Method for determining an imbalance of at least one cylinder

Abstract

A method for determining an imbalance of at least one cylinder in an arrangement of at least two cylinders in a system is provided, which includes an internal combustion engine, the imbalance of the at least one cylinder being present in relation to at least one property of an exhaust gas of the internal combustion engine and the determination of the imbalance of the at least one cylinder occurring with a sensor device for detecting at least one property of the exhaust gas of the internal combustion engine. A diagnostic threshold is ascertained through a dynamic characterization of the system with the sensor device, the dynamic characterization taking place after an excitation of the system. The diagnostic threshold makes it possible to delimit a range of potential erroneous detections which are caused due to a dispersion of an evaluating signal resulting from the imbalance of the at least one cylinder.

Claims

1. A method for determining an imbalance of at least one cylinder in an arrangement of at least two cylinders in a system having at least one control unit and a sensor device, which includes an internal combustion engine, the method comprising: determining the imbalance of the at least one cylinder with the sensor device for detecting at least one property of an exhaust gas of the internal combustion engine, the imbalance of the at least one cylinder being present in relation to at least one property of the exhaust gas of the internal combustion engine, the determining the imbalance comprising: applying an abrupt excitation to the system, wherein the abrupt excitation is a sudden change in an injection quantity of fuel into the system that applies a forced amplitude to the internal combustion engine at a time outside of a normal operation of the internal combustion engine, the abrupt excitation resulting in a dynamic characterization of the system; setting a chronological progression of a lambda value based on the forced amplitude such that the determined imbalance of the at least one cylinder is determined outside of a range in which a lambda 1 ripple occurs; and ascertaining a variable diagnostic threshold based on the dynamic characterization of the system with the aid of the sensor device, the variable diagnostic threshold varying as a function of the dynamic characterization; determining that an error is present in the system based on the determined imbalance of the at least one cylinder and the ascertained variable diagnostic threshold; and displaying and/or storing the error via an on board diagnostic system.

2. The method of claim 1, wherein the variable diagnostic threshold is ascertained by comparing at least one parameter of the dynamic characterization with at least one predefined value.

3. The method of claim 1, wherein for the purpose of the dynamic characterization of the system, an abrupt response of the system to the abrupt excitation of the system is used, and wherein at least one rise time of the abrupt response to the abrupt excitation and/or a down time between the abrupt excitation and the abrupt response is used as the at least one parameter.

4. The method of claim 1, wherein the sensor device detects an oxygen content of the exhaust gas, the oxygen content of the exhaust gas being indicated in the form of the lambda value.

5. The method of claim 1, wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set unequal to 1.0.

6. The method of claim 1, wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set to a value of at least 0.9.

7. The method of claim 1, wherein a first period, during which the lambda value is set to a first value below 1.0, is followed by a second period, during which the lambda value is set to a second value above 1.0, or a first period, during which the lambda value is set to a first value above 1.0, is followed by a second period, during which the lambda value is set to a second value below 1.0.

8. The method of claim 1, wherein the second period is followed by an interval during which the lambda value is set to the value of 1.0, the interval being followed by the first period, the interval having a duration which exceeds the duration of the first period and/or the second period, the duration of the first period and/or of the second period being maximally 3 s.

9. The method of claim 1, wherein the sensor device includes at least one filtering device for detecting and/or suppressing at least one spectral component in a frequency spectrum of an evaluating signal resulting from the imbalance of the at least one cylinder.

10. The method of claim 1, wherein the variable diagnostic threshold is ascertained by comparing at least one parameter of the dynamic characterization with at least one predefined value, wherein the predefined value is retrieved from at least one of a characteristics map and a characteristics function.

11. The method of claim 1, wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set to a value of at least 0.95.

12. The method of claim 1, wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set to a value of maximally 1.1.

13. The method of claim 1, wherein the lambda value, at which the determination of the imbalance of the at least one cylinder takes place, is set to a value of maximally 1.05.

14. The method of claim 1, wherein the second period is followed by an interval during which the lambda value is set to the value of 1.0, the interval being followed by the first period, the interval having a duration which exceeds the duration of the first period and/or the second period, the duration of the first period and/or of the second period being maximally 1 s.

15. The method of claim 1, further comprising displaying the error via the on board diagnostic system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B show the chronological progression of the lambda value within multiple operating cycles of an internal combustion engine (a) as well as the evaluating signal which is detected therefrom with the aid of a sensor device and ascertained with the aid of an algorithm (b).

(2) FIG. 2 shows a schematic representation of the evaluating signal as a function of the real imbalance of at least one cylinder in an arrangement of at least two cylinders in a system, which includes an internal combustion engine, a dispersion of the evaluating signal possibly resulting in a potential erroneous detection.

(3) FIGS. 3A and 3B schematically show an abrupt excitation of the system at a point in time t0 (a) and the abrupt response of the sensor device which is used for the dynamic characterization of the system (b).

(4) FIG. 4 shows a schematic representation of the abrupt response with and without a lambda 1 ripple.

(5) FIG. 5 shows a schematic representation of a typical frequency spectrum of the evaluating signal.

DETAILED DESCRIPTION

(6) FIG. 1a) shows the chronological progression of the lambda value within multiple operating cycles 110 of an internal combustion engine, while in FIG. 1b), the chronological progression of the lambda value is illustrated in the way in which it is recorded by the sensor device for detecting at least one property of the exhaust gas of the internal combustion engine. An evaluating signal 116, which may also be referred to as an air/fuel imbalance monitoring (AFIM) signal, may be ascertained from a difference between a maximum lambda value 112 and a minimum lambda value 114 with the aid of an evaluation algorithm which is suitable therefore.

(7) As is apparent from FIG. 2, evaluating signal 116 correlates with how high imbalance 118 of the observed cylinder is in relation to the remaining at least two cylinders in the arrangement in a system, which includes the internal combustion engine. However, evaluating signal 116 is subject to a dispersion 120 with regard to the real imbalance of the observed cylinder. Dispersion 120 results in that an imbalance may only be reliably recognized above a range 122 of potential erroneous detections, i.e., in a range 124 of the reliable recognition, whereas a potential erroneous detection may occur if evaluating signal 116 is subject to dispersion 120 in range 122.

(8) Here, in the case of a constellation of influence variables on evaluating signal 116, which result in an excessively high evaluating signal 126, a fixed diagnostic threshold 130 may already be exceeded in range 122 of the potential erroneous detections, although the maximally admissible imbalance of the observed cylinder has not been reached yet. Conversely, in the case of a constellation of influence variables, which result in an excessively low evaluating signal 128, a fixed threshold 130 is not exceeded yet in range 122 of the potential erroneous detections, although the maximally admissible imbalance of the observed cylinder has already been exceeded.

(9) For this reason, it is provided according to the present invention to ascertain a variable diagnostic threshold 132 by a dynamic characterization of the system with the aid of the sensor device according to FIG. 1b). According to the present invention, diagnostic threshold 132 is ascertained by comparing at least one parameter of the dynamic characterization with at least one predefined value, whereby an increase or a decrease of diagnostic threshold 132 may result as a function of the dynamic of the observed system. In this way, it may be ensured that a reliable detection of the imbalance of the at least one cylinder is made possible according to the present invention even in range 122 in which in the case of a fixed threshold 130, a potential erroneous detection may occur according to the related art.

(10) FIG. 3 shows a particular exemplary embodiment for ascertaining the dynamic characterization of the system with the aid of the sensor device. For this purpose, as shown in FIG. 3a), an abrupt excitation 134 of the system takes place at point in time t0 with the aid of an external impulse and/or an external stimulus, e.g., by suddenly increasing the injection quantity of fuel into the at least one cylinder. The system, which includes the arrangement of at least two cylinders, responds to this abrupt excitation 134 with the aid of an abrupt response 136 which may be characterized by a down time 138 and a subsequent rise time 140. In particular, down time 138 and/or rise time 140 of abrupt response 136 of the system are suitable parameters for the dynamic characterization. For this purpose, rise time t.sub.10-63, i.e., the time within which the differential signal between the maximum lambda value and the minimum lambda value increases from 10% to 63% may be used, for example. Alternatively, a rise time which is different therefrom may be used, e.g., rise time t.sub.10-90 which is the time within which the differential signal rises from 10% to 90%. The measured values ascertained therefrom may be compared with at least one predefined value, the predefined value being in particular retrievable from a characteristics map and/or a function. The dynamic characterization of the system ascertained in this way makes possible the determination of diagnostic threshold 132 as a function of the dynamic of the system, whereby range 122 may be considerably delimited by potential erroneous detections.

(11) FIG. 4 schematically illustrates the phenomenon of so-called lambda 1 ripple 142 which shows a non-monotone evaluating signal at lambda=1. If however, as is furthermore provided according to the present invention, a diagnosis of the system, which includes the internal combustion engine, is carried out at a lambda value of lambda?1, a monotone evaluating signal 144 without the lambda 1 ripple is obtained, since a lambda 1 sequence does not take place for a lambda value of lambda?1.

(12) In FIG. 5, a frequency spectrum of the lambda value is schematically illustrated as a function of frequency f. Here, n identifies the engine speed, the following spectral components being typically observable: at U0, i.e., at half of engine speed 1/2 n, at U1, i.e., at engine speed n, at 3/2 at engine speed 3/2 n, possibly further harmonic waves, the dynamic pressure dependence DDA at twice the engine speed 2n, as well as an interference signal HEK which is caused by the heating element of the lambda sensor which is acted on by a pulse-width-modulated voltage PWM at a frequency f.sub.PWM.

(13) By using a device for filtering, one or multiple desirable spectral components may be filtered out from frequency spectrum 146 and/or undesirable spectral components are suppressed. Alternatively or additionally, one or multiple spectral components, also desirable spectral components, may be selected or undesirable spectral components may be suppressed with the aid of an algorithm, in particular a Fourier transform, a discrete Fourier transform, a fast Fourier transform and/or a Goertzel algorithm.