GB2502796A - Optimising a Desulphation Strategy of an Internal Combustion Engine - Google Patents

Abstract

The invention provides method of optimizing a desulphation (DeSOx) process in an exhaust system of an internal combustion engine of an automotive system, the exhaust system comprising at least one after-treatment devices (280, fig.1), the after-treatment device being a lean NOx trap (LNT) (281, fig.3), the method comprising: identifying an automotive system mission profile 17 relating to the current driving conditions, a stored sulphur quantity 18 in the lean NOx trap , an air/fuel ratio set point 20 and a lean NOx trap ageing level 19 which may be an external input based on mileage and exhaust gas temperature, correcting 21 a maximum time under rich combustion condition, during said desulfation process, as function of the above parameters, and sending 22 said corrected maximum time under rich combustion conditions to a wobbling manager. The invention alleviates the hydrogen sulphide (H2S) issues relating to the DeSOx process (The rotten egg smell sometimes given off).

Description

METHOD OF OPTIMIZING DESULPI-JATION STRATEGY OFAN 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 and, more in detail, to the reduction of the Hydrogen sulfide (H2S) production during such regeneration.

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.

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

The reactions that are enhanced to desulphate the trap could produce different sulphur compounds: among them, there is the hydrogen sulphide (1-125), which has a particular bad smell and is produced under determined conditions during the combustion rich mode.

Therefore a need exists to identify the LNT conditions that would lead to an excessive production of H25 and use them to adapt the DeSOx Rich duration.

Therefore, an object of this invention is to provide a method which optimizes the desuiphation wobbling strategy and in particular defines an adaptive rich time which takes into account the real boundary conditions (mission profile, stored sulphur quantity, ageing status, air fuel ratio, among others) in order to minimize the l-12S production 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 optimizing a desulphation process in an exhaust system of an internal combustion engine of an automotive system, the exhaust system comprising at least one after-treatment devices, the after-treatment device being at least a lean NOx trap, the method comprising: -identifying an automotive system mission profile, a stored sulphur quantity in the lean NOx trap, an air/fuel ratio set point and a lean NOx trap ageing level, -correcting a maximum time under rich combustion condition, during said desulphation process, as function of the above parameters, -sending said corrected maximum time under rich combustion conditions to a Consequently, an apparatus is disclosed for optimizing a desulphation strategy in an exhaust system of an intemal combustion engine of an automotive system, the apparatus comprising: -means for identifying an automotive system mission profile, a stored sulphur quantity in the lean NOx trap, an air/fuel ratio set point and a lean NOx trap ageing level, -means for correcting a maximum time under rich combustion condition, during said desulphation process, as function of the above parameters, -means for sending said corrected maximum time under rich combustion conditions to a wobbling manager.

An advantage of this embodiment is that it provides a method of optimizing a desulphation strategy of a DeSOx process, particularly, improving customer satisfaction thus avoiding nasty impacts of desulphation on the vehicle user and on other potential customers.

According to a further embodiment of the invention, said correction of the maximum time under rich combustion condition is based on the following steps: -a first correction is calculated as the output of a map, function of the air/fuel ratio set point and the stored sulphur quantity, referred to a new lean NOx trap and as the output of a map, function of the air/fuel ratio set point and the stored sulphur quantity, referred to an aged lean NOx trap, -a second correction is obtained through interpolation of the two maps output by an ageing factor, -a third correction is obtained multiplying the output of said interpolation by a factor Which is a function of the automotive system mission profile, -a final correction is then obtained by subtracting said third correction to a calibrated fixed maximum time under rich combustion conditions.

Consequently the above apparatus further comprises: -means for calculating a first correction as the output of a map, function of the air/fuel ratio set point and the stored sulphur quantity, referred to a new lean NOx trap and as the output of a map, function of the air/fuel ratio set point and the stored sulphur quantity, referred to an aged lean NOx trap, -means for obtaining a second correction through interpolation of the two maps output by an ageing factor, -means for obtaining a third correction multiplying the output of said interpolation by a factor which is a function of the automotive system mission profile, -means for obtaining a final correction by subtracting said third correction to a calibrated fixed maximum time under rich combustion conditions.

An advantage of this embodiment is that it allows to take into account the most important factors, which influence the H2S production during the desulphation process, among them a high sulphur content and a low air-fuel ratio inside the LNT.

According to an aspect of the invention said interpolation by an ageing factor of the two maps output, respectively a new LNT map and an aged LNT map, is linear.

An advantage of this aspect is to property take into account, in an effective and easy way, of the ageing status of the LNT catalyst.

According to another aspect of the invention, said factor, which is a function of the automotive system mission profile, is in a range 0-1.

An advantage of this aspect is that it allows to discern if the main focus is on the DesOx efficiency (correction factor to be as small as possible, i.e. DeSOx rich time as high as possible) or on customer satisfaction (remarkable correction factor to be applied to limit H2S emission, DeSOx rich time penalized).

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 S 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 signal1 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 phases.

Fig. 5 is a graph depicting the sulphur compound production during the LNT desuiphation.

Figure 6 is a flowchart of a method for managing DeSOx regeneration maximum rich time according to the present invention.

Figure 7 is a more detailed flowchart of a method for managing DeSOx regeneration maximum rich time 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 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 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. 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 storage1 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.

Tuming back to exhaust system 270, the proposed invention relies on the optimization of the DeSOx rich time duration for a Lean NOx Trap 281, in order to minimize the H2S production during the regeneration itself. 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 B 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 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 sulphur poisoning, which heavily reduces the Wax 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 desulphation 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 S content in the trap and to bum the HC 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 reactions that are enhanced to desulphate the trap could produce different sulphur compounds: among them, very important is the hydrogen suiphide (H2S), whose production, because of its obnoxious rotten egg odor, has to be avoided as much as possible. In particular, H28 production should be kept to a minimum when the customer is in a urban driving profile, in the traffic or in any conditions when the vehicle speed is low and the H2S smell could be felt by other drivers or in general by people close to the vehicle that is performing the DeSOx.

Two parameters are mainly responsible of the hydrogen sulphide formation: -high sulphur content inside the LNT: the quantity that can be desuiphated in a rich event is proportional to the stored sulphur. At fixed engine conditions (temperature, air-fuel ratio, mass flow) and at fixed ageing level of the LNT, the percentage of sulphur, emitted in form of 1-125 is the same: therefore, the higher the stored sulphur, the higher the amount of H2S, emitted in each rich event; -low air-fuel ratio inside the LNT, which means very low oxygen content inside the exhaust gases that flow through the LNT. Rich combustion is needed to desuiphate the LNT, while the ratio between H28 and 802 emission (which are the two main compounds emitted during desulphation) is strongly dependent on the oxygen available to oxidize the Sulphur.

Moreover, the LNT has the property, in lean combustion conditions, to store a certain amount of oxygen, the extent of which goes under the name of Oxygen Storage Capacity (OSC). This property cause, during the regeneration rich phase, a first time period in which the LNT is in leaner conditions than expected: in other words, in the first seconds of the rich event, the main desuiphation product is 602, instead of H2S. This can be seen in Fig. 5, which describes a standard DeSOx rich emission, with 15 seconds of rich profile, at 650°C temperature set point and 0.9 air/fuel ratio. Curve 530 indicates S02 concentration, while curve 540 is the l-t2S concentration.

The standard DeSOx strategy is based on a "wobbling" with a fixed maximum rich time, set by a single calibration. The proposed method takes into accourit the effect of the mission profile 17, the stored sulphur quantity 18, the ageing level 19 and the air/fuel ratio (lambda) 20 on the production of H2S and uses this information to establish when it is most necessary and effective to correct (extend or reduce) 21 the maximum DeSOx rich time to be forwarded 22 to the wobbling manager. The mission profile is determined by the driving style and the environment conditions, among others, engine speed, engine torque, gear ratio ambient temperature and pressure. The stored sulphur quantity depends on the exhaust gas flcwrate through the LNT, the exhaust gas temperature and the air/fuel ratio. The airlfuel ratio (lambda) set point depends on engine speed1 engine torque, sulphur amount in the fuel, ambient temperature and pressure. Finally, the ageing factor depends on mileage and exhaust gas temperature.

In the method according to the invention, the maximum DeSOx rich time is the difference 21 between the base DeSOx rich time, fixed by a calibration, and a rich time correction.

Rich time correction is, at first, calculated as the output of a map function of the airlfuel ratio set point and the stored sulphur quantity at the beginning of the considered DeSOx rich event. Two maps are present, one 23' for a "new" LNT, the other 23" corresponding to an "aged" LNT (which has usually a smaller oxygen storage quantity), to take into account the containing effect on H2S production, due to the consumption of the oxygen Storage Capacity at the beginning of each DeSOx rich event. Assuming that an ageing factor is present to evaluate the current condition of the LNT, the rich time correction is obtained through interpolation 24 between the two maps. The output of the interpolation is then multiplied 26 by a factor which is a function of the current mission profile 25, obtaining the final rich time correction that is subtracted 21 to the base DeSOx rich time and sent 22 to the DeSOx wobbling manager.

The method defines the correction to be applied to the base calibration 16 as a function of the air/fuel ratio set point and the stored sulphur quantity, considering the following phenomena: -the main driver of the H2S emission (opposed to the more desirable 502 emission) is the low oxygen content in the exhaust gas, during rich combustion. Therefore, with the same sulphur amount in the LNT, at the beginning of a DeSOx rich phase, a lower air/fuel ratio will cause a higher H2S/S02 ratio and consequently a higher H2S emission.

-the sulphur quantity, which is desorbed during a DeSOx rich event regardless of the compounds distribution (depending on the exhaust gas composition), is driven by the sored sulphur quantity in the LNT at the beginning of a DeSOx rich phase.

An ageing factor (which is assumed to be as an external input for this invention) is used to define the current level of ageing of the LNT, whose limits are identified by no ageing (New LNT map) 23' and maximum ageing (aged LNT map) 23". The final value is the output of an interpolation based on the ageing factor. Preferably said interpolation is linear.

Too high H2S emission represents a customer satisfaction problem, when it is possible to perceive its typical rotten egg odor and when other customers can feel it as well. In all the cases in which the odor can't be felt by the customers, DeSOx rich phase should be as long as possible, to guarantee an efficient DeSOx regeneration: with a mission profile linked to city driving conditions, H2S emission is critical, due to the slow speed and the possible presence of other vehicles and pedestrians in the surrounding of the considered vehicle; with a mission profile linked to highway driving conditions, vehicle speed and vehicle-to-vehicle distance are both high enough to avoid H2S odor feeling. The mission profile (which is assumed to be an external input for this invention and for example may be an election knob or an automatic driving style) carries the information on the current driving conditions, in order to discern if the main focus is on the DeSOx efliciency (correction factor to be as small as possible, i.e. DeSOx rich time as high as possible) or on customer satisfaction (remarkable correction factor to be applied to limit H2S emission, DeSOx rich phase penalized). Each mission profile is therefore associated with a scaling factor applied to the correction coming from the interpolation between new and aged maps that can reduce/enhance the mapped correction according to the driving conditions. Preferably this scaling factor is in a range between 0 and 1.

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 embodiment, 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

16 block 17 block 18 block 19 block block 21 block 22 block 23', 23" blocks 24 block block 26 block data carrier automotive system 110 internal combustion engine engine block cylinder cylinder head camshaft 140 piston crankshaft combustion chamber cam phaser fuel injector l7ofuelrail 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 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 VOl 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 Exhaust temperature 510 Combustion mode, lean phase 520 Combustion mode, rich phase 530 602 concentration 540 EQS concentration