GB2490940A

GB2490940A - Method for operating a lean NOx trap

Info

Publication number: GB2490940A
Application number: GB1108395.3A
Authority: GB
Inventors: Roberto Argolini; Cristian Taibi; Alberto Giordano
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2011-05-19
Filing date: 2011-05-19
Publication date: 2012-11-21
Also published as: GB201108395D0

Abstract

A method for operating a Lean NOx Trap 225 in an Internal Combustion Engine 110, where the method comprises starting a regeneration event of the Lean NOx Trap using a first value of an air to fuel ratio and then subsequently varying the air to fuel ratio from the first value to a second value, wherein the second value is maintained until the end of the regeneration event. Preferably the second value of the air to fuel ratio is smaller than the first and the ratio is varied when a time threshold, determined as a function of engine speed and engine load, is reached. The first and second ratio values may be functions of engine speed and engine load and the air to fuel ratios may be measured by a lambda probe 227 upstream of the Lean NOx Trap and in communication with an ECU 450. An automotive system is also claimed.

Description

METHOD FOR OPERATING A LEAN NO, 17<AP IN AN INTERNAL CCMBUSTION ENGINE TEQ*L flEW The present disclosure relates to a method for operating a Lean NO Trap in an Internal Combustion Engine.

BACKGPCXD

It is known that exhaust gas after-treatment system of a Diesel engine can be provided, among other devices, with a Lean NO Trap (LNT) which represents a cost efficient alternative to SCR (Selective Catalytic Reduction).

A Lean NO Trap (LNT) traps nitrogen oxides (NO) contained in the exhaust gas and is located in the exhaust line.

In fact, a LNT is a catalytic device containing catalysts, such as Rhodium, Pt and Pd, and adsorbents, such as barium based elements, which provide active sites suitable for binding the nitrogen oxides (NO) contained in the exhaust gas, in order to trap them within the device itself.

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

For this reason, the Lean NO Traps (LNT) are operated cyclically, for example by switching the engine from a lean burn operation to a rich operation, performing a regeneration phase also referenced as DeNOX regeneration.

In fact, internal combustion engines are currently operated with multi-injection patterns, namely for each engine cycle, a plurality of injection pulses is performed typically, starting from a pilot injection pulse and a following main injection pulse.

Typically, in order to achieve the needed temperature for these regeneration events, so called fuel after-injections are employed; these are fuel injections that are activated during the regeneration process and that occur after the Top Dead Center (TX) of the piston.

During normal operation of the engine and a lean burn operation, the ND are stored on an adsorbent surface. When the engine is switched to a rich operation mode, the NOX stored on the adsorbent site react with the reductants in the exhaust gas and are desorbed and converted to nitrogen, thereby regenerating the adsorbent site of the catalyst.

These regeneration phases may last for a short period each, such as S to 8 seconds, in which the rich mixture is used and the Lean NO Trap may be cleaned up and its storage capacity recovered.

Lean NO Traps (LNT) may also be subjected to SO, regeneration phases, namely after some thousand kilometers, sulphur contained in the diesel fuel poisons the Lean NOx trap and a desulphurization phase is needed.

This is performed by means of several rich combustion phases executed at high temperature, where gas temperature at LNT inlet may be around 65000.

Therefore to perform both NO, arid SO> regenerations, it is necessary to have a rich combustion and this means to control the air-to-fuel ratio, as expressed for example by the lambda values upstream and downstream the catalyst measured by respective lambda probes.

In the prior art, this is done by setting a fixed lambda target for a specific regeneration, but it must be considered that the choice of the lambda value to be used gives rise to problems directly related to the performances of the whole engine system in terms of fuel consumption and of emission efficiency.

In practical terms, a fixed value of lambda upstream of the Lean NO Trap is selected and a closed-loop control ensure a rich combustion for regenerating.

This prior art method has the problem that, if lambda is chosen with a high value (e.g: 0.94) the regeneration may not be completed leaving a high NO residue in the trap.

Also, there may be a high fuel penalty, since the trap must be regenerated more frequently.

Moreover, if lambda is chosen with a low value (e.g: 0.9) a high NO slip, namely a high proportion of NO,<not trapped, may occur.

Also, there may be a very high fuel consumption, since the trap is regenerated using an extremely rich mixture.

An object of an embodiment of the invention disclosed is to manage regeneration events, be they NO or SQ regeneration events, in order to achieve a beneficial effect on both fuel economy and emission reduction.

A further obj ect of an embodiment of the invention is to manage successive regeneration events in order to avoid subsequent storage efficiency drops.

Another object is to provide an iniproved Lean NO Trap method of operation without using complex devices and by taking advantage from the computational capabilities of the Electronic Control Unit (ECU) of the vehicle.

SThtRY An embodiment of the disclosure provides a method for operating a Lean NO,( Trap in an Internal Combustion Engine, the method comprising: -starting a regeneration event of the Lean NO Trap using a first value of an air to fuel ratio, -subsequently varying the air to fuel ratio from the first value to a second value, where the second value is maintained until the end of the regeneration event.

An advantage of this embodiment is that it improves the overall Lean NO Trap management, causing a reduction in fuel consumption and, at the same time, an increase of NO>, conversion efficiency and storage capacity.

According to another embodiment of the invention, the second value of the air to fuel ratio is smaller than the first valueS An advantage of this embodiment is that in the first phase of the regeneration event, when the first value of the air to fuel ratio is used, there is a relatively low desorption rate, then a moderate increase of the desorption rate occurs when the second value of the air to fuel ratio is used, but the second air to fuel ratio used allows a complete regeneration of the trap.

According to another embodiment of the invention, the air to fuel ratio is varied from the first value to the second value when a time threshold is reached, the time threshold being determined ad a function of engine speed and engine load.

An advantage of this embodiment is that the period of time during which the second value of the air to fuel ratio is used depends on engine conditions and operation.

According to a further embodiment of the invention, the first value and the second value of the air to fuel ratio are functions of engine speed and engine load.

According to a further embodiment of the invention, the first and the second value of the air to fuel ratio are measured by a lambda probe upstream of the Lean NO Trap and in corrmunication with the Electronic Control Unit.

I

An advantage of this embodiment is that it uses a sensor that is generally already present on current automotive systems.

According to still another embodiment of the invention, the regeneration event of the Lean NO, Trap is performed by means of fuel after injections into the internal combustion engine.

According to a further embodiment of the invention, the regeneration event of the Lean NO Trap is performed by means of fuel injections into the exhaust line of the internal combustion engine upstream of the Lean NO Trap.

A further embodiment of the invention is comprised of an automotive system comprising: an internal combustion engine, managed by an engine Electronic Control Unit, the engine having at least a cylinder equipped with a fuel injector and being equipped with an exhaust line, having a Lean NO Trap Catalyst and an upstream lambda probe in corrmunication with the Electronic Control Unit, for measuring an air to fuel ratio wherein the Electronic Control Unit is configured to monitor the signals from the upstream lambda probe and to start a regeneration event of the Lean NO Trap using a first value of an air to fuel ratio, and to subsequently vary the air to fuel ratio from the first value to a second value, wherein the second value is maintained until the end of the regeneration event 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 equipped with a Lean NO> Trap, the engine comprising an Electronic Control Unit configured for carrying out the method according to any of the preceding claims.

BR DEScRIP'nct OF THE DRAWINGS The various embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an Internal Combustion Engine equipped with a Lean NO Trap (LNT); Figure 2 is graph schematically representing the logical steps of an errbodirnent of the invention; Figure 3 is graph that depicts an air to fuel ratio over time, upstream and downstream of Lean NO Trap (LNT), according to an embodiment of the invention; Figure 4 is a schematic representation of an automotive system equipped with an electronic control unit (ECU) for performing the various embodiments of the invention; and Figure 5 is schematic representation of an Internal Combustion Engine of the automotive system of figure 4 in cross-section.

DETAIlED DESCRIP'flCE OF THE DRAWINGS Preferred embodiments iiill now be described with reference to the enclosed drawings.

Figure 1 shows an internal combustion engine 110 managed by an engine Electronic Control Unit (ECU) 450 equipped with a data carrier 460, the internal combustion engine 110 being equipped with an exhaust line 224 with a Lean NQ< Trap Catalyst 225 and a tailpipe 275.

An upstream lambda probe 227 and a downstream lambda probe 228 are respectively placed in the exhaust line 224 upstream and downstream of the Lean NO, Trap 225.

In operation, the upstream lambda probe 237 and the ECU 450 work in closed loop controlling the quantity of fuel that is injected by means of injectors 160 into the engine 110. If the upstream lambda probe 227 generates a signal that represents a low value of lambda, that means that the mixture is rich and the ECU 450 may reduce the injection time in order to have a leaner mixture. Conversely, if the upstream lambda probe 227 generates a signal that represents a high value of lambda, that means that the mixture is lean and the ECU 450 may increment the injection time in order to increment the fuel amount injected into the engine 110.

The lambda value is therefore directly correlated to air to fuel ratio in the mixture injected into the engine 110.

The downstream lambda probe 228 measures the lambda value downstream of Lean NQ, Trap 225 in order to have a measure of its performance.

The various embodiments of the invention are based on the following considerations.

The lower is the value of the upstream air to fuel ratio as measured by the upstream lambda probe 227, the higher is the NO desorption rate and the trap regeneration, leaving a low NO< residual.

However, a higher NO desorption rate creates significant NO slip, namely a high proportion of NO not trapped, if it occurs when the trap is loaded.

The choice of a right value of the upstream air to fuel ratio is also complicated by the fact that a low value of it implies greater fuel consumption, but if the Lean NO)( Trap 225 is not completely regenerated, a higher air to fuel ratio may also lead to greater fuel consumption because more frequent regenerations are necessary.

Therefore, according to an embodiment of the invention, first a relatively high value of the upstream air to fuel ratio is applied and then a lower value of said ratio is applied after a certain amount of time, namely when the trap 225 has been partially regenerated.

In other words, in order to optimize the trade off between fuel consumption and regeneration efficiency an embodiment of the invention provides for starting a regeneration event of the Lean NO Trap 225, using a first value 52 of an air to fuel ratio, and then varying the air to fuel ratio from the first value 52 to a second value 53, where the second value 53 is maintained until the end of the regeneration event.

More specifically, the final dynamic lambda target presented here can be calculated according to the logic depicted schematically in figure 2.

As soon as the lambda control has been enabled, a timer 50 is started. A time threshold 51 is set, the time threshold 51 being a function of engine speed and load.

A first value 52 of the air to fuel ratio is measured by the upstream lambda probe 227 and sent to the Electronic Control Unit 450.

On the basis of such first value 52, a regeneration event of the Lean NO Trap 225 is started and performed by means of fuel after injections into the internal combustion engine 110.

Tüternatively, a regeneration event of the Lean NO> Trap 225 may be performed by means of fuel injections into the exhaust line 224 of the internal combustion engine 110, upstream of the Lean NO Trap 225.

When the timer 50 reaches the time threshold 51, the electronic control unit 450 varies the lambda target is varied from the first value 52 to the second value 53, also measured by the upstream lambda probe 227.

The variation may be performed gradually by means of a rate of variation limiter 54. The rate limiter 54 may be function of engine speed and load.

Also, the first value 52 and the second value 53 of the air to fuel ratio may preferably be determined as a function of engine speed and engine load.

Figure 3 is graph that depicts an air to fuel ratio over time upstream and downstream of Lean NO Trap 225 according to an embodiment of the invention.

Curve A represents the value of the upstream air to fuel ratio, as measured by the upstream lambda probe 227 and curve B represents the value of the downstream air to fuel ratio, as measured by the downstream lambda probe 228.

According to this graph it can be seen that following the method according to the various embodiment of the invention, a low NO residual value is obtained meaning that the regeneration is fully completed, as expressed by a low downstream air to fuel ratio measured by the downstream lambda probe 228.

The fuel consumption is also low because only few seconds of rich mixture are needed to perform the method.

The method allows for only a moderate and acceptable NO slip since in the first phase when the first value 52 of the air to fuel ratio is used there is a relatively low desorption rate, then a moderate increase of the desorption rate occurs when the second value 53 of the air to fuel ratio is used.

Some embodiments described may be performed in an automotive system 100, as shown in Figures 4 and 5, that includes the 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 corrrnunication 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 rotating in time with the crankshaft 145. The valves 215 selectively allow air into the combustion charter 150 from the port * 25 210 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 dir. 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 aftertreatrnent devices 280 include, but are not limited to, catalytic converters (two and three way), oxidation catalysts, lean NO traps 225, hydrocarbon adsorbers, selective catalytic reduction (SCR) systems, and particulate filters. 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 corimunication with one or more sensors and/or devices associated with the ICE 110. 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 cern 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 corrffnunication with a memory system or data carrier 460 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/fran the various sensors and control devices. The program may ertody the methods disclosed herein, allowing the CPU to carryout out the steps of such methods and control the ICE 110.

While at least one exemplary eithodiment has been presented in the foregoing surruriary 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.

REFCE MJbERS timer 51 time threshold 52 first value of air to fuel ratio 53 second value of air to fuel ratio 54 rate limiter automotive system internal combustion engine (ICE) engine block 125 cylinder cylinder head camshaft piston crankshaft 150 combustion charter cam phaser fuel injector fuel rail fuel pump 190 fuel source intake manifold 205 air intake duct 210 intake air port 215 valves of the cylinder 220 exhaust gas port 224 exhaust line 225 exhaust manifold 227 lambda probe 228 air flow meter 230 turbocharger 240 compressor 250 turbine 255 Lean NQ Trap 260 intercooler 270 exhaust system 275 exhaust pipe 280 exhaust aftertreatment device 290 VGT actuator 300 EGR 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 and oil temperature and level sensors 400 fuel rail pressure sensor 410 cam position sensor 420 crank position sensor 430 exhaust pressure and temperature sensor 445 accelerator pedal position sensor 450 electronic control unit (ECU) 460 data carrier 500 first value of fuel quantity 510 final value of fuel quantity 520 linear evolution 530 ramp time 540 lean operation mode 550 offset aam

Claims

1. Method for operating a Lean NO Trap (225) in an Internal Combustion Engine (110), the method comprising: -starting a regeneration event of the Lean NO Trap (225) using a first value (52) of an air to fuel ratio, -subsequently varying the air to fuel ratio from the first value (52) to a second value (53), wherein the second value (53) is maintained until the end of the regeneration event.

2. Method according to claim 1, wherein the second value (53) of the air to fuel ratio is smaller than the first value (52).

3. Method according to claim 1, wherein the air to fuel ratio is varied from the first value (52) to the second value (53) when a time threshold (51) is reached, the time threshold (51) being determIned as a function of engine speed and engine load.

4. Method according to claim 1, wherein the first value (52) and the second value (53) of the air to fuel ratio are functions of engine speed and engine load.

5. Method according to claim 1, wherein the first (52) and the second value (53) of the air to fuel ratio are measured by a lambda probe (227) upstream of the Lean NO Trap (225) and in coninunication with the Electronic Control Unit (450).

6. Method according to claim 1, wherein the regeneration event of the Lean N0< Trap (225) is performed by means of fuel after injections into the internal combustion engine (110).

7. Method according to claim 1, wherein the regeneration event of the Lean NQ< Trap (225) is performed by means of fuel injections into the exhaust line (224) of the internal combustion engine (110) upstream of the Lean NO Trap (225).

8. An automotive system comprising an internal combustion engine (110), managed by an engine Electronic Control Unit (450), the engine (110) having at least a cylinder (125) equipped with a fuel injector (160) and being equipped with an exhaust line (224), having a Lean NO Trap Catalyst (225) and an upstream lambda probe (227) in corrmunication with the Electronic Control Unit (450) for measuring an air to fuel ratio, wherein the Electronic Control Unit (450) is configured to monitor the signals from the upstream lambda probe (227) and to start a regeneration event of the Lean NO Trap (225) using a first value (52) of an air to fuel ratio, and to subsequently vary the air to fuel ratio from the first value (52) to a second value (53), wherein the second value (53) is maintained until the end of the regeneration event

9. Internal combustion engine, in particular Diesel engine, the combustion engine (110) being equipped with a Lean NQ Trap (225), the engine (110) being equipped with an Electronic Control Unit (450) having a data carrier (460) and an interface bus for connection to sensors measuring parameters of the engine (110), the Electronic Control Unit (450) being configured to execute instructions stored in the data carrier (460) for the actuation of the method according to claims 1-7.

10. A computer program comprising a computer-code suitable for performing the method according to any of the claims 1-7.

11. Computer program product on which the computer program according to claim 9 is stored.

12. Control apparatus for an internal combustion engine (110) equipped with a Lean NO Trap (225), the control apparatus comprising an Electronic Control Unit (450), a data carrier (460) associated to the Electronic Control Unit (450) and a computer program according to claim 9 stored in the data carrier (460).

13. An electromagnetic signal modulated as a carrier for a sequence of data bits representing the computer program according to claim 9.