U.S. patent application number 12/696685 was filed with the patent office on 2011-08-04 for adaptive desulfation control algorithm.
This patent application is currently assigned to Eaton Corporation. Invention is credited to Christian Thomas Chimner, James Edward McCarthy, JR., Jiyang Yan, Hanlong Yang.
Application Number | 20110185708 12/696685 |
Document ID | / |
Family ID | 44147515 |
Filed Date | 2011-08-04 |
United States Patent
Application |
20110185708 |
Kind Code |
A1 |
McCarthy, JR.; James Edward ;
et al. |
August 4, 2011 |
Adaptive Desulfation Control Algorithm
Abstract
The exhaust from a diesel-fueled internal combustion engine is
treated by a lean NO.sub.X trap. The maximum temperature used for
desulfating the lean NO.sub.X trap is kept relatively lower during
early life and progressively increased as the trap ages. Designing
for adequate late life performance entails excess capacity during
early life. The method utilizes the excess capacity available
during early life to slow aging of the trap and thereby extend the
trap lifetime. The method facilitates meeting durability
requirements for diesel-powered vehicles with exhaust
aftertreatment.
Inventors: |
McCarthy, JR.; James Edward;
(Canton, MI) ; Yan; Jiyang; (Troy, MI) ;
Chimner; Christian Thomas; (Royal Oaks, MI) ; Yang;
Hanlong; (Novi, MI) |
Assignee: |
Eaton Corporation
Cleveland
OH
|
Family ID: |
44147515 |
Appl. No.: |
12/696685 |
Filed: |
January 29, 2010 |
Current U.S.
Class: |
60/286 ;
60/299 |
Current CPC
Class: |
F01N 11/002 20130101;
F01N 3/0814 20130101; F01N 3/0842 20130101; Y02T 10/24 20130101;
F01N 2570/14 20130101; Y02A 50/2344 20180101; Y02T 10/12 20130101;
F01N 2570/04 20130101; F01N 13/0097 20140603; F01N 3/0871 20130101;
F01N 2560/026 20130101; F01N 2250/02 20130101; F01N 2560/06
20130101; F01N 2250/12 20130101; F01N 2240/30 20130101; Y02T 10/40
20130101; Y02T 10/47 20130101; F01N 2610/03 20130101 |
Class at
Publication: |
60/286 ;
60/299 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/10 20060101 F01N003/10 |
Claims
1. A method of operating a diesel power generation system in which
the exhaust from a diesel-fueled internal combustion engine is
treated using a lean NO.sub.X trap, the method comprising:
operating the engine to produce a lean exhaust comprising NO.sub.X
and SO.sub.X; treating the exhaust using the lean NO.sub.X trap;
determining when to denitrate the lean NO.sub.X trap and
denitrating the lean NO.sub.X trap accordingly; from time-to-time,
desulfating the lean NO.sub.X trap by heating the lean NO.sub.X
trap to within a desulfating temperature range having an upper
limit temperature and exposing the heated lean NO.sub.X trap to
rich conditions under which the lean NO.sub.X trap desulfates;
aging the lean NO.sub.X trap through many desulfations; and in
response to the lean NO.sub.X trap's aging, increasing the upper
limit temperature.
2. A diesel power generation adapted, configured, and functional to
operate according to the method of claim 1.
3. The method of claim 1, wherein: heating the lean NO.sub.X trap
to within a desulfating temperature range having an upper limit
temperature consists of controlling the heating of the lean
NO.sub.X trap to bring a characteristic temperature for the lean
NO.sub.X trap to a predetermined target value; and increasing the
upper limit temperature consists of increasing the target
value.
4. The method of claim 1, wherein: the aging takes place over two
or more consecutive time intervals, each interval comprising at
least 1,000 hours of operating the engine and at least 10
desulfations; and the highest upper limit temperature used within
each interval after the first is at least 5.degree. C. greater than
the highest upper limit temperature used within of the preceding
interval.
5. The method of claim 4, wherein the highest upper limit
temperature used within the last interval is at least 30.degree. C.
greater than the highest upper limit temperatures used within the
first interval.
6. The method of claim 4, wherein the highest upper limit
temperature within the last interval is at least 50.degree. C.
greater than the highest upper limit temperature within the first
interval.
7. The method of claim 4, wherein there are three or more of the
intervals.
8. The method of claim 4, wherein there are five or more of the
intervals.
9. A method of operating a diesel power generation system in which
the exhaust from a diesel-fueled internal combustion engine is
treated using a lean NO.sub.X trap, the method comprising:
operating the engine to produce a lean exhaust comprising NO.sub.X
and SO.sub.X; treating the exhaust using the lean NO.sub.X trap;
determining when to denitrate the lean NO.sub.X trap and
denitrating the lean NO.sub.X trap accordingly; from time-to-time,
desulfating the lean NO.sub.X trap by heating the lean NO.sub.X
trap to a desulfating temperature and providing the heated lean
NO.sub.X trap with a reducing environment; aging the lean NO.sub.X
trap through many desulfations; and compensating for the effects of
the aging by increasing the desulfating temperature.
10. A vehicle comprising a diesel power generation system adapted,
configured, and functional to operate according to the method of
claim 9.
11. The method of claim 9, wherein the increases to the desulfating
temperature are made incrementally and progressively over the
lifetime of the lean NO.sub.X trap, whereby the lifetime is
appreciably extended.
12. The method of claim 9, wherein the desulfating temperature is a
main desulfating temperatures and further comprising: between the
foregoing desulfations, performing mild desulfations that comprise
heating the lean NO.sub.X trap to a mild desulfating temperature
that is less than the main desulfating temperature and providing
the lean NO.sub.X trap at the mild desulfating temperature with a
reducing environment.
13. The method of claim 12, wherein the mild desulfating
temperature is also increased to compensate for the LNT's
aging.
14. A method of operating a diesel power generation system in which
the diesel exhaust is treated using a lean NO.sub.X trap, the
method comprising: operating an engine to produce a lean exhaust
comprising NO.sub.X and SO.sub.X; treating the exhaust using the
lean NO.sub.X trap; from time-to-time, denitrating the lean
NO.sub.X trap; from time-to-time, desulfating the lean NO.sub.X
trap by heating the lean NO.sub.X trap to within a desulfating
temperature range having an upper limit temperature and providing
the heated lean NO.sub.X trap with reductants; determining the
efficacy of the desulfations; and in response to the efficacy
determination, increasing the upper limit temperature, whereby the
upper limit temperature increases as the LNT ages.
15. A vehicle comprising a diesel power generation system adapted,
configured, and functional to operate according to the method of
claim 14.
16. The method of claim 14, wherein determining the efficacy of the
desulfations comprises evaluating NO.sub.X mitigation data
collected between desulfations.
17. The method of claim 14, wherein determining the efficacy of a
desulfation comprises comparing the performance of the lean
NO.sub.X trap after the desulfation to the performance of the lean
NO.sub.X trap before the desulfation.
18. The method of claim 14, wherein determining the efficacy of a
desulfation comprises determining whether the desulfation brings
the lean NO.sub.X trap to a satisfactory level of performance,
wherein performance relates to NO.sub.X storage capacity or
NO.sub.X trapping efficiency.
19. The method of claim 14, wherein a duration for the desulfations
is adapted according to the efficacy determination and the upper
limit temperature is not increased unless the duration is at an
upper limit.
20. The method of claim 14, wherein a duration for the desulfations
is adapted according to the efficacy determination and the upper
limit temperature is not increased unless increasing the duration
at the current upper limit temperature has failed to satisfactorily
increase the efficacy of the desulfations.
Description
FIELD OF THE INVENTION
[0001] The invention relates to systems having diesel-fueled
internal combustion engines with exhaust aftertreatment and methods
of operating those systems.
BACKGROUND
[0002] Diesel-fueled internal combustion engines are used to power
vehicles such as medium and heavy duty trucks. Diesel engines are
also used in stationary power generation systems. While exhaust
aftertreatment systems for gasoline engine have been widely used
since the 1970s, diesel engine aftertreatment systems have only
recently coming into widespread use.
[0003] Whereas gasoline engines use spark ignition, diesel engines
use compression ignition. As a consequence, the composition of
diesel exhaust is much different from that of gasoline engines. The
major pollutants in gasoline engine exhaust are carbon monoxide,
unburned hydrocarbons, and some NO.sub.x. The major pollutants in
diesel engine exhaust are NO.sub.X and particulate matter
(soot).
[0004] A catalytic converter, which is an exhaust treatment device
comprising a so-called three-way catalyst, can effectively control
gasoline engine emissions by oxidizing carbon monoxide and unburned
hydrocarbons while also reducing NO.sub.X. This approach is
unsuitable for diesel engine exhaust because diesel exhaust
contains from about 4 to 20% oxygen. The excess oxygen and dearth
of oxygen accepting species (reductants) makes catalytic converters
ineffective for reducing NO.sub.X in diesel exhaust.
[0005] Several solutions have been proposed for controlling
NO.sub.X emissions from diesel-powered vehicles. One set of
approaches focuses on the engine. Techniques such as exhaust gas
recirculation and partially homogenizing fuel-air mixtures are
helpful, but these techniques alone will not eliminate NO.sub.X
emissions. Another set of approaches remove NO.sub.X from the
vehicle exhaust. These include the use of lean-burn NO.sub.X
catalysts, selective catalytic reduction (SCR) catalysts, and lean
NO.sub.X traps (LNTs).
[0006] Lean-burn NO.sub.X catalysts promote the reduction of
NO.sub.x under oxygen-rich conditions. Reduction of NO.sub.X in an
oxidizing atmosphere is difficult. It has proven challenging to
find a lean-burn NO.sub.x catalyst that has the required activity,
durability, and operating temperature range. A reductant such as
diesel fuel must be steadily supplied to the exhaust for lean
NO.sub.X reduction, adding 3% or more to the engine's fuel
requirement. Currently, the sustainable NO.sub.X conversion
efficiencies provided by lean-burn NO.sub.X catalysts are
unacceptably low.
[0007] SCR generally refers to selective catalytic reduction of
NO.sub.X by ammonia. The reaction takes place even in an oxidizing
environment. The NH.sub.3 can be temporarily stored in an adsorbent
or ammonia can be fed continuously into the exhaust. SCR can
achieve high levels of NO.sub.X reduction, but there is a
disadvantage in the lack of infrastructure for distributing ammonia
or a suitable precursor. Another concern relates to the possible
release of ammonia into the environment.
[0008] LNTs are devices that adsorb NO.sub.X under lean conditions
and reduce and release the adsorbed NO.sub.X under rich conditions.
An LNT generally includes a NO.sub.X adsorbent and a catalyst. The
adsorbent is typically an alkali or alkaline earth compound, such
as BaCO.sub.3 and the catalyst is typically a combination of
precious metals including Pt and Rh. In lean exhaust (exhaust
containing an excess of oxygen and other oxidizing species in
comparison to reducing compounds), the catalyst speeds reactions
that lead to NO.sub.X adsorption. In a rich exhaust (containing
reductants in excess of oxidizing compounds), the catalyst speeds
reactions by which reductants are consumed and adsorbed NO.sub.X is
reduced and desorbed. In a typical operating protocol, a rich
condition (reducing environment) is created within the exhaust from
time-to-time to regenerate (denitrate) the LNT.
[0009] In addition to accumulating NO.sub.X, LNTs accumulate
SO.sub.X. SO.sub.X is the product of combusting sulfur-containing
fuels. Even with low sulfur diesel fuels, the amount of SO.sub.X
produced by combustion is significant. SO.sub.X adsorbs more
strongly than NO.sub.X and necessitates a more stringent, though
less frequent, regeneration (desulfation). Desulfation requires
elevated temperatures, e.g., 700.degree. C.
[0010] A desulfation process requires much more time than a
denitration process. Whereas denitration can be completed in a few
seconds, desulfation takes several minutes, commonly on the order
of 5-15 minutes. Desulfation could be carried out more rapidly if
higher temperatures were used, but normal desulfating temperatures
already approach the point at which the LNT will undergo rapid
thermal degradation. For example, a temperature of 800.degree. C.
may cause a particular LNT to deteriorate and lose a substantial
portion of its functionality after a single desulfation.
[0011] U.S. Pat. No. 6,637,198 proposes a desulfation process in
which several partial desulfations are performed between each full
desulfation. The partial desulfations use lower temperatures and
have shorter duration than the main desulfations. The patent
asserts that this process facilitates making opportunistic use of
higher than normal exhaust temperatures to reduce the amount of
fuel expended heating the LNT for desulfations.
[0012] In spite of advances, there continues to be a long felt need
for an affordable and reliable diesel exhaust aftertreatment system
that is durable, has a manageable operating cost (including fuel
requirement), and reduces NO.sub.X emissions to a satisfactory
extent in the sense of meeting U.S. Environmental Protection Agency
(EPA) regulations effective in 2010 and other such regulations that
limit NO.sub.X emissions from trucks and other diesel-powered
vehicles.
SUMMARY
[0013] The invention is a method of operating a diesel power
generation system in which the exhaust from a diesel-fueled
internal combustion engine is treated by a lean NO.sub.X trap. The
invention extends to encompass systems configured to implement that
method. The systems and methods are particularly concerned with how
the lean NO.sub.X trap is desulfated. According to the invention,
the maximum temperature used for desulfating the lean NO.sub.X trap
is kept relatively lower during early life and increased as the
trap ages.
[0014] The storage performance of a lean NO.sub.X trap will
invariably diminish over time. A lean NO.sub.X trap designed to
provide adequate late life performance must have excess capacity
during early life. The method utilizes the excess capacity
available during early life to slow aging of the trap and thereby
extend the trap's lifetime. The method facilitates meeting
durability requirements for diesel-powered vehicles with exhaust
aftertreatment systems.
[0015] In one embodiment, the invention is a method of operating a
diesel power generation system in which the diesel exhaust is
treated using a lean NO.sub.X trap. The engine produces exhaust
containing NO.sub.X and SO.sub.X and the exhaust is treated using
the lean NO.sub.X trap. From time-to-time, the lean NO.sub.X trap
is desulfated by heating it to a desulfating temperature, or
equivalently, to within a desulfating temperature range. The heated
trap is exposed to rich conditions, under which the lean NO.sub.X
trap desulfates. The lean NO.sub.X trap is aged through many
desulfations. As the lean NO.sub.X trap ages, the highest
temperature used for the desulfations is increased. The highest
desulfating temperatures used are therefore lower during early lean
NO.sub.X trap life as compared to mid and late lean NO.sub.X trap
life.
[0016] The primary purpose of this summary has been to present
certain of the inventors' concepts in a simplified form to
facilitate understanding of the more detailed description that
follows. This summary is not a comprehensive description of every
one of the inventors' concepts or every combination of the
inventors' concepts that can be considered "invention". Other
concepts of the inventors will be conveyed to one of ordinary skill
in the art by the following detailed description together with the
drawings. The specifics disclosed herein may be generalized,
narrowed, and combined in various ways with the ultimate statement
of what the inventors claim as their invention being reserved for
the claims that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a flow chart of an exemplary method of adaptive
desulfation according to the present invention.
[0018] FIG. 2 is a schematic illustration of an exemplary power
generation system that can be configured to operate according to
the present invention.
[0019] FIG. 3 is a flow chart of an exemplary method for
determining when desulfation is currently required.
[0020] FIG. 4 is a flow chart of an exemplary method for executing
a desulfation request.
[0021] FIG. 5 is a flow chart of another exemplary method for
adapting desulfating conditions.
DETAILED DESCRIPTION
[0022] FIG. 1 is a flow chart of an exemplary adaptive desulfation
method 100 according to the present invention. The method 100
begins with a determination 101 whether desulfation is required. If
desulfation is required the method 100 proceeds to step 102, which
analyzes lean NO.sub.X trap performance over a preceding interval
to determine the effectiveness of prior desulfations using a
current set of desulfating conditions. The conditions include a
desulfating temperature and optionally additional parameters,
duration in this example. If one or more desulfations have been
successfully completed using the current desulfating conditions but
have failed to produce a satisfactory result, step 103 directs the
method 100 to step 105. Otherwise the method 100 proceeds with step
104 and carries out a desulfation using the extant conditions.
[0023] Step 105 determines whether measures can be taken to improve
desulfation without increasing the desulfating temperature. In this
example, step 105 determine whether the desulfating time currently
in use is less than a maximum. If not, the method 100 increases the
desulfating time and proceeds with the desulfation. If the
desulfating time is already at a maximum, the method proceeds to
step 107.
[0024] Step 107 determines whether the desulfating temperature is
already at a limit. The method 100 raises the desulfating
temperature over the lifetime of a lean NO.sub.X trap, but still
uses a lifetime limit on how high the desulfating temperature can
go. If the limit has been reached, a fault is indicated in step 110
and the desulfation is carried out without further increasing the
severity of the desulfating conditions. If the limit has not yet
been reached, the method 100 proceeds with step 108 in which the
desulfating temperature is increased. The desulfating time is
reduced in step 109 and the desulfation carried out in step
104.
[0025] The method 100 will increases the desulfating time again,
later, as necessary, while retaining the increased desulfating
temperature. Further increases to the desulfating temperature will
be deferred until at least one successfully completed desulfation
using the higher desulfating and the maximum desulfating time has
proven inadequate. Optionally, decreases to desulfation time can be
made between steps 103 and 104, whereby the duration of desulfation
is adapted either upwards or downwards according to desulfation
performance as determined from the data analyzed in step 102.
Increases to the maximum temperature used for desulfation, however,
are preferably monotonic.
[0026] FIG. 2 provides a schematic illustration of an exemplary
power generation system 200 that can be configured, adapted, and
functional to implement methods of the invention. The exemplary
power generation system 200 can be installed in a vehicle or in a
stationary power application, such as a back-up generator for a
hospital. While the invention may be described and claimed in terms
of power generation systems, it should be appreciated that the full
value of the invention may be realized in a larger system, such as
vehicle. For example, the invention extends the period over which a
vehicle can operate within the confines of emission control
regulations without requiring replacement of any aftertreatment
system components. This can be critical in meeting regulatory and
customer requirements.
[0027] The exemplary power generation system 200 comprises an
engine 201, a manifold 202, and an exhaust aftertreatment system
203. The exhaust aftertreatment system 203 comprises an exhaust
line 204 configured to channel exhaust from the manifold 202
through, in order, a fuel reformer 205, a thermal mass 206, an LNT
207, a DPF 208, and an SCR catalyst 209. A fuel injector 211 is
configured to inject fuel into the exhaust line 204 upstream from
the fuel reformer 205 at times and at rates determined by the
controller 210. Implementation of the present invention does not
require either the fuel reformer 205, the thermal mass 206, the DPF
208, the SCR catalyst 209, and the fuel injector 211. The major
device that relate to the invention are the engine 201, the LNT
207, and the controller 210.
[0028] The controller 210 may be a control unit for the engine 201
or a separate control unit. If separate, the controller 210
preferably communicates with the engine control unit. Configuring
the system 200 to practice a method of the invention generally
involves providing the controller 210 with suitable programming.
With suitable programming and any other necessary adaptations, the
system 200 will be functional to carry out the method.
[0029] The controller 210 receives data from various sensors, such
as a temperature sensor 212. The sensor 212 is configured to sense
a characteristic temperature for the LNT 207. Other sensors that
may be provided include, without limitation, a temperature sensor
for the fuel reformer 205 and one or more exhaust composition
sensors that can be used to monitor performance of the LNT 207.
Usually there will be at least one composition sensor downstream
from the LNT 207, such as a NO.sub.X sensor. Suitable locations
include locations upstream and downstream from the SCR catalyst
209.
[0030] The engine 201 can be any engine that operates to produce a
lean exhaust stream comprising NO.sub.X and SO.sub.X. Generally the
engine 201 is a diesel-fueled compression ignition internal
combustion engine that produces an exhaust containing from 2 to 20%
oxygen. The diesel exhaust is typically at temperatures in the
range from about 200 to about 500.degree. C., with temperatures in
the range from 250 to 450.degree. C. beginning common. The manifold
202 couples the exhaust aftertreatment system 203 to an exhaust
stream from the engine 201. Preferably the exhaust system 203
comprises a single exhaust line 204 that receives the entire
exhaust from the engine 201.
[0031] The exhaust aftertreatment system 203 and the exhaust line
204 preferably have no valves or dampers that control the flow of
exhaust. Exhaust system valves and dampers provide control over the
distribution of exhaust between a plurality of flow paths. Such
control is desirable in terms of limiting fuel usage. Reducing the
flow of exhaust to the fuel reformer 205 and the LNT 207 during
rich regeneration would reduce the amount of fuel expended
eliminating oxygen from the exhaust in order to provide rich
conditions. The reduced flow rate would also increase residence
times, and thus the efficiency with which reductants are used.
Nevertheless, it is preferred that the exhaust treatment system 203
operate without exhaust line valves or dampers in order to avoid
failures resulting from reliance on such devices.
[0032] The LNT 207 is a device that adsorbs NO.sub.X under lean
conditions and reduces NO.sub.X releasing the reduction products
(N.sub.2 and NH.sub.3) under rich conditions. Some alternate terms
for a lean NO.sub.X trap (LNT) are NO.sub.X absorber-catalyst and
NO.sub.X trap-catalyst. An LNT generally comprises a NO.sub.X
absorbent and a precious metal catalyst in intimate contact on an
inert support. Examples of NO.sub.X adsorbent materials include
certain oxides, carbonates, and hydroxides of alkaline earth metals
such as Mg, Ca, Sr, and Ba or alkali metals such as K or Cs. The
adsorption can be physical or chemical, but is generally primarily
chemical. The precious metal typically comprises one or more of Pt,
Pd, and Rh. The support is typically a monolith, although other
support structures can be used. The monolith support is typically
ceramic, although other materials such as metal and SiC are also
suitable for LNT supports. The LNT 207 may be provided as two or
more separate bricks.
[0033] The fuel reformer 205 and the fuel injector 211 are part of
a system for producing the rich conditions and providing the
reductant required for denitration and desulfation. A reductant is
a compound that is reactive to accept oxygen and become oxidized.
The reductant is generally diesel fuel or a substance derived from
diesel fuel by partial combustion and or steam reforming reactions.
A rich condition for the exhaust is one in which the concentration
of reductants is more than stoichiometric for combustion with any
oxygen and other oxidizing compounds present. In other words, a
rich environment is one in which there is an excess of reductant
and the overall composition is reducing rather than oxidizing.
[0034] Optionally, the engine 201 is used to assist in producing
rich conditions. If the engine 201 can be operated with rich
combustion or with post-combustion fuel injection, than the engine
201 can provide a rich mixture and the exhaust line fuel injector
211 is optional. The engine 201 can also facilitate generating rich
conditions by measures that reduce the exhaust oxygen flow rate.
Such measures may include, for example, throttling an air intake
for the engine 201, increasing exhaust gas recirculation (EGR),
modifying cylinder injection controls, and shifting gears to reduce
the engine speed.
[0035] It is preferable for the aftertreatment 203 to be capable of
providing the rich conditions for denitration and desulfation of
the LNT 207 while making few or no changes to the operation of the
engine 201 in order to avoid having regenerations (denitrations and
desulfations) adversely affect drivability and also to provide
greater independence between the designs and configurations of the
aftertreatment system 203 and the engine 201. It would not be
unusual for the engine 201 to be manufactured by one company while
the power generation system 200 comprising the engine 201 is
assembled by another company. A third company may build a vehicle
using the assembled power generation system 200.
[0036] The fuel reformer 205 is a device that is functional to
reform diesel fuel into reformate, especially CO and H.sub.2.
Reformate is a better reductant than diesel fuel for denitrating
the LNT 207. Reformate is more reactive than diesel fuel and
results in less NO.sub.X slip. NO.sub.X slip is the release of
unreduced NO.sub.X from the LNT 207 during denitration.
[0037] Preferably, the fuel reformer 205 has a low thermal mass and
comprises both oxidation and steam reforming catalysts. A low
thermal mass allows the fuel reformer 205 to be heated to steam
reforming temperatures for each denitration without requiring an
excessive amount of time or fuel. Steam reforming temperatures have
a minimum in the range from about 500 to about 600.degree. C.,
typically requiring at least 550.degree. C. At steam reforming
temperatures, energy from oxidation and partial oxidation, which
are exothermic, can drive steam reforming, which is endothermic.
This improves the efficiency with which reformate is produced and
decreasing the amount of waste heat. A sufficiently low thermal
mass can be achieved by constructing the fuel reformer 205 around a
monolith substrate formed of thin metal foils, e.g., 130 microns or
less. Preferably the foils are 100 microns or less, and more
preferably 50 microns or less. The preferred structure can be
heated from a typical diesel exhaust temperature in the range from
250 to 300.degree. C. to steam reforming temperatures in 2 or 3
seconds or less.
[0038] The exhaust from the engine 201 generally comprises at least
2% oxygen. When fuel is added to the exhaust to produce a rich
condition for denitration or desulfation, this oxygen is eliminated
by combustion. In the system 200, this combustion takes place in
the fuel reformer 205. If the combustion does not take place
upstream from the LNT 207, it will generally take place within the
LNT 207. The precious metal catalysts typically used by the LNT 207
are functional as oxidation catalysts. If too much combustion takes
place within the LNT 207, it can cause undesirable temperature
excursions which are particularly problematic if they take place
during denitrations. Such temperature excursions can cause wear and
result in the release of unreduced NO.sub.X.
[0039] Optionally, a burner or any device that is functional to
bring about combustion, such as an oxidation catalyst, a three-way
catalyst, or a suitably catalyzed diesel particulate filter can be
used instead of the fuel reformer 205. Like the fuel reformer 205,
these device can cause combustion to take place upstream from the
LNT 207. They may also accomplish a certain amount of fuel
reformate through partial oxidation reactions.
[0040] When combustion takes place upstream from the LNT 207 in
preparation for or during denitration, the heat is preferably held
temporarily within devices upstream from the LNT 207 to be released
only slowly over a prolonged period. According to the preferred
design, the fuel reformer 205 has a low thermal mass (thermal
inertia) and is not very effective for holding heat. In the system
200, the thermal mass 206 provides the desired heat retention
function.
[0041] The thermal mass 206 is any device that is effective for
exchanging heat with the exhaust and storing the heat produced by
the fuel reformer 205 over the course of a denitration without
heating excessively. A suitable device can be simply a catalyst
substrate, with or without a catalyst. A suitable device is, for
example, an inert monolith substrate, either metal or ceramic.
Preferably, the thermal mass 206 has a thermal inertia that is
greater than that of the fuel reformer 205. The DPF 208 can be used
as the thermal mass 206, although in the exemplary system 200, the
DPF is downstream from the LNT 208 and instead serves to help
protect the SCR catalyst 209 from high temperatures during
desulfations.
[0042] The DPF 208 and the SCR catalyst 209 contribute to meeting
emission control limits and durability requirements. The DPF 208
removes particulate matter from the exhaust, which is the major
pollutant in diesel exhaust other than NO.sub.X. The SCR catalyst
209 provides supplementary NO.sub.X mitigation. It improves
durability by allowing sufficient NO.sub.X mitigation to be
maintained with less frequent denitration and desulfation of the
LNT 208. When some NO.sub.X is reduced downstream from the LNT 207,
the LNT 207 does not need to be maintained at as high a level of
efficiency.
[0043] A DPF is a device that traps particulates matter (soot),
removing it from the exhaust flow. The DPF 208 can be a wall flow
filter, which uses primarily cake filtration, or a flow-through
filter, which uses primarily deep-bed filtration. The DPF 208 can
have any suitable structure. Examples of suitable structures
include monoliths. A monolith wall flow filter is typically made
from a ceramic such as cordierite or SiC, with alternating passages
blocked at each end to force the flow through the walls. A
flow-through filter can be made from metal foil.
[0044] Trapped soot can be removed from the DPF 208 continuously by
catalyzing reactions between soot and NO.sub.X, but typically the
DPF 208 must be heated from time-to-time to a temperature at which
it regenerates by combustion of trapped soot. The temperature
required for soot combustion can be reduced by a catalyst. Suitable
catalysts include precious metals and oxides of Ce, Zr, La, Y, and
Nd. Soot combustion is exothermic and can be self-sustaining once
ignited.
[0045] The SCR catalyst 209 is an ammonia-SCR catalyst functional
to catalyze reactions between NOx and NH.sub.3 to reduce NOx to
N.sub.2 in lean exhaust. Examples of SCR catalysts include oxides
of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt,
Rh, Rd, Mo, W, and Ce, zeolites, such as ZSM-5 or ZSM-11, exchanged
with metal ions such as cations of Cu, Co, Ag, Zn, or Pt.
[0046] The engine 201 operates to produce a lean exhaust comprising
NO.sub.X, particulate matter, and SO.sub.X. The expression NO.sub.R
designates the family of molecules consisting of nitrogen and
oxygen atoms, primarily NO and NO.sub.2. The subscript X indicates
the family includes multiple species with varying proportions
between nitrogen and oxygen atoms. The notation SO.sub.X is
similar.
[0047] Under lean conditions, the LNT 207 absorbs a portion of the
NO.sub.X and a portion of the SO.sub.X in the exhaust. If the SCR
catalyst 209 contains stored ammonia, an additional portion of the
NO.sub.X is reduced therein. The DPF 208 removes at least a portion
of the particular matter from the exhaust.
[0048] From time-to-time, the controller 210 determines to
denitrate the LNT 207. For denitration, the fuel reformer 205 is
heated to steam reforming temperatures by injecting fuel into the
exhaust line 204 through the fuel injector 211 under the control of
the controller 210 at rates that leave the exhaust lean. Under lean
condition, most of the injected fuel combusts in the fuel reformer
205, heating it. After the fuel reformer 205 has reached steam
reforming temperatures, as may be determined using a temperature
sensor, the fuel injection rate is controlled to make the exhaust
condition rich for a period of time (rich phase) over which the LNT
207 denitrates.
[0049] During the rich phase, injected fuel mixed with lean exhaust
enters the fuel reformer 205. A portion of the fuel combusts,
consuming most of the oxygen from the exhaust. Another portion of
the fuel is converted to reformate (syn gas), which is primarily
H.sub.2 and CO. The reformate enters the LNT 207 where it reacts to
reduce and release trapped NO.sub.X. Most of the NO.sub.X released
during the rich phase is reduced to N.sub.2 or NH.sub.3, although
it is typical for a small amount to be released (slip) without
being reduced. The NH.sub.3 is mostly trapped by the SCR 209, where
it is generally consumed reducing NO.sub.X over the course of the
following lean phase.
[0050] NO.sub.X slip occurs primarily at the beginning of the rich
phase and may be lessened by varying the reductant concentration
over the course of the denitration. The preferred reductant
concentration profile has the reductant concentration relatively
low at the start of the rich phase and gradually increasing over at
least a first portion of the rich phase.
[0051] When the fuel reformer 205 heats for denitration, the
thermal mass 206 also heats, but to a lesser degree. After
denitration is complete, fuel injection ceases and the fuel
reformer 205 cools down to exhaust temperatures. The thermal mass
206 also cools. The LNT 207 will heat over the course of the
denitration and a short period following, but only to a modest
degree.
[0052] From time-to-time, the LNT 207 must be heated substantially
in order to carry out a desulfation. In the system 200, the LNT 207
can be heated by injecting fuel into the exhaust. The injected fuel
combusts, primarily in the fuel reformer 205. Over the course of a
few minutes, the thermal inertias of the thermal mass 206 and of
the LNT 207 are overcome and the LNT 207 reaches desulfating
temperatures. Alternative and supplemental means of heating the LNT
207 include, without limitation, engine measures, such as operating
the engine to produce a hot exhaust, burners, and electrical
heaters.
[0053] In the system 200, after the LNT 207 has reached desulfating
temperatures, the fuel injection rate is controlled to make the
exhaust rich. It might be considered ideal to maintain the rich
condition until the LNT 207 has desulfated to a desired degree. In
the system 200, however, it proves difficult to continuously
maintain rich conditions while also maintaining the fuel reformer
205 and the LNT 207 within desired temperature ranges. In general,
it is necessary to pulse the fuel injection over the course of a
desulfation.
[0054] In the context of maintaining the desired conditions for
desulfation, pulsing the fuel injection means creating alternating
rich and lean phases (periods). During the rich phases, the
reformer 205 heats and produces reformate. During the lean phase,
fuel injection ceases and the fuel reformer 205 cools. Typically,
the durations of the rich phases are in the range from about 4 to
about 30 seconds, with periods in the range from about 5 to about
15 seconds being preferred.
[0055] When lean and rich phases are alternated in this manner,
some combustion takes place in the LNT 207. The LNT 207 typically
comprises oxygen storage materials, which are materials that are
functional to store oxygen. These materials accumulate oxygen
during the lean phases, typically to the point of saturation.
During the rich phases, the reductants react with the stored
oxygen, consuming the stored oxygen and producing heat. The amount
of heat can be significant, e.g., enough to make the LNT 207 50 to
100.degree. C. hotter than the fuel reformer 205. The amount of
this heating is generally proportional to the oxygen storage
capacity of the LNT 207 and to the frequency of switches between
lean and rich phases.
[0056] In a preferred embodiment, the fuel injection rate is
selected to provide a desired reformate production rate or
concentration while the temperatures of the fuel reformer 205 and
the LNT 207 are controlled by varying two parameters that dictate
the lean-rich pulse pattern. The lean rich pulse pattern consists
of the lean phase lengths and the rich phase lengths or two other
parameters that define these, such as the pulse frequency and the
ratio between lean time and rich time.
[0057] In one exemplary control strategy, the rich phases are
terminated when the fuel reformer 205 reaches a pre-defined upper
limit temperature, such as 650.degree. C. The lean phase durations
are then adjusted in a closed loop control algorithm to maintain
the temperature of the LNT 207 within the desired temperature
range. In a variation of this method, the adjusted parameter is a
temperature to which the fuel reformer 205 is required to cool
before terminating. The LNT temperature target by control is
preferably a maximum temperature that the LNT reaches over the
course of a lean-rich cycle, but could alternatively be another
temperature, such as an average temperature. Making the pulse
periods shorter by reducing the durations of the lean phases raises
temperatures within the LNT 207. The temperatures rise because the
fuel reformer 205 is on average hotter resulting in more heat
convection to the LNT 207 and because the more frequent alternation
between lean and rich phases results in more combustion within the
LNT 207. Conversely, lengthening the lean phases lowers
temperatures within the LNT 207.
[0058] In another exemplary control strategy, the durations of the
rich phases are predetermined and both the upper and lower
temperatures of the fuel reformer 205 (or two equivalent
parameters) are set in order to achieve the desired rich phase
length while maintaining the LNT 207 at the desired temperature. In
this example, the temperature to which the fuel reformer 205 falls
during a lean phase is raised or lowered to raise or lower the
temperature of the LNT 207. During the rich phases, the reformer
heats by an amount that depends on the predetermined rich phase
duration. By selecting the rich phase duration within suitable
limits, the fuel reformer 205 can be prevented from either
overheating or cooling excessively, e.g., cooling below steam
reforming temperatures. Preselecting the rich phase duration can be
beneficial in managing hydrocarbon emissions during desulfations.
In terms of limiting hydrocarbon emissions, a suitable length for
the rich phases is on the order 10 to 20 seconds for a fresh
catalyst, decreasing to about 50-70% as much as the catalyst
ages.
[0059] It is generally also necessary to regenerate the DPF 208
from time-to-time. Regenerating the DPF 208 comprises heating the
DPF 208 to temperatures at which soot trapped with the DPF 208
combusts. The DPF 208 can be heated in the same way as the LNT 207
is heated for desulfation. Soot combustion is generally
self-sustaining. Once the DPF 208 is heated to soot combustion
temperatures, it is generally not necessary to supply any
additional heat. The upstream devices, including the fuel reformer
205 and the LNT 207, can be allowed to cool while soot combustion
is proceeding to completion.
[0060] The DPF 208 is typically of the wall flow filter variety and
must be regenerated often enough to avoid excessive back pressure.
Ideally, regenerating the DPF 208 each time the LNT 207 is
regenerated provides sufficient frequency. The DPF 208 is heated to
soot combustion temperatures each time the LNT 207 is heated for
desulfation. If the DPF 208 has sufficient capacity to require
regeneration no more often than the LNT 207 is regenerated,
supplemental fuel expenditure and additional heating of the LNT 207
for the sole purpose of regenerating the DPF 208 can be
avoided.
[0061] One approach that can facilitate not having to regenerate
the DPF 208 more often than the LNT 207 is desulfated is to provide
a second DPF downstream from the fuel reformer 205 and upstream
from the LNT 207. This second DPF can be used as the thermal mass
206, but is preferably a low thermal mass device upstream from the
thermal mass 208. Preferably, this second DPF is of the
flow-through type. Preferably, its thermal mass is sufficiently low
that it heats and regenerates each time the fuel reformer 205 is
heated to supply reformate for denitration. Accordingly, in this
embodiment, there is a second DPF that regenerates as often as the
LNT 207 is denitrated.
[0062] The times at which the LNT 207 is denitrated are determined
by the controller 210 and can be determined in any suitable manner.
Typically, certain threshold must be met before allowing a
denitration to begin. Threshold criteria can be, for example, one
or both the LNT 207 and the fuel reformer 205 being at minimum
temperatures, the oxygen concentration being below a maximum (e.g.,
less than 15%), the flow rate being above a minimum (e.g.,
significantly greater than at idle), the engine speed variance, as
determined by a moving average, being below a maximum, at a minimum
time elapsed since the last denitration, and a gear shift not
currently imminent or in progress. If the threshold criteria are
met, regeneration will begin if an additional condition (or
conditions) are met. An additional condition generally relates to a
measure of how urgently denitration is needed and is optionally
weighed against the suitability of current conditions for beginning
a denitration.
[0063] The urgency of the need to denitrate generally relates to
one or more of NO.sub.X loading of the LNT 207, remaining NO.sub.X
storage capacity, NO.sub.X trapping efficiency (optionally
normalized for such factors as the LNT temperature and exhaust flow
rate), NO.sub.X concentration in the exhaust at a point downstream
from the LNT 207, and cumulative NO.sub.X emissions since the last
denitration (optionally normalized by the engine's toque
production). A measure of suitability can relate to one or more of
such factors as the exhaust oxygen concentration (low is
preferred), the engine speed variance (low is preferred), and the
exhaust flow rate. One procedure for weighing the urgency of the
need to denitrate against the suitability of current conditions to
denitration is to assign numerical values to the urgency and the
conduciveness, multiplying the two together, and denitrating based
on whether the result exceeds a predetermined critical value.
[0064] Likewise, the times at which the LNT 207 is desulfated are
determined by the controller 210 in any suitable manner. Threshold
criteria may be employed similar to those used for denitration and
a measure of suitability of current conditions to desulfation can
be weighed against a measure of the urgency of the need for
desulfation. The urgency of the need to desulfated can be based on,
for example, one or more of an estimate of the amount sulfur
trapped in the LNT, the frequency with which denitration is being
required, an estimate of the post-denitration NO.sub.X storage
capacity of the LNT 207, the amount of time since the last
desulfation, the number of denitrations since the last desulfation,
and an estimate of the average LNT efficiency following the last
desulfation, or over the last several desulfations. The LNT
efficiency can be normalized to separate changes intrinsic to the
LNT 207 from changes in the operating regime of the engine 201.
Alternatively, normalization can be limited, whereby less sulfur
loading is tolerated when the engine is in an operating regime that
requires peak LNT efficiency, e.g., when the engine 201 is in a
high speed-high load condition. Preferably, the determinations of
when to desulfate the LNT 207 include a dynamic measure of LNT
performance, whereby adjustments to the desulfation timing and the
desulfating conditions can be adapted to measurable indications of
aging.
[0065] FIG. 3 provides a flow chart of an exemplary method 300 that
determines whether there is a need to desulfate the LNT 207. After
initialization 301, the method 300 begins with a threshold
determination 302. In this example, the threshold determination is
whether a minimum number of denitrations, A, have taken place since
the last desulfation. The use of a threshold determination provides
a failsafe for preventing overly frequent desulfations. Alternate
or additional criteria can be used to make this threshold
determination, such as whether a minimum time of operation has
elapsed since the last desulfation.
[0066] If the threshold criteria are satisfied, the method 300 sets
a desulfation request flag in step 306 if any of several criteria
are met. These criteria are tested through a series of steps,
303-305. The first criteria examines the median of a normalized LNT
efficiency over the last several lean-rich cycles of NO.sub.X
trapping followed by denitration. The efficiency is normalized for
the LNT temperature and the exhaust flow rate to provide a value
that is largely independent of the engine 201's speed and load. If
the efficiency is below a threshold value B, the desulfation
request flag is set. Step 304 checks the elapsed operating time
(engine running hours) since the last desulfation. An alternative
criteria could be based on the number of denitrations since the
last desulfation. Step 305 checks whether the SO.sub.X loading is
greater than a critical value D.
[0067] The SO.sub.X loading is estimated. The estimate generally
takes into account at least the amount of fuel used since the last
desulfation and the estimated sulfur content of that fuel. The
accumulation rate is optionally modified by an accumulation
efficiency that depends on the temperature of the LNT 207 and or
the exhaust flow rate. Optionally, the estimate includes an amount
remaining after the last desulfation, which would be particularly
relevant if the last desulfation was aborted prematurely.
[0068] If the method 300 determines there is a need for
desulfation, the desulfation request is set in step 306. The actual
start of the desulfation may be postponed until conditions are
suitable, but a desulfation will begin in response to the
determination. A separate algorithm checks the suitability of
current conditions, The method 300 is executed periodically, e.g.,
after each denitration, within the course of a more broadly
functioning control algorithm. If the desulfation request flag is
set in step 306 before the routine exists through the return step
308, the parameters for desulfation are set in step 307.
[0069] The parameters for desulfation include a desulfating
temperature. Another parameter is typically set to determine a
duration for the desulfation. This could be, for example, a total
time at rich conditions, a total amount of reductant to be provided
to the LNT 207 at desulfating temperature, or a total amount of
sulfur to be removed. The later envisages a dynamic determination
of the sulfur removal rate as a function of measured values such as
the LNT temperature. Another parameter that may be set is a
duration for individual rich phases to be used over the course
desulfation.
[0070] The example shows the determination of parameters for
desulfation being made immediately after setting the desulfation
request flag. Optionally the selection of parameters is postponed
until it has been determined that conditions are suitable for
beginning the desulfation. Postponement in this manner allows a
desulfation duration parameter to be adjusted to account for
additional sulfur accumulation that may occur between setting the
desulfation request flag and finding that conditions are suitable
to begin desulfating.
[0071] Step 307, selecting the parameters to use for the
desulfation, invokes another method. This method is exemplified by
the process 500, which is illustrated with a flow chart in FIG. 5.
The method 500 selectively modifies the desulfation parameters from
previously used values based on how efficacious the previous
desulfations have been in improving the performance of the LNT
207.
[0072] After initialization 501, the first step in the method 500
is to determine 502 whether the last desulfation was completed
successfully. This determination of whether a desulfation was
successful may consider factors in addition to whether the
desulfation was completed. Examples of additional factors are the
number of rich pulses required to complete the desulfation, the
median rich pulse duration, and the mean LNT temperature during the
rich pulses. The desulfation may be considered unsuccessful if the
number of rich pulses required to make up the rich time was
excessive, if the median rich pulse duration was too short, or if
the mean LNT temperature was too far below the intended temperature
range. Only desulfation qualified as successful are considered in
deciding whether to adapt the desulfation conditions. Step 502
prevents any adjustment to the current desulfating parameters
before they have proven inadequate to restore the LNT 207 to
effectiveness. If one or more desulfations using the current
desulfating conditions have been completed successfully, the method
500 proceeds with step 503, in which the efficacy of the last
desulfation, or the last several desulfation, in improving LNT
performance is evaluated.
[0073] The assessment of LNT performance improvement can be made in
any suitable manner. Any suitable measure of LNT performance can be
used. In one example, LNT performance is determined according to a
normalized LNT efficiency averaged over a period following the last
desulfation. The efficiency is preferably measured with input from
a NO.sub.X sensor within the exhaust line 204 at a position
downstream from the LNT 207. The average is preferably taken over
at least several cycles of NO.sub.X trapping followed by
denitration. As another example, the LNT performance can be
determined based on how frequently it has been necessary to
denitrate the LNT in order to meet pollution control targets. If a
target degree of pollution control can be maintained with less
frequent denitration, the LNT is performing better.
[0074] The determination of efficacy can be made by evaluating the
LNT performance in either relative or absolute terms. An example of
a relative determination is one comparing the average LNT
efficiency over a period before the last desulfation to an average
LNT efficiency over a period following the last desulfation. In
that case, the efficacy determination is based on the degree of
improvement in performance. An example of an absolute determination
is one comparing the LNT efficiency following the last desulfation
against a fixed reference. A desulfation is effective if it
improves LNT performance to a satisfactory degree.
[0075] If the previous desulfation was not sufficiently effective,
step 504 determines whether a desulfation duration parameter is
already raised to its upper limit. When desulfation with current
parameters is not producing a satisfactory result (is not proving
sufficiently effective), the method 500 responds by increasing
desulfating time before increasing the desulfating temperature. The
method 500 uses a predetermined maximum, which can be a fixed value
or a function of a parameter that correlates to the LNT's aging,
such as the desulfation temperature. If the maximum desulfation
time is variable, then it preferably diminishes as the LNT ages. An
aged LNT has less functional storage capacity that is amendable to
restoration by desulfation. Also, desulfation proceeds more quickly
at higher temperatures.
[0076] As an alternative to using a predetermined maximum duration,
step 504 can instead analyze LNT performance data to determine
whether the last increase in desulfation time produced a
satisfactory degree of improvement in desulfation efficacy. In this
alternative, the desulfation time is increased as required until
the data shows these increases have reached a point of diminishing
returns. In any event, if further increases to the desulfation time
are tenable, because an upper limit or point of diminishing returns
has not yet been met, then the step 504 directs an increase in
desulfation time, step 511. Otherwise the method proceeds with step
505.
[0077] Step 505 avoids premature increases to the desulfating
temperature by preventing any increase unless desulfation using the
current desulfating temperature and the maximum desulfating time
has proven unsatisfactory through several attempts. Increases to
the maximum temperature used for desulfation are preferably made
monotonically. Also, the increases are preferably made only when it
is clear that the LNT 207 cannot be operated satisfactorily without
employing at least some desulfations at an increased temperature.
These preferences provide the greatest deferment of LNT aging. If
step 505 has been reached several times in a row due to several
desulfations using the current desulfating temperature and maximum
time failing to provide a satisfactory result (as determined by
steps 503 and 504), then the method 500 proceeds with step 506,
from which the desulfating temperature can be raised. Otherwise,
the method 500 proceeds with step 513, which directs that the
parameters for the last desulfation be used again. Steps 503 and
513 are no different. They are illustrated separately to make the
process flow easier to display.
[0078] Step 506 makes a final check before raising the desulfating
temperature. This step ensures that the desulfating temperature is
never raised above a predetermined upper limit. The upper limit
temperature is set at the point where the deterioration experienced
by the LNT is likely to outweigh the benefits of deeper desulfation
regardless of how much the LNT 207 has already aged.
[0079] A typical upper limit is the one specified by the LNT
manufacturer, or a slightly higher temperature. While an aged LNT
may be less vulnerable than a fresh LNT to the effects of high
desulfating temperatures, the primary mechanism of the invention is
to take advantage of overdesign. A fresh LNT is overdesigned in
anticipation of diminishing performance over time. Overdesign for
early life is necessary to ensure satisfactory late life
performance. The invention takes advantage of this overdesign to
use lower desulfating temperatures during early life and thereby
slow the process of aging. The diminished effectiveness of low
temperature desulfations is tolerated as long as possible in order
to maintain functionality over a longer period.
[0080] If the upper limit temperature has not yet been reached,
then the method 500 proceeds with step 511 in which the desulfating
temperature is raised. The magnitudes of the increases made in step
511 can be chosen in any suitable manner. Generally the magnitudes
of the increases in desulfating temperature are predetermined.
Examples of predetermined increases include, for example 5.degree.
C. or 10.degree. C. each time the desulfating temperature is
raised. Any suitable series of preselected temperatures can be
used. Another example provides a series of temperatures that rise
linearly on a logarithmic scale from the lowest desulfation
temperature, used for a fresh catalyst, to the upper limit
temperature. The upper limit temperature is not used until the LNT
207 has aged into mid or late life.
[0081] If step 506 is reached with the upper limit desulfating
temperature already in use, then the method proceeds with step 512,
Step 512 signals a fault and the need for service. When this fault
condition is reached, operation may continue as shown. While the
desulfations may be unsatisfactory, the aftertreatment system 203
may remain functional albeit outside specification in terms of
either unsatisfactory pollution control or excessive fuel usage.
The system 200 may begin denitrating very frequently in attempts to
maintain the overly aged LNT at a satisfactory level of NO.sub.X
trapping efficiency. To limit this type of behavior, step 512 may
implement other fault procedures. One option is to terminate the
LNT regeneration cycle entirely until the unit has been
serviced.
[0082] FIG. 4 provides a flow chart of an exemplary method 400 in
which a desulfation is carried out using the selected desulfating
parameters. The method 400 is executed periodically whenever the
desulfation request flag is set. For example, it might be executed
after each denitration, or every few seconds. Following
initialization, the first step in the method 400 is to determine
whether conditions are suitable for starting a desulfation. Similar
to the threshold determination for beginning a denitration, step
402 can require one or more of a minimum temperature for the LNT
207 or the fuel reformer 205, a maximum exhaust oxygen
concentration, a minimum exhaust flow rate, a maximum engine speed
variance as determined by a moving average, and a check that a gear
shift is not currently in progress. Optionally, the conduciveness
of conditions to beginning a desulfation is weighed against the
time since the desulfation request flag was set, or a similar
criteria relating to the urgency of the need to desulfate, whereby
more flexible criteria such as the engine speed variance can be
given greater latitude over time in order to avoid unnecessary and
excessive waiting for preferred conditions to arrive.
[0083] Once step 402 determines that conditions are satisfactory,
the method 400 proceeds with step 403 which checks the soot loading
level of the DPF 208. The amount of soot can be determined in any
suitable manner. One option is to calculate the amount, based for
example on the engine's speed load history since the last DPF
regeneration (DeSoot operation) in combination with knowledge of
the engine 201's particulate matter production rate as a function
of speed and load. Another option is to determine the amount of
soot from the pressure differential across the DPF, the temperature
of the DPF, and the engine exhaust flow rate. If it is determined
that too much soot has accumulated, then step 404, a special DeSoot
operation 404 is performed before proceeding to heat the LNT 207 to
desulfating temperatures in step 405.
[0084] The special DeSoot operation 404 is performed to avoid
overheating the DPF 208. When the DPF 208 is heavily loaded with
soot, there is a danger that once heating initiates soot
combustion, the combustion will further heat the DPF 208 to a point
where damage takes place. To minimize this risk, step 404 ceases
the provision of supplemental heating once the DPF 208 is hot
enough for soot combustion to be underway. After the DPF 208 has
had time to regenerate, the step 404 allows the method 404 to
resume heating for desulfation with step 405.
[0085] Step 405 checks whether the LNT 207 is at a suitable
temperature to begin the preferred cycles of alternating lean and
rich phases to provide temperature control and desulfation
conditions as described above. Step 405 may also check the
temperature of the fuel reformer 205. If the LNT 207 is not yet hot
enough, the method 400 proceeds with step 406, which heats the LNT
207. Heating may be accomplished in any suitable manner. For
example, fuel may be injected into the exhaust line 204 at a rate
where the fuel combusts within the reformer 205 to raise it to its
maximum operating temperature and then maintain the reformer 205 at
that temperature until the LNT 207 is adequately heated.
[0086] Once the LNT 207 is adequately heated, the method 400
proceeds with step 407 in which desulfation proceeds. In this
example, desulfation proceeds through pulsed fuel injection as
described above. In this context, pulsed fuel injection alternates
periods of fuel injection that creates rich condition with period
of no fuel injection that leave the exhaust lean. Pulsing in this
context should not be confused with pulse width modulated flow
control, which may be used by the fuel injector 211 to provide a
desired fuel injection rate. Pulse width modulated flow control has
a high frequency and results in an essentially constant fuel dosing
rate as opposed to low frequency pulsing which creates alternating
lean and rich conditions within the exhaust line 204.
[0087] After each rich phase or with other suitable timing, the
method 400 proceeds to step 408 in which a progress variable for
the desulfation is advanced. The progress variable can be of the
types described previously. For example, the progress can be
measured by the accumulated time at which the LNT 207 has been at
rich conditions or the amount of reductant that has been provided
to the LNT 207 at desulfating temperature. After advancing the
progress variable, the method 400 proceeds with step 409 in which
the value of the progress variable is tested to determine whether
desulfation is complete. In this example, the total time at rich
conditions is compared to the desulfation time set by the method
500.
[0088] If desulfation is complete, the desulfation request flag is
reset (turned off) in step 411 and the process 400 completes. If,
not the method 400 proceeds to step 410, which determines whether
conditions remain suitable for desulfation. This determination can
be similar to step 402, but generally with less stringent criteria.
The criteria ensure that the exhaust conditions are amendable to
controlling temperatures in the reformer 205 and the LNT 207.
[0089] The method 400 illustrates only one of several possible
procedures that may be followed if conditions become unsuitable for
continuing a desulfation that is in progress. The procedure shown
has the desulfation aborting and resetting the desulfation request
flag. Optionally, the desulfation flag can remain set, whereby
desulfation resumes once conditions are again suitable. Optionally,
step 410 waits for a period to determine if the problematic
condition is fleeting before aborting. During this waiting period,
fuel reformer and LNT temperatures can be maintained by fuel
injection. The procedure taken can be made dependent on the
condition that caused the interruption. For example, if the problem
condition is excessive engine speed variance, it is likely to pass
soon and waiting is preferred. If the problem condition is idle
(assuming that desulfation at idle is problematic in the system
200), then aborting the desulfation may be the better option.
[0090] Each complete desulfation takes from about 3 to about 30
minutes, more typically 5 to 10 minutes, e.g., 7.5 minutes. Shorter
desulfations are generally avoided because it usually takes several
minutes to heat the LNT 207 to desulfating temperatures. Longer
desulfation is generally unnecessary.
[0091] Desulfation comprises heating the LNT 207 to a desulfating
temperature, or equivalently, to within a range of desulfating
temperatures. A desulfating temperature refers to any
characteristic temperature for the LNT 207. A characteristic
temperature can be, for example, a measured temperature, an
estimated temperature, or an average of several measured or
estimated temperatures for one or more points within the LNT 207 or
the exhaust within or immediately downstream from the LNT 207. A
characteristic temperature can be an actual value, an estimated
value, a target value, or a blend of the foregoing. A target value
is an objective value (set point) used by a control system
controlling the heating of the LNT 207.
[0092] As a practical matter, most measures of LNT temperature have
a degree of variability. During desulfation, the LNT 207 will have
a range of temperatures within its volume, particularly if it is
being heated. Temperatures constantly vary over time due to
perturbations in exhaust conditions, noise in temperature
measurements, and lag in the system's response to temperature
control measures. This is all in addition to the ranging of
temperatures inherent if a control method in with lean-rich cycling
is employed. Accordingly, even if a method purports to raise the
LNT 207 to one particular desulfating temperature, it would be more
accurate to say the LNT 207 is raised to within a desulfating
temperature range.
[0093] Descriptions of the present invention refer to a either a
desulfating temperature, or a desulfating temperature range having
an upper limit (peak or maximum). These descriptions are
coextensive. A "desulfating temperature" can be understood as
either a single characteristic temperature or the peak or average
of a range for a characteristic temperature. A description
referring to a range of desulfating temperatures is inclusive of
cases that purport to hold the LNT 207 at a single desulfating
temperature. The range can be considered either the single value
(upper and lower limits the same), or the single value plus or
minus a measure of uncertainty or variability, e.g. .+-.25.degree.
C.
[0094] The invention can be employed regardless of how the LNT
temperature is characterized. If the examination of any one
characteristic temperature demonstrates the invention is being
employed, then the invention is being employed regardless of
whether that characteristic temperature is used by a controller. An
increase in any one characteristic temperature or upper limit
temperature will inherently result in increases to all other
characteristic temperatures and upper limit temperatures.
[0095] Aging of the LNT 207 refers to irreversible physical or
chemical changes that occur over time with use. Aging causes a
progressive deterioration in functionality. The affected
functionality includes at least NO.sub.X uptake efficiency. Aging
is not relieved by desulfation. Aging can occur through a variety
of mechanisms, which may or may not be elucidated. Typically,
however, an important mechanism of aging is thermal aging or
sintering. At elevated temperatures, small catalyst particles
gradually coalesce into larger particles, the rate depending on
temperature. This coalescence results in a reduction in surface
area. As catalyst surface area goes down, so does catalyst
activity. Aging can also occur through essentially irreversible
poisoning. For example, SO.sub.X or another compound can become
bound in such a way that the poison cannot be removed by a standard
desulfation process. The time over which LNT aging occurs is time
in operation, especially time at desulfating temperatures. The
higher the temperature, the more quickly aging is taking place.
Aging will occur more quickly if high sulfur fuels, necessitating
more frequent desulfations, are used.
[0096] While aging can occur catastrophically, more typically aging
is a process that occurs gradually over the lifetime of the LNT
207, including many desulfations. Typically, the LNT 207 has a
service life spanning several hundred thousand kilometers of
highway driving or the equivalent. The service life is typically
thousands of hours over which hundreds of desulfations take
place.
[0097] Increases in maximum desulfating temperature are made over a
substantial portion of the lifetime and many desulfations. While a
small increase can be made with each desulfation, use of the
present invention is better characterized by increases in the
maximum desulfating temperature within successive periods, each
period comprising many desulfations, such as 10, 25, 50, or 100
desulfations.
[0098] The increases in maximum desulfation are spread out over a
substantial portion of the lifetime of the product, e.g., from
about 25% to about 75% of the lifetime. Cumulative increase over
the lifetime are typically from about 30 to about 100.degree. C.,
e.g., about 50.degree. C. A typical rate of increase is on the
order of about 5 to about 10.degree. C. over periods of about 50 to
100 desulfation cycles.
[0099] The intervals between desulfations are typically from about
10 to 100 operating hours, e.g. 30 hours. The intervals depend on
the sulfur content of the fuel and generally decrease over time due
to diminishing storage capacity. Increases of 5 or 10.degree. C. in
maximum desulfating temperature are typically made over periods of
about 500 to 5,000 hours of operation.
[0100] The operating lifetime can be divided into consecutive time
intervals, I.sub.1, I.sub.2, . . . I.sub.N, with N.gtoreq.2, each
interval comprising at 1,000 operating hours and at least 10
desulfations. The intervals can be selected so that the highest of
the desulfating temperatures used within each interval I.sub.n is
at least 5.degree. C. greater than that of the preceding interval,
I.sub.n-1. The total increase from I.sub.1 to I.sub.N is at least
30.degree. C., preferably at least 50.degree. C. Preferable, N is
at least 3 so that the increases are made through a series of
stages. Still more preferably, N is at least 5 so that the
increases are gradual, e.g., about 10.degree. C. or less per
interval.
[0101] The increases in maximum desulfating temperature can be
stepwise or continuous. They can result from the application of a
continuous function to a quantified measure of the LNT's aging, or
they can be made through a series of gated stages. Each desulfation
need not use a higher maximum temperature than the proceeding one.
The relevant increases are those being made between successive
intervals each of which comprises many desulfation. The relevant
increases concern the highest among the range of temperatures used
within each interval.
[0102] For example, several mild desulfations with reduced maximum
temperature can be used between deeper desulfations at the current
maximum temperature. The mild desulfation temperatures or the
fraction of desulfations that are mild can be increased, like
maximum desulfation time, as a first response to desulfation
ineffectiveness before raising the maximum desulfation
temperature.
[0103] The timing of the increases to the maximum desulfation
temperature can be predetermined or dynamically determined.
Predetermined increases can be made, for example, according to the
number of desulfations executed or the accumulated time spent at
desulfating temperatures. If the aging of a particular LNT is
predictable and well characterized, a predetermined schedule for
increasing the maximum desulfating temperature over time has the
advantage of simplicity and avoiding the possibility of premature
increases resulting from measurement error. Optionally, a
predetermined timing can be used to provide a minimum period to
wait before increasing desulfating temperatures regardless of
whether an increase is indicated by a dynamically assessment of
whether a temperature increase is warranted.
[0104] A dynamic method of increasing maximum desulfating
temperatures comprises analyzing data to assess the current state
or performance of the LNT 207. Such an analysis seeks to determine
whether the LNT 207 can be adequately desulfated without further
increases to the maximum desulfation temperature. The function of
desulfation is to restore the LNT 207 to adequate levels of
performance. When current desulfation condition no longer restore
the LNT 207 to adequate levels of performance, an increase in the
maximum desulfating temperature may be indicated. Thus, a dynamic
method analyzes performance, or a measure of state that relates to
performance, in order to determine whether there is a need to raise
the maximum desulfation temperature.
[0105] The data to be examined relates to the performance of the
LNT 207 in terms of trapping NO.sub.X during lean phases and
reducing NO.sub.X during rich phases. Measures that can be used
include, for example and without limitation, NO.sub.X trapping
efficiency, NO.sub.X storage capacity, frequency with which
denitration is required, reductant usage during denitration, and
oxygen storage capacity of the LNT 207. Any suitable data can be
used. The data actually used may depend on what is available to the
controller 210 as a result of the design choices made for
scheduling and controlling denitration. Typically, the data
considered will span at least a plurality of desulfations in order
that the effects of LNT aging can be distinguished from other
sources of variability in desulfation effectiveness and subsequent
LNT performance.
[0106] Some of the forgoing examples analyze LNT efficiency data to
determine whether to desulfate the LNT 207 or whether to adapt the
desulfating conditions. This efficiency data can be normalized to
eliminate factors other than catalyst aging and sulfur loading that
affect LNT performance. Such factors include LNT temperature and
the exhaust flow rate. It can, however, be difficult to make data
collected under disparate operating conditions comparable.
[0107] A method for analyzing LNT performance, which is applicable
to a variety of methods for determining whether to desulfate an LNT
or adapt LNT desulfation conditions, sorts LNT performance data
into separate groups according to the conditions under which the
data was collected. Each group corresponds to a distinct range of
conditions. For example, there may be 16 groups indexed according
to two parameters. The two parameters can be, for example, engine
out exhaust gas temperature and engine out oxygen concentration.
The span of exhaust temperatures is divided into four ranges, the
span of exhaust oxygen concentrations is divided into four ranges,
whereby the possible combination create the 16 distinct groups.
[0108] When a decision is to be made based on LNT performance, the
decision criteria is analyzed separately for each group's data. If
the criteria is met based on the data from any one group for which
a statistically significant sample has been collected, the
decision, such as a decision to desulfate or adapt desulfating
conditions, can be made. Variations on the method include using
only the group with the most recently obtained statistically
significant data, only the group with the most statistically
significant data, or voting among the groups having data that is
both sufficiently recent and statistically significant.
[0109] The number of parameters to use, the identity of the
parameters, the number of ranges to divide each parameter into, are
all choices that can be flexibly made. Suitable parameters include,
without limitation those that characterize the engine's operating
state, such as torque, speed, and load, and those that characterize
the exhaust or LNT condition, such as air-to-fuel ratio, oxygen
concentration, flow rate, exhaust temperature, and LNT
temperature.
[0110] The number or parameters and their ranges are preferably
defined in such a way that the majority of operating conditions
fall within a relatively small or moderate number of groups, such
as 4 to 20. The number of groups is preferably limited to ensure
that at least one group generally accumulates a statistically
significant set of data in a timely fashion for making the relevant
decision.
[0111] Several sets of data can be retained. For example there may
be one set corresponding to LNT performance within limited
intervals that follow successfully completed desulfations. That
data set is used for adapting desulfating conditions. There may be
another data set that contains the most recent LNT performance
data. That data, optionally in combination with the post
desulfation data, can be used to decide whether it is time for
desulfation.
[0112] Within each group, only a limited number of data points
corresponding to the most recently collected data may be retained.
For example, each group may retain a number of date points in the
range from 3 to 7, for example 5. As new data is collected, the
oldest is discarded. Alternatively, the number of data points
retained is not limited by number, but is limited by age. In using
the data, an average or median value can be taken. In one example,
5 data points are retained within each set of each group. Analysis
of lean NO.sub.X trap performance uses the averages taken after
discarding the highest and lowest values.
* * * * *