U.S. patent application number 10/807236 was filed with the patent office on 2004-12-09 for estimating method of nox occlusion amount.
Invention is credited to Kohketsu, Susumu, Murata, Minehiro, Nakayama, Shinji, Sunohara, Daisuke, Tanabe, Keiki.
Application Number | 20040244361 10/807236 |
Document ID | / |
Family ID | 33399059 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040244361 |
Kind Code |
A1 |
Tanabe, Keiki ; et
al. |
December 9, 2004 |
Estimating method of NOx occlusion amount
Abstract
The present invention is a method for estimating a NOx occlusion
amount of a NOx occlusion catalyst interposed in an exhaust passage
in an engine, which is constituted such that a NOx occlusion amount
is estimated by using a polynomial reflected with NOx occlusion
characteristics of a NOx occlusion catalyst and each coefficient of
the polynomial is sequentially corrected on the basis of NOx
purification rates actually measured.
Inventors: |
Tanabe, Keiki; (Tokyo,
JP) ; Kohketsu, Susumu; (Tokyo, JP) ;
Nakayama, Shinji; (Tokyo, JP) ; Murata, Minehiro;
(Tokyo, JP) ; Sunohara, Daisuke; (Tokyo,
JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
33399059 |
Appl. No.: |
10/807236 |
Filed: |
March 24, 2004 |
Current U.S.
Class: |
60/274 ;
60/301 |
Current CPC
Class: |
F01N 2560/026 20130101;
F02D 2200/0806 20130101; F02D 41/146 20130101; F01N 3/0842
20130101 |
Class at
Publication: |
060/274 ;
060/301 |
International
Class: |
F01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2003 |
JP |
2003-83650 |
Claims
What is claimed is:
1. A method for estimating a NOx occlusion amount of a NOx
occlusion catalyst interposed in an exhaust passage in an engine,
characterized in comprising the steps of: estimating said NOx
occlusion amount using a polynomial reflected with a NOx occlusion
characteristics of said NOx occlusion catalyst, and correcting each
coefficient of said polynomial sequentially on the basis of NOx
purification rates actually measured.
2. A method for estimating a NOx occlusion amount according to
claim 1, characterized in that the polynomial for obtaining the NOx
occlusion amount x which is used in said estimating step includes a
NOx purification rate r, an exhaust gas temperature y and an
exhaust gas flow velocity z, and said polynomial is a polynomial
obtained by multiplying said exhaust gas temperature y and said
exhaust gas flow velocity z by respective coefficients.
3. A method for estimating a NOx occlusion amount according to
claim 2, characterized in that said polynomial is expressed by the
following equation; x=[r-(k.sub.0+k.sub.2y+k.sub.3z . . .
)]/(k.sub.1+k.sub.4y+ . . . ) here, ki (i=1, 2, . . . ) are
coefficients.
4. A method for estimating a NOx occlusion amount according to
claim 2, characterized in that said correcting step comprises, in
an occasion of correcting said coefficient sequentially: estimating
the (N+1)-th NOx purification rate r on the basis of the N-th (N is
a natural number) NOx occlusion amount x obtained from said
polynomial, and correcting each coefficient such that said
estimated (N+1)-th NOx purification rate r becomes the NOx
purification rate r actually measured.
5. A method for estimating a NOx occlusion amount according to
claim 4, characterized in that the coefficient is corrected by
using the method of least square.
6. A method for estimating a NOx occlusion amount according to
claim 1, characterized in that a NOx discharging amount in said NOx
occlusion catalyst is calculated according to the following
equation. NOx discharging amount=.intg. (reducing agent
concentration at catalyst inlet.times.reducing agent utilization
rate-0.5.times.oxygen concentration in catalyst
inlet).times.exhaust gas flow rate
7. A method for estimating a NOx occlusion amount according to
claim 6, characterized in that: said reducing agent utilization
rate is set on the basis of exhaust gas temperature y and exhaust
gas flow velocity z, and at the same time the characteristics of
the reducing agent utilization rate are stored in a reducing agent
utilization rate setting map.
8. A method for estimating a NOx occlusion amount according to
claim 6, characterized in that: said reducing agent utilization
rate is estimated using a polynomial which is reflected with a NOx
discharging characteristics of the NOx occlusion catalyst, and the
coefficients of said polynomial are sequentially corrected on the
basis of the concentration of reducing agent.
9. A method for estimating a NOx occlusion amount according to
claim 8, characterized in that: the polynomial for obtaining the
reducing agent utilization rate r' includes a catalyst inlet
reducing agent concentration x', an exhaust gas temperature y and
an exhaust gas flow velocity z, and said polynomial is a polynomial
obtained by multiplying said catalyst inlet reducing agent
concentration x', said exhaust gas temperature y and said exhaust
gas flow velocity z by respective coefficients.
10. A method for estimating a NOx occlusion amount according to
claim 9, characterized in that the polynomial for obtaining the
reducing agent utilization rate r' is expressed by the following
equation; 3 r ' = f ( x ' , y , z ) = m 0 + m 1 x ' + m 2 y + m 3 z
+ m 4 x ' y + m 5 yz + m 6 zx ' + m 7 x ' 2 y + m 8 x ' y 2 + here,
m.sub.i (i=1, 2, . . . ) are coefficients.
11. A method for estimating a NOx occlusion amount according to
claim 1, is characterized in that: said engine is constituted such
that switching can be performed between a lean operation where an
exhaust gas air-fuel ratio is lean and a rich operation where said
exhaust gas air-fuel ratio is rich, and said coefficients of the
polynomial are held during said rich operation, and when a
difference between the NOx purification rate obtained by using said
held coefficients at a starting time of the lean operation and said
NOx purification rate actually measured is equal to or more than a
threshold value, said NOx occlusion amount is corrected.
12. A method for estimating a NOx occlusion amount according to
claim 11, characterized in that the NOx occlusion amount is
corrected, when a difference between an actually measured value of
the NOx purification rate r at the starting time of the lean
operation of said engine and an estimated value thereof is equal to
or more than a threshold value.
13. A method for estimating a NOx occlusion amount according to
claim 12, characterized in that said NOx occlusion amount is
corrected based upon a judgment that a NOx occlusion amount
calculated at the starting time of the lean operation is incorrect,
when a difference between said NOx purification rate estimated by
the polynomial and the NOx purification rate obtained by actual
measurement immediately after switching is performed from the rich
operation of said engine to the lean operation thereof is equal to
or more than a predetermined value.
14. A method for estimating a NOx occlusion amount according to
claim 1, characterized in judging that said catalyst is abnormal,
when an average value of said each coefficient in a predetermined
period is deviated from a predetermined range.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The present invention relates to a method for estimating an
occlusion amount of NOx occluded in a NOx occlusion type reducing
catalyst provided in an exhaust passage.
[0003] (2) Description of Related Art
[0004] In a diesel engine, generally, exhaust gas contains a lot of
oxygen, which makes exhaust gas to be oxidizing atmosphere due to
its structural characteristics (that is, exhaust gas air-fuel ratio
becomes lean). Also, in the case of a gasoline engine which is
capable of lean burning, exhaust gas air-fuel ratio becomes lean
during lean burning. Therefore, a lean NOx catalyst has
conventionally been developed so that NOx can be purified even in
such an oxidizing atmosphere.
[0005] As such a lean NOx catalyst, there is a NOx occlusion type
reducing catalyst (hereinafter, referred to as "a NOx occlusion
catalyst" or "a catalyst" simply). A NOx occlusion catalyst is
constituted to occlude NOx contained in exhaust gas in oxidizing
atmosphere and to discharge the occluded NOx when CO exists under a
low oxygen concentration, whereby almost all of the NOx discharged
from the NOx occlusion catalyst is reduced to harmless N.sub.2 by
reducing agent such as unburned HC, CO or the like exhausted from
the engine and then exhausted to the atmosphere.
[0006] In a vehicle equipped with a NOx occlusion catalyst,
so-called rich spike control is carried out in general. That is,
since the continuous lean operation under lean air-fuel ratio makes
a NOx occlusion catalyst to be in saturated state where no further
NOx occlusion can occur, the oxygen concentration in the exhaust
gas is lowered by forcibly making the air-fuel ratio rich
temporarily at an appropriate timing (conducting a rich spike
control) and let NOx be discharged from the NOx occlusion catalyst
by supplying a reducing agent, so that the NOx occlusion catalyst
is restored to the state where it can occlude NOx.
[0007] Now, in order to conduct such the rich spike control as
described above, it is necessary to estimate (or detect) the NOx
occlusion amount in a NOx occlusion catalyst accurately. As the
estimating means of a NOx occlusion amount, there has been known,
for example, the following arts.
[0008] (1) First Conventional Art: NOx Occlusion Amount Estimating
Means using NOx Sensors:
[0009] In the first conventional art, as shown in FIG. 6, NOx
sensors 102 and 103 detecting the concentration of NOx are provided
upstream and downstream of a catalyst 101 respectively, and an
airflow sensor (not shown) which detects exhaust gas flow rate is
provided in the exhaust passage.
[0010] Then, a NOx occlusion amount is estimated in an ECU 104
according to the following equation on the basis of information
from the NOx sensors 102 and 103 and information from the airflow
sensor.
NOx occlusion amount=.intg. exhaust gas flow rate.times.(NOx
concentration upstream of catalyst-NOx concentration downstream of
catalyst)
[0011] (2) Second Conventional Art: Estimating Means According to a
Mathematical Model (Refer to, for Example, JP-A09-72235):
[0012] In the second conventional art, a mathematical catalyst
model is provided based upon a chemical phenomenon and a physical
phenomenon of a catalyst such as occlusion reaction,
oxidation-reduction reaction, releasing reaction and the like and a
NOx occlusion amount is estimated according to an equation of the
catalyst model.
[0013] In the above-described first conventional art, however, it
is possible to calculate a NOx occlusion amount during lean
operation, but it is impossible to detect a NOx discharge amount
during rich operation (the reduced amount of occlusion amount) by a
NOx sensor. For this reason, in the case that the duration of rich
operation is insufficient, there is a problem that calculation
error in NOx occlusion amount occurs due to accumulation of NOx
remaining in the catalyst. Further, there is a problem of cost
increase because it is necessary to provide NOx sensors upstream
and downstream of the catalyst respectively.
[0014] In the second conventional art, it is possible to estimate a
NOx occlusion amount by only providing one NOx sensor at least
downstream of a catalyst. However, since the art is mainly intended
to apply to the air-fuel ratio control of a three way catalyst, the
model structure is different from that of a NOx occlusion catalyst,
which raises a problem that the application to a NOx occlusion
catalyst is difficult. Further, even when the NOx occlusion amount
is estimated by other catalyst models, characteristic value
variations depending on catalyst types require modification of the
model equation according to the types and a further precise
modeling is additionally required in order to correspond to
deterioration of the catalyst.
SUMMARY OF THE INVENTION
[0015] The present invention has been made in view of these
problems, and the object thereof is to provide an estimating method
of NOx occlusion amount which is devised to estimate an NOx
occlusion mount in a NOx occlusion catalyst with high accuracy
through reflecting the latest state to the catalyst model at any
time.
[0016] Therefore, an estimating method of NOx occlusion amount of
the present invention is a method for estimating NOx occlusion
amount of a NOx occlusion catalyst interposed in an exhaust passage
in an engine, characterized in comprising the steps of: estimating
the NOx occlusion amount by using a polynomial reflected with NOx
occlusion characteristics of the NOx occlusion catalyst, and
correcting each coefficient of the polynomial sequentially on the
basis of NOx purification rates actually measured.
[0017] Further, it is preferable that the polynomial for obtaining
the NOx occlusion amount x which is used in the estimating step
includes a NOx purification rate r, an exhaust gas temperature y
and exhaust gas flow velocity z, and the polynomial is a polynomial
obtained by multiplying the exhaust gas temperature y and the
exhaust gas flow velocity z by respective coefficients.
[0018] Further, it is preferable that the polynomial expressed by
the following equation is used.
x=[r-(k.sub.0+k.sub.2y+k.sub.3z . . . )]/(k.sub.1+k.sub.4y . . .
)
[0019] Here, ki (i=1, 2, . . . ) are coefficients.
[0020] Furthermore, it is preferable that the correcting step
comprises, in an occasion of correcting each coefficient
sequentially: estimating the (N+1)-th NOx purification rate r on
the basis of the N-th (N is a natural number) NOx occlusion amount
x obtained from the polynomial, and correcting each coefficient
such that the estimated (N+1)-th NOx purification rate r becomes
the NOx purification rate r actually measured.
[0021] In this case, it is preferable that the coefficients are
corrected by using the method of least square.
[0022] Moreover, it is preferable that a NOx discharging amount in
the NOx occlusion catalyst is calculated according to the following
equation.
NOx discharging amount=.intg.(reducing agent concentration at
catalyst inlet.times.reducing agent utilization
rate-0.5.times.oxygen concentration in catalyst
inlet).times.exhaust gas flow rate
[0023] Further, it is preferable that the reducing agent
utilization rate is set on the basis of the exhaust gas temperature
y and the exhaust gas flow velocity z, and at the same time the
characteristics of the reducing agent utilization rate are stored
in a reducing agent utilization rate setting map.
[0024] In addition, it is preferable that the reducing agent
utilization rate is estimated by using a polynomial which is
reflected with a NOx discharging characteristics of the NOx
occlusion catalyst, and the coefficients of the polynomial are
sequentially corrected on the basis of the concentration of the
reducing agent.
[0025] Moreover, it is preferable that the polynomial for obtaining
the reducing agent utilization rate r' includes a catalyst inlet
reducing agent concentration x', the exhaust gas temperature y and
the exhaust gas flow velocity z, and the polynomial is a polynomial
obtained by multiplying the catalyst inlet reducing agent
concentration x', the exhaust gas temperature y and the exhaust gas
flow velocity z by respective coefficients.
[0026] Furthermore, it is preferable that the polynomial for
obtaining the reducing agent utilization rate r' is expressed by
the following equation.
r'=f(x', y,
z)=m.sub.0+m.sub.1x'+m.sub.2y+m.sub.3z+m.sub.4x'y+m.sub.5yz+m.-
sub.6zx'+m.sub.7x'.sup.2y+m.sub.8x'y.sup.2+ . . .
[0027] Here, m.sub.i (i=1, 2, . . . ) are coefficients.
[0028] Further, it is preferable that the engine is constituted
such that switching can be performed between a lean operation where
an exhaust gas air-fuel ratio is lean and a rich operation where
the exhaust gas air-fuel ratio is rich, and the coefficients of the
polynomial are held during the rich operation, and when a
difference between the NOx purification rate obtained by using the
held coefficients at the starting time of the lean operation and
the NOx purification rate actually measured is equal to or more
than a threshold value, the NOx occlusion amount is corrected.
[0029] In addition, it is preferable that the NOx occlusion amount
is corrected, when a difference between an actually measured value
of the NOx purification rate r at the starting time of lean
operation of the engine and an estimated value thereof is equal to
or more than a threshold value.
[0030] Furthermore, it is preferable that the NOx occlusion amount
is corrected based on a judge that the NOx occlusion amount
calculated at the starting time of lean operation is incorrect,
when a difference between the NOx purification rate estimated by
the polynomial and the NOx purification rate obtained by actual
measurement immediately after switching is performed from the rich
operation of the engine to the lean operation thereof is equal to
or more than a predetermined value.
[0031] Further, it is preferable to judge that the catalyst is
abnormal, when an average value of the each coefficient in a
predetermined period is deviated from a predetermined range.
[0032] As a result, according to the estimating method of a NOx
occlusion amount of the present invention, anyone of the following
advantages can be obtained.
[0033] First, since the NOx occlusion amount is estimated by using
the polynomial reflected with NOx occlusion characteristics of a
NOx occlusion catalyst and also each coefficient of the polynomial
is sequentially corrected on the basis of NOx purification rates
actually measured, such an advantage can be achieved that the NOx
occlusion amount can be estimated with high accuracy. Further, by
estimating the NOx occlusion amount accurately, an optimal rich
operation control based upon the NOx occlusion amount can be
performed and fuel consumption can be improved. Furthermore, even
when the NOx occlusion catalyst is changed, each coefficient is
updated corresponding to the characteristics of the changed
catalyst. Therefore there is an advantage that it is unnecessary to
modify the model equation.
[0034] Moreover, since the NOx discharging amount is calculated on
the basis of the reducing agent utilization rate, the NOx
discharging amount can be estimated with relatively high
precision.
[0035] In addition, since the reducing agent utilization rate is
estimated by using the polynomial reflected with the NOx
discharging characteristics of the catalyst and each coefficient of
the polynomial is sequentially corrected on the basis of the
concentration of the reducing agent, there is such an advantage
that the estimation accuracy of the NOx discharging amount can be
further improved.
[0036] Further, by holding the coefficient of the polynomial during
rich operation and correcting the NOx occlusion amount when the
difference between the estimated NOx purification rate and the
actual NOx purification rate is equal to or more than the threshold
value, the estimation accuracy of the NOx occlusion amount can be
further improved.
[0037] Moreover, when the average value of each coefficient in the
polynomial for a predetermined period is deviated from a
predetermined range, the catalyst is judged abnormal. Therefore,
there is an advantage that the catalyst abnormality judge can be
made with high accuracy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is an illustrative block diagram for explaining an
estimating method of a NOx occlusion amount according to one
embodiment of the present invention;
[0039] FIG. 2 is a map for explaining an estimating method of a NOx
occlusion amount according to one embodiment of the present
invention, which shows characteristics of a NOx catalyst during NOx
occlusion;
[0040] FIG. 3 is a map for explaining an estimating method of a NOx
occlusion amount according to one embodiment of the present
invention, which shows characteristics of a NOx catalyst during NOx
discharge;
[0041] FIG. 4 is a flowchart for explaining an estimating method of
a NOx occlusion amount according to one embodiment of the present
invention;
[0042] FIG. 5 is a flowchart for explaining an estimating method of
a NOx occlusion amount according to one embodiment of the present
invention;
[0043] FIG. 6 is an illustrative block diagram for explaining a
conventional art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] An estimating method of a NOx occlusion amount according to
one embodiment of the present invention will be explained below
with reference to the drawings. FIG. 1 is an illustrative block
diagram showing the whole constitution of an exhaust system of a
vehicle to which the present invention is applied.
[0045] As illustrated, a NOx occlusion catalyst 1 is interposed in
an exhaust passage 2, and a NOx sensor 3 which detects NOx
concentration at a catalyst outlet is provided downstream of the
NOx occlusion catalyst 1.
[0046] Further, though not illustrated, an O.sub.2 sensor, a
temperature sensor, an air flow sensor (AFS), and the like are
connected to the exhaust passage 2, and oxygen concentration,
exhaust gas temperature and exhaust gas flow rate in the exhaust
passage 2 are respectively detected by these sensors.
[0047] Furthermore, each sensor 3 is connected to an ECU 4 serving
as a control unit. Here, the ECU 4 is provided with an input/output
unit, a storage unit (a ROM, a RAM, a non-volatile RAM or the
like), a processing unit (CPU), a timer counter and the like, and
estimation of a NOx occlusion amount in the NOx occlusion catalyst
1 is performed by the ECU 4.
[0048] Further, various maps are provided in the ECU 4, where
respective concentrations of NOx, CO and HC at the catalyst inlet
(hereinafter, referred to as catalyst inlet [NOx], catalyst inlet
[CO] and catalyst inlet [HC], respectively) are readout from the
maps by utilizing, for example, the acceleration opening degree or
the number of rotation of the engine as parameters. In this
embodiment, these values are obtained from the maps, but these
values may be directly detected from the various sensors provided
in the exhaust passage 2.
[0049] Now, the above-described second conventional art is
constituted such that the characteristics themselves of the NOx
occlusion catalyst are described with a mathematical expression to
obtain a NOx occlusion amount, meanwhile the present invention is
constituted such that a NOx occlusion amount is calculated by using
a quartic linear polynomial.
[0050] That is, the polynomial as shown below in the equation (1)
is stored in the ECU 4, and a NOx purification rate and a NOx
occlusion percentage in the NOx occlusion catalyst 1 are calculated
on the basis of the polynomial. 1 r = f ( x , y , z ) ( 1 ) = k 0 +
k 1 x + k 2 y + k 3 z + k 4 xy + k 5 yz + k 6 zx + k 7 x 2 y + k 8
xy 2 +
[0051] In the equation (1), r is a NOx purification rate, x is a
NOx occlusion percentage, y is an exhaust gas temperature, z is a
SV value (or an exhaust gas flow velocity), and k.sub.0, k.sub.1,
k.sub.2, . . . are coefficients.
[0052] Here, the NOx occlusion characteristics of the NOx occlusion
catalyst 1 can be known by such as an experiment in advance, and it
can be approximated with a polynomial. For example, if there are
three parameters (cubic), the characteristics of the catalyst can
be represented with a curved face as shown in FIG. 2, and it can be
expressed by a polynomial. Of course, since the equation (1) is
quartic, the characteristics does not form such a curved face as
shown in FIG. 2, but the same concept as the cubic case can be
applied.
[0053] Further, each coefficient ki (i=1, 2, . . . ) is input with
a proper value as an initial value on the basis of the
characteristics preliminarily obtained from experiments or the
like.
[0054] Then, the latest catalyst condition can always be
represented according to the equation (1) by correcting each
coefficient ki sequentially on the basis of detected values of the
NOx sensor 3 downstream of the catalyst during engine
operation.
[0055] Hereinafter NOx estimation means will be explained in
further detail. First, the NOx purification rate r in the equation
(1) can also be obtained by the following equation (2).
r=NOx concentration at catalyst outlet/NOx concentration at
catalyst inlet (2)
[0056] Here, the NOx concentration at catalyst outlet is a value
detected at the NOx sensor 3, and the NOx concentration at catalyst
inlet can also be obtained from a map. Therefore, the NOx
purification rate r obtained by the equation (2) can be said to be
a sensor value (an actually measured value), while the NOx
purification rate r obtained by the equation (1) can be said to be
an estimated value (a calculated value).
[0057] Further, actually measured data or map values are applied to
the exhaust gas temperature [y] and SV value [z] in the equation
(1). Thereby, the unknown value in the equation (1) is only the NOx
occlusion percentage [x] now, while other values except this are
known. Accordingly, the NOx occlusion percentage [x] can be
obtained by the following equation (3), which is modification of
equation (1).
x=[r-(k.sub.0+k.sub.2y+k.sub.3z . . . )]/(k.sub.1+k.sub.4y+ . . . )
(3)
[0058] Then, the NOx occlusion percentage [x] obtained by the
equation (3) is returned back in the equation (1) again, and it is
used for calculation in the next calculation cycle. That is, in the
equation (1), the purification rate r is calculated by using the
NOx occlusion percentage [x] obtained the previous time, and newly
detected [y] and [z].
[0059] Here, the purification rate r calculated by the equation (1)
is an estimated value, but when each coefficient k.sub.i is an
accurate value, the estimated value of the purification rate r and
the actually measured value obtained by the equation (2) should
coincide with each other.
[0060] Therefore, in this invention, the estimated value and the
actually measured value of the purification rate r are compared
with each other, and when there is a difference between the two,
each coefficient k.sub.i is sequentially corrected by using the
method of least square so that the estimated value coincides with
the actually measured value. Then, the polynomial of the equation
(1) can be modified at any time to the equation reflected with the
state of the NOx occlusion catalyst 1 accurately by repeating such
calculations to correct sequentially each coefficient ki.
[0061] That is, though the characteristics of the NOx occlusion
catalyst 1 will vary according to its operating situation, its age
deterioration or the like, the equation (1) is kept to serve as an
equation representing the state of the catalyst 1 accurately by
means of repetitive corrections of the coefficient ki, hence the
NOx occlusion amount can be estimated with a high accuracy.
Incidentally, the NOx occlusion percentage [x] obtained according
to the equation (3) is an instantaneous NOx occlusion amount in the
calculation cycle of the initial stage, but an accumulated value of
the NOx occluded in the catalyst 1 can be obtained by returning the
calculation result to the equation (1) repeatedly to calculate the
purification rate r.
[0062] Thus, during lean operation where NOx is occluded, the
catalyst outlet [NOx] is captured from the NOx sensor provided
downstream of the catalyst and the captured value and the estimated
value according to the equation (2) are compared with each other,
then an accurate NOx occlusion amount can be obtained by correcting
the coefficient ki by the method of least square for each
comparison.
[0063] By the way, since NOx is discharged during rich operation,
unless an accurate NOx discharge amount can be detected or
estimated, an initial value of the NOx occlusion amount at the next
lean operation cannot be calculated accurately.
[0064] Therefore, the coefficient ki is held during the rich
operation and the NOx discharging amount is calculated according to
the following equations (4) and (5), and the NOx occlusion amount
during rich operation is calculated by subtracting the NOx
discharging amount calculated according to the equations (4) and
(5) from the NOx occlusion amount just before the start of the rich
operation.
NOx discharging amount=.intg. (reducing agent concentration at
catalyst inlet.times.reducing agent utilization rate
[r']-0.5.times.catalyst inlet [O.sub.2]).times.exhaust gas flow
rate (4)
Reducing agent utilization rate [r']=f(y, z) (5)
[0065] Here, a reducing agent utilization rate setting map as shown
in FIG. 3 is provided in the ECUL. The characteristics of the
reducing agent utilization rate based upon the exhaust gas
temperature [y] and the SV value [z] are stored in the map, so that
the reducing agent utilization rate is set on the basis of the
parameters [y] and [z].
[0066] Further, the catalyst inlet reducing agent concentration
(catalyst inlet [CO] and catalyst inlet [HC] ) can be obtained from
the map utilizing the acceleration opening degree or the number of
engine revolution as parameters, and other parameters can also be
obtained as map values or detected values.
[0067] Therefore, when switching is preformed from the lean
operation to the rich operation, each coefficient ki at the time of
lean operation termination is held and the NOx occlusion amount is
stored in the ECU4, and when switching is performed from the rich
operation to the lean operation again, the remaining amount of NOx
is calculated by subtracting the NOx discharged amount at the time
of rich operation termination from the NOx occlusion amount.
[0068] Moreover, when a difference between an actually measured
value and an estimated value of the NOx purification rate r at the
time of lean operation start is equal to or more than a threshold
value, the NOx occlusion amount is corrected in the ECU 4. That is,
when a difference between the NOx purification rate estimated
according to the equation (1) and the NOx purification rate
obtained according to the equation (2) immediately after switching
has been performed from the rich operation to the lean operation is
equal to or more than a predetermined value, judgement is made that
the NOx remaining amount calculated at the starting time of lean
operation (=the NOx occlusion amount at the time of the latest lean
operation termination -the NOx discharged amount at the time of the
rich operation termination) is incorrect, and the NOx remaining
amount [x] (that is, the NOx occlusion amount at the time of lean
operation start) is corrected on the basis of the equation (3).
[0069] In addition, the ECU 4 has such a function that, when the
coefficient ki becomes an abnormal value due to an accidental
irregularity of the catalyst 1 or the like, the ECU 4 detects such
a fact and notifies it to a driver. Specifically, regarding each
coefficient ki, moving averages in a predetermined continuous
calculation cycle are calculated constantly and when the value of a
moving average is deviated from the predetermined range set for
each coefficient, an abnormality judgment is made such that
deterioration or damage has occurred in the catalyst 1.
Incidentally, the predetermined range is established by adding
.+-..alpha. (.alpha. is a constant value) to an initial value of
each coefficient ki, for example.
[0070] Further, when the ECU4 judges that the catalyst 1 is
abnormal, an alarming lamp on the instrument panel, for example, is
turned on and the rich operation is inhibited.
[0071] Thus, inhibiting the rich operation at an abnormal occasion
of the catalyst 1 can avoid an event like CO discharge in advance
properly.
[0072] Since the estimating method of a NOx occlusion amount
according to one embodiment of the present invention is constituted
as described above, the NOx occlusion amount is estimated in the
following manner.
[0073] In a lean operation, first, the actual NOx purification rate
r is calculated according to the equation (2) and the exhaust gas
temperature [y] and the SV value [z] are obtained from the actually
measured data or the map values. Also, in the polynomial of
equation (1), a suitable value obtained by experiments or the like
is inputted for each coefficient k.sub.i as an initial value. Then,
each value is substituted for the equation (3) obtained by
modifying the equation (1) to calculate the NOx occlusion
percentage [x].
[0074] Next, the NOx occlusion percentage [x] obtained according to
the equation (3), the newly obtained exhaust gas temperature [y]
and the SV value [z] are substituted for the equation (1) to obtain
a NOx purification rater (an estimated value). Then, the estimated
NOx purification rate r and the actual NOx purification rate r
newly calculated according to the equation (2) are compared with
each other, and each coefficient ki is corrected by the method of
least square so that the estimated NOx purification rate is
coincident with the actual NOx purification rate.
[0075] Then, such calculations are performed repeatedly to update
each coefficient k.sub.i at any time, which makes the equation (1)
to be an expression representing an accurate state of the catalyst
1, and realizes that the NOx occlusion amount can be estimated with
a high accuracy. Further, the accumulated value of the NOx occluded
in the catalyst 1 can be obtained by repeated calculation of the
NOx occlusion percentage [x] using the equation (3).
[0076] In addition, as NOx is discharged during a rich operation,
the calculation of the NOx occlusion amount according to the
equation (1) is suspended at this time and each coefficient ki is
stored, meanwhile the NOx discharging amount is calculated
according to the equations (4) and (5) described above.
[0077] Then, when switching is performed from rich operation to
lean operation again, the NOx remaining amount at the time of lean
operation start is calculated by subtracting the NOx discharged
amount at the time of rich operation termination from the NOx
occlusion amount at the time of lean operation termination.
[0078] Further, when a difference between the actually measured
value and the estimated value of the NOx purification rate r at the
time of lean operation start is equal to or more than a threshold
value, the NOx occlusion amount is corrected. Here, the correction
means for the NOx occlusion amount will be explained with reference
to a flow chart shown in FIG. 4. First, each coefficient k.sub.i of
the polynomial (1) is sequentially corrected during the lean
operation to identify each coefficient k.sub.i (Step S1).
[0079] Then, when it is judged that a rich operation has started
(Step S2), each coefficient ki is held (Step S3). Thereafter, when
the rich operation is terminated (Step S4), an actually measured
value and an estimated value of the NOx purification rate are
compared with each other and judgement is made whether or not a
difference between the both is equal to or more than the threshold
value (Step S5).
[0080] When the difference between the actually measured value and
the estimated value of the NOx purification rate is less than the
threshold value, returning back to Step 1 again, and updating each
coefficient k.sub.i in the lean operation is continued, while the
difference is equal to or more than the threshold value, the NOx
occlusion amount is corrected to a value calculated according to
the equation (3) (Step S6) and the flow returns.
[0081] Thus, the estimation accuracy of the NOx occlusion amount
can further be enhanced by correcting means of the NOx occlusion
amount in this manner.
[0082] On the other hand, when deterioration or irregularity of the
catalyst 1 is judged, the judgement is notified to the driver by
turning on the alarm lamp in the instrument panel and the rich
operation is inhibited. This procedure will be explained according
to a flowchart shown in FIG. 5. Each coefficient k.sub.i of the
polynomial (1) is sequentially corrected and identified (Step S11),
and the moving average of each coefficient k.sub.i for a fixed
period is then calculated (Step S12).
[0083] Next, judgement is made about whether or not the moving
average of each coefficient k.sub.i calculated falls with in a
predetermined range (Step S13). When the moving average of any
coefficient is within a predetermined range, the flow returns back
to Step S2 again. However, when at least one of the moving averages
of the coefficients k.sub.i is out of the corresponding
predetermined range, judgement is made that abnormality has
occurred in the catalyst 1 to give an alarm (Step S14) and inhibit
the rich operation (Step S15).
[0084] As described above in detail, according to the estimating
method of NOx occlusion amount according to the embodiment of the
present invention, since the NOx occlusion amount is estimated by
using the polynomial reflected with the NOx occlusion
characteristics of the NOx occlusion catalyst 1 and each
coefficient k.sub.i of the polynomial is sequentially corrected on
the basis of the actually measured NOx purification rates, there is
an advantage that the polynomial which is modeled consistently with
the latest catalyst state can be obtained and the NOx occlusion
amount can be estimated with high accuracy. Further, by estimating
the NOx occlusion amount accurately, the optimal rich operation
control can be conducted on the basis of the NOx occlusion amount
and fuel consumption can be improved.
[0085] Further, even if the NOx occlusion catalyst 1 is changed,
since updating to each coefficient ki corresponding to the
characteristics of the changed catalyst 1 is conducted, there is
such an advantage that modification of the model equation is not
required. On the other hand, when a mathematical model equation is
applied as the conventional art, there occurs such a problem that
works (fitting) for identifying coefficients corresponding to the
characteristics of each catalyst is needed, which requires
tremendous labor. In the present invention, however, the NOx
occlusion amount can be estimated accurately without conducting
such fitting works.
[0086] Since the NOx discharging amount is calculated on the basis
of the reducing agent utilization rate, the NOx discharged amount
can be estimated with a relatively high precision.
[0087] Furthermore, the coefficient k.sub.i of the polynomial is
held during rich operation, and when the difference between the NOx
purification rate estimated by using the coefficient held at the
time of lean operation start and the actually measured NOx
purification rate is equal to or more than a threshold value, the
NOx occlusion amount is corrected, therefore the estimation
precision of the NOx occlusion amount can further be enhanced.
[0088] When the average value of each coefficient k.sub.i for a
predetermined period is deviated from a predetermined range, it is
judged that the catalyst 1 is abnormal, therefore the abnormality
of the catalyst 1 can be judged with a high precision. Further,
since an alarm is sent out to the driver at the time of such
abnormality judgement, the driver can recognize the abnormality of
the catalyst 1 immediately, which promotes the driver to exchange
catalysts 1 early. Further, since the rich operation is inhibited
in case of such an abnormality of catalyst 1, the event that CO is
exhausted as it is can be avoided.
[0089] Incidentally, the estimating method of a NOx occlusion
amount of the present invention is not limited to the
above-described embodiments, but it may be modified in various ways
within the range not deviated from the scope and spirit of the
invention. For example, the discharging amount of NOx can be
obtained from an approach similar to the estimating method of a NOx
occlusion amount.
[0090] Namely, though the reducing agent utilization rate [r'] used
for calculating the NOx discharged amount is obtained from the
reducing agent utilization rate setting map shown in FIG. 3, it
maybe obtained according to a polynomial of catalyst inlet reducing
agent concentration [x'], exhaust gas temperature [y] and SV value
[z] as shown in the following equation (6). 2 r ' = f ( x ' , y , z
) = m 0 + m 1 x ' + m 2 y + m 3 z + m 4 x ' y + m 5 yz + m 6 zx ' +
m 7 x ' 2 y + m 8 x ' y 2 + ( 6 )
[0091] Here, m.sub.i (i=1, 2, . . . ) are coefficients. Further,
the equation (6) is a polynomial reflected with NOx discharging
characteristics of the NOx occlusion catalyst, where each
coefficient m.sub.i is inputted with a proper initial value
obtained from experiments or the like as an initial value similarly
to the equation (1).
[0092] In this case, additionally, a sensor (a CO sensor) for
detecting a reducing agent (specifically CO) concentration is
provided at least in the exhaust passage 2, and the reducing agent
utilization rate r' is obtained by sequentially updating each
coefficient m.sub.i from actually measured values of the reducing
agent concentration [x'] obtained from the CO sensor and estimated
values of the reducing agent concentration [x'] obtained according
to the equation (6), and then the NOx discharged amount can be
obtained by substituting the value thus obtained for the
above-described equation (4).
[0093] The estimation precision of the NOx discharged amount can be
improved by obtaining the NOx discharged amount in this manner.
* * * * *