In other words, the duration during which the engine incorrectly operates in a degraded mode is significant. 
     SUMMARY OF THE INVENTION 
     Therefore, the aim of an aspect of the invention is to overcome this disadvantage by proposing an effective solution for quickly determining the end of a transient failure of the signal originating from a crankshaft sensor. 
     An aspect of the invention relates to a method for validating the elimination of a transient failure of the signal originating from a crankshaft sensor of an internal combustion engine of a vehicle, said engine comprising: 
     a plurality of cylinders; 
     a crankshaft capable of being driven by the pistons of the cylinders during the operation of the engine and comprising a toothed wheel comprising a free space of teeth corresponding to a reference position of said crankshaft; 
     said crankshaft sensor being configured to detect the teeth and the free space of said toothed wheel; 
     at least one camshaft, comprising a toothed wheel, the teeth of which are of different lengths and/or are unevenly spaced apart; and 
     a camshaft sensor configured to detect the teeth of the toothed wheel of said camshaft; 
     said vehicle comprising a computer configured to determine the angular position of the crankshaft from the detected free space and, in a mode called “normal” mode, to determine the angular position of the engine from the angular position of the crankshaft (synchronized by virtue of the profile of the camshaft) and, in a mode called “degraded” mode, in the event of the failure of the signal originating from the crankshaft sensor, to determine the angular position of the engine only from the angular position of the camshaft, the method, which is implemented by the computer, being characterized in that it comprises the following steps: 
     detecting, in a degraded mode and when the computer detects a signal from the crankshaft sensor, the free space and the teeth of the toothed wheel of the crankshaft from the signal generated by the crankshaft sensor during the rotation of the crankshaft; 
     determining the minimum rotation speed of the crankshaft in the vicinity of each combustion top dead center, determined from the duration of the edges of teeth received by the crankshaft sensor upon each rotation of said crankshaft, with the position of the vicinity of each combustion top dead center being determined from the free space and the detected teeth; 
     determining the angular positions of the crankshaft corresponding to the determined minimum rotation speeds; 
     switching to normal mode when, for each combustion top dead center, the difference between the determined angular position of the crankshaft and a corresponding reference angular position value is below a predetermined position threshold for at least one crankshaft revolution. 
     By virtue of the method according to an aspect of the invention, the end of a transient failure of the signal originating from the crankshaft sensor, allowing the switch to the normal mode, is determined in a quick and reliable manner by comparing the angular positions of the minimum speed in the vicinity of the combustion top dead centers of the crankshaft with corresponding reference values, for example, for a given value of the angular opening between the free space of the toothed wheel of the crankshaft and the combustion top dead center of the first cylinder, thus allowing the computer to quickly return to the normal operating mode. In particular, the method is particularly effective since, irrespective of the engine speed, the minimum speed of the crankshaft in the vicinity of each combustion top dead center is clearly identifiable. Moreover, the synchronization of the engine is automatically verified with the method according to an aspect of the invention due to the fact that the position of the free space of the toothed wheel of the crankshaft is distinguished from among those that are possible in an engine cycle. 
     Advantageously, the method comprises, prior to the step of determining the minimum rotation speed in the vicinity of each combustion top dead center, a step of generating a modeled curve of the speed of the crankshaft by correlating, from a time measurement, the detection of the teeth and of the free space. This allows the minimum rotation speed of each combustion top dead center to be easily determined by analyzing the curve, for example, as taught in document FR 3065283, which is included in the present description by reference. Preferably, the speed curve is modelled by a parabola, which is obtained by the mathematical method of least squares. 
     Preferably, the speed curve is generated in the vicinity, preferably between −40° and +40°, of each determined angular position of the crankshaft, in order to limit the computations required to generate the curve. Indeed, the tested teeth are those that are located within an angular window, for example, between −40° and +40° of each top dead center. 
     Advantageously, the rotation speed of the crankshaft is determined when detecting a tooth from the time that has elapsed since the detection of the preceding tooth. This allows the rotation speed to be easily determined from the measured duration and from the known positions of the teeth. 
     Advantageously, the method comprises a preliminary step of determining reference angular position values, said preliminary step comprising a step of rotating the crankshaft in normal mode, a step of the crankshaft sensor detecting the free space and the teeth of the toothed wheel of the crankshaft, a step of determining the minimum rotation speed in the vicinity of each combustion top dead center of the crankshaft from the detected free space and from the detected teeth during the rotation of said crankshaft, and a step of determining the angular positions of the crankshaft corresponding to the determined minimum rotation speeds, with the value of the reference angular positions being equal to the determined angular positions. 
     Advantageously, the switch to normal mode is carried out if, for each combustion top dead center, the difference between the determined angular position of the crankshaft and the reference angular position value is below the corresponding predetermined position threshold during a determined number of consecutive cycles of the engine, preferably less than 40. This allows the operating duration in degraded mode to be limited, whilst ensuring reliability in terms of determining the end of a failure. Advantageously, the method comprises a step of computing the difference between the determined rotation speed of the crankshaft, preferably determined from the duration between two detected successive teeth, and each corresponding point of the generated curve, with the switch to normal mode being carried out if the computed difference is less than a speed dependent threshold. This advantageously allows a failure to be detected. Preferably, the speed curve is modeled by a parabola, which is obtained using the mathematical method of least squares, in a manner that is simple and quick per se. 
     Preferably, the computation of the difference between the rotation speed of the crankshaft and the generated curve is carried out using the computation of a correlation coefficient. 
     An aspect of the invention also relates to a computer for a vehicle, said vehicle comprising an internal combustion engine, said engine comprising a plurality of cylinders, a crankshaft capable of being driven by the pistons of the cylinders during the operation of the engine and comprising a toothed wheel comprising a free space of teeth corresponding to a reference position of said crankshaft, a crankshaft sensor configured to measure the angular position of said crankshaft from said toothed wheel and at least one camshaft, at least one camshaft, comprising a toothed wheel, the teeth of which are of different lengths and/or are unevenly spaced apart, and a camshaft sensor configured to detect the teeth of the toothed wheel of the camshaft, said computer being configured to determine the angular position of the crankshaft from the detected free space and, in a mode called “normal” mode, to determine the angular position of the engine from the angular position of the crankshaft and from the angular position of the camshaft, and, in a mode called “degraded” mode, in the event of a transient failure of the signal originating from the crankshaft sensor, to determine the angular position of the engine only from the angular position of the camshaft, said computer also being configured to implement the method as described above. 
     An aspect of the invention further relates to a vehicle comprising an internal combustion engine and a computer as described above, said engine comprising a plurality of cylinders, a crankshaft capable of being driven by the pistons of the cylinders during the operation of the engine and comprising a toothed wheel comprising a free space of teeth corresponding to a reference position of said crankshaft, a crankshaft sensor configured to measure the angular position of said crankshaft from said toothed wheel and at least one camshaft, comprising a toothed wheel, the teeth of which are of different lengths and/or are unevenly spaced apart, and a camshaft sensor configured to detect the teeth of the toothed wheel of the camshaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of aspects of the invention will become more clearly apparent from reading the following description. This description is purely illustrative and must be read with reference to the accompanying drawings, in which: 
         FIG. 1  schematically illustrates an embodiment of a vehicle according to the invention comprising a V-engine viewed from the side; 
         FIG. 2  schematically illustrates an example of a toothed wheel for a crankshaft of the engine of  FIG. 1 ; 
         FIG. 3  schematically illustrates an example of a toothed wheel for a camshaft of the engine of  FIG. 1 ; 
         FIG. 4  schematically illustrates signals emitted by a crankshaft sensor and a camshaft sensor mounted opposite the toothed wheels of  FIGS. 2 and 3  on an engine cycle; and 
         FIG. 5  schematically illustrates a rotation speed curve of the crankshaft determined from the signal of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An aspect of the invention will be described hereafter in the light of implementation in a motor vehicle. However, any implementation in a different context, in particular for any vehicle comprising an internal combustion engine, for which the angular position needs to be determined, is also covered by an aspect of the present invention. 
       FIG. 1  schematically shows a side view of an internal combustion engine  10  of a motor vehicle  1 . The vehicle  1  further comprises the engine  10 , a computer  20 . 
     The internal combustion engine  10  comprises, in this non-limiting example, four cylinders  11  each demarcating a combustion chamber  11 A, in which a piston  12  slides, connected to a crankshaft  13  and the movement of which is driven by the compression and the expansion of the gases originating from the compression of an air and fuel mixture introduced into the combustion chambers  11 A. 
     In this example, the engine  10  is of the four-stroke engine type. Furthermore, during operation of the engine  10 , four operating phases are required for each cylinder  11 : an air and fuel intake phase in the combustion chamber  11 A of the cylinder  11 , a phase of compressing the obtained mixture, on completion of which phase its combustion will occur, a phase of expanding the gases originating from the combustion of the mixture generating the thrust of the piston  12  and a phase of exhausting the gases out of the combustion chamber  11 A. These four phases form an engine cycle that repeats. During the intake phase and the expansion phase, the piston  12  descends to the low position. During the compression phase and the exhaust phase, the piston  12  rises to the high position. The high position is denoted TDC (Top Dead Center). An engine cycle  10  thus comprises four combustion top dead centers TDC 1 , TDC 2 , TDC 3 , TDC 4  in  FIG. 5 , with each combustion top dead center TDC 1 , TDC 2 , TDC 3 , TDC 4  by convention identifying the cylinder at the start of the combustion phase. The rotation speed of the engine  10 , and in particular the rotation speed of the crankshaft  13  of the engine  10 , varies during a cycle and has a minimum value (denoted V mini ) at the top dead centers TDC 1 , TDC 2 , TDC 3 , TDC 4 , as will be described hereafter. 
     The air and the gases are respectively introduced and expelled via intake valves  14 A and exhaust valves  14 B connected to a camshaft  15 . The camshaft  15  activates the intake valves  14 A and the exhaust valves  14 B. More specifically, the rotating camshaft  15  allows the intake valves  14 A and the exhaust valves  14 B of each combustion chamber  11 A to be alternately opened and closed. Alternatively, the engine  10  of the vehicle could also equally comprise two camshafts, one dedicated to the intake valves  14 A and the other dedicated to the exhaust valves  14 B. Similarly, in this example, each cylinder  11  is connected to an intake valve  14 A and an exhaust valve  14 B; however, each cylinder  11  also could be connected to a plurality of intake valves  14 A and to a plurality of exhaust valves  14 B. 
     With reference to  FIG. 1 , the set of pistons  12  is connected to a crankshaft  13 , the rotation of which is carried out by the thrust of each piston  12  and allows energy to be transferred by a flywheel and a gearbox (not shown), resulting in the rotation of the wheels of the vehicle  1 . 
     The crankshaft  13  comprises a coaxially mounted toothed wheel  130  (commonly called a target by a person skilled in the art), an example of which is illustrated in  FIG. 2 . This toothed wheel  130  comprises a predetermined number of evenly spaced apart teeth  131 , as well as a free space  132  of teeth corresponding to a reference position of the crankshaft  13 . It should be noted that the toothed wheel  130  of the crankshaft  13  could comprise more than one free space  132 , in particular two free spaces  132 , in another embodiment. 
     With further reference to  FIG. 1 , a position sensor, called crankshaft sensor  16 , is mounted opposite the toothed wheel  130  of the crankshaft  13 . This crankshaft sensor  16  generates a signal  51 , an example of which is illustrated in  FIG. 4 , comprising rising and falling edges representing the rising and falling edges of the teeth  131  of the toothed wheel  130  of the crankshaft  13 . This signal  51  allows the computer  20  to determine the angular position, ranging between 0° and 360°, denoted “°CRK”, of the crankshaft  13  relative to the reference position of the toothed wheel  130 . In an engine cycle  10 , the position of the crankshaft  13  and the number of the revolution in which it is located thus provide the “angular position of the engine  10 ” that corresponds to the angular position of the crankshaft  13  ranging between 0 and 720°CRK (between 0 and 360°CRK for the first revolution and between 360 and 720°CRK for the second revolution of the engine cycle). As a variant, it should be noted that the crankshaft sensor  16  could be configured to itself detect the free space  132 , count the teeth  131  and send this information to the computer  20 , without this limiting the scope of the present invention. 
     The camshaft  15  comprises a coaxially mounted toothed wheel  150 , an example of which is illustrated in  FIG. 3 . This toothed wheel  150  comprises a predetermined number of unevenly spaced apart teeth  151 ,  152 ,  153 , in a manner per se known. With reference to  FIG. 1 , a position sensor, called camshaft sensor  17 , is mounted opposite the toothed wheel  150  of the camshaft  15  so as to allow the angular position of said camshaft  15  to be determined. More specifically, the camshaft sensor  17  is configured to deliver a signal S 2 , an example of which is illustrated in  FIG. 4 , comprising rising and falling edges representing the rising and falling edges of the teeth of the toothed wheel  150  of the camshaft  15  and that allows the computer  20 , using the signal  51 , to determine the angular position, ranging between 0 and 360°CAM, of the camshaft  15  relative to the reference position of the toothed wheel  130  of the crankshaft  13 . Since this determination is per se known, it will not be described in further detail herein. As a variant, it should be noted that the camshaft sensor  17  could be configured to itself detect the position of the teeth and send this information to the computer  20 , without this limiting the scope of the present invention. 
     The crankshaft sensor  16  and the camshaft sensor  17  can be, in particular, in the form of sensors, for example, Hall-effect sensors, which are per se known, detecting the rising and falling edges. Alternatively, the computer  20  can be configured to process only the rising edges or the falling edges of the crankshaft sensor  16  and/or the camshaft sensor  17  in order to limit costs. 
       FIG. 4  shows an example of a signal  51  generated by the crankshaft sensor  16  and an example of a signal S 2  concomitantly generated by the camshaft sensor  17  during an engine cycle  10 . During an engine cycle  10 , the crankshaft  13  rotates twice when the camshaft  15  rotates only once. In other words, the crankshaft  13  rotates twice as much as the camshaft  15 . Two free spaces  132  are thus detected on this signal  51  during an engine cycle  10 . Furthermore, when a free space  132  is detected, the crankshaft  13  can be in two different positions. However, the fuel injection time depends on the position of the crankshaft  13  relative to the camshaft  15 . Furthermore, in order to allow the engine  10  to operate, the camshaft  15  and the crankshaft  13  must be synchronized in order to know the absolute position of the engine  10  and thus optimize the control of fuel injection into the cylinders  11  of the engine  10 . 
     The computer  20 , for example, of the type known as an ECU (Electronic Control Unit) or an EMS (Engine Management System), is configured to determine the position of the engine  10  in order to optimize its operation, in particular by optimizing the time of combustion in the engine cycle  10 . To this end, the computer  20  is configured to receive the signals  51 , S 2  respectively emitted by the crankshaft sensor  16  and by the camshaft sensor  17 . The computer  20  is configured to identify, on each of these signals  51 , S 2 , the teeth  131 ,  151 ,  152 ,  153  of the targets  130 ,  150 . The computer  20  is also configured to identify, on the signal  51  emitted by the crankshaft sensor  16 , the one or more free space(s)  132  of the toothed wheel  130  of the crankshaft  13 , with this or these free space(s)  132  each being detected twice during an engine cycle  10 . 
     The computer  20  is configured to switch between two modes for determining the position of the engine  10 : a normal mode, in which the computer  20  is configured to determine the angular position of the engine  10  from the angular position of the crankshaft  13 , previously synchronized by virtue of the signal originating from the camshaft sensor S 2 , and a degraded mode in the event of the failure of the signal originating from the crankshaft sensor, wherein the computer  20  is configured to determine the angular position of the engine  10  only from the angular position of the camshaft  15 . In other words, in the degraded mode, the computer  20  is configured to determine the position of the engine  10  only from the signal S 2  emitted by the camshaft sensor  15 . The computer  20  is, for example, particularly configured to switch to degraded mode after an error is detected on the signal  51  emitted by the crankshaft sensor  16 . 
     In normal mode, the computer  20  is configured to, from the position of the camshaft  15  (determined from the signal S 2  emitted by the camshaft sensor  17 ), identify whether the one or more free space(s)  132  have been detected during the first or the second revolution of the crankshaft  13  during an engine cycle  10  in order to synchronize the engine  10 . Since such synchronization is known, it will not be described in further detail herein. 
     The computer  20  is configured to determine the rotation speed of the crankshaft  13  from the signal  51  generated by the crankshaft sensor  16 . To this end, the computer  20  is configured to measure the duration between two teeth  131  successively detected on the signal  51 . Alternatively, the computer  20  can be configured to determine the time, in relation to a clock of the computer  20 , at which each tooth  131  is detected in order to determine the duration between two successively detected teeth  131 . The computer  20  is then configured to compute the speed of the crankshaft  13  when a tooth  131  is detected from the duration determined from the detection of the preceding tooth  131 . 
     The computer  20  is then configured to generate a curve C representing the rotation speed of the crankshaft  13  over time, an example of which is illustrated in  FIG. 5 . This curve C is generated from the different speeds determined for each tooth  131  of the toothed wheel  130  of the crankshaft  13 . Such a curve C particularly can be generated in a known manner using the method of least squares. Such a curve C has, at the minimums corresponding to the combustion top dead centers TDC 1 , TDC 2 , TDC 3 , TDC 4 , a parabolic shape representing the speed variations of the crankshaft  13  around said minimums. 
     The computer  20  is configured to determine, from the generated curve C, the minimum speed V mini  of rotation of each combustion top dead center TDC 1 , TDC 2 , TDC 3 , TDC 4  of the crankshaft  13 . This minimum speed V mini  is detected in the vicinity of a combustion top dead center TDC 1 , TDC 2 , TDC 3 , TDC 4  of the engine cycle  10 . With the engine cycle  10  comprising four top dead centers TDC 1 , TDC 2 , TDC 3 , TDC 4  in this example, the computer  20  is configured to determine four minimum speeds V mini , with each minimum speed V mini  corresponding to one of these top dead centers TDC 1 , TDC 2 , TDC 3  or TDC 4 . 
     The computer  20  is configured to determine the angular positions of the crankshaft  13  corresponding to the determined minimum speeds V mini . Each angular position particularly can be determined by extrapolation from the known position of the detected teeth  131 . In the example illustrated in  FIG. 5 , the angular position corresponding to a minimum speed V mini  of rotation ranges between two consecutive teeth  131 - 1 ,  131 - 2  of the toothed wheel  130 . Furthermore, determining this angular position by extrapolation allows a precise position to be obtained. 
     Advantageously, with the angular position of the combustion top dead centers TDC 1 , TDC 2 , TDC 3 , TDC 4  being known per structure and being recorded in the ECU, the curve C is only generated in the vicinity of the combustion top dead centers TDC 1 , TDC 2 , TDC 3 , TDC 4  in order to limit the computations. In particular, the curve C can be generated from the measurements carried out for the four preceding teeth and the four teeth following a combustion top dead center TDC 1 , TDC 2 , TDC 3 , TDC 4 . 
     The computer  20  is configured to compare, at each combustion top dead center TDC 1 , TDC 2 , TDC 3 , TDC 4 , the determined angular position with a reference angular position value associated with the minimum speed V mini  of rotation of the considered crankshaft  13 . The computer  20  is configured to determine the end of the transient failure of the signal originating from the crankshaft sensor  16  and to switch the operation of the engine  10  to normal mode when the difference between the determined angular position and the reference value is below a position threshold during a predetermined number of revolutions of the crankshaft  13 , for example, at least 10, preferably of the order of 40 revolutions. 
     The reference value is predetermined when the engine  10  operates in normal mode. To this end, the computer  20  is configured to generate a speed curve of the crankshaft  13  in order to determine a minimum speed V mini , as previously described, but this time in normal mode in order to determine the reference value. This predetermination can be carried out in the factory or over the lifetime of the vehicle  1 . This reference value is stored in a non-volatile, or permanent, memory of the computer in order to be able to act as a reference. 
     The computer  20  is also configured to detect whether a measured speed is far removed from the generated curve C. Such a difference represents a failure of the signal originating from the crankshaft sensor  16 , which allows the engine  10  to be kept in a degraded mode. The difference between a measured speed and the generated curve C particularly can be determined from the computation of a correlation coefficient, denoted R 2 . This coefficient, which is per se known to a person skilled in the art, allows the difference between the generated curve C and the measured values to be quantified. 
     Using the correlation coefficient R 2  allows a failure to be detected, in particular two failures that compensate each other and thus cannot be detected when determining the minimum speed V mini . If, for example, the crankshaft sensor  16  views, in the same revolution (between two free spaces), an additional tooth and a missing tooth, the total number of total teeth will be correct, while an error may have occurred. The correlation coefficient R 2  allows an additional tooth and/or a missing tooth to be detected on the sample, with the measured speed values not corresponding to the values determined on the curve C (with the difference being significant for the measurements with an erroneous position). 
     The previous description will be advantageously used in a degraded mode when the failure of the crankshaft sensor  16  is no longer detected, in order to requalify the received signal and thus return to normal mode. 
     An embodiment of the method according to the invention will now be described. 
     Firstly, the engine  10  is started. In other words, the crankshaft  13  and the camshaft  15  are rotated by the combustion produced in the cylinders  11 . 
     In a preliminary step, the engine  10  operates in normal mode. In other words, no failure is detected. 
     The crankshaft sensor  16  and the camshaft sensor  17  then respectively detect the teeth  131 ,  151 ,  152 ,  153  and the one or more free space(s)  132  of the toothed wheel  130 ,  150 , opposite which they are placed, and generate signals  51 , S 2 . The crankshaft sensor  16  transmits the signal  51  that it generates to the computer  20 , and the camshaft sensor  17  transmits the signal S 2  that it generates to the computer  20 , in order to allow the computer  20  to determine the position of the engine  10 . 
     The computer  20  also determines the rotation speed of the crankshaft  13  when a tooth  131  is detected, in particular from the measurement of the duration since the detection of the preceding tooth  131 . A speed curve is then generated from the different speeds determined using a method of the least squares type. 
     Finally, the computer  20  determines the minimum speed V mini  of rotation of the crankshaft  13  during an engine cycle  10  for each combustion top dead center TDC 1 , TDC 2 , TDC 3 , TDC 4 . Then, the computer  20  determines the angular position of the crankshaft  13  corresponding to the minimum speed V mini  of rotation thus determined for each combustion top dead center TDC 1 , TDC 2 , TDC 3 , TDC 4 . Each angular position is then recorded in a non-volatile memory zone of the computer  20  as a reference angular position value. 
     During the operation of the engine  10 , the computer  20  also detects a failure of the signal originating from the crankshaft sensor  16 . To this end, the computer  20  determines the number of teeth  131  detected during one revolution of the crankshaft  13 , i.e. between two detections of the free space  132 . If the number of detected teeth  131  is different, in particular lower or higher, from the number of teeth  131  that the toothed wheel  130  comprises, then the computer  20  detects a failure. 
     The computer  20  then switches the operation of the engine  10  to a degraded mode. In this degraded mode, the computer  20  no longer uses the signal  51  generated by the crankshaft sensor  16  to determine the position of the engine  10 , unlike the normal mode. 
     During the degraded mode, and when the computer detects a signal originating from the crankshaft sensor, the crankshaft sensor  16  detects the teeth  131  and the one or more free space(s)  132  of the toothed wheel  130  and generates a signal  51 . The crankshaft sensor  16  transmits the signal  51  that it generates to the computer  20 . 
     The computer  20  also determines the rotation speed of the crankshaft  10  when a tooth  131  is detected and generates a speed curve C from the different determined rotation speeds, as previously described. 
     The computer  20  then determines the minimum speeds V mini  of rotation of the crankshaft  13  during an engine cycle, as well as the angular positions of the crankshaft  13  corresponding to the determined minimum speeds V mini  of rotation. The computer can define these speeds and the associated positions once per engine cycle, or between two consecutive top dead centers in order to determine these parameters, without this limiting the scope of the present invention. 
     The computer  20  then compares the determined angular positions with the reference values. The computer  20  repeats these operations during a plurality of revolutions of the crankshaft  13 , for example, around ten (that is approximately forty comparisons for a four-cylinder engine). If the difference between a determined angular position for each revolution of the crankshaft  13  and the corresponding reference value is consecutively below a position threshold, preferably of the order of 2°CRK, the computer  20  switches the operation of the engine  10  to normal mode. 
     Preferably, the computer  20  comprises a counter, the value of which is incremented upon each crankshaft revolution  13  if the difference between the determined angular position upon one revolution of the crankshaft  13  and the considered reference value is below the position threshold. The switch to normal mode occurs when the value of the counter reaches a determined value, for example, forty. 
     In the degraded mode, the computer  20  also determines the correlation coefficient R 2  of the generated curve C in order to detect failures. To this end, the computer  20  compares the value of the determined coefficient with a speed dependent reference value. If the value of the coefficient is greater than this reference value, the operation of the engine  10  does not switch to normal mode. 
     Similarly, if the computer  20  detects a failure during the degraded mode, in particular by counting the number of teeth  131  detected during one revolution of the crankshaft  13 , the switch to normal mode is interrupted, and the previously described steps will be repeated when the failure has been eliminated. 
     By virtue of these many tests, the switch to normal mode from the degraded mode is reliable. In other words, the switch to normal mode is only carried out if no failure has been detected by one of these tests. Moreover, this enables switching to normal mode after a limited number of revolutions of the crankshaft, for example, less than ten, which limits the operating duration of the engine  10  in degraded mode and thus limits polluting discharges.