GB2504354A

GB2504354A - Method of calculating the NOx storage capacity of a Lean NOx Trap device

Info

Publication number: GB2504354A
Application number: GB1213420.1A
Authority: GB
Inventors: Angela Vitiello; Roberto Argolini
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2012-07-27
Filing date: 2012-07-27
Publication date: 2014-01-29
Also published as: GB201213420D0

Abstract

Disclosed is a method of evaluating the NOx storage capacity of a Lean NOx Trap device 281 in an exhaust system 270 of an internal combustion engine 110 of an automotive system 100. The method comprises: calculating a BaseCap variable at a fixed NOx_base constant as function of catalyst temperature and sulphur content, calculating a slope variable as function of catalyst temperature and sulphur content; subtracting said NOx_base constant by a filtered NOx_in value and then multiplying such result by the slope variable and adding the previous result to said BaseCap variable, thus providing a NOx storage capacity. The age of the LNT may also be taken into account in the method which assumes a linear relationship between the NOx storage capacity and the amount of NOx reaching the LNT. A control system using the method and the associated computer program are also disclosed

Description

S METHOD OF ESTIMATING NOx STORAGE CAPACITYOFA LEAN NOx TRAP

OF AN INTERNAL COMBUSTION ENGINE

TECHNICAL FIELD

The present disclosure relates to a method of estimating the NOx storage capacity of a Lean NOx Trap device in an exhaust system of an internal combustion engine.

Particularly, this new method is based on a physical model of the Lean NOx Trap.

BACKGROUND

It is known that the exhaust gas after-treatment systems of a Diesel engine can be provided1 among other devices, with a Lean NO, Trap (LNT).

A Lean NO, Trap 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 (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.

Unfortunately, the NOx storage capacity of Lean NOx Trap is not just dependent on catalyst technology but it is influenced by different parameters. The most important parameters are catalyst temperature, NOx engine out concentration, sulphur content, space velocity and ageing status of the component. Without an accurate estimation of the NiOx storage capacity, the management of the LNT regeneration phase is badly influenced.

Therefore a need exists for a method that can accurately estimate the NOx storage capacity in a LNT during all engine operating conditions and, preferably, also during the device lifetime.

An object of this invention is to provide a method which evaluates the NOx storage capacity of a Lean NOx Trap device in an exhaust system of an internal combustion engine. The method should be based on a physical model of NOx storage capacity, taking into account the above most influent parameters.

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 invention provides a method of estimating the NOx storage capacity of a Lean NOx Trap device in an exhaust system of an internal combustion engine of an automotive system, the method comprising: a) calculating a BaseCap variable at a fixed NOx_base constant as function of catalyst temperature and sulphur content; b) calculating a Slope variable as function of catalyst temperature and sulphur content; c) subtracting said NOx_base constant by a filtered NOx_in value and then multiplying such result per said Slope variable d) adding the previous result to said BaseCap variable, thus providing a NOx storage capacity.

Consequently, an apparatus is disclosed for evaluating the NOx storage capacity of a Lean NOx Trap device in an exhaust system of an internal combustion engine of an automotive system, the apparatus comprising: a) means for calculating a BaseCap variable at a fixed NOx_base constant as function of catalyst temperature and sulphur content; b) means for calculating a Slope variable as function of catalyst temperature and sulphur content; c) means for subtracting said NOx_base constant by a filtered NOx_in value and then means for multiplying such result per said Slope variable d) means for adding the previous result to said BaseCap variable, thus providing a NOx storage capacity.

An advantage of this embodiment is that it provides a method of estimating the NOx storage capacity of a Lean NOx Trap device, thus allowing an improvement in the LNT management thanks to a more efficient regeneration in terms of performances, fuel consumption and ageing.

According to another embodiment of the invention, said method is repeated twice for a fresh Lean NOx Trap and for an aged Lean NOx Trap and, after step d), further comprises: e) interpolating a NOx storage capacity for a fresh Lean NOx Trap and a NOx storage capacity for an aged Lean NOx Trap by means of a calibratable ageing factor, thus providing a NOx storage capacity, also determined by the Lean NOx Trap ageing status.

An advantage of this embpdiment is that it provides an estimation of the NOx storage capacity of a Lean NOx Trap, also taking into account the ageing status of the catalyst.

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 computerprogram 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 invention.

Figure 4 is a flowchart of a method of estimating the NOx storage capacity, according to an embodiment of the invention.

Figure 5 is a flowchart of a method of estimating the NOx storage capacity, which also takes into account the LNT ageing status, according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Some embodiments may include an automotive system 1001 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 dud 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, particulate filters (DPF) 282 or a combination of the last two devices, i.e. selective catalytic reduction system comprising a particulate filter (SCRF). 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 andlor 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, including1 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 bu& The CPU is

B

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 totfrom 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 the exhaust system 270, the proposed invention relies on a method which evaluates the NOx storage capacity of a Lean NOx Trap after-treatment device 281 (Fig. 3). Advantageously, the after treatment system could also comprise a particulate filter (DPF) 282 to trap particulate emitted in case of a Diesel engine.

Upstream and downstream the LNT, NOx sensors 283, 284 and temperature sensors 285, 286 can be provided.

Preferably, the LNT 281 could be positioned as close as possible to the exit of the turbocharger 230 to take advantage of the high temperature conditions which are beneficial for its catalytic reactions.

The LNT reduces engine-out exhaust gas constituents (CO and HC) with high efficiency and stores NOx during normal operating conditions of the engine, which are characterized by a lean combustion. When the storage capacity is reached a regeneration of the lean NOx trap 281 is needed. The LNT regeneration phase is characterized by a rich combustion. The high temperature, the lack of oxygen and the presence of a reductant catalyst (for example, Rhodium) promote the NOx reduction and release as N2 and NH3.

To properly manage the regeneration phase is important to know the LNT NOx storage capacity, and this invention is related to a method, which estimates this capacity. To define the method, an experimental campaign has been performed to assess the influence of several parameters on the NOx storage capacity. It has been supposed that the storage capacity can be a function of the following parameters: main catalyst temperature, space velocity, sulfur content, engine out NOx concentration (that is to say, the NOx ppm at the catalyst inlet). Of course, the LNT ageing status, mainly due to the exposition at high temperatures, could also influence the NOx storage capacity.

Experimental activities on the actual LNT technology have been performed in order to assess the impact of each parameter. In particular, the tests have been carried out using: U 3 levels of Space velocity (25k -50k -lOOk) [1] U 3 levels of NOx inlet concentration (75-150-300) [ppm] 0 3 levels of Sulfur loading (0 -1 -2) [gIl] U 5 levels of temperature (75-150-250-350-450)[°C] The results of such experimental campaign indicate that: -the space velocity (dimensionally [sd]) has no remarkable influence on the NOx storage capacity. Space velocity inside LNT is calculated as the ratio between the exhaust mass flow upstream LNT, the density and the LNT volume. The density is calculated as the ratio between the LNT pressure (average of pressure between LNT upstream pressure and LNT downstream pressure) and the product between 2 5 LNT main temperature (average temperature between LNT upstream temperature and LNT downstream temperature) and the ideal gas constant. Due to the experimental results, this parameter has not been considered as input of the method of evaluating the NOx storage capacity; the sulphur content is strongly influent on NOx Storage Capacity, that decreases as sulphur increases (having a not linear behavior); the same trend in both aged and fresh catalyst can be observed, but with different impact; therefore the sulphur content has been entered as input for the method according to this invention; for this purpose, the sulphur content is calculated as output of a specific SOx model, not forming part of the present invention. Such model evaluates the amount of sulphur trapped inside the LNT according to fuel mass flow, fuel sulphur content, oil consumption rate and oil sulphur content. The model also takes into account the fact that, during a rich phase, part of the sulphur trapped inside LNT is released, according to a submodel based on temperature, LNT upstream airlfuel ratio, space velocity and rich condition duration. Therefore this model is able to provide the SOx stored inside LNT in real time.

Engine out NOx concentration is strongly influent on NOx storage capacity that increases as NOx concentration increases (having an almost linear behavior); the same trend in both aged and fresh catalyst can be observed, but with different impact; this trend can be explained, since the catalyst becomes more active when higher NOx engine out concentration are present. Therefore the engine out NOx concentration has been entered in the new method too; such parameter is available by measuring it (NOx sensors 283, 284) and filtering it (by using a common low pass filter) or by a specific combustion model, not forming part of the present invention.

Main temperature is also strongly influent on NOx storage capacity having a nonlinear behavior with a peak around 250°C; the same trend in both aged and fresh catalyst can be observed, but with different impact. The main catalyst temperature also enters as input for the present method and its value can be obtained by measuring it (LNT upstream temperature sensor 285 and LNT downstream temperature sensor 266) and modeling it (weighted average or other suitable methods).

In conclusion, the experimental campaign has shown the following conclusions: -the NOx storage capacity is mainly influenced by: main catalyst temperature, sulphur content and engine out NOx concentration (NOx ppm at catalyst inlet); -the ageing status, particularly a thermal ageing status also influences the NOx storage capacity; -the impact of temperature and sulphur content do not show a linear behavior and have been modeled with a dedicated maps structure; -the impact of NOx concentration at catalyst inlet shows a linear behavior on NOx Storage Capacity and has been modeled with a linear structure; -the impact of component status (aged, fresh) do not show a linear behavior and has been modeled with a dedicated maps structure.

According to these considerations, in a preferred embodiment, which does not consider the ageing status of the catalyst, the NOx storage capacity has been modeled through a linear curve function of the engine out NOx concentration at catalyst inlet: NOxCap = BaseCap ÷ Slope * (NOx_In -NOx_base) (1) where BaseCap = NOx Storage capacity evaluated at a fixed engine out NOx concentration at catalyst inlet (NOx_base), Slope = the slope of the curve, evaluated for two or more generic engine out NOx quantities (as a correction map), NOx_base = constant, reference engine out NOx concentration NOx_In = Engine out NOx concentration, measured and filtered.

Moreover one linear curve for each sulfur content and temperature condition has been considered.

Therefore in Fig. 4 a block diagram of a controller is shown for the implementation of the method. It consists of calculation block 20 comprising a map for determining a BaseCap as a function of temperature and sulphur content, wherein the map is generated by a set up measurement which is done with a fixed NOx_base constant. Said NOx_base constant is a reference engine out NOx concentration, experimentally determined.

Further, a calculation block 21 is used for calculating a Slope variable as function of catalyst temperature and sulphur content; then, in order to model the above equation (1) about the NOx storage capacity, means for subtracting 22 said NOx_base constant by a filtered NOx_in value and means for multiplying 23 such result per said Slope variable are available in said controller; finally means adding 24 the previous result to said BaseCap variable are further comprised in the controller thus allowing the method as claimed to obtain the NOx storage capacity.

According to another embodiment, which also takes into account the ageing status of the catalyst, two dedicated structures for a fresh and for an aged catalyst have been setup.

The controllers, the logic and the algorithms are exactly the same, as shown in Fig. 5.

The output of such structures will be a NOx storage capacity for a fresh LNT 281 and a NOx storage capacity for an aged LNT 281'. These two values will be entered in suitable means for interpolating 25. The interpolation will be performed by means of a calibratable ageing factor and will provide a NOx storage capacity, also determined by the catalyst ageing status. According to a preferred embodiment, the interpolation is a linear interpolation, according to the following scheme: NOx_copacity = NOx_capacity Fresh x (1-A going factor) + NOx_capacity Aged x Ageing factor Where the Ageing factor runs from 0 (new catalyst) to 1 (aged catalyst).

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

20, 20' blocks 21,21' blocks 22, 22' blocks 23, 23' blocks 24, 24' blocks block data carrier automotive system 110 internal combustion engine engine block cylinder cylinder head camshaft 140 piston crankshaft combustion chamber cam phaser fuel injector 170 fuel rail 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), fresh catalyst 281' lean NOx trap (LNT), aged catalyst 282 diesel particulate filter (DPF) 283 LNT upstream NOx sensor 284 LNT downstream NOx 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 BaseCap = NOx Storage capacity evaluated at a fixed engine out NOx concentration at catalyst Inlet Slope = the slope of the curve, evaluated for two or more generic engine out NOx quantities (as a correction map) NOx_base = constant, reference engine out NOx concentration NOxIn = Engine out NOx concentration, measured and filtered

Claims

CLAIMS1. Method of evaluating the NOx storage capacity of a Lean NOx Trap device (281) in an exhaust system (270) of an internal combustion engine (110) of an automotive system (100), the method comprising: a) calculating (20) a BaseCap variable at a fixed NOx_base constant as function of catalyst temperature and sulphur content; b) calculating (21) a Slope variable as function of catalyst temperature and sulphur content; c) subtracting (22) said NOx_base constant by a filtered NOx_in value and then multiplying (23) such result per said Slope variable d) adding (24) the previous result to said BaseCap variable, thus providing a NOx storage capacity.
2. Method according to claim 1, wherein said method is repeated twice for a fresh Lean NOx Trap (281) and for an aged Lean NOx Trap (281') and, after step d), further comprises: e) interpolating (25) a NOx storage capacity for a fresh Lean NOx Trap (281) and a NOx storage capacity for an aged Lean NOx Trap (281') by means of a calibratable ageing factor, thus providing a NOx storage capacity, also determined by the Lean NOx Trap ageing status.
3. 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, 281'), said exhaust system also comprising at least a NOx sensor (283, 284) and at least a temperature sensor (285, 286), the automotive system (100) comprising an electronic control unit (450) configured for carrying out the method according to claims 1-2.
4. A computer program comprising a computer-code suitable for performing the method according to any of the claims 1-2.
5. Computer program product on which the computer program according to claim 4 is stored.
6. 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 4 stored in the data carrier (40).
7. An electromagnetic signal modulated as a carrier for a sequence of data bits representing the computer program according to claim 4.