FR2739139A1 - Diagnostic device for determining efficiency of exhaust gas catalyser - Google Patents

Abstract

The diagnostic device takes a signal (S1) from an oxygen probe (10) downstream of a catalyser (4) which receives exhaust from two rows of cylinders (2a,2b). The signals may give a false indication of catalyser efficiency due to phase opposition in the two exhaust channels. A better indicator may be obtained by producing a sliding average of the signal variations which is then passed through a low pass filter. The low pass filter has a time constant which varies inversely with the instantaneous value of the original signal (S1) and the final indicating signal decreases more slowly than the sliding average.

Description

 The invention relates to the diagnosis of the efficiency of exhaust gas catalysts of a combustion engine with two rows of cylinders.

 It relates more particularly to engines of this type in which the fuel richness of the gaseous mixture admitted into the cylinders is regulated by means of two probes respectively placed in two exhaust ducts connecting the two rows of cylinders to a main catalyst, a precatalyst possibly being arranged on at least one of the two conduits, downstream of the corresponding probe.

 This regulation causes a variation in the oxygen rate of the exhaust gases, according to a frequency of the order of one hertz, so that each catalyst acts on these gases alternately as a reducer of nitrogen oxides and by oxidizing d 'hydrocarbons and carbon monoxide, by damping the variation of the oxygen rate. An oxygen sensor placed downstream of the main catalyst makes it possible to diagnose the effectiveness of the latter: if the signal amplitude of this downstream sensor is very low, the catalyst is considered to be in good condition, and if this amplitude is greater than a determined threshold, it is considered defective.

 In the case of an exhaust line with two conduits where at least one includes a precatalyst, the probe placed downstream of the catalyst is generally sufficient to diagnose all of the catalysts.

However, it may happen that despite the possible absence of damping of the oxygen rate variations by this set of defective catalysts, these variations are at times in phase opposition in the two conduits, from which it follows that the probe downstream then captures only a very small amplitude of variations and does not detect the defect, however real. This situation occurs periodically, randomly and for a more or less long period of the order of a few seconds, the probe emits a variable signal corresponding successively to good and poor efficiency of the catalysts.

 The invention aims to overcome this drawback by means of a particular processing of the signal emitted by the downstream probe, processing such that the diagnosis in question is little influenced by this signal during the abovementioned phase opposition periods.

 It thus relates to a device for diagnosing the efficiency of an exhaust gas catalyst of a combustion engine with two rows of cylinders and two exhaust ducts connecting these two rows of cylinders to the catalyst, the device comprising , downstream of the catalyst, a probe emitting a signal which varies with the oxygen level of the gases leaving the catalyst, and this signal being processed electrically so as to bring its average value to zero, then to rectify it and to filter it for obtain an indicator representing a sliding average of the amplitude of the variations of the transmitted signal.

 According to the invention, this sliding average is filtered by a low-pass filter, the time constant of which varies as a function of the instantaneous value of the signal emitted by the probe or of a signal obtained by processing the emitted signal, this time constant being all the longer as the variation of the transmitted signal is of lower amplitude, so as to obtain a final indicator whose decreases as a function of time are smaller than those of the first indicator.

An embodiment of the diagnostic device according to the invention is described below, with reference to the appended drawings among which:
- Figure 1 is a schematic view of a combustion engine equipped with the device;
- Figure 2 is a graph of the signal emitted by a probe of the device;
FIG. 3 is a graph of a signal obtained by known processing of said transmitted signal,
FIG. 4 is a graph of values of time constants used in the processing according to the invention,
- And Figure 5 is a graph of an indicator resulting from the processing according to the invention.

 FIG. 1 shows a combustion engine 1 with two rows of cylinders 2a and 2b and with two exhaust ducts 3a and 3b connecting these two rows to the same main catalyst 4. In each of these ducts is arranged a precatalyst 5a , 5b upstream of which an oxygen probe 6a, 6b is placed, the signal of which is transmitted to a computer 7a, 7b controlling fuel injectors 8a, 8b so as to regulate the richness of the mixture admitted into the cylinders.

As a variant, the exhaust line may comprise a precatalyst only on one of the two conduits, or may not include any precatalyst.

 This regulation results in a permanent variation - with a frequency of the order of one hertz - of the oxygen rate in each exhaust duct and consequently in a common duct 9 for entry into the main catalyst 4.

 An oxygen sensor 10 is placed downstream of this catalyst 4 to diagnose its effectiveness. If it is in good condition, its alternative action of reducing and oxidizing gases entering it through line 9 allows it to attenuate -or dampen-, if not to cancel, the variations in the oxygen rate of these gases. If it is defective, this damping is insufficient, and the gases which exit from it have an oxygen rate whose variations are relatively large. The downstream probe 10 captures these variations as a function of time t, as shown in the graph in the figure. 2, where we see strong variations in amplitude of the signal S1 emitted by this probe in periods T1 and T3, and a significant attenuation of this amplitude in the period T2, due to a temporary phase opposition of the rate variations oxygen in the two exhaust pipes.

 The signal S1 emitted by the probe is in a known manner electrically processed by high-pass filtering bringing its average value to zero, then by rectification and low-pass filtering producing a signal -or indicator- S2 (FIG. 3) corresponding to a sliding average of the amplitude of the signal S1. If this filtering is unimportant, a very unstable signal S2 is obtained as a function of the abovementioned phase oppositions. If we want to limit these instabilities of signal S2, we must filter strongly, but in this case the difference in values of this signal between good catalysts and deteriorated catalysts is reduced; the measurement dispersion and the different tolerances of the components involved can then disturb the result of the diagnosis.

 To avoid this drawback, according to the invention an additional processing is applied to the signal S2, consisting of filtering the latter by a low-pass filter whose time constant is a function of the instantaneous value of the transmitted signal S1 or of a signal such as S2 obtained by processing the signal S1. This last filtering is weak, that is to say that its time constant is short - less than or equal to approximately 10 seconds - in periods such as T1 or T3 where the variations of the signal emitted S1 are significant, and the filtering is all the stronger, that is to say that its time constant is all the longer - of the order of 30 to 50 seconds - that the variations of the signal emitted S1 are of weaker amplitude, as in the T2 period.

 FIG. 4 shows such a variation of the time constant C in question for the periods considered T1, T2, T3, from which results a signal -or indicator- S3 (FIG. 5) of which each decrease is much slower than that of the corresponding S2 indicator since taking into account the previous period T1 large variations in the signal emitted by the probe.

 This indicator S3 thus constitutes a sliding average of the amplitudes of the signal S1, like the indicator S2 but with a much weaker influence of the attenuations of variations of signals S1 due to the oppositions of temporary phases.

 The indicator S3 is finally compared with a determined threshold H calibrated in the computer, if it is greater than this threshold, this means that the amplitude of the signal emitted by the downstream probe is excessive and that the catalysts do not dampen sufficiently the variations in the oxygen rate of the gases passing through them, and are therefore defective.

As a variant, the time constant C of the filtering of the indicator S2 is defined as a function of the instantaneous value of the derivative of one of the signals S1 or S2 with respect to time, and instead of applying to the signal
S2, it can be applied to a report of signals emitted respectively by the downstream probe and by at least one of the two upstream probes 6a and 6b.

Claims (1)

CLAIM  1 - Device for diagnosing the efficiency of an exhaust gas catalyst (4) of a combustion engine (1) with two rows of cylinders and two exhaust ducts (3a, 3b) connecting to the catalyst these two rows of cylinders, the device comprising, downstream of the catalyst (4), a probe (10) emitting a signal (S1) which varies with the oxygen level of the gases leaving the catalyst, and this signal being electrically processed so as to bring its average value to zero, then to rectify it and to filter it to obtain an indicator (S2) representing a sliding average of the amplitude of the variations of the transmitted signal (S1), characterized in that this sliding average (S2 ) is filtered by a low-pass filter whose time constant (C) varies as a function of the instantaneous value of the signal (S1) emitted by the probe (10) or of a signal (S2) obtained by processing the emitted signal , this time constant (C) being all the longer as the variation of the sign l emitted (S1) is of lower amplitude, so as to obtain a final indicator (S3) whose decreases as a function of time are smaller than those of the first indicator (S2).