Method for detecting an abnormal disturbance of an internal combustion engine torque

A method for detecting an abnormal disturbance of an internal combustion engine torque and for suspending the functioning of a system in which combustion misfiring has been diagnosed. The method operates by analysis of values of a quantity representing the quality of combustion by observing engine crank shaft rotation. A stability criterion is defined of the quantity for each of the engine cylinders. Then, after each combustion the stability criterion is compared with a predetermined threshold, and an abnormal torque disturbance is detected when the stability criterion is greater than the threshold for a given number of consecutive times for at least one cylinder.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a process for detection of an abnormal
 perturbation of the torque of an internal combustion engine, especially
 one mounted in a motor vehicle. The present invention relates more
 particularly to a process capable of suspending the diagnosis of misfires
 when abnormal perturbations caused in particular by rough roadway pavement
 or by any other factor create excessive noise in the signal used for
 diagnosis of misfires.
 2. Discussion of the Background
 In the scope of antipollution regulations such as the American OBD
 standards (On Board Diagnostics), it is required that electronic engine
 control systems of motor vehicles must be able to diagnose certain engine
 dysfunctions which influence pollutant emissions.
 For this reason it is planned, for future electronic engine control
 systems, to install systems capable of diagnosing proper operation of the
 oxygen sensor or of the EGR loop (exhaust gas recirculation), wherein
 recognition of a malfunction which affects pollutant emissions must trip
 activation of less intensive modes of operation and/or must turn on a
 light on the dashboard to warn the driver (relaxed regulations) or must
 stop the vehicle (strict regulations).
 In particular, the American and European standards provide for requiring
 detection of misfires and for identification of the cylinder or cylinders
 responsible. Such detection must, for example, furnish the percentage of
 misfires occurring in a given number of engine cycles.
 The equipment designers and automobile manufacturers have therefore
 developed a certain number of techniques for detection of misfires.
 As an example, there can be cited the accelerometer method, which comprises
 detecting a misfire by analysis of the variation of longitudinal
 acceleration of the vehicle, or the method using an oxygen-proportional
 sensor disposed in the exhaust line. There can also be cited the method
 using pressure sensors communicating with the combustion chambers, or that
 based on measurement of arc voltage or ionization current of the spark
 plugs (for controlled ignition engines).
 The most commonly used method, however, is that which deduces the existence
 of misfires by measurement of the instantaneous speed of the crankshaft.
 The use thereof is actually extremely simple, since it involves merely
 applying software processing of the signal furnished by the angular
 position sensor of the crankshaft, a signal which is already used by the
 engine control system to control fuel injection, and so no specific
 equipment-related device is required for implementation of this method.
 The analysis solely of the signal furnished by the crankshaft position
 sensor to detect possible misfires nevertheless suffers from certain
 disadvantages.
 The method of detection of misfires is in fact based on the postulate that
 a misfire is manifested by a drop of the gas torque, which in turn
 generates a corresponding change of the instantaneous speed of the
 crankshaft. To identify misfires, therefore, it is sufficient to record
 the changes in instantaneous speed of the crankshaft.
 However, the instantaneous speed of revolution of the crankshaft and of the
 engine flywheel integral therewith reflects not only the operation of the
 engine and the alternating thrust of the connecting rods under the effect
 of combustion of the carbureted mixture, but also the operation of the
 entire kinematic chain connecting the engine to the tire/ground interface.
 In fact, the mechanical energy at the end of the crankshaft is transmitted
 to the wheels by a transmission system which traditionally comprises a
 clutch, a speed-change box and a differential, this transmission
 possessing its own damping and stiffness. As a result, therefore, any
 abrupt variation of torque involving any of the elements of the kinematic
 chain, such as, for example, involving the vehicle's wheels because of
 poor condition of the roadway surface, is fed back to the crankshaft in
 the form of oscillations, the magnitude of which will depend on the
 characteristics of the transmission system and of the perturbation.
 As a result, the abrupt changes in instantaneous speed of the crankshaft
 are therefore caused not solely by misfires but also by all the
 perturbations capable of affecting the kinematic transmission chain, and
 therefore especially a roadway in poor condition.
 For correct and exclusive identification of misfires, therefore, it seems
 important to be able to distinguish, among the changes of crankshaft
 speed, those due effectively to drops of gas torque from those having
 other causes, so that the latter are not counted.
 To achieve this, additional strategies for deactivation of the program for
 detection of misfires have been developed, which strategies are based on
 recognition of perturbations affecting the kinematic transmission chain.
 Thus there can be cited strategies which use the information of wheel
 speed furnished by a specific sensor or else those requiring an
 accelerometer. There can also be cited the method described in British
 Patent GB A 2290870, which attempts to identify such perturbations by fuel
 motion in the fuel tank.
 It nevertheless appears at present that none of the proposed methods is
 capable of distinguishing, in simple and economic fashion and with
 sufficient precision and reliability, the changes of crankshaft speed
 caused by perturbations which affect the kinematic transmission chain.
 SUMMARY OF THE INVENTION
 The object of the present invention is therefore to improve the process
 described in the foregoing by providing a process for detection of
 abnormal torque perturbations other than those related to true misfires,
 which process is capable of temporarily suspending the diagnosis of
 misfires.
 The process for detection of an abnormal perturbation of an internal
 combustion engine torque and suspending the operation of a system for
 diagnosis of misfires is of the type that operates by analysis of the
 values of a variable representative of firing quality by observation of
 the rotation of the engine crankshaft.
 According to the invention, the process for detection of an abnormal torque
 perturbation is characterized in that it comprises:
 defining a criterion of stability of the variable representative of firing
 quality for each cylinder of the engine and for each firing;
 comparing, after each firing, this stability criterion with a predetermined
 threshold, and detecting an abnormal torque perturbation when this
 stability criterion exceeds, for at least one cylinder, the threshold for
 a given number of consecutive times.
 According to another characteristic of the process according to the
 invention for detecting an abnormal torque perturbation, the said variable
 representative of firing quality is the gas torque.
 According to another characteristic of the process according to the
 invention for detecting an abnormal torque perturbation, the stability
 criterion quantifies, for a given cylinder and given firing, the absolute
 deviation between the value of the said variable representative of firing
 quality and the value of a statistical variable representative of the mean
 of the different values of the variable representative of firing quality,
 the values being measured successively over a given measurement horizon.
 According to another characteristic of the process according to the
 invention for detecting an abnormal torque perturbation, the statistical
 variable representative of the mean of the different values of the
 variable representative of firing quality is obtained by a first-order
 low-pass filter with a given filtering constant.
 According to another characteristic of the process according to the
 invention for detecting an abnormal torque perturbation, the threshold is
 deduced from the variations of the variable representative of firing
 quality observed during normal engine operation, and it therefore depends
 on the engine operating point.
 According to another characteristic of the process according to the
 invention for detecting an abnormal torque perturbation, the firings being
 diagnosed as misfires initiate a particular treatment in the calculation
 of the stability criterion.
 According to another characteristic of the process according to the
 invention for detecting an abnormal torque perturbation, the firings being
 diagnosed as misfires initiate a particular treatment in the comparison of
 the stability criterion with the predetermined threshold.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 1, there can be seen a simplified sketch of an engine
 control system using the process according to the present invention for
 detecting misfires. Only the constituent parts necessary for understanding
 of the invention have been shown.
 Internal combustion engine 1 is designed more particularly as equipment for
 a motor or road vehicle. Engine 1 is connected to a transmission device
 suitable for transmitting motion to the vehicle's wheels. This
 transmission device classically comprises a clutch, a speed-change box 2
 and a differential, not illustrated.
 The four-cycle multi-cylinder engine 1 is equipped with a fuel-injection
 device of the multi-point type with electronic control, by virtue of which
 each cylinder is supplied with fuel from a specific electronic injector 5.
 Opening of each electronic injector 5 is controlled by the electronic
 engine control system 7, which adjusts the quantity of fuel injected and
 the moment of injection into the cycle as a function of engine operating
 conditions, in such a way that the richness of the combustible air-fuel
 mixture admitted to the cylinders is adjusted automatically and precisely
 to a predetermined setpoint value.
 The electronic engine control system 7 classically comprises a
 microprocessor 70, read-write memories 71, read-only memories 72,
 analog-to-digital converters 74 and various input and output interfaces.
 Microprocessor 70 is provided with electronic circuits and software
 routines appropriate for processing the signals originating from suitable
 sensors, for determining the states of the engine and for initiating
 predefined operations in order to generate control signals destined in
 particular for the injectors (and for the ignition coils in the case of a
 controlled ignition engine), so as to ensure optimum management of the
 firing conditions in the engine cylinders.
 The input signals of microprocessor 70 include in particular those
 addressed by a crankshaft sensor 4. This sensor 4, of the variable
 reluctance type, for example, is mounted immovably on the engine frame at
 a position in front of a measuring gear 12 fixed to one end of the
 crankshaft.
 This gear 12 is provided on its periphery with a succession of teeth and
 spaces, which are identical with the exception of one tooth, which has
 been removed in order to define an absolute reference with which there can
 be deduced the moment at which a given reference cylinder, in the present
 case cylinder No. 1, passes top dead center.
 Sensor 4 delivers a signal Dn corresponding to the procession of the teeth
 of gear 12, which signal is processed to generate a TDC signal during each
 half-revolution of the crankshaft, thus making it possible to identify in
 alternation the passes of cylinders No. 1, 3, 4, 2 through top dead
 center.
 Processing of signal Dn emitted by sensor 4 also makes it possible to
 measure the speed of procession of the teeth of gear 12, and thus to
 record the instantaneous speed of revolution of the engine.
 Microprocessor 70 therefore transforms signal Dn to produce a
 representative variable characteristic of the quality of firings occurring
 in each of the engine cylinders and to perform the diagnosis of misfires
 according to a process known in itself, which will not be described in
 detail since it is not the object of the present invention.
 The representative variable characteristic of firing quality can be, for
 example, the instantaneous crankshaft acceleration (see German Patent
 Applications DE 3939113 or DE 4002208), or else the torque, the value of
 which is obtained by spectral analysis of the instantaneous crankshaft
 speed (see French Patent Applications No. 91/11273 and 91/11274 filed by
 the Applicant in relation to a "process and device for measuring the gas
 torque of an internal combustion heat engine").
 According to the embodiment described below, the gas torque Cg is the
 representative variable chosen to control both the system for diagnosis of
 misfires and the system capable of suspending the diagnosis of misfires
 when perturbations external to engine operation are interfering with this
 diagnosis.
 During operation of the engine, appropriate calculating means activated by
 microprocessor 70 therefore furnish successive torque values Cg,n,i to
 characterize the value of the gas torque Cg corresponding to the n-th
 firing of cylinder No. i.
 These values are then processed by first calculation means designed to
 perform the diagnosis of misfires and by second calculation means,
 specific to the present invention, designed to identify the occurrence of
 torque perturbations capable of falsifying the diagnosis of misfires,
 which perturbations are due, for example, to the transmission device and
 in particular to the condition of the roadway pavement on which the
 vehicle is traveling.
 According to the invention, the process for detecting the occurrence of
 torque perturbations capable of falsifying the diagnosis of misfires is
 performed by analysis of the torque variations cylinder by cylinder. In
 the practical example described hereinafter, it is sufficient to detect
 the occurrence of torque perturbations in a single cylinder in order to
 suspend the diagnosis of misfires for all cylinders.
 Quite obviously it can be provided, in alternative embodiments, that the
 occurrence of torque perturbations in a single cylinder suspends the
 diagnosis of misfires only for the cylinder in question, or else it can be
 provided that the occurrence of torque perturbations must be detected in a
 predetermined number of cylinders in order to suspend the diagnosis of
 combustion misfires for all cylinders.
 Similarly, in the event of detection of abnormal torque perturbations,
 inhibition of the diagnosis of misfires can be tripped immediately
 thereafter or can even be effected retroactively on a given period
 preceding the detection of the perturbations, in order to allow for the
 response time, the length of which depends on the chosen embodiment of the
 process for detection of perturbations.
 In the process according to the invention, therefore, the first step is
 calculation of a statistical variable MCg,n,i representative of the mean
 of the different torque values measured successively on cylinder No. i
 over a given horizon of torque measurements.
 Calculation of the statistical variable MCg,n,i is achieved, for example,
 by a first-order low-pass filter with given filtering constant .tau.. In
 this case, statistical value MCg,n,i for any new torque value Cg,n,i is
 determined from the previously calculated statistical value, MCg,n-1,i, on
 the basis of the following recurrence formula:
EQU MCg,n,i=MCg,n-1,i+((Cg,n,i-(MCg,n-1,i))/.tau.)
 The value thus calculated from the statistical variable MCg,n,i
 representative of the mean of the different torque values Cg,n,i measured
 successively for cylinder No. 1 over a given horizon of torque
 measurements is then used to determine the value ECg,n,i, defined as the
 absolute deviation between the torque value Cg,n,i and the statistical
 value MCg,n,i:
EQU ECg,n,i=.vertline.MCg,n,i-cg,n,i.vertline.
 This deviation ECg,n,i is therefore representative of the engine torque
 variations for the cylinder in question and therefore of the stability of
 this torque.
 In an alternative embodiment, ECg,n,i can also be obtained, not from the
 value MCg,n,i, but instead from the value MCg,n-1,i calculated in the
 preceding cycle, specifically in order to calculate ECg,n,i more rapidly.
 This yields:
 ECg,n,i=.vertline.Mcg,n-1,i-Cg,n,i.vertline.
 In an alternative embodiment of the process according to the invention, it
 is also possible to eliminate from the calculation of MCg,n,i the effect
 of misfires which occurred during the observed period and which were
 detected by the said first calculation means. For this purpose, the torque
 value corresponding to a misfire is replaced in the calculation of the
 statistical variable MCg,n,i by the last value that did not correspond to
 a misfire or else, by blocking the filter, to its preceding value
 MCg,n,i=MCg,n-1,i.
 Quite obviously, if the cylinder continuously exhibits misfires (ignition
 defect, etc.), detection of abnormal perturbations is then suspended for
 that cylinder until the fault is cleared.
 Regardless of the formula for calculating ECg,n,i, it is then sufficient,
 in order precisely to isolate the abnormal torque perturbations (rough
 roadway pavement, transmission jolts, etc.) capable of interfering with
 the diagnosis of misfires, to compare this deviation with a predetermined
 fixed stability threshold Sstab, which depends on the engine operating
 point.
 The stability threshold Sstab is deduced from the maximum torque excursions
 encountered during normal operation. It therefore depends on the engine
 operating point and is obtained, for example, from the following formula:
 Sstab=.alpha..multidot..delta.Cg
 According to this formula, .alpha. is a calibrated factor
 (0&lt;.alpha.&lt;1) and .delta.Cg is the expected torque drop at the
 operating point. Sstab, which is listed in tables or obtained by
 calculation, can also evolve with aging of the engine. Of course, any
 other representation of the nominal torque delivered by the engine can be
 used instead of the torque drop.
 The graphs of FIGS. 2a, 2b and 2c illustrate the principle of the
 invention.
 For stabilized engine operating conditions, when the transmission device is
 not subject to any large perturbation, and in the absence of misfire (FIG.
 2a), the values of the gas torque Cg,n,i for a given cylinder i then vary
 only slightly on both sides of the value MCg,n,i. The criterion ECg,n,i
 then remains close to 0 and therefore is always below the threshold Sstab.
 When misfires occur (FIG. 2b), the criterion ECg,n,i reaches extreme values
 during each misfire, because the instantaneous torque Cg,n,i then drops
 abruptly while the value MCg,n,i remains at a high value, whereas for the
 other firings the values of gas torque Cg,n,i vary only slightly on both
 sides of the value MCg,n,i and the criterion ECg,n,i then remains close to
 0. Thus the threshold Sstab is exceeded only sporadically, and only during
 the occurrence of misfires.
 In contrast, during perturbations external to cylinder operation, as in the
 case of poor roadway condition (see FIG. 2c), the torque Cg,n,i is then
 randomly noisy, as is the stability criterion ECg,n,i, which reaches a
 much higher mean level. The threshold Sstab is then frequently exceeded.
 It is therefore sufficient to note the frequency at which the threshold
 Sstab is exceeded in order to detect the occurrence of abnormal torque
 perturbations. For example, it is sufficient, in order to deduce the
 occurrence of abnormal torque perturbations, to record p consecutive
 overshoots of the threshold Sstab by the criterion ECg,n,i, where p is a
 natural integer greater than or equal to 2, and to suspend the diagnosis
 of misfires.
 Nevertheless, in order not to confuse the occurrence of abnormal torque
 perturbations with a cylinder dysfunction which continuously generates
 misfires, overshoots of the threshold Sstab can be counted only in the
 case of a misfire.
 As illustrated in FIG. 3, a process according to the invention therefore
 comprises the following stages, which are executed after each firing
 indexed n and for a given cylinder No. i:
 (i) acquisition of the value of the gas torque Cg,n,i generated by firing,
 and calculation of the value ECg,n,i (using MCg,n-1,i);
 (ii) comparison of ECg,n,i with Sstab
 if ECg,n,i is less than Sstab, counter N is reset to 0 (N=0);
 if ECg,n,i is greater than or equal to Sstab, counter N is incremented by
 one unit (N=N+1);
 (iii) at the same time, MCg,n,i is calculated;
 (iv) comparison of N with p
 if the value of counter N is less than p, it is therefore assumed that
 abnormal torque perturbations have not been detected
 if, on the other hand, the value of counter N is greater than or equal to
 p, it is then assumed that abnormal torque perturbations have been
 detected, and the diagnosis of misfires is then suspended.
 The described process for suspending the diagnosis of misfires therefore
 has the advantage of being a particularly simple and rapid method of
 achieving detection of abnormal torque perturbations capable of falsifying
 the diagnosis of misfires, without extra cost due to a component.
 Furthermore, this process is remarkably reliable and therefore trips
 inhibition of the diagnosis of misfires only when necessary.
 Of course, the invention is in no way limited to the described and
 illustrated embodiment, which has been presented only as an example.
 To the contrary, the invention comprises all technical equivalents of the
 described means as well as combinations thereof if they are effected in
 accordance with its spirit.
 Thus, it is possible to operate not directly on the values Cg,n,i but on
 filtered values.
 As regards implementation of the device for detection of misfires, it can
 be achieved in diverse forms regardless of the variant chosen:
 with analog electronic components, in which case the summing units,
 comparators and other filters are achieved by means of operational
 amplifiers;
 or with digital electronic components, which would achieve the function by
 hard-wired logic;
 or by a signal-processing algorithm loaded in the form of a software
 module, which is a component of an engine control software system for
 operating the microcontroller of an electronic calculator,
 or even by a specific (custom) chip, whose hardware and software resources
 will have been optimized to achieve the functions according to the
 invention; chip microprogrammable or not, encapsulated separately or else
 comprising all or part of a coprocessor implanted in a microcontroller or
 microprocessor, etc.
 Similarly, the invention comprises all technical equivalents applied to an
 internal combustion engine, regardless of its combustion cycle (2-cycle,
 4-cycle), of the fuel used (diesel or gasoline), or even of the number of
 its cylinders.