U.S. patent application number 10/012577 was filed with the patent office on 2003-05-29 for nox purge air/fuel ratio selection.
Invention is credited to Hepburn, Jeffrey Scott, Ingram, Grant Alan, Roth, John M., Surnilla, Gopichandra, Theis, Joseph Robert.
Application Number | 20030097833 10/012577 |
Document ID | / |
Family ID | 21755620 |
Filed Date | 2003-05-29 |
United States Patent
Application |
20030097833 |
Kind Code |
A1 |
Ingram, Grant Alan ; et
al. |
May 29, 2003 |
NOX PURGE AIR/FUEL RATIO SELECTION
Abstract
A method for improving a purge conversion efficiency of a Lean
NOx Trap coupled downstream of a lean-burn internal combustion
engine is presented. This method recognizes that during a purge of
the LNT, its temperature increases due to the exothermic reactions
in the LNT. Once the LNT temperature exceeds a certain threshold,
further increases lead to a reduction in the NOx storage capacity,
and therefore an increase in NOx emissions during the purge of the
LNT. Therefore, it is proposed to cool the LNT temperature once the
threshold is exceeded. This method improves emission control and
fuel economy during purge.
Inventors: |
Ingram, Grant Alan; (West
Lafayette, IN) ; Surnilla, Gopichandra; (West
Bloomfield, MI) ; Hepburn, Jeffrey Scott;
(Birmingham, MI) ; Roth, John M.; (Grosse Ile,
MI) ; Theis, Joseph Robert; (Rockwood, MI) |
Correspondence
Address: |
FORD GLOBAL TECHNOLOGIES, INC
SUITE 600 - PARKLANE TOWERS EAST
ONE PARKLANE BLVD.
DEARBORN
MI
48126
US
|
Family ID: |
21755620 |
Appl. No.: |
10/012577 |
Filed: |
November 29, 2001 |
Current U.S.
Class: |
60/274 ; 60/277;
60/287; 60/298; 60/301 |
Current CPC
Class: |
F02D 41/0275 20130101;
Y02T 10/22 20130101; F01N 3/0871 20130101; F02D 41/1456 20130101;
Y02T 10/12 20130101; F02D 2200/0406 20130101; F02D 2041/0265
20130101; F01N 3/05 20130101; F02D 2011/102 20130101; Y02T 10/24
20130101; F01N 13/009 20140601; F01N 2240/02 20130101; F01N 2430/08
20130101; F01N 3/0842 20130101; F01N 2430/06 20130101; F02D 41/1441
20130101; F01N 3/0814 20130101; F01N 2570/14 20130101; Y02A 50/20
20180101; Y02A 50/2344 20180101 |
Class at
Publication: |
60/274 ; 60/277;
60/287; 60/298; 60/301 |
International
Class: |
F01N 003/00; F01N
007/00; F01N 003/10 |
Claims
1. A method for improving a performance of an exhaust gas
aftertreatment device during a purge, the device coupled downstream
of a lean-burn internal combustion engine, the method comprising:
providing an indication of a device operating condition, wherein
said condition is reached when a capacity of the device to store an
exhaust gas component decreases with increases in a temperature of
the device; and in response to said indication, adjusting an
operating parameter, thereby decreasing said device
temperature.
2. The method as cited in claim 1 wherein the exhaust gas
aftertreatment device is a Lean NOx Trap.
3. The method as set forth in claim 1 wherein said exhaust gas
component is NOx.
4. The method as cited in claim 1 wherein said indication is
provided based on a signal from a temperature sensor disposed
inside the exhaust gas aftertreatment device.
5. The method as cited in claim 1 wherein said indication is
provided based on an estimate of an engine operating condition.
6. The method as cited in claim 5 wherein said engine operating
condition is an engine speed.
7. The method as cited in claim 5 wherein said engine operating
condition is an air-fuel ratio.
8. The method as cited in claim 1 wherein adjusting said operating
condition comprises reducing a temperature of an exhaust gas
mixture entering the device.
9. A method for controlling emissions during a purge in a lean-burn
internal combustion engine having an exhaust gas aftertreatment
device connected downstream by a manifold comprising a first branch
and a second branch longer than the first branch, the manifold
further comprising a valve, the method comprising: providing an
indication that the device temperature is within a predetermined
range, wherein within said range a capacity of the device to store
an exhaust gas component decreases with increases in said device
temperature; and in response to said indication, adjusting the
valve such that a flow of an exhaust gas mixture through the first
branch is substantially disabled.
10. The method as cited in claim 9 wherein the exhaust gas
aftertreatment device is a lean NOx trap.
11. The method as cited in claim 9 wherein said exhaust gas
component is NOx.
12. The method as cited in claim 9 wherein said indication is
provided based on a signal from a temperature sensor disposed
inside the exhaust gas aftertreatment device.
13. The method as cited in claim 9 wherein said indication is
provided based on an estimate of an engine operating condition.
14. The method as cited in claim 13 wherein said engine operating
condition is an engine speed.
15. The method as cited in claim 13 wherein said engine operating
condition is an air fuel ratio.
16. A system for improving a performance of an exhaust gas
aftertreatment device coupled downstream of an internal combustion
engine, the system comprising: a manifold connecting the engine and
the device, said manifold comprising a first branch and a second
branch; a valve disposed in said manifold, said valve capable of
directing a flow of an exhaust gas mixture entering said manifold
through said first branch and said second branch; and a controller
determining that a temperature of the device is above a
predetermined threshold, in response to said determination,
calculating a rate of change of a NOx storage capacity of the
device as a function of said device temperature, and controlling
said valve such that the flow of said exhaust gas mixture is
substantially directed through said second branch when said
capacity rate of change is below a precalculated value.
17. The system as set forth in claim 16 wherein the exhaust gas
aftertreatment device is a three-way catalyst.
18. The system as set forth in claim 16 wherein said three-way
catalyst is a lean NOx trap.
19. The system as set forth in claim 16 wherein said precalculated
value is substantially zero.
Description
FIELD OF INVENTION
[0001] The present invention relates to a system and a method for
controlling a lean-burn internal combustion engine, and more
particularly, to minimizing NOx emissions during a purge of the
LNT.
BACKGROUND OF THE INVENTION
[0002] Internal combustion engines are typically coupled to an
emission control device known as a three-way catalytic converter
(TWC) designed to reduce combustion by-products such as carbon
monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx).
Engines can operate at air-fuel mixture ratios lean of
stoichiometry, thus improving fuel economy. For lean engine
operation, an additional three-way catalyst commonly referred to as
a Lean NOx Trap (LNT), is usually coupled downstream of an upstream
three-way catalyst. The LNT, like the TWC, stores exhaust gas
constituents such as, for example, nitrogen oxides, NOx, when the
engine is operating at a lean air-fuel ratio, and reduces (purges)
them when the engine is operating at a rich or stoichiometric
air-fuel ratio.
[0003] Because continued lean operation will ultimately saturate
the LNT with NOx, the prior art teaches periodically varying the
air-fuel ratio from a nominally lean setting to a rich setting,
during which stored NOx are released from the LNT and reduced by
the available hydrocarbons and carbon monoxides in the enriched
operating condition.
[0004] The inventors herein have recognized that during the purge,
some of the released NOx is not reduced, and is therefore emitted
into the atmosphere. The inventors have further recognized that the
LNT temperature raises during the purge due to the exothermic
reaction created by the reduction of NOx by CO in the rich air fuel
mixture entering the LNT, and once the temperature exceeds a
predetermined threshold, the LNT's NOx storage capacity decreases
with increasing temperature. Therefore, the inventors recognized
that this decrease in the NOx storage capacity as the LNT
temperature is increasing, would cause the NOx to be displaced from
the LNT until the absorbed NOx is equivalent to the reduced LNT
capacity at higher temperature. The NOx thus displaced would exit
the LNT as emissions, and therefore the performance of the LNT
would be degraded.
SUMMARY OF THE INVENTION
[0005] In solving the above problem, a system and a method are
provided for decreasing NOx emissions during an LNT purge by active
temperature control of the LNT.
[0006] In carrying out the above solution, features and advantages
of the present invention, a system and a method for improving a
performance of an exhaust gas aftertreatment device during a purge,
the device coupled downstream of a lean-burn internal combustion
engine, include: providing an indication of a device operating
condition, wherein said condition is reached when a capacity of the
device to store an exhaust gas component decreases with increases
in a temperature of the device; and in response to said indication,
adjusting an operating parameter, thereby decreasing said device
temperature.
[0007] An advantage of the present invention is improved emission
control, due to the fact that maintaining the temperature of the
LNT in the region where NOx storage capacity does not reduce with
increases in temperature, reduces NOx emissions during purge.
[0008] The above advantages and other advantages, objects and
features of the present invention will be readily apparent from the
following detailed description of the preferred embodiments when
taken in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The objects and advantages described herein will be more
fully understood by reading an example of an embodiment in which
the invention is used to advantage, referred to herein as the
Description of Preferred Embodiment, with reference to the
drawings, wherein:
[0010] FIG. 1 is a block diagram of an internal combustion engine
illustrating various components related to the present
invention;
[0011] FIG. 2 is a plot of the LNT NOx storage capacity during a
purge vs. the LNT temperature; and
[0012] FIG. 3 is a block diagram of the embodiment in which the
invention is used to advantage.
DESCRIPTION OF PREFERRED EMBODIMENTS(S)
[0013] As will be appreciated by those of ordinary skill in the
art, the present invention is independent of the particular
underlying engine technology and configuration. As such, the
present invention may be used in a variety of types of internal
combustion engines, such a s conventional engines in addition to
direct injection stratified charge (DISC) or direct injection spark
ignition engines (DISI).
[0014] A block diagram illustrating an engine control system and
method for a representative internal combustion engine according to
the present invention is shown in FIG. 1. Preferably, such an
engine includes a plurality of combustion chambers only one of
which is shown, and is controlled by electronic engine controller
12. Combustion chamber 30 of engine 10 includes combustion chamber
walls 32 with piston 36 positioned therein and connected to
crankshaft 40. In this particular example, the piston 30 includes a
recess or bowl (not shown) for forming stratified charges of air
and fuel. In addition, the combustion chamber 30 is shown
communicating with intake manifold 44 and exhaust manifold 48 via
respective intake valves 52a and 52b (not shown), and exhaust
valves 54a and 54b (not shown). A fuel injector 66 is shown
directly coupled to combustion chamber 30 for delivering liquid
fuel directly therein in proportion to the pulse width of signal
fpw received from controller 12 via conventional electronic driver
68. Fuel is delivered to the fuel injector 66 by a conventional
high-pressure fuel system (not shown) including a fuel tank, fuel
pumps, and a fuel rail.
[0015] Intake manifold 44 is shown communicating with throttle body
58 via throttle plate 62. In this particular example, the throttle
plate 62 is coupled to electric motor 94 such that the position of
the throttle plate 62 is controlled by controller 12 via electric
motor 94. This configuration is commonly referred to as electronic
throttle control, (ETC), which is also utilized during idle speed
control. In an alternative embodiment (not shown), which is well
known to those skilled in the art, a bypass air passageway is
arranged in parallel with throttle plate 62 to control inducted
airflow during idle speed control via a throttle control valve
positioned within the air passageway.
[0016] Exhaust gas sensor 76 is shown coupled to exhaust manifold
48 upstream of catalytic converter 70. In this particular example,
sensor 76 is a universal exhaust gas oxygen (UEGO) sensor, also
known as a proportional oxygen sensor. The UEGO sensor generates a
signal whose magnitude is proportional to the oxygen level (and the
air-fuel ratio) in the exhaust gases. This signal is provided to
controller 12, which converts it into a relative air-fuel
ratio.
[0017] Advantageously, signal UEGO is used during feedback air-fuel
ratio control in to maintain average air-fuel ratio at a desired
air-fuel ratio as described later herein. In an alternative
embodiment, sensor 76 can provide signal EGO, exhaust gas oxygen
(not shown), which indicates whether exhaust air-fuel ratio is lean
or rich of stoichiometry. In another alternate embodiment, the
sensor 76 may comprise one of a carbon monoxide (CO) sensor, a
hydrocarbon (HC) sensor, and a NOx sensor that generates a signal
whose magnitude is related to the level of CO, HC, NOx,
respectively, in the exhaust gases.
[0018] Those skilled in the art will recognize that any of the
above exhaust gas sensors may be viewed as an air-fuel ratio sensor
that generates a signal whose magnitude is indicative of the
air-fuel ratio measured in exhaust gases.
[0019] Conventional distributorless ignition system 88 provides
ignition spark to combustion chamber 30 via spark plug 92 in
response to spark advance signal SA from controller 12.
[0020] Controller 12 causes combustion chamber 30 to operate in
either a homogeneous air-fuel ratio mode or a stratified air-fuel
ratio mode by controlling injection timing. In the stratified mode,
controller 12 activates fuel injector 66 during the engine
compression stroke so that fuel is sprayed directly into the bowl
of piston 36. Stratified air-fuel layers are thereby formed. The
stratum closest to the spark plug contains a stoichiometric mixture
or a mixture slightly rich of stoichiometry, and subsequent strata
contain progressively leaner mixtures.
[0021] In the homogeneous mode, controller 12 activates fuel
injector 66 during the intake stroke so that a substantially
homogeneous air-fuel mixture is formed when ignition power is
supplied to spark plug 92 by ignition system 88. Controller 12
controls the amount of fuel delivered by fuel injector 66 so that
the homogeneous air-fuel ratio mixture in chamber 30 can be
selected to be substantially at (or near) stoichiometry, a value
rich of stoichiometry, or a value lean of stoichiometry. Operation
substantially at (or near) stoichiometry refers to conventional
closed loop oscillatory control about stoichiometry. The stratified
air-fuel ratio mixture will always be at a value lean of
stoichiometry, the exact air-fuel ratio being a function of the
amount of fuel delivered to combustion chamber 30. An additional
split mode of operation wherein additional fuel is injected during
the exhaust stroke while operating in the stratified mode is
available. An additional split mode of operation wherein additional
fuel is injected during the intake stroke while operating in the
stratified mode is also available, where a combined homogeneous and
split mode is available.
[0022] Lean NOx trap 72 is shown positioned downstream of catalytic
converter 70. Both devices store exhaust gas components, such as
NOx, when engine 10 is operating lean of stoichiometry. These are
subsequently reacted with HC, CO and other reductant and are
catalyzed during a purge cycle when controller 12 causes engine 10
to operate in either a rich mode or a near stoichiometric mode.
[0023] Exhaust gas manifold 74 has a control valve 76 disposed in
it. The valve is controlled by controller 12, which sends a signal
to open or close the valve, thus enabling or disabling passage of
the exhaust gas through it. Additionally, exhaust gas manifold 74
has a cooling loop 78. When the LNT temperature needs to be
increased, valve 76 is open, and most of the hot exhaust gas
travels via the short path, thus causing the LNT temperature to
rise. When the LNT temperature needs to be lowered, valve 76 is
closed, thus routing the gas through the cooling loop, where it
cools down prior to entering the LNT, thus lowering the LNT
temperature. Alternatively, the LNT temperature could be lowered or
raised by adjusting engine-operating parameters such as spark
timing, air-fuel ratio, compression ratio, etc.
[0024] Controller 12 is shown in FIG. 1 as a conventional
microcomputer including but not limited to: microprocessor unit
102, input/output ports 104, an electronic storage medium for
executable programs and calibration values, shown as read-only
memory chip 106 in this particular example, random access memory
108, keep alive memory 110, and a conventional data bus.
[0025] Controller 12 is shown receiving various signals from
sensors coupled to engine 10, in addition to those signals
previously discussed, including: measurement of inducted mass air
flow (MAF) from mass air flow sensor 100 coupled to throttle body
58; engine coolant temperature (ECT) from temperature sensor 112
coupled to cooling sleeve 114; a profile ignition pickup signal
(PIP) from Hall effect sensor 118 coupled to crankshaft 40 giving
an indication of engine speed (RPM); throttle position TP from
throttle position sensor 120; and absolute Manifold Pressure Signal
MAP from sensor 122. Engine speed signal RPM is generated by
controller 12 from signal PIP in a conventional manner and manifold
pressure signal MAP provides an indication of engine load.
[0026] Fuel system 130 is coupled to intake manifold 44 via tube
132. Fuel vapors (not shown) generated in fuel system 130 pass
through tube 132 and are controlled via purge valve 134. Purge
valve 134 receives control signal PRG from controller 12.
[0027] Exhaust sensor 140 is a NOx/UEGO sensor located downstream
of the LNT. It produces two output signals. First output signal
(SIGNAL1) and second output signal (SIGNAL2) are both received by
controller 12. Exhaust sensor 140 can be a sensor known to those
skilled in the art that is capable of indicating both exhaust
air-fuel ratio and nitrogen oxide concentration.
[0028] In a preferred embodiment, SIGNAL1 indicates exhaust
air-fuel ratio and SIGNAL2 indicates nitrogen oxide concentration.
In this embodiment, sensor 140 has a first chamber (not shown) in
which exhaust gas first enters where a measurement of oxygen
partial pressure is generated from a first pumping current. Also,
in the first chamber, oxygen partial pressure of the exhaust gas is
controlled to a predetermined level. Exhaust air-fuel ratio can
then be indicated based on this first pumping current. Next, the
exhaust gas enters a second chamber (not shown) where NOx is
decomposed and measured by a second pumping current using the
predetermined level. Nitrogen oxide concentration can then be
indicated based on this second pumping current. In an alternative
embodiment, a separate NOx sensor could be used in conjunction with
an air-fuel sensor, which could be a UEGO or a HEGO sensor.
[0029] FIG. 2 is a plot of the LNT NOx storage capacity vs. the LNT
temperature during a NOx purge. As can be seen, at lower LNT purge
temperatures (Region A), NOx storage capacity increases with
increasing LNT temperatures, until T.sub.opt, an LNT temperature at
which maximum NOx storage capacity is achieved. At higher LNT purge
temperatures (Region B), NOx storage capacity decreases with
increasing LNT temperatures. The drop in NOx storage capacity with
increasing LNT temperatures is especially noticeable in region C,
when the LNT temperature exceeds T.sub.crit. As can be seen in the
plot, if the LNT is saturated with NOx at temperatures above
T.sub.crit, an increase in temperature would cause NOx to be
displaced and released into the atmosphere till the absorbed NOx is
equal to the new reduced LNT NOx storage capacity. Therefore, in
order to reduce the amount of NOx released during a NOx purge, the
LNT temperature needs to be controlled to be below T.sub.crit. This
can be accomplished by using the cooling loop as described with
particular reference in FIG. 2 above.
[0030] The diagram in FIG. 3 generally represents operation of one
embodiment of a system or method according to the present
invention. As will be appreciated by one of ordinary skill in the
art, the diagram may represent one or more of any number of
processing strategies such as event-driven, interrupt-driven,
multi-tasking, multi-threading, and the like. As such, various
steps or functions illustrated may be performed in the sequence
illustrated, I parallel, or in some cases omitted. Likewise, the
order of processing is not necessarily required to achieve the
objects, features and advantages of the invention, but is provided
for ease of illustration and description.
[0031] Although not explicitly illustrated, one of ordinary skill
in the art will recognize that one or more of the illustrated steps
or functions may be repeatedly performed depending on the
particular strategy being used.
[0032] Referring now to FIG. 3, first, in step 50, a determination
is made whether a NOx purge should be performed. If the answer to
step 50 is NO, the routine exits. If the answer to step 50 is YES,
the routine proceeds to step 100, where a determination is made
whether the LNT temperature is less than or equal to T.sub.1 (250
deg. C. in this example). If the answer to step 100 is YES, the
valve 76 described above with particular reference to FIG. 1 is
opened thus allowing most of the exhaust gas mixture to travel to
the LNT via manifold 74 described above with particular reference
to FIG. 1, and heat the LNT. The routine then proceeds to step 800
wherein a purge mixture air-fuel ratio is selected as a function of
the LNT temperature. The routine then proceeds to step 900 wherein
a NOx purge is performed. If the answer to step 100 is NO, i.e.,
the LNT temperature is above T.sub.1, the routine proceeds to step
300 wherein .circle-solid.T, a temperature rise in the LNT due to
the exothermal reaction created by the LNT purge is determined from
a look-up table based on the amount of NOx and oxygen stored in the
LNT. The amounts of NOx and O.sub.2 stored in the LNT could be
estimated, for example, from engine operating conditions, such as
engine speed, load, air-fuel ratio, etc. Next, in step 400,
T.sub.pg.sub..sub.--.sub.max, the maximum desired LNT temperature
at the beginning of the NOx purge, is calculated:
T.sub.pg.sub..sub.--.sub.max=T.sub.crit-.DELTA.T,
[0033] wherein T.sub.crit is the temperature above which the LNT
NOx storage capacity decreases with further temperature increases
due to exothermic reactions in the LNT (in this example, 450 deg.
C.). The value of T.sub.crit is typically experimentally determined
from the physical and chemical characteristics of the LNT. Next, in
step 500, a determination is made whether T.sub.LNT, the current
LNT temperature is greater than T.sub.pg.sub..sub.--.sub.max. If
the answer to step 500 is YES, further increases in the LNT
temperature will cause a reduction in NOx storage capacity, and
consequently cause an increase in NOx emissions during the purge.
Therefore, in accordance with the present invention, the routine
proceeds to step 700, wherein valve 76 is closed thus routing the
exhaust gas mixture exiting the engine via the cooling loop 78.
Thus cooled exhaust gas mixture enters the LNT and reduces its
temperature below T.sub.pg.sub..sub.--.sub.max so that NOx
emissions during a purge are reduced. The routine then proceeds to
step 800 wherein a purge air-fuel ratio is selected, and then to
step 900 wherein a NOx purge commences. The routine then exits. If
the answer to step 500 is YES, i.e., the temperature of the LNT is
below the maximum desired LNT purge temperature, the routine
proceeds to step 600 wherein valve 76 is opened thus allowing most
of the exhaust gas mixture exiting the engine to travel via
manifold 74. The routine then proceeds to step 800 described
above.
[0034] Therefore, according to the present invention, it is
possible to reduce the amount of NOx emissions released during a
NOx purge of the LNT by determining a critical temperature above
which the NOx storage capacity of the LNT reduces with further
increases in temperature, and by reducing the LNT temperature once
the threshold is reached. The reduction can be accomplished by
cooling off of the exhaust gas mixture entering the LNT either by
routing the mixture via a cooling loop, or by adjusting engine
parameters, such as the air-fuel ratio, spark timing, compression
ratio, etc. Further, using this method, the LNT temperature during
the purge can be maintained close to the optimal temperature for
achieving peak NOx storage capacity.
[0035] This concludes the description of the invention. The reading
of it by those skilled in the art would bring to mind many
alterations and modifications without departing from the spirit and
the scope of the invention. Accordingly, it is intended that the
scope of the invention be defined by the following claims:
* * * * *