GB2502832A - Controlling a Desuiphation Process of a Lean NOx trap - Google Patents

Abstract

The invention provides a method of controlling a desuiphation process of a lean NOx trap (281), the method comprising cycles of the following steps, starting 20 a lean phase (510, fig.4) of the desuiphation process (DeSOx), defining 21' an adaptive minimum duration for said lean phase (510, fig.4) based on the rich phase duration of the previous cycle, starting 22 a rich phase (520, fig,4) of a desuiphation process, if activation conditions 21" of said rich phase are met, checking 23 deactivation conditions of said rich phase (520, fig.4) and ending the rich phase, when such deactivation conditions are met. The rich phase activation and deactivation conditions can be that the engine is operating inside a permitted area of an engine map, and the exhaust temperature (500, fig.4) is in a range between a lower and upper thresholds.

Description

METHOD OF OPTIMIZING DESULPHATION STRATEGY OF AN INTERNAL

COMBUSTION ENGINE

TECHNICAL FIELD

The present disclosure relates to a method of optimizing the desulphation strategy of an internal combustion engine. In particular, the method is dedicated to the sulphur regeneration of a Lean NOx Trap located in the exhaust system of the internal combustion engine.

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 (NOX) contained in the exhaust gas, in order to trap them within the device itself.

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-burn operation to operation whereby an excess amount of fuel is available, referred also 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.

Due to the presence of suplhur into the fuel, the LNT is 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 The standard DeSOx strategy considers a fixed minimum lean time between two consecutive rich events. Anyway, during real driving conditions, the rich event can be shorter than the desired time. Since the regeneration duration cannot overcome a predetermined period of time and the sulphur stored into the LNT is removed only during the rich events, keeping a fixed lean time leads to a reduction of the DeSOx regeneration efficiency.

Therefore a need exists for a method that optimizes the desuiphation strategy, in order to have an adaptive lean time, thus improving the DeSOx regeneration strategy.

Therefore, an object of this invention is to provide a method which optimizes the desulphation wobbling strategy versus the real boundary conditions due to the driving style, adapting the minimum lean time between two consecutive rich events in order to maximize DeSOx efficiency.

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 comprising cycles of the following steps: -starting a lean phase of the desulphation process, -defining an adaptive minimum duration for said lean phase, -starting a rich phase of a desuiphation process, if activation conditions of said rich phase are met, -checking deactivation conditions of said rich phase and ending the rich phase, when such deactivation conditions are met Consequently, an apparatus is disclosed for controlling a desuiphation process of a lean NOx trap, the apparatus comprising: -means for starting a lean phase of the desulphation process, -means for defining an adaptive minimum duration for said lean phase, -means for starting a rich phase of a desulphation process, if activation conditions of said rich phase are met, -means for checking deactivation conditions of said rich phase and means for ending the rich phase, when such deactivation conditions are met.

An advantage of this embodiment is that it provides a method of optimizing a desulphation strategy of a DeSOx process, particularly defining an adaptive minimum lean time, thus improving the trade-off between fuel consumption and oil dilution versus the desulphation efficiency in any driving conditions.

According to a further embodiment of the invethion, said adaptive minimum duration for said lean phase is based on the rich phase duration of the previous cycle.

An advantage of this embodiment is that the previous rich phase duration is a good indicator time needed, during the next DeSOx lean phase, to restore the oxygen storage capacity and to get back to the temperature target.

According to a further embodiment, said rich phase activation and deactivation conditions are: -the engine is operating inside a permitted area of an engine map, -the exhaust temperature is in a range between a lower and a upper threshold.

An advantage of this embodiment is that it guarantees the engine combustion stability, no mechanical failures in the exhaust system and at the same time a good regeneration efficiency.

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 computer program all the steps of the method described above are carried 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 inventiorn Figure 4 is a scheme of the DeSOx regeneration phases.

Figure 5 is a flowchart of a method for managing DeSOx regeneration according to a known strategy.

Figure 6 is a flowchart of a method for managing DeSOx regeneration according to 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 fuel 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 allow 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 aftertreatmeflt devices 280 include1 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 (DFF) 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 EGR 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. Note1 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 io 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 the optimization of the DeSOx lean time minimum duration 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) 262 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 F-IC) with high efficiency and stores NOx during lean operating conditions. During rich operating conditions, the NOx is released and converted. The LNT works properly 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 suplhur into the fuel, the LNT is exposed to the sutphur poisoning, which heavily reduces the NOx storage capacity of the catalyst. The sulphur contained in the fuel is easily oxidized in lean atmosphere: it is stored as barium and aluminum sulphates, which are more stable compounds than the corresponding nitrates.

This process reduces the efficiency of the Trap in terms of NOx Storage: this efficiency can be restored through a desulohation process, that requires high temperature and rich atmosphere. In particular, it consists in a fast alternation of lean and rich atmosphere at high temperature during engine working condition.

The DeSOx regeneration is defined as the process which leads to the desulphation 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, shown in Fig. 4, that is to say, an alternation of lean 510 and rich 520 phases at high temperature 500. The DeSOx rich phase is needed to destabilize the chemical links between the sulphur and the barium and/or aluminium, making the barium/aluminium sites free for the NOx and restoring the NOx trapping capability 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 burn the HG cumulated in the trap during the rich phase.

The effective sulphur removal occurs in the rich phase and the released quantity is a function of the sulphur already present in the trap. As easily understandable, the higher is the time spent in rich atmosphere during the DeSOx, the higher is the efficiency of the regeneration. For this reason any attempt to increase the rich time, keeping the same duration of DeSOx regeneration, will lead to an increase of DeSOx efficiency without any deterioration of other constrain (fuel consumption, oil dilution, etc.).

On the other side, it must be observed that, particularly for a Diesel engine, it is not always possible to provide a rich atmosphere at the engine outlet. This is due to several factors, among them combustion stability, too high inlet turbine temperatures-Therefore, rich engine conditions are only permitted in a delimited area of the engine ma: the engine speed must be in the range 1000 rpm -3000 rpm and the brake mean effective pressure between 2 bar and 10 bar, just to avoid, as mentioned above a unstable combustion and a severe stress to the turbine.

The exhaust temperature, on the other side, must lie between a lower threshold and an upper threshold because a too low temperature would allow only a poor regeneration efficiency and a too high temperature would compromise the engine safety from a mechanical resistance point of view. Therefore, according to a preferable embodiment, the upstream LNT temperature should be in the range 525°C and 675°C. :ii

The standard DeSOx strategy is based on a "wobbling" with a fixed minimum lean phase duration. Due to the fact that the rich phase can be aborted, often depending on the boundary conditions as described above, the desulphation can be highly inefficient especially for some driving profiles, like the heavy urban, in which the rich events are aborted several times after only few seconds. For example, if the driver is slowing down, the engine speed is below the rich combustion condition.

The proposed method aims to maximize the time spent in rich condition during the DeSOx regeneration, by enabling an adaptive minimum lean time depending on the rich time spent in the real operating conditions and the temperature behavior. In this way, the DeSOx regeneration management becomes flexible and adapted to the real driving conditions.

Once a DeSOx regeneration has been started and a DeSOx lean mode 510 has been authorized, a DeSOX rich mode 520 shall be requested only if the DeSOx lean minimum time is above a calibratable threshold, where the threshold shall be function of the time spent in the previous rich phase. Of course, for the DeSOx rich mode 520 to be requested, the LNT downstream temperature must be within an interval, defined by the lower and the upper thresholds, for a calibratable debouncing time, the DeSOx rich inhibition flag should not be present and no DeSOx end criteria have to be active as well.

The known standard strategy is shown in Fig. 5. The wobbling of the DeSOx regeneration begins with a DeSOx lean start 20, the lean phase has a fixed minimum time 21, which is calibrated and represents an enabling condition for a rich phase to be actuated, then the DeSOx rich phase starts 22 and ends when the deactivation conditions 23 are satisfied. Once the rich phase ends a new cycle begins with the start of another lean phase. According to this strategy, the DeSOx lean minimum duration is always fixed, regardless of the actual time needed during a DeSOx lean phase to restore the oxygen storage capacity (which has been used during the previous DeSOx rich phase, according to its duration) and to get back to the temperature target (which decreases during the DeSOx rich, always according to its duration). This means a useless DeSOx lean time, if the previous DeSOx rich effect has not degraded the two conditions for which the DeSOx lean phase is used.

The logic according to the present invention is shown in Fig ft After the DeSOx regeneration begins with a DeSOx lean start 20, said lean phase has an adaptive minimum duration 21'; then the rich phase activation conditions 21" are checked and the DeSOx rich phase starts 22 and ends when the deactivation conditions 23 are satisfied.

The rich phase duration 24 will be used as input to define the adaptive minimum duration of a new lean phase, while the rich phase activation and deactivation conditions are, among others, to stay inside the permitted area of the engine map and to have the exhaust temperature 500 inside a lower and a upper threshold.

Therefore, in the proposed logic, the DeSOx lean minimum duration is depending on the previous DeSOx Rich duration, that is a good indicator of the stored oxygen consumption and of the temperature decrease. The DeSOx lean minimum duration is optimized to restore the two conditions (02 and Temperature), thus after this action, the next DeSOx rich phase can be started as soon as possible. This means reduced DeSOx lean time if the previous DeSOx rich effect is not requiring a longer lean phase. The benefit is to increase the DeSOx rich time, maintaining the same DeSOx total duration, thus improving the desulphation process.

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 block

21,21,21" blocks 22 block 23 block 24 block 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 fuel pump 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 aftertreatmeflt 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 VOT 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 365 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 Exhaust temperature 510 combustion mode lean phase 520 combustion mode, rich phase