GB2504975A

GB2504975A - Method of controlling a DeSOx regeneration process of a Lean NOx Trap

Info

Publication number: GB2504975A
Application number: GB201214585A
Authority: GB
Inventors: Andrea De Filippo
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2012-08-15
Filing date: 2012-08-15
Publication date: 2014-02-19
Also published as: CN103590913A; GB201214585D0

Abstract

A method of controlling a desulphation process of a Lean NOx Trap (LNT) comprises wobbling of combustion between lean 500 and rich phases. At least one of the rich combustion phases is a stepped phase comprising a rich combustion phase 510 and a very rich combustion phase 520. The step of the stepped phase is realised either: by a first air/fuel ratio (AFR) being followed by a second AFR, the first AFR being lower, i.e. richer, than the second; or, alternatively, by a first exhaust temperature being followed by a second exhaust temperature, the first exhaust temperature being higher than the second. Preferably, there is instantaneous transition 530 from the lean combustion phase to the very rich combustion phase and ramped transition 540,550 from the very rich combustion phase to the rich combustion phase and from the rich combustion phase back to the lean combustion phase. Ideally, the very rich combustion phase is at 700Â°C with an AFR of 0.9 and the rich combustion phase is at 650Â°C with an AFR of 0.95. The method is particularly suited to optimising desulphation of a LNT in an exhaust system of an internal combustion engine running under urban driving conditions.

Description

METHOD OF OPTIMIZING THE DES ULPHA TION STRATEGY OFAN INTERNAL

COMBUSTION ENGINE IN URBAN DRIVING CONDITION

TECHNICAL FIELD

The present disclosure relates to a method of optimizing the desulphatian strategy of an internal combustion engine, particularly the method is dedicated to the sulphur regeneration of a Lean NOx Trap located in the exhaust system of the internal combustion engine, when running under urban driving condition.

BACKGROUND

It is known that the exhaust gas after-treatment systems of a Diesel engine can be provided, among other devices, with a Lean NO Trap (hereafter, also LNT).

A LNT is provided for trapping nitrogen oxides NO contained in the exhaust gas and is located in the exhaust line.

A LNT is a catalytic device containing catalysts, such as Rhodium, Platinum and Palladium, and adsorbents, such as barium based elements, which provide active sites * suitable for binding the nitrogen oxides (NOr) contained in the exhaust gas, in order to trap them within the device itself. a

Lean NO Traps are subjected to periodic regeneration processes, whereby such regeneration processes are generally provided to release and reduce the trapped nitrogen oxides (NOr) from the LNT.

The LNT are operated cyclically, for example by switching the engine from lean-bum operation to operation whereby an excess amount of fuel is available, also referred as rich operation or regeneration phase. During normal operation of the engine, the NO are stored on a catalytic surface. When the engine is switched to rich operation, the NO stored on the adsorbent site react with the reductants in the exhaust gas and are desorbed and converted to nitrogen and ammonia, thereby regenerating the adsorbent site of the catalyst.

Unfortunately, due to the presence of sulphur into the fuel, the LNT is also exposed to the sulphur poisoning, which heavily reduces the NOx storage capacity of the catalyst. In order to re-enable the capability of the after-treatment system, a desulphation (also called DeSOx regeneration or simply DeSOx) is needed. The DeSOx consists in a fast alternation of lean and rich atmosphere (hereafter, wobbling) at high temperature during engine working condition.

However, the known strategy for managing the DeSOx process suffers a remarkable drawback. In fact, there are specific actions which deactivate the DeSOx before its ideally completion, such as disabling the fuel request (cut-off, key off) or going at lower engine speed. These actions are very frequent in urban driving conditions and the consequence is that the DeSOx is almost ineffective because there is no time to remove the sulphur species from the catalyst: the DeSOx mode terminates after just few seconds from its activation which are not sufficient to guarantee the LNT performances again.

Another consequence is that the DeSOx will be eventually requested at every DPF regeneration which implies drawbacks in terms of fuel consumption and C02 emissions in addition to a quicker thermal aging of the catalyst.

Therefore a need exists for a method that optimizes the desulphation strategy, in order to avoid the above drawbacks.

An object of this invention is to provide a method which optimizes the desulphation wobbling strategy, especially during urban driving conditions, improving the effectiveness of the process, thus reducing its whole duration and consequently reducing the risk that the process is aborted.

Another object is to provide an apparatus which allows to perform the above method.

These objects are achieved by a method, by an apparatus, by an engine, by a computer program and computer program product, and by an electromagnetic signal having the features recited in the independent claims.

The dependent claims delineate preferred and/or especially advantageous aspects.

SUMMARY

An embodiment of the disclosure provides a method of controlling a desulphation process of a lean NOx trap, the method comptising a wobbling of combustion lean phases and combustion rich phases, wherein at least one of said rich phases is stepped phases, wherein at least one of said rich phases is stepped and wherein -the step of the at least one of said rich phases is realized by a first value of the air/fuel ratio followed by a second value of the air/fuel ratio and wherein said first value of the air/fuel ratio is lower than said second value of the airlfuel ratio, or the step of the at least one of said rich phases is realized by a first value of the exhaust temperature followed by a second value of said temperature and wherein said first value of the exhaust temperature is higher than said second value of the exhaust temperature.

Consequently, an apparatus is disclosed for controlling a desulphation process of a lean NOx trap, the apparatus comprising: -means for realizing a wobbling of combustion lean phases and combustion rich phases, wherein at least one of said tich phases is stepped and -means for realizing a first value of the air/fuel ratio followed by a second value of the air/fuel ratio and wherein said first value of the air/fuel ratio is lower than said second value of the air/fuel ratio, or -means for realizing a first value of the exhaust temperature followed by a second value of said temperature and wherein said first value of the exhaust temperature is higher than said second value of the exhaust temperature.

An advantage of this embodiment is that it provides a reduction of the time for the release of the sulphur species and an increased amount of the emitted sulphur species, reducing the risk of any interruption of the DeSOx especially during urban driving conditions. This advantage is realized by keeping the air/fuel ratio target value at richer conditions than the current ones for the first portion of the DeSOx rich time. Altematively same effect can be obtained by keeping an exhaust temperature higher than the normally used one for the first portion of the DeSOx rich time. This embodiment, therefore, reduces the risk to require more frequent desulphation processes, thus penalizing the fuel consumption and increasing the ageing of the catalyst.

S

According to a further embodiment of the invention, transitions between combustion lean phases and first step of combustion rich phases have a time duration as close as possible to zero.

An advantage of this, high slew rate of the air/fuel ratio is that it would immediately activate the first step of the rich combustion mode (i.e. the richer combustion mode) and consequently the effectiveness of the desulphatiori process.

According to a further embodiment, transitions from first step of combustion rich phases to second step of combustion rich phases and transitions from second step of combustion rich phases to combustion lean phase are realized by means of a calibrated ramp behavior.

An advantage of this embodiment is that it will take into account and improve the driveability of the vehicle: a step-transition, instead of a ramp, will likely be perceived by the driver.

According to a still further embodiment the actuation of said stepped rich phases is calibrated based either on their number or on a time duration.

An advantage of this embodiment is that limiting richer air/fuel ratio values for a part of the whole DeSOx process is recommended to avoid risk of hardware reliability.

According to another embodiment of the invention, an enabling condition for the method to be applied is a vehicle speed lower than a calibrated threshold.

An advantage of this embodiment is that the method will only be applied when useful, i.e. when the vehicle speed is low, in other words under urban driving conditions.

The method according to one of its aspects can be carried out with the help of a computer program comprising a program-code for carrying out all the steps of the method described above, and in the form of computer program product comprising the computer program.

The computer program product can be embodied as a control apparatus for an internal combustion engine, comprising an Electronic Control Unit (ECU), a data carrier associated to the ECU, and the computer program stored in a data carrier, so that the control apparatus defines the embodiments described in the same way as the method. In this case, when the control apparatus executes the computerprogram all the steps of the method described above are carded out.

The method according to a further aspect can be also embodied as an electromagnetic signal, said signal being modulated to carry a sequence of data bits which represents a computer program to carry out all steps of the method.

A still further aspect of the disclosure provides an internal combustion engine specially arranged for carrying out the method claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an automotive system.

Figure 2 is a section of an internal combustion engine belonging to the automotive system of figure 1.

Figure 3 is a schematic view of the after-treatment system according to the invention.

Figure 4 is a scheme of the DeSOx regeneration phase according to a known strategy.

Figure 5 is a scheme of the DeSOx regeneration phase according to the method of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Some embodiments may include an automotive system 100, as shown in Figures land 2, that includes an internal combustion engine (ICE) 110 having an engine block 120 defining at least one cylinder 125 having a piston 140 coupled to rotate a crankshaft 145.

A cylinder head 130 cooperates with the piston 140 to define a combustion chamber 150.

A fuel and air mixture (not shown) is disposed in the combustion chamber 150 and ignited, resulting in hot expanding exhaust gasses causing reciprocal movement of the piston 140. The fuel is provided by at least one fuel injector 160 and the air through at least one intake port 210. The fuel is provided at high pressure to the fuei injector 160 from a fuel rail 170 in fluid communication with a high pressure fuel pump 180 that increase the pressure of the fuel received a fuel source 190, Each of the cylinders 125 has at least two valves 215, actuated by a camshaft 135 rotating in time with the crankshaft 145. The valves 215 selectively al low air into the combustion chamber 150 from the port 210 and alternately allow exhaust gases to exit through a port 220. In some examples, a cam phaser 155 may selectively vary the timing between the camshaft 135 and the crankshaft 145.

The air may be distributed to the air intake port(s) 210 through an intake manifold 200.

An air intake duct 205 may provide air from the ambient environment to the intake manifold 200. In other embodiments, a throttle body 330 may be provided to regulate the flow of air into the manifold 200. In still other embodiments, a forced air system such as a turbocharger 230, having a compressor 240 rotationally coupled to a turbine 250, may be provided. Rotation of the compressor 240 increases the pressure and temperature of the air in the duct 205 and manifold 200. An intercooler 260 disposed in the duct 205 may reduce the temperature of the air. The turbine 250 rotates by receiving exhaust gases from an exhaust manifold 225 that directs exhaust gases from the exhaust ports 220 and through a series of vanes prior to expansion through the turbine 250. The exhaust gases exit the turbine 250 and are directed into an exhaust system 270. This example shows a variable geometry turbine (VGT) with a VGT actuator 290 arranged to move the vanes to alter the flow of the exhaust gases through the turbine 250. In other embodiments, the turbocharger 230 may be fixed geometry and/or include a waste gate.

The exhaust system 270 may include an exhaust pipe 275 having one or more exhaust aftertreatment devices 280. The aftertreatment devices may be any device configured to change the composition of the exhaust gases. Some examples of aftertreatment devices 280 include, but are not limited to, catalytic converters (two and three way), oxidation catalysts, lean NOx traps 281, hydrocarbon adsorbers, selective catalytic reduction (SCR) systems 282, particulate filters (DPF) or a combination of the last two devices, i.e. selective catalytic reduction system comprising a particulate filter (SCRF) 283. Other embodiments may include an exhaust gas recirculation (EGR) system 300 coupled between the exhaust manifold 225 and the intake manifold 200. The EGR system 300 may include an EGR cooler 310 to reduce the temperature of the exhaust gases in the EGR system 300. An EGR valve 320 regulates a flow of exhaust gases in the EGR system 300.

The automotive system 100 may further include an electronic control unit (ECU) 450 in communication with one or more sensors and/or devices associated with the ICE 110 and equipped with a data carrier 40. The ECU 450 may receive input signals from various sensors configured to generate the signals in proportion to various physical parameters associated with the ICE 110. The sensors include, but are not limited to, a mass airflow and temperature sensor 340, a manifold pressure and temperature sensor 350, a combustion pressure sensor 360, coolant and oil temperature and level sensors 380, a fuel rail pressure sensor 400, a cam position sensor 410, a crank position sensor 420, exhaust pressure and temperature sensors 430, an EGS temperature sensor 440, and an accelerator pedal position sensor 445. Furthermore, the ECU 450 may generate output signals to various control devices that are arranged to control the operation of the ICE 110, including, but not limited to, the fuel injectors 160, the throttle body 330, the EGR Valve 320, the VGT actuator 290, and the cam phaser 155. Note, dashed lines are used to indicate communication between the ECU 450 and the various sensors and devices, but some are omitted for clarity.

Turning now to the ECU 450, this apparatus may include a digital central processing unit (CPU) in communication with a memory system and an interface bus. The CPU is configured to execute instructions stored as a program in the memory system, and send and receive signals to/from the interface bus. The memory system may include various storage types including optical storage, magnetic storage, solid state storage, and other non-volatile memory. The interface bus may be configured to send, receive, and modulate analog and/or digital signals to/from the various sensors and control devices.

The program may embody the methods disclosed herein, allowing the CPU to carryout out the steps of such methods and control the ICE 110.

Turning back to exhaust system 270, the proposed invention relies on ta new strategy to manage the DeSOx process for a Lean NOx Trap 281. Fig. 3 shows the scheme of the aftertreatment system 280 which, advantageously, could also comprise a particulate filter (DPF) 282 to trap particulate emitted in case of a Diesel engine. Upstream and downstream the LNT, air/fuel ratio sensors 283, 284 and temperature sensors 285, 286 can be provided.

Preferably, the LNT catalyst could be positioned as close as possible to the exit of the turbocharger to take advantage of the high temperature conditions which are beneficial for it.

The LNT reduces engine-out exhaust gas constituents (CO and HC) with high efficiency and stores NOx during lean operating conditions. During rich operating conditions, the NOx is released and converted. The LNT works property thanks to presence in the catalyst of barium and aluminum that define its NOx storage capacity during the diesel engine lean operation.

However, due to the presence of sulphur into the fuel, the LNT is exposed to the sulphur poisoning, which heavily reduces the NOx storage capacity of the catalyst. The sulphur contained in the fuel is easily oxidized in lean atmosphere as S02 or SOS. The latter is stored into the barium sites in the form of sulphates, which are more stable compounds than the corresponding nitrates. This process reduces the efficiency of the LNT in terms of NOx Storage: this efficiency can be restored through a desulphation process, which requires high temperature and rich atmosphere.

The DeSOx regeneration is defined as the process which leads to the desuiphation of the Lean NOx Trap. It is critical from the point of view of thermal degradation of the trap, because of the requested high temperature. It is penalizing in terms of fuel consumption as well, because of the additional injected fuel needed to provide a rich atmosphere at the inlet of the Trap. The DeSOx regeneration is based on the "wobbling" concept, that is to say, an alternation of lean combustion modes and rich combustion modes, the latter at higher temperature The DeSOx rich phase is needed to destabilize the chemical links between the sulphur and the barium and/or aluminum, making the barium/aluminum sites free for the NOx and restoring the NOx trapping capabi!ity of the LNT. The DeSOx lean phase is used to maintain a stable and high temperature, to restore the oxygen content in the trap and to bum the HC cumulated in the trap during the rich phase.

In Fig. 4 is represented a known strategy applied to the desulphation process. The wobbling of the DeSOx regeneration begins with a DeSOx lean combustion mode 500, having a calibrated duration (12 s in the example of Fig. 4) and an air/fuel ratio value higher than 1. Then the DeSOx rich combustion mode 510 starts, also with a calibrated duration (15 s in the shown example) and an air/fuel ratio value lower than 1 (0,95 in the example). The wobbling foresees a certain number of completed lean and rich phases (normally higher than 5, depending on the amount of stored sulphur) for the desulphation process to be completed.

With a fixed DeSOx wobbling strategy (Rich/Lean time, air/fuel ratio value, high temp conditions), for the desulphation frequency, there are in principle the following possibilities: a) a DeSOx every DPF regeneration. An advantage of this strategy is to always have low loaded sulphur into the LNT and, consequently, high efficiencies over time. On the other side, the LNT is frequently exposed to high temperature conditions, which accelerate the thermal ageing of the catalyst; b) a DeSOx after a certain fixed mileage, for example after 4000 km. An advantage of this strategy is to have less frequent exposure to high temperature conditions for the LNT, while a drawback is to reach a higher amount of loaded sulphur into the catalyst which represent a risk to fulfill emission limits; c) a DeSOx after a certain mileage, depending on the amount of estimated sulphur loading by model approach: the advantage is that the request of DeSOx at a specific amount of loaded sulphur can be calibrated to avoid over-stressing of the catalyst and to fulfill the emission limits, Of course, a drawback is that it requires a detafled and complicated management.

The last solution is the more attractive but, however, as the other options, is dependent on the driving style. In particular, there are specific actions, which deactivate the DeSOx before its ideally completion, such as disabling the fuel request (cut-off, key off, etc.) or going at lower engine speed. These actions are very frequent in urban driving conditions and the consequence is that the DeSOx is almost ineffective because there is no time to remove the sulphur species from the catalyst: the DeSOx mode terminates after just few seconds from its activation which are not sufficient to guarantee the LNT performances again. Another consequence is that the DeSOx will be eventually requested at every DPF regeneration, which implies drawbacks in terms of fuel consumption and C02 emissions in addition to a quicker thermal aging of the catalyst.

Aim of the present invention is a new management of the rich combustion mode, either by an air/fuel ratio target or an exhaust temperature target, in order to improve the DeSOx effectiveness in critical driving conditions, such as the urban driving conditions. In fact, in these conditions, the driving style is characterized by actions (like cut-off, key-off, etc.) which disable the DeSOx just after few seconds from its activation. In this short time, the release of sulphur species from the LNT is minimal or zero, thus keeping the amount of loaded sulphur at a critical level for the catalyst efficiency. As a consequence, a DeSOx will be requested almost at every DPF regeneration, which implies drawbacks in terms of fuel consumption and C02 emissions in additions to a quicker thermal aging of the catalyst.

The present invention arises from consideration that, after a DeSOx activation, to reduce the time for the sulphur release (and consequently restore in quicker time the catalyst performances) reducing the risk of any interruption of the DeSOx procedure, would be useful to control and keep the air/fuel ratio target at richer conditions than the current one (for example, 090 instead of 0.95) for the first portion of the DeSOx rich time.

Alternatively, same effect can be obtained by controlling an exhaust temperature and keeping it higher than the normally used one (for example, 700°C instead of 650°C) for the first portion of the DeSOx rich time. This action produces more reactant species (such as H2 and/or HC5) which immediately interact with the sulphur trapped into the LNT storage sites and/or over the PGM (Platinum Group Metals). The two alternatives are almost equivalent, since a lower air/fuel ratio normally correspond to a higher exhaust temperature.

This action is extremely important in the first seconds after the activation of the rich mode because it gives the possibility to make efficient the DeSOx almost instantaneously, reducing the need to activate the DeSOx every DPF regeneration, especially in critical conditions such as urban driving conditions.

Therefore, the new strategy modifies the desired air/fuel ratio or exhaust temperature target as soon as the DeSOx rich combustion mode is activated, moving to a richer lambda target (for example 0.90) or to a higher exhaust temperature (700°C instead of 650°C, for example, according to the specific catalyst feature) for the first few seconds of the rich combustion mode. Then, if the driving conditions are not disabling the DeSOx, there is no need to stay at richer air/fuel ratio value (or higher temperature) and a transition to higher air/fuel ratio value (i.e. 0.95) or to a lower temperature (i.e. 650°C) is suggested to avoid excessive fuel consumption. A timer would determine the duration of the two time-splits of the rich DeSOx mode. In Fig. 5, this new strategy is schematized: the wobbling of the DeSOx regeneration begins with a DeSOx lean combustion mode 500, having a calibrated duration (12 $ in the example of Fig. 5) and an air/fuel ratio value higher than 1. Then the DeSOx rich combustion mode starts, with a first step at a lower air/fuel ratio value (that is to say at a richer combustion mode 520) also having a calibrated duration (5 s in the example of Fig. 5). The air/fuel ratio value could be 090.

for example. Then a second step in rich combustion mode 510 follows, having a higher air(fuel ratio (0.95 in the example, as in the standard rich combustion mode, see Fig. 4) and a duration for example equal to 10 s. The number of stepped rich combustion modes could be equal to 3, but in any case needs to be calibrated. If, alternatively, we choose to control the exhaust temperature instead of the air/fuel ratio, Fig.5 still properly schematized this alternative, if we substitute the air/fuel ratio 0.95 with the exhaust temperature of 650°C and the airffuel ratio 0.9 with the exhaust temperature of 700°C.

Advantageously, to make this strategy still more effective, two further features can be implemented: the transition lean combustion mode 500 to richer combustion mode 520 must be as fast as possible (ideally, with a duration equal to 0 s, as represented by the transition 530). This would immediately activate the richer combustion mode 520 and consequently the effectiveness of the desulphation process. On the other side, the transition from the richer phase 520 to the "standard" rich phase 510, could be advantageously a first ramp 540 and the transition from the "standard" rich phase 510 to lean phase 500 could also be a calibrated ramp 550 (see dotted lines in Fig. 5). This feature has no major effects on the desulphation process, but will take into account and improve the driveability of the vehicle: a step-transition, instead of a ramp, will likely be perceived by the driver.

It is clear that the strategy needs to be calibrated and that the values shown in the above Fig. 5 are only exemplifying. In particular: the duration of each split (richer vs. rich combustion mode) must be calibrated and their air/fuel ratio values (or, alternatively, temperature values) as well. Also as already mentioned, the ramp time between the transitions richer to rich and rich to lean must be calibrated. Furthermore, the length of this strategy can be calibrated based either on the number of DeSOx stepped rich modes (i.e. for the first 3-4 modes) or on a time duration (i.e. in the first minute of the DeSOx process). Keeping a richer air/fuel ratio (or a higher exhaust temperature) value for the whole DeSOx process is not recommended because of hardware reliability, which is still to be investigated. A possible drawback would be a higher H2S emission which can be solved by using a DPF with EQS suppression capabilities (for example, DPF converting H2S into 502) 1 5 Enabler for this new DeSOx management could be represented by the vehicle driving profile. For instance, if the vehicle speed is below a calibratable threshold, for instance km/h, the new proposed management will be enabled, while if the vehicle speed is above the threshold, the current management will be kept.

Even if using a richer air/fuel ratio value would request slightly higher fuel consumption, this drawback is balanced by the fact that improving the DeSOx effectiveness will reduce the DeSOx frequency and the thermal ageing of the LNT catalyst.

Few tests were preliminarily performed and demonstrated that the new strategy reduces the delay time for the release of the sulphur species and increases the amount of the emitted sulphur species. Among the two effects, the first one is the most important and makes this invention attractive for its benefits in critical driving conditions.

While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment1 it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.

REFERENCE NUMBERS

data carrier automotive system internal combustion engine 120 engine block cylinder cylinder head camshaft piston 145 crankshaft combustion chamber cam phaser fuel injector fuel rail 180 fuelpump fuel source intake manifold 205 air intake pipe 210 intake port 215 valves 220 port 225 exhaust manifold 230 turbocharger 240 compressor 245 turbocharger shaft 250 turbine 260 intercooler 270 exhaust system 275 exhaust pipe 280 aftertreatment devices 281 lean NOx trap (LNT) 282 diesel particulate filter (DPF) 283 LNT upstream air/fuel ratio sensor 284 LNT downstream air/fuel ratio sensor 285 LNT upstream temperature sensor 286 LNT downstream temperature sensor 290 VGT actuator 300 exhaust gas recirculation system 310 EGR cooler 320 EGR valve 330 throttle body 340 mass airflow and temperature sensor 350 manifold pressure and temperature sensor 360 combustion pressure sensor 380 coolant temperature and level sensors 385 lubricating oil temperature and level sensor 390 metal temperature sensor 400 fuel rail pressure sensor 410 cam position sensor 420 crank position sensor 430 exhaust pressure and temperature sensors 440 EGR temperature sensor 445 accelerator position sensor 446 accelerator pedal 450 ECU 500 Combustion mode, lean phase 510 Combustion mode, rich phase 520 Combustion mode, richer phase 530 Transition lean to richer phase 540 Ramp between richer and rich phase 550 Ramp between rich and lean phase

Claims

CLAIMS1. Method of controlling a desuiphation process of a lean NOx trap (281), the method comprising a wobbling of combustion lean phases (500) and combustion rich phases, wherein at least one of said rich phases is stepped and wherein -the step of the at least one of said rich phases is realized by a first value of the air/fuel ratio (520) followed by a second value of the air/fuel ratio (510) and wherein said first value of the air/fuel ratio is lower than said second value of the air/fuel ratio, or -the step of the at least one of said rich phases is realized by a first value of the exhaust temperature followed by a second value of said temperature and wherein said first value of the exhaust temperature is higher than said second value of the exhaust temperature.
2. Method according to claim 1, wherein transitions between combustion lean phases (500) and first step of combustion rich phases (520) have a time duration as close as possible to zero.
3. Method according to claim 1 or 2, wherein transitions from first step of combustion rich phases (520) to second step of combustion rich phases (510) and transitions from second step of combustion rich phases (510) to combustion lean phase (500) are realized by means of a calibrated ramp behavior (540, 550).
4. Method according to one of the previous claims, wherein the actuation of said stepped rich phases is calibrated based either on their number or on a time duration.
5. Method according to one of the previous claims, wherein its enabling condition is the a vehicle speed lower than a calibrated threshold.
6. Internal combustion engine (110) of an automotive system (100) equipped with an exhaust system (270), comprising at least an aftertreatment device (280), the after-treatment device being a lean NOx trap (281), the automotive system (100) comprising an electronic control unit (450) configured for carrying out the method according to claims 1-4.
7. A computer program comprising a computer-code suitable for performing the method according to any of the claims 1-4.
8. Computer program product on which the computer program according to claim 6 is stored.
9. Control apparatus for an internal combustion engine, comprising an Electronic Control Unit (450), a data carrier (40) associated to the Electronic Control Unit (450) and a computer program according to claim 6 stored in the data carrier (40).
10. An electromagnetic signal modulated as a carrier for a sequence of data bits representing the computer program according to claim 6.