GB2502795A

GB2502795A - Method of enhancing ammonia generation in an exhaust system of an internal combustion engine

Info

Publication number: GB2502795A
Application number: GB1210024.4A
Authority: GB
Inventors: Andrea De Filippo; David Brian Brown; Rahul Mital
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2012-06-06
Filing date: 2012-06-06
Publication date: 2013-12-11
Also published as: GB201210024D0

Abstract

Disclosed is a method of enhancing ammonia generation in an exhaust system 270 of an internal combustion engine 110 of an automotive system 100. The exhaust system comprises at least one after-treatment device 280, the after-treatment device being a lean NOx trap 281. The method is applied during a lean NOx trap LNT regeneration condition and comprises determining a post main injection amount of fuel that can be used to produce excess ammonia during a rich engine operating phase used for purging the LNT. The excess ammonia can then be used be a selective catalytic reduction SCR system downstream of the LNT. The method makes use of a combination of open-loop and closed-loop modelling methods. An automotive system and control software are also disclosed.

Description

METHOD OF ENHANCING AMMONIA GENERATION IN AN EXHAUST SYSTEM OF

AN INTERNAL COMBUSTION ENGINE

TECHNICAL FIELD

The present disclosure relates to a method of enhancing ammonia generation in an exhaust system of an 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 (LNT).

A Lean NO Trap (LNT) is provided for trapping nitrogen oxides NO contained in the exhaust gas and is located in the exhaust me.

A LNT is a catalytic device containing catalysts, such as Rhodium, Platinum and Palladium1 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 (LNT) are subjected to periodic regeneration processes, whereby such regeneration processes are generally provided to release and reduce the trapped nitrogen oxides (NOr) from the LNT.

The LNT are operated cyclically, for example by switching the engine from lean-bum operation to operation whereby an excess amount of fuel is available, referred also as nitrogen oxides (NQ) from the LNT.

The LNT are operated cychcally, 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.

In the art are also known exhaust gas treatment systems for the emissions reduction and in particular of particulates and oxides of nitrogen (NO) from the diesel engine exhaust gas. These systems are provided with after-treatment devices installed along the exhaust line of the engine and typically comprise a diesel particulate filter (DPF) for control of particulates, and selective catalytic reduction (8CR) system for NOx control.

It is also known in the art, in some exhaust system configuration, to inject a reagent (catalyst) fluid in the exhaust line of the diesel engine in order to reduce emissions by means of the afore-mentioned aftertreatment devices. In particular, a fluid catalyst such as urea, or ammonia, or a combination thereof (generally in a water solution) are injected into the exhaust line of the diesel engine in order to promote the reduction of nitrogen oxides (NOr) in the selective catalytic reduction system (SCR).

Such a configuration, due to the number of components leads to an high system complexity, high costs of production, and also to a reduced installation flexibility of the exhaust gas treatment system.

A possible semplification of such complex exhaust system is to combine a Lean NOx Trap (LNT) upstream to a NH3 storage device (a SCR, selective catalytic reduction system or a SCRF, a particulate filter comprising a selective catalytic reduction system).

The big potential of such an architecture would be from one side the reduction of the after-treatment device numbers and on the other side the fact that an external urea/ammonia injector would not be needed anymore. In fact, during 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 too, The generated ammonia could be advantageously used for the downstream SCRISCRF to remove excess NOx, while the filter removes particulate matter. In such a way, no urea injector is needed, thus allowing a cheaper exhaust system architecture. Of course the ammonia generation must be carefully controlled and optimized: a lower amount of it is insufficient to help performing the complete nitrogen reduction without any external catalyst injection, while a higher amount causes an ammonia slip downstream the SCR or the SCRF. Furthermore, a sufficient amount of ammonia can be produced by requesting lower air/fuel ratio or long rich phase duration: this strategy has the drawback in providing higher hydrocarbon and CO emission level.

Therefore a need exists for a method that enhances ammonia generation during the rich phase, i.e. during the regeneration process of the LNT.

An object of this invention is to provide a method which enhances the ammonia generation in an exhaust architecture, combining a Lean NOx Trap (LNT) upstream to a SCR or a SCRF. In particular, the optimization is related to the management of a faster lean rich transition, to enhance the ammonia 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 andlor especially advantageous aspects.

SUMMARY

An embodiment of the disclosure provides a method of enhancing ammonia generation in an exhaust system of an internal combustion engine of an automotive system, the exhaust system comprising at least one after-treatment device, the after-treatment device being a lean NOx trap, the method, applied during a lean NOx trap regeneration condition, comprising: -determining the after injection quantity Qafter in open loop control -setting the time with only contribution of Qafter open loop control equal to zero -determining the actual air/fuel ratio -closing the loop on the air/to fuel ratio and determining the after injection quantity Qaner in closed loop control.

Consequently, an apparatus is disclosed for enhancing ammonia generation in an exhaust system of an internal combustion engine of an automotive system, the apparatus comprising: -means for determining the after injection quantity °after in open loop control -means for setting the time with only contribution of 0ar open loop control equal to zero -means for determining the actual air/fuel ratio -means for closing the loop on the air/to fuel ratio and determining the after injection quantity Qar, in closed loop control, Advantages of this embodiment are a higher amount of produced ammonia, without requesting lower air/fuel ratio or longer rich phase duration, during LMT regeneration, and a reduction of the total number of rich mode pulses triggered for the ammonia production According to another embodiment, closed loop enabling conditions are modified to anticipate their activation, by eliminating a condition air/fuel ratio equal to a predetermined value".

An advantage of this embodiment is that the switch from the after injection quantity Qaner in open loop control to both after injection quantity open and closed loop control is immediately realized.

According to a further embodiment1 said after injection quantity Qar in open loop is determined upon a speed/torque map.

An advantage of this embodiment is that the method does not require any additional engine map.

According to a still further embodiment, said air/fuel ratio closed loop control comprises a proportional factor KP and an integral factor KI.

An advantage of this embodiment is that the air/fuel ratio closed loop control is very

stable.

According to a still further embodiment, said proportional factor KP has been increased of more than 10% and said integral factor KI has been increased of more than 100%, to get a quicker closed loop control of the after injection quantity Qaner.

An advantage of this embodiment is an improvement of the overall LNT efficiency, since the released NOx will be instantaneously converted to NH3 by means of reactions with either hydrocarbons or hydrogen.

According to another embodiment, an internal combustion engine of an automotive system is provided1 said internal combustion engine being equipped with an exhaust system, comprising at least two after-treatment devices, the after-treatment devices being at least a lean NOx trap and a selective catalytic reduction system or a selective catalytic reduction system comprising a particulate filter, the automotive system comprising an electronic control unit configured for carrying out the above method according to one of its embodiments.

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 invention.

Figure 4 is a graph depicting the result of the ammonia production according to a known control method.

Figure 5 is a graph depicting the behavior of the interventions on the faster transition of the lean-rich combustion mode, according to the present invention.

Figure 6 is a graph depicting the result of the ammonia production according to the present control method.

Figure 7 is a control algorithm of a method for enhancing the ammonia generation in an internal combustion engine, according to an embodiment of the 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

B

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 (SOP) 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 ECR 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 1i0, including, but not limited to, the fuel injectors 160, the throttle body 330, the EGR Valve 320, the VGT actuator 290, and the cam phaser 155. Note, dashed lines are used to indicate communication between the ECU 450 and the various sensors and devices, but some are omitted for clarity.

Turning now to the ECU 450, this apparatus may include a digital central processing unit (CPU) in communication with a memory system and an interface bus. The CPU is configured to execute instructions stored as a program in the memory system, and send and receive signals to/from the interface bus. The memory system may include various storage types including optical storage, magnetic storage, solid state storage, and other nan-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 after-treatment system, which features the combination of two NOx after-treatment devices. The combined system consists (Fig. 3) of an upstream LNT 281 and a downstream SGR 282 or SCRF (SCR an DPF) 283 positioned in all possible close-coupled and/or under floor configurations.

Preferably, the first 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 both LNT and SCR/SCRF. 11*

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, after injecting fuel by the so called after injection (Qaner), the NOx is released and converted.

However during rich operating conditions1 NH3 (ammonia) is also generated even though this is not typically monitored when the LNT catalyst is used as stand alone or with a downstream DPF. According to known methods, the amount of generated ammonia depends on the specific management of the rich combustion conditions, as for example the lambda value, the duration of each single DeNOx events and the temperature conditions. Moreover, the ammonia production, according to the present method, could be further enhanced, by acting on the control method. In particular, the method consists in realizing a quicker lean -rich combustion mode transition, by immediately enabling the after injection closed loop control with respect to the after injection open loop control.

In detail, the method utilizes (see Figure 7) a P1 (proportional + integral) controller 25 controlling the after injection in closed loop, via the air/fuel ratio. The set point is represented by the airtfuel ratio demand 22 and the controlled quantity is represented by the actual air/fuel ratio 24. The error between them is corrected twice, by the controller proportional factor KP and by the controller integral factor KI. Then, the after injection in closed loop 26 is given by the sum of the closed loop controller output. Finally, the actuated after injection 29 results as the sum of the contribution of the open loop 23 and the contribution of the closed loop 26. Of course, the actuated after injection determines a new value of the actual airlfuel ratio, to be used in the P1 controller. The closed and the open loop control could be activated depending on the state of the DeNOx demand 20 and the C.L. enable conditions 21. As mentioned above, with respect to the known strategy, the new method for enhancing the ammonia generation immediately enables the closed loop contribution.

Experimental activities on the actual LNT technology have been performed, in order to assess the real capability to generate more ammonia, by changing the control method.

S The results are shown in Fig 4-6 and will be now commented.

The test conditions have been set up as follows: -vehicle test -steady state on vehicle test bench at 70 kpmlhrs (1650 rpm x 80 Nm) -NOx stored before the rich mode request: -0.20 g -LNT upstream temperature: 275°C -desired air/fuel value: 0.95 -rich time duration: 15 sec In Fig. 4 and 6, 500 is a step curve, indicating the combustion mode behavior. The first step is the transition lean-rich, the second step the transition rich-lean. 510 is the curve of the air/fuel ratio, measured by an oxygen sensor, 520 is the amount of the ammonia production, measured by a NI-13 sensor, 530 is the amount of engine NOx, measured by a NOx sensor. In Fig. 5, left side represents the result according to a known control method, while right side are the result due to the faster lean-rich transition according to the invention. 540 represents the duration of Qafter open loop control, while 550 is the duration of Qafter closed loop control.

In Fig. 4, the results of the LNT regeneration, by applying a known control method are shown. As evident, the air/fuel ratio 510 target (0.95) is reached smoothly. On one side, small air/fuel ratio undershoot can be observed. On the other side, the peak of the ammonia 520, which is released by the [NT after 15 sec of rich combustion mode is only about 460 ppm.

Turning to Fig. 5, left side, it has to be observed that the activation of the closed loop control 550 occurs after a delay, during which Qafter is only determined by the open loop control 540, which can be based on an existing speed/torque engine map. This leads to an air/fuel value of about 1.02 and then the closed loop control is activated. Therefore, a potential to enhance the ammonia production has been seen in quicker enabling the closed loop control and empowering the correction applied by the air/fuel closed loop controL Looking at both sides of Fig. 5, a comparison between the known method and the present one can be done: the new method prescribed that the closed coop controller is suddenly engaged; in other words, the time, during which only the open loop contribution is active, has been set to zero. Therefore, the correction, applied by the closed loop controller is much quicker. On one side, see also Fig. 6, the air/fuel ratio target (0.95) is reached in a shorter time and consequently this leads to some air/fuel ratio undershoot; on the other side, the increased amount of hydrocarbons in the LNT environment enhances the ammonia production (about 785 ppm, almost doubled) after 15 sec of rich combustion mode.

Summarizing and following Fig. 7, according to a preferred embodiment, the method of enhancing the ammonia generation starts from determining 20 a [NT 281 regeneration condition, enabling 21 a closed loop control on the after injection quantity and determining 22 an air/fuel ratio demand. The first two conditions will enable the P1 controller 25 to operate the closed loop control, the third condition is the set-point variable for the controller itself. The method goes further determining 23 the after injection quantity Qar in open loop control and setting the time with only contribution of °after open loop control equal to zero. In other words, immediately enabling the closed loop control. After determining 24 the actual airffuel ratio, the P1 controller closes the loop 25 on the air/to fuel ratio and determining 26 the after injection quantity in closed loop. Finally, according to the method, the contribution Qar in open loop control 23 and Qer in closed oop control 26 are summed 28 to obtain the actuated after injection quantity 29.

As already stated the fundamental of the method is to anticipate the activation of the closed loop enabling conditions. A preferable way is to eliminate the condition "air/fuel ratio equal to a predetermined value". In fact, the switch of the after injection quantity control from only open loop control to both open and closed loop control is realized, according to known methods, only when the airffuel ratio has reached a fixed value, for instance 1.02.

Advantageously, said air/fuel ratio closed loop control comprises a proportional factor KP and an integral factor KI.

According to a preferred embodiment, said proportional factor KP has been increased of more than 10% with respect to the one used in the known method and said integral factor 1(1 has been increased of more than 100%, to get a quicker closed loop control of the after injection quantity Qafter The method according to one or more of the presented embodiments, has different advantages with respect to the known methods. First of all, a higher amount of ammonia is produced, without requesting lower air/fuel ratio or longer rich phase duration during LNT regeneration; consequently, a reduction of the total number of rich mode pulses triggered for the ammonia production is possible; finally, an improvement of the overall LNT efficiency can be reached, since the released NOx will be instantaneously converted to Nl-13, by means of reactions with either hydrocarbons or hydrogen.

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 block

21 block 22 block 23 block 24 block block 26 block 27 block 28 block data carrier automotive system internal combustion engine engine block 125 cylinder cylinder head camshaft piston crankshaft 150 combustion chamber cam phaser fuel injector fuel rail fuel pump l9ofuelsource * 200 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 after-treatment devices 281 lean NOx trap 282 selective catalytic reduction (8CR) system 283 selective catalytic reduction systems comprising a particulate filters (SCRF) 284 NH3INOx dual sensor upstream the SCR/SCRF 285 NH3/NOx dual sensor downstream the SCR/SCRF 290 VGT actuator 300 exhaust gas recirculation system 310 EGR cooler 320 EGR valve 330 throttle body 340 mass airflow and temperature sensor 350 manifold pressure and temperature sensor 360 combustion pressure sensor 380 coolant temperature and level sensors 385 lubricating oil temperature and level sensor 390 metal temperature sensor 400 fuel rail pressure sensor 410 cam position sensor 420 crank position sensor 430 exhaust pressure and temperature sensors 440 EGR temperature sensor 445 accelerator position sensor 446 accelerator pedal 450 ECU 500 combustion mode 510 air/fuel ratio 520 NH3 production 530 engine NOx 540 after injection in open loop control 550 after injection in closed loop control 2 D Qofter after injection fuel quantity KP controller proportional factor KI controller integral factor

Claims

CLAIMS1. Method of enhancing ammonia generation in an exhaust system (270) of an internal combustion engine (110) of an automotive system (100), the exhaust system comprising at least one after-treatment device (280), the after-treatment device being a lean NOx trap (281), the rtiethod, applied during a lean NOx trap regeneration condition, comprising: - -determining (23) an after injection quantity (Qafter) in open loop control -selling the time with only contribution of (Qafter) open loop control equal to zero -detemiining (24) the actual air/fuel ratio -closing the loop (25) on the air/to fuel ratio and determining (26) an after injection quantity (Qafte,) in closed loop control.
2. Method according to claim 1, wherein closed loop enabling conditions are modified to anticipate their activation, by eliminating a condition "air/fuel ratio equal to a predetermined value".
3. Method according to claim 1, wherein said after injection quantity (Qafter) in open loop is determined (23) upon a speedftorque map.
4. Method according to claim 1, wherein said air/fuel ratio closed loop control comprises a proportional factor (KP), and an integral factor (KI).
5. Method according to claim 3, wherein said proportional factor (KP) has been increased of more than 10% and said integral factor (KI) has been increased of more than 100%, to get a quicker closed loop control of the after injection quantity (Qarter).
6. Internal combustion engine (110) of an automotive system (100) equipped with an exhaust system (270), comprising at least two after-treatment devices (280), the after-treatment devices being at least a lean NOx trap (281) and a selective catalytic reduction system (282) or a selective catalytic reduction system comprising a particulate filter (283), the automotive system (100) comprising an electronic control unit (450) configured for carrying out the method according to claims 1-5.
7. A computer program comprising a computer-code suitable for performing the method according to any of the claims 1-5.
8. Computer program product on which the computer program according to claim 7 is stored.
9. Control apparatus for an internal combustion engine, comprising an Electronic Control Unit (450), a data carrier (40) associated to the Electronic Control Unit (450) and a computer program according to claim 7 stored in the data carrier (40).
10. An electromagnetic signal modulated as a carrier for a sequence of data bits representing the computer program according to claim 7.