Method for adjusting the richness of a spark-ignition internal combustion engine equipped with a three-way catalyst

Abstract

This method for adjusting the richness of a spark-ignition internal combustion engine (1) equipped with a three-way type catalyst (5) configured to treat pollutant emissions from the engine (1), comprises the steps of : - modulation of the richness at a first predetermined frequency and with a predetermined richness amplitude centred around a constant mean richness; - monitoring of the voltage at the terminals of a binary richness probe (7b) positioned downstream of the catalyst (5) and identification of voltage peaks representative of a leak; - calculation of a second frequency representative of a frequency of occurrence of the identified voltage peaks; - comparison between the second frequency and the first frequency; and - reduction of the richness amplitude when the absolute value of the difference between the second and first frequencies is less than or equal to a predetermined threshold value.

Description

[0001] DESCRIPTION

[0002] TITLE : Method for adjusting the richness of a spark-ignition internal combustion engine equipped with a three-way catalyst

[0003] Technical area

[0004] The technical field of the invention is the depollution of exhaust gases from a spark-ignition internal combustion engine equipped with a three-way catalyst.

[0005] Previous techniques

[0006] In internal combustion engines of the spark-ignition type, used in particular in motor vehicles and running on petrol, it is known that the engine's combustion gases are depolluted by means of a three-way catalyst fitted to the engine exhaust.

[0007] Such devices for treating pollutant emissions make it possible to reduce nitrogen oxide (NOx) molecules and oxidise unburnt hydrocarbon (HC) and carbon monoxide (CO) molecules emitted by the engine during its operation.

[0008] Typically, two lambda probes quantify the oxygen concentration respectively upstream and downstream of the catalyst while the richness varies between rich and lean compositions at a given frequency and amplitude, in order to stimulate the conversion of pollutants within the catalyst and achieve optimum efficiency in the treatment of pollutant emissions .

[0009] This richness modulation strategy works correctly as long as the catalyst has sufficient oxygen storage capacity (known by its acronym OSC), defined by a lower limit called OS_min and an upper limit called OS_max. When the oxygen stored in the catalyst (known by its acronym OS for "Oxygen Storage") is between these two limits, the catalyst is capable of correctly treating polluting emissions.

[0010] If the OS is below the lower limit OS_min, there is no longer enough oxygen in the catalyst to treat the rich species, in particular the CO emissions . If the OS is greater than the upper limit OS_max, there is too much oxygen in the catalyst to treat the poor species, in particular NOx emissions .

[0011] The OSC value of a catalyst decreases as it ages .

[0012] It is therefore understandable that it is not possible to adopt the same richness modulation amplitude for a new catalyst as for an aged catalyst.

[0013] In current systems, the richness modulation amplitude is generally chosen to optimise the treatment of polluting emissions when the catalysts have aged. Depollution is therefore not optimal over the entire life of the catalysts, particularly when the catalysts are new.

[0014] Explanation of the invention

[0015] In view of the above, the invention aims to provide a method for adjusting the richness of a spark-ignition type internal combustion engine equipped with a three-way catalyst which optimises the treatment of polluting emissions over the entire life of the catalyst.

[0016] The invention relates to a method for adjusting the richness of a spark-ignition internal combustion engine equipped with a three-way type catalyst configured to treat pollutant emissions from the engine. The method comprises the steps of : modulation of the richness at a first predetermined frequency and with a predetermined richness amplitude centred around a constant mean richness, so as to produce an alternation of engine operating phases in rich mode and in lean mode; monitoring of the voltage at the terminals of a binary richness probe arranged downstream of the catalyst and identification of voltage peaks representative of a leak when the voltage at the terminals of the probe repeatedly exceeds a predetermined threshold voltage greater than a predetermined nominal voltage of the probe; calculation of a second frequency representative of a frequency of occurrence of the identified voltage peaks ; comparison between the second frequency and the first frequency; and reduction of the richness amplitude when the absolute value of the difference between the second and first frequencies is less than or equal to a predetermined threshold value, so as to eliminate voltage peaks .

[0017] This process optimises the treatment of pollutant emissions over the entire life of the catalyst.

[0018] According to one feature, the average richness has a value of between 1 ,001 and , .002, and preferably is equal to 1 ,0015. This average richness value represents a very good compromise for the treatment of NOx and CO.

[0019] Advantageously, the voltage is monitored over stabilised motor operating phases.

[0020] According to another feature, the engine is operated in rich mode and lean mode respectively with a first constant richness and a second constant richness, the second richness being lower than the first richness, the difference between the first richness and the second richness corresponding to the richness amplitude. This type of operation allows linear variation in the OS of the catalyst.

[0021] Preferably, the richness amplitude is adjusted as a function of engine speed and load. This type of richness adjustment provides optimum treatment of pollutant emissions .

[0022] For example, the second frequency is calculated from a frequency analysis of the voltage signal at the probe terminals .

[0023] According to another aspect, the invention relates to an internal combustion engine equipped with a three-way type catalyst configured to treat pollutant emissions from the engine, said engine further comprising an electronic control unit configured to implement a process as defined above.

[0024] Another aspect of the invention relates to a motor vehicle equipped with an engine as defined above.

[0025] Brief description of the drawings Other aims, features and advantages of the invention will become apparent from the following description, given solely by way of nonlimiting example, and made with reference to the appended drawings in which :

[0026] [Fig. 1 ] is a schematic view of an engine according to one example of embodiment of the invention;

[0027] [Fig 2] illustrates the richness / voltage feature of a binary richness probe on the engine in [Fig 1 ] ;

[0028] [Fig. 3] is a flow chart illustrating the various stages of a process for adjusting the richness of the engine of [Fig. 1 ] according to an example of an embodiment of the invention; and

[0029] [Fig 4] to [Fig 6] illustrate an example of implementation of the process described in [Fig 3] .

[0030] Detailed description of at least one embodiment

[0031] In the example shown in Figure 1 , the internal combustion engine 1 is of the atmospheric spark ignition type. In an embodiment not shown, the engine 1 can be supercharged, for example by a turbocharger or a positive displacement compressor.

[0032] The engine 1 draws in air in the direction of arrow E via an intake pipe 2, and discharges its exhaust gases via an exhaust pipe 3 to a depollution device 4.

[0033] The depollution device 4 comprises a three-way catalyst 5 and preferably a particulate filter 6. In the example shown in Figure 1 , the particulate filter 6 is housed in the same after-treatment volume as the three-way catalyst 5 to promote its temperature rise. Alternatively, the particulate filter 6 may not be housed in the same post-treatment volume as the catalyst 5.

[0034] Upstream of the depollution device 4, the exhaust pipe 3 is fitted with a proportional richness probe 7a for regulating the richness of the air-fuel mixture in the engine 1.

[0035] Downstream of the depollution device 4, the exhaust pipe 3 is fitted with a binary richness probe 7b which, in a manner known per se, returns a voltage value as a function of the richness level (Figure 2) . For example, the richness probe 7b can be set to return a nominal voltage of 700 mV, equivalent to a richness of 1 ,0015, which optimises the treatment of NOx and CO emissions.

[0036] The fuel, for example petrol, a mixture of petrol and ethanol, or even pure ethanol, is fed to the engine 1 by means of an injection system (not shown), for example a direct injection system, which comprises a feed rail common to the cylinders 8 and at least one fuel injector per cylinder capable of injecting the fuel directly into each of the cylinders 8. In the example shown in Figure 1 , the engine has three cylinders arranged in line. For example, the engine 1 may comprise a different number of cylinders . Other cylinder configurations may be envisaged, without departing from the scope of the invention.

[0037] At the outlet of exhaust circuit 3 , the exhaust gases are discharged into the outside atmosphere in the direction of arrow S .

[0038] In the air intake duct 2, in a non-limiting way, an air filter 9 can be found to eliminate the dust contained in the air and an intake flap 10, or throttle box 10 which regulates the flow admitted to the engine 1 by more or less obstructing the intake duct 2. The air is distributed to the cylinders 8 via an intake manifold 1 1 located downstream of the throttle 10.

[0039] In the example shown in Figure 1 , an exhaust manifold 12 is integrated directly into a cylinder head 13 of the engine 1 in order to minimise the distance between the exhaust valves of the cylinders 8 and the catalyst 5. The catalyst 5 is thus positioned close to an exhaust outlet 3a of the cylinder head 13. This configuration allows the temperature of the catalyst to rise rapidly, maximising the number of calories available for heating the catalyst 5. Alternatively, the exhaust manifold 12 can be positioned outside the cylinder head 13.

[0040] The engine 1 also includes an electronic control unit 14 configured to control the various elements of the engine 1 on the basis of data collected by sensors at different points on the engine.

[0041] The electronic control unit 14 comprises a calculation module 15, a measurement module 16 and a control module 17. The measurement module 16 can, for example, receive measurements from the richness probes 7a and 7b.

[0042] The control module 17 is able, for example, to control the fuel injection system and the opening and closing of the throttle body 10 in order to adjust the richness value of the air-fuel mixture of the engine 1 to a set value.

[0043] Figure 3 illustrates the various stages of a procedure for adjusting the richness of engine 1 according to one example of the invention.

[0044] In particular, the process is implemented by means of an electronic control unit 14 of the engine 1.

[0045] The process begins with a step 20 of modulating the richness at a first predetermined frequency F l and with a predetermined richness amplitude A centred around a constant mean richness Rmoy, so as to achieve an alternation of phases of operation of the engine in rich mode and in lean mode. As previously indicated, this alternation of engine operating phases in rich and lean mode makes it possible to stimulate the conversion of pollutants within the catalyst 5 and thus to increase the efficiency of the treatment of polluting emissions .

[0046] For example, the average richness value Rmoy has a value between 1 ,001 and 1 ,002, and preferably is equal to 1 ,0015. According to an example illustrated in Figures 4 to 6, the average richness value Rmoy is equal to 1 .

[0047] Advantageously, the value of the richness amplitude A is adjusted as a function of the rotation speed and load of the engine 1.

[0048] According to one embodiment, the engine is operated in rich mode and lean mode respectively at a first constant richness R 1 and a second constant richness R2 that is lower than the first richness R1 (Figure 4) . The difference between the first richness R1 and the second richness R2 corresponds to the richness amplitude A

[0049] The process continues with a step 21 of monitoring the voltage at the terminals of the binary richness probe 7b arranged downstream of the catalyst 5, and identifying voltage peaks representative of a leak when the voltage at the terminals of the probe 7b repeatedly reaches a predetermined threshold voltage greater than the predetermined nominal voltage of the probe 7b. For example, the predetermined nominal voltage may have a value of about 700 mV and the predetermined threshold voltage may have a value of about 850 mV. Preferably, the voltage at the terminals of probe 7b is monitored during stabilised operating phases of engine 1 .

[0050] After step 21 for identifying voltage peaks, the electronic control unit 14 calculates a second frequency F2 representative a frequency of appearance of the identified voltage peaks (step 22) . For example, the second frequency F2 is calculated from a frequency analysis of the voltage signal at the terminals of the probe 7b.

[0051] In the next comparison step 23, the second frequency F2 is compared with the first frequency F l .

[0052] If the absolute value of the difference between the second and first frequencies F2, Fl is less than or equal to a predetermined threshold value a, the richness amplitude A is reduced so that the voltage peaks disappear (step 24) .

[0053] If the absolute value of the difference between the second and first frequencies F2, F l is greater than the predetermined threshold value a, the process resumes at step 21 for monitoring the voltage of the probe 7b .

[0054] Figures 4 to 6 illustrate an example of the process described in Figure 3. It should be noted that identical or similar elements bear the same references from one figure to another.

[0055] Figure 4 is an example of the richness modulation of engine 1 associated with a new catalyst 5. Curve 30 illustrates the voltage in the terminals of the binary richness probe 7b. Curve 31 represents the variation over time in the quantity of oxygen stored OS in the catalyst 5. Curve 32 illustrates the variation over time of the richness upstream of the catalyst 5 as measured by the proportional richness probe 7a.

[0056] The richness upstream of the catalyst 5 is controlled by the electronic control unit 14 so that the engine alternates between rich and lean operating phases. The engine alternates between rich and lean operating phases at a period dt equivalent to a frequency F l equal to 1 / dt

[0057] In this case, richness slots are made with an amplitude A equal to 0, 1 centred around a unitary average richness Rmoy. This value of amplitude A makes it possible to keep the OS level of the new catalyst outside the lean 33 and rich 34 leakage zones . It should be noted that the value of the amplitude A can be adjusted as a function of the engine speed and load of engine 1 in order to optimise the treatment of pollutant emissions .

[0058] Figure 5 shows the effect of catalyst 5 ageing on the OS value in the catalyst. As illustrated in figure 5, in the case of an aged catalyst the OSC is smaller and the leakage zones 33 and 34 are closer together. By keeping the same frequency F l and amplitude A modulation as when the catalyst is new, it can be seen that the OS periodically re-enters the leakage zone 34 and voltage peaks 35 appear at the terminals of probe 7b. Analysis of the voltage signal at the terminals of probe 7b shows that the frequency F2 of the voltage peaks 35 corresponds to the frequency F l of the richness modulation. In fact, the same period dt is observed for both the richness modulation and the appearance of the voltage peaks 35.

[0059] Figure 6 shows the effect of reducing the richness amplitude A on the OS value in the aged catalyst 5. According to the method, the richness amplitude A is reduced so as to eliminate the voltage peaks 35. Here, it can be seen that a reduction in the amplitude A of 0,06 makes it possible to eliminate the voltage peaks 35 and return the OS value of the catalyst 5 to outside the leakage zone 34

Claims

CLAIMS1. Method for adjusting the richness of a spark-ignition internal combustion engine ( 1 ) equipped with a catalyst (5) of the three-way type configured to treat pollutant emissions from the engine ( 1 ), characterised in that the method comprises steps of : modulation of the richness at a first predetermined frequency (Fl ) and with a predetermined richness amplitude (A) centred around a constant mean richness (Rmoy), so as to produce an alternation of engine operating phases in rich mode and in lean mode; monitoring of the voltage at the terminals of a binary richness probe (7b) arranged downstream of the catalyst (5) and identification of voltage peaks (35) representative of a leak when the voltage at the terminals of the probe (7b) repeatedly exceeds a predetermined threshold voltage greater than a predetermined nominal voltage of the probe (7b); calculation of a second frequency (F2) representative of a frequency of occurrence of the identified voltage peaks (35); comparison between the second frequency (F2) and the first frequency (Fl ); and reduction of the richness amplitude (A) when the absolute value of the difference between the second and first frequencies (F2, F l ) is less than or equal to a predetermined threshold value (a), so as to make the voltage peaks (35) disappear.

2. Method according to claim 1 , in which the average richness (Rmoy) has a value of between 1 ,001 and 1 ,002, and preferably is equal to 1 ,0015.

3. Method according to claim 1 or 2, in which the voltage is monitored over stabilised operating phases of the motor ( 1 ).

4. Method according to any one of claims 1 to 3 , in which the engine is operated in rich mode and lean mode respectively at a first constant richness (Rl ) and at a second constant richness (R2), the second richness (R2) being lower than the first richness (R l ), thedifference between the first richness and the second richness corresponding to the richness amplitude (A) .

5. Method according to any one of claims 1 to 4, in which the richness amplitude (A) is adjusted as a function of the rotation speed and the load of the engine ( 1 ) .

6. Method according to any one of claims 1 to 5 , in which the second frequency (F2) is calculated from a frequency analysis of the voltage signal at the terminals of the probe (7b) .

7. Internal combustion engine ( 1 ) equipped with a three-way type catalyst (5) configured to treat pollutant emissions from the engine( 1 ), said engine ( 1 ) further comprising an electronic control unit ( 14) configured to implement a method according to any one of claims 1 to 6.

8. Motor vehicle equipped with an engine ( 1 ) according to claim 7.

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