U.S. patent application number 10/101892 was filed with the patent office on 2003-03-20 for method and apparatus for determining whether a sensor which is responsive to both a reactant and a substance to be reduced by such reactant is responding to either un-reacted portions of the substance or un-reacted portions of the reactant.
Invention is credited to Soltis, Richard E., Van Nieuwstadt, Michiel J..
Application Number | 20030051468 10/101892 |
Document ID | / |
Family ID | 24739731 |
Filed Date | 2003-03-20 |
United States Patent
Application |
20030051468 |
Kind Code |
A1 |
Van Nieuwstadt, Michiel J. ;
et al. |
March 20, 2003 |
Method and apparatus for determining whether a sensor which is
responsive to both a reactant and a substance to be reduced by such
reactant is responding to either un-reacted portions of the
substance or un-reacted portions of the reactant
Abstract
A method wherein a reactant is added to a substance to react
with such substance. The product of such reaction along with
un-reacted portions of the substance and un-reacted portions of the
reactant are directed to a sensor. The sensor produces an output
signal in response to detection of both the un-reacted portions of
the substance and the un-reacted portions of the reactant. The
method includes changing the amount of reactant added to the
substance. A measurement is made to determine whether the change in
the amount of reactant and the change the output signal are in the
same direction or in opposite directions. The changes in reactant
and output signal are multiplied an integrated to form a correction
on the quantity of reactant to be injected.
Inventors: |
Van Nieuwstadt, Michiel J.;
(Ann Arbor, MI) ; Soltis, Richard E.; (Saline,
MI) |
Correspondence
Address: |
Richard M. Sharkansky
Daly, Crowley & Mofford, L.L.P.
Suite 101
275 Turnpike Street
Canton
MA
02021-2310
US
|
Family ID: |
24739731 |
Appl. No.: |
10/101892 |
Filed: |
March 19, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10101892 |
Mar 19, 2002 |
|
|
|
09682445 |
Sep 4, 2001 |
|
|
|
Current U.S.
Class: |
60/286 |
Current CPC
Class: |
Y02T 10/24 20130101;
B01D 53/9495 20130101; F02D 2041/1468 20130101; Y02T 10/47
20130101; F01N 2560/026 20130101; F02D 41/1463 20130101; F01N
2550/05 20130101; F01N 2560/021 20130101; F01N 2610/02 20130101;
F02D 41/1461 20130101; F02D 2250/36 20130101; Y02A 50/20 20180101;
F01N 2550/02 20130101; F02D 41/146 20130101; Y02C 20/10 20130101;
Y02T 10/12 20130101; F01N 3/2066 20130101; Y02A 50/2325 20180101;
F01N 11/00 20130101; B01D 53/90 20130101; F02D 41/221 20130101;
Y02T 10/40 20130101 |
Class at
Publication: |
60/286 |
International
Class: |
F01N 003/00 |
Claims
What is claimed is:
1. A method, comprising: (a) adding a reactant into a substance to
react with the substance, the product of such reaction along with
un-reacted portions of the substance and un-reacted portions of the
reactant being directed to a sensor, such sensor producing an
output signal in response to detection of both the un-reacted
portions of the substance and the un-reacted portions of the
reactant, such method including changing at least one of the amount
of reactant added to the substance and the amount of the substance;
and (b) determining from said at least one of the change in the
amount of reactant and the change in the amount of the substance,
and from the change in the output signal, whether the sensor is
responding to the substance or to the reactant.
2. A method, comprising: (a) adding a reactant into a substance to
react with the substance, the product of such reaction along with
un-reacted portions of the substance and un-reacted portions of the
reactant being directed to a sensor, such sensor producing an
output signal in response to detection of both the un-reacted
portions of the substance and the un-reacted portions of the
reactant, such method including changing the amount of reactant
added to the substance; and (b) determining from the change in the
amount of reactant and the change the output signal whether the
sensor is responding to the substance or to the reactant.
3. A sensing system, comprising: (a) a sensor for producing an
output signal in response to either a first substance of a reaction
or to a second substance of the reaction; (b) an injector for
changing the level of one of the substances in the reaction; (c) a
processor responsive to the output signal for determining whether
the sensor is responding to the first substance to the second
substance.
4. A NOx sensing system, comprising: (a) a NOx sensor for producing
an output signal in response to NO.sub.x and urea; (b) a processor
responsive to the output signal for determining whether the sensor
is responding to NOx or to urea.
5. A method, comprising: (a) adding a reactant to the substance to
react with a substance, the product of such reaction along with
un-reacted portions of the substance and un-reacted portions of the
reactant being directed to a sensor, such sensor producing an
output signal in response to detection of both the un-reacted
portions of the substance and the un-reacted portions of the
reactant, such method including changing the amount of reactant
added to the substance; and (b) determine whether the change in the
amount of reactant and the change the output signal are in the same
direction or in opposite directions.
6. A method for use in reducing a substance with a reactant added
to the substance to react with such substance, the product of such
reaction along with un-reacted portions of the substance and
un-reacted portions of the reactant being directed to a sensor,
such sensor producing an output signal in response to detection of
both the un-reacted portions of the substance and the un-reacted
portions of the reactant, such method comprising: (a) changing the
amount of reactant added to the substance; (b) determining whether
the output signal increases or decreases with the changed amount of
reactant; and (c) determining therefrom whether the sensor is
responding to the un-reacted reactant or to the un-reacted
substance.
7. A method, comprising: changing an amount of a reactant added to
a substance to be reacted , and thereby reduced by, the reactant,
the time history of such change in the reactant being represented
by: (t) where t is time; such reaction taking place in the presence
of a catalyst; wherein un-reacted portions of the reactant and the
un-reacted portions of the substance are passed by a sensor, such
sensor producing an output signal in response to the un-reacted
portions of the reactant or the un-reacted portions of the
substance, the change in the output signal from the change in the
amount of reactant added to the substance may be represented as:
{circumflex over (V)}(t) wherein such change takes place over a
period of time T, and with a delay between the change in the amount
of the reactant, T0, to the time the reaction is effected by such
change in the amount of reactant being represented by D; producing
a signal, I, to determine whether the output signal increases or
decreases with the changed amount of reactant, such signal being
produced in accordance with 5 I = t0 t0 + D + T e x c V ^ ( t + D )
u ^ ( t ) t determining whether, I>0 or I<0; concluding that
the sensor is responding to the un-reacted reactant if I>0 and
concluding that the sensor is responding to the un-reacted
substance if I<0.
8. The method recited in claim 7 including determining whether I=0,
and is so concluding that the injector is faulty.
9. The method recited in claim 7 including determining whether V is
greater than a calibratable threshold and if so concluded that the
catalyst is faulty.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of co-pending
U.S. patent application Ser. No. 09/682,445 entitled "Method and
Apparatus for Controlling the Amount of Reactant to be Added to a
Substance Using a Sensor which is Responsive to both the Reactant
and the Substance", filed on Sep. 4, 2001, the entire subject
matter thereof being incorporated herein by reference. This
application claims the benefit of the Sep. 4, 2001 filing date of
said co-pending U.S. patent application Ser. No. 09/682,445 under
the provisions of 35 U.S.C. .sctn.120.
TECHNICAL FIELD
[0002] This invention relates generally to methods and apparatus
for determining whether a sensor which is responsive to both a
reactant and a substance to be reduced by such reactant is
responding to either un-reacted portions of the substance or
un-reacted portions of the reactant. More particularly the
invention relates to methods and apparatus for detecting NOx to be
reduced with urea using a sensor which is responsive to un-reacted
portions of the NOx and un-reacted portions of the urea.
BACKGROUND
[0003] As is known in the art, in many applications it is desirable
to detect the effectiveness of a reaction used to reduce a
substance. One such application is in measuring the effectiveness
in urea based selective catalytic reduction (SCR) in reducing
nitrogen (NOx) in the exhaust gas of a diesel engine. More
particularly, an aqueous solution of urea is injected into the
exhaust gas of the engine upstream of a catalyst. In order for the
method to reduce NOx in the exhaust effectively, it is important
that the amount of urea injected into the exhaust be accurately
controlled. Injection of too little urea may result in sub-optimal
(i.e., incomplete) NOx conversion. Injection of too much urea may
produce nitrates in the exhaust which can reduce the life of the
exhaust system downstream of the catalyst, may produce an
unpleasant odor, and may also produce increases in regulated
emissions.
[0004] Thus, it is desirable to have a sensor downstream of the
catalyst which can detect the presence of NOx after the reaction.
However, the inventors herein have recognized that currently
available sensors which are practical, from a size and cost
perspective, for automotive use cannot differentiate between NOx
and urea. The inventors have further recognized that degraded
performance may result when using such a sensor with conventional
urea injection methods that adjust the injected urea based on one
of measured NOx or measured urea,. In particular, if it is assumed
that NOx is measured, yet actually urea is exiting the catalyst,
the system may improperly increase injected urea since the system
believes excess NOx is being produced and thus more urea is
necessary. This can results in increased urea slip. Similarly, if
it is assumed that urea is measured, yet actually NOx is exiting
the catalyst, the system may improperly reduce injected urea since
the system believes excess urea is being produced and thus less
urea is necessary. This can result in increased NOx emission.
SUMMARY
[0005] In accordance with the present invention, a method is
provided wherein a reactant is added into a substance to react with
the substance. The product of such reaction along with un-reacted
portions of the substance and un-reacted portions of the reactant
are directed to a sensor. The sensor produces an output signal in
response to detection of both the un-reacted portions of the
substance and the un-reacted portions of the reactant. The method
includes changing at least one of the amount of reactant added to
the substance and the amount of the substance. A determination is
made from said at least one of the change in the amount of reactant
and the change in the amount of the substance, and from the change
in the output signal, whether the sensor is responding to the
substance or to the reactant.
[0006] In one embodiment, a method is provided wherein a reactant
is added to a substance to react with such substance. The product
of such reaction along with un-reacted portions of the substance
and un-reacted portions of the reactant are directed to a sensor.
The sensor produces an output signal in response to detection of
both the un-reacted portions of the substance and the un-reacted
portions of the reactant. The method includes changing the amount
of reactant added to the substance and determining from the
response of the sensor to such change in the amount of reactant
whether the output signal is responding to the substance or to the
reactant.
[0007] In accordance with another aspect of the invention, a
sensing system is provided wherein a sensor is provided for
producing an output signal in response to either a first substance
of a reaction or to a second substance of the reaction. The system
includes: an injector for changing the level of one of the
substances in the reaction; and a processor responsive to the
output signal and a signal representative of the change in level of
one of the substances, for determining whether the sensor is
responding to the first substance or to the second substance.
[0008] In one embodiment, the sensing system is a NOx sensing
system. Such system includes: a NOx sensor for producing an output
signal in response to NOx and urea and a processor responsive to
the output signal for determining whether the sensor is responding
to NOx or to urea.
[0009] In accordance with another aspect of the invention, a method
is provided wherein a reactant is added to a substance to react
with such substance. The product of such reaction along with
un-reacted portions of the substance and un-reacted portions of the
reactant are directed to a sensor. The sensor produces an output
signal in response to detection of both the un-reacted portions of
the substance and the un-reacted portions of the reactant. The
method includes changing the amount of reactant added to the
substance, determining whether the change in the amount of reactant
and the change in the output signal are in the same direction or in
opposite directions, and from such determination determining
whether the output signal is responding to the substance or to the
reactant.
[0010] In one embodiment, if the directions are the same, reactant
is added to the substance. If the directions are opposite, reactant
is reduced from the substance.
[0011] In one embodiment of the invention, a change is made in the
amount of a reactant added to a substance to be reacted and thereby
reduced by, the reactant, the time history of such change in the
reactant being represented by:
[0012] (t)
[0013] where t is time.
[0014] The products of the reaction, along with un-reacted portions
of the reactant and the un-reacted portions of the substance are
passed by a sensor. The sensor is unable to effectively
differentiate between the un-reacted reactant portions and the
un-reacted portions of the substance. That is, the sensor produces
an output signal V(t) in response to the un-reacted portions of the
reactant and the un-reacted portions of the substance. The change
in the output signal from the change in the amount of reactant
added to the substance may be represented as:
[0015] {circumflex over (V)}(t)
[0016] The change in the amount of the reactant is recorded over a
period of time T. The delay between the change in the amount of the
reactant occurring at a time t0, to the time the reaction is
effected by such change in the amount of reactant is a time delay,
D. A signal, I, is produced to determine whether the output signal
increases or decreases with the changed amount of reactant. In one
embodiment, 1 I = t0 t0 + D + T e x c V ^ ( t + D ) u ^ ( t ) t
[0017] where Texc is the time duration of the excitation (t).
[0018] If the change in the level of the reactant is in the same
sense (i.e., direction) as the resulting change in the output
signal, (i.e., if the amount of reactant increases and the output
signal increases, or if the amount of reactant decreases and the
output signal decreases), I>0. In such case, the sensor is
determined to be responding to the un-reacted reactant.
[0019] On the other hand, if the change in the level of reactant is
in the opposite to resulting change in the output signal, (i.e., if
the amount of reactant increases and the output signal decreases,
or if the amount of reactant decreases and the output signal
increases), I<0. In such case the sensor is determined to be
responding to un-reacted substance.
[0020] If I=0, there was no change in the reactant and therefore it
may be concluded that the injector is faulty.
[0021] Further, if V is much greater than 0 (i.e., V is greater
than some calibratable threshold), it may be concluded that both
the un-reacted substance, and un-reacted reactant are both being
measured and therefore a catalyst used to facilitate the reaction
is determined to be faulty.
[0022] Thus, from the signal I, a determination may be made as to
whether the sensor is responding to the un-reacted reactant, to the
un-reacted substance, and whether the injector or the catalyst is
faulty.
[0023] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0024] FIG. 1 is a diagrammatical sketch of an engine exhaust
system according to the invention having a processor programmed for
determining the effectiveness of injected urea in reducing NOx
produced by the engine;
[0025] FIG. 2 is a diagram showing the relationship between an
output signal produced by a sensor used in the exhaust system of
FIG. 1 as a function of either nitrogen monoxide (NO), nitrogen
dioxide (NO.sub.2) or urea;
[0026] FIG. 3 is a diagram showing the output signal produced by
the sensor used in the exhaust system of FIG. 1 in response to
increases and decreases in the amount of urea injected into the
engine exhaust in the system of FIG. 1; and
[0027] FIG. 4 is a flow diagram of the process used by the
processor in FIG. 1 to determine the effectiveness of injected urea
in reducing NOx produced by the engine exhaust in the system of
FIG. 1;
[0028] FIG. 5 is a functional block diagram of a NOx reduction
system according to another embodiment of the invention;
[0029] FIG. 6 are diagrams showing as a result of computer
simulations various parameters produced in the system of FIG. 5 for
an assumed Urea to NOx ratio correction factor of +0.32; and
[0030] FIG. 7 are diagrams showing as a result of computer
simulations various parameters produced in the system of FIG. 5 for
an assumed Urea to NOx ratio correction factor of -0.3.
[0031] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0032] Referring now to FIG. 1, is a diagrammatical sketch of an
engine exhaust system 10 is shown having a processor 12 programmed
for determining the effectiveness of injected urea in reducing
NO.sub.x produced by an engine 14. The system 10 includes a
reactant injector 16, here a urea injector, adapted to inject urea
into the exhaust 18 of the engine in response to an excitation
signal u(t), where t is time. The injection signal u(t) is made up
of two components: a nominal level u.sub.0 and a changeable
component (t). The nominal level u.sub.0 may be produced by
providing a sensor 13 in the engine exhaust upstream of the
injection of the urea. Thus, here u.sub.0=k_base*NOx1, where NOx1
is the signal produced by this upstream sensor and k_base is a
conversion factor stored in a look-up table 15 relating the a
priori assumed stoichiometric amount of urea needed to convert the
NOx in the upstream engine exhaust.
[0033] The exhaust 18, together with the injected urea, is directed
to a catalyst 20 though the engine's exhaust pipe. The catalyst 20
is used to facilitate the reaction between the NOx and the urea.
The injected urea reacts with NOx which may be present in the
exhaust 18 in the catalyst 20 to produce a product 22 which
includes the reaction products as well as any un-reacted portions
of the injected urea or un-reacted portions of the NOx.
[0034] As will be described, the processor 12 is also programmed to
determine whether the injector 16 is faulty and whether the
catalyst 20 is faulty.
[0035] More particularly, the output of the catalyst 22 is sensed
by a second NOx sensor 26. While it would be desirable that the
sensor 26 sense only the presence of any NOx at the output of the
catalyst, current practical automotive NOx sensors, as noted above,
produce an output signal in response to NOx and urea. Thus, these
sensors cannot differentiate between NOx and urea. That is, the
sensor 26 is a non-selective NOx sensor. This is illustrated from
FIG. 2. Curve 30 shows the relationship, i.e., transfer function,
between sensor 26 output signal (V, volts) and measured urea in the
presence of only urea. It is noted that the slope of curve 60 is
here k_urea. Curve 32 shows the relationship, i.e., transfer
function, between sensor 26 output signal (V, volts) and measured
NO in the presence of only NO. It is noted that the slope of curve
60 is here k_no Curve 34 shows the relationship, i.e., transfer
function, between sensor 26 output signal (V, volts) and measured
NO.sub.2 in the presence of only NO.sub.2. It is noted that the
slope of curve 60 is here k.sub.13 no2. (It is noted that the
response of the sensor to NOx is interpolated from the responses to
NO and NO2. For example a 50-50 mixture of NO and NO2 would result
in the slope (k_no+k_no2)/2. The family of interpolated slopes is
collectively referred to as k_nox.
[0036] We have discovered a method which enables differentiation
between NOx and urea detection using theses practical NOx
non-selective sensors. More particularly, we have discovered that a
urea excitation technique to be described allows one to determine
whether the non-selective sensor 26 is responding to NOx or to
urea. According to the method, the amount of urea injected into the
engine exhaust 18 is modulated, or changed in a particular, a
priori known direction, or sense. The output of the non-selective
NOx sensor 26 is processed by processor 12 along with knowledge of
the direction in the change in the amount of injected urea. The
result of the processing yields an indication of whether the sensor
is sensing urea or NOx by determining whether the change in urea
injection and the change in sensor 26 output signal are in the same
direction or in opposite directions.
[0037] Here, in the system shown in FIG. 1, a small, periodic
negative amplitude excitation signal (t) of duration Texc is
superimposed on the excitation signal u.sub.0 in a summer 28. The
summer 28 produces u(t) which provides the excitation signal for
the injector urea injector 16. Since during these negative
excursions in the excitation signal u(t) the amount of urea is
reduced, less NOx is being reduced, if any. If the sensor 26 output
signal, here a voltage V(t), goes lower in response to the negative
excursion in the excitation signal u(t), too much urea is being
injected into the exhaust 18 and the sensor 26 output signal change
{circumflex over (V)}(t) was due to un-reacted urea. If, on the
other hand, the sensor 26 produces a higher voltage V(t), not
enough urea was being injected into the exhaust to reduce all of
the NO.sub.x and the sensor output signal change {circumflex over
(V)}(t) was due to un-reacted NOx. Thus, if we reduce the amount of
urea and the voltage V produced by the sensor reduces, (i.e., the
direction of the change in urea is in the same direction in the
change in the output signal, V(t), of the sensor) the sensor 26 is
now known to be responding to urea. On the other hand, if we reduce
the amount of urea and the voltage V(t) produced by the sensor
increases, (i.e., the direction of the change in urea is in the
opposite direction to change in the output signal, V(t), of the
sensor) the sensor 26 is now known to be responding to NOx. FIG. 3
illustrates this effect.
[0038] The processor 12 (FIG. 1) is provided to detect the
direction of the change in injected urea u(t) relative to the
change in the output signal V(t) of the sensor 26. That is, the
processor determines whether the direction of the change in output
signal V(t) of the sensor 26 is the same as, or opposite to, the
direction of the change in injected urea u(t). As described above,
if they are the same direction, the sensor 26 is detecting urea
whereas if the directions are opposite one another the sensor 26 is
detecting NOx. Here such determination is made by multiplying a
signal representative of {circumflex over (V)}(t) (i.e., the change
in the output signal V(t) produced by the sensor 28) with the
change in the excitation signal (t). Thus, the change in the output
of the sensor, {circumflex over (V)}(t), is proportional to the
change in the output signal of the sensor V(t). Here, the
determination in {circumflex over (V)}(t) is made when the system
is in a steady-state condition which can be checked by monitoring
rate of change of rpm, load, space velocity, etc.
[0039] Thus, for time [t0-Tss, t0], we determine the steady-state
output voltage, Vss= 2 t0 - T s s t o V ( t ) t
[0040] Thus, during time from t0, to t0+D+Texc:
{circumflex over (V)}(t)=V(t)-Vss,
[0041] Where Vss is the steady state output voltage of the sensor
28 and where the excitation is repeated every Ttot seconds, where
Ttot>D+Tss+Texc where Tss is used to take into account the time
used to determine Vss.
[0042] If the mathematical product of {circumflex over (V)}(t) and
(t) is positive, the sensor 26 is determined by the processor 12 to
be detecting urea. If, on the other hand, the mathematical product
is negative, the sensor 26 is determined by the processor 12 to be
detecting NOx. In FIG. 1, the multiplication is shown by a
multiplier 41 which is fed {circumflex over (V)}(t) produced by the
high pass filter 47 (i.e., a signal proportional to the change in
sensor output signal) and by a signal representative of the change
in the urea excitation, i.e. (t). It should be understood that the
processor 12 is preferably a digital processor which performs the
process shown in FIG. 4.
[0043] It should be noted that due to adsorption and desorption of
urea in the catalyst 20 and reaction kinetics the time history
profile of the voltage V(t) produced by the sensor 26 will not be a
rapidly changing, pulse, but more a low pass filtered version of a
pulse. Further, due to transport delay (i.e., the delay between the
time the urea is injected into the exhaust 18 and the time the of
reaction in the catalyst 20), the voltage V(t) produced by the
sensor will be delayed an amount D from the commencement of the
change in the injection of the urea. The delay D may be mapped a
priori as a function of engine operating conditions, e.g., engine
speed using a look-up table.
[0044] Here, the output of the multiplier 41 is integrated in the
processor 12 as represented by integrator 43. Thus, if the
pulse-width of the change in the urea excitation signal (t) has a
pulse width Texc and starts at a time t0, the output of the
integrator 43 may be represented as: 3 I = t0 t0 + D + T e x c V ^
( t + D ) u ^ ( t ) t
[0045] Thus, if I>0, (i.e., if the integrated mathematical
product of {circumflex over (V)}(t) and (t) is positive), the
sensor 26 is detecting urea. Here, such condition is indicated by a
logic 1 produced by comparator 44. Thus, here the amount of urea is
V(t)*k_urea
[0046] If, on the other hand, I<0 (i.e., the integrated
mathematical product of {circumflex over (V)}(t) and (t) is
negative), the sensor 26 is detecting NO.sub.x. Here, such
condition is indicated by a logic 1 produced by comparator 46.
Thus, here the amount of NOx is V(t)*k_nox
[0047] Having discriminated between sensing urea and sensing NOx,
if I<0, the sensor 26 output signal V(t) can be fed to the
transfer function shown by curves 32 or 34 in FIG. 2 and a
measurement of the NOx may be obtained. If, one the other hand,
I.ltoreq.0, the sensor 26 output signal V(t) can fed to the
transfer function shown by curve 30 in FIG. 2 and a measurement of
the urea may be obtained.
[0048] Here, as noted above, the excitation is repeated every Ttot
seconds, where Ttot>D+Texc+Tss.
[0049] It should be noted that if I=0, the commanded change in urea
was not applied and a diagnosis is that of a faulty injector. Here,
such condition is indicated by a logic 1 produced by comparator
48.
[0050] If V>>0 (i.e., greater than some calibrate threshold),
one may conclude that both the engine exhaust NO.sub.x and the urea
are still measured after the catalyst 20 and one can conclude a
faulty catalyst 20. Here, such condition is indicated by a logic 1
produced by comparator 50.
[0051] It should be noted that since here we decrease the amount of
urea for a short time, an increase in tail pipe emissions may
result. This is the case if I<0. This can be compensated by
applying a positive correction+ after T0+T1+T2, when I<0. If
I>0, too much urea is being injected and a positive correction
is not required. This procedure keeps the overall tailpipe
emissions neutral.
[0052] It should also be noted that the procedure works best in the
steady state, when, as noted above, it is easy to determine
{circumflex over (V)}(t), by subtracting the mean over the previous
period. If knowledge of engine emissions and catalyst behavior is
very accurate, the method may be applied to transients. In this
case, it may be able to gain further information about the system
catalyst conditions, urea injection system, and NOx sensor by
measuring the transient sensor response.
[0053] Referring now to FIG. 4, the processor is programmed in
accordance with the flow diagram shown therein. Thus, a
determination is made as the open loop quantity of urea u.sub.0 to
be added. Next, a negative excitation signal is applied to the urea
injector for T2 seconds. A measurement is made as to the change in
the sensor 26 output signal. The signal I is computed as described
above. If I<0, the NOx transfer function is used and a positive,
increase, is made in the amount of urea. If, on the other hand,
I<0, the urea transfer function is used.
[0054] Having described a method and system for determining whether
the sensor 26 is responding to NOx or Urea, a method and system
using these techniques will be described which controls the amount
of urea to be injected into the engine exhaust to produce correct
stoichiometric urea and diagnostics.
[0055] Referring now to FIG. 5, a functional block diagram is shown
of a NOx reduction system 10'. Here a NOx sensor 60 is included
upstream of the point 19 where urea is added into the engine 14
exhaust 18 via injector 16. The NOx sensor 26 is disposed
downstream of the catalyst 20, as shown. As noted above in
connection with FIG. 1, the NOx sensors 26 and 60 are sensitive to
both urea and NOx. The catalyst 20 is positioned between the point
19 where the urea is injected and the position of the NOx sensor
26. Output signals nox1, nox2 (where nox2 is referred to as V in
connection with FIGS. 1-4) produced by sensors 60, 26 respectively
are processed by a programmed processor 12', in a manner to be
described, to produce the urea injection signal to urea injector
16. It is noted that part of the output nox2 is due to urea
slip.
[0056] Here, the processor 12' is shown by a functional block
diagram for purposes of understanding the signals processed by such
processor 12'. It should be understood that preferably the
processor 12' is a digital processor programmed to execute an
algorithm to be described below. Suffice it to say here that the
processor 40' includes a square wave generator 62 which produces an
excitation voltage u_exc (where u_exc was referred to as (t) in
connection with FIGS. 1-4) which may be represented as:
u.sub.--exc:=A.sub.--du*k.sub.--nox/k.sub.--urea if
t<0.5*T.sub.--du
u.sub.--exc:-=A.sub.--du if t>0.5*T.sub.--du;
[0057] where:
[0058] A_du is the amplitude of the negative portion of the steady
state excitation signal fed to the urea injector 16 (i.e., a
negative excitation signal reduces the amount of urea injected into
the engine exhaust);
[0059] T_du is the period of the excitation signal u_exc;
(T_du=Texc.times.2).
[0060] k_urea is the sensitivity of the NOx sensor 26 with respect
to urea;
[0061] k_NOx is the sensitivity of the NOx sensor 26 with respect
to NOx.
[0062] Thus, a correction factor k_nox/k_urea is inserted to
account for different sensitivities of the sensor 26 to urea and
NOx.
[0063] The output of the square wave generator 62 is fed, via a
switch 64, to a low pass filter 66. The function of the low pass
filter 66 is to take into account a reaction delay, D, between the
time urea is injected into the exhaust and the reaction in the
catalyst 20. The processor 40' provides this low pass filter 66
digitally in accordance with:
u.sub.--rk:=kf.sub.--rk*u.sub.--rk+(1-kf.sub.--rk)*u.sub.--exc,
[0064] where:
[0065] u_rk is the output of the low pass filter 66;
[0066] kf_rk is time constant of NOx-urea reaction kinetics.
[0067] The output signal produced by the sensor 26 is here, as
noted above, represented as: nox2. The signal nox2 is fed to a high
pass filter 42 to produce an output signal nox2_hp representing the
change in the output signal produced by the sensor 26. Here, the
processor 40 provides this high pass filter output in accordance
with:
[0068] nox2_hp:=nox2-nox2_lp; (where nox2_hp was referred to as
{circumflex over (V)}(t) in FIGS. 1-4).
[0069] where nox2_lp:=kf_lp_nox2*nox2_lp+(1-kf_lp_nox2)nox2;
[0070] where: kf_lp_nox2 is the filter gain for the low pass filter
nox2.
[0071] The output of the high pass filter 42 (which represents the
change in the output signal produced by the sensor 26) and the
signal produced by the low pass filter 66 (which represents the
change in the excitation signal u_exc to be fed to the urea
injector 16, in a manner to be described, are fed to a multiplier
41. It should be noted that if the direction of change in the
excitation signal u_rk is the same as the direction in the change
in the sensor 26 signal, the mathematical product produced by the
multiplier 41 will be positive. On the other hand, if the direction
of change in the excitation signal u_rk is opposite to the
direction in the change in the sensor 26 signal, the mathematical
product produced by the multiplier 41 will be negative.
[0072] The mathematical product produced by the multiplier 41 is
normalized by the signal nox1 produced by the sensor 60 in a
divider 45 to produce:
dydu:nox-hp*u.sub.--rk/nox1
[0073] The signal dydu is scaled by ki and integrated by integrator
43 to produce a correction signal k_corr in accordance with:
k_corr:=.intg.(ki*dydu)dt;
[0074] where: ki>0 is the integral gain used to correct the
nominal urea:NOx ratio (which is k_base)
[0075] Thus, if dydu>0 (i.e., the change in the sensor 26 and
the change in the urea excitation are the in the same direction),
the amount of urea should be reduced whereas, if dydu<0, (i.e.,
the change in the sensor 26 and the change in the urea excitation
are the in opposite directions),the amount of urea should be
increased.
[0076] The signal k_corr is used to add (if k_corr>0) or
subtract from (if k_corr<0) the nominal a priori determined
amount of urea for correct stoichiometry which is k_base*nox1. The
actual amount of urea added by the injector is
k_base*nox1*k_injector, where k_injector is the injector transfer
function which is unknown because of aging, etc. Thus, with the
processor 40, the signal k_corr together with u_exc will both
modulate the signal k_base*nox1 to produce the correct
stoichiometric urea independent of k_injector. and thus
automatically adjust the amount of urea which should be added to
the exhaust.
[0077] More particularly, the final urea quantity to be applied via
the injector 16 is:
u.sub.--tot.sub.--ppm:=(k.sub.--base+k.sub.--corr)*nox1+u.sub.--exc;
[0078] where k_base is the nominal urea:NOx ratio.
[0079] To put it another way, a priori determined injection signal
u.sub.0=nox1*k_base is modulated by both the correction signal
k_corr*nox1 and the square wave signal u_exc. At correct
stoichiometry, (k_corr+k_base)*k_injector*nox1 results in the
injector 16 delivering stoichiometric urea to the engine exhaust
upstream of the catalyst 20.
[0080] The signal u_tot_ppm is in parts per million of urea and is
converted to mg/sec of urea by using mass air flow (Maf) of the
exhaust, the upstream temperature (TMP) of the catalyst and the
fuel flow (Wf) in a converter 61.
[0081] At the end of each period T_du, the integral int_dydu_last
is evaluated in a comparator 47 as follows: 4 t - T _ d u t y u ( t
) t
[0082] between the limits t-T_du and t, where t is the current
time. The integral represents the incremental correction to the
urea:NOx ratio accumulated over the last excitation period T_du. If
int_dydu_last<k_dydu_thres, (where k_dydu_thres is the threshold
for correction contribution to determine whether further adjustment
is needed), this incremental correction is considered small enough
to terminate adjusting the urea and the switch 64 is open from its
initial closed condition. Otherwise, the switch 64 remains
closed.
[0083] If k_corr>k_corr_lmx, where k_corr_lmx is the maximum
limit for the correction factor k_corr to be declared a failure,
(i.e., a blocked injector 16 or a catalyst 20 failure) as
determined by comparator 47, a system failure has occurred (i.e.,
the catalyst 20 is inactive or the injector 16 is blocked).
[0084] If k_corr<k_corr_lmn<0, where k_corr_lmn is the
minimum limit for the correction factor k_corr to declare a system
failure for a leaking urea injector, as determined by comparator
49, a system failure has occurred, i.e., the injector is leaking,
and excess urea will shorten the life of the exhaust system. For
example, for detection of a 50 percent increase, k_corr_lmx is
set=0.5.
[0085] Referring now to FIG. 6, such FIG. 6 shows, as a result of
computer simulations various parameters produced in the system of
FIG. 5 for a resulting Urea to NOx ratio correction factor of +0.32
(i.e., k_base=0.68) FIG. 7 shows, as a result of computer
simulations various parameters produced in the system of FIG. 5 for
a resulting Urea to NOx ratio correction factor of -0.3 (i.e.,
k_base=1.3).
[0086] The process described above in connection with FIG. 5 may be
summarized as follows:
[0087] The following measured inputs are used:
[0088] Nox1:Nox sensor signal measured before the SCR brick (in
ppm)
[0089] Nox2:Nox sensor signal measured after the SCR brick (part of
this output is due to urea slip) (in ppm)
[0090] MAF:mass air flow
[0091] T1:temperature upstream of the SCR brick
[0092] Wf:fuel flow
[0093] (Maf, T1 and Wf are used to convert the urea ppm quantity to
a quantity in mg/sec.)
[0094] The following gains are used and to be calibrated based on
experimental data. They may dependent on engine operating
conditions (speed and load) and exhaust temperature.
[0095] A_du:the amplitude of the negative part of the
excitation
[0096] T_du:the period of the excitation is T_du
[0097] k_urea:sensitivity of the Nox sensor wrt urea
[0098] k_nox:sensitivity of the Nox sensor wrt urea
[0099] (A_du, k_urea and k_nox determine the amplitude of the
positive part of the excitation).
[0100] kf_lp_nox2:filter gain for low pass filtered nox2.
[0101] kf_rk:time constant of nox-urea reaction kinetics.
[0102] k_base:nominal urea:nox ratio
[0103] ki:integral gain to correct the nominal urea:nox ratio
[0104] k_dydu_thres:threshold for correction contribution to
determine whether further adjustment is needed.
[0105] k_corr_lmx:maximum limit for correction factor to declare an
OBD failure (blocked injector or catalyst malfunction).
[0106] k_corr_lmn:minimum limit for correction factor to declare a
system failure (leaking injector).
[0107] As noted above, the processor 12' is a digital processor.
The following flow diagram describes the program executed by such
digital processor:
THE PROCESS FLOW
[0108] Step 1 The excitation signal is the repetitive signal with
period T_du defined by:
[0109] u_exc:=A_du*k_nox/k_urea if t<0.5*T_du
[0110] u_exc:=-A_du if t>0.5*T_du
[0111] The correction factor k_nox/k_urea is inserted to account
for different sensitivities to Nox and urea. These gains follow
from sensor characteristics.
[0112] Step 2 Compute the switch on_logic (specified later).
[0113] Step 3 If on_logic=TRUE, apply u_exc, otherwise don't and
return to 1.
[0114] Step 4 Compute the excitation signal delayed by reaction
kinetics:
[0115] u_rk :=kf_rk*u_rk+(1-kf_rk)*u_exc.
[0116] Step 5 Obtain the measurement voltage from the second Nox
sensor, nox2.
[0117] Step 6 Compute the low pass filtered version nox2_lp, with
filter constant kf_lp_nox2:
[0118] nox2_lp:=kf_lp_nox2*nox2_lp+(1-kf_lp_nox2)*nox2.
[0119] Step 7 Compute the high pass filtered
[0120] nox2_hp:=nox2-nox2_lp.
[0121] Step 8 Obtain the measurement voltage from the first Nox
sensor:nox1.
[0122] Step 9 Compute the convolution:
[0123] dydu:=nox2_hp*u_rk/nox1
[0124] Step 10 Integrate the convolution into a correction
factor:
[0125] k_corr:=k_corr+ki*dydu
[0126] Step 11 At the end of every period T_du evaluate the
integral
[0127]
int_dydu_last:=ki*.intg..sup.t.sub.t-T.sub..sub.--.sub.dudydu(t) dt
between the limits t-T_du and t, where t is the current time. This
integral represents the incremental correction to the urea:nox
ratio accumulated over the last excitation period. If
int_dydu_last<k_dydu_t- hres, this incremental correction is
small enough to stop adjusting, and set on_logic=false.
[0128] Otherwise on_logic is true.
[0129] Step 11 If
[0130] k_corr>k_corr_lmx
[0131] a system failure has occurred:the catalyst is inactive or
the injection system is blocked.
[0132] If
[0133] k_corr<k_corr_lmn
[0134] a system failure has occurred: the injection system is
leaking. The limit k_corr_lmx corresponds directly to OBD limits.
For detection of a 50% increase in a regulated exhaust component,
set k_corr_lmx=0.5.
[0135] Step 12 The final urea quantity to be applied is:
[0136] u_tot_ppm:=(k_base+k_corr)*nox1+u_exc.
[0137] Step 13 Use MAF, Wf, T1 to convert u_tot_ppm to a quantity
in mg/sec: u_tot_mgsec.
[0138] Step 14 Go to Step 1.
[0139] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, other embodiments are within
the scope of the following claims.
* * * * *