U.S. patent application number 13/443181 was filed with the patent office on 2012-10-11 for particulate matter treatment system.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Shinichi Mitani, Eiji Murase, Hiroshi Nomura.
Application Number | 20120255284 13/443181 |
Document ID | / |
Family ID | 46965021 |
Filed Date | 2012-10-11 |
United States Patent
Application |
20120255284 |
Kind Code |
A1 |
Mitani; Shinichi ; et
al. |
October 11, 2012 |
PARTICULATE MATTER TREATMENT SYSTEM
Abstract
A particulate matter treatment system includes an electrode
provided in an exhaust passage of an internal combustion engine, a
power supply connected to the electrode and operable to apply a
voltage to the electrode, a particle number detector that detects
the number of particles of particulate matter downstream of the
electrode, and a determining device that determines that the system
is at fault when an absolute value of the amount of change in the
number of particles of particulate matter detected by the particle
number detector when the voltage applied from the power supply to
the electrode is changed is smaller than a threshold value.
Inventors: |
Mitani; Shinichi;
(Susono-shi, JP) ; Nomura; Hiroshi; (Gotenba-shi,
JP) ; Murase; Eiji; (Gotenba-shi, JP) |
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
46965021 |
Appl. No.: |
13/443181 |
Filed: |
April 10, 2012 |
Current U.S.
Class: |
60/275 |
Current CPC
Class: |
Y02T 10/47 20130101;
F01N 2560/05 20130101; F01N 11/00 20130101; F01N 2550/04 20130101;
F01N 2560/12 20130101; Y02T 10/40 20130101 |
Class at
Publication: |
60/275 |
International
Class: |
F01N 3/01 20060101
F01N003/01 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 11, 2011 |
JP |
2011-087257 |
Claims
1. A particulate matter treatment system, comprising: an electrode
provided in an exhaust passage of an internal combustion engine; a
power supply connected to the electrode and operable to apply a
voltage to the electrode; a particle number detector that detects
the number of particles of particulate matter downstream of the
electrode; and a determining device that determines that the
particulate matter treatment system is at fault when an absolute
value of an amount of change in the number of particles of
particulate matter detected by the particle number detector when
the voltage applied from the power supply to the electrode is
changed is smaller than a threshold value.
2. The particulate matter treatment system according to claim 1,
wherein the threshold value is changed according to the number of
particles detected by the particle number detector.
3. The particulate matter treatment system according to claim 2,
wherein the threshold value is set to a larger value as the number
of particles detected by the particle number detector is
larger.
4. The particulate matter treatment system according to claim 1,
further comprising an exhaust gas amount detector that detects or
estimates an amount of exhaust gas emitted from the internal
combustion engine, wherein the threshold value is changed according
to the amount of exhaust gas detected by the exhaust gas amount
detector.
5. The particulate matter treatment system according to claim 4,
wherein the threshold value is set to a larger value as the amount
of exhaust gas detected by the exhaust gas amount detector is
smaller.
6. The particulate matter treatment system according to claim 2,
further comprising an exhaust gas amount detector that detects or
estimates an amount of exhaust gas emitted from the internal
combustion engine, wherein the threshold value is changed according
to the amount of exhaust gas detected by the exhaust gas amount
detector.
7. The particulate matter treatment system according to claim 6,
wherein the threshold value is set to a larger value as the amount
of exhaust gas detected by the exhaust gas amount detector is
smaller.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to Japanese Patent
Application No. 2011-087257 filed on Apr. 11, 2011, which is
incorporated herein by reference in its entirety including the
specification, drawings and abstract.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a particulate matter treatment
system.
[0004] 2. Description of Related Art
[0005] For example, Japanese Patent Application Publication No.
2006-194116 (JP-A-2006-194116) discloses a technology of providing
a discharge electrode in an exhaust passage of an internal
combustion engine, and charging particulate matter (which will also
be called "PM") by allowing the discharge electrode to generate
corona discharge, so as to aggregate the PM. With the PM thus
aggregated, the number of particles of the PM can be reduced. Also,
the particle size of the PM is increased, so that the PM can be
more easily trapped by a filter provided downstream of the
electrode.
[0006] In some cases, the particulate matter treatment system fails
or deteriorates, which makes it difficult to aggregate the PM.
Accordingly, it is also important to detect a failure of the
system.
SUMMARY OF THE INVENTION
[0007] The invention provides a particulate matter treatment system
that detects a failure of the system.
[0008] A particulate matter treatment system according to one
aspect of the invention includes an electrode provided in an
exhaust passage of an internal combustion engine, a power supply
connected to the electrode and operable to apply a voltage to the
electrode, a particle number detector that detects the number of
particles of particulate matter downstream of the electrode, and a
determining device that determines that the particulate matter
treatment system is at fault when an absolute value of an amount of
change in the number of particles of particulate matter detected by
the particle number detector when the voltage applied from the
power supply to the electrode is changed is smaller than a
threshold value.
[0009] When a voltage is applied to the electrode, the PM can be
charged. The charged PM moves toward the inner wall of the exhaust
passage under coulomb force and flow of exhaust gas. The PM that
has reached the inner wall of the exhaust passage releases
electrons to the exhaust passage, so that electricity flows to the
ground side rather than the electrode. Then, the PM particles that
have released electrons aggregate or clump together with other PM
particles present in the vicinity of the above PM, so that the
number of particles can be reduced.
[0010] The number of particles of particulate matter detected by
the particle number detector when a voltage is applied to the
electrode is the number of particles detected after aggregation of
the PM. If the voltage applied to the electrode is increased, a
larger number of electrons are released from the electrode. As a
result, the aggregation of the PM can be promoted, and the number
of PM particles can be further reduced. Namely, when the
particulate matter treatment system is normal, the number of
particles of particulate matter detected by the particle number
detector changes according to the voltage applied to the electrode.
Accordingly, when the voltage applied to the electrode is changed,
the number of PM particles detected by the particle number detector
should change if the particulate matter treatment system is normal.
The absolute value of the amount of change in the number of PM
particles responsive to a change of the applied voltage is
relatively large if the particulate matter treatment system is
normal, and is relatively small if the system is at fault. Namely,
even if an attempt to change the voltage applied to the electrode
is made, the voltage may not actually change, or the change of the
voltage may be insufficient when the particulate matter treatment
system is at fault. For example, when no voltage is applied to the
electrode due to a failure, the voltage does not actually change
even if an attempt to change the voltage applied to the electrode
is made, and the amount of change in the number of PM particles is
equal to zero.
[0011] Thus, if the particulate matter treatment system is at
fault, the aggregated PM is reduced. As a result, the absolute
value of the amount of change in the number of particles of
particulate matter detected by the particle number detector when
the voltage is changed becomes relatively small. Accordingly, it
can be determined that the system is at fault when the absolute
value of the amount of change in the number of PM particles is
smaller than the threshold value. The threshold value may be set to
a lower limit value of the absolute value of the amount of change
in the number of particles when the particulate matter treatment
system is normal. It may be determined that the particulate matter
treatment system is at fault when the number of PM particles
detected by the particle number detector does not change before and
after the applied voltage is changed.
[0012] When a failure detection is performed, the amount of change
in the number of particles can be promptly detected by positively
changing the voltage applied to the electrode; therefore, the
process of making a failure decision can be promptly completed.
Also, a failure decision may be made based on an absolute value of
the amount of change in the number of particles while the applied
voltage is varied within a preset range.
[0013] With the voltage thus changed, the number of particles
detected by the particle number detector may increase or may
decrease. The amount of change is a positive value when the number
of particles increases, and is a negative value when the number of
particles decreases. Therefore, the absolute value of the amount of
change is used. It may also be determined that the particulate
matter treatment system is at fault when the amount of change in
the number of particles is within a specified range that ranges
from a negative value to a positive value.
[0014] According to the above aspect of the invention, a failure of
the particulate matter treatment system can be detected.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Features, advantages, and technical and industrial
significance of exemplary embodiments of the invention will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements, and wherein:
[0016] FIG. 1 is a view schematically showing the construction of a
particulate matter treatment system according to a first embodiment
of the invention;
[0017] FIG. 2 is a flowchart illustrating the control flow of
failure determination according to the first embodiment;
[0018] FIG. 3 is a view concerned with a second embodiment of the
invention, showing the relationships among the applied voltage, the
number of PM particles, and the percentage of reduction of the
number of PM particles; and
[0019] FIG. 4 is a view concerned with the second embodiment of the
invention, showing the relationships among the applied voltage, the
number of PM particles, and the percentage of reduction of the
number of PM particles.
DETAILED DESCRIPTION OF EMBODIMENTS
[0020] FIG. 1 schematically shows the construction of a particulate
matter treatment system 1 according to a first embodiment of the
invention. The particulate matter treatment system 1 is provided in
an exhaust passage 2 of a gasoline engine, for example. The
particulate matter treatment system 1 may also be provided in an
exhaust passage of a diesel engine.
[0021] The particulate matter treatment system 1 includes a housing
3 connected at opposite ends to an exhaust passage 2. The housing 3
is formed of stainless steel. The housing 3 is formed in the shape
of a circular cylinder having a larger diameter than that of the
exhaust passage 2. The opposite end portions of the housing 3 are
tapered such that the cross-sectional area of the housing 3
decreases toward the opposite ends thereof. In operation, exhaust
gas flows in the direction of the arrow indicated in FIG. 1, and
flows into the housing 3. Thus, the housing 3 may be regarded as a
part of the exhaust passage 2.
[0022] The exhaust passage 2 and the housing 3 are connected to
each other via insulating portions 4. Each of the insulating
portions 4 is sandwiched by and between a flange 21 formed on an
end portion of the exhaust passage 2, and a flange 31 formed on an
end portion of the housing 3. The exhaust passage 2 and the housing
3 are fastened to each other with, for example, bolts and nuts. The
bolts and nuts are also subjected to electric insulating treatment,
so that electricity does not flow through the bolts and nuts. With
this arrangement, no electricity flows between the exhaust passage
2 and the housing 3.
[0023] An electrode 5 is mounted in the housing 3. The electrode 5
penetrates a side face of the housing 3, and extends from the side
face of the housing 3 toward the center axis of the housing 3.
Then, the electrode 5 bends in a direction opposite to the
direction of flow of exhaust gas at around the center axis, and
extends toward the upstream side of the flow of exhaust gas, in
parallel with the center axis. Thus, an end portion of the
electrode 5 is located in the vicinity of the center axis of the
housing 3. The electrode 5 is provided with an insulator portion 51
consisting of an electric insulator, so that no electricity flows
between the electrode 5 and the housing 3. The insulator portion 51
is located between the electrode 5 and the housing 3, and has the
functions of insulating electricity, and also fixing the electrode
5 to the housing 3.
[0024] The electrode 5 is connected to a power supply 6 via a
power-supply-side electric wire 52. The power supply 6 applies
current to the electrode 5, and is able to change the applied
voltage. The power supply 6 is connected to a controller 7 and a
battery 8 via electric wires. The controller 7 controls the voltage
applied from the power supply 6 to the electrode 5.
[0025] Also a ground-side electric wire 53 is connected to the
housing 3, and the housing 3 is grounded via the ground-side
electric wire 53. The ground-side electric wire 53 is provided with
a detector 9 that detects current passing through the ground-side
electric wire 53. The detector 9 detects the current by measuring a
potential difference between opposite ends of a resistor provided
at some midpoint in the ground-side electric wire 53. The detector
9 is connected to the controller 7 via an electric wire, so that
the controller 7 receives the current detected by the detector 9.
The power-supply-side electric wire 52 may be provided with a
detector, like the detector 9, for detecting current passing
through the power-supply-side electric wire 52.
[0026] A particle number sensor 75 that detects the number of
particles of PM (particulate matter) in exhaust gas is provided in
the exhaust passage 2 downstream of the housing 3. The particle
number sensor 75 detects the number of particles of PM per unit
volume in the exhaust gas. The particle number sensor 75 is
connected to the controller 7 via an electric wire, so that the
controller 7 receives the number of particles of PM detected by the
particle number sensor 75. In this embodiment, the particle number
sensor 75 corresponds to the above-indicated particle number
detector of the invention.
[0027] An acceleration stroke sensor 71, a crank position sensor
72, a temperature sensor 73, and an airflow meter 74 are connected
to the controller 7. The acceleration stroke sensor 71 generates an
electric signal representing the amount of depression of the
accelerator pedal by the driver of the vehicle in which the
internal combustion engine is installed, so as to determine the
engine load. The crank position sensor 72 detects the engine speed.
The temperature sensor 73 detects the temperature of a coolant or a
lubricating oil of the internal combustion engine, so as to
determine the temperature of the internal combustion engine. The
airflow meter 74 detects the amount of intake air of the internal
combustion engine.
[0028] In the particulate matter treatment system 1 constructed as
described above, a negative DC high voltage is applied from the
power supply 6 to the electrode 5, so that electrons are released
from the electrode 5. Namely, the potential of the electrode 5 is
made lower than that of the housing 3, so that electrons are
released from the electrode 5. Then, the electrons thus released
can negatively charge PM contained in the exhaust gas. The
negatively-charged PM moves under the influences of coulomb force
and gas flow. Then, when the PM reaches the housing 3, the
electrons that negatively charged the PM are released to the
housing 3. The PM particles that have released electrons to the
housing 3 aggregate or clump together so that the particle size of
the PM is increased. With the PM thus aggregated, the number of
particles of PM is reduced. Namely, it is possible to increase the
particle size of the PM and reduce the number of particles of the
PM, by applying a voltage to the electrode 5.
[0029] While the electrode 5 is bent to the upstream side, namely,
in the direction opposite to the direction of flow of the exhaust
gas, in this embodiment, the electrode 5 may be bent to the
downstream side, namely, in the direction of flow of the exhaust
gas. If the electrode 5 is bent toward the upstream side of the
flow of exhaust gas as in this embodiment, the PM is less likely or
unlikely to be deposited on the insulator portion 51. Namely, the
PM can be charged at the upstream side of the insulator portion 51,
and the charged PM is directed onto the inner circumferential
surface of the housing 3. Therefore, the amount of PM that collides
with the insulator portion 51 is reduced, and thus the PM is less
likely or unlikely to be deposited on the insulator portion 51.
However, if the electrode 5 is bent toward the upstream side of the
flow of exhaust gas, the electrode 5 is more likely to be deformed
when receiving force from the flow of the exhaust gas. Therefore,
the arrangement of this embodiment is suitable for the case where
the electrode 5 is short. On the other hand, if the electrode 5 is
bent toward the downstream side of the flow of exhaust gas, the PM
is more likely to be deposited on the insulator portion 51, but the
electrode 5 is less likely or unlikely to be deformed even if it
receives force from the flow of the exhaust gas. In this case,
therefore, the durability and reliability of the electrode 5 can be
enhanced, and the length of the electrode 5 can be increased.
[0030] The controller 7 is configured to change the voltage applied
to the electrode 5, and determine whether the particulate matter
treatment system 1 is at fault, based on the amount of change in
the number of PM particles detected by the particle number sensor
75 before and after the change of the applied voltage. In this
embodiment, an absolute value of the amount of change is used as a
failure criterion.
[0031] If the particulate matter treatment system 1 is normal,
electrons are released from the electrode 5 when a voltage is
applied to the electrode 5, resulting in aggregation of the PM
particles. As the applied voltage becomes larger, a larger number
of electrons are released from the electrode 5, and therefore, the
amount of aggregated particles in the PM increases. Namely, if the
particulate matter treatment system 1 is normal, the amount of
change in the number of PM particles when the applied voltage is
changed becomes relatively large.
[0032] If, on the other hand, the particular matter treatment
system 1 is at fault, no electrons may be released from the
electrode 5, or the amount of release of electrons may not be
sufficiently large. As a result, the amount of aggregated PM
particles is reduced, and therefore, the amount of change in the
number of PM particles when the applied voltage is changed becomes
relatively small. For example, when a failure or problem that no
voltage is applied to the electrode 5 occurs, the applied voltage
does not change but remains zero even if an attempt to change the
applied voltage is made. Therefore, the number of PM particles
detected by the detector 9 is equal to the same value before and
after the applied voltage is changed. Namely, the amount of change
in the number of PM particles is equal to zero.
[0033] Accordingly, if a threshold value is set for the amount of
change in the number of PM particles when the applied voltage is
changed, it can be determined that the particulate matter treatment
system 1 is at fault when the detected amount of change is smaller
than the threshold value. The threshold value is determined in
advance by experiment, or the like, as a lower limit of the amount
of change in the number of PM particles when the particulate matter
treatment system 1 is normal.
[0034] FIG. 2 is a flowchart illustrating the control flow of
failure determination according to this embodiment. The routine of
FIG. 2 is repeatedly executed by the controller 7 at given time
intervals.
[0035] In step S101, operating conditions of the internal
combustion engine are acquired. For example, values, such as those
of the engine speed, engine load, and the temperature of the
internal combustion engine, which will be required for subsequent
processing are read. The engine speed is detected by the crank
position sensor 72, and the engine load is detected by the
acceleration stroke sensor 71. The temperature of the engine (e.g.,
the temperature of the lubricating oil or the temperature of the
coolant) is detected by the temperature sensor 73.
[0036] In step S102, a voltage applied to the electrode 5 is
calculated. The applied voltage is set in accordance with the
estimated number of PM particles (per cm.sup.3), for example. The
number of PM particles is the number of PM particles discharged
from the internal combustion engine, which number is measured
before the exhaust gas flows into the housing 3. The number of PM
particles is correlated with the engine speed, engine load, and the
temperature of the internal combustion engine (e.g., the
temperature of the lubricating oil or the temperature of the
coolant), and is therefore calculated based on these values. A
plurality of maps for calculating the number of PM particles, from
the engine speed and the engine load, may be stored in relation to
the temperature of the engine, and the number of PM particles may
be calculated based on a selected one of the maps.
[0037] A sensor that detects the number of PM particles may be
mounted in the exhaust passage 2 upstream of the housing 3, and the
number of PM particles may be detected by the sensor. Also, a
detection value of the particle number sensor 75 obtained when no
voltage is applied to the electrode 5 may be used.
[0038] Then, the applied voltage is calculated based on the number
of PM particles and the amount (g/sec.) of exhaust gas of the
internal combustion engine. This relationship may be obtained in
advance by experiment, or the like, and may be mapped. The amount
of exhaust gas of the engine is correlated with the amount of
intake air of the engine, and can be thus obtained based on the
intake air amount detected by the airflow meter 74.
[0039] As the amount of exhaust gas is smaller, the inertial force
of the PM is reduced, and therefore, an influence of electrostatic
actions is relatively increased. As a result, the PM particles are
more likely to aggregate. Accordingly, as the amount of exhaust gas
is smaller, the PM particles aggregate with a smaller voltage
applied to the electrode 5. Therefore, the applied voltage is
reduced as the amount of exhaust gas is smaller. As the number of
PM particles is larger, the distance between the PM particles
becomes shorter, and therefore, an influence of electrostatic
actions is relatively increased. Therefore, as the number of PM
particles is larger, the PM particles aggregate with a smaller
voltage applied to the electrode 5. Thus, the applied voltage is
reduced as the number of PM particles is larger.
[0040] The applied voltage may also be set to a value at which the
percentage of reduction of the number of PM particles becomes equal
to a give value (e.g., 40%). The percentage of reduction of the
number of PM particles is the ratio of the number of PM particles
reduced in the housing 3 to the number of PM particles flowing into
the housing 3. Also, the applied voltage may be set to a
predetermined, specified value. After the applied voltage is
calculated in this manner, the voltage is applied to the electrode
5, and the control goes to step S103.
[0041] In step S103, it is determined whether it is the time for
making a failure decision. For example, the determination of step
S103 is made by determining whether the accumulated value of the
operating time of the internal combustion engine as measured from
the time when a failure decision was made the last time reaches a
predetermined value. Namely, a failure decision is made each time
the accumulated value of the operating time reaches the
predetermined value. For example, it may be determined that it is
the time for making a failure decision when the accumulated
operating time of the engine is a multiple of the predetermined
value. Also, it may be determined that it is the time for making a
failure decision when the travel or running distance of the vehicle
on which the engine is installed is equal to a multiple of a
predetermined value. The above-indicated predetermined values are
set in advance as values at which a failure decision is required to
be made. If an affirmative decision (YES) is made in step S103, the
control goes to step S104. If a negative decision (NO) is made in
step S103, the routine of FIG. 2 ends.
[0042] In step S104, it is determined whether the internal
combustion engine is in steady operation. The number of PM
particles varies depending on the operating conditions of the
engine; therefore, it may be difficult to make a failure decision
if the operating conditions of the engine change while a failure
decision is being made. Accordingly, in this embodiment, a failure
decision is made during steady operation of the engine. It is
determined that the engine is in steady operation, when the amounts
of change of the engine speed obtained by the crank position sensor
72 and the engine load obtained by the acceleration stroke sensor
71 over a given period of time are within predetermined ranges in
which the engine can be said to be in a steady state. If an
affirmative decision (YES) is made in step S104, the control goes
to step S105. If a negative decision (NO) is made in step S104, the
routine of FIG. 2 ends.
[0043] In step S105, the number of PM particles detected by the
particle number sensor 75 is obtained. In this step, the number of
PM particles before the applied voltage is changed is detected. The
number of PM particles obtained in this step is denoted as
"pre-voltage-change number of particles PM1".
[0044] In step S106, the applied voltage is changed. For example,
the applied voltage is reduced by a predetermined value. The
applied voltage may also be set to zero. As a result, the PM is
less likely to aggregate, and therefore, the number of PM particles
detected by the detector 9 increases. The applied voltage may be
increased, so that the PM is more likely to aggregate, and the
number of PM particles is reduced.
[0045] In step S107, the number of PM particles detected by the
particle number sensor 75 is obtained. In this step, the number of
PM particles after the applied voltage is changed is detected. The
number of PM particles obtained in this step is denoted as
"post-voltage-change number of particles PM2".
[0046] In step S108, the amount of change in the number of PM
particles is calculated. This amount is expressed as an absolute
value. Namely, the amount of change is calculated according to the
following equation (1).
Amount of Change=|PM2-PM1| (1)
[0047] In step S109, it is determined whether the amount of change
in the number of PM particles calculated in step S108 is smaller
than a threshold value. In this step, it is determined whether the
number of PM particles is sufficiently changed due to the change of
the applied voltage. The threshold value is a lower limit of the
amount of change in the number of PM particles when the particulate
matter treatment system 1 is normal, and is obtained in advance by
experiment, or the like. If an affirmative decision (YES) is made
in step S109, the control goes to step S110. If a negative decision
(NO) is made in step S109, the routine of FIG. 2 ends since no
failure is found in the particulate matter treatment system 1. In
this embodiment, the controller 7 that executes step S109
corresponding to the determining device of the invention.
[0048] In step S110, a failure flag is set to ON. The failure flag
is set to ON when the particulate matter treatment system 1 is at
fault, and is set to OFF when the system 1 is not at fault. The
initial value of the failure flag is OFF. If the failure flag is
set to ON, a warning light is turned on so as to inform the driver
of the vehicle of the presence of a failure.
[0049] As explained above, according to this embodiment, a failure
of the particulate matter treatment system 1 can be determined,
based on the amount of change in the number of PM particles
calculated from the numbers of PM particles before and after the
applied voltage is changed. Also, the current or applied voltage
need not be detected when a failure decision is made. It is
possible to enhance the accuracy of the failure determination by
changing the applied voltage two or more times, and calculating the
amount of change in the number of PM particles two or more times.
It is also possible to enhance the accuracy of the failure
determination by calculating the amount of change in the number of
PM particles at each applied voltage when the applied voltage is
reduced or increased in steps.
[0050] Next, a second embodiment of the invention will be
described. In the second embodiment, the threshold value used in
step S109 of FIG. 2 is changed based on at least one of the number
of PM particles and the amount of exhaust gas of the internal
combustion engine. In the second embodiment, the devices, etc.
other than the controller 7 are identical with those of the first
embodiment, and therefore, will not be explained.
[0051] Initially, the case where the threshold value is changed
according to the number of PM particles (the number of particles
detected by the particle number detector) will be explained.
[0052] The threshold value may be changed according to the number
of particles detected by the particle number detector when no
voltage is applied from the power supply to the electrode. Also,
the threshold value may be changed according to the number of
particles of particulate matter discharged from the internal
combustion engine. The number of particles of the particulate
matter discharged from the engine may be detected by a sensor, or
may be estimated based on the operating conditions of the engine.
As the number of PM particles is larger, the distance between the
PM particles becomes shorter, and an influence of electrostatic
actions is relatively increased. Therefore, as the number of PM
particles is larger, the PM particles aggregate at a smaller
applied voltage. Accordingly, as the number of PM particles is
larger, an absolute value of the amount of change in the number of
particles when the applied voltage is changed is increased. Thus,
the threshold value can be changed in accordance with the number of
PM particles. Namely, since the PM particles are more likely to
aggregate as the number of PM particles is larger, the absolute
value of the amount of change in the number of PM particles should
become larger if the particulate matter treatment system is normal.
Therefore, the threshold value can be increased as the number of PM
particles is larger, and can be reduced as the number of PM
particles is smaller. Consequently, the accuracy with which a
failure decision is made can be further enhanced.
[0053] The above-described situation will be described in greater
detail with reference to FIG. 3. FIG. 3 is a graph indicating the
relationships among the applied voltage, the number of PM
particles, and the percentage of reduction of the number of PM
particles. FIG. 3 shows four patterns of relationships (indicated
by circles, triangles, diamonds, and inverted triangles) involving
different numbers of PM particles flowing into the housing 3. The
applied voltage is equal to 0 (kV) when application of voltage to
the electrode 5 is stopped. Namely, the number of PM particles when
the applied voltage is 0 (kV) corresponds to the number of PM
particles flowing into the housing 3. The number of PM particles
flowing into the housing 3 is largest in the case of the circles
indicated in FIG. 3, and becomes smaller in the order of the
triangles, diamonds, and inverted triangles as indicated in FIG.
3.
[0054] In the case of the circles in which the number of PM
particles flowing into the housing 3 is largest, the percentage of
reduction of the number of PM particles reaches the highest level
among the four patterns indicated in FIG. 3. On the other hand, in
the case of the inverted triangles in which the number of PM
particles flowing into the housing 3 is smallest, the percentage of
reduction of the number of PM particles is lowest among the four
patterns. It is thus understood from FIG. 3 that the percentage of
reduction of the number of PM particles increases as the number of
PM particles flowing into the housing 3 is larger. It is also
understood that the amount of change in the number of PM particles
when the applied voltage is changed is larger as the number of PM
particles flowing into the housing 3 is larger.
[0055] As the number of PM particles is larger, the distance
between the PM particles becomes shorter, and an influence of
electrostatic actions is relatively increased. Therefore, as the
number of PM particles is larger, the PM particles aggregate at a
smaller applied voltage. Thus, if the particulate matter treatment
system 1 is normal, the amount of change in the number of PM
particles when the applied voltage is changed increases as the
number of PM particles flowing into the housing 3 is larger. The
number of PM particles can vary due to individual differences among
internal combustion engines and/or chronological changes of the
internal combustion engine.
[0056] Accordingly, the accuracy of the failure determination can
be enhanced by changing the threshold value according to the number
of PM particles. If the particulate matter treatment system 1 is
normal, the amount of change in the number of PM particles when the
applied voltage is changed should increase as the number of PM
particles is larger. Therefore, the threshold value is increased as
the number of PM particles is larger. For example, the relationship
between the number of PM particles and a correction factor is
obtained in advance by experiment, or the like. Then, the
correction factor is obtained from the detected or estimated number
of PM particles upstream of the housing 3, and the threshold value
is changed by multiplying the threshold value by the correction
factor.
[0057] Thus, the accuracy of the failure determination can be
enhanced by increasing the threshold value as the number of PM
particles is larger.
[0058] The amount of change in the number of PM particles when the
applied voltage is changed also varies depending on the amount of
exhaust gas of the internal combustion engine.
[0059] Thus, an exhaust gas amount detector for detecting or
estimating the amount of exhaust gas emitted from the engine may be
further provided, and the threshold value may be changed according
to the exhaust gas amount detected by the exhaust gas amount
detector.
[0060] As the exhaust gas amount is smaller, the inertial force of
the PM is reduced, and an influence of electrostatic actions is
relatively increased. Therefore, the PM particles are more likely
to aggregate. Accordingly, as the exhaust gas amount is smaller,
the PM particles aggregate at a smaller applied voltage. Therefore,
as the exhaust gas amount is smaller, (the absolute value of) the
amount of change in the number of PM particles when the applied
voltage is changed increases. Thus, the threshold value can be
changed in accordance with the exhaust gas amount. Namely, since
the PM particles are more likely to aggregate as the exhaust gas
amount is smaller, the absolute value of the amount of change in
the number of PM particles should become larger if the particulate
matter treatment system is normal. Thus, the threshold value can be
increased as the exhaust gas amount is smaller, and can be reduced
as the exhaust gas amount is larger. Consequently, the accuracy of
the failure determination can be further enhanced.
[0061] The above-described situation will be described in greater
detail with reference to FIG. 4. FIG. 4 is a graph indicating the
relationships among the applied voltage, the number of PM
particles, and the percentage of reduction of the number of PM
particles. FIG. 4 shows four patterns of relationships (indicated
by circles, triangles, diamonds, and inverted triangles) involving
different amounts of exhaust gas emitted from the internal
combustion engine. The exhaust gas amount is largest in the case of
the circles indicated in FIG. 4, and becomes smaller in the order
of the triangles, diamonds, and inverted triangles. The applied
voltage is equal to 0 (kV) when application of voltage to the
electrode 5 is stopped. The operating conditions of the engine are
controlled so that the number of PM particles when the applied
voltage is 0 (kV) is equal to substantially the same value no
matter how large the exhaust gas amount is.
[0062] In the case of the circles in which the exhaust gas amount
is largest, the percentage of reduction of the number of PM
particles is lowest among the four patterns indicated in FIG. 4. On
the other hand, in the case of the inverted triangles in which the
exhaust gas amount is smallest, the percentage of reduction of the
number of PM particles is highest among the four patterns. It is
thus understood from FIG. 4 that as the exhaust gas amount is
smaller, the amount of change in the number of PM particles when
the applied voltage is changed becomes larger.
[0063] Accordingly, the accuracy of failure detection can be
enhanced by changing the threshold value according to the exhaust
gas amount. As the exhaust gas amount is smaller, the amount of
change in the number of PM particles when the applied voltage is
changed should become larger if the particulate matter treatment
system 1 is normal. Therefore, the threshold value is increased as
the exhaust gas amount is smaller. For example, the relationship
between the exhaust gas amount and the correction factor is
obtained in advance by experiment, or the like. Then, a correction
factor is obtained from the detected exhaust gas amount, and the
threshold value is changed by multiplying the threshold value by
the obtained correction factor. Since the exhaust gas amount
(g/sec) of the internal combustion engine is correlated with the
intake air amount of the engine, the exhaust gas amount may be
obtained based on the intake air amount detected by the airflow
meter 74. In this embodiment, the controller 7 that calculates the
exhaust gas amount corresponds to the above-indicated exhaust gas
amount detector of the invention.
[0064] Thus, the accuracy of failure determination can be further
enhanced by increasing the threshold value as the exhaust gas
amount is smaller.
[0065] Furthermore, the threshold value may be changed based on
both the number of PM particles and the exhaust gas amount. For
example, the relationships among the number of PM particles, the
exhaust gas amount, and the correction factor are obtained in
advance by experiment, or the like. Then, a correction factor is
obtained from the detected number of PM particles and exhaust gas
amount, and the threshold value is changed by multiplying the
threshold value by the obtained correction factor. Consequently,
the accuracy of failure detection can be further enhanced.
[0066] According to the embodiment as described above, the
threshold value is changed based on at least one of the number of
PM particles and the exhaust gas amount of the internal combustion
engine, so that the accuracy of failure detection can be further
enhanced.
* * * * *