U.S. patent application number 10/515529 was filed with the patent office on 2006-04-13 for spark ignition engine including three-way catalyst with nox storage component.
Invention is credited to Robert James Brisley, Daniel Swallow, Martyn Vincent Twigg.
Application Number | 20060075740 10/515529 |
Document ID | / |
Family ID | 9937339 |
Filed Date | 2006-04-13 |
United States Patent
Application |
20060075740 |
Kind Code |
A1 |
Twigg; Martyn Vincent ; et
al. |
April 13, 2006 |
Spark ignition engine including three-way catalyst with nox storage
component
Abstract
A spark ignition engine with an exhaust system having a
catalyst, which includes a three-way catalyst (TWC) and a NOx
storage component, and an engine control unit is provided. The
engine control unit is programmed to control the air-to-fuel ratio
of the engine to run at the stoichiometric air-to-fuel ratio during
normal running conditions and to run lean of the stoichiometric
air-to-fuel ratio during a defined portion of an engine speed/load.
The engine control unit also determines the amount of NOx
contacting the TWC during lean running operation in response to
data input from a sensor means, thereby a remaining NOx storage
capacity of the TWC is determined. The control unit is programmed
to return the air-to-fuel ratio to stoichiometry when the NOx
storage capacity is below a pre-determined value. The engine and
its components are arranged such as to substantially prevent
passing more NOx to atmosphere during an engine cycle compared with
a spark ignition engine run continuously at stoichiometric
conditions.
Inventors: |
Twigg; Martyn Vincent;
(Cambridge, GB) ; Brisley; Robert James; (Cambs,
GB) ; Swallow; Daniel; (Herts, GB) |
Correspondence
Address: |
RATNERPRESTIA
P O BOX 980
VALLEY FORGE
PA
19482-0980
US
|
Family ID: |
9937339 |
Appl. No.: |
10/515529 |
Filed: |
May 21, 2003 |
PCT Filed: |
May 21, 2003 |
PCT NO: |
PCT/GB03/02200 |
371 Date: |
August 24, 2005 |
Current U.S.
Class: |
60/285 ;
60/301 |
Current CPC
Class: |
Y02A 50/20 20180101;
Y02T 10/12 20130101; F02D 2200/0806 20130101; B01D 53/9495
20130101; F02D 41/0275 20130101; B01D 53/9445 20130101; Y02T 10/22
20130101; Y02A 50/2324 20180101; F01N 3/0814 20130101; F01N 3/0842
20130101; F01N 2570/16 20130101 |
Class at
Publication: |
060/285 ;
060/301 |
International
Class: |
F01N 3/00 20060101
F01N003/00; F01N 3/10 20060101 F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 24, 2002 |
GB |
0211971.7 |
Claims
1. A spark ignition engine comprising an exhaust system comprising
a catalyst and an engine control unit programmed to control the
air-to-fuel ratio of the engine to run at a stoichiometric
air-to-fuel ratio during normal running conditions and to run lean
of the stoichiometric air-to-fuel ratio during a defined portion of
an engine speed/load map, which catalyst comprising a three-way
catalyst (TWC) including a NOx storage component, wherein the
engine control unit is programmed to determine an amount of NOx
contacting the TWC during lean running operation in response to
data input from sensor means, thereby a remaining NOx storage
capacity of the TWC is determined, and the control unit is
programmed to return the air-to-fuel ratio to stoichiometry when
the NOx storage capacity is below a pre-determined value.
2. An engine according to claim 1, wherein the defined portion of
the engine speed/load map is engine idle.
3. An engine according to claim 1, wherein the defined portion of
the engine speed/load map comprises driving conditions, wherein a
level of NOx emitted by the engine at the driving conditions is up
to ten times more than the level of NO.sub.x emitted at engine idle
conditions.
4. An engine according to claim 1, further comprising a clock,
wherein the data input sensor means includes a predetermined or
predicted time elapsed from the start of lean running
operation.
5. An engine according to claim 4, wherein the predicted time is
subsequently adjusted in response to data input.
6. An engine according to claim 1, wherein the sensor means detects
a value of airflow over the TWC and the data input includes that
detected value.
7. An engine according to claim 1, wherein the sensor means detects
a manifold vacuum value and the data input includes that detected
value.
8. An engine according to claim 1, wherein the sensor means detects
an ignition timing value and the data input includes that detected
value.
9. An engine according to claim 1, wherein the sensor means detects
an engine speed value and the data input includes that detected
value.
10. An engine according to claim 1, wherein the sensor means
detects a throttle position value and the data input includes that
detected value.
11. An engine according to claim 1, wherein the sensor means is a
lambda value sensor, and the data input includes the lambda value
detected upstream and/or downstream of the TWC.
12. An engine according to claim 1, wherein the sensor means
detects a quantity of fuel injected in the engine and the data
input includes that detected quantity.
13. An engine according to claim 1, further comprising an exhaust
gas recirculation (EGR) circuit, wherein the sensor means detects
an amount of exhaust gas recirculation by the position of an EGR
valve and the data input includes the detected amount of EGR.
14. An engine according to claim 1, wherein the sensor means
detects an engine coolant temperature value and the data input
includes that detected value.
15. An engine according to claim 1, wherein the sensor means
comprises a NOx sensor and the data input includes an amount of NOx
detected by the NO.sub.x sensor upstream and/or downstream of the
TWC.
16. An engine according to claim 1, wherein the engine is a
gasoline engine.
17. An engine according to claim 16, wherein the engine is a port
fuel injection engine.
18. An engine according to claim 16, wherein the engine is a direct
injection engine.
19. An engine according to claim 1, wherein the engine is fuelled
by a fuel selected from the group consisting of liquid petroleum
gas, natural gas, methanol, and hydrocarbon mixtures including
ethanol or hydrogen gas.
20. An engine according to claim 1, wherein the TWC comprises at
least one platinum group metal (PGM).
21. An engine according to claim 20, wherein the at least one PGM
is selected from the group consisting of platinum (Pt), palladium,
rhodium (Rh), ruthenium, osmium or iridium and combinations of any
two or more thereof.
22. An engine according to claim 1, wherein the NOx storage
component comprises an alkali metal, an alkaline-earth metal or a
rare-earth metal or a combination of any two or more thereof.
23. An engine according to claim 22, wherein the alkali metal is
potassium or caesium.
24. An engine according to claim 22, wherein the alkaline-earth
metal is magnesium, calcium, strontium or barium.
25. An engine according to claim 22, wherein the rare earth metal
is a lanthanide group metal.
26. An engine according to claim 1, wherein the TWC comprises an
oxygen storage component (OSC).
27. An engine according to claim 26, wherein the OSC comprises a
component selected from the group consisting of stabilised ceria,
perovskites, NiO, MnO.sub.2, manganese-based compounds supported on
alumina-containing mixed oxide, a mixed oxide of manganese and
zirconium, Pr.sub.2O.sub.3 and combinations of any two or more
thereof.
28. An engine according to claim 27, wherein the ceria stabiliser
selected from the group consisting of zirconium, lanthanum,
aluminium, yttrium, praseodymium and neodymium.
29. An engine according to claim 1, wherein the TWC comprises an
inner layer comprising a first PGM and the NOx storage component,
and an outer layer comprising an OSC and a second PGM.
30. A vehicle including an engine according to claim 1.
31. A vehicle according to claim 30, wherein the TWC is in a
close-coupled position.
32. A vehicle according to claim 30, wherein a fresh TWC includes a
sufficient amount of the NOx storage component to retain sufficient
NOx storage capacity after high temperature ageing.
33. An engine control unit for a spark ignition engine comprising
an exhaust system comprising a TWC including a NOx storage
component, which engine control unit is programmed to control the
air-to-fuel ratio of the engine to run at a stoichiometric
air-to-fuel ratio during normal running conditions and to run lean
of stoichiometry during a defined portion of an engine speed/load
map and to determine an amount of NOx contacting the TWC during
lean running operation in response to data input from sensor means,
thereby a remaining NOx storage capacity of the TWC is determined,
and the control unit is programmed to return the air-to-fuel ratio
to stoichiometry when the NOx storage capacity is below a
pre-determined value.
34. A method of treating exhaust gas of a spark ignition engine run
at the stoichiometric air-to-fuel ratio during normal running
conditions, which engine comprising an exhaust system comprising a
TWC including a NOx storage component, which method comprising the
steps of controlling the engine air-to-fuel ratio to run lean of
stoichiometry during a defined portion of an engine speed/load map,
determining the amount of NOx contacting the TWC during lean
running operation in response to data input from sensor means
thereby determining the remaining NOx storage capacity of the TWC,
and returning the air-to-fuel ratio to stoichiometry when the
remaining NOx storage capacity is below a pre-determined value.
35. (canceled)
36. (canceled)
37. An engine according to claim 29, wherein the first PGM is
platinum.
38. An engine according to claim 29, wherein the second PGM is
rhodium.
39. An engine according to claim 11, wherein the lambda value
sensor is a linear lambda sensor.
40. An engine according to claim 25, wherein the rare earth metal
is lanthanum.
Description
[0001] The present invention relates to a spark ignition engine
comprising an exhaust system comprising a catalyst and an engine
control unit programmed to control the air-to-fuel ratio of the
engine to run at the stoichiometric air-to-fuel ratio during normal
running conditions and to run lean of the stoichiometric
air-to-fuel ratio during a defined portion of an engine speed/load
map. In particular, the present invention relates to such an engine
wherein the catalyst is a three-way catalyst (TWC) including a NOx
storage component.
[0002] A heterogeneous catalyst capable of simultaneous conversion
of nitrogen oxides (NOx), carbon monoxide (CO) and unburnt
hydrocarbons (HC) in exhaust gas from a stoichiometrically
operated, spark-ignited combustion engine is known as a three-way
catalyst (TWC). NOx reduction readily occurs over the TWC when the
air-to-fuel ratio is rich of stoichiometric, whereas CO and HC
reactions are hindered by insufficient oxygen (O.sub.2). On the
lean side, the CO and HC conversions are high, but NOx reduction is
difficult because of the excess of oxidising species. Accordingly,
effective three-way conversion occurs in a relatively narrow
air-to-fuel ratio window. In practice an oxygen sensor is used to
detect the lambda composition of the exhaust gas upstream of the
TWC and to adjust the air-to-fuel ratio accordingly to equilibrate
the exhaust gas.
[0003] A typical TWC comprises platinum (Pt) and/or palladium (Pd)
as an oxidation catalyst and rhodium (Rh) as a reduction catalyst
on a suitable high surface area oxide support, such as alumina
(AM.sub.2O.sub.3), and an oxygen storage component (OSC), e.g. a
ceria-zirconia mixed oxide. Various minor amounts of base metal
catalyst promoters, stabilisers and hydrogen sulphide suppressers
can be included. For flirter details, see WO 98/03251 (incorporated
herein by reference).
[0004] A consequence of using detected oxygen concentration to
control the air-to-fuel ratio is that there is a time lag
associated in adjusted air-to-fuel ratio. This results in
perturbation around the control set point. Thus, when operating
rich, there is a need to provide a small amount of O.sub.2 to
consume the unreacted CO and HC. Conversely, when the exhaust gas
goes slightly oxidising, the excess O.sub.2 needs to be consumed.
One development in TWC technology adopted in order to reduce the
problem of emissions associated with perturbations was to
incorporate an O.sub.2 storage component in the TWC composition.
This component adsorbs (or absorbs) O.sub.2 in the lean environment
and releases it in the rich environment, thus effectively extending
the time at which the exhaust gas is at the set point Where more
significant amounts of HC fuel are required to maintain the
air-to-fuel ratio, such as during acceleration, this can be
provided e.g. by adjusting the fuel injection period.
[0005] More recently, there has been a move towards running
gasoline combustion engines lean, for example, in gasoline direct
injection engines. The rationale is to improve fuel economy (thus
decreasing the emissions of CO.sub.2) by running lean of
stoichiometry. The principal problem in pursuing this strategy is
that the lean environment inhibits NOx reduction in the TWC. One
technology which has been developed to meet this problem is called,
variously, a NOx absorber/catalyst, lean NOx trap (LNT) or NOx trap
and is based on acid-base washcoat chemistry. It involves
adsorption (or absorption) and storage of NOx in the catalyst
washcoat during lean driving conditions and release under rich
operation. The released NOx is catalytically converted to nitrogen
just as it is in TWCs.
[0006] A typical NOx trap composition comprises Pt and Rh on a high
surface area oxide support, such as Al.sub.2O.sub.3, and a NOx
storage component such as barium oxide (BaO (see, e.g. EP 0758713,
incorporated herein by reference)). Generally, loadings of NOx
storage components in NOx trap washcoats can be up to 50 wt % or
even higher
[0007] A major problem with the use of NOx trap technology is that
it requires very careful and complicated control of the engine in
order to provide for rich regeneration of the NOx storage component
Additionally, a number of feedback sensors are used to control the
function of the NOx trap NOx storage capacity, e.g. sensors to
estimate cumulative engine-out NOx production utilising stored
engine maps and NOx trap temperature sensors, because the
efficiency of the NOx storage component to absorb NOx is
temperature dependent.
[0008] It is known in the art of TWCs, to stabilise a
gamma-Al.sub.2O.sub.3 component from sintering during high
temperature ageing using small amounts (1-3 percent) of BaO or
lanthanum oxide (La.sub.2O.sub.3). Base metal catalyst promoters,
such as barium (Ba), cerium, lanthanum, magnesium, calcium and
strontium can also be used (see WO 98/03251, mentioned above).
[0009] In our WO 99/00177 (incorporated herein by reference) we
describe a catalytic converter for a lean burn internal combustion
engine, such as a direct injection gasoline engine, comprising a
catalyst component capable of storing NOx. In one embodiment, the
catalytic converter comprises a supported layered catalyst having a
first, inner layer containing a first platinum group metal (PGM),
e.g. Pt, and a NOx storage component, e.g. Ba, and a second, outer
layer containing a second different PGM, such as Rh supported on a
non-Al.sub.2O.sub.3 support, and optionally an OSC such as a mixed
oxide of ceria and zirconia.
[0010] Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652
(incorporated herein by reference) describes a catalyst comprising
a substrate, and an alkaline-earth metal compound and Pt loaded on
the substrate. The description observes that during urban driving a
vehicle is frequently accelerated and decelerated. Consequently,
this can cause the air-to-fuel ratio to vary frequently from the
range of values adjacent to the stoichiometric point during more
steady conditions, e.g. idling, to the fuel rich side. In order to
lower the fuel consumption during e.g. urban driving conditions,
the gasoline engine is run on the fuel lean side, such as an
air-to-fuel ratio of up to 23:1 (wt./wt.). The catalyst is designed
to adsorb (or absorb) NOx on the alkaline-earth metal during lean
running conditions and to use the natural fluctuation of the
air-to-fuel ratio to the rich side to enable stored NOx to be
released and reduced, thus regenerating the NOx storage capacity of
the alkaline-earth metal compound.
[0011] U.S. Pat. No. 5,575,983 (incorporated herein by reference)
observes that the NOx absorbing capacity of the catalyst of
Japanese Unexamined Patent Publication No. 5-317,652 is poisoned by
sulfate. In order to combat this, it proposes a catalyst comprising
Pt or Pd and alkali metals, alkaline-earth metals and rare-earth
elements including lanthanum (La) supported on lithium-stabilised
Al.sub.2O.sub.3.
[0012] We believe that the use of lean burn gasoline engines other
than direct injection gasoline engines, such as those described in
Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 and
U.S. Pat. No. 5,575,983 has not received widespread acceptance in
the vehicle industry and that one reason for this is the difficulty
in controlling NOx emissions to meet existing and future emission
legislation.
[0013] We have now found, very surprisingly, that it is possible to
operate a spark ignition engine, such as a port fuel injection
gasoline engine, which engine comprising a TWC including a NOx
storage component, in such a way as to benefit from the increased
fuel economy available during lean running conditions whilst
avoiding the requirement for expensive and complicated control
systems.
[0014] According to one aspect, the invention provides A spark
ignition engine comprising an exhaust system comprising a catalyst
and an engine control unit programmed to control the air-to-fuel
ratio of the engine to run at the stoichiometric air-to-fuel ratio
during normal running conditions and to run lean of the
stoichiometric air-to-fuel ratio during a defined portion of an
engine speed/load map, which catalyst comprising a three-way
catalyst (TWC) including a NOx storage component, characterised in
that the engine control unit is further programmed to determine the
amount of NOx contacting the TWC during lean running operation in
response to data input from sensor means, thereby to determine the
remaining NOx storage capacity of the TWC and to return the
air-to-fuel ratio to stoichiometry when the remaining NOx storage
capacity is below a pre-determined value, the arrangement being
such as to substantially prevent passing more NOx to atmosphere
during an engine cycle compared with a spark ignition engine run
continuously in stoichiometric mode.
[0015] By "engine cycle" herein we mean the period between key on
and key off.
[0016] The present invention takes advantage of the natural
fluctuation in the composition of an exhaust gas of a spark
ignition engine operated at the stoichiometric air-to-fuel ratio to
rich lambda values, e.g. during acceleration, to regenerate the NOx
storage component. NOx stored on the NOx storage component is
generally not released, and the NOx storage component is not
regenerated, under lambda=1 conditions, i.e. conditions rich of
lambda=1 are required. We have also devised a catalyst that
facilitates the regeneration of a NOx storage component in a TWC,
which catalyst is used in a preferred embodiment according to the
invention.
[0017] The present invention provides a number of very substantial
advantages. One such advantage is that it enables a vehicle powered
by a spark ignition engine to be run at a fuel saving over a
similar vehicle operated continuously at substantially
stoichiometric conditions. Such increased efficiency can result in
lower CO.sub.2 emissions in a legislative test cycle for a vehicle.
Lower CO.sub.2 emissions in a legislative test cycle translates to
lower CO.sub.2 emissions in "real world" driving conditions.
Accordingly, a vehicle according to the invention can be more
"environmentally friendly". Furthermore, in countries where
vehicles are taxed depending on the amount of CO.sub.2 they emit (a
so-called "green tax"), such as the UK, it can reduce the tax
burden to the consumer.
[0018] A second such advantage is that it can allow existing
vehicles including spark ignition engines to receive the benefit of
the invention by retrofitting certain components. This can be done
by simply replacing the existing TWC with a TWC including
sufficient NOx storage component and the engine control unit for
one programmed: (i) to run the engine lean of the stoichiometric
air-to-fuel ratio during a defined portion of an engine speed/load
map; and (ii) to determine the amount of NOx contacting the TWC
during lean running operation in response to data input from sensor
means, thereby to determine the remaining NOx storage capacity of
the TWC and to return the air-to-fuel ratio to stoichiometry when
the remaining NOx storage capacity is below a pre-determined value,
thereby substantially to prevent passing more NOx to atmosphere
during an engine cycle compared with a spark ignition engine run
continuously in stoichiometric mode.
[0019] According to a particularly preferred embodiment, the
defined portion of the engine speed/load map is engine idle. This
is a particularly advantageous arrangement in that it provides a
fail-safe system for regenerating the NOx storage component This is
because the only thing that can happen to the engine after idle is
that it is accelerated, following which the exhaust gas contacting
the TWC is temporarily rich before the engine control unit returns
the exhaust gas to equivalence. Even if the engine is switched off
following idle, NOx can be stored to key on, following which the
engine will be accelerated. Accordingly, this preferred arrangement
takes advantage of the natural fluctuations in the composition of
the exhaust gas to the rich side during acceleration to regenerate
the NOx storage component For similar reasons, the defined portion
of the engine speed/load map can comprise low speed driving wherein
the level of NOx emitted by the engine is up to ten times more,
such as five times or twice more, than at engine idle.
[0020] A very substantial advantage of this preferred arrangement
is that it avoids the requirement for complicated and expensive
sensors and controls in order to meet emission legislation that
presently burdens the adoption of NOx traps.
[0021] A number of means for inputting data to the engine control
unit to determine the amount of NOx contacting the TWC, and hence
the remaining NOx storage capacity of the NOx storage component,
can be used either singly or in any mechanically/electronically
viable combination. Many of the sensor means required to collect
this information are already included in the engine and/or vehicle
fitted with the engine and are used by the engine control unit for
controlling other functions of the engine and/or vehicle. This is
one reason why it is possible to adopt the invention by
retrofitting the vehicle engine control unit, together with a TWC
including a NOx storage component.
[0022] Such detected data that can be used to monitor remaining NOx
storage capacity in the TWC of the invention include: predetermined
or predicted time elapsed from the start of lean running operation,
by sensing the status of a suitable clock means; airflow over the
TWC or manifold vacuum; ignition timing; engine speed; throttle
position; exhaust gas redox composition, for example using a lambda
sensor, preferably a linear lambda sensor; quantity of fuel
injected in the engine; where the vehicle includes an exhaust gas
recirculation (EGR) circuit, the position of the EGR valve and
thereby the detected amount of EGR; engine coolant temperature; and
where the exhaust system includes a NOx sensor, the amount of NOx
detected upstream and/or downstream of the TWC. Where the clock
embodiment is used, the predicted time can be subsequently adjusted
in response to data input.
[0023] The spark ignition engine can be any capable of operating
during normal running conditions at the stoichiometric air-to-fuel
ratio. In one embodiment, the engine can be powered by gasoline and
the engine can be of the port fuel injection or direct injection
type. Optionally, the engine can be fuelled using an alternative
fuel such as liquid petroleum gas (LPG), natural gas (NG),
methanol, hydrocarbon mixtures including ethanol or hydrogen gas.
The invention can be used on all grades of sulfur-containing fuel,
but with particular efficiency with grades containing less that 50
ppm by weight of sulfur, and most preferably less than 10 ppm by
weight of sulfur.
[0024] The present invention can utilise any known TWC composition
provided that sufficient NOx storage component is included in order
to perform the desired function.
[0025] A typical TWC composition comprises at least one PGM, and
can be selected from the group consisting of Pt, Pd, Rh, ruthenium,
osmium and iridium and any combination of two or more thereof.
[0026] Many NOx storage components are disclosed in the prior art,
and any can be utilised in the present invention. Typical NOx
storage components comprise alkali metals, such as potassium or
caesium, alkaline-earth metals, e.g. magnesium, calcium, strontium
or Ba, rare-earth metals lanthanide group metal, preferably La, or
any viable combination, e.g. a mixed oxide, of any two or more
thereof.
[0027] A common component of state-of-the-art TWCs is the OSC and
these too can be included in the TWC, with utility, according to
the present invention. Indeed, it is trite knowledge of the person
skilled in the art that NOx trap compositions do not include OSC,
because an OSC assists in the combustion of HC at the
stoichiometric point and slightly rich thereof. This property runs
counter to the requirement of a system including a NOx trap
composition which is to regenerate the NOx storage component using
reducing species, such as HCs in the exhaust, resulting from
air-to-fuel ratio modulation. Accordingly, the presence of an OSC
in a NOx trap composition would cause increased fuel consumption
for the same amount of NOx storage component regeneration relative
to an OSC-free NOx trap composition.
[0028] Known OSC include optionally stabilised ceria, perovskites,
NiO, MnO.sub.2, manganese-based compounds supported on
Al.sub.2O.sub.3-containing mixed oxides (see PCT/GB01/05124,
incorporated herein by reference) a mixed oxide of manganese and
zirconium (see WO 99/34904, incorporated herein by reference),
Pr.sub.2O.sub.3 or a combination of any two or more thereof. The
ceria stabiliser can be zirconium, lanthanum, aluminium, yttrium,
praseodymium or neodymium.
[0029] A preferred TWC for use in the present invention comprises a
first PGM, preferably Pt, and the NOx storage component in a first,
inner layer and an OSC and a second PGM, preferably Rh, in a
second, outer layer.
[0030] This arrangement is advantageous for the following reasons.
During stoichiometric running the Rh/OSC component is active for
NOx reduction and other reactions while the Pt component is active
for oxidation reactions. During oxygen rich conditions the Rh is
relatively inactive, the Pt is active for NO, HC and CO oxidation
while the NO.sub.2 produced is stored in the adsorber as
nitrate.
[0031] On subsequent return to lambda=1, the OSC component in the
second layer prevents the stored NOx "seeing" reducing gas so that
the NOx remains stored as nitrate. The Rh is active for NOx
reduction by CO and the Pt is active for oxidation of HC and
CO.
[0032] On acceleration the gas mix becomes rich biased, reducing
the OSC material so that the stored NOx is released, as the nitrate
becomes thermodynamically unstable. The released NOx is then
reduced by the Rh layer with the excess reductant,
[0033] According to a further aspect the invention provides a
vehicle comprising an engine according to the present
invention.
[0034] TWC are generally placed in one or both of two positions in
a vehicle according to the intended purpose: the close-coupled
position, in which the TWC is disposed as close to the exhaust
manifold as possible; and the underfloor position. The reason for
placing a TWC in the close-coupled position is to control emissions
immediately following cold start, as much of the controlled
emissions are emitted during the legislative test cycle immediately
following cold-start. By positioning a TWC close to the engine, the
catalyst is contacted by hot exhaust gases immediately after key on
and accordingly reaches the light-off temperature for CO and HC
oxidation sooner than a TWC in the cooler, underfloor location.
However, once the exhaust system is up to temperature for efficient
three-way conversion, the underfloor catalyst shoulders much of the
burden of treating the exhaust gas. Meanwhile the close-coupled TWC
is exposed to very high temperatures e.g. up to 1000.degree. C.
Indeed one vehicle manufacturer requires testing of close-coupled
TWCs for 50 hours at catalyst bed temperatures of at least
970.degree. C. and up to 1010.degree. C. At these sort of
temperatures, catalysts can lose activity as materials lose their
surface area through sintering events, migration of active species
into pores and component interactions. Accordingly, a TWC in the
close-coupled position can be expected to lose some of its activity
compared with a fresh catalyst. The NOx storage component can also
lose NOx storage capacity through this high temperature ageing by
loss-of surface area.
[0035] Whilst loss of NOx storage capacity through high temperature
ageing, particularly in TWCs located in the close-coupled position,
is undesirable in the present invention, the benefit of the
invention is still obtained provided that a proportion of NOx
storage activity is retained. Accordingly, in an embodiment of the
invention, the fresh TWC includes sufficient of the NOx storage
component to retain sufficient NOx storage capacity after high
temperature ageing, for example in the close-coupled position.
[0036] According to a further aspect, the invention provides An
engine control unit for a spark ignition engine comprising an
exhaust system comprising a TWC including a NOx storage component,
which engine control unit is programmed to control the air-to-fuel
ratio of the engine to run at the stoichiometric air-to-fuel ratio
during normal running conditions and to run lean of stoichiometry
during a defined portion of an engine speed/load map and to
determine the amount of NOx contacting the TWC during lean running
operation in response to data input from sensor means, thereby to
determine the remaining NOx storage capacity of the TWC and to
return the air-to-fuel ratio to stoichiometry when the remaining
NOx storage capacity is below a pre-determined value, the
arrangement being such as to substantially prevent passing more NOx
to atmosphere during an engine cycle compared with a spark ignition
engine run continuously in stoichiometric mode.
[0037] According to a further aspect, the invention provides A
method of treating exhaust gas of a spark ignition engine run at
the stoichiometric air-to-fuel ratio during normal running
conditions, which engine comprising an exhaust system comprising a
TWC including a NOx storage component, which method comprising the
steps of controlling the engine air-to-fuel ratio to run lean of
stoichiometry during a defined portion of an engine speed/load map
and determining the amount of NOx contacting the TWC during lean
running operation in response to data input from sensor means,
thereby to determine the remaining NOx storage capacity of the TWC
and to return the air-to-fuel ratio to stoichiometry when the
remaining NOx storage capacity is below a pre-determined value, the
arrangement being such as to substantially prevent passing more NOx
to atmosphere during an engine cycle compared with a spark ignition
engine run continuously in stoichiometric mode.
[0038] In order that the invention may be more fully understood,
the following Examples are provided by way of illustration only and
with reference to the accompanying drawings, in which:
[0039] FIG. 1 is a graph showing HC conversion against temperature
after ageing of a TWC including a NOx storage component according
to the invention, a state-of-the-art TWC and a NOx trap; and
[0040] FIGS. 2, 3 and 4 are graphs showing % conversion of CO, HC
and NOx against lambda for a TWC including a NOx storage component
according to the invention, a state-of-the-art TWC and a NOx trap
formulation respectively.
EXAMPLE 1
Total Hydrocarbon Light Off
[0041] Three catalyst washcoats were tested. Comparative catalyst A
is a state-of-the art Pt/Rh TWC on a thermally stable, high surface
area support at a ratio of 5Pt:1Rh and a total precious metal
loading of at 60 g ft.sup.-3.
[0042] Catalyst B is a TWC including a NOx storage component
according to the present invention supported on an identical
substrate. It comprised a first, inner layer of a high surface area
Al.sub.2O.sub.3 impregnated with Pt and a NOx storage component,
such as BaO, and a second, outer layer of a mixed oxide OSC
impregnated with Rh. The ratio of Pt:Rh and the total precious
metal loading was the same as for catalyst A.
[0043] Comparative catalyst C is a NOx trap composition comprising
a high surface area Al.sub.2O.sub.3-based mixed oxide support
impregnated with Pt, Rh and a NOx storage component. The ratio of
Pt:Rh was 6:1 and the total precious metal loading was 70 g
ft.sup.-3.
[0044] Each washcoat was coated on a 4.66.times.6 inch
(11.9.times.15.2 cm) ceramic substrate of 400 cells per square inch
((cpsi) 62 cells cm.sup.-2) of 0.15 mm wall thickness and the
resulting coated substrate was hydrothermally aged at 800.degree.
C. for 5 hours under 10% O.sub.2/0% H.sub.2O balance nitrogen.
[0045] The catalysts were fitted to the exhaust of a four cylinder
2.0 litre Port Fuel Injection bench mounted engine controlled by a
Bosch ME7 control system. The catalyst temperature was increased by
adjustment of a heat exchanger fitted to the exhaust line before
the catalyst. The temperature ramp rate was 14.degree.
C./minute.
[0046] The results for the HC light-off (the temperature at which
the reaction is catalysed to 50% efficiency) are shown in FIG. 1,
from which it can be seen that the HC light-off temperature for
catalyst B is similar to that of the comparative catalyst A. It can
also be seen that the HC light-off temperature of comparative
catalyst C is approximately 30.degree. C. higher than comparative
catalyst A.
[0047] This result shows that the TWC including a NOx storage
component (catalyst B) has very similar activity for HC activity
compared with a state-of-the-art TWC (comparative catalyst A),
despite the presence of the NOx storage component. The NOx trap
(comparative catalyst C) performs less well than either catalyst B
or comparative catalyst A, despite having a higher Pt loading.
EXAMPLE 2
Perturbed Lambda Scan
[0048] The same engine as in Example 1 was used to give a catalyst
inlet temperature of 450.degree. C. The lambda scans were done
using a 10% perturbation (cycling between a value 10% below
lambda=1, i.e. less 0.147 lambda, to a value of 10% above lambda=1
(1.147 lambda)) and at a frequency of 1 Hz. These conditions were
chosen to simulate the exhaust gas composition at the inlet to a
TWC disposed in an exhaust system including a lambda sensor
upstream of the catalyst inlet providing feedback to the engine
control unit in order to maintain lambda=1 conditions.
[0049] The results are shown in FIGS. 2, 3 and 4. As can be seen,
the NOx trap (comparative catalyst C) is poorer for lambda scan
performance (showing poorer conversion), despite having more Pt
while the TWC including a NOx storage component (catalyst B)
performs similarly to the TWC (comparative catalyst A).
EXAMPLE 3
Engine Test
[0050] In a bench test cell, a 4 cylinder, 1.8 litre, 1997 model
year, Mitsubishi direct injection engine from a vehicle calibrated
for the Japanese market was installed with a direct current
dynamometer. A catalyst substrate prepared according to Example 1
was fitted in the close-coupled position approximately 30 cm from
the engine exhaust manifold. The substrate volume represented of
22% engine swept volume (ESV). Concentrations of NOx, HC, CO.sub.2,
CO and O.sub.2 were measured using a dual bank of MEXA (Motor
Exhaust Gas Analyser) 9500 sensors to allow continuous measurement
of gas concentrations upstream and downstream of the catalyst The
catalyst inlet temperature was measured by thermocouple.
[0051] The engine was operated from one of two sets of maps: one
for the homogeneous mode and the other for the lean, stratified
mode. Basic maps for ignition and injection timing and duration
were generated by firstly logging data from the ECU of a vehicle
including the same model of engine as the one used for the bench
test and then basing the maps on this information by reverse
engineering. In the homogeneous mode, the engine was run at a range
of engine speeds and loads and a supplementary map to the basic map
was generated for the best emissions of NOx, CO and HC emissions
under .lamda.=1 operation. The lean, stratified mode was mapped by
matching the torque achieved in homogeneous mode at the same speed
and load demand.
[0052] Prior to testing, the engine was thoroughly warmed up in
idling condition. In homogeneous mode, the engine was then run so
that the inlet temperature to the close-coupled catalyst was
300.degree. C. It was then switched to lean, stratified operation
and the EGR valve position was adjusted until the engine-out NOx
was 300 ppm. The EGR valve position was recorded and was referred
to as the lean set point The engine was switched back to
homogeneous mode, and the EGR valve was closed. A rich set point
was obtained by increasing the fuel injector pulse width to obtain
lambda 0.80. A series of lean/rich cycles were run as follows. In
the lean mode, the EGR valve was at the lean set-point position
until the NOx efficiency of the system had dropped below 75%. The
engine was then switched back to homogeneous mode for 15 seconds,
with injector duration at the rich set point. The cycles were
repeated five times and the results obtained for each of the cycles
were logged. Exhaust systems were fitted to the engine and the
protocol above was followed and data collected in the lean,
stratified mode at 300.degree. C. catalyst inlet temperature, a
typical catalyst inlet temperature for a close-coupled TWC in the
exhaust system of a port fuel injection during idling.
[0053] Table 1 shows the NOx storage efficiency with which each of
comparative catalysts A and C and catalyst B store NOx, and in
particular, how the efficiency with which each catalyst stores 30,
40, 50 and 60 mg of NOx. Thus the NOx trap (comparative catalyst C)
stores 60 mg NOx with 97% efficiency, i.e. 97% of the NOx
contacting the catalyst is stored, whereas the TWC (comparative
catalyst A) stores 60 mg of NOx with 9% efficiency, i.e. for the
time it takes to store 60 mg of NOx, the catalyst has slipped 91%
of the NOx contacting it. TABLE-US-00001 TABLE 1 NOx storage
efficiency Catalyst B Comparative (TWC including Comparative
Catalyst C a NOx storage Catalyst A NOx stored (mg) (NOx Trap)
component) (TWC) 30 98% 77% 18% 40 98% 73% 16% 50 97% 68% 12% 60
97% 64% 9%
[0054] The results of Examples 1, 2 and 3 show that the TWC
including a NOx storage component maintains TWC performance despite
including a NOx storage component, and that NOx storage capacity is
several times higher than a state-of-the-art TWC composition.
* * * * *