Method and device for detecting misfires in an internal combustion engine of a motor vehicle

Abstract

Method for detecting misfires in an internal combustion engine of a motor vehicle, in which a misfire characteristic is calculated based on a current signal available in an engine control unit of the motor vehicle and the calculated misfire characteristic is compared with a predetermined threshold, wherein at least one misfire is detected if the misfire characteristic deviates from the threshold, characterized in that a current signal from an electric motor of the motor vehicle is used as the current signal.

Description

[0001] The present invention relates to a method and a corresponding arrangement for detecting misfires in an internal combustion engine of a motor vehicle. State of the art

[0002] Missing fuel injection is a major cause of cylinder-specific misfires in diesel engines. Misfires result in a slower engine speed increase after what is known as top dead center of the ignition process in a misfiring cylinder. Misfire detection, known by the abbreviation MisfDet, detects a misfiring cylinder when the engine speed increase after top dead center (TDC) is too low. In the United States of America, monitoring of periodically occurring misfires in diesel engines is required by law. Therefore, in the United States of America, diagnostic results from misfire detection must be entered in a fault log as errors relevant for on-board diagnostics (OBD). In addition, the diagnostic results from misfire detection are used for workshop diagnostics.

[0003] The aforementioned misfire detection system MisfDet is already state of the art in the diesel engines mentioned.

[0004] A primary cause of misfiring in gasoline engines is the lack of the spark necessary for ignition, which is largely related to a defective spark plug. If no ignition occurs, an unburned fuel mixture enters the corresponding exhaust tract. The fuel mixture may then combust in the hot exhaust tract and cause damage, for example, to an existing catalytic converter. An engine-speed-based misfire detection system, abbreviated as DMD, detects a misfiring cylinder in gasoline engines. To prevent the aforementioned component damage, the injection systems are automatically shut off in the event of a misfire.

[0005] Misfire monitoring is also required by law for gasoline engines. Therefore, diagnostic results from the aforementioned misfire detection (DMD) system are entered into a corresponding error log as errors relevant for on-board diagnostics (OBD).

[0006] The so-called misfire detection DMD is also state of the art for gasoline engines.

[0007] EP 0 408 877 B1 discloses a method for determining the speed of an internal combustion engine with a known number of cylinders. A signal processing arrangement is used to determine the speed from a signal occurring at the generator or at a load. The signal contains a first signal component, which corresponds to an alternating voltage component from a generator driven by the internal combustion engine with a given transmission ratio. The signal occurring at the generator and at the load contains a second, lower-frequency signal component, which is caused by the discontinuous operation of the internal combustion engine, due to combustion in the individual cylinders. The first signal component is frequency-modulated with the second signal component.The signal processing arrangement determines the transmission ratio between the internal combustion engine and the generator from the ratio of the frequency of the first signal component and the frequency of the second signal component.

[0008] The evaluation of a generator current signal to detect misfires is already known from DE 10 2010 035 612 A1.

[0009] Further methods for detecting misfires are already known from DE 103 50 856 B4 and DE 102011 018 552 A1.

[0010] Against the background of the prior art, it is now an object of the present invention to provide a simple and robust misfire detection system for both diesel and gasoline engines with hybrid powertrain concepts. Disclosure of the invention

[0011] The above-mentioned object is achieved by a method having the features of patent claim 1 and an arrangement having the features of patent claim 7. Further embodiments are described in the corresponding subclaims.

[0012] According to the invention, a method is provided for detecting misfires in an internal combustion engine of a motor vehicle, in which method a misfire characteristic is calculated based on a current signal available in an engine control unit of the motor vehicle and the calculated misfire characteristic is compared with a predetermined threshold. If the misfire characteristic deviates from the threshold by a predetermined amount, at least one misfire is detected. Furthermore, the electrification of the drive train is becoming increasingly important, and the prevalence of hybrid vehicles, including in daily operation, is increasing. In hybrid vehicles, i.e. motor vehicles with an electric motor or an electric machine in the drive train, the current signal of the electric motor of the motor vehicle is used as the current signal in the method according to the invention.

[0013] In an embodiment of the method not according to the invention, a current signal from a motor vehicle alternator is used as the current signal. The alternator in a motor vehicle is a generator that generates the necessary current for the vehicle's electrical system. The alternator is usually connected to the internal combustion engine, in particular the motor, via a belt drive. Three-phase generators are primarily known from the prior art; they generate a three-phase alternating current that is rectified via power diodes.

[0014] The speed-based misfire detection known from the prior art has the disadvantage that, due to, among other things, the nonlinear transmission behavior of a dual-mass flywheel in the drivetrain of the corresponding motor vehicle, a speed signal is distorted over wide operating ranges. These speed disturbances complicate the aforementioned speed-based misfire detection.

[0015] In general, it can be said that cylinders with misfiring do not contribute to torque generation. In a low speed range, i.e. < 2500 rpm, the corresponding torque M is mainly made up of a gas torque component. In the low speed range, a linear physical relationship M ∼ q applies, where q corresponds to the injected fuel quantity. Since the cylinder-individual torques are directly reflected in the speed signal and a misfiring characteristic can therefore be determined based on the speed, speed-based misfiring detection is a logical option and has also proven successful in a large number of projects in series production. However, speed-based misfiring detection is made more difficult if, in addition to the engine or internal combustion engine, there are other vibrating elements in the drive train, such as jerking vibrations or an electric motor or electric machine. In these cases, detection and / orThis makes the assignment of multiple dropouts particularly difficult. This problem can be addressed very efficiently and simply using the method according to the invention.

[0016] In a further embodiment of the method according to the invention, the current signal used to calculate the misfiring characteristic is acquired at a first sampling rate that is greater than or equal to a second sampling rate used to acquire a rotational speed of the internal combustion engine. Preferably, the first sampling rate corresponds to the second sampling rate.

[0017] In a further embodiment of the method according to the invention, a period of the rotational speed of the internal combustion engine is divided into successive segments by the first sampling rate, and in each of these segments the current signal is determined at a predetermined sampling time and kept constant until the next sampling time in the subsequent segment.

[0018] Furthermore, in the case of a plurality of consecutively arranged cylinders of the internal combustion engine, the current signal is determined in the segment immediately preceding top dead center and in the segment immediately following top dead center as the current signal for the cylinder that is the next cylinder to pass top dead center, according to the order of the cylinders, and then switches to the immediately following cylinder. This means that, for example, in an engine with four cylinders, the four cylinders are examined one after the other with regard to misfires, with the cylinder that is about to pass top dead center always being considered. A current signal is then measured in each case in the segment immediately preceding top dead center and in the segment immediately following top dead center.Once these two current signals have been recorded for each cylinder, the same procedure is repeated for the second cylinder. This is repeated until all existing cylinders have been tested.

[0019] Based on the respective current signals, a respective misfire characteristic is then calculated, assigned to the respective cylinder and checked for misfires on a cylinder-by-cylinder basis.

[0020] The present invention further relates to an arrangement for detecting misfires in an internal combustion engine of a motor vehicle. The arrangement according to the invention comprises a sampling current detection unit designed to sample and detect a current signal available in an engine control unit of the motor vehicle, a computing unit designed to calculate a misfire characteristic based on the detected current signal, and a comparison unit designed to compare the calculated misfire characteristic with a predetermined threshold, wherein at least one misfire is detected if the misfire characteristic deviates from the threshold by a predetermined amount.

[0021] In an embodiment, the arrangement according to the invention further comprises a display unit with the aid of which a detected misfire can be displayed.

[0022] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0023] It is understood that the features mentioned above and to be explained below can be used not only in the combinations specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention. Short description of the drawings Fig. 1 shows a motor vehicle in which an embodiment of an arrangement not according to the invention is provided. Fig. Figure 2 shows a schematic representation of a typical current characteristic of a motor vehicle alternator. Fig. 3 shows in Fig. 3a in the upper area a curve of a crankshaft speed as a function of a crankshaft position and in the lower area a curve of a generator current as a function of the crankshaft position, with no misfiring, and in Fig. 3b also shows a curve of the crankshaft speed and the generator current as a function of the crankshaft position, whereby misfiring is detected here. Fig. 4 shows a schematic representation in Fig. 4a a misfiring feature of a 4-cylinder engine at idle with no misfiring and in Fig. 4b the misfiring feature with double misfires in cylinder 1 and cylinder 2. Fig. 5 shows a block diagram according to which an embodiment of the method according to the invention is carried out. Fig. Figure 6 shows a misfire feature of a 4-cylinder diesel engine at idle with no misfires in Fig. 6a and with misfiring of cylinder 1 in Fig. 6b. Embodiment of the invention

[0024] The invention is illustrated schematically in the drawings using embodiments and is described in detail below with reference to the drawings. The invention is explained using the example of evaluating the current signal of an alternator, which is not according to the invention. According to the invention, the current signal of an electric motor of the motor vehicle is used.

[0025] Fig. 1 shows a motor vehicle 1 in which, for example, an embodiment of an arrangement not according to the invention for detecting misfires can be installed. The vehicle 1 comprises a lighting system 10, which is coupled to a starter battery 14 and to a generator 17 via appropriate electrical wiring. The generator 17, in turn, is coupled to an internal combustion engine or motor 19 of the vehicle 1 via a belt drive 18. A supply battery 16 can also be charged via the generator 17, as well as other on-board electrical consumers 15. The current generated by the generator 17 can be used to identify misfires of a motor 19 correspondingly coupled to the generator 17.

[0026] Fig. Figure 2 shows a current characteristic curve 20 of a motor vehicle alternator. The abscissa 21 represents the speed of the alternator, also referred to as generator speed n, in units of min -1plotted. On an ordinate 22, a generator current I, ie a current of the alternator in the unit A, is plotted. This shows a curve 20 of the generator current over the generator speed n, whereby the generator current I initially increases very sharply with increasing generator speed n and then slowly transitions to saturation up to a maximum current I max24. This current characteristic curve 20 illustrates the effect of a coupling between the alternator and the crankshaft of the internal combustion engine, since the alternator is usually coupled to the internal combustion engine via a belt drive. This means that the coupling of the alternator to the crankshaft is reflected in the generator speed n and thus, via the current characteristic curve 20, in the generator current I. This also makes cylinder-specific torque differences, for example caused by misfiring due to the aforementioned coupling, visible in the generator current I. Accordingly, the alternator's current signal provides a well-suited feature for detecting misfiring, i.e., a misfiring feature already described above.

[0027] The following Fig. 3 shows an idle measurement of a 4-cylinder diesel engine.

[0028] In Fig. In Figure 3a, the upper section plots a crankshaft speed n over a crankshaft position, and the lower section plots a generator current over the same range of crankshaft positions. The crankshaft position is plotted in °KW on the abscissa 31, while the speed n in rpm is indicated on the ordinate 32, and the generator current in A is indicated on the ordinate 33. Fig. Figure 3a shows a typical crankshaft speed curve and a corresponding generator current curve, with no misfires visible here. The crankshaft position range shown can be divided into eight segments 1 to 8, with two consecutive segments each assigned to one of the four cylinders: segments 1 and 2 cylinder 1, segments 3 and 4 cylinder 2, segments 5 and 6 cylinder 3, and segments 7 and 8 cylinder 4.

[0029] In contrast, if we consider Fig. 3b, one can see in the upper graph of the Fig. 3b, where the crankshaft speed is plotted on an ordinate 32 versus the crankshaft position on an abscissa 31, as well as in the lower graph, in which the corresponding generator current is plotted on an ordinate 33 versus the crankshaft position, shows that two consecutive misfires are evident in the first four segments. This means that a misfire was detected in both the first and the subsequent second cylinder.

[0030] The segments are again numbered from 1 to 8, although the area of ​​the crankshaft position shown is different from that in Fig. 3a, but again a period is passed through.

[0031] Fig. 4 shows a misfire characteristic of a 4-cylinder engine at idle. 41 segments 1 to 8 are plotted on the abscissa, into which a cycle duration of a 4-cylinder engine can be divided. This results in a total of eight segments, i.e. two segments per cylinder. The battery current or generator current is plotted on the ordinate 42. The misfire characteristic is represented by the drawn lines 44, with two current signals being sampled for each cylinder. A current signal was sampled in a segment immediately before the ignition TDC, which is passed through by the respective cylinder, and in a segment that immediately follows the ignition TDC. This results in two current signals per cylinder, a maximum and a minimum, whereby the time at which the current signal is sampled during the respective segment can be chosen arbitrarily, for example at the beginning.The current signal is then kept constant until the next sampling time, resulting in a constant curve over a segment. In . Fig. 4a no misfiring is detectable, while in Fig. 4b, in which the misfiring feature is represented by the respective lines 45, corresponding to Fig. 3b, a misfire is detectable in both cylinder 1 and cylinder 2. This can be seen by specifying a threshold, as shown here by the solid line 46. Misfires in the cylinder can be identified using this predetermined threshold 46. For a threshold comparison, for example, only minima 45_2, 45_4, 45_6, and 45_8 are used in Fig. 4b. The threshold is set so that in the normal case, ie when there are no misfires, the value falls below the threshold. A misfire is detected when the minimum misfire characteristic associated with the cylinder exceeds the threshold. In the present case, this is the case, as in Fig. 4b, this is the case for both cylinder 1 with minimum 45_2 and cylinder 2 with minimum 45_4, so that a misfire is detected in each case.

[0032] If a misfire characteristic does not have sufficient quality and an alternative misfire characteristic has been determined by calculating a moving average of the generator current sampling times before and after ignition TDC, misfires can also be detected using this alternative characteristic via a threshold comparison. The method allows for the detection of both single and multiple misfires.

[0033] Fig. Figure 5 shows a diagram illustrating how an embodiment of a method not according to the invention functions. Input variable 50 is a current signal from the alternator. After a higher-level check of release conditions, a conditioned current signal 52 is provided by block 51, which can be referred to as signal conditioning. The conditioned current signal 52 indicates the misfiring feature and is shown as an example in the following Fig. 6. In block 54, also referred to as cylinder assignment, an assignment is made between the misfiring characteristic and the respective cylinders. In block 55, referred to as the detection algorithm, a cylinder-by-cylinder check is performed based on the misfiring characteristic to determine whether misfiring is present. If misfiring is detected in block 55, a defect diagnostic result 56 is issued. If no misfiring is present, an OK diagnostic result 57 is issued.

[0034] Fig. Figure 6 shows a misfiring characteristic of a four-cylinder diesel engine at idle. Fig. 6a a battery current is plotted on an ordinate 62 over segments 1 to 8 on an abscissa 61. The same plot is shown in Fig. 6b. Two consecutive segments are assigned to each cylinder, so that cylinder 1 is assigned segments 1 and 2, cylinder 2 is assigned segments 3 and 4, cylinder 3 is assigned segments 5 and 6, and cylinder 4 is assigned segments 7 and 8. In segment 1, a current signal is sampled in a segment immediately before cylinder 1 passes top dead center (TDC). This results in a maximum 66_1. In the immediately following segment 2, a current signal is sampled again for the same cylinder, resulting in a minimum 66_2. The current signal is sampled once within a segment and kept constant for the remainder of the segment. The same procedure is carried out for the following cylinders 2, 3, and 4. In Fig. 6a shows a normal expected course of a characteristic high-low pattern 66. In Fig.6b, however, a corresponding high-low pattern 67 is broken and it can be seen that in cylinder 1 the expected minimum 67_2 in segment 2 is above a threshold 68 to be set, so that a misfire is detected here.

Claims

[1] Method for detecting misfires in an internal combustion engine of a motor vehicle, in which a misfire characteristic is calculated based on a current signal available in an engine control unit of the motor vehicle and the calculated misfire characteristic is compared with a predetermined threshold, wherein at least one misfire is detected if the misfire characteristic deviates from the threshold by a predetermined amount, characterized by that a current signal from an electric motor of the motor vehicle is used as the current signal. [2] Method according to claim 1, wherein, in order to calculate the misfiring feature, the current signal is acquired at a first sampling rate which is greater than or equal to a second sampling rate at which a rotational speed of the internal combustion engine is acquired. [3] The method of claim 2, wherein the first sampling rate corresponds to the second sampling rate. [4] Method according to claim 3, in which a period of the rotational speed of the internal combustion engine is divided into successive segments by the first sampling rate and in each of these segments the current signal is determined at a predetermined sampling time and is kept constant until the next sampling time in the following segment. [5] Method according to claim 4, in which, in the case of a plurality of cylinders of the internal combustion engine arranged one after the other, the current signal is determined as the current signal in the segment immediately preceding the top dead center and in the segment immediately following the top dead center, in accordance with the order of the cylinders one after the other, and then switched to the immediately following cylinder. [6] Method according to claim 5, in which a respective misfiring characteristic is calculated on the basis of the respective current signals, assigned to the respective cylinder and checked for misfiring on a cylinder-by-cylinder basis. [7] Arrangement for detecting misfires in an internal combustion engine of a motor vehicle, comprising a sampling and current detection unit designed to sample and detect a current signal available in an engine control unit of the motor vehicle, a computing unit designed to calculate a misfire characteristic based on the detected current signal, and a comparison unit designed to compare the calculated misfire characteristic with a predetermined threshold, wherein at least one misfire is to be detected if the misfire characteristic deviates from the threshold by a predetermined amount, characterized by that a current signal from an electric motor of the motor vehicle is used as the current signal. [8] Arrangement according to claim 7, further comprising a display unit for displaying a detected misfire.

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