WO2006079407A1

WO2006079407A1 - Secondary air diagnosis in an internal combustion engine

Info

Publication number: WO2006079407A1
Application number: PCT/EP2005/013871
Authority: WO
Inventors: Michael-Rainer Busch; Rudolf Klein; Johannes Koenders
Original assignee: Daimlerchrysler Ag
Priority date: 2005-01-26
Filing date: 2005-12-22
Publication date: 2006-08-03
Also published as: DE102005003591A1

Abstract

The invention relates to a method and device for monitoring a secondary air system of an exhaust system mounted on a motor vehicle. The aim of said invention is to improve the recognition of tightness loss and disturbances in the secondary air system of an exhaust system mounted on a motor vehicle. For this purpose, the inventive method consists in detecting the gas pressure time sequences between a secondary air pump and a catalyst by means of a pressure sensor (block 1), in carrying out a frequency analysis of the gas pressure time sequences (block 5) and in determining the disturbances in the secondary air system by evaluating the frequency analysis.

Description

Secondary air diagnosis of an internal combustion engine

The invention relates to a method according to the preamble of claim 1 and a device according to the preamble of claim 6.

From DE 102 05 966 A1 a method for monitoring a secondary air system comprising a secondary air pump of an internal combustion engine is known, in which a signal characterizing the secondary air flow is determined. For this purpose, at least one signal of at least one operating characteristic of the secondary air pump is determined during the determination of the signal characterizing the secondary air flow, and it is concluded that there is an error in the secondary air system if the signal of the at least one operating parameter lies outside a predefinable interval.

The object of the present invention is to improve the detection of leaks and malfunctions of a secondary air system of an exhaust system arranged in a motor vehicle.

This object is achieved by a method having the features of claim 1 and an apparatus having the features of claim 6. The inventive method is characterized in that between a secondary air pump and a catalyst, the gas pressure is detected in its time course by means of a pressure sensor, that a frequency analysis of the time course of the gas pressure is performed and that an error in the secondary air system is determined by evaluating the frequency analysis. Thus, a fault in the secondary air system can be detected solely by the evaluation of the gas pressure. A recording and evaluation of operating variables of the secondary air system is not necessary.

A frequency analysis resolves the time course of the gas pressure into its periodic components. In this case, the largest periodic proportion of the combustion process in the cylinders of the internal combustion engine of the motor vehicle. This vibration is changed by the geometry of the exhaust system. In addition, there is a vibration damping overlay by the uniform air flow of the secondary air supply. In addition, there are further overlays due to further influencing factors. One of these further influencing variables may be a leak in the region between the secondary air pump and the catalytic converter. A frequency analysis allows to differentiate between the different superimposed parameters by separating the different periodic components. Thus, a change in the time course of the curve after frequency analysis of a causative influence factor can be assigned. Since even the smallest holes or leaks change the temporal pressure curve, the device is suitable for detecting even the smallest leaks in the secondary air system.

In one embodiment of the method, the frequency analysis method is a Fourier analysis. The Fourier analysis is an accurate and secure way to perform a frequency analysis.

In one embodiment, a frequency range to be considered is selected as a function of an engine speed and an ambient air pressure, and the maximum value of the gas pressure in this frequency range is determined. The highest pressure value of the frequency analysis is in the range of the frequency of the power strokes of the engine and is thus dependent on the engine speed. , Thus, the frequency range in which the highest pressure value is located, in a simple manner can be limited.

In one embodiment of the method, this maximum value of the gas pressure is compared with a threshold value and when the threshold value is undershot, a leak in the secondary air system is detected. This makes it possible to detect even the smallest holes in the area between the secondary air pump and the secondary air valve as well as between the secondary air valve and the catalyst.

In one embodiment of the method, a sliding time average of the gas pressure is additionally formed, this mean value is compared with a threshold value and the secondary air pump is detected as defective if the mean value is smaller than the threshold value.

The sliding time average is understood to be an average value over a time interval in which the interval is shifted by one or more measuring points during each averaging, but overlaps with the preceding time interval. Typically, in the present case, the moving average is formed over a period corresponding to a crankshaft angle of the engine of 720 ° ent. In this period have at an internal combustion - machine all cylinders once worked. This minimizes fluctuations in the mean value due to unequal cylinder work. The mean value of the gas pressure is dependent on secondary air mass flow rate. A decrease in the gas pressure can be interpreted as a drop in the secondary air mass flow rate. This means that the secondary air pump is not working anymore. So it can be set a threshold below which the secondary air pump is considered defective. This makes it possible to detect a malfunction of the secondary air pump only with a pressure sensor and an evaluation unit connected thereto. Information about operating parameters of the secondary air pump is not necessary.

The device according to the invention is characterized in that a pressure sensor for detecting the time profile of the gas pressure between the secondary air supply and catalyst and an evaluation unit connected to the pressure sensor are provided, that a frequency analysis unit associated with the evaluation unit is provided for transforming the time profile of the gas pressure detected by the pressure sensor and means are provided for detecting an error by evaluating the frequency analysis.

The time profile of the gas pressure can be detected in a variant A between a secondary air pump and a secondary air valve or in a variant B between the secondary air valve and a catalyst. Variant A has the advantage that the pressure sensor is exposed to lower thermal loads. Variant B, on the other hand, has the advantage that the signals are easier to evaluate, since the signals are larger and small disturbances are better recognizable. The curve of the temporal pressure curve results from a superposition of different influences. Of course, the main influence is the combustion process in the cylinders of the internal combustion engine of the motor vehicle and the geometry of the exhaust system. In addition, there is an overlay by the secondary air supply and further overlays by other influencing variables. One of these further influencing variables may be a leak in the region between the secondary air pump and the catalytic converter.

A frequency analysis of the time course of the gas pressure makes it possible to separate the different, superimposed, influencing variables. Thus, a change in the time course of the curve after frequency analysis of a causative influence factor can be assigned. Since even the smallest holes change the time profile of the gas pressure, the device is suitable for detecting even the smallest holes and leaks in the secondary air system.

Further features and combinations of features result from the description as well as the drawings and measuring curves. In the following, the invention is illustrated with reference to the drawing and measurement curves and explained in more detail in the following description. Showing:

Fig. 1 an embodiment of the invention

Method,

Fig. 2 two pressure pulsation curves, Fig. FIG. 3 is an illustration of the dependence of the average gas pressure on the air mass flow rate, FIG. 4 different frequency-analyzed gas pressure curves.

Fig. 1 shows an embodiment of the invention Method. In block 1, the pressure in the exhaust system is recorded. For this purpose, for example, a pressure sensor is provided in a region between the secondary air pump and the catalyst. The pressure sensor records the time course of the gas pressure. For this purpose, the gas pressure is recorded continuously or discontinuously. It can also be provided several pressure sensors for gas pressure detection.

In the simplest embodiment of the method according to the invention, the result of the pressure detection in block 5 is supplied to a frequency analysis. This frequency analysis may be performed as a Fourier analysis or the like. Likewise, mathematically simpler analysis methods can also be used. Finally, the highest peak of the frequency analysis is determined and its maximum value pmax determined.

In block 6, pmax is compared to a threshold S_pmax. This threshold depends on the design of the exhaust system and engine and is determined individually for each vehicle or each type.

If pmax is less than S_pmax then there is a leak. In this case, it is detected in block 7 that there is a leak in the secondary air system. This information can also be read out externally.

If pmax is greater than S_pmax then there is no leak. In this case, it is detected in block 7 that the secondary air system is faultless (ok). This information can also be read out externally.

In the in Fig. 1 embodiment shown, the blocks are preceded by blocks 5 to 8, the blocks 1 to 4. By blocks 1 to 4 is checked whether the secondary air pump is working properly.

In block 2, a moving average value pmitt of the pressure is determined from the data of the pressure value detection of block 1. Moving mean value means that the mean value is recalculated again and again, whereby the values for the mean value calculation lie in a time interval, which is shifted by the average time since the last calculation in each average value calculation.

The length of the time interval is typically selected to correspond to a time the engine needs at the current engine speed to rotate the crankshaft through an angle of 720 °.

In block 3, the current mean value pmitt is compared with a threshold value S_pmitt. If the threshold falls below, then the secondary air pump is faulty.

If pmitt is smaller than S_pmitt, then there is a fault of the secondary air pump. In this case, it is detected in block 4 that there is an error in the secondary air pump. This information can also be read out externally.

If pmitt is greater than S_pmitt, then there is no error. Block 5 records accordingly that the secondary air system is operating correctly. This information can also be read out externally in one embodiment.

In the in Fig. 1, the blocks 2 and 3 are arranged between block 1 and block 5. However, other embodiments of the invention are possible. For example, blocks 2 to 4 may go to block 6 be arranged. In particular, the blocks 2 and 3 may be arranged between block 6 and block 8.

The blocks 2 to 4 can also be arranged downstream of the blocks 5 to 8 in an alternative embodiment.

Fig. 2 shows the temporal gas pressure profile at a pressure sensor. In this case, curve 9 shows the pressure curve at a pressure sensor, which is arranged between the secondary air pump and secondary air valve in the vicinity of the secondary air valve. Curve 10 shows the pressure curve at a pressure sensor which is arranged between the secondary air valve and the catalytic converter. It can be seen that at the measuring location of the curve 10, the pressure pulsations are significantly more pronounced than at the measuring location of the curve 9. In this case, the temporal change of the gas pressure at the measuring location is understood as pressure pulsations.

Fig. 3 shows how the mass flow rate of the secondary air pump affects the averaged gas pressure at the measuring location. In this case, the gas pressure is averaged over a firmly selected in its width sliding interval. This fixed width is typically 720 ° crankshaft angle rotation, but may be chosen differently (e.g., 360 ° crankshaft angle or fixed time intervals). In this case, the time interval required to rotate the crankshaft through an angle of 720 °, depending on the speed of the engine, be different lengths.

Fig. 3 shows that the mean gas pressure pmitt at the measuring location decreases as the mass flow rate of the secondary air pump decreases. An error in the secondary air pump can therefore be detected on the basis of the average gas pressure pmitt at the measuring location. It can to a threshold S_jpmitt for the average gas pressure pmitt are selected, below which the secondary air pump is detected as faulty.

Fig. 4 shows a schematic representation of various frequency-analyzed gas pressure curves. As a database for carrying out the frequency analysis, the measured value interval is preferably considered, which was also selected for calculating the moving average value. The result of the frequency analysis shows a main maximum, a limited frequency range, which is associated with a particularly high pressure. Such maxima are referred to in the present document as a peak. Further secondary peaks are outside the frequency range shown in FIG. In Fig. 4, various curves 11 through 15 are shown.

Curve 11 shows the frequency-analyzed pressure curve at the measuring location with 100% working secondary air pump.

Curve 12 shows the frequency-analyzed pressure curve at the measuring location with the air mass flow rate of the secondary air pump reduced to 64%. Since the lower air mass flow rate of the secondary air pump less dampens the pressure fluctuations caused by the engine, the main peak is increased compared to the curve 11.

Curve 13 shows the frequency-analyzed pressure curve at the measuring location with an air mass flow rate of the secondary air pump reduced to 47%. Since the lower air mass flow rate of the secondary air pump attenuates the pressure fluctuations caused by the engine less, the main peak is increased compared to the curve 11 and the curve 12.

Curve 14 shows the frequency-analyzed pressure curve at the measuring location with 100% working secondary air pump in the presence a leak between secondary air valve and catalyst. The peak is less high than in curves 11-13.

Curve 15 shows the frequency-analyzed pressure curve at the measuring location with 100% working secondary air pump in the event of a leak between secondary air pump and secondary air valve. The peak is less high than curves 11 through 13. The comparison between curve 14 and curve 15 shows that a leak after the secondary air valve affects the peak height somewhat less than a leak before the secondary air valve.

The process can also be carried out on a peak other than the highest (secondary peak). It must be ensured that only the frequency range corresponding to this secondary peak is considered. The threshold is then set correspondingly lower.

As shown in FIG. 4, the threshold S_pmax is to be selected so that the highest peak at 100% operating secondary air pump is always safely above this threshold S_pmax. If the peak falls below this threshold, a leak in the secondary air system is detected. The entire area between the secondary air pump and the catalytic converter is considered part of the secondary air system.

In extension of the invention, it is conceivable to introduce a threshold S2_pmax above the peak of the curve 11. If this threshold is exceeded, the secondary air pump is considered faulty. Thus, this threshold S2_pmax would correspond in its function to the threshold S_pmitt. It can serve as additional protection of the threshold s_pmitt. Alternatively, the threshold S2_pmax can also replace the threshold S_pmitt. In this case, block 2 of FIG. 1 block 3 is subordinated to block 5 and now checks whether pmax> S2_pmax.

If two independent exhaust systems are present, for example in a V engine, then each exhaust system has a pressure sensor and the evaluation for the two exhaust systems takes place independently of one another.

Claims

claims

1. A method for monitoring a secondary air system of an exhaust system arranged in a motor vehicle, characterized

- That between a secondary air pump and a catalyst, the gas pressure is detected in its time course by means of a pressure sensor (block 1),

- that a frequency analysis of the time course of the gas pressure is performed (block 5)

- And that an error in the secondary air system is determined by evaluation of Freguenzanalyse (block 6).

2. The method according to claim 1, characterized in that the frequency analysis method (block 5) is a Fourier analysis.

3. The method according to claim 1 or 2, characterized

that a frequency range to be considered is selected as a function of an engine speed and an ambient air pressure,

- And that the maximum value (pmax) of the gas pressure in this frequency range is determined (block 5).

4. The method according to claim 3, characterized

that this maximum value (pmax) of the gas pressure is compared with a threshold value (S_pmax) (block 6),

- And that falls below the threshold (S_pmax) a leak in the secondary air system is detected (block 7).

5. The method according to any one of claims 1 to 4, characterized

- that in addition a sliding time average (pmitt) of the gas pressure is formed (block 2),

- That this average (pmitt) with a threshold (S_pmitt) is compared (block 3) and

- That the secondary air pump is detected as faulty (block 4), when the average value (pmitt) is less than the threshold value (S_pmitt).

6. An apparatus for monitoring a secondary air system of an exhaust system arranged in a motor vehicle, wherein the exhaust system comprises a catalytic converter, characterized

that a pressure sensor for detecting the time profile of the gas pressure between the secondary air supply and the catalyst and an evaluation unit connected to the pressure sensor are provided,

a frequency analysis unit assigned to the evaluation unit is provided for transforming the time profile of the gas pressure detected by the pressure sensor,

- That means are provided for detecting an error by evaluating the frequency analysis.