PROCESS FOR DETECTING A MISFIRE IN AN INTERNAL COMBUSTION ENGINE AND SYSTEM FOR CARRYING OUT SAID PROCESS

Abstract: A process for detecting a misfire in one or more cylinders (3, 3') of an internal combustion engine, including the following operative steps: sampling the exhaust gas pressure values during at least one engine cycle, the sampling frequency being proportional to the crankshaft rotational speed; analyzing the sampled signal in the frequency domain; calculating a misfire index as a function of the results of said analysis; comparing said index with one or more threshold values. Said frequency domain analysis preferably includes a Fourier transform of the sampled signal. The present invention also relates to a system carrying out said process.

Description: - 1 - "PROCESS FOR DETECTING A MISFIRE IN AN INTERNAL COMBUSTION ENGINE AND SYSTEM FOR CARRYING OUT SAID PROCESS" The present invention relates to a process for detecting a misfire in an internal combustion engine, and in particular a process which can be used for detecting a misfire in one or more cylinders of an internal combustion engine. The present invention also relates to a system for carrying out said process. It is known that in order to monitor the performance of an internal combustion engine, in particular a racing engine with a high number of cylinders, it is desirable to detect the occurrence of the misfire of the fuel mixture in one or more cylinders. A known process for carrying out said detection, which presently plays an important role with respect to the ever stricter rules for the control of polluting exhausts, consists of measuring the sudden fluctuations in the rotational speed of the crankshaft by means of an electronic sensor located close to the fly-wheel. This sensor is connected to the control unit positioned inside the car, which receives all the data concerning the engine and transmitted by suitable sensors. By calculating the fluctuations in the speed according to the delivered torque it is possible to identify a possible misfire in one cylinder of the engine. However, this process does not allow to precisely identify hi which cylinder the misfire occurred and moreover has a quite high error probability, particularly in the case of the traveling car being subjected to sharp oscillations, e.g. caused by defects in the road surface, which temporarily affect the rotational speed of the crankshaft. In order to overcome these drawbacks, it has been devised to measure the fluctuations in time of the pressure of the engine exhaust gas. Though pressure sensors available on the market are veiy accurate and provide a response almost in real time, the known processes for detecting the misfire on the basis of the measurement of the pressure fluctuations in the exhaust gas are still very inaccurate and poorly reliable, particularly when applied to engines with a high number of cylinders. Therefore the object of the present invention is to provide a process for detecting the misfire which is free from the above-mentioned drawbacks. Another object of the present invention is to provide a system which carries out said process. - 2 - These objects are achieved by means of a process and a system whose main characteristics are disclosed in the first and eighth claim respectively. Thanks to the sampling and the subsequent frequency analysis of the pressure values detected in the exhaust pipes, the process according to the present invention provides a higher accuracy and reliability with respect to prior art processes. In fact, if the engine filing is regular, the periodical openings of the cylinder exhaust valves generate pressure pulses in the exhaust pipes having the same periodicity and similar waveforms. On the contraiy, in the case of misfire in one of the cylinders, the corresponding pressure pulse is changed, thus changing the periodical pattern of the pressure values. The reference for the synchronization with the pulse frequency is readily derivable from the sensors detecting the rotational speed of the crankshaft and/or camshaft. Another advantage of the process according to the present invention is that through the frequency analysis of the sampled signal it is possible to determine whether only one or more misfires occurred during a single engine cycle. In fact, the amplitude of the modulus of the various harmonics of the sampled signal depends on the number of cylinders wherein the misfire occurred. A further advantage of the process according to the present invention is that through the frequency analysis of the sampled signal it is possible to determine not only the misfiie, but also the position of the cylinder where it occurred. In fact, the knowledge of the cylinder firing sequence and the comparison of the phase of the fust harmonic of the sampled signal with the phase of the first cylinder provide a phase difference which indicates the position of the cylinder where the misfire occurred. These and other advantages and characteristics of the process and system according to the present invention will be clear to those skilled in the art from the following detailed description of an embodiment thereof, with reference to the annexed drawings wherein: - Figure 1 shows a diagrammatic view of the system according to the present invention; - Figure 2 shows a flow chart of the process according to the present invention; - Figures 3 a, 3b and 3 c show three diagrams of the pressure as a function of the - 3 - crankshaft rotation; - Figures 4a, 4b and 4c show other three diagrams of the pressure as a function of the crankshaft rotation; - Figures 5a, 5b and 5c show three diagrams of the misfiie index as a function of the number of engine cycles; - Figure 6 shows a Fourier transform of the diagram of figure 3a; and - Figures 7a, 7b and 7c show three diagrams in polar coordinates of the mahi harmonic of the pressure in the diagrams of figures 3 a, 3b and 3 c. With reference to figure 1, there is seen that the system according to the present invention includes in a known manner a control unit 1 (indicated by a dotted line) which in turn includes a pah of mutually connected electronic controllers 2. 2 ' each of which provides the control over one of two rows of cylinders 3, 3' of the engine, hi the present embodiment there is described a VI 2 engine having two rows of six cylinders 3, 3' each, but in other embodiments the number of cylinders and/or rows may obviously change. The controllers 2, 2' are connected in a known rnarmer to a pah of coolant temperature sensors 4, 4' and to two pairs of sensors 5, 5' and 6, 6' respectively detecting the temperature and pressure of the air in the intake manifolds 7, 7'. The controllers 2, 2' are also connected to a pair of lambda sensors 8, 8' for analyzing the oxygen content in the exhaust pipes 9, 9', to two series of injectors 10, 10' which inject the fuel into the intake pipes 11, 1 1' of the cylinders 3, 3', as well as to a pah of ignition coils 12, 12'. The exhaust pipes 9, 9' are preferably provided also with a pah of temperature sensors 13, 13' connected to the controllers 2, 2\ The system according to the present embodiment of the invention suitably includes a sensor 14 detecting the rotational speed of the fly-wheel 15 integral with the crankshaft and a further pair of sensors 16, 16' detecting the rotation of the camshaft 17. These sensors 14, 16 and 16' are connected to the controllers 2, 2' so that the latter, on the basis of the received data, can calculate in real time the speed and angle of rotation of the crankshaft during an engine cycle. The presence of the sensors 14, 16 and 16' is made necessary by the fact that the fly-wheel 15 in a four- stroke engine makes two revolutions (720°) per cycle, whereby the reference provided by the sensors 16, 16' allows to distinguish the first revolution from the second one. In order to cany out the process according to the present invention, in the two exhaust pipes 9, 9' there are properly arranged two high-precision pressure sensors 18, 18' connected to the controllers 2, 2', said sensors transmitting in real time an electric signal whose voltage is proportional to the measured pressure. Furthermore, the controllers 2, 2' are connected to a pah of warning hglits 19, 19' positioned inside the car, to a port 20 for the connection to an external processor, as well as to a sensor 21 detecting the position of the engine throttle 22. Referring now to figure 2, there is seen that the process according to the present invention includes, after a certain period of time from the engine stait, a first step of periodical check, e.g. each second, of the engine running state. In fact, in order to obtain reliable results from the process, it is preferable that the latter be earned out only if some engine parameters are within a preset range of values. In particular, the process according to the present invention is activated only when the coolant temperature measured by sensors 4, 4', the ah temperature measured by sensors 5, 5' and the ah pressure measured by sensors 6, 6' in the manifolds 7, T are above certain thresholds stored in the memory of the controllers 2, 2'. Moreover, these controllers check that the revolutions per minute (rpm) detected by sensor 14 are within a preset range of values. Table 1 hereunder shows an example of values meeting the conditions for the start of the process. - 5 Minimum number of revolutions 990 rprn Maximum number of revolutions 7550 rpm State check period I s Delay from engine start 10 s Minimvun coolant temperature 20° C Minimum ah temperature 20° C Minimum absolute pressure in manifolds 7, T 250 mmHg Table 1 : start conditions A further condition for starting the process may be reaching a certain opening of the throttle 22 as detected by sensor 21. If the conditions above are met, at the beginning of an engine cycle, corresponding to a certain position of the camshaft 17 as detected by sensors 16, 16', the controllers 2, 2' start sampling the electric signals transmitted by sensors 18, 18' and proportional to the pressure inside the exhaust pipes 9, 9'. These analogue signals are converted in a known manner into digital form and then stored in a buffer memoiy within each controller 2, 2'. The sampling frequency is suitably synchronized with the rotational speed of the fly-wheel 15 as detected by sensor 14, so that at the end of the engine cycle, detected through sensors 16 and 16', there is stored a preset number, e.g. 64, of pressure samples. Though the response of the pressure sensors 18, 18' is almost immediate, in order to syuchionize precisely with the engine, the controllers 2, 2' take into account the lag, almost constant, caused by the time required by the pressure pulse to travel from the exhaust valves of the cylinders 3, 3' to the pressure sensors 18, 18' along the exhaust pipes 9, 9'. Thanks to the temperature sensors 13, 13' it is possible to compensate for the very small fluctuations in said lag caused by the fluctuations in the temperature within the pipes 9, 9' . After having been sampled, the pressure values corresponding to an engine cycle are processed by the controllers 2, 2' which, at the same time, sample another series of pressure values which are stored in a further buffer memoiy for a subsequent processing. 6 - This processing carried out by each processor of the controllers 2, 2' suitably includes an analysis in the frequency domain, and in particular a Fourier transform of the sampled signal, through which there are obtained two seiies of coefficients corresponding to the real part and the imaginary part of the fust harrnonics of the signal. In particular, in the present embodiment there are calculated the coefficients of the first 32 harmonics of the sampled signal, but in other embodiments it is obviously possible to calculate a different number of harmonics according to the needs. These coefficients are used to calculate in a known way the modulus of the first harmonics, e.g. the first three, and then, by combining the values of these moduli, to obtain an index which allows to detect a misfiie in one or more of the cylinders 3, 3'. This misfire index can be calculated in various ways, e.g. by adding or multiplying the moduh of the harrnonics. Prior to this addition or multiphcation, the moduh may possibly be multiplied or raised to a power with a different coefficient for each hainiouic, so as to obtain a weighed addition or multiphcation. In the present embodiment, the misfiie index is calculated by simply adding the moduh of the first three harmonics. Once said index has been calculated, it is compared with preset threshold values stored in the controllers 2, 2'. Table 2 hereunder shows an example of threshold values of the misfire index experimentally obtained as a function of the engine rpm detected by sensor 14 and of the pressure in the manifolds 7, T as detected by sensors 6, 6'. rpm - mmHg 4- 1100 2000 3000 4000 5000 6000 7000 7500 300 110 110 124 130 144 148 156 168 450 115 120 148 156 180 188 196 204 600 130 140 180 188 236 248 256 268 760 150 162 224 264 292 300 312 320 Table 2: Threshold values of the misfiie index The controller 2 or 2' which detects the exceeding of said threshold, indicates - 7 through the warning hght 19 or 19' that a misfiie occuned in the conesponding row of cylinders 3 or 3'. At this moment, the controller 2 or 2' which detected the misfiie preferably compares the modulus of each of the first three harmonics with preset thieshold values also stored as a function of the engine ipm and of the pressure hi the conesponding manifold 7 or T . If all three moduh are within a range of values between a rn iimuni threshold and a maximum thieshold, a single misfiie is detected, i.e. a misfire occurred in one only of the cylinders 3 or 3', otherwise a multiple misfiie is detected, i.e. a misfiie occuned in at least two of the cylinders 3 or 3' belonging to a row. The following tables 3.1, 3.2, 4.1, 4.2, 5.1 and 5.2 show examples of minimum values and amplitudes of the thieshold ranges for the moduh of the first three harmonics. rpm 4- mmHg - 1100 2000 3000 4000 5000 6000 7000 7500 300 8 28 48 40 56 84 84 84 450 8 24 56 40 80 96 96 96 600 12 36 60 72 116 104 104 104 760 24 44 72 108 160 144 144 144 Table 3.1: Minimum thieshold values for the modulus of the first harmonic rpm 4- mmHg - 1100 2000 3000 4000 5000 6000 7000 7500 300 128 108 92 180 144 188 192 196 450 120 140 148 184 168 192 196 200 600 96 128 176 144 192 200 224 244 760 68 144 196 188 244 224 228 232 Table 3.2: Range amplitude for the modulus of the first harmonic rpm 4- mmHg 4- 1100 2000 3000 4000 5000 6000 7000 7500 300 20 8 8 4 12 16 24 28 450 24 12 8 8 16 16 28 36 600 20 12 8 16 24 16 24 32 760 24 16 8 28 40 36 36 36 Table 4.1: Mhinnum threshold values for the modulus of the second harmonic rpm 4 ' mmHg 4- 1100 2000 3000 4000 5000 6000 7000 7500 300 48 64 72 96 80 72 56 52 450 48 80 112 92 104 68 52 48 600 72 108 140 124 136 96 80 60 760 96 124 172 168 160 136 88 72 Table 4.2: Range amphtude for the modulus of the second harmonic rpm 4- mmHg 4- 1100 2000 3000 4000 5000 6000 7000 7500 300 0 0 0 0 0 0 0 0 450 0 0 0 0 0 0 0 0 600 8 4 4 4 8 4 0 0 760 4 4 4 12 12 8 4 4 Table 5.1: Minimum threshold values for the modulus of the thud hannonic - 9 rpm 4- mmHg 4- 1100 2000 3000 3000 5000 6000 7000 7500 300 92 72 52 124 40 132 144 152 450 92 88 84 88 64 84 64 60 600 88 104 112 68 96 48 28 24 760 88 124 160 136 160 80 80 80 Table 5.2: Range amphtude for the modulus of the third hannonic If a misfiie is detected in only one of the six cylinders 3 or 3', the relevant controller 2 or 2' can determine the position of the cylinder where the misfiie occurred by first calculating in a known manner the phase of the first harmonic. Thereafter, by subtracting the phase of the first hannonic from the phase of the first cylinder of the engine cycle, stored in the controllers 2, 2' by means of a table as a function of the engine ipm, there is obtained a phase difference which approximately conespouds to the phase of the cylinder where the misfire occurred. For example, if at given engine rpm the phase of the first cylinder of the engine cycle is 210°, a misfiie occuned in the first, second, third, fourth, fifth or sixth cylinder in firing order when the phase of the first harmonic is respectively between 180° and 240°, 120° and 180°, 60° and 120°, 0° and 60°, 300° and 360° or 240° and 300°. Table 6 hereunder shows the relationship between the engine rpm and the phase of the first cylinder in order to determine the position of the cylinder where the misfire occuned. 10 rpm phase 510 164° 990 140° 1500 106° 2010 80° 2490 58° 3000 36° 3510 16° 3990 0° 3990 360° 4500 348° 5010 338° 5490 328° 6000 320° 6510 312° 6990 302° 7500 292° Table 6: relationship between engine rpm and phase of the first cylinder Each detection of a misfiie in one of the engine cylinders, as well as the conesponding cylinder position in case of single misfire, is stored in suitable counters in the memory of controUers 2, 2'. This memory can be read thiough port 20 by an external processor during the car servicing, so as to diagnose possible engine failures. Referring now to figures 3a to 3c, there is seen, thiough measurements made in experimental tests where misfires were caused in the tested engine, how the signal transmitted by sensors 18, 18' changes as a function of the misfire in one of the cylinders 3, 3'. In particular, figure 3a shows that at about 2000 rpm with an engine load around 15%, the voltage (given in Nolts) at the terminals of the pressure sensors 18, 18' proportional to the pressure in the exhaust pipes 9, 9' is almost regular with six periodical oscillations during an engine cycle (indicated by the crankshaft rotation - 11 - angle from -180° to 540°). This voltage is indicated by a thin line, whereas a thick line indicates the voltage in the case of misfiie in the first cylinder. In this case, it is clearly seen that the voltage pattern has a first ^regularity around 240° and a second inegularity around 480°. However, figure 3b shows that at about 4000 ipm with an engine load approximately at 100%, the voltage pattern in case of regular filing is more comphcated with respect to the preceding case. Nonetheless, the voltage pattern in case of misfiie in the first cylinder (still indicated by the thick hue) moves away around 400° from the regular filing voltage pattern (still indicated by the thin hue). Also figure 3 c shows that at about 6000 ipm with an engine load approximately at 100%, the voltage pattern of the pressure sensors 18, 18' is different in the case of misfiie in the first cylinder, in particular around 470°. Similarly, with reference to figures 4a to 4c, there is seen, still through measurements made in experimental tests, how the signal transmitted by the pressure sensors 18, 18' changes as a function of the misfiie in one of the cylinders 3, 3', regardless of the misfiie being caused by a lack of fuel injection or ignition in the cylinder. In fact, there is seen that the voltage pattern in case of lack of injection (indicated by the thick line) is substantially equal to the voltage pattern in case of lack of ignition (indicated by the dotted line). This conespondence can be found both at low rprn, i.e. at about 2000 rpm with an engine load around 15% (figure 4a), at intermediate rpm, i.e. at about 4000 ipm with an engine load around 55% (figure 4b), and at high rprn, i.e. at about 6000 rpm with an engine load approximately at 100% (figure 4c). Referring now to figures 5a to 5c, there is seen that the misfire index measured as a function on the engine cycles (indicated on the horizontal axis) shows readily detectable peaks, which conespond to the moments when a misfire was experimentally caused in one of the engine cylinders. This can be found both at low rprn, i.e. at about 1000 rpm with an engine load around 15% (figure 5a), at intermediate rprn, i.e. at about 3000 rpm with an engine load around 55% (figure 5b), and at high rprn, i.e. at about 5000 rpm with an engine load approximately at 100% (figure 5c). With reference to figure 6, there is seen that the modulus of the first ten harmonics of the signal (in Nolts) transmitted by sensors 18, 18' changes quite - 12 - apparently fiom the case of regular filing in all cylinders (indicated by the white bars) to the case of misfiie in the first cylinder (indicated by the grey bars). The figure shows the modulus of the first ten harmonics calculated with the engine at 2000 ipm and a load around 15%, i.e. the case shown in figure 3a and figure 4a. The figure clearly shows that in the case of regular filing the modulus of the sixth harmonic is much higher than all other moduh, whereas in the case of misfiie in the first cylinder there is also a significant contribution of the moduh of the first harrnonics, in particular of the first three. It is clear that the contribution of the modulus of each hannonic depends on some factors which have to be considered when setting the thieshold values of the misfire index. These factors include, for example, the shape of the exhaust pipes 9, 9', the number and the firing sequence of the cylinders 3, 3' of each row. Finally referring to figures 7a to 7c, there is seen that the phase of the first harmonic changes as a function of the position of the cylinder where the misfiie occurred. In fact, it is possible to identify six separate areas, each area corresponding to an engine cylinder, where the polar coordinates of the modulus and phase of the first hannonic at the moment of the misfiie are concentrated. In particular, there is seen that said coordinates concentrate in six sectors having an extension of 60° each, whose sequence is defined by the cylinder firing sequence, which in the present embodiment is 1-4-2-6-3-5 for the row of cylinders 3. Taking into account the engine phase, this conespondence can be found both at low rprn, i.e. at about 2000 rpm with an engine load around 15% (figure 7a), at intermediate rpm, i.e. at about 4000 ipm with an engine load approximately at 100% (figure 7b), and at high rprn, i.e. at about 6000 rpm with an engine load approximately at 100% (figure 7c). Possible additions and/or modifications may be made by those skilled in the art to the above-described and illustrated embodiment, yet without departing from the scope of the invention. In fact it is obvious that the type of sampling, frequency analysis and particularly the method for calculating the misfiie index may change according to the type of engine to be monitored. Similarly, also the threshold values may change according to the experimental tests carried out on each type of engine. Finally, it is obvious that the process according to the present invention can be used in combination with one or more prior art processes.

Claims: - 13 - CLAIMS 1. A process for detecting a misfiie in one or more cylinders (3, 3') of an internal combustion engine, characteiized in that it includes the following operative steps: - sampling the exhaust gas pressure values during at least one engine cycle, the sampling frequency being proportional to the crankshaft rotational speed; - analyzing the sampled signal in the frequency domain; - calculating a misfiie index as a function of the results of said analysis; - comparing said index with one or more thieshold values. 2. A process according to the preceding claim, characterized in that said frequency domain analysis includes a Fourier transform of the sampled signal. 3. A process according to the preceding claim, characteiized in that the calculation of the misfire index includes the combination of the modulus of some harmonics of the sampled signal. 4. A process according to the preceding claim, characteiized in that the calculation of the misfiie index includes the addition of the modulus of at least the first three harmonics of the sampled signal. 5. A process according to one of the preceding claims, characteiized in that the sampling of the pressure values is started at the beginning of an engine cycle. 6. A process according to one of the preceding claims, characterized in that it includes the comparison of the modulus of at least one harmonic of the sampled signal with one or more thieshold values. 7. A process according to the preceding claim, characteiized in that it includes the calculation of the phase of the first hannonic of the sampled signal, and the calculation of the difference between said phase and the phase of at least one engine cylinder (3, 3'). 8. A system for caπying out the process according to one of the preceding clahns, characterized in that it includes at least one control unit (1, 2, 2') connected to at least one sensor (18, 18') detecting the pressure in the exhaust pipes (9, 9') and - 14 - to at least one sensor (14) detecting the crankshaft rotation. 9. A system according to the preceding claim, characteiized in that it mcludes at least one sensor (16, 16') detecting the camshaft (17) rotation. 10. A system according to claim 8 or 9, characteiized in that said control unit ( 1, 2, 2') includes means for the analogue-to- digital conversion of the electric signal transmitted by the sensor (18, 18') detecting the pressure hi the exhaust pipes (9, 9'), means for sampling the signal converted into digital form, as well as memoiy means for storing the sampled signal. 11. A system according to the preceding claim, characteiized in that the sampling frequency of said sampling means is adjusted according to the signal transmitted by the sensor (14) detecting the crankshaft rotation. 12. A system according to one of the clahns 8 to 11, characterized in that said control unit (1, 2, 2') is connected to at least one sensor (4, 4') detecting the coolant temperature, and to at least two sensors (5, 5', 6, 6') respectively detecting the temperature and pressure of the ah in the intake manifolds (7, 7'). 13. A system according to one of the clahns 8 to 12, characteiized in that said control unit (1, 2, 2') is connected to at least one warning hght (19, 19') indicating a misfire in at least one engine cylinder. 14. A system according to one of the clahns 8 to 13, characteiized in that said control unit (1, 2, 2') is connected to a sensor (21) detecting the position of the engine throttle (22). 15. A system according to one of the clahns 8 to 14, characteiized in that said control unit (1, 2, 2') is connected to at least one sensor (13, 13') detecting the temperature in the exhaust pipes (9, 9'). 16. A car characteiized in that it includes a system according to one of the clahns 8 to 15 for detecting a misfiie in one or more engine cylinders (3, 3').

Inventors: BETTINI RICCARDO, CAMPI PIERO, CECCARANI MASSIMO, REBOTTINI CORRADO, BETTINI, RICCARDO, CAMPI, PIERO, CECCARANI, MASSIMO, REBOTTINI, CORRADO
Applicants: AUTOMOBILI LAMBORGHINI S P A, BETTINI RICCARDO, CAMPI PIERO, CECCARANI MASSIMO, REBOTTINI CORRADO, AUTOMOBILI LAMBORGHINI S.P.A., BETTINI, RICCARDO, CAMPI, PIERO, CECCARANI, MASSIMO, REBOTTINI, CORRADO
Application year: 1998
Main classification: G01M15
IPC codes: F02D45, F02P17, G01M15