U.S. patent application number 09/944770 was filed with the patent office on 2002-03-14 for system for reducing nox transient emission.
This patent application is currently assigned to Engelhard Corporation. Invention is credited to Dettling, Joseph C., Roth, Stanley A., Yassine, Mahmoud.
Application Number | 20020029564 09/944770 |
Document ID | / |
Family ID | 24020968 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020029564 |
Kind Code |
A1 |
Roth, Stanley A. ; et
al. |
March 14, 2002 |
System for reducing NOx transient emission
Abstract
A catalyst reduction system using diesel fuel as a reductant for
vehicles equipped with diesel engines reduces NOx transient
emissions produced during acceleration. Impending engine
accelerations are sensed to produce metered pulse(s) of fuel oil
simultaneously with and preferably in advance of, the NOx transient
emission. The fuel pulse is sufficient in quantity to reduce the
NOx transient emission when the NOx and HC resulting from the
cracked fuel oil are present at spatially equal distances within
the reducing catalytic converter. Optionally, the washcoat of the
reducing catalyst is formulated to delay the adsorption/desorbtion
of one of the gases on the washcoat to assure proper timing of the
NOx transient and fuel pulse(s).
Inventors: |
Roth, Stanley A.; (Yardley,
PA) ; Dettling, Joseph C.; (Howell, NJ) ;
Yassine, Mahmoud; (Edison, NJ) |
Correspondence
Address: |
Stephen I. Miller
Engelhard Corporation
101 Wood Avenue
P. O. Box 770
Iselin
NJ
08830-0770
US
|
Assignee: |
Engelhard Corporation
|
Family ID: |
24020968 |
Appl. No.: |
09/944770 |
Filed: |
September 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09944770 |
Sep 4, 2001 |
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09507999 |
Feb 22, 2000 |
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6311484 |
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Current U.S.
Class: |
60/286 ; 60/274;
60/285; 60/301 |
Current CPC
Class: |
Y02T 10/12 20130101;
B01D 53/9431 20130101; F01N 3/0835 20130101; F01N 3/0871 20130101;
F01N 3/2821 20130101; F01N 3/20 20130101; B01D 53/9495 20130101;
F01N 13/009 20140601; Y02A 50/20 20180101; F01N 2610/03 20130101;
F01N 2570/18 20130101; F01N 3/2066 20130101; F01N 3/0814
20130101 |
Class at
Publication: |
60/286 ; 60/274;
60/285; 60/301 |
International
Class: |
F01N 003/10; F01N
003/00 |
Claims
Having thus defined the invention, it is claimed:
1) A system for reducing transient and steady-state NOx emissions
in the exhaust gases of a vehicle powered by a diesel fueled
internal combustion engine comprising: a) a reducing catalytic
converter downstream of said engine having a plurality of channels
with a washcoat surface on the walls thereof and a reducing
catalyst deposited over a portion of said washcoat surface, said
channels having a set cross-sectional area and extending through
said catalytic converter from an inlet to an exit thereof; b) a
source of substantially long chain, unbranched hydrocarbons having
a majority of hydrocarbon molecules containing more than 10 carbon
atoms per molecule in its liquid phase; c) a fuel metering valve
for pulsing variably set quantities of said hydrocarbons from said
source pursuant to a valved pulse command signal; d) a plurality of
vehicle sensors generating sensor signals indicative of an
operating condition of said engine; said sensors including at least
a temperature sensor generating a signal indicative of the
temperature of said exhaust gases and an acceleration sensor
generating a signal indicative of the change in speed and/or load
of said engine set by the vehicular operator; and e) an engine
control unit having a plurality of programmed routines for
controlling said engine in response to said plurality of sensor
signals; at least a first routine setting a constant pulsed
quantity of said hydrocarbons when said engine is operating at
steady state conditions sufficient to reduce a portion of NOx
emissions produced at the steady state condition and a second
routine activated when said acceleration sensor generates a signal
indicative of impending acceleration, said second routine i)
calculating a transient quantity of hydrocarbons necessary to
reduce the NOx emissions generated during the time the engine is
accelerating and ii) generating a pulse command signal sufficient
to meter said transient quantity of said hydrocarbons at a set time
prior to the engine producing said transient NOx emissions when
said washcoat has an acidic pH and after the engine has produced
said transient NOx emissions when said washcoat has an alkaline
pH.
2) The system of claim 1 wherein said engine has at least one fuel
injector and said metering valve is said fuel injector.
3) The system of claim 2 wherein each piston in said engine has a
fuel injector for each cylinder and said metering valve injects
additional fuel beyond that needed for engine operation into each
cylinder.
4) The system of claim 1 wherein said fuel metering valve is
positioned in front of said inlet of said reducing catalytic
converter.
5) The system of claim 1 wherein said source of hydrocarbons is the
diesel fuel tank of said vehicle.
6) The system of claim 1 wherein each channel extends in a
longitudinal direction from said inlet to said outlet and each
channel is skewed in said longitudinal direction whereby turbulent
flow of gases through each channel occurs to minimize hydrocarbon
slip.
7) The system of claim 6 wherein said channels are formed in a
metal or cordierite monolith, and each channel has longitudinally
extending zones with certain zones having an acidic washcoat and
certain zones having an alkaline or base washcoat.
8) The system of claim 7 further including an oxidation catalyst or
said reducing catalyst having an oxidation catalyst portion
downstream of said reducing catalyst, said oxidation catalyst or
oxidation catalyst portion having longitudinally skewed
channels.
9) The system of claim 1 wherein said catalyst is a washcoat
including ZSM-5 zeolite and a metal.
10) The system of claim 9 wherein said second routine of element
(e) causes said metering command at about 0 to 2 seconds in advance
of the time said engine produces a transient pulse of NOx
emissions.
11) A method for reducing NOx emissions produced in the exhaust gas
of an internal combustion engine including gasoline engines in a
vehicle operating at stoichiometric ratios in excess of 1.03
comprising the steps of: a) providing a liquid source of long chain
hydrocarbons at least 50% of which have at least 10 carbon atoms
per hydrocarbon molecule; b) providing a reducing catalytic
converter downstream of said engine through which said exhaust
gases pass; c) sensing a set of first engine operating parameters
including the space velocity of the exhaust gases and the
temperature of the exhaust gases; d) calculating the concentration
of hydrocarbon and NOx emissions in the exhaust gases produced at
any given time by said engine from the sensed set of first
operating parameters; e) metering a steady state rate of said
hydrocarbons into the exhaust gases upstream of said reducing
catalytic converter sufficient to reduce a desired percentage of
NOx emissions when the exhaust gases leave said reducing catalytic
converter provided that the sensed set of first engine operating
parameters remain generally constant and the temperature of the
exhaust gases is within a set range; f) sensing an operator
inputted command causing an acceleration of said engine or said
vehicle such as by a pedal depression sensor or fuel demand sensor;
g) calculating the concentration of transient NOx emissions
expected to occur while the engine is accelerating to meet said
operator acceleration command utilizing the current NOx emissions
produced as determined by said first set of parameters and a sensed
rate of change as determined by said operator inputted command; and
h) pulsing an additional quantity of said hydrocarbons at a rate
determined by said space velocity of said exhaust gases sufficient
to reduce said transient NOx emissions provided the exhaust gas
temperature is within a set range; said pulsing of said additional
quantity of hydrocarbons occurring at a set time in advance of the
time said transient NOx emissions are produced by said engine.
12) The method of claim 11 wherein said reducing catalytic
converter has a plurality of channels with an acidic washcoat on
the channel walls, said advanced set time determined as a function
of the total washcoat surface area of said reducing catalytic
converter.
13) The method of claim 11 wherein said additional quantity of
hydrocarbons are pulsed within a time range commencing with a time
period which is co-incident with the generation of transient
emission by said engine.
14) The method of claim 13 wherein said time range ends at a time
period which is not earlier than about two seconds in advance of
the time said NOx transient emissions are produced by said
engine.
15) The method of claim 11 wherein said step of sensing an operator
inputted signal determines whether said operation is causing said
vehicle to decelerate and said method includes the step of stopping
any pulsed quantity of hydrocarbons during the time said engine is
decelerating.
16) The method of claim 12 wherein said channels in said reducing
catalytic converter are formed to create a tortuous path, said
hydrocarbons and said exhaust gases traveling said tortuous path as
they pass through said reducing catalytic converter whereby
hydrocarbon slip is minimized.
17) The method of claim 11 wherein said sensed operating conditions
include the fuel-to-air ratio of the engine.
18) A method for reducing NOx emissions produced by a vehicle
powered by a diesel engine having a de-nox catalyst through which
the exhaust gases pass and sensors for determining NOx
concentrations at steady state engine operating conditions, the
improvement comprising the steps of: a) sensing a fuel demand pedal
sensor signal to determine an impending acceleration of the
vehicle; and, b) injecting a set quantity of diesel fuel reductant
into the exhaust gases of the engine sufficient to substantially
react with transient emissions produced by the engine during
acceleration, the injection occurring at a time in advance of and
not later than the time the engine produces the transient
emissions.
19) The method of claim 18 wherein the vehicle has a hydrocarbon
trap catalyst or the de-nox catalyst has a hydrocarbon trap
catalyst portion upstream of the de-nox catalyst, the method
comprising the steps of detecting a rise in the temperature of the
hydrocarbon and/or de-nox catalyst during warm-up of the vehicle
and injecting a quantity of diesel fuel sufficient to react with
NOx transient emissions caused by the temperature rise at or prior
to the time the temperature rise produces a NOx transient
emission.
20) The method of claim 19 further including the step of stopping
injection of the diesel fuel reductant when the engine is
decelerating.
21) The method of claim 20 further including the step of injecting
the diesel fuel reductant at a set rate when the temperature of the
de-nox catalyst is within a first temperature range and the engine
is operating at steadystate conditions and injecting the diesel
fuel at a variable rate when the temperature of the de-nox catalyst
is within a second broader temperate range and the engine is
accelerating.
22) The method of claim 21 further including the step of causing
the exhaust gases including the diesel fuel to traverse tortuous
paths defined by specially configured zig-zag channels formed in
the de-nox catalyst.
23) The method of claim 22 further including the step of causing
the exhaust gases with the diesel fuel in the form of hydrocarbons
to travel through different acidic and alkaline zones formed in the
catalyst channels.
24) The method of claim 23 further including the step of passing
the exhaust gases through an oxidation catalyst or oxidation
catalyst portion of the de-nox catalyst downstream of the de-nox
catalyst, the exhaust gases traversing tortuous paths defined by
specially configured zig-zag channels formed in the oxidation
catalyst or oxidation catalyst portion.
25) The method of claim 19 further including the step of
substantially cracking the fuel oil before the fuel oil enters the
inlet of the de-nox catalyst.
Description
[0001] This invention relates generally to NOx emissions and more
particularly to systems for reducing NOx emissions in mobile or
vehicular applications.
[0002] The invention is particularly applicable to and will be
described with specific reference to a system for reducing NOx
transient emissions in vehicles powered diesel engines. However,
those skilled in the art, will recognize that the invention has
broader application and could be used in mobile applications
powered by gasoline engines operated at "lean burn" conditions.
INCORPORATION BY REFERENCE
[0003] The following documents are incorporated by reference and
made a part hereof:
[0004] U.S. Pat. No. 5,804,155 to Farrauto et al., issued Sep. 8,
1998, entitled "Basic Zeolites as Hydrocarbon Traps for Diesel
Oxidation Catalysts";
[0005] U.S. Pat. No. 5,522,218 to Lane et al., issued Jun. 4, 1996,
entitled "Combustion Exhaust Purification System and Method";
[0006] SAE Paper No. 950747, dated Feb. 27-Mar. 2, 1995, entitled
"Abatement of NOx from Diesel Engines: Status and Technical
Challenges", by Jennifer S. Feeley, Michel Deeba, and Robert J.
Farrauto;
[0007] SAE Paper No. 952491, dated Oct. 16-19, 1995, entitled
"Catalytic Abatement of NOx from Diesel Engines: Development of
Four Way Catalyst", by Michel Deeba, Jennifer Feeley, and Robert J.
Farrauto.
[0008] None of these documents incorporated by reference herein
form any part of the present invention. They are incorporated by
reference so that details of the technology to which this invention
relates need not be repeated nor described in detail in the
Detailed Description of the Invention set forth below.
BACKGROUND OF THE INVENTION
[0009] This invention is directed to the removal of nitrogen oxides
(NOx) from the exhaust gases of internal combustion engines,
particularly diesel engines, which operate at combustion conditions
with air in large excess of that required for stoichiometric
combustion, i.e., lean. It is well known that fuel efficiency
improvements in excess of 10% can be achieved in gasoline engines
operated at "lean burn" conditions when compared to today's engines
which cycle the air to fuel ratio about stoichiometric. Diesel
engines, by their nature, operate at lean conditions and achieve
20-30% better fuel economy than stoichiometric gasoline
engines.
[0010] Unfortunately, the presence of excess air makes the
catalytic reduction of nitrogen oxides difficult. Emission
regulations impose a limit on the quantity of specific emissions,
including NOx, that a vehicle can emit during a specified drive
cycle such as an FTP ("federal test procedure") in the United
States or an MVEG ("mobile vehicle emission group") in Europe. The
regulations are increasingly limiting the amount of nitrogen oxides
that can be emitted during the regulated drive cycle.
[0011] There are numerous ways known in the art to remove NOx from
a waste gas. This invention is directed to a catalytic reduction
method for removing NOx. A catalytic reduction method essentially
comprises passing the exhaust gas over a catalyst bed in the
presence of a reducing gas to convert the NOx into nitrogen. Two
types of catalytic reduction are practiced. The first type is
non-selective catalyst reduction (NSCR) and the second type is
selective catalyst reduction (SCR). This invention relates to
hydrocarbon (HC) lean-NOx reaction which can be either NSCR or
SCR.
[0012] In the selective catalyst reduction method, a reducing agent
or reductant is supplied to the exhaust stream and the mixture is
then contacted with a catalyst. A common nitrogen oxide reducing
agent typically used in industrial processes is urea or ammonia,
which despite the number of prior art automotive patents, is not
favored for vehicular applications because of the infrastructure
required for reductant sale to the public. Additionally, any SCR
method using a separate reducing agent requiring separate on-board
holding tanks and environmental provisions (such as provisions to
keep the tank from freezing) present difficult engineering problems
to overcome.
[0013] Perhaps one of the more sophisticated approaches to using
urea/ammonia system in a mobile application is disclosed in a
series of patents which include U.S. Pat. No. 5,833,932 issued Nov.
10, 1998; U.S. Pat. No. 5,785,937, issued Jul. 28, 1998; U.S. Pat.
No. 5,643,536, issued Jul. 1, 1997; and U.S. Pat. No. 5,628,186,
issued May 13, 1997. While these patents discuss reducing reagents
in a general sense, they are clearly limited to urea/ammonia
reductants. According to this system, a catalytic converter having
composition defined in the '932 patent, has a reducing agent
storage capacity per unit length that increases in the direction of
gas flow. This allows for positioning of instrumentation along the
length of the catalyst as disclosed in the '536 patent to determine
the quantity of ammonia stored in the catalyst. The catalyst is
thus charged with the reducing agent such that transient emissions
can be converted by the reducing agent stored in the catalytic
converter. As explained in the '186 patent, should the vehicle
experience a sudden increase in load or acceleration, and without
having to wait for an increase in the temperature of the catalytic
converter, the stored reducing agent is utilized to reduce NOx,
thereby preventing overloading of the catalytic converter, i.e.,
ammonia breakthrough. The ammonia is metered on/off by the length
sensors to "charge" the catalyst with stored ammonia. On
acceleration, the metering is stopped to prevent ammonia slip (col.
7, '536 patent).
[0014] Urea/ammonia SCR systems are characterized in that the
catalysts have the ability to store ammonia at temperatures of the
exhaust gases, at least at the relatively low operating temperature
ranges of exhaust gases produced by diesel engines. Ammonia SCR
systems can therefore be developed by catalyst sizing and ammonia
slip control techniques (such as described above) to assure a
sufficient quantity of ammonia is present to reduce the NOx
emissions generated by the engine and particularly the increased
NOx emissions produced, transiently, during engine acceleration or
engine load increase periods.
[0015] Because of the infrastructure limitations of a urea/ammonia
SCR system in mobile applications, there is prior art for the use
of hydrocarbons (HC) as a selective reducing reagent for NOx
emissions. While reducing catalytic converters (principally base
metal zeolites, copper or cobalt ZSM-5 for high temperatures) are
able to reduce NOx emissions in the presence of HC at relatively
high temperatures (300 to 450.degree. C.), they are not able to
store and release the HC at the higher temperatures. It is well
known that HC can be adsorbed in zeolite based catalysts at
temperatures below 200.degree. C. which are then desorbed at
temperatures of about 200.degree. C. or higher (see any number of
HC trap patents, for example, U.S. Pat. No. 5,804,155 incorporated
by reference herein and SAE paper No. 950747 incorporated by
reference herein). There is also prior art for use of HC as a
non-selective reducing agent or reductant for NOx emissions. The
reducing catalytic converters in this case are precious metal based
catalysts and more usually platinum zeolites typically based on Pt
ZSM-5 which are active for NOx reduction at lower temperatures (180
to 250.degree. C.).
[0016] Because normal operating temperatures of diesel engines
produce exhaust gas temperatures above 200.degree. C., it is not
usually possible to store and release HC as in ammonia systems.
This is a fundamental difference between ammonia based SCR systems
and HC based reaction systems.
[0017] Despite this fundamental distinction, there is a segment of
the prior art that teaches HC can be adsorbed and desorbed (stored
and released) at temperatures which include a portion of the normal
operating range of the diesel engine. This conclusion appears to be
based on the observation that zeolite containing catalysts show
better NOx reduction conversion percentages than non-zeolite
containing catalysts.
[0018] In Mercedes-Benz U.S. Pat. No. 5,935,530, issued Aug. 10,
1999, a three stage catalytic converter is disclosed having an
intermediate section which is said to store HC when the engine runs
at reduced load and release the stored HC when the engine is at
load so that the secondary injection of HC would not have to change
in synchronization with the changing engine load (col. 6). The
known adsorber catalyst is defined to include a precious metal
catalyst. The data disclosed in the '530 patent is based on an
artificial gas composition heated at various temperatures and to
which a fixed propane/propene ratio is added. The data shows, (as
noted in the SAE references), that for a given temperature range,
propene propane will achieve a high NOx reduction. However, there
is no evidence that transient NOx emissions can be controlled by
this catalyst design.
[0019] Johnson Matthey U.S. Pat. No. 5,943,857 issued Aug. 31,
1999, is also directed to a storage of HC, but storage occurring
below a temperature range of 190.degree. C. and a desorbtion of the
stored HC at temperature ranges stated to be at 198.degree. C. to
200.degree. C. The '857 patent shows NOx reduction levels achieved
between catalyst with and without zeolites and shows that zeolite
containing catalysts have a higher NOx conversion efficiency than
non-zeolite containing catalysts. The '857 patent shows "transient"
test data but what is plotted is not the transient NOx emissions
during a FTP or MVEG cycle. In a FTP or MVEG cycle, transient
emissions occurring during acceleration significantly increase NOx
ppm. In the '857 patent, a constant gas mixture is reduced at
varying exhaust gas temperatures modeling temperature variations in
a drive cycle and the NOx conversion results are plotted. The data
shows an overall increase in NOx reduction using a zeolite
catalyst. Significantly, even with a constant gas composition, the
data shows that temperature changes produce NOx spikes. The '857
patent attributes the spikes to the catalyst heating up (col. 4).
While the statement is correct, for reasons discussed in the
Detailed Description below, the spikes result from differences in
the NOx conversion, percentages attributed to the changing
temperature. A careful reading of the '857 and '530 patents simply
show that reduction levels of NOx can be increased with catalysts
containing zeolites which is a known adsorber. Neither patent shows
the catalyst is able to store HC similar to the ammonia systems to
reduce NOx transient emissions during a regulated drive cycle.
[0020] Further, while many arrangements use diesel fuel as the HC
source, there are segments within the prior art in which the HC is
said to comprise short chain hydrocarbon. For example, Daimler-Benz
U.S. Pat. No. 5,921,076, issued Jul. 13, 1999 shows a staged
arrangement for injecting i) H.sub.2, ii) H.sub.2 and short chain
HC and iii) short chain HC into the exhaust stream as a reductant.
Air Products U.S. Pat. No. 5,524,432, issued Jun. 11, 1996 shows
methane injection. As late as 1995, assignee's SAE paper No. 950747
recommended low molecular weight HCs with high volatility as the
reductant. For reasons which will be discussed below, this
invention is limited to long straight chain saturated HCs and
unsaturated olefins (of any chain length) which are present in fuel
oil or diesel fuel.
[0021] It should also be noted that within the diesel fuel SCR
prior art a number of arrangements exist for injecting the diesel
fuel into the exhaust gas. These include injecting excess fuel into
the combustion chamber during the expansion stroke, either through
individual injectors or utilization of a common injector rail, and
any number of injector designs, including those utilizing pulsation
techniques, which dispense fuel oil or diesel fuel into the exhaust
gas upstream of the catalytic converter.
[0022] There are a number of control schemes or techniques used to
control the diesel fuel admitted to the catalyst. For example,
Volkswagen's European patent No. EP0881367, dated Dec. 02, 1998,
measures residual concentrations of hydrocarbons after the
catalytic converter to adjust the reducing fuel oil by a regulating
algorithm once certain temperatures have been attained. Similarly,
Daimler-Benz U.S. Pat. No. 5,845,487, issued Dec. 8, 1998, uses a
nitrogen-oxide sensor after an operating temperature has been
achieved to control the system. Unfortunately, applicants have not
been able to obtain a commercially acceptable NOx sensor having the
response sensitivity needed to control NOx emissions at regulated
levels.
[0023] Caterpillar U.S. Pat. No. 5,522,218, issued Jun. 4, 1996
illustrates a control methodology typically followed by most HC
reducing systems in that certain operating conditions of the engine
are mapped and correlated with temperature to perform a
mathematical routine, usually by a CPU or the engine's ECU, to
determine a quantity of reducing agent which is pulsed metered into
the system. Systems which measure engine operating conditions to
produce a variable metering of the reductant to the catalyst are
generally based upon steady-state engine maps. These systems
typically measure or calculate engine speed and/or load, space
velocity of the exhaust gas and the exhaust gas and/or catalyst
temperature to determine a quantity of NOx produced by the engine
and a quantity of HC reductant to be metered into the exhaust gas.
Once the operating parameters are known, the quantity of NOx
emissions produced and consequently the quantity of HC reductant
(amount in addition to HC concentration normally present in exhaust
gas) to be metered are known vis-a-vis conventional mapping
techniques. (See SAE paper No. 950747 and SAE paper No. 952491.)
However, it is well known that transient changes in the operating
conditions of the engine, specifically EGR (exhaust gas
recirculation) and variable geometry turbocharging (VGT), upon
acceleration produce transient NOx emissions which are significant.
Those emissions will cause current vehicles to fail future NOx
emission standards notwithstanding the fact that such vehicles
could meet standards at steady state conditions. In this regard, it
must also be noted that response time improvements in
microprocessor based control systems have led to improvements in
EGR control systems reducing NOx transients. However, the NOx
transient is instantaneously formed so a response latency exists in
any feedback system. Further, while it is recognized that EGR
limits NOx formation by lowering in cylinder oxygen levels, the HC
present in EGR systems for diesel engines is limited.
[0024] Apart from the prior art segment which erroneously concludes
that reducing catalysts can effectively store and release HC
reductants at normal engine operating temperatures (discussed at
some length above), attempts to account for NOx transient
variations in HC reducing systems have been based on temperature
sensing systems. Because transient emissions are accompanied by a
significant increase in exhaust gas temperatures (see discussion in
'857 patent), an early detection of the temperature rise coupled
with a reduction of HC added to control NOx may keep the
temperature within the NOx temperature reduction window at which
SCR zeolite based systems are known to function. While that
strategy reduces NOx transient spikes, it must be recognized that
the engine instantaneously produces the NOx transient which has
flowed through the system before the catalyst temperature has
measurably changed. Reference can be had to Toyota's U.S. Pat. No.
5,842,341, issued Dec. 1, 1998, for a discussion of such an
approach. The '341 patent discloses a conventional steady state
system which measures space velocity and outlet exhaust gas
temperature to determine a quantity of fuel oil to be metered to
the catalyst. The '341 patent recognizes that transient engine
conditions will increase the temperature of the exhaust gas which,
in turn, will raise the temperature of the catalytic converter to
the point where the temperature "window" at which NOx conversion
occurs may be exceeded. To prevent this, exhaust gas temperature is
measured upstream and downstream of the catalytic converter and the
reductant flow is decreased from the steady-state programmed flow
when the upstream gas temperature differential exceeds a set value.
By reducing the HC reductant, the exothermic reactions attributed
to oxidation of the reactant is reduced and the mass of the
catalytic converter will not be heated, at a later time, to as high
a temperature as it would be at if the HC reductant were present.
The belief is that the NOx temperature window of the reducing
catalytic converter will not be exceeded during the engine
acceleration and the catalyst will still be able to function.
However, there may be insufficient reductant to dispose of the NOx
emissions when the HC is reduced. Apart from this, in practice,
this strategy will not work under real engine conditions because
the temperature increase in the catalyst due to the HC+NOx reaction
(or other HC oxidation reactions) lags the flow transient.
Therefore, the gas transient has passed through the catalyst before
the temperature sensor can call for reduction of the HC flow.
SUMMARY OF THE INVENTION
[0025] Accordingly, it is a principal object of the present
invention to provide a system which detects the impending
occurrence of a transient NOx emission and timely meters a fuel oil
reductant sufficient in quantity to substantially react with the
transient NOx emissions within the catalytic converter.
[0026] This object along with other features of the invention is
achieved in a system (method and apparatus) for reducing transient
and steady state NOx emissions in the exhaust gases of a vehicle
powered by a diesel fueled, internal combustion engine which
includes a reducing catalytic converter downstream of the engine
having a plurality of channels with a reducing catalyst deposited
over a portion of the washcoat surface, the channels having a set
cross-sectional area and extending through the catalytic converter
from an inlet to an exit thereof. A source of substantially long
chain, unbranched hydrocarbons, having a majority of hydrocarbon
molecules containing more than ten carbon atoms per molecule in its
liquid phase, is in fluid communication with a fuel metering valve
for pulsing variably set quantities of the hydrocarbons from the HC
source pursuant to a valved pulse command signal. An engine control
unit receives a plurality of vehicle sensor signals each of which
is indicative of an operating condition of the engine and the
sensors include at least a temperature sensor generating a signal
indicative of the temperature of the exhaust gases and an
acceleration sensor signal indicative of an impending change in
speed and/or load of the engine as set by the vehicular operator.
The ECU in response to the sensor signals, performs at least a
first routine setting a constant pulsed quantity of the
hydrocarbons when the engine is operating at steady state
conditions sufficient to reduce a portion of the NOx emissions
produced at that steady state condition and a second routine
activated when the acceleration sensor generates a signal
indicative of impending acceleration. The second routine i)
calculates a transient quantity of hydrocarbons necessary to reduce
the NOx emissions generated during the time the engine is
accelerating and ii) generates a valve command signal sufficient to
pulse meter the transient quantity of hydrocarbons at a set time
prior to the engine producing a transient NOx emission when the
washcoat has an acidic pH and after the engine has produced the
transient NOx emissions when the washcoat has an alkaline pH
whereby a time control strategy correlated to the sizing and
composition of the reducing catalytic converter as well as the
engine conditions which define temperature and space velocity
account for transient emissions produced during acceleration phases
of a test drive cycle.
[0027] In accordance with another aspect of the invention, the
reducing catalytic converter has a metal or cordierite monolith
substrate with a catalyst washcoat including an acid zeolite such
as ZSM-5 zeolite and a metal whereby the reducing catalytic
converter is acidic and the advanced time period at which the
transient hydrocarbons are introduced into the reducing catalytic
converter extends from a time commencing about two seconds in
advance of the generation of NOx transient emissions to a time
commencing simultaneously or co-incident with the generation of NOx
transient emissions.
[0028] In accordance with another aspect of the invention, the
acceleration sensor may be either a pedal depression sensor or fuel
demand sensor, either of which has an inherent lag or response time
associated therewith and the system senses not only acceleration
but deceleration with the second routine terminating hydrocarbon
metering during the time the vehicle is decelerating in response to
an operator set speed command.
[0029] In accordance with another aspect of the invention, each
passage in the monolith is formed with a plurality of
longitudinally skewed adjacent sections which define a tortuous
path through which the exhaust gases and hydrocarbons pass as the
reducing catalytic converter is traversed from inlet to outlet
whereby the HC and NOx reaction is improved for NOx reduction and
hydrocarbon slip is minimized.
[0030] In accordance with another aspect of the invention, the
washcoat composition of the reducing catalytic converter is
formulated to produce zones within the catalyst having slightly
different alkaline pH compositions to retard or slow the NOx and
zones within the catalyst having slightly different acidic pH
compositions to retard or slow the HC whereby variations in the
traveling time of HC and NOx through the catalyst are caused to
occur to better insure simultaneous presence of HC and NOx in the
catalyst channels where the reduction reaction can occur.
[0031] In accordance with yet another aspect of the invention,
different acidic/alkaline zones are formed along the length of each
catalyst channel and each catalyst channel is longitudinally skewed
to form a tortuous path so that the travel time of the HC and NOx
components within each channel is varied to increase the likelihood
of simultaneous contact of HC and NOx gases within each channel
causing NOx conversion. Optimally, the alkaline zones contain
alkali, alkaline earth or rare earth metal oxides or
carbonates.
[0032] In accordance with another aspect of the invention, a method
for reducing NOx emissions produced in the exhaust gas of an
internal combustion engine in a vehicle operating at stoichiometric
ratios in excess of 1.03 including lean burn gasoline engines is
provided. The method includes the steps of providing a liquid
source of long chain hydrocarbons, at least 50% of which have at
least ten carbon atoms per hydrocarbon molecule and a reducing
catalytic converter downstream of the engine through which the
exhaust gases pass. A set of engine operating parameters is sensed
sufficient to determine the space velocity of the exhaust gases and
the temperature of the exhaust gases. The method senses an operator
inputted command causing an acceleration of the engine, such as by
a pedal depression sensor or a fuel demand sensor, and calculates
the concentration of transient NOx emissions which is expected to
occur when the engine is accelerated to meet the operator
acceleration command. Provided the exhaust gas temperature is
within a set range, the transient calculation utilizes the current
NOx emissions produced as determined by the engine operating
parameters and a sensed rate of change of those parameters as
determined by the operator inputted command. An additional quantity
of hydrocarbons sufficient to reduce the transient NOx emissions
produced during the acceleration is pulse metered into the exhaust
gases. Importantly, the hydrocarbons are pulsed at a set time in
advance of but not later than simultaneously with the time the
transient NOx emissions are produced by the engine whereby it is
likely that the hydrocarbons and the transient NOx emissions occur
at the same time and place in the channels of reducing catalytic
converter.
[0033] It is a general object of the invention to provide a system
for reducing NOx emission produced by an internal combustion
engine.
[0034] Another general object of the invention is to provide a time
advanced, HC active lean-NOx reaction system for reducing NOx
transient emissions in vehicles powered by diesel engines.
[0035] It is a more specific but general object of the invention to
provide an HC active lean-NOx system for reducing transient NOx
emissions produced in mobile, internal combustion engine
applications operated at excess air in which the reductant is
diesel fuel or fuel oil.
[0036] Still another object of the invention is to provide an NOx
reducing system for diesel engines operated in a mobile environment
by means of a hydrocarbon reductant characterized in that the
likelihood of hydrocarbon slip occurring during hydrocarbon
metering is minimized.
[0037] Still yet another object of the invention is to provide a
hydrocarbon SCR NOx reduction system for mobile diesel engine
applications in which active hydrocarbons are timely metered to
reducingly react with NOx transient emissions.
[0038] Another object of the invention is to provide a liquid HC
active lean-NOx catalyst which significantly extends the reducing
operating temperature window by the timely admission of a pulse of
reductant sufficient to react with the increased transient NOx
emissions whereby excess HC exothermic oxidation reactions that
reduce NOx at steady state are not an important criteria.
[0039] An important object of the invention is to provide a diesel
fuel NOx reduction system which uses a desired washcoat formulation
in the reducing catalyst to delay the time one of the gas emissions
pass through the catalyst by causing an adsorption/desorption
reaction with the washcoat so that fuel pulsing can be timely
generated to match NOx transient emissions.
[0040] Still yet another distinction of the invention is the
utilization of a time advanced active lean-NOx HC reaction system
to reduce NOx transient emissions resulting from catalyst
"light-off" attributed to HC desorbtion of an HC trap catalyst
during engine warm-up.
[0041] These and other objects, features and advantages of the
present invention will become apparent to those skilled in the art
upon reading and understanding the Detailed Description of the
Invention set forth below taken in conjunction with the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] The invention may take form in certain parts and an
arrangement of certain parts taken together and in conjunction with
the attached drawings which form a part of the invention and
wherein:
[0043] FIG. 1 is a graph of NOx emissions produced in the exhaust
gases of a diesel powered vehicle during the European MVEG
cycle;
[0044] FIG. 2 is a graph of two traces showing NOx conversion
percentages as a function of i) catalyst inlet transient
temperature for an active HC lean-NOx system and ii) catalyst inlet
steady state temperature for a passive NOx System;
[0045] FIG. 3 is a graph plotting NOx conversion percentages as a
function of the hydrocarbon to NOx ratio;
[0046] FIG. 4 is a graph of NOx conversion percentages plotted as a
function of catalyst inlet temperature for several traces which
vary the time at which hydrocarbons are introduced into the inlet
of the reducing catalytic converter relative to the time that NOx
emissions are introduced into the inlet of the catalytic
converter;
[0047] FIG. 5 is a graph showing NOx ppm reductions when pulses of
HC are periodically injected into an exhaust gas having a steady
state NOx concentration state level;
[0048] FIGS. 6a, 6b and 6c are graphs similar to FIG. 5 but
periodically pulsing concentrations of HC at various times relative
to the pulses of NOx, i.e., transients;
[0049] FIG. 7 is a bar graph showing average NOx conversion
percentages for various times at which pulses of HC concentrations
are introduced into the catalytic converter relative to the time at
which pulses of NOx concentrations are introduced into the
catalytic converter;
[0050] FIG. 8 is a schematic representation of an HC and NOx pulse
traveling in a monolith channel of the catalytic converter;
[0051] FIG. 9 is a graph showing NOx conversion percentages for
various long chain HCs at various temperatures;
[0052] FIG. 10 is a schematic representation of some of the control
components used in the system of the present invention;
[0053] FIG. 11 is a flow chart of the process employed in the
system of the present invention; and,
[0054] FIG. 12 is a pictorial, prior art representation of a
preferred type monolith which can be used in the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0055] Referring now to the drawings wherein the showings are for
the purpose of illustrating a preferred embodiment of the invention
only and not for the purpose of limiting same, there is shown in
FIG. 1 an MVEG test conducted with a test vehicle equipped with a
1.9 liter turbo-charged, direct injection (TDI) diesel engine. The
invention will be described throughout as applicable to a diesel
engine, but as indicated above, the invention, in its broader
sense, is applicable to gasoline fuel type engines operated lean or
with "lean burn" engine fuel strategy.
[0056] In FIG. 1, the European MVEG cycle is plotted in seconds on
the x-axis with the nitrogen oxides (NOx) emitted by the engine
during the drive cycle plotted on the left-hand y-axis and the
vehicle speed plotted in km/hr on the right-hand y-axis. The lower
trace identified by the reference numeral 10 is a plot of the
vehicle speed over the timed portion of the drive cycle. The
uppermost plot identified by reference numeral 12 are the NOx
emissions produced in the exhaust gases of the engine with the
vehicle traveling at the speeds as shown in speed plot 10. The
graph shown in FIG. 1 is typical of NOx emissions produced by
conventional diesel engines during a regulated drive cycle and is
characterized rather dramatically by "spikes" of NOx transient
emissions. In marked contrast, laboratory tests where it is
possible to achieve 80% NOx conversion efficiencies or higher are
typically conducted with propylene at steady state conditions. The
transient spikes account for efficiency drops to less than 30% when
the laboratory reduction systems are applied to commercial
vehicles. Several factual observations concerning the graph of FIG.
1 should be noted as follows:
[0057] 1) When the vehicle is traveling at a constant speed, the
NOx emissions are somewhat constant. This can be shown, for
example, by looking at that portion of the vehicle speed plot
designated by reference numeral 10A and comparing it to the
generally flat portion of NOx emissions generated during that time
period in the NOx plot section designated by reference numeral
12A.
[0058] 2) When the engine accelerates, such as indicated by the
acceleration designated as reference numeral 10B, the NOx emissions
correspondingly dramatically increase or spike as shown by spike
12B and the spike or pulse or increase in NOx emissions is commonly
referred to as an NOx transient. Further, the faster the
acceleration or the rate of change, the greater the NOx transient
emission.
[0059] 3) When the engine decelerates, such as at the deceleration
designated by reference numeral 10C, the NOx emissions drop and
drop below the NOx concentration which occurs at steady state such
as indicated by the corresponding NOx emission drop shown by
reference numeral 12C.
[0060] In general summary, FIG. 1 shows that NOx transient
emissions comprise a significant portion of the NOx emissions
emitted by a diesel engine during a regulated drive cycle. Also, as
a matter of definition, the term "acceleration" when used herein
and in the claims is not limited to merely rate changes in the
engine rpm but also includes increases in engine load whether or
not accompanied by changes in engine speed. As is well known, a
load increase on the engine at constant speed, such as when a truck
travels up a hill at constant speed, will cause an NOx transient.
NOx transients caused by load changes, per se, fall within
"acceleration" as used herein. It is appreciated that NOx transient
emissions are caused by changes in operating parameters of the
total engine "system" such as for example by a change in EGR flow
or composition or actuation of a turbo charger. Those changes
typically are associated with speed/load changes of the engine.
Thus, sensing an acceleration command, as provided for in this
invention, accounts for NOx transients caused, by the engine
"system" components.
[0061] As explained in some detail in the SAE papers incorporated
by reference herein, the NOx reduction reaction in the presence of
HC proceeds in accordance with equation No. 1.
(--CH.sub.2--)+2NO+1/2O.sub.2.fwdarw.CO.sub.2+H.sub.2O+N.sub.2
[0062] and/or
(--CH.sub.2--)+NO+NO.sub.2.fwdarw.CO.sub.2+H.sub.2O+N.sub.2
Equation No. 1.
[0063] The hydrocarbon to NOx ratio (C.sub.1/NOx) must be
approximately 0.5 to meet the stoichiometric requirements for the
NOx reduction reaction shown in equation 1. Diesel exhausts
typically have lower instantaneous C.sub.1/NOx ratios than this
during NOx transient emissions and thus, additional fuel, i.e.,
on-board diesel fuel or drive fuel, must be supplied. However, the
selectivity of the catalyst and the speed of the reduction
reaction, is somewhat limited. Significantly, the excess air
present in the diesel exhaust gases cause competing exothermic
oxidation reactions with the HC proceeding in accordance with
equation No. 2.
(--CH.sub.2--)+3/2O.sub.2.fwdarw.CO.sub.2+H.sub.2O Equation No.
2
[0064] In practice, because of competition between NOx and oxygen
for the added HC and other considerations, significantly larger
quantities of HC producing HC/NOx ratios more than 0.5 are required
to produce desired conversion percentages of reduced NOx. Reference
should be had to FIG. 3 which plots the HC/NOx ratios for producing
various NOx conversion percentages for diesel exhaust gases at
various catalytic converter inlet temperatures. More specifically,
the trace plotted through triangles identified by reference numeral
14 was produced with diesel exhaust gases at a catalytic inlet
temperature of 210.degree. C. The trace passing through squares
indicted by reference numeral 15 was generated from diesel gas
passing through the catalyst at a catalyst inlet temperature of
250.degree. C. Gas temperatures between 210-250.degree. C. would
produce HC/NOx ratio traces falling between curves 14 and 15. The
reducing catalyst used when the FIG. 3 curves were generated was a
zeolite based catalyst defined by this invention.
[0065] Referring now to FIG. 2, there is shown two plots of NOx
conversion percentages possible with HC reductant added at desired
HC/NOx ratios (i.e., FIG. 3) for diesel exhaust gases at various
catalyst inlet temperatures. The trace passing through triangles
and identified by reference numeral 18 is a plot of steady state
gas inlet temperatures and clearly shows that there is a
temperature window within which NOx conversion of exhaust gases are
reduced when the exhaust gases pass over a zeolite based catalyst.
If gas inlet temperatures fall outside the window, little if any HC
reduction of NOx is possible. Again, trace 18 is based on steady
state conditions and resembles graphs shown in U.S. Pat. No.
5,943,857 to Ansell et al. and U.S. Pat. No. 5,935,530 to Langer et
al. discussed at some length in the Background. In contrast to
trace 18, the trace indicated by reference numeral 19 and passing
through squares is a plot of exhaust gases containing NOx transient
emissions. That is, into exhaust gases of constant composition at a
constant temperature, a pulse of NOx gas was injected and the NOx
conversion percentage for that NOx transient was determined. In
marked contrast to the steady state temperature window of trace 18,
the transient temperature window of trace 19 is significantly wider
(greater temperature range) and higher (greater NOx conversion
range). Trace 19 forms an important underpinning of the present
invention for several reasons. First, it is possible to convert and
significantly convert, transient emissions even if the temperature
of the exhaust gas is outside the steady state catalyst operating,
temperature window. Second, the "map" for transient NOx emissions
cannot follow the steady state map conventionally used for NOx
control. That is, the HC injection pulse is to be followed from a
transient pulse map as explained further below. As is well known,
conventional techniques measure engine speed (and load) and fueling
to determine NOx emissions emitted by the engine. The space
velocity (speed of exhaust gases through catalyst) and temperature
of the catalyst are then sensed to generate a secondary hydrocarbon
injection demand to set the HC injection. Trace 19 shows that the
temperature for the active lean NOx can extend significantly
further with a higher conversion ratio for transient NOx emissions
than possible for steady state emissions. As applied to the present
invention, injection of diesel fuel occurs when the reducing
catalyst is within a temperature window. That temperature window is
at one range when steady state engine operation occurs and at a
second range when NOx transient emissions occur.
[0066] Referring next to FIG. 5, there is shown the effects of
pulsing at timed intervals set quantities of hydrocarbons into a
250.degree. C. exhaust gas containing a steady state or constant
concentration of NOx emissions. Unless otherwise indicated, the
data and graphs discussed herein were generated with exhaust gases
produced from a 2.5 liter TDI diesel engine running at contact
speed of about 2000 rpm. The pulses or transients were generated
with a Melt-Ranger Timer made by Automatic Timing Controls Co. This
controlled the width, timing and injection period of the pulse. A
metering pump was used to inject HC from a diesel fuel tank. The
NOx spikes as transients were generated from a 2% cylinder of NOx
with balance N.sub.2. The HC injection was about 2' in front of
catalytic converter inlet and NOx transient was about 40" in front
of the catalyst. The pulse widths were about 2 seconds. The
catalytic converter was a conventional, platinum exchanged ZSM-5
zeolite based catalyst applied over a metal monolith substrates
formed in a honeycomb pattern.
[0067] Some explanation must be given for what is portrayed in FIG.
5 which explanation will likewise apply to discussion of FIGS. 6a,
6b and 6c. The time scale plotted on the x-axis of FIG. 5 is
relative and is to be read in conjunction with right-hand facing
arrow designated by reference numeral 20 and left-hand arrow
designated by reference numeral 21 appearing at the top of FIG. 5.
Right-hand and left-hand arrows 20, 21 are bisected by a dividing
line 23 which represents the reducing catalytic converter. That
portion of the graph extending from dividing line 23 in the
direction of left-hand arrow 21 indicates the gas conditions in
front of the reducing catalytic converter and that portion of the
graph extending from dividing line 23 in the direction of
right-hand arrow 20 indicates the condition of the exhaust gas at
the outlet of the reducing catalytic converter. The NOx
concentration in the exhaust gas is indicated by the lower trace
designated by reference numeral 24 and as shown, is fairly constant
over that portion of the trace covered by left hand arrow 21. The
NOx concentration is shown on the left-hand y-axis. The upper trace
(extending over the left-hand portion of FIG. 5) designated by
reference numeral 26 are hydrocarbons (diesel fuel) pulsed at
periodic intervals to the exhaust gas containing the steady
concentration of NOx. In the trace shown in FIG. 5, there are five
pulses designated 26A, 26B, 26C, 26D and 26E. The injection or the
pulse of HC occurs at the leading edge 26' of each pulse trace. The
long trailing edge of each pulse trace shown as 26" in reality does
not exist. It is an artifact attributed to the HC sampling system.
(All pulses were injected over a 2 second time period.) Note, that
for each HC pulse (26A-26E) injected into the gas stream upstream
of catalytic converter, i.e., 23, there is produced downstream of
the reducing catalytic converter, a corresponding NOx reducing
pulse respectively identified by reference numerals 24A, 24B, 24C,
24D, etc. FIG. 5 shows that it is possible to generate transient HC
pulses which produce corresponding transient NOx reduction pulses
which temporarily reduce the concentration of NOx emissions in the
exhaust gas. FIG. 5 shows the HC reductant is not adsorbed into the
washcoat such as occurs with ammonia SCR systems and is completely
removed by the reactions of equations 1 and 2.
[0068] Referring now to 6a, there is shown a graph similar to that
described in FIG. 5 in that a reducing catalytic converter is
represented by dividing line 23 with the conditions upstream of the
reducing catalytic converter covered by that portion of the graph
extending in the direction of left-hand arrow 21 and the conditions
of the gas concentrations downstream of reducing catalytic
converter 23 covered by that portion of the graph extending in the
direction of right-hand arrow 20. In the graph shown in FIG. 6a,
the exhaust gas has a fairly constant concentration of NOx
emissions designated by the trace passing through circles and
indicated by reference numeral 30 except that periodically, a pulse
of additional NOx emissions is caused to occur. This NOx transient
emission spike is designated by reference numerals 30A, 30B, 30C
and 30D in FIG. 6a. As in FIG. 5, pulses of HC indicated by the
trace passing through rectangles and designated as reference
numeral 31 are also periodically injected into the gas stream. As
explained with reference to FIG. 5, the leading edge of each HC
pulse designated by reference numeral 31' is indicative of the time
during which the HC is injected into the gas stream and the
trailing edge of each HC pulse designated by reference numeral 31"
is a sampling artifact to be ignored. In FIG. 6a, there are four HC
pulses designated 31A, 31B, 31C and 31D which are timed to be
injected into the gas stream four seconds before the NOx transient
pulses 30A-30D have been injected. It can be seen that the NOx
transient emissions downstream of reducing catalytic converter 23
have been reduced relative to the transient NOx emissions injected
into the gas stream upstream of reducing catalytic converter 23.
That is, the peak of the downstream NOx transient designated by
reference numerals 30A', 30B', 30C' and 30D' have been reduced
relative to the peaks of the upstream NOx transients 30A, 30B, 30C
and 30D respectively. It should also be noted that each NOx
transient spike emission downstream of the catalytic converter has
a reducing "dip", designated by reference numerals 30A", 30B", 30C"
and 30D" which leads or occurs earlier than NOx spike 30A', 30B',
30C' and 30D'. This indicates that the HC was injected early
because a portion of the HC pulse was reacting with the steady
state concentration of NOx emissions before the NOx transient
emissions arrived in the reducing catalyst.
[0069] Referring now to FIG. 6b, a similar test to that conducted
and explained with reference to FIG. 6a is shown but in this test,
the HC pulsed injection occurred 4 seconds after the NOx transient
pulses were induced. The same reference numerals used in describing
the test results depicted in graph form in FIG. 6a will be likewise
used in describing the test results depicted in graph form for FIG.
6b. FIG. 6b shows that the NOx transient emissions occurring
downstream of reducing catalytic converter 23 (i.e., 30A', 30B',
30C' and 30D') are for all intents and purposes, the same transient
emissions induced in the exhaust gases upstream of catalytic
converter 23, i.e., 30A, 30B, 30C and 30D. Note that the reducing
dips 30A", 30B", 30C" and 30D" are occurring after the transient
spikes 30A'-30D' and are the same dips shown for FIG. 5. FIG. 6b
shows that if the HC is injected at a time period which is very
late or after the NOx transient has occurred, the NOx transient
will not be affected. FIG. 6b also shows that HC is not stored in
the washcoat, at least at the operating temperatures of the
engine.
[0070] Referring now to FIG. 6c, there is shown graphically, the
results of yet another test identical to that described for FIGS.
6a and 6b except that the HC pulses were timed to occur two seconds
prior to the NOx injected pulse. Reference numerals used in FIGS.
6a and 6b for describing the test results will likewise be used
with respect to FIG. 6c. It is rather dramatically shown that the
NOx transient spikes 30A', 30B', 30C' and 30D' occurring at the
outlet of reducing catalytic converter are significantly reduced
when compared to the upstream NOx transient spikes 30A-30D. Note,
there are little if any NOx reducing dips in the NOx emissions
downstream of the reducing catalytic converter even though the HC
pulses were injected two seconds prior to the injections of the NOx
transient pulses.
[0071] Again, it should be noted when considering FIGS. 5, 6a, 6b
and 6c, that the HC pulses do not significantly affect the NOx
concentrations relative to the steady state conditions, i.e.,
reference numeral 24 in FIG. 5. This shows there is little
adsorption occurring within the catalytic converter at 250.degree.
C. diesel operating exhaust gas temperatures of this
experiment.
[0072] The methodology explained with reference to FIGS. 6a, 6b and
6c was repeated in a series of tests with the exhaust gases at
various temperatures and HC pulsing occurring at various time
intervals relative to the timing of the NOx transient induced
pulse. Test results are plotted in FIG. 4. In FIG. 4, the inlet
temperature of the exhaust gases is plotted on the x-axis and the
NOx conversion percentage is plotted on the y-axis for a series of
HC injection pulses timed relative to the NOx transient injection
pulse. More specifically, the trace passing through circles and
indicated by reference numeral 40 occurred when injecting the HC
pulses four seconds after the NOx transient spike was caused to
occur. The trace passing through stars and indicated by reference
numeral 41 was generated by causing the HC pulses to occur two
seconds after the NOx transient emission spikes occurred. The trace
passing through diamonds and indicated by reference numeral 42 was
generated by injecting the HC pulses simultaneously with the
generation of the NOx transient spiked emission. The trace passing
through triangles and designated by reference numeral 43 was
generated by injecting the HC pulses two seconds prior to the time
the NOx transient spikes were generated. Finally, the trace passing
through squares and designated by reference numeral 44 was
generated by injecting the HC pulses four seconds in advance of the
time the NOx transient spikes were caused to occur. FIG. 4 shows
that over the entire window temperature range of the reducing
catalytic converter, injecting HC pulses at or prior to the time
the NOx transient emissions occur, result in significant
conversions of NOx. It should again be noted that the catalyst was
acidic for these tests.
[0073] Referring now to FIG. 7, there is shown in bar chart form
the average conversion of NOx at 250.degree. C. when HC pulses or
spikes of hydrocarbon are caused to occur in timed relationship to
the NOx transient emissions. When the HC pulses are retarded or
occur after the NOx transient spikes occur (designated by -1, -2,
-3 and -4 second increments on the x-axis) conversion of the NOx
transient emission does not significantly occur. When the HC
transients are caused to occur simultaneously or in advance of the
time of the NOx transients (indicated by the 0, 1, 2, 3 and 4
second increments on the x-axis) there is a dramatic increase in
the average overall conversion of the NOx transient emission.
[0074] While the invention is not necessarily limited to any
specific theory accounting for the observations illustrated in
FIGS. 4-7, there are two plausible explanations which can be viewed
as consistent with one another.
[0075] The first explanation may perhaps be best explained by
reference to FIG. 8 which shows schematically one of a plurality of
channels 50 making up a honeycomb catalytic converter. Channel 50
is formed of a substrate 51 which is a metal or cordierite monolith
upon which a washcoat 52 containing a catalyst and other substances
is deposited. The channel defines a relatively long, somewhat
narrow passage having an inlet 53 and an outlet 54.
[0076] Assume that a pulse of HC designated by reference numeral 56
and a pulse of NOx designated by reference numeral 57 are
simultaneously injected at inlet 53 at the same time To. An analogy
is drawn to capillary gas chromatography theory in that the
washcoat 52 (the coated walls of the monolith) can by hypothesized
to be analogous to the adsorptive properties of the stationary
phase of a gas chromatography column. Gas chromatography theory
states that any short gas pulse or transient will be broadened and
delayed as the gas pulse interacts (i.e., adsorbs and desorbs) with
the coated walls or washcoat 52. Furthermore, each gas component
will interact differently as a function of the temperature of the
gas and the chemical characteristics of the gas component. Because
HC and NOx interact differently with any washcoat, there will be a
tendency for these gas components to separate spatially as they
travel down the length of passage 50. This is diagrammatically
represented by the HC pulse within channel 50 designated by
reference numeral 56' at time T.sub.1 and NOx pulse in channel 50
designated by reference numeral 57' at time T.sub.1. Each pulse is
broadened and extends between the time intervals T1' and T1",
although the breadth of the pulse is not necessarily the same for
each. Each pulse is delayed as indicated by the relative positions
of HC pulse 56' and NOx pulse 57'. In order to react HC and NOx
within channel 50, they must be present at the same place and time
within channel 50 and at a temperature where the selective
reduction reaction can favorably occur. Thus, in order that maximum
overlap of HC and NOx occurs down the length of channel 50, it may
be necessary that the HC and NOx elements impact the front face or
inlet 53 of channel 50 at different times. More specifically, if
the catalyst washcoat 52 has a function which adsorbs HC, then it
will be beneficial to have the HC impact the catalyst prior to the
NOx. On the other hand, if the catalyst has a function which
adsorbs NOx, then it may be beneficial to have the NOx impact the
catalyst prior to the HC. Furthermore, and as will be discussed
with respect to the second explanation, if a pretreatment of one
component is required in a slow reaction step, then it will be
important to have that component which is pretreated impact the
front face of the catalyst first. More particularly, if washcoat 52
has an acidic function (pH less than 6) which reacts with the HC
(to form an oxygenated or partially cracked reactive species) then
it will be beneficial to have the HC impact the catalyst prior to
NOx. In the examples discussed above, the zeolite based reducing
catalyst is highly acidic and will crack or partially crack the HC
thus delaying the reaction. On the other hand, if the washcoat has
a basic function (pH greater than 8) which reacts with NOx (to form
an adsorbed nitrite/nitrate such as barium nitrate from barium
present in washcoat 52) or performs a catalyst function that
oxidizes NO to NO.sub.2, then it may be beneficial to have the NOx
impact the catalyst prior to the time the HC contacts the
catalyst.
[0077] In accordance with another explanation of how the transient
reactions described above occur, and which is somewhat consistent
with the first explanation, is that the fuel oil or diesel fuel,
when cracked at the appropriate temperature within channel 53, will
produce activated carbon. The activated carbon resulting from the
cracking of the fuel will react with the oxygen in the NOx to
reduce the NOx. The activated carbon will also react with the
oxygen in the exhaust gas. Thus, the injection of the fuel oil into
the exhaust gas has to take into account the time for the fuel to
be cracked so that activated carbon is produced or deposited at the
time the NOx transient comes into contact with HC. Studies
conducted by the assignee of the present invention have determined
that when fuel oil, which by definition includes a majority of
molecules having more than 10 atoms per molecule in a liquid phase,
is cracked or partially cracked to produce unsaturated or
oxygenated long chain hydrocarbons, the carbons in the long chain
hydrocarbons are active in the reduction of NOx. As is well known,
cracking of the fuel oil is a function of the gas temperature, time
and the catalyst and proceeds in accordance with the radial chain
theory splitting off short chain molecules from longer chain
molecules.
[0078] In the preferred embodiment, it is contemplated that diesel
fuel (preferably fuel oil No. 2) will be pulsed in liquid form
through any number of conventional arrangements to provide the
reductant for the NOx conversion. Cracking will occur, if not
earlier, in the reducing catalyst producing specific HC compounds
formed as a function of the temperature of the exhaust gases (and
space velocity). Certain HC long chain hydrocarbons will be
produced having a beneficial reduction effect. However, the
invention will function if the fuel oil is cracked outside the
exhaust system to produce desirable HC reductants and the HC
reductant pulse metered as a reducing gas at the inlet of the
reducing converter. Studies conducted by the assignee have shown
that normal or unbranched aliphatic HCs are very good reductants as
long as the chain length is greater than C7, i.e., the organic
molecules have more than 7 carbon atoms. For example, decane and
dodecane are shown to be excellent reductants while propane (three
carbon chain), ethane and methane are not. Olefins are believed
good reductants, regardless of chain length. Propylene has been one
of the key HC reductants somewhat universally reported in studies
and in literature and is demonstrated as a good reductant.
Aldehydes and ketones also exhibit good reducing properties. In
contrast, branched aliphatic HCs are poor reductants. Aromatic HCs
are better reductants than branched aliphatics but are not the best
HC reductants. In general, good HC reductants include normal,
unbranched aliphatics having a chain length where the number of
carbon atoms per molecule is equal to or greater than 7 and
olefins, of any size. If selectivity is possible, such as would
occur if the vehicle was equipped with a cracking catalyst,
branched aliphatics and aromatic HC reductants should be
minimized.
[0079] Referring now to FIG. 9, there is graphed NOx reduction
percentages at various exhaust gas temperatures at the catalyst
inlet for certain long chain hydrocarbons. More particularly,
n-Hexadecane (C16) is shown by trace designated by reference
numeral 59A passing through triangles; n-Dodecane (C12) is shown by
trace designated by reference numeral 59B passing through diamonds;
n-Decane (C10) is shown by trace designated by reference numeral
59C passing through circles and n-Heptane (C7) is shown by trace
designated by reference numeral 59D passing through squares.
Consistent with the previous discussion, FIG. 9 shows the
desirability of long chain HC's as the reducing reagent for NOx
reduction.
[0080] The reducing catalytic converter is a de-nox catalyst or an
active lean catalyst. Currently, lean-NOx catalysts are of two
types: 1) low temperature lean-NOx catalysts which are platinum
based (Pt-based) and 2) high temperature lean-NOx which have base
metal/zeolite compositions, for example, CU/ZSM-5. The Pt in the
low temperature type 1 catalyst is best atomically dispersed and
would produce an amorphous and not crystalline structure. The Pt
catalyst does not have to have the zeolite present to be active but
Pt/zeolite catalyst are better and appear to have better
selectivity against formation of N.sub.2O as a byproduct than other
catalysts, i.e., Pt/alumina. Zeolite alone (e.g., H-ZSM-5) has some
activity for NOx reduction but is not as good a lean-NOx catalyst.
When exchanged with copper, it constitutes an active catalyst for
high temperature NOx reduction.
[0081] The preferred reducing catalyst for use in this invention
are composites of zeolites with base metals or platinum (i.e.,
precious metals group). The zeolites within the preferred group can
be any of the conventional acidic, hydrothermally stabilized
zeolites but preferred, as noted above, is ZSM-5. While the metal
may or may not be crystalline, the zeolites have a crystalline
structure exhibiting porosity. Reference can be had to assignee's
U.S. Pat. Nos. 4,961,917 and 5,516,497, incorporated by reference
herein. The crystalline structures of zeolites exhibits a complex
pore structure having more or less recurring connections,
intersections, and the like in dimensional planes. Preferred
zeolites for use in the reducing catalysts are those which have
relatively large diameter pores interconnected within all three
crystallographic dimensions, typically about 5-6 Angstroms.
However, the invention will function with any zeolite catalyst bed
having a porosity of 2-3 Angstroms to about 8-10 Angstroms. It is
to be noted that some of the preferred HC have longer chain
components of a size which will not permeate the preferred zeolite
bed. Nevertheless, the longer chain aliphatics are preferred since
improved reactivity results from the activated carbon resulting
from these compounds.
[0082] Referring now to FIG. 10 there is schematically shown a
general arrangement of the principle system components used to
control NOx transient emissions produced by a diesel engine 60. An
intake valve 61 controls admission of a fuel/air mixture to the
engine's combustion chamber 62 from an intake manifold 63. An
exhaust valve 65 controls the emission of exhaust gases produced in
combustion chamber 62 to an exhaust manifold 66, in turn, connected
to an exhaust pipe 67. Attached to exhaust pipe is a reducing or
de-nox catalytic converter 68 followed by an oxidation catalyst 69.
Oxidation catalyst 69 is optional in that its function can be
included or built into reducing catalytic converter 68. Oxidation
catalyst 69 is conventional. It is a base metal oxide such as an
oxide of copper, cobalt, chromium, cerium, etc. or a noble metal
catalyst containing palladium or platinum supported on
Al.sub.2O.sub.3, TiO.sub.2, CeO, etc.
[0083] A fuel injector 70 receives pressurized diesel fuel from
fuel tank 71 for pulse metering of the fuel into intake manifold
63. A fuel demand command controlled by the vehicle's operator
through an accelerator or fuel demand pedal 73 controls fueling and
consequently the speed/load of the vehicle.
[0084] In the embodiment shown in FIG. 10, a separate hydrocarbon
feed line 75 connected to fuel tank 71 is provided for admitting
diesel fuel in front of reducing catalytic converter 68. Within HC
feed lines 75 is a metering valve 76, i.e., solenoid valve,
controlling admission of fuel oil to exhaust pipe 67. This is an
optional arrangement and is illustrated because it would be the
arrangement used if a separate fuel oil cracking unit (not shown)
would be employed in the invention. In such instance, the fuel oil
would be cracked into desired hydrocarbon chain lengths and pulsed
metered to the inlet of reducing catalytic converter 68 by metering
valve 76. In the preferred embodiment, a common rail fuel injector
70 would be operated in the conventional known manner to inject an
additional quantity of diesel fuel into combustion chamber 62 in
excess of that required by the set A/F ratio to produce HC for
reducing purposes. The additional fuel is typically injected after
TDC (top dead center) during the expansion stroke of the engine
cycle. The exhaust gases, i.e., products of combustion, evaporate
the fuel oil which eventually crack in reducing catalytic converter
68 producing at least some quantity of activated carbon required
for reduction. Still alternatively, a known design could be
employed where an additional fuel injector is used to inject
additional quantities of diesel fuel on demand to any cylinder of
diesel engine 60.
[0085] The operation of engine 60 is under the control of an ECU
(engine control unit) 80. ECU is a microprocessor based control
system containing a conventional CPU with RAM, nonvolatile RAM,
ROM, look-up tables for engine mapping purposes, etc. ECU 80
receives input sensor signal information, processes the data by
programed routines and generates actuator output signals. While a
dedicated processor could be supplied to control the transient HC
metering system of the present invention, because the input sensor
data, for the most part, is now utilized by ECU 80 to control
engine 60, it is preferable that ECU 80 likewise control the HC
metering system.
[0086] Typical sensor input signals that can be utilized by the
present invention include a speed/load signal from a speed/load
pickup 81 (i.e., a speed sensor and a torque sensor such as used in
the engine's transmission) on speed/load sensor line 82, a fuel
demand input signal from fuel demand pedal 73 on fuel demand input
signal line 83, a mass flow signal generated from a pressure or
flow sensor 84 on a mass flow sensor line 85 and an exhaust gas
temperature shown sensed by a temperature probe 87 at the inlet of
reducing catalytic converter 68 inputted on exhaust gas sensing
line 88. All sensors currently exist so that additional hardware,
with the possible exception of modifying the fuel demand pedal, do
not have to be employed. Again, the sensors noted are merely for
illustration purposes. For example, mass flow sensor 84 is utilized
by ECU to not only generate a pulsed fuel signal on an injector
actuator line 89 but is also used in connection with other signals
to determine space velocity of exhaust gases. The invention needs
to determine an impending engine speed command signal inputted by
the vehicle's operator, the current speed/load condition of the
engine, the space velocity and temperature of the exhaust gas.
These conditions can be directly sensed as shown or, as is known in
the art, be generated or modeled from other sensor signals through
mathematical routines stored in ECU 80. For example, it is known
that the exhaust gas temperature (or an approximation thereof) can
be calculated from other sensors on the vehicle such as a sensor
measuring the ambient temperature and a sensor measuring the
engine's coolant temperature which, in combination with other data
input, allows ECU 80 to model the exhaust gas temperature. It is to
be understood that all such known arrangements for sensing and/or
modeling the input operating parameters of engine 60 are included
within the scope of this invention.
[0087] ECU 80 receives the input sensor signals, performs
programmed routines and generates a pulsed metering signal on pulse
signal line 90 to metering valve 76, or alternatively, as already
explained, on injector actuator line 89 to injector 70 or to a
common rail injector (not shown). The current state of the art
generates a map of steady state engine operating conditions for
each engine and for each catalytic converter system mated to that
engine, and stores the map in a look up table in ECU 80. The map is
accessed at current engine operating conditions determined by the
sensors and ECU 80 performs a first routine which determines the
NOx emissions, the HC emissions and oxygen produced at that
operating condition, and calculates an HC reductant quantity to be
pulse metered through metering valve 76 to reducing catalytic
converter 68. When determining the HC reductant quantity the
temperature of the catalyst and the space velocity of the exhaust
gases through the catalyst are also considered. This is at steady
state or constant speed/load engine operating conditions. Reference
can be had to U.S. Pat. No. 5,522,218 for a more detailed
description of the mechanics than that presented herein.
[0088] Referring now to FIG. 11, there is shown, generally, a flow
process chart used by the system of the present invention. The
operating parameters of engine 60 are monitored by sensors, as
described with reference to FIG. 10, at block 93.
[0089] As discussed above, the preferred embodiment utilizes a fuel
demand sensor input signal to make systemic decisions although
other sensors could be used in addition or in lieu of the fuel
demand pedal such as a clutch position sensor, or to a lesser
extent, transmission sensor. Preferably, the fuel demand pedal
sensor senses not only the absolute position of the throttle (which
is conventional) but it also senses the position at short periodic
intervals so that rate of change signals can be ascertained within
very short time periods. Thus, the pedal sensor may have to be
modified so that a timing circuit or clock signal can be associated
with its position signal. Manufacturers currently build into the
fuel demand pedal a fractional second delay for driver convenience
(response) and there is an inherent response latency within the
engine before the engine is actually operating at the commanded
fuel demand input. It is appreciated that when the engine is
accelerating NOx transients are being produced. Nevertheless, the
delay times are sufficient to enable utilization of conventional
feed forward control techniques to generate an advance fuel demand
signal. Preferably, the rate of change of the fuel demand signal
i.e., signal magnitude, is then applied to an empirically developed
acceleration curve which is exponential. The curve is read at the
sensed acceleration to extrapolate a factor which multiplies the
current steady state HC addition to arrive at a transient fed
forward to metering valve 76. Alternatively, a separate
acceleration map can be generated to arrive at a factor applied to
the steady state concentration of the HC reductant. Acceleration
map will then account for additional HC generated during the
transient, etc. A still further alternative is simply to calculate
the rate of change of the pedal position as a linear slope to
produce a factored number for varying the, amount of HC reductant
which is currently being fed to the system.
[0090] Referring still to FIG. 11, if the fuel demand pedal
indicates that engine 60 is in a steady state condition as at
decision block 94, the system then determines the temperature of
the exhaust gas, i.e., temperature probe 87, to ascertain whether
or not the exhaust temperature is within the operating temperature
window of reducing catalytic converter 68 at decision block 95.
This is the temperature range of trace 18 discussed with reference
to FIG. 2. If exhaust gas temperature is outside the window,
metering valve 76 is shut off at stop block 96. If exhaust gas
temperatures are within the catalytic converter temperature
operating window, the steady state map previously discussed is
interpolated to set a constant metered pulse at constant pulse
block 97 as conventionally done in the prior art. As noted, this
value is determined from the speed/load of the engine and the
intake air flow (i.e., space velocity, temperature being previously
sensed in block 94). From these operating engine parameters it is
possible to determine the concentration of NOx emissions, HC
emissions, and oxygen, all of which are considered in determining a
fixed quantity of HC to be periodically metered through metering
valve 76 either continuously or through periodic pulses of HC.
[0091] If the fuel demand pedal is detecting a rate of change, then
the rate of change is determined to cause an acceleration or
deceleration of engine 60 in rate of change block 98. If a
deceleration is detected, metering valve 76 is shut off at stop
block 96. If an acceleration is detected, the temperature of
reducing catalyst 68 is checked to see if it is within the
transient temperature window at decision block 99. This is trace 19
discussed with reference to FIG. 2. If outside, the system shuts
off metering at stop block 96 to prevent the oxidation catalyst
from being flooded with excess HC. If within the temperature
window, then the NOx transient and the additional quantity of HC
which must be supplied to match the transient is determined by
using any of the techniques discussed above in NOx transient block
100. The time of injection is determined at time block 101 and the
injection occurs at block 102. Conceptually, the time duration of
the pulsed HC injection, i.e., the pulse width, will be equal to
the time at which the transient emission is produced, i.e., the
acceleration time. In practice, the HC pulse will be made up of a
series of short duration pulses correlated to some number of
rate-of-change signals detected while throttle peddle is being
depressed. The time duration of each HC pulse may be constant. In
such instance, the height or amplitude of each pulse will equal the
set quantity of HC determined by the acceleration map for the sum
of the HC pulses to make up the transient amount of HC to be
supplied. In this way, if the operator should vary the
acceleration, the system will somewhat account for the variation in
the NOx transient. Other engine operating conditions (block 93)
which can lower engine out NOx such as EGR or VGT or a driving
event like a deceleration, can stop HC injection.
[0092] It is or should be appreciated that load changes on the
engine without speed change, such as when the vehicle travels up a
hill, are accompanied with a pedal depression and thus picked up as
an acceleration change in rate of change block 98.
[0093] As noted, with respect to the discussion of the washcoat
composition of the catalyst with reference to FIG. 8, it is
possible to delay the NOx pulse if the washcoat is alkaline or
basic tending to promote the formation of nitrites or nitrates,
i.e., barium nitride or copper nitride, etc., which, in turn, will
delay the passage of the NOx transient as it travels through
passage 50 and adsorbs and desorbs relative to the washcoat. Also,
from the discussion noted above, there is a delay attributed to
further cracking of the evaporated diesel fuel within reducing
catalytic converter 68. Finally, it is noted that ideally, for an
acidic washcoat, an HC injection of about 2 seconds prior to the
NOx transient will produce ideal conversions. As a practical
matter, an advance timing of 2 seconds from the time the operator
decides to accelerate the vehicle to the time that the vehicle
engine is commencing acceleration (at which time the NOx transients
are being produced)is likely not possible. In this respect, FIG. 7
shows that if the HC transient can be injected simultaneously with
the NOx transient at the front face of reducing catalytic converter
68, a significant conversion of the NOx transient is possible. This
is entirely feasible especially with injection through the injector
70.
[0094] It is important to recognize that a NOx sensor does not
obsolete the invention and could be used in a system incorporating
the invention. As discussed in the Background, currently a NOx
sensor does not commercially exist that can give the fast response
times needed in vehicle emission control systems. Assume for
discussion purposes that a fast NOx sensor did exist and the NOx
sensor was mounted upstream of the catalyst. The space velocity of
the exhaust gas is such that at the time the NOx sensor would
detect the NOx transient emission, the NOx transient emission would
be through the catalyst. The fuel demand pedal or some other
advanced recognition sensor must be used to allow feed forwarding
of the HC injection signal.
[0095] It is possible to "tweak" the performance of reducing
catalyst 68 by its construction and formulation to impose a delay
time on the reactants such as to cause the reactants to be
spatially coincident within passage 50 at the same time allowing a
reduction of the transient reactants. With respect to converter
construction, passages 50 should be made in as small a
cross-sectional area as possible without producing adverse back
pressures on the engine. An L/D ratio of about 20 to about 200 is
acceptable. While a ceramic monolith can be constructed within the
L/D ratio range, metal monoliths can also be constructed with
relatively long L/D ratios. The object is to formulate passages 50
so as to assimilate a capillary tube.
[0096] With respect to the composition, it is preferred that
alkaline elements be added to the washcoat or to at least portions
of the washcoat, to delay the NOx transient. The alkaline
composition optimally include alkali, or alkaline earth or rare
earth metal oxides or carbonates. The NOx "delay" is to try and
match the "delay" attributed to HC cracking to produce desired HC
reductant chains. The effect of an optimal washcoat composition is
expected to reduce the optimum advance conversion time from the
observed 2 seconds discussed with reference to the graphs above to
as early as 1/2 second or so which is clearly within the time of
the feed forward techniques discussed above. When adding alkaline
components, it is necessary to avoid NOx release phenomena as
reported in the '857 patent.
[0097] It is potentially possible to have an HC breakthrough or
slip which will exceed the HC emission requirements set in the test
drive cycles. Test results have indicated that if channel 50 was
formed to provide a tortuous path such as by bends or zigzags in
the channel, HC breakthrough or slip is not likely to occur. An
example of a tortuous channel path is illustrated in a prior art
honeycomb catalytic converter shown in FIG. 12. Preferably, the
tortuous path channels are defined by offset longitudinally
extending sections (in the x and y directions) such as indicated by
reference numerals 110 and 111 in FIG. 12. Change in direction of
gas flow increases the likelihood of HC oxidation as well as
enhancing reduction reaction with NOx. More particularly, the
channel configuration should be such that turbulent flow can be
created within channels 50 without producing adverse backpressure
to produce NOx and HC contact for the desired NOx reduction
reactions to occur. Laminar flow and its associated boundary
layers, to the extent possible, is to be avoided or reduced.
Finally, it is preferred to have a tortuous path monolith in the
oxidation catalyst 69 as well as in the de-nox catalyst 68 for even
further improvement in the area of HC slip.
[0098] The preferred arrangement for de-nox catalyst 68 is to
provide zones in each channel 50 having alkaline or acidic
composition. For example, as shown in FIG. 12, an alkaline washcoat
can be provided at a channel zone 110A followed by an acidic
washcoat at adjacent channel zone 110B and the pattern repeated
through the zig-zag length of the channel. The alkaline and acidic
concentrations in each zone can progressively vary from inlet to
outlet. To some extent, the tortuous path displaces the relative
position of the HC and NOx gas streams as the gas streams travel
the length of the channel so that certain portion of the gas
streams may contact various zones in the channel tending to hold or
momentarily adsorb one of the gases. The likelihood that the HC and
NOx gases contact one another at some point along the channel
length to produce the desired NOx reduction reaction increases.
[0099] As noted in the Background and as discussed in assignee's
U.S. '155 patent incorporated by reference herein, zeolite based
trap catalysts are conventionally used to trap and store HC
emissions during warm-up of the engine. The trap catalyst is
usually configured as extending over a portion of de-nox catalyst
68. When exhaust temperature exceeds about 200.degree. C. the HC is
desorbed and since an excessive amount of HC is present, a rather
fast catalyst temperature rise in the catalyst will occur. This
phenomena similar to TWC "light-off" is a following event. The HC
has desorbed and passed through reducing catalyst 68 by the time
the catalyst experiences its temperature rise. The temperature rise
produces an NOx transient which is significant and which occurs, as
noted, after the trapped HC has been released. The active lean-NOx
invention disclosed herein can sense the inevitable temperature
rise at catalyst light-off and meter HC to avoid the NOx release
transient (as opposed to a transient delivered from the engine). In
this instance, the NOx transient is not caused by the acceleration
of the engine but by the rapid temperature rise. There are
temperature modeling techniques which can be implemented in the ECU
which can predict when the temperature rise will occur. For
purposes of understanding the invention, a temperature sensor at
catalyst outlet can predict when the catalyst temperature reaches
200.degree. C. (or some other temperature) where NOx transients are
formed. The HC injection can then be timed to occur at or slightly
before the NOx transient is formed as was done with the
acceleration embodiment discussed above.
[0100] The invention has been described with reference to diesel
engines for which it is particularly suited. Conceptually, the
invention is not limited to diesel engines and may have application
to vehicles powered by gasoline engines operated lean or with "lean
burn" engine fueling strategy. For definitional purposes a "lean"
engine as used herein and in the claims is an engine that operates
at an air-to-fuel (A/F) ratio such that the nitrogen oxide
emissions cannot be continuously treated by conventional three-way
catalysts (TWC). Conventional TWCs are able to treat gasoline
powered engines using fueling strategies that cycle lambda (A/F
necessary to produce stoichiometric combustion) at lean conditions
as high as 1.03. Again, although diesel engines operate at
significantly higher A/F ratios than "lean burn" gasoline engines,
the invention can nevertheless function with lean burn engines. In
such instance, the infrastructure of the vehicle would have to
change to include a tank for fuel oil or diesel fuel. An external
injector arrangement such as shown in FIG. 10, would have to be
utilized and the engine operating parameters that are measured to
determine NOx would also have to be expanded to account for the A/F
ratio used, not only during steady state, but also during the time
NOx transients occur. With these additional changes, the invention
will function in the manner described above.
[0101] The invention has been described with reference to a
preferred embodiment. Obviously, alterations and modifications will
become apparent to those skilled in the art upon reading and
understand the Detailed Description set forth herein. It is
intended to include all such modifications and alterations insofar
as they come within the scope of the present invention.
* * * * *