U.S. patent application number 11/490913 was filed with the patent office on 2008-01-24 for coupled dpf regeneration and lnt desulfation.
This patent application is currently assigned to Eaton Corporation. Invention is credited to James Edward McCarthy, Johannes Walter Reuter, Dmitry Arie Shamis, Jiyang Yan.
Application Number | 20080016852 11/490913 |
Document ID | / |
Family ID | 38970122 |
Filed Date | 2008-01-24 |
United States Patent
Application |
20080016852 |
Kind Code |
A1 |
Shamis; Dmitry Arie ; et
al. |
January 24, 2008 |
Coupled DPF regeneration and LNT desulfation
Abstract
A diesel engine exhaust aftertreatment system including a DPF
and a LNT in that order is operated with simultaneous soot
combustion and LNT desulfation. When a control signal to desulfate
the LNT is generated, the DPF is heated to ignite combustion of
trapped soot. As the trapped soot is combusting in the DPF,
reductant is injected downstream of the DPF, but upstream of the
LNT at a rate that leaves the exhaust rich, whereby the LNT
undergoes desulfation. Soot combustion reduces the fuel penalty for
desulfation by removing oxygen from the exhaust. When a reformer is
configured upstream of the LNT, soot combustion helps stabilize the
reformer operation. In one embodiment, there are two fuel
injectors; one upstream of the DPF and one between the DPF and the
fuel reformer. Methods are provided for using this type of
configuration to operate the reformer when the DPF is not being
regenerated.
Inventors: |
Shamis; Dmitry Arie;
(Commerce Twp, MI) ; McCarthy; James Edward;
(Canton, MI) ; Reuter; Johannes Walter;
(Ypsilanti, MI) ; Yan; Jiyang; (Troy, MI) |
Correspondence
Address: |
PAUL V. KELLER, LLC
4585 LIBERTY RD.
SOUTH EUCLID
OH
44121
US
|
Assignee: |
Eaton Corporation
Cleveland
OH
|
Family ID: |
38970122 |
Appl. No.: |
11/490913 |
Filed: |
July 21, 2006 |
Current U.S.
Class: |
60/286 ; 60/285;
60/295; 60/297 |
Current CPC
Class: |
F01N 9/002 20130101;
F01N 2410/00 20130101; Y02T 10/40 20130101; Y02T 10/47 20130101;
F01N 2610/03 20130101; F01N 3/0253 20130101; Y02A 50/20 20180101;
F01N 3/106 20130101; Y02A 50/2325 20180101; F01N 13/0097 20140603;
F01N 2610/1453 20130101; F01N 3/36 20130101; F02B 37/00 20130101;
F01N 3/0885 20130101; F01N 2240/30 20130101; F01N 3/0842 20130101;
F01N 2610/14 20130101 |
Class at
Publication: |
60/286 ; 60/285;
60/295; 60/297 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Claims
1. A method of operating a diesel engine exhaust aftertreatment
system, comprising: passing the exhaust through a DPF and a LNT in
that order, whereby under lean conditions the LNT adsorbs SO.sub.x
from the exhaust and stores the SO.sub.x; generating a control
signal to regenerate the LNT to remove accumulated SO.sub.x; in
response to the control signal, heating the DPF to ignite
combustion of trapped soot; and as the trapped soot is combusting
in the DPF, injecting a reductant downstream of the DPF, but
upstream of the LNT at a rate that leaves the exhaust rich, whereby
the LNT releases stored SO.sub.x and is regenerated.
2. The method of claim 1, wherein there is a device comprising an
oxidation catalytic between the LNT and the DPF and the reductant
is injected upstream of the device.
3. The method of claim 1, wherein: the reductant is diesel fuel;
there is a fuel reformer between the LNT and the DPF; and the
diesel fuel is injected upstream of the fuel reformer.
4. The method of claim 3, wherein the aftertreatment system
selectively increases an injection rate of reductant upstream of
the DPF in order to reduce the temperature of the reformer.
5. The method of claim 1, wherein the DPF is regenerated as often
as the LNT is denitrated.
6. The method of claim 1, wherein the DPF and LNT are of sizes
whereby the DPF needs to be regenerated to remove accumulated soot
approximately as often as the LNT needs to be regenerated to remove
accumulated SO.sub.x.
7. The method of claim 1, wherein heating the DPF comprises
injecting reductant into the exhaust upstream of the DPF, whereby
combustion of the reductant heats the DPF.
8. The method of claim 7, wherein the reductant injected into the
exhaust upstream of the DPF is injected downstream of the
engine.
9. A method of desulfating a LNT in a diesel engine exhaust
aftertreatment system, comprising: passing the exhaust through a
DPF and a LNT in that order; injecting fuel into the exhaust
upstream of the DPF, whereby at least a portion of the fuel
combusts to heat the DPF to a temperature at which soot trapped in
the DPF combusts to regenerate the DPF; and as the trapped soot is
combusting in the DPF, injecting a reductant upstream of the LNT,
but downstream of the DPF, at a rate that leaves the exhaust rich,
whereby SO.sub.x stored in the LNT is released and the LNT
regenerates
10. The method of claim 9, wherein there is a device comprising an
oxidation catalytic between the LNT and the DPF and the reductant
is injected upstream of the device.
11. The method of claim 9, wherein: the reductant is diesel fuel;
there is a device comprising a fuel reformer between the LNT and
the DPF; and the diesel fuel is injected upstream of the fuel
reformer.
12. The method of claim 9, wherein the DPF and LNT are of sizes
whereby the DPF needs to be regenerated to remove accumulated soot
approximately as often as the LNT needs to be regenerated to remove
accumulated SO.sub.x.
13. The method of claim 9, wherein the aftertreatment system
selectively increases the injection of reductant upstream of the
DPF in order to reduce the temperature of the reformer during LNT
desulfation.
14. The method of claim 9, wherein the exhaust aftertreatment
system further comprises a SCR catalyst downstream of the LNT.
15. A vehicle comprising an exhaust aftertreatment system and a
controller configured to operate the exhaust aftertreatment system
according to the method of claim 9.
16. The method of claim 9, wherein the fuel injection upstream of
the DPF is continued after the DPF has finished heating.
17. The method of claim 9, wherein: soot combustion in the DPF
reaches a self sustaining rate; the rate of soot combustion
declines to below a self-sustaining rate due to consumption of
soot; and soot combustion is continued after the decline by
maintaining the temperature of the DPF with additional fuel
injection into the exhaust upstream of the DPF.
18. A method of operating a diesel engine exhaust aftertreatment
system, comprising: passing the exhaust through an exhaust line in
which are installed a fuel reformer and a LNT, in that order, the
LNT being adapted to adsorb and store NOx under lean conditions and
to reduce the stored NOx and regenerate under rich exhaust
conditions; generating a control signal to regenerate the LNT; and
in response to the control signal, injecting fuel into the exhaust
using two separate fuel injectors including a first fuel injector
located upstream of the a second fuel injector; wherein a
substantial portion of the fuel injected with the first fuel
injector mixes within the exhaust line with fuel injected with the
second fuel injector to form a rich mixture that enters the fuel
reformer, which removes oxygen from the exhaust, produces
reformate, and thereby regenerates the LNT; wherein the fuel from
the first fuel injector undergoes substantially greater mixing with
the exhaust and dispersion along the direction of exhaust flow than
the fuel injected with the second fuel injector.
19. The method of claim 18, wherein the first fuel injector is an
engine fuel injector that injects the fuel into the exhaust prior
to its leaving the engine.
20. The method of claim 18, wherein the first fuel injector injects
the fuel into the exhaust as the exhaust travels through an engine
manifold upstream of a turbocharger.
21. The method of claim 18, wherein the first fuel injector injects
the fuel directly into the exhaust line.
22. The method of claim 18, wherein the control signal is a signal
to regenerate the LNT to remove stored NOx.
23. The method of claim 18, wherein: the process of regenerating
the LNT involves a period of heating the fuel reformer under lean
exhaust conditions followed by a period of producing reformate
under rich exhaust conditions; and immediately following the
transition from lean to rich, the majority of fuel supplied to the
fuel reformer comes from the second fuel injector, but as the rich
period progresses, the proportion of fuel supplied to the fuel
reformer by the first fuel injector increases.
24. The method of claim 18, wherein a first exhaust system device
is installed in the exhaust line downstream of the first fuel
injector, but upstream of the second fuel injector.
25. The method of claim 24, wherein a portion of the fuel injected
with the first fuel injector combusts in the first exhaust system
device to remove a substantial portion of oxygen from the
exhaust.
26. The method of claim 24, wherein the first exhaust system device
is a DPF.
27. A method of operating a diesel engine exhaust aftertreatment
system, comprising: passing the exhaust through an exhaust line in
which are installed a first exhaust system device comprising an
oxidation catalyst, a fuel reformer, and a LNT, in that order, the
LNT being adapted to adsorb and store NOx under lean conditions and
to reduce the stored NOx and regenerate under rich exhaust
conditions; generating a control signal to regenerate the LNT; and
in response to the control signal, injecting fuel into the exhaust
using two separate fuel injectors including a first fuel injector
located upstream of the first exhaust system device and a second
fuel injector located between the first exhaust system device and
the fuel reformer; whereby a portion of the fuel injected with the
first fuel injector combusts in the first exhaust system device to
remove a substantial portion of oxygen contained in the exhaust
from the exhaust; the combined fuel injections result in a rich
exhaust mixture that enters the fuel reformer; the fuel reformer
produces reformate that enters the LNT, which is thereby
regenerated.
28. The method of claim 27, wherein the first fuel injector injects
the fuel into the exhaust as the exhaust travels through an engine
manifold upstream of a turbocharger.
29. The method of claim 27, wherein the control signal is a signal
to regenerate the LNT to remove stored SO.sub.x.
30. The method of claim 27, wherein the first exhaust system device
is a DPF.
31. The method of claim 27, wherein the exhaust aftertreatment
system further comprises a SCR catalyst downstream of the LNT.
32. The method of claim 27, wherein the amount of fuel injected
into the exhaust by the first fuel injector is selectively
increased in order to mitigate an increase in temperature of the
fuel reformer.
33. A vehicle comprising an exhaust aftertreatment system and a
controller configured to operate the exhaust aftertreatment system
according to the method of claim 27.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to pollution control devices
for diesel engines.
BACKGROUND
[0002] NO.sub.x and particulate matter (soot) emissions from diesel
engines are an environmental problem. Several countries, including
the United States, have long had regulations pending that will
limit NO.sub.x and particulate matter (soot) emissions from trucks
and other diesel-powered vehicles. Manufacturers and researchers
have put considerable effort toward meeting those regulations.
Diesel particulate filters (DPFs) have been proposed for
controlling particulate matter emissions. A number of different
solutions have been proposed for controlling NOx emissions.
[0003] In gasoline powered vehicles that use stoichiometric
fuel-air mixtures, NO.sub.x emissions can be controlled using
three-way catalysts. In diesel-powered vehicles, which use
compression ignition, the exhaust is generally too oxygen-rich for
three-way catalysts to be effective.
[0004] One set of approaches for controlling NOx emissions from
diesel-powered vehicles involves limiting the creation of
pollutants. Techniques such as exhaust gas recirculation and
partially homogenizing fuel-air mixtures are helpful in reducing
NOx emissions, but these techniques alone are not sufficient.
Another set of approaches involves removing NOx from the vehicle
exhaust. These approaches include the use of lean-burn NO.sub.x
catalysts, selective catalytic reduction (SCR), and lean NO.sub.x
traps (LNTs).
[0005] Lean-burn NOx catalysts promote the reduction of NO.sub.x
under oxygen-rich conditions. Reduction of NOx 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. Lean-burn NO.sub.x
catalysts also tend to be hydrothermally unstable. A noticeable
loss of activity occurs after relatively little use. Lean-burn NOx
catalysts typically employ a zeolite wash coat, which is thought to
provide a reducing microenvironment. The introduction of a
reductant, such as diesel fuel, into the exhaust is generally
required and introduces a fuel economy penalty of 3% or more.
Currently, peak NOx conversion efficiencies for lean-burn NOx
catalysts are unacceptably low.
[0006] SCR generally refers to selective catalytic reduction of NOx
by ammonia. The reaction takes place even in an oxidizing
environment. The NOx can be temporarily stored in an adsorbent or
ammonia can be fed continuously into the exhaust. SCR can achieve
high levels of NOx 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.
[0007] To clarify the state of a sometime ambiguous nomenclature,
it should be noted that in the exhaust aftertreatment art, the
terms "SCR catalyst" and "lean NOx catalyst" are occasionally used
interchangeably. Where the term "SCR" is used to refer just to
ammonia-SCR, as it often is, SCR is a special case of lean NOx
catalysis. Commonly when both types of catalysts are discussed in
one reference, SCR is used with reference to ammonia-SCR and lean
NOx catalysis is used with reference to SCR with reductants other
than ammonia, such as SCR with hydrocarbons.
[0008] LNTs are devices that adsorb NOx under lean exhaust
conditions and reduce and release the adsorbed NOx under rich
exhaust conditions. A LNT generally includes a NOx adsorbent and a
catalyst. The adsorbent is typically an alkaline earth compound,
such as BaCO.sub.3 and the catalyst is typically a combination of
precious metals, such as Pt and Rh. In lean exhaust, the catalyst
speeds oxidizing reactions that lead to NOx adsorption. In a
reducing environment, the catalyst activates reactions by which
adsorbed NOx is reduced and desorbed. In a typical operating
protocol, a reducing environment will be created within the exhaust
from time-to-time to remove accumulated NOx and thereby regenerate
(denitrate) the LNT.
[0009] Creating a reducing environment for LNT regeneration
involves eliminating most of the oxygen from the exhaust and
providing a reducing agent. Except where the engine can be run
stoichiometric or rich, a portion of the reductant reacts within
the exhaust to consume oxygen. The amount of oxygen to be removed
by reaction with reductant can be reduced in various ways. If the
engine is equipped with an intake air throttle, the throttle can be
used. However, at least in the case of a diesel engine, it is
generally necessary to eliminate some of the oxygen in the exhaust
by combustion or reforming reactions with reductant that is
injected into the exhaust.
[0010] The reactions between reductant and oxygen can take place in
the LNT, but it is generally preferred for the reactions to occur
in a catalyst upstream of the LNT, whereby the heat of reaction
does not cause large temperature increases within the LNT at every
regeneration.
[0011] Reductant can be injected into the exhaust by the engine
fuel injectors or separate injection devices. For example, the
engine can inject extra fuel into the exhaust within one or more
cylinders prior to expelling the exhaust. Alternatively, or in
addition, reductant can be injected into the exhaust downstream of
the engine.
[0012] U.S. Pat. Pub. No. 2004/0050037 (hereinafter "the '037
publication") describes an exhaust treatment system with a fuel
reformer placed in the exhaust line upstream of a LNT. The reformer
includes both oxidation and reforming catalysts. The reformer both
removes excess oxygen and converts the diesel fuel reductant into
more reactive reformate.
[0013] The operation of an inline reformer can be modeled in terms
of the following three reactions:
0.684 CH.sub.1.85+O.sub.2.fwdarw.0.684 CO.sub.2+0.632H.sub.2O
(1)
0.316 CH.sub.1.85+0.316H.sub.2O.fwdarw.0.316 CO+0.608H.sub.2
(2)
0.316 CO+0.316H.sub.2O.fwdarw.0.316 CO.sub.2+0.316H.sub.2 (3)
wherein CH.sub.1.85 represents an exemplary reductant, such as
diesel fuel, with a 1.85 ratio between carbon and hydrogen.
Reaction (1) is exothermic complete combustion by which oxygen is
consumed. Reaction (2) is endothermic steam reforming. Reaction (3)
is the water gas shift reaction, which is comparatively thermal
neutral and is not of great importance in the present disclosure,
as both CO and H.sub.2 are effective for regeneration.
[0014] The inline reformer of the '037 publication is designed to
be rapidly heated and to then catalyze steam reforming.
Temperatures from about 500 to about 700.degree. C. are said to be
required for effective reformate production by this reformer. These
temperatures are substantially higher than typical diesel exhaust
temperatures. The reformer is heated by injecting fuel at a rate
that leaves the exhaust lean, whereby Reaction (1) takes place.
After warm up, the fuel injection rate is increased to provide a
rich exhaust. Depending on such factors as the exhaust oxygen
concentration, the fuel injection rate, and the exhaust
temperature, the reformer tends to either heat or cool as reformate
is produced. Reformate is an effective reductant for LNT
denitration.
[0015] U.S. Pat. No. 6,006,515 suggests that a LNT may be
regenerated more efficiently by either longer chain or shorter
chain hydrocarbons, depending on the LNT composition and the
temperature at which regeneration takes place. In order to be able
to control the selection between long and short chain hydrocarbons,
the patent proposes two fuel injectors, one in the exhaust manifold
upstream of the turbocharger and one in the exhaust line
immediately before the LNT. Due to the high temperatures in the
exhaust upstream of the turbocharger, fuel injected with the
manifold fuel injector is said to undergo substantial cracking to
form shorter chain hydrocarbons.
[0016] During denitrations, much of the adsorbed NOx is reduced to
N.sub.2, although a portion of the adsorbed NOx is released without
having been reduced and another portion of the adsorbed NOx is
deeply reduced to ammonia. The NOx release occurs primarily at the
beginning of the regeneration. The ammonia production has generally
been observed towards the end of the regeneration.
[0017] U.S. Pat. No. 6,732,507 proposes a system in which a SCR
catalyst is configured downstream of the LNT in order to utilize
the ammonia released during denitration. The LNT is provided with
more reductant over the course of a regeneration than required to
remove the accumulated NOx in order to facilitate ammonia
production. The ammonia is utilized to reduce NOx slipping past the
LNT and thereby improves conversion efficiency over a stand-alone
LNT.
[0018] U.S. Pat. Pub. No. 2004/0076565 (hereinafter "the '565
publication") also describes hybrid systems combining LNT and SCR
catalysts. In order to increase ammonia production, it is proposed
to reduce the rhodium loading of the LNT. In order to reduce the
NOx release at the beginning of the regeneration, it is proposed to
eliminate oxygen storage capacity from the LNT.
[0019] In addition to accumulating NOx, LNTs accumulate SO.sub.x.
SO.sub.x is the combustion product of sulfur present in ordinarily
fuel. Even with reduced sulfur fuels, the amount of SO.sub.x
produced by combustion is significant. SO.sub.x adsorbs more
strongly than NOx and necessitates a more stringent, though less
frequent, regeneration. Desulfation requires elevated temperatures
as well as a reducing atmosphere. The temperature of the exhaust
can be elevated by engine measures, particularly in the case of a
lean-burn gasoline engine, however, at least in the case of a
diesel engine, it is often necessary to provide additional heat.
Typically, this heat can be provided through the same types of
reactions as used to remove excess oxygen from the exhaust. Once
the LNT is sufficiently heated, the exhaust is made rich by
measures like those used for LNT denitration.
[0020] Diesel particulate filters must also be regenerated.
Regeneration of a DPF is to remove accumulated soot. Two general
approaches are continuous and intermittent regeneration. In
continuous regeneration, a catalyst is provided upstream of the DPF
to convert NO to NO.sub.2. NO.sub.2 can oxidize soot at typical
diesel exhaust temperatures and thereby effectuate continuous
regeneration. A disadvantage of this approach is that it requires a
large amount of expensive catalyst.
[0021] Intermittent regeneration involves heating the DPF to a
temperature at which soot combustion is self-sustaining in a lean
environment. Typically this is a temperature from about 400 to
about 600.degree. C., depending in part on what type of catalyst
coating has been applied to the DPF to lower the soot ignition
temperature. A challenge in using this approach is that soot
combustion tends to be non-uniform and high local temperatures can
lead to degradation of the DPF.
[0022] Because both DPF regeneration and LNT desulfation require
heating, it has been proposed to carry out the two operation
successively. The main barrier to combining desulfation and DPF
regeneration has been that desulfation requires rich condition and
DPF regeneration requires lean conditions. U.S. Pat. Pub. No.
2001/0052232 suggests heating the DPF to initiate soot combustion,
and afterwards desulfating the LNT, whereby the LNT does not need
to be separately heated. Similarly, U.S. Pat. Pub. No. 2004/0113249
describes adding reductant to the exhaust gases to heat the DPF,
ceasing the addition of reductant to allow the DPF to regenerate,
and then resuming reductant addition to desulfate the LNT.
[0023] U.S. Pat. Pub. No. 2004/0116276 suggests close coupling a
DPF and a LNT, with the DPF upstream of the LNT. The publication
suggests that this close-coupling allows CO produced in the DPF
during DPF regeneration to assist regeneration of the downstream
LNT by removing NOx during DPF regeneration in a lean
environment.
[0024] In spite of advances, there continues to be a long felt need
for an affordable and reliable exhaust treatment system that is
durable, has a manageable operating cost (including fuel penalty),
and is practical for reducing NOx emissions from diesel engines to
a satisfactory extent in the sense of meeting U.S. Environmental
Protection Agency (EPA) regulations effective in 2010 and other
such regulations.
SUMMARY
[0025] One of the inventors' concepts relates to a diesel engine
exhaust aftertreatment system including a DPF and a LNT in that
order. When a control signal to desulfate the LNT is generated, the
DPF is heated to ignite combustion of trapped soot. As the trapped
soot is combusting in the DPF, reductant is injected downstream of
the DPF, but upstream of the LNT at a rate that leaves the exhaust
rich, whereby the LNT undergoes desulfation simultaneously with DPF
soot combustion. Soot combustion reduces the fuel penalty for
desulfation by removing oxygen from the exhaust. When a reformer is
configured upstream of the LNT, soot combustion helps stabilize the
reformer operation.
[0026] In one embodiment, there are two fuel injectors; one
upstream of the DPF and one between the DPF and the fuel reformer.
Configurations of this type are useful in operating the reformer
even when the DPF is not being regenerated. Fuel from the upstream
injector becomes well mixed with the exhaust, but becomes
relatively dispersed along the direction of exhaust flow. Combining
an injection from the upstream injector with an injection from the
downstream injector provides a balance between good mixing and
accurate control of the fuel supply rate to the fuel reformer. In
another embodiment, due to high temperatures or intervening
devices, the fuel from the upstream injector extensively combusts
before reaching the fuel reformer. In this embodiment, injection
from the upstream injector stabilizes the reformer temperature and
permits more accurate control of the reformer fuel dosing.
[0027] 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
[0028] FIG. 1 is a schematic illustration of an exemplary power
generation system in which some of the inventors' concepts can be
implemented.
[0029] FIG. 2 is a schematic illustration of another exemplary
power generation system in which some of the inventors' concepts
can be implemented.
[0030] FIG. 3 is a plot showing a preferred reformer fuel profile
for LNT regeneration.
[0031] FIG. 4 is a schematic illustration of another exemplary
power generation system in which some of the inventors' concepts
can be implemented.
[0032] FIG. 5 is a schematic illustration of another exemplary
power generation system in which some of the inventors' concepts
can be implemented.
[0033] FIG. 6 is a schematic illustration of another exemplary
power generation system in which some of the inventors' concepts
can be implemented.
[0034] FIG. 7 is a schematic illustration of another exemplary
power generation system in which some of the inventors' concepts
can be implemented.
[0035] FIG. 8 is a schematic illustration of another exemplary
power generation system in which some of the inventors' concepts
can be implemented.
[0036] FIG. 9 is a schematic illustration of another exemplary
power generation system in which some of the inventors' concepts
can be implemented.
[0037] FIG. 10 is a schematic illustration of an exemplary fuel
injector for use with some of the inventors' concepts can be
implemented.
[0038] FIG. 11A is a schematic illustration of an exemplary
pressure intensifier in a fuel intake configuration.
[0039] FIG. 11B is a schematic illustration of an exemplary
pressure intensifier in a fuel expelling configuration.
[0040] FIG. 12 is a schematic illustration of another exemplary
fuel injector for use with some of the inventors' concepts can be
implemented.
DETAILED DESCRIPTION
[0041] One of the inventors' concepts is to carry out soot
combustion and LNT denitration simultaneously. FIG. 1 provides a
schematic illustration of an exemplary power generation system 1
configured to implement this concept. The system 1 comprises an
engine 9 connected by a manifold 8 to an exhaust aftertreatment
system 2. The exhaust aftertreatment system 2 comprises an exhaust
line 16 in which are configured a first injector 6, a DPF 10, a
second injector 7, and a LNT 11, in that order with respect to the
direction of exhaust flow from the engine 9. A controller 8
controls reductant flow through the injectors 6 and 7 using
information from the engine 9, and a temperature sensor 3.
[0042] The controller 8 may be an engine control unit (ECU) that
also controls the exhaust aftertreatment system 2 or may include
several control units that collectively perform these functions.
The controller 8 may have different connections and draw data from
different sensors than those illustrated in FIG. 1, depending on
the control strategy for the exhaust aftertreatment system 2.
[0043] The preferred reductant injected by the injectors 6 an 7 is
diesel fuel, in which case these are fuel injectors. The advantage
of using diesel fuel as the reductant is that it is readily
available on diesel-powered vehicles. Nevertheless, the inventors'
concepts extend to systems using other reductants. Examples of
other reductants include gasoline, short chain hydrocarbon gases,
and syn gas.
[0044] Instead of the injector 6, a fuel injector for the engine 9
can be used. A diesel engine fuel injector can inject fuel into the
exhaust before it leaves the engine. For example, fuel injection
can take place during a cylinder exhaust stroke. Another
alternative is to position the injector 6 to inject the reductant
into the exhaust manifold 5.
[0045] The engine 9 is typically a diesel engine operational to
produce a lean exhaust. Lean exhaust generally contains from about
4 to about 20% oxygen. Lean exhaust also generally contains NOx and
soot. The engine 9 can be operated to reduce the production of
either NOx or soot, but reducing the output of one pollutant
typically increases the output of the other. Typical untreated
diesel engine exhaust contains environmentally unacceptable amounts
of both NOx and soot.
[0046] The DPF 10 is operative to remove most of the soot from the
exhaust. The LNT 11 is operative to adsorb and store a substantial
portion of the NOx from the exhaust, provided the LNT 11 is in an
appropriate temperature range. Over time, the DPF 10 becomes filled
with soot and begins to lose activity or cause unacceptable
backpressure on the engine 9. Also over time, the LNT 11 becomes
saturated with NOx and begins to lose its effectiveness as well.
Accordingly, both devices must be regenerated from time to
time.
[0047] The DPF 10 is regenerated by heating it to a temperature at
which the accumulated soot undergoes combustion. Combustion is
exothermic. If the temperature of the DPF 10 is sufficiently high,
there is sufficient soot loading in the DPF 10, and there is
sufficient oxygen in the exhaust, soot combustion is
self-sustaining. LNT 11 is regenerated by supplying it with
reductant at a rate that leaves the exhaust rich.
[0048] Regeneration of the DPF 10 is begun by heating the DPF 10.
The DPF 10 is heated by injecting reductant using the injector 6.
At least a portion of this reductant combusts to heat the DPF 10.
The combustion may take place in the DPF 10, provided the DPF 10
has a suitable catalyst, or the combustion may take place in
another device upstream of the DPF 10, such as a separate oxidation
catalyst. The DPF 10 is heated at least until soot combustion
initiates. After soot combustion has initiated, it may be desirable
to stop injecting reductant using the fuel injector 6 in order to
slow the rate at which the DPF 10 heats, although in certain
configurations ceasing reductant injection can actually lead to
higher DPF temperatures as discussed more fully below.
[0049] LNT regeneration is begun by injecting reductant using the
reductant injector 7. Reductant is injected at a rate that leaves
the exhaust downstream of the injector 7 rich. LNT regeneration may
begin while the DPF 10 is being heated, or as soot combustion
begins. In either case, a portion of the oxygen in the exhaust will
have been consumed upstream of the injector 7 either by combustion
of soot or combustion of reductant from the injector 6.
[0050] Simultaneously regenerating the LNT 11 and the DPF 10 can
reduce the fuel penalty for regenerating the LNT 11 in at least two
ways. One is that reductant used to heat the DPF 10 can serve a
dual use; the reductant heats the DPF 10 and the reductant removes
oxygen from the exhaust that must be removed to regenerate the LNT
11. The other way is that the oxygen removed from the exhaust by
soot combustion does not have to be removed by reductant
injection.
[0051] This later function is present regardless of how the DPF 10
is heated. Thus, the inventors' concept extends to systems in which
the DPF 10 is heated without consuming oxygen from the exhaust. For
example the DPF 10 can be heated electrically. Once the DPF 10 is
sufficiently hot, the inventors' concept can be implemented by
injecting reductant using the injector 7 to make the exhaust rich
and regenerate the LNT 11 as soot is combusting in the DPF 10.
[0052] The concept of simultaneous LNT and DPF regeneration is
particularly useful when the reductant is fuel and the exhaust line
16 comprises a fuel reformer 12 upstream of the LNT 11. FIG. 2 is a
schematic illustration of an exemplary power generation system 20
comprising these and other additional components. The additional
components include an oxidation catalyst 15, the fuel reformer 12,
a thermal mass 13, and a SCR catalyst 14.
[0053] The oxidation catalyst 15 is functional to combust reductant
from the injector 6 to generate heat for warming the DPF 10.
Optionally, the oxidation catalyst 15 is also functional to convert
some NO to NO.sub.2. NO.sub.2 can contribute to the regeneration of
the DPF 10 even under lean conditions, provided the DPF 10 has an
appropriate catalyst. NO.sub.2 may also remove carbonaceous
deposits from the fuel reformer 12 and the LNT 11, be adsorbed more
efficiently than NO by the LNT 11, and provide the exhaust with an
NO to NO.sub.2 ratio that results in more efficient NOx reduction
by the SCR catalyst 14.
[0054] The reformer 12 converts injected fuel into more reactive
reformate. An oxidation catalyst could be used in place of the
reformer 12, although a fuel reformer is preferred. A reformer that
operates at diesel exhaust gas temperatures requires a large amount
of catalyst and may excessively increase the cost of an exhaust
aftertreatment system. Accordingly, the reformer 12 is preferably
of the type that has low thermal mass and must be heated to be
operational.
[0055] The thermal mass 13 is another optional component placed
upstream of the LNT 11. The thermal mass 13 acts to reduce the
magnitude of temperature excursion experienced by the LNT 11 due to
heat generated in upstream devices. Frequent large temperature
excursions can reduce the lifetime of the LNT 11.
[0056] The SCR catalyst 14 functions to adsorb and store ammonia
generated by the LNT 11 during rich regeneration phases. During the
lean phases between regenerations of the LNT 11, the SCR catalyst
uses this stored ammonia to reduce NOx slipping past the LNT 11
thus increasing the overall extent of NOx mitigation.
[0057] In the system 20, combustion to heat the DPF 10 and soot
combustion in the DPF 10 reduce the amount of oxygen that must be
removed by the reformer 12 in order for the reformer 12 to produce
reformate. In addition, heat generated by these processes can
reduce the amount of fuel that must be injected to heat the
reformer 12 to an operating temperature.
[0058] In one embodiment, upon receiving a signal to commence
regeneration, fuel injection through injector 6 begins. The fuel
combusts in the oxidation catalyst 15, heats the DPF 10 and, to a
lesser extent, heats the reformer 12. Once the DPF 10 reaches a
sufficiently high temperature, soot combustion begins. Fuel
injection through the injector 7 can begin at any time, but
preferably begins after fuel injection through the injector 6
begins, more preferably at about the time that soot combustion
begins or shortly thereafter.
[0059] If the reformer 12 is not yet warm enough when fuel
injection through the injector 7 begins, fuel injection through the
injector 7 is at a rate that leaves the exhaust lean, whereby
essentially all of the injected fuel is combusted to heat the
reformer 12. Once the reformer 12 is sufficiently warm, the fuel
injection rate through the injector 7 is increased to a point that
leaves the exhaust rich, whereupon reformate production begins.
Fuel injection through the injector 7 is terminated when the LNT 11
has been regenerated to a satisfactory extent. Fuel injection
through the injector 6 can be terminated once the DPF 10 has
reached a temperature where soot combustion is self-sustaining,
however, fuel injection through the injector 6 can be continued as
long as it does not cause overheating of the DPF 10. Preferably,
the period over which the reformer 12 is producing reformate
overlaps the period in which soot is combusting within the DPF
10.
[0060] In a prior art method, soot combustion in the DPF 10
continues until there is no longer sufficient soot to sustain
combustion temperatures. According to another of the inventors'
concepts, however, soot combustion can be continued and soot
removed to a greater degree. Soot combustion can be continued by
injecting fuel through the fuel injector 6 to provide sufficient
heat to sustain soot combustion temperatures in the DPF 10. A fuel
injection that had been stopped when the DPF 10 first reached a
sufficient temperature for self-sustaining soot combustion may be
resumed for this purpose. This additional fuel might be considered
underutilized if LNT regeneration were not simultaneous. Using the
inventors' concept, however, this is fuel that would be required in
any event to continue regeneration of the LNT 11.
[0061] The systems 1 and 20 can be configured so that the DPF 10
and the LNT 11 are always regenerated simultaneously. However, it
is possible to regenerate one device more frequently than the
other. The DPF 10 can be regenerated independently of the LNT 11 by
using only the injector 6. The LNT 11 can be regenerated
independently of the DPF 10 by using only the injector 7 in order
that the DPF 10 can be heated quickly with a low fuel penalty and
in order that a large portion of the heat generated in the DPF 10
is quickly transported downstream, the DPF 10 preferably has a
small thermal mass. A small thermal mass is achieved by having a
small size and thin walls. The DPF 10 can be a wall flow filter or
a pass through filter and can use primarily either depth filtration
of cake filtration. Any DPF with a suitably low pressure drop can
be used, but one that uses primarily depth filtration may be more
conducive to maintaining a small thermal mass while keeping engine
back pressure within acceptable limits.
[0062] Cake filtration is the primary filter mechanism in a wall
flow filter. In a wall flow filter, the soot-containing exhaust is
forced to pass through a porous medium. Typical pore diameters are
from about 0.1 to about 1.0 .mu.m. Soot particles are most commonly
from about 10 to about 50 nm in diameter. In a fresh wall flow
filter, the initial removal is by depth filtration, with soot
becoming trapped within the porous structure. Quickly, however, the
soot forms a continuous layer on an outer surface of the porous
structure. Subsequent filtration is through the filter cake and the
filter cake itself determines the filtration efficiency. As a
result, the filtration efficiency increases over time. In the prior
art, the filter cake was generally allowed to build to a thickness
from about 15 to 50 .mu.m deep before regeneration began. In the
present invention, if a wall flow filter is used, regeneration
begins before the cake is about 10 .mu.m deep, more preferably
before the cake about 5 .mu.m deep, still more preferably before
the cake is about 2 .mu.m deep.
[0063] For a wall flow filter, a small size is typically about
1/10th the engine displacement or less. Preferably, the size is
about 1/20th the engine displacement or less. The diameter of the
DPF 10 is preferably about the same as that of an upstream or
downstream abutting exhaust pipe. Wall flow filters are typically
made from ceramics, especially cordierite or SiC.
[0064] In contrast to a wall flow filter, in a flow through filter
the exhaust is channeled through macroscopic passages and the
primary mechanism of soot trapping is depth filtration. The
passages may have rough walls, baffles, and bends designed to
increase the tendency of momentum to drive soot particles against
or into the walls, but the flow is not forced though micro-pores.
The resulting soot removal is considered depth filtration, although
the soot is generally not distributed uniformly with the depth of
any structure of the filter. Preferably, the filter has metal
walls, which can be made very thin to keep the thermal mass low.
Emitec.TM. produces such filters. A flow through filter can also be
made from temperature resistant fibers, such as ceramic or metallic
fibers, that span the device channels. A flow through filter can be
larger than a wall flow filter having equivalent thermal mass
[0065] Reducing the size of the DPF 10 generally involves a
reduction in soot storage capacity. This is acceptable in that the
DPF 10 can be regenerated much more frequently than a conventional
DPF, which would be regenerated much less frequently than the LNT
11. In order to maintain the functionality of the DPF 10, the DPF
10 must generally be regenerated at least about 20% as often as the
LNT 11, more typically at least about 50%, and still more typically
at least about 70% as often. In other terms, the DPF 10 generally
needs to be regenerated at least about once every 10 minutes, more
typically at least about once every 5 minutes, still more typically
at least about once every 3 minutes.
[0066] It is acceptable if the DPF 10 is regenerated more often
than necessary, but the above regeneration requirements are
indicative of the DPF 10 being optimally sized for use in
conjunction with the inventors' concepts. Having a somewhat greater
capacity in the DPF 10 than in the LNT 11 facilitates a simplified
control scheme, where only the criteria for LNT regeneration is
examined by the controller 8, it being assumable that if the LNT 11
is being regenerated often enough, the DPF 10 is being regenerated
often enough as well.
[0067] If it is difficult to achieve a target level of particulate
emission control while maintaining a sufficiently small size of the
DPF 10, one option is to install a second DPF downstream of the LNT
11. For example, this second DPF might be used as the thermal mass
13. The second filter can be of the wall flow type and much large
than the DPF 10. Preferably, however, the majority of the
particulates are removed by the DPF 10. The second DPF can be
heated for regeneration in conjunction with heating of the LNT 11
for desulfation.
[0068] The time at which to regenerate the LNT 11 to remove
accumulated NOx can be determined by any suitable method. Examples
of methods of determining when to begin a regeneration include
initiating a regeneration upon reaching a thresholds in any of a
NOx concentration in the exhaust, a total amount of NOx emissions
per mile or brake horsepower-hour over a previous period or since
the last regeneration, a total amount of engine out NOx since the
last regeneration, an estimate of NOx loading in the LNT 11, and an
estimate of adsorption capacity left in the LNT 11. Regeneration
can be periodic or determined by feed forward or feedback control.
Regeneration can also be opportunistic, being triggered by engine
operating conditions that favor low fuel penalty regeneration. A
threshold for regeneration can be varied to give a trade off
between urgency of the need to regenerate and favorability of the
current conditions for regeneration. The time at which to
regenerate the LNT 11 can be determined by the controller 8, which
generates a control signal that initiates the regeneration
process.
[0069] In addition to the option of carrying out denitration
simultaneously with soot combustion, the inventors have also
conceived the idea of carrying out desulfation simultaneously with
soot combustion. This latter concept can be implemented with
systems having the same schematic structures as the systems 1 and
20 illustrated in FIGS. 1 and 2. The main differences from the
previous description are in terms of the size of the DPF 10 and the
method of operation.
[0070] When implementing the concept of simultaneous soot
combustion and desulfation, the operation of systems 1 and 20
between regenerations remains the same as previously described. The
DPF 10 accumulates soot and the LNT 11 stores a portion of the
exhaust NOx. In the system 20, the SCR 14 reduces a portion of the
NOx slipping past the LNT 11 using stored ammonia.
[0071] During LNT denitration, the DPF 10 is generally not heated
significantly and continues to accumulate soot. LNT denitration is
carried out with reductant injection, which may be carried out
using either or both the injectors 6 & 7. For the system 20,
fuel is first injected at a rate that leaves the exhaust lean and
heats the reformer 12, then at a rate that leaves the exhaust rich,
causing reformate to be produced and regenerating the LNT 11.
[0072] The LNT 11 can be desulfated independently of regenerating
the DPF 10 and the DPF 10 can be regenerated independently of
desulfating the LNT 11, however, preferably the DPF 10 is
regenerated each time the LNT 11 is desulfated. More preferably
regeneration of the DPF 10 and desulfation of the LNT 11 are always
simultaneous.
[0073] If desulfation is to be carried out simultaneously with soot
combustion, the DPF 10 is preferably large enough to only require
soot combustion approximately as often as the LNT 11 requires
desulfation. A conventionally sized wall-flow DPF may serve this
purpose. Generally the LNT 11 must be desulfated at least about 20%
as often as the DPF 10 needs to be regenerated, more typically at
least about 50%, and still more typically at least about 70% as
often.
[0074] It is acceptable if the LNT 11 is desulfated more often than
necessary, but the above regeneration requirements are indicative
of the DPF 10 being optimally sized for use in conjunction with the
inventors' concept of making LNT desulfation simultaneous with soot
combustion. If one of the DPF 10's storage capacity for soot (in
terms of lengths of times between regenerations) and the LNT 11's
storage capacity for sulfur is greater than the other, the
frequency of simultaneous regenerations can be based on the
requirements of the device needing the more frequent
regenerations.
[0075] The times to regenerate the DPF 10 and desulfate the LNT 11
can be determined in any suitable fashions. When the DPF 10 is a
wall flow filter, the time to regenerate the DPF 10 can be
determined by monitoring the pressure drop across the DPF 10.
Desulfation may be scheduled periodically, e.g., after every 30
hours of operation. Alternatively, desulfation may be scheduled
based on an estimate of the amount on SO.sub.x stored in the LNT
11. The amount of stored SO.sub.x can be assumed to increase in
proportion to fuel usage and to decrease in a manner dependent on
the extent of desulfations. A further option is to determine the
need for desulfation based on system performance, e.g., based on
the activity of the LNT 11 following an extensive denitration or
based on the frequency with which denitration is required
[0076] To initiate soot combustion and DPF regeneration, reductant
is injected through the injector 6. The injected reductant
combusts, heating the DPF 10 and, to a lesser extent, the
downstream LNT 11. Eventually soot combustion in the DPF 10 begins.
Soot combustion provides additional heat the DPF 10.
[0077] Heat from the DPF 10 will eventually warm the LNT 11, but
rather than waiting for this processes, the LNT 11 can be
separately heated by injecting reductant through the injector 7 at
a rate that leave the exhaust lean. In the case of the system 20,
the rate of this injection may need to be limited to avoid
overheating the reformer 12.
[0078] Once the DPF 10 has reached a sufficient temperature for
self-sustaining soot combustion, the reductant injection through
the injector 6 may be discontinued. Once the LNT 11 has reached a
sufficient temperature for desulfation, the reductant through the
injector 7 is increased to a rate that makes the exhaust rich and
causes desulfation in the LNT 11. As in the case of simultaneous
soot combustion and LNT denitration, soot combustion in the DPF 10
reduces the amount of reductant that must be injected to consume
excess oxygen and thereby reduces the fuel penalty for desulfating
the LNT 11. Additional fuel is saved in that heat from the DPF 10
warms the LNT 11.
[0079] Soot combustion during desulfation also promotes stable
operation of the reformer 12. As discussed more fully below, the
reformer 12 may have a tendency to overheat when operated steadily
for long periods. Removing some of the oxygen from the exhaust
mitigates this overheating problem. More heat is generated in the
DPF 10 by soot combustion to approximately the same extent that
less heat is produced in the reformer 12 due to less remaining
oxygen, and the heat from the DPF 10 tend to be transported to the
reformer 12, however, overheating of the reformer 12 is nonetheless
mitigated due to heat losses to the surroundings and more uniform
heat distribution.
[0080] Even when the DPF 10 is sized for simultaneous soot
combustion and desulfation, rather than simultaneous soot
combustion and denitration, the injector 6 may be used for LNT
denitration. Fuel injected using the injector 6 that does not fully
combust before it reaches the reformer 12 becomes better mixed with
the exhaust than fuel injected using the injector 7. The fuel
injected with injector 6 also tends to become more dispersed along
the direction of flow, which could be a disadvantage if injector 6
were the only one used. Combining the two fuel injectors, however,
allows a balance between good mixing and precise control of exhaust
air-fuel ratios at the reformer 12. Such a balance is easier to
achieve if there is no oxidation catalyst 15 and the DPF 10 has a
relatively low catalyst loading whereby a substantial portion of
the fuel injected by the injector 6 makes it to the reformer
12.
[0081] FIG. 3 illustrates a preferred reductant fuel injection
profile for denitrating a LNT positioned in an exhaust line
downstream of a fuel reformer. Line 31 is the fuel injection rate,
line 32 is the exhaust oxygen flow rate (controlled through an
engine intake air throttle), line 33 is the resulting reformer
temperatures, and line 34 is the resulting reformate flow rate.
After an initial heating period, the fuel injection 31 is
controlled through an approximately Gaussian profile, which causes
the reformate flow rate 34 to begin relatively low, increase to a
maximum, and then decreases toward the end. This type of reformate
profile has been found to provide superior denitration fuel
efficiency in comparison to a constant reformate flow profile. The
superior efficiency is in terms of less NOx slip during
denitration, more conversion of stored NOx per unit fuel used, and
more ammonia production during regeneration.
[0082] A theory that explains the functionality of this preferred
reductant flow rate or concentration profile is that the reductant
supply rate approximately matches the NOx release rate. At the
beginning of regeneration, reductant is consumed by reaction with
oxygen stored in the LNT 11. Until this stored oxygen is removed,
reduction of NOx is not effective, particularly not deep reduction
of NOx to NH.sub.3.
[0083] Regeneration does not take place uniformly throughout the
LNT 11. Oxygen is first removed near the entrance. The point of
oxygen removal is believed to form a front that moves towards the
exit of the LNT 11. As this front moves through the LNT 11, a
greater and greater portion of the LNT 11 is essentially free of
stored oxygen and begins to undergo release of stored NOx. As this
portion of the LNT 11 increases, the NOx release rate also
increases. By progressively increasing the reductant supply rate,
this release rate can be approximately matched by the reductant
supply rate while oxygen is being removed at a relatively constant
speed. Eventually, after essentially all of the stored oxygen is
removed and the NOx release rates in the oxygen-free zones are
ebbing due to depleting reserves of stored NOx, the overall NOx
release rate decreases. By decreasing the reductant supply rate
toward the end of the regeneration, the reductant supply rate can
be approximately matched to the NOx release rate in the latter part
of the regeneration as well.
[0084] A highly dispersed fuel injection from the injector 6 can
naturally provide a Gaussian profile of the type desired. When a
more exact control of the fueling rate is desired, for LNT warm-up
or perhaps upon the transition from lean to rich, the fuel injector
7 can used. Together, the two can provide any desired profile and a
balance between precise control of profile shape and excellent
mixing of fuel and exhaust.
[0085] If there are devices comprising oxidation catalysts between
the upstream inject 6 and the reformer 12, some of the injected
fuel will not reach the reformer 12. The reformer temperatures may
be different as a result of this oxidation, but the reformate
production rates will be much the same in that, excepting the
effect on reformer temperatures, it makes little difference whether
oxidation takes place in the reformer 12 or upstream of it.
[0086] The different distribution of heat depending on whether fuel
is injected using the injector 6 or the injector 7 can be used to
stabilize operation of the fuel reformer 12. When a fuel reformer
of the preferred type is operated to produce reformate at high
exhaust oxygen concentrations, e.g., 8-15%, there is a tendency of
the reformer 12 to overheat. In principle, overheating could be
reduced by increasing the fuel injection rate, which would be
expected to increase the rate of endothermic reaction (2) while the
rate of exothermic reaction (1) remains constant. In practice,
however, the reformer 12 and the LNT 11 generally cannot operate
efficiently with such high fueling rates. An alternative solution
is to pulse the fuel injection to the reformer 12, allowing the
reformer 12 to cool between pulses. Disadvantages to fuel pulsing
include loss of efficiency due to reductant from rich phases
reacting with oxygen from lean phases.
[0087] The structure illustrated in FIG. 2 provides a different way
to control heating in the fuel reformer 12. Even if the DPF 11 is
not being regenerated, a portion of the fuel required to make the
exhaust lean and produce a target amount of reformate can be
injected using the injector 6. Much of the injected fuel combusts
over the oxidation catalyst 15 or in the DPF 10, removing a portion
of the oxygen from the exhaust and releasing heat.
[0088] Even though the same amount of heat is generated,
overheating of the reformer 12 can be reduced. If the DPF 10 is not
fully heated, as in during denitrations when soot combustion is not
also being carried out, the heat can be stored in the DPF 10 and
slowly released. If the DPF 10 heats to a steady state temperature,
as during a prolonged desulfation, a greater portion of the total
heat generated is lost to the surroundings upstream of the reformer
12. That heat that does reach the reformer 12 from the DPF 10 is
less problematic than if it were generated in the reformer 12 in
that the heat is more evenly distributed. Overheating tends to
occur in local hot spots.
[0089] The concept of controlling the reformer temperature by using
selective distribution of fuel between two injection points can be
implemented without the DPF 12 using, for example, a system as
illustrated in FIG. 4 where there is an oxidation catalyst 15
between the upstream point of fuel injection and the reformer 12.
The fuel required by the reformer 12 in FIG. 4 can be selectively
distributed between the fuel injectors 6 and 7. In order to
facilitate temperature control by this method, the oxidation
catalyst 15 can be specifically designed to readily lose heat to
the surroundings. Such a design may involve a high external surface
area and conductive rather than insulating packaging.
[0090] This same concept can be applied, perhaps with even greater
effect, to controlling the temperature of the DPF 10. FIG. 5
provided an exemplary power generation system 50 for implementing
this concept. To begin DPF regeneration in the system 50, the DPF
10 can be heated by injecting reductant through the injector 6. The
reductant combusts in the oxidation catalyst 15, generating heat
that warms the DPF 10.
[0091] Once soot combustion reaches a self sustaining temperature,
the fuel injection optionally ceases, however, if soot combustion
threatens to overheat the DPF 10, fuel injection can
counter-intuitively be increased. Rather than aggravating the
overheating problem, this mitigates the problem. The injected fuel
combusts in the oxidation catalyst 15, removing oxygen from the
exhaust. This oxygen is no longer available in the DPF 10, thus
reducing the soot combustion rate. The same total amount of heat
may be released, but the distribution is significantly different.
The heat may temporarily reside in the oxidation catalyst 15 and
the DPF 10 may have already begun to cool by the time the heat is
transferred downstream. There will be greater heat lost to the
surroundings upstream of the DPF 10 due to the higher temperatures.
Finally, soot combustion tends to occur along narrow fronts.
Whereas the heat produced from these fronts is concentrated, the
heat transferred to the DPF 10 through the exhaust is rather evenly
distributed. If temperatures in the DPF 10 can be effectively
controlled, a less expensive substrate can be used resulting in
significant cost savings. In particular, cordierite can be used
instead of the more expensive SiC.
[0092] FIG. 6 provides another power generation system 60 in which
the concept of limiting DPF temperatures using reductant injection
can be implemented. The power generation system 60 contains several
bricks between the upstream injector 6 and the DPF 10. These bricks
include the reformer 12, the thermal mass 13, the LNT 11, and the
SCR catalyst 14. Soot combustion can be initiated using the
downstream injector 7. When soot combustion rates become too high,
reductant injection through the injector 6 can commence. The fuel
injected upstream will be combusted in the reformer 12, consuming
oxygen from the exhaust, but causing little heating of the DPF 10
due to the thermal mass of the reformer 12 and the various
intervening devices.
[0093] The concept of limiting the DPF temperature using upstream
reductant injection can be implemented with either feedback or feed
forward control. Feed back control involves the use of one or more
temperature sensors, like the sensor 3. If the sensor readings are
subject to significant delays, it may be desirable to correct them
by extrapolation or some other method to obtain and estimate of the
current temperature. A typical feedback control strategy is PID
control, with the degree of fueling tending to increase in
proportion to the extent to which a DPF temperature is in excess of
a target maximum.
[0094] In some cases, feed forward control may be more desirable.
One reason to use feed forward control is that hot spots may tend
to occur locally in the DPF 10, with the hot spots moving as soot
combustion progresses. Because the hot spots are local and not
always in the same location, it may be inadequate to rely on
sensors. On the other hand, a model, particularly one that is
corrected using some sensor data, can predict local hot spots.
[0095] Another option is to simply control the fuel injection
according to a pre-determined program. A characteristic of such a
program implementing the inventors' concepts is that the injection
rate through the injector 6 is maintained or increased after soot
combustion within the DPF 10 has reached a self-sustaining rate. In
an exemplary program using just one fuel injector, the reductant
injection begins at a rate designed to heat the DPF 10. After soot
combustion begins, the rate is maintained. As soot combustion
completes, the rate of reductant injection is gradually
decreased.
[0096] In an exemplary method using two fuel injectors, fuel
injection begins immediately upstream of the DPF 10 at a rate
designed to heat the DPF 10. After soot combustion has reached a
self-sustaining rate, fuel injection immediately upstream of the
DPF 10 ceases. After soot combustion has begun in the DPF 10, but
before soot combustion reaches an excessive rate, fuel injection
begins at a point further upstream, whereby combustion
significantly reduces the flow rate of oxygen into the DPF 10. The
upstream fuel injection may be maintained until the DPF 10 is
nearly regenerated.
[0097] Additional measures may be used to limit the exhaust oxygen
flow rate. Examples of such measures include increasing exhaust gas
recirculation (EGR), throttling the engine air intake, and shifting
gears to make the engine run at lower speeds. All of these methods
can be used together with the inventors' concepts to limit the rate
of soot combustion and heating in the DPF 10.
[0098] Another related concept involves placing an oxidation
catalyst upstream of a fuel reformer or an oxidation
catalyst-containing DPF. An example with a fuel reformer is the
power generation system 70 schematically shown in FIG. 7. The power
generation 70 comprises a diesel engine 9, an oxidation catalyst
15, a reductant injector 7, a fuel reformer 12, a LNT 11, and a SCR
catalyst 14.
[0099] Although the reformer 12 itself contains an oxidation
catalyst, the upstream oxidation catalyst 15 can perform several
functions. One function is to reduce the oxygen content of the
exhaust by combusting hydrocarbons also contained in the exhaust.
As in the other concepts, the same amount of heat is generated, but
at a point displaced from the reformer 12. During regeneration of
the LNT 11, the engine 9 can be operated to provide additional
hydrocarbon to augment this function. In addition to simply
injecting the hydrocarbons, the engine 9 can provide additional
hydrocarbon by operating near or beyond the smoke limit.
[0100] Additional hydrocarbons may also be provided as a natural
consequence of other measures used to reduce the oxygen
concentration of the exhaust during regeneration of the LNT 11.
Examples of such measures may include increasing EGR rates,
throttling the engine air intake, and shifting gears to reduce the
engine speed. Additional hydrocarbons can also be provided by
increasing engine fueling rates. These additional hydrocarbons can
be combusted in the oxidation catalyst 15 while causing only
attenuated heating of the reformer 12
[0101] The oxidation catalyst 15 can perform additional functions
as well. One such function is that it can operate to heat the
exhaust slightly even when the LNT 11 is not being regenerated.
This additional heat can extend the operating temperature range of
the reformer 12, allowing the reformer 12 to be started at lower
exhaust temperatures. This function can be facilitated by placing
the oxidation catalyst 15 as close to the engine as possible,
whereby the exhaust will keep the catalyst 15 at relatively higher
temperatures.
[0102] A further advantage of catalyzing or fueling combustion to
remove some of the exhaust oxygen upstream of the reformer 12 is
that it facilitates more precise control of the reformer 12. The
oxygen concentration or lambda value of the exhaust between the
oxidation catalyst and the reformer can be measured for use in this
control. Because less oxygen needs to be removed, the fuel dose
immediately upstream of the reformer 12 is smaller and can be
controlled more accurately.
[0103] Another potential use for the oxidation catalyst 15 is
converting NO to NO.sub.2. The NO.sub.2 can function to remove
carbonaceous deposits from the reformer 12 and the LNT 13.
Increasing the proportion of the NO.sub.2 in the exhaust can also
enhance NOx removal by the LNT 12 and the SCR catalyst 14.
Generally, more catalyst is required to effectuate the NO to
NO.sub.2 function than the more basic hydrocarbon oxidation
function.
[0104] The upstream oxidation catalyst is also useful when the
reformer 12 is in a separate branch from the main exhaust line.
FIG. 8 provides a schematic illustration of a power generation
system 80 having this type of branching. In the system 80, the fuel
reformer 12 is in the main exhaust line, but it is essentially the
same if the fuel reformer 12 is in the bypass line 81 and the main
exhaust line in parallel with the branch is empty.
[0105] The power generation system 80 is designed without exhaust
system valves. An exhaust system valve or damper can be used to
control the distribution of exhaust between branches. Such control
is desirable in terms of limiting fuel penalty, but exhaust
treatment systems with exhaust valves may be less reliable than
exhaust treatment systems without valves.
[0106] An additional improvement that is applicable to several of
the above-described concepts is to place a fuel injector in an
exhaust manifold upstream of a turbocharger. FIG. 9 provides a
schematic illustration of an exemplary power generation system 90
implementing this concept. The engine 9 operates to produce exhaust
which passes through the exhaust manifold 5 to the exhaust line 16.
The exhaust line 16 contains a fuel reformer 12 and a LNT 11,
although the concept is not limited to these exhaust line
components. The manifold contains turbocharger 91, which is
configured to provide pressurized air to the inlet 93 of the engine
9. An injector 92 is configured to inject a reductant into the
manifold 5 upstream of the turbocharger 91.
[0107] One advantage of manifold reductant injection is that the
reductant undergoes intense mixing with the exhaust as it pass
through the turbocharger 91. Thorough mixing promotes better
utilization of the reductant in downstream devices, which is
particularly important if the reductant is diesel fuel. The types
of devices that can benefit from this mixing include fuel
reformers, oxidation catalysts, and DPFs.
[0108] Another advantage is that the exhaust is hotter upstream of
the turbocharger 91. At these higher temperatures, the reductant
can undergo reactions. In the case of diesel fuel, these reactions
include cracking of the diesel fuel into smaller and more reactive
molecules. These reactions generally involve expansion of the gases
and the release of heat. On the one hand, these reactions can
provide a boost to the turbocharger 91. On the other hand, these
reactions can release heat and consume oxygen, thus displacing heat
from a downstream device as is done with the oxidation catalyst 15
in FIG. 5. The function is also similar to the oxidation catalyst
15 in FIG. 5 in that the release of heat and the production of
smaller more reactive reductant molecules can facilitates low
temperature start-up of downstream devices, such as a DPF 10 or a
reformer 12.
[0109] If the first downstream device is a DPF 10, the use of the
manifold fuel injector 92 can reduce the amount of oxidation
catalyst required. The DPF 10 may be loaded with oxidation catalyst
to allow light-off through the addition of fuel that combusts in
the DPF 10. The same oxidation catalyst can promote soot
combustion. By reducing the amount of catalyst, not only can the
cost be reduced, but also soot combustion rates and problems with
excessively high DPF temperatures during soot combustion.
[0110] A potential problem with the manifold injector 90 and other
exhaust system injectors is cooking. Coke can form from residual
fuel left in the injector when the injector is off, particularly if
the injector is off for long periods of time during which it is
subject to high temperatures. Coke can deteriorate injector
performance and cause failures.
[0111] FIG. 10 illustrates a fuel injector 100 adapted to implement
an air purge method to the cooking problem. The fuel injector 100
is shown installed within a wall 101 of an exhaust passage, which
may be an exhaust line or an exhaust manifold. The fuel injector
100 comprises a valve body 102, a needle 103, a solenoid 104 for
controlling the position of need 103, a fuel source 105, a valve
106 for controlling the fuel flow from the fuel source 105, an air
supply 107, and a valve 108 for controlling the flow of air from
the air supply 107.
[0112] When fuel injection is required, the valve 106 opens to
admit fuel from the fuel source 105. Optionally, the valve 106 is
opened and closed rapidly in a controlled manner to regulate the
fuel flow rate. Once the fuel dosing is complete and the valve 106
is finally closed, valve 108 is opened briefly to admit air from
air supply 107. The air flows through the valve body 102, flushing
the passages therein of fuel, whereby little or no fuel remains to
form coke.
[0113] The air supply 107 can be any suitable source of pressurized
air. Examples of pressurized air sources are an air pump, an intake
manifold pressurized by a turbocharger, and an exhaust manifold
upstream of a turbocharger (providing the injector 100 is not
itself installed in an exhaust manifold). In a preferred
embodiment, the pressurized air is drawn from a truck braking
system.
[0114] The fuel supply 105 can be any suitable source of fuel. A
standard fuel pump can be used to obtain fuel from a vehicle fuel
tank. To promote atomization, vaporization, and mixing, however, it
can be desirable to obtain higher pressures than the 3 to 6 bars
provided by a standard electric fuel pump. To obtain higher
pressures, it is preferred to use a pressure intensifier.
[0115] FIGS. 11A and 11B illustrates a pressure intensifier 110.
The pressure intensifier contains a body 111 and a piston 112
defining an upper chamber 113, a middle chamber 114, and a lower
chamber 115. The intensifier is charged with low pressure fuel from
pump 116 as illustrated in FIG. 11A by opening valve 117 and 118
and closing valve 119 an 121. Fuel enters the lower chamber through
valve 118, forcing the piston 112 to rise, forcing fuel out of the
upper chamber 113, through valve 117, to a reservoir from which the
pump 116 draws fuel.
[0116] Fuel is expelled at high pressure through valve 120 as
illustrated in FIG. 11B by closing valves 117 and 118 and opening
valves 119 and 121. The pump 116 pumps fuel into the upper chamber
113 through the valve 119. The fuel in the upper chamber acts on
the piston 112 to force fuel out of the lower chamber 115 through
the valve 121. Because the area of the upper surface of the piston
112, which is acted on by the fuel in the upper chamber 113 at the
pump pressure, is greater than the area of the lower surface of the
piston 112, which acts on the fuel in the lower chamber 115, the
fuel in the lower chamber 115 can be pressurized in proportion to
the difference in area. Preferably, the fuel is pressurized by at
least a factor of 2, more preferably by at least a factor of three.
The middle chamber 114 accumulates fuel slipping between the piston
112 and the walls of the body 111. The accumulated fuel is returned
to the pump reservoir through port 122
[0117] In addition to cooking, manifold and exhaust system fuel
injectors may be susceptible to overheating. One method to avoid
overheating is to provide an excess fuel flow to the fuel injector.
The excess fuel flow is returned to the fuel reservoir, carrying
away heat. Circulating fuel in this manner also prevents cooking.
The fuel flow can be maintained as long as the injector is subject
too high temperatures.
[0118] FIG. 12 illustrates and exemplary fuel injector 120 designed
to accommodate an excess fuel flow. The fuel injector 120 comprises
a valve body 125 having internal passages 126, a needle 123, a
solenoid 124 for controlling the position of need 123, the fuel
source 105, an exhaust port 127, and a check valve 128 for
controlling the flow of fuel through the exhaust port 127. The fuel
injector 120 is shown installed within a wall 101 of an exhaust
passage.
[0119] The check valve 128 can be set just below the pressure of
the fuel source 105, whereby there is a continuous fuel flow
through the valve 127 and the internal passages 126 when the needle
valve 123 is closed. This fuel is returned to a reservoir for the
fuel source 105. When the needle valve 123 is open, the pressure
drops and the flow is primarily out the opening created by needle
valve 123.
[0120] While the engine 9 is preferably a compression ignition
diesel engine, the various concepts of the inventor are applicable
to power generation systems with lean-burn gasoline engines or any
other type of engine that produces an oxygen rich, NOx-containing
exhaust. For purposes of the present disclosure, NOx consists of NO
and NO.sub.2.
[0121] The power generation system can have any suitable types of
transmission. A transmission can be a conventional transmission
such as a counter-shaft type mechanical transmission, but is
preferably a CVT. A CVT can provide a much larger selection of
operating points than a conventional transmission and generally
also provides a broader range of torque multipliers. The range of
available operating points can be used to control the exhaust
conditions, such as the oxygen flow rate and the exhaust
hydrocarbon content. A given power demand can be met by a range of
torque multiplier-engine speed combinations. A point in this range
that gives acceptable engine performance while best meeting a
control objective, such as minimum oxygen flow rate, can be
selected.
[0122] In general, a CVT will also avoid or minimize interruptions
in power transmission during shifting. Examples of CVT systems
include hydrostatic transmissions; rolling contact traction drives;
overrunning clutch designs; electrics; multispeed gear boxes with
slipping clutches; and V-belt traction drives. A CVT may involve
power splitting and may also include a multi-step transmission.
[0123] A preferred CVT provides a wide range of torque
multiplication ratios, reduces the need for shifting in comparison
to a conventional transmission, and subjects the CVT to only a
fraction of the peak torque levels produced by the engine. This can
be achieved using a step-down gear set to reduce the torque passing
through the CVT. Torque from the CVT passes through a step-up gear
set that restores the torque. The CVT is further protected by
splitting the torque from the engine, and recombining the torque in
a planetary gear set. The planetary gear set mixes or combines a
direct torque element transmitted from the engine through a stepped
automatic transmission with a torque element from a CVT, such as a
band-type CVT. The combination provides an overall CVT in which
only a portion of the torque passes through the band-type CVT.
[0124] A fuel reformer is a device that converts heavier fuels into
lighter compounds without fully combusting the fuel. A fuel
reformer can be a catalytic reformer or a plasma reformer.
Preferably, the reformer 12 is a partial oxidation catalytic
reformer comprising a steam reforming catalyst. Examples of
reformer catalysts include precious metals, such as Pt, Pd, or Ru,
and oxides of Al, Mg, and Ni, the later group being typically
combined with one or more of CaO, K.sub.2O, and a rare earth metal
such as Ce to increase activity. A reformer is preferably small in
size as compared to an oxidation catalyst or a three-way catalyst
designed to perform its primary functions at temperatures below
450.degree. C. The reformer is generally operative at temperatures
from about 450 to about 1100.degree. C.
[0125] The LNT 11 can comprise any suitable NOx-adsorbing material.
Examples of NOx adsorbing materials include oxides, carbonates, and
hydroxides of alkaline earth metals such as Mg, Ca, Sr, and Ba or
alkali metals such as K or Cs. Further examples of NOx-adsorbing
materials include molecular sieves, such as zeolites, alumina,
silica, and activated carbon. Still further examples include metal
phosphates, such as phosphates of titanium and zirconium.
Generally, the NOx -adsorbing material is an alkaline earth oxide.
The absorbent is typically combined with a binder and either formed
into a self-supporting structure or applied as a coating over an
inert substrate.
[0126] The LNT 11 also comprises a catalyst for the reduction of
NOx in a reducing environment. The catalyst can be, for example,
one or more transition metals, such as Au, Ag, and Cu, group VIII
metals, such as Pt, Rh, Pd, Ru, Ni, and Co, Cr, or Mo. A typical
catalyst includes Pt and Rh. Precious metal catalysts also
facilitate the adsorbent function of alkaline earth oxide
absorbers.
[0127] Adsorbents and catalysts according to the present invention
are generally adapted for use in vehicle exhaust systems. Vehicle
exhaust systems create restriction on weight, dimensions, and
durability. For example, a NOx adsorbent bed for a vehicle exhaust
systems must be reasonably resistant to degradation under the
vibrations encountered during vehicle operation.
[0128] The ammonia-SCR catalyst 14 is a catalyst effective 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,
substituted with metal ions such as cations of Cu, Co, Ag, Zn, or
Pt, and activated carbon. Preferably, the ammonia-SCR catalyst 14
is designed to tolerate temperatures required to desulfate the LNT
11.
[0129] Although not illustrated in any of the figures, a clean-up
catalyst can be placed downstream of the other aftertreatment
device. A clean-up catalyst is preferably functional to oxidize
unburned hydrocarbons from the engine 9, unused reductants, and any
H.sub.2S released from the NOx absorber-catalyst 11 and not
oxidized by the ammonia-SCR catalyst 14. Any suitable oxidation
catalyst can be used. To allow the clean-up catalyst to function
under rich conditions, the catalyst may include an oxygen-storing
component, such as ceria. Removal of H.sub.2S, where required, may
be facilitated by one or more additional components such as NiO,
Fe.sub.2O.sub.3, MnO.sub.2, CoO, and CrO.sub.2.
[0130] The invention as delineated by the following claims has been
shown and/or described in terms of certain concepts, components,
and features. While a particular component or feature may have been
disclosed herein with respect to only one of several concepts or
examples or in both broad and narrow terms, the components or
features in their broad or narrow conceptions may be combined with
one or more other components or features in their broad or narrow
conceptions wherein such a combination would be recognized as
logical by one of ordinary skill in the art. Also, this one
specification may describe more than one invention and the
following claims do not necessarily encompass every concept,
aspect, embodiment, or example described herein.
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