U.S. patent application number 13/900693 was filed with the patent office on 2013-12-26 for exhaust purifier of an internal combustion engine.
This patent application is currently assigned to DENSO CORPORATION. The applicant listed for this patent is DENSO CORPORATION. Invention is credited to Ataru ICHIKAWA.
Application Number | 20130340412 13/900693 |
Document ID | / |
Family ID | 49713823 |
Filed Date | 2013-12-26 |
United States Patent
Application |
20130340412 |
Kind Code |
A1 |
ICHIKAWA; Ataru |
December 26, 2013 |
EXHAUST PURIFIER OF AN INTERNAL COMBUSTION ENGINE
Abstract
An exhaust purifier purifies an exhaust gas of an internal
combustion engine used in an exhaust purification system that has
an addition unit and a catalyst. The addition unit, disposed in an
exhaust pipe of the exhaust purification system, adds an additive
agent to the exhaust pipe, and the catalyst, disposed on a
downstream side of the addition unit in the exhaust pipe, purifies
the exhaust gas with the additive agent added by the addition unit
and stored therein. The exhaust purifier determines a total amount
of the additive agent to be added, and calculates an exhaust flow
velocity of the exhaust gas. The purifier controls the addition
unit to enable the additive agent to be added to the exhaust pipe
by the total amount determined, such that the purifier controls the
addition unit to decrease a per-unit-time addition amount of the
additive agent as the exhaust flow velocity decreases.
Inventors: |
ICHIKAWA; Ataru;
(Kariya-city, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DENSO CORPORATION |
Kariya-city |
|
JP |
|
|
Assignee: |
DENSO CORPORATION
Kariya-city
JP
|
Family ID: |
49713823 |
Appl. No.: |
13/900693 |
Filed: |
May 23, 2013 |
Current U.S.
Class: |
60/286 |
Current CPC
Class: |
Y02A 50/2325 20180101;
F01N 2900/1622 20130101; F01N 2900/0601 20130101; F01N 2610/02
20130101; F01N 2900/1602 20130101; F01N 2900/1411 20130101; Y02T
10/12 20130101; Y02T 10/24 20130101; F01N 3/08 20130101; F01N 3/208
20130101; Y02A 50/20 20180101 |
Class at
Publication: |
60/286 |
International
Class: |
F01N 3/08 20060101
F01N003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 21, 2012 |
JP |
2012-140010 |
Claims
1. An exhaust purifier for purifying an exhaust gas of an internal
combustion engine used in an exhaust purification system that has
an addition unit and a catalyst, wherein the addition unit,
disposed in an exhaust pipe of the exhaust purification system,
adds a predetermined additive agent to the exhaust pipe, and the
catalyst, disposed on a downstream side of the addition unit in the
exhaust pipe, purifies the exhaust gas with the additive agent
added by the addition unit and stored therein, the exhaust purifier
comprising: an addition amount determination unit determining a
total amount of the additive agent to add from the addition unit;
an addition control unit controlling the addition unit to enable
the additive agent to be added to the exhaust pipe by the total
amount determined by the addition amount determination unit; and a
flow velocity calculation unit calculating an exhaust flow velocity
of the exhaust gas, wherein the addition control unit controls the
addition unit to decrease a per-unit-time addition amount of the
additive agent as the exhaust flow velocity calculated by the flow
velocity calculation unit decreases.
2. The exhaust purifier of claim 1, wherein the addition control
unit adds the total amount of the additive agent in the exhaust
pipe at one time.
3. The exhaust purifier of claim 1, wherein the addition control
unit adds the total amount of the additive agent in the exhaust
pipe according to a division number that defines a number of times
the additive agent is added, and the addition control unit
increases the division number as the exhaust flow velocity
decreases to lower the per-unit-time addition amount of the
additive agent.
4. The exhaust purifier of claim 1, wherein the addition control
unit adds the additive agent to the exhaust pipe according to a
duty-control of the addition unit, and the addition control unit
decreases a driving duty as the exhaust flow velocity decreases to
lower the per-unit-time addition amount of the additive agent.
5. The exhaust purifier of claim 1, wherein the addition control
unit adds the additive agent to the exhaust pipe according to a
frequency-control of the addition unit, the frequency-control
drives the addition unit periodically for a predetermined period,
and the addition control unit increases the predetermined period as
the exhaust flow velocity decreases to lower the per-unit-time
addition amount of the additive agent.
6. The exhaust purifier of claim 1, the addition amount
determination unit further comprising: a temperature acquisition
unit acquiring a temperature of the catalyst; a first calculation
unit calculating a target storage amount indicating an amount of
the additive agent stored in the catalyst as a
temperature-dependent amount according to the temperature acquired
by the temperature acquisition unit; a second calculation unit
calculating an estimated storage amount indicating an amount of the
additive agent currently stored in the catalyst; and a third
calculation unit calculating a deviation between the target storage
amount from the first calculation unit and the estimated storage
amount from the second calculation unit, wherein the addition
amount determination unit determines the total amount of the
additive agent to be added to the catalyst based on the deviation
from the third calculation unit.
7. The exhaust purifier of claim 6, wherein the first calculation
unit calculates the target storage amount as an amount that is
smaller than a maximum storable amount of the additive agent in the
catalyst at the acquired temperature.
8. The exhaust purifier of claim 1, wherein the flow velocity
calculation unit calculates the exhaust flow velocity as a space
velocity of the exhaust gas in the catalyst.
9. The exhaust purifier of claim 1, wherein the addition unit adds
urea aqueous solution as the additive agent, the catalyst stores
ammonia after converting the urea aqueous solution to ammonia, and
is a NOx selective reduction catalyst that chemically reduces NOx
in the exhaust gas by using the stored ammonia in the catalyst.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application is based on and claims the benefit
of priority of Japanese Patent Application No. 2012-140010 filed on
Jun. 21, 2012, the disclosure of which is incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present disclosure generally relates to an exhaust
purifier of an internal combustion engine in an exhaust gas
purification system, which purifies a predetermined harmful
ingredient by using a catalyst arranged in an exhaust pipe.
BACKGROUND
[0003] One type of conventional exhaust gas purification systems is
known as a urea selective catalytic reduction (SCR) system. The
urea SCR system uses a catalyst in an exhaust pipe, which
selectively reduces and purifies NOx in an exhaust gas (i.e., an
SCR catalyst, or a NOx selective reduction catalyst). An addition
valve is disposed along an upstream side of the catalyst for
spraying urea aqueous solution (i.e., an additive agent) into the
exhaust pipe. The catalyst stores the urea aqueous solution that
comes from the addition valve by converting the urea aqueous
solution to ammonia, and resolves (i.e., chemically reduces) NOx in
the exhaust gas to nitrogen and water by using the stored
ammonia.
[0004] A storable amount of ammonia in the catalyst may change
depending on a catalyst temperature. Therefore, it is necessary to
optimize the stored amount of ammonia according to the catalyst
temperature to increase the purification rate of NOx by using the
catalyst. For example, if the stored amount of ammonia is too
little, NOx reduction cannot be sufficiently performed. On the
other hand, if the stored amount of ammonia exceeds the storable
amount of the catalyst due to a steep change of the catalyst
temperature or the like, an ammonia slip may be caused in which an
excessive amount of ammonia at the current temperature is
discharged from the catalyst. Such a drawback may be prevented by
accurately estimating the stored amount of ammonia in the catalyst
and by adequately adding the urea aqueous solution from the
addition value based on the estimation of the stored amount of
ammonia.
[0005] Conventionally, the stored amount of ammonia in the catalyst
is estimated based on a balance among an exhaust amount of NOx from
the internal combustion engine, an added amount of the urea aqueous
solution from the addition valve (i.e., an NH3 amount), and an
amount of ammonia that is consumed by the catalyst. Such a
estimation technique is disclosed in Japanese patent No. 3,951,774
(U.S. Pat. No. 6,755,014 B2).
[0006] For an accurate estimation of the stored amount of the
additive agent (i.e., ammonia) in the catalyst, it is necessary for
the additive agent that is added from the addition valve to flow
through the exhaust pipe to reach the catalyst with the exhaust
gas. However, it is sometimes difficult for the additive agent to
flow through the pipe without landing or condensing on a wall of
the exhaust pipe, especially when the exhaust gas has a low energy
flow at a low flow velocity, which prevents the additive agent to
reach the catalyst. In such a case, the estimation of the stored
amount of ammonia in the catalyst has some error, thereby leading
to an addition of a not-so-adequate amount of the additive agent
and deteriorating the purification rate by the catalyst.
SUMMARY
[0007] In an aspect of the present disclosure, an exhaust purifier
purifies an exhaust gas of an internal combustion engine used in an
exhaust purification system that has an addition unit and a
catalyst. The addition unit, which is disposed in an exhaust pipe
of the exhaust purification system, adds a predetermined additive
agent to the exhaust pipe, and the catalyst, which is disposed on a
downstream side of the addition unit in the exhaust pipe, purifies
the exhaust gas with the additive agent added by the addition
unit.
[0008] The exhaust purifier includes an addition amount
determination unit, an addition control unit, and a flow velocity
calculation unit. The addition amount determination unit determines
a total amount of the additive agent to be added from the addition
unit. The flow velocity calculation unit calculates an exhaust flow
velocity of the exhaust gas.
[0009] The addition control unit controls the addition unit to
enable the additive agent to be added to the exhaust pipe by the
total amount determined by the addition amount determination unit.
In particular, the addition control unit controls the addition unit
to decrease a per-unit-time addition amount of the additive agent
as the exhaust flow velocity calculated by the flow velocity
calculation unit decreases.
[0010] According to the present disclosure, when the additive agent
is added from the addition unit by an amount that is determined by
the addition amount determination unit, the amount added per unit
time is decreased according to the decrease of the exhaust flow
velocity. When the exhaust flow velocity is low, an energy of the
exhaust gas for conveying the additive agent to the catalyst
becomes low. However, in such a case, the amount of the additive
agent added per unit time is decreased. Therefore, the additive
agent is efficiently mixed with the exhaust gas to achieve an
efficient conveyance of the agent to the catalyst. In other words,
the additive agent is prevented from landing or condensing on the
wall of the exhaust pipe.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Other objects, features and advantages of the present
disclosure will become more apparent from the following detailed
description disposed with reference to the accompanying drawings,
in which:
[0012] FIG. 1 is an illustration of an exhaust gas purification
system of the present disclosure;
[0013] FIG. 2 is a flowchart of a process performed by an ECU of
the exhaust gas purification system in a first embodiment;
[0014] FIG. 3 is a first example of a calculation method of a space
velocity SV as an exhaust flow velocity;
[0015] FIG. 4 is a second example of a calculation method of the
space velocity SV as the exhaust flow velocity;
[0016] FIG. 5 is a graph of a maximum NH3 storage amount against a
catalyst temperature;
[0017] FIG. 6 is a graph of the maximum NH3 storage amount on one
line and a target NH3 storage amount on another line against the
catalyst temperature;
[0018] FIG. 7 is a map illustrating addition of urea aqueous
solution by increasing a division number in a continuous manner
according to a decrease of the exhaust flow velocity;
[0019] FIG. 8 is a map illustrating addition of urea aqueous
solution by increasing the division number in a stepwise manner
according to a decrease of the exhaust flow velocity;
[0020] FIG. 9 is a map illustrating addition of urea aqueous
solution by selectively switching the division number at a
threshold of the exhaust flow velocity;
[0021] FIGS. 10A and 10B are graphs of an added amount of urea
aqueous solution and the NH3 storage amount in an SCR catalyst
against the exhaust flow velocity of the first embodiment;
[0022] FIG. 11 is a flowchart of a process performed by the ECU in
a second embodiment;
[0023] FIG. 12 is a map illustrating a driving duty for driving an
addition value against the exhaust flow velocity of the second
embodiment;
[0024] FIGS. 13A and 13B are graphs of the added amount of urea
aqueous solution and the NH3 storage amount in the SCR catalyst
against the exhaust flow velocity of the second embodiment;
[0025] FIG. 14 is a flowchart of a process performed by the ECU in
a third embodiment;
[0026] FIG. 15 is a map illustrating a driving period against the
exhaust flow velocity of the third embodiment; and
[0027] FIGS. 16 A and 16B are graphs of the added amount of urea
aqueous solution and the NH3 storage amount in the SCR catalyst
against the exhaust flow velocity of the third embodiment.
DETAILED DESCRIPTION
First Embodiment
[0028] An exhaust purifier of an internal combustion engine
regarding the present disclosure is described in the following with
reference to the drawings. With reference to FIG. 1, an exhaust gas
purification system 100 installed in a vehicle is configured as a
urea SCR system for purifying a NOx containing exhaust that is
discharged by a diesel engine 101, which is an internal combustion
engine. The engine 101 has an exhaust pipe 13 connected thereto,
and the exhaust gas from the engine 101 goes through the exhaust
pipe 13 to be discharged outside of the vehicle.
[0029] The exhaust pipe 13 has an SCR catalyst 1 (i.e., a NOx
selective reduction catalyst) disposed therein for selectively
reducing NOx in the exhaust gas. The SCR catalyst 1 converts urea
aqueous solution (i.e., an additive agent, or a reduction agent)
added from a urea aqueous solution addition valve 2 into ammonia
(NH3) by hydrolysis, and stores ammonia. The SCR catalyst 1 causes
the following reactions represented by equation 1 or equation 2
between the stored ammonia and NOx in the exhaust gas, for the
resolution (i.e., purification) of NOx to water and nitrogen. The
urea aqueous solution addition valve 2 may be referred to as the
addition valve 2 for brevity.
4NO+4NH3+O2.fwdarw.4N2+6H2O (Equation 1)
6NO2+8NH3.fwdarw.7N2+12H2O (Equation 2)
[0030] The storable amount of ammonia in the SCR catalyst 1 is not
unlimited, and thus, has a certain limitation. With reference to
FIG. 5, the maximum storable amount of ammonia in the SCR catalyst
1 changes depending on temperature of the SCR catalyst 1 (i.e., a
catalyst temperature). Specifically, as the catalyst temperature
increases, the maximum storable amount of ammonia (NH3)
decreases.
[0031] The addition valve 2 adds or releases the urea aqueous
solution in the exhaust pipe 13, and is disposed on an upstream
side of the SCR catalyst 1 in the exhaust pipe 13. The addition
valve 2 has a structure that is substantially the same as an
injector (not illustrated) that injects fuel into a cylinder of the
engine 101. In particular, the addition valve 2 is provided as an
electro-magnetic valve that includes a drive unit, such as an
electromagnetic solenoid, and a valve body that has a urea passage
for passing the urea aqueous solution and a needle that
opens-closes a tip jet nozzle, in which the valve opens-closes
according to a drive signal from an ECU 11. Based on the drive
signal, the electromagnetic solenoid receives electricity and the
needle is moved to a valve open direction by such supply of
electricity to open the nozzle, thereby adding (i.e., spraying or
injecting) the urea aqueous solution from the tip jet nozzle.
[0032] The addition valve 2 is provided with a supply of the urea
aqueous solution from a urea aqueous solution tank 7 as required.
The configuration of a urea aqueous solution supply system is
described in the following. For illustration purposes, assuming
that the urea aqueous solution in the supply system flows from the
tank 7 to the valve 2, a tank side (i.e., a position closer to the
urea aqueous solution tank 7) is described as an upstream side of
the urea aqueous solution supply system, and a valve side (i.e., a
position closer to the addition valve 2) is described as a
downstream side of the urea aqueous solution supply system.
[0033] The urea aqueous solution tank 7 includes an airtight
container having a liquid supply cap, and stores therein a
predetermined amount of the urea aqueous solution of predetermined
density. The urea aqueous solution tank 7 and the addition valve 2
are connected with each other by a supply pipe 15, which has
defined passage in which the urea aqueous solution travels. An
inlet to suck in the urea aqueous solution is formed at a tip of
the supply pipe 15 on the urea aqueous solution tank 7 side, and
such inlet is immersed in the urea aqueous solution when the tank 7
is filled with the urea aqueous solution.
[0034] A pump 6 is provided along the supply pipe 15. The pump 6 is
a line-type electric pump driven in a rotating manner according to
a drive signal from the ECU 11. In the present embodiment, the pump
6 is disposed in the urea aqueous solution of the urea aqueous
solution tank 7 in an immersed state. However, the pump 6 may be
disposed outside of the urea aqueous solution tank 7.
[0035] A filter 3 having a porosity element is disposed along the
supply pipe 15 for filtering the urea aqueous solution. Foreign
matter is filtered by the filter 3, for protecting the valve 2 and
the tank 7 from such foreign matter.
[0036] A pressure regulator 5 is provided on the downstream side of
the pump 6, for adjusting a urea aqueous solution addition pressure
to have a predetermined value. As a result of such adjustment by
the regulator 5, a surplus of the urea aqueous solution is returned
to the tank 7. Also, a pressure sensor 4, which is disposed along
the supply pipe, detects a pressure of the urea aqueous solution in
the supply pipe 15. Instead of mechanically controlling the urea
aqueous solution addition pressure by the regulator 15, the urea
aqueous solution addition pressure may be controlled through a
drive control of the pump 6 based on the pressure detected by the
sensor 4.
[0037] An exhaust flow sensor 9 for detecting an exhaust flow
amount from the engine 101 is provided on an upstream side of the
SCR catalyst 1 along the exhaust pipe 13. Specifically, the exhaust
flow sensor 9 is provided on an upstream side of the addition valve
2.
[0038] Along the exhaust pipe 13 between the valve 2 and the
catalyst 1 an exhaust temperature sensor 12 and an upstream side
NOx sensor 81 are disposed. The exhaust temperature sensor 12
detects an exhaust temperature on the upstream side of the catalyst
1, and the upstream side NOx sensor 81 detects a density of NOx on
the upstream side of the SCR catalyst 1. On a downstream side of
the SCR catalyst 1 along the exhaust pipe 13, a downstream side NOx
sensor 82 is disposed for detecting a density of NOx on the
downstream side of the catalyst 1.
[0039] The exhaust gas purification system also includes an
atmospheric pressure sensor 10 for detecting an atmospheric
pressure. Each of the sensors 9, 12, 81, 82, 10 are connected to
the ECU 11, and a detection value from each of the sensors 9, 12,
81, 82, 10 is inputted to the ECU 11.
[0040] The ECU 11 is equipped with a well-known microcomputer, for
controlling an addition of the urea aqueous solution to the exhaust
pipe 13 from the addition valve 2 based on the detection values of
various sensors. Further, the ECU 11 includes a memory 111 to store
various data (i.e., various maps mentioned later), which are used
for a process performed by the ECU 11. The ECU 11 may serve as an
"exhaust purifier of an internal combustion engine" in the present
disclosure. Details of the process performed by the ECU 11 are
described in the following with reference to FIG. 2. The process of
FIG. 2 is started when, for example, the engine 101 is started, and
is repeatedly performed at predetermined intervals until the engine
101 is stopped.
[0041] At S11, the ECU 11 acquires various data such as an exhaust
temperature Tex (.degree. C.), an atmospheric pressure Pa (Pa), and
an exhaust flow amount Mf (kg/s) from the exhaust temperature
sensor 12, the atmospheric pressure sensor 10, and the exhaust flow
sensor 9, respectively. In the present embodiment, the exhaust flow
amount Mf is provided as a mass of exhaust flow (kg/s).
Alternatively, instead of using the detection value of the exhaust
flow sensor 9, a detection value of an air flow meter, which
detects an amount of an intake air taken into the engine 101, may
be used as a replacement of the exhaust flow amount.
[0042] Subsequently, the ECU 11 calculates an exhaust flow velocity
Vex, at S12. In the present embodiment, the ECU 11 calculates a
space velocity SV of the exhaust gas in the SCR catalyst 1 as the
exhaust flow velocity Vex.
[0043] To determine the space velocity SV, an exhaust density
(kg/m3) of the gas on the upstream side of the SCR catalyst 1 is
first calculated. The exhaust density may be calculated by, for
example, substituting a temperature Tc (i.e., a catalyst
temperature) of the SCR catalyst 1 and a pressure of the exhaust
gas for variables of an equation of state of an ideal gas. Here,
the catalyst temperature Tc is replaced with the exhaust
temperature Tex. Further, the catalyst temperature Tc may be
calculated based on the exhaust temperature Tex by preparing a map
that represents a relationship between the catalyst temperature Tc
and the exhaust temperature Tex.
[0044] By ignoring a pressure difference before and after the SCR
catalyst 1, the atmospheric pressure Pa that is acquired at S11 is
used as the pressure of the exhaust gas. After calculating the
exhaust density, a volume flow amount (m3/s) of the exhaust gas is
calculated by dividing the exhaust flow amount Mf (kg/s), which is
received at S11, by the exhaust density (kg/m3). Subsequently, by
dividing the volume flow amount (m3/s) by a volume (m3) of the SCR
catalyst 1, the space velocity SV (1/s) is calculated. The volume
of the SCR catalyst 1 is stored in the memory 111 in advance.
[0045] The space velocity SV is interrelated with the exhaust flow
amount Mf, the atmospheric pressure Pa, and the catalyst
temperature Tc (i.e., the exhaust temperature Tex). Therefore, at
S12, the ECU 11 uses an SV calculation equation (i.e., SV (Mf, Pa,
Tc)), as shown in FIG. 3, to calculate the space velocity SV by
using the exhaust flow amount Mf, the atmospheric pressure Pa, and
the catalyst temperature Tc, which are stored in the memory 111, as
its parameters. Alternatively, as shown in FIG. 4, a map 201 that
maps the space velocity SV against the catalyst temperature Tc, the
exhaust flow amount Mf, and the atmospheric pressure Pa may be
stored in the memory 111, and the space velocity SV may be
calculated with reference to the map 201. The map 201 of the
exhaust flow amount Mf and the atmospheric pressure Pa is prepared
for different catalyst temperatures Tc.
[0046] With continuing reference to FIG. 2, the ECU 11 calculates a
maximum NH3 storage amount stored in the catalyst at a current
temperature Tc at S13. Specifically, with reference to FIG. 5, a
map 203 representing a relationship between the catalyst
temperature Tc and the maximum NH3 storage amount is stored in the
memory 111. Based on the map 203, the ECU 11 calculates the maximum
NH3 storage amount. Further, in this case, the exhaust temperature
Tex, which is received at S11, is used in place of the catalyst
temperature Tc, and the ECU 11 calculates the maximum NH3 storage
amount STmax (Tex) for such exhaust temperature Tex.
[0047] Subsequently, the ECU 11 calculates a target storage amount
of ammonia (i.e., a target NH3 storage amount) STtg (Tex) in the
SCR catalyst 1 at S14. With reference to FIG. 6, a line 203 of the
maximum NH3 storage amount at a certain catalyst temperature and a
line 204 of the target NH3 storage amount are shown, where the line
203 is the same as the map 203 of FIG. 5. By designating the
maximum NH3 storage amount at a certain catalyst temperature T1, as
STmax(T1), the target NH3 storage amount may be set as STmax(T1)
itself or an amount 202 that is close to STmax(T1). Based on such
assumption, a situation of changing a driving state of the engine
101 from an idle state to an acceleration state is considered, in
which the catalyst temperature (i.e., the exhaust temperature)
steeply rises and the maximum NH3 storage amount steeply falls. As
a result, excessive ammonia in the SCR catalyst 1 relative to the
maximum NH3 storage amount of ammonia is discharged from the SCR
catalyst 1 (i.e., an NH3 slip).
[0048] Therefore, in order to prevent the NH3 slip at a time of
steep rise of the catalyst temperature, the ECU 11 sets the target
NH3 storage amount to an amount having a certain margin to the
maximum NH3 storage amount at S14. For example, the target NH3
storage amount is set at 60% to 70% level of the maximum NH3
storage amount. Further, the excessive amount of ammonia exceeding
the maximum NH3 storage amount will not actually be discharged and
wasted. That is, a part of the discharged amount of ammonia is
consumed for the chemical reduction of NOx. The line 204 takes into
account such a consumption of ammonia for setting the relationship
between the catalyst temperature and the target NH3 storage amount.
At S14, the ECU 11 may directly calculate the target NH3 storage
amount from a map 204 (i.e., line 204) without going through the
calculation of the maximum NH3 storage amount. In particular, the
map 204 (i.e., the line 204), which represents a relationship
between the catalyst temperature and the target NH3 storage amount,
may be stored in the memory 111 in advance. In such a case, S13 of
FIG. 2 may be omitted.
[0049] After S14, the ECU 11 calculates an amount of ammonia
currently stored in the SCR catalyst 1 as an estimated NH3 storage
amount STr at S15. To determine the estimated NH3 storage amount
STr, the ECU 11 may first calculate a NOx discharge amount from the
engine 101 based on a map that represents a relationship between
(i) the NOx discharge amount and (ii) an engine rotation number and
a fuel injection amount. A NOx purification rate of the SCR
catalyst 1 is then determined based on the detection values from
the NOx sensors 81, 82 provided along the upstream and downstream
sides, respectively, of the SCR catalyst 1.
[0050] The ECU 11 further calculates a NOx amount purified of the
SCR catalyst 1 (i.e., a NOx purification amount) based on the NOx
discharge amount and the NOx purification rate. An ammonia
consumption amount consumed by the SCR catalyst 1 is then
determined based on the NOx purification amount. Since there is a
correlation between the NOx purification amount and the ammonia
consumption amount, the calculation of the ammonia consumption
amount may be performed based on such correlation. In particular a
map that represents the correlation may be stored in the memory 111
in advance. The ECU 11 calculates the estimated NH3 storage amount
at a current time by subtracting the ammonia consumption amount
from a pre-correction estimated NH3 storage amount. Alternatively,
various methods including a method disclosed in Japanese patent No.
3,951,774 (patent document 1) may be employed as an estimation
method for calculating the estimated NH3 storage amount STr.
[0051] The ECU calculates a deviation .DELTA.ST between the target
NH3 storage amount STtg and the estimated NH3 storage amount STr at
S16. In particular, the ECU 11 calculates STtg-STr=.DELTA.ST.
[0052] Based on the deviation .DELTA.ST calculated at S16, the ECU
11 calculates an added amount Qu (.DELTA.ST) of urea aqueous
solution (i.e., a urea aqueous solution addition amount) that is
injected from the addition valve 2, at S17. There are two kinds of
chemical reduction of NOx: (i) a reduction by ammonia stored in the
SCR catalyst 1 and (ii) a direct reduction directly performed by
the urea aqueous solution (i.e., ammonia) from the addition valve
2. The ratio between the two kinds of chemical reduction may change
depending on conditions such as the catalyst temperature. At S17,
the ECU calculates the urea aqueous solution addition amount Qu by
considering the amount of the urea aqueous solution that is
consumed by the direct reduction (i.e., by considering the ratio of
the two kinds of chemical reduction) for satisfying the target NH3
storage amount STtg. Specifically, the ECU 11 calculates the urea
aqueous solution addition amount Qu by pre-storing an equation for
calculating the urea aqueous solution addition amount Qu in the
memory 111. The equation uses various conditions, such as the
deviation .DELTA.ST and the catalyst temperature, as parameters.
For instance, the urea aqueous solution addition amount Qu may be
determined by the following equation: Qu=Qu1+(a*Qu2)+((1-a)*Qu3).
Per the equation of "Qu", "Qu1" is NH3 storage shortage amount and
is based on the deviation .DELTA.ST; "Qu2" is an amount of NH3 that
is directly consumed for chemical reduction and is based on NOx
discharge amount; "Qu3" is an amount of NH3 that is consumed for
the chemical reduction by the stored NH3 and is based on the NH3
storage amount and the catalyst's temperature; "a" is a ratio of
direct chemical reduction and is based on the catalyst's
temperature.
[0053] Based on the urea aqueous solution addition amount Qu
calculated at S17, the ECU 11, at S18, determines a division number
N, which represents the number of times the urea aqueous solution
should be added from the addition valve 2. The division number N is
a number used to divide the amount Qu into portions. In particular,
when the exhaust flow velocity Vex (i.e., the space velocity SV)
calculated at S12 is lower, the division number N should be
greater. For instance, the division number N may be calculated
based on a map 301 (FIG. 7), a map 302 (FIG. 8), or a map 303 (FIG.
9), which may be pre-stored in the memory 111.
[0054] In FIG. 7, when the exhaust flow velocity Vex is equal to or
greater than a threshold Vth, the map 301 shows that the addition
of the urea aqueous solution is performed only once by setting the
division number N to 1 (i.e., the urea aqueous solution addition
amount Qu is not divided into multiple portions). When the exhaust
flow velocity Vex is less than the threshold Vth, the division
number N continuously increases as the velocity Vex decreases. By
calculating the division N according to the map 301, the addition
of the urea aqueous solution is fine-tuned according to the exhaust
flow velocity.
[0055] In FIG. 8, when the exhaust flow velocity Vex is equal to or
greater than the threshold Vth, the map 302 shows that the addition
of the urea aqueous solution is performed only once by setting the
division number N to 1. When the exhaust flow velocity Vex is less
than the threshold Vth, the division number N is increased in a
stepwise manner as the exhaust flow velocity Vex decreases. In
addition, as long as the urea aqueous solution is enabled to reach
the SCR catalyst 1 without landing on a wall of the exhaust pipe
13, an interval .DELTA.N between two adjacent division numbers N
and an interval .DELTA.Vex between two exhaust flow velocities Vex
may be arbitrarily set. By calculating the number N based on the
map 302, a frequent change of the number N is prevented, which may
otherwise be caused as a re-calculation of the number N whenever
the exhaust flow velocity Vex changes.
[0056] In FIG. 9, the map 303 shows that the division number N is
switched selectively for a high exhaust flow velocity and a low
exhaust flow velocity. In particular, when the exhaust flow
velocity Vex is equal to or greater than the threshold Vth, the map
303 sets the division number N to a value N1 (e.g., 1), and when
the exhaust flow velocity Vex is less than the threshold Vth, the
map 303 sets the division number N to a value N2 (i.e., N2>N1).
By calculating the division number N based on the map 303, the
calculation process of the division number N is simplified since
the calculation is based only on a simple determination of the
velocity Vex, which is either greater or smaller than the threshold
Vth.
[0057] The ECU 11, at S19, then drives the addition valve 2 to add
(i.e., release) the urea aqueous solution in the pipe 13 by a
controlled amount that equal the urea aqueous solution addition
amount Qu (i.e., a total amount in claims) that is calculated at
S17. In such manner, the urea aqueous solution from the addition
valve 2 is converted to ammonia by the SCR catalyst 1, and the
ammonia is stored in the SCR catalyst 1. The amount of ammonia
stored in the SCR catalyst 1 is then controlled to the target NH3
storage amount that is calculated in S14.
[0058] Further, at S19, when the division number N=1, the urea
aqueous solution, which has the addition amount Qu, is added at one
time. Alternatively, when the division number N is greater than or
equal to 2, the amount of the urea aqueous solution added at each
division time (i.e., each injection) is calculated by dividing the
amount Qu equally by the division number N. Thus, the urea aqueous
solution is added in equal amounts at each injection for N times.
With reference to FIGS. 10A and 10B, the addition of the urea
aqueous solution changes according to the exhaust flow velocity,
which may be high or low, and the NH3 storage amount in the SCR
catalyst 1 changes according to the change of the addition of the
urea aqueous solution. FIG. 10A is a graph of a high exhaust flow
velocity case, and FIG. 10B is a graph of a low exhaust flow
velocity case. A line 21 provides a time change of the NH3 storage
amount after the urea aqueous solution addition. Lines 22, 24
provide the time change of an actual NH3 storage amount (i.e., an
estimated NH3 storage amount). A pulse line 23 and a pulse line 25
along the time t represent an interval at which the urea aqueous
solution is added and the amount of the urea aqueous solution added
at each interval. That is, the number of pulses in each of the
pulse lines 23, 25 represents the number of times the urea aqueous
solution is injected (i.e., division number N), where each pulse is
an injection of the urea aqueous solution. Also, the size of each
pulse of the pulse lines 23, 25 represents the amount of the urea
aqueous solution added at each injection.
[0059] FIG. 10A illustrates an example of adding the urea aqueous
solution at one time (i.e., N=1), and FIG. 10B illustrates an
example of adding the urea aqueous solution by dividing the amount
Qu into five portions (division number N=5), such that the urea
aqueous solution is added five times at equal amounts.
[0060] When the exhaust flow velocity is high as shown in FIG. 10A,
the division number N (i.e., 1 in FIG. 10A) is smaller than the
division number N of the low exhaust flow velocity of FIG. 10B,
which increases the amount of the urea aqueous solution added at
each injection. Therefore, the estimated NH3 storage amount 22 is
controlled to quickly (i.e., in a short time) approach the target
NH3 storage amount 21. In contrast, as shown in FIG. 10B, when the
exhaust flow velocity is low, the division number N (i.e., 5 in
FIG. 10B) is greater than the division number N of the high exhaust
flow velocity in FIG. 10A, which decreases the amount of the urea
aqueous solution added at each injection. Therefore, the urea
aqueous solution added is efficiently conveyed to the SCR catalyst
1 by preventing the urea aqueous solution from landing on the wall
of the exhaust pipe 13. However, the time required for the
estimated NH3 storage amount 24 to approach the target NH3 storage
amount 21 in FIG. 10B is longer than the required time in FIG. 10A.
When addition of the urea aqueous solution is performed at multiple
times (FIG. 10B) an interval .DELTA.t between two injections at S19
is a constant value regardless of the division number N. After
performing S19, the process of FIG. 2 is finished.
[0061] As described above, the addition of the urea aqueous
solution in the present embodiment is performed so that the amount
of added is controlled to approach the target NH3 storage amount
that accords with the catalyst temperature. Therefore, even when
the catalyst temperature changes, the deterioration of the NOx
purification rate as well as the NH3 slip is prevented. Further, at
the time the urea aqueous solution is added, when the exhaust flow
velocity is low, the amount of the urea aqueous solution is
decreased by increasing the number of times the urea aqueous
solution is added (i.e., increasing the division number N), which
prevents the urea aqueous solution from landing on the wall of the
pipe. As a result, an estimation error in an estimation of the
stored amount of ammonia in the SCR catalyst (i.e., an estimated
NH3 storage amount) is decreased, which is beneficial for
maintaining a high NOx purification rate.
Second Embodiment
[0062] The second embodiment of the exhaust purifier of the
internal combustion engine regarding the present disclosure is
described in the following, and focuses on differences from the
first embodiment. In the second embodiment, an addition method of
adding the urea aqueous solution from the urea aqueous solution
addition valve is different from the first embodiment.
Specifically, the addition valve is duty-controlled for adding the
urea aqueous solution from the addition valve according to the
driving duty.
[0063] The configuration of the exhaust gas purification system of
the present embodiment is same as the configuration of the first
embodiment in FIG. 1. FIG. 11 shows a flowchart of the process
performed by the ECU 11. In FIG. 11, like numbers show like steps
of the process of FIG. 2. In FIG. 11, S181 and S191 in are
different from FIG. 2.
[0064] After S17, the ECU 11, at S181, calculates a driving duty D
(i.e., a drive time .tau. against an addition period T: D=.tau./T)
when the process drives the addition valve 2. With reference to
FIG. 12, the ECU 11 calculates the driving duty D based on a map
401, which is pre-stored in the memory 111. The map 401 defines a
relationship between the exhaust flow velocity Vex and the driving
duty D. As readily understood from FIG. 12, the map 401 defines a
continuous decrease of the driving duty D when the exhaust flow
velocity Vex decreases. Further, at S181, the driving duty D may be
decreased in a stepwise manner as the exhaust flow velocity Vex
decreases, similar to FIG. 8, or the driving duty D may be
selectively switched between high and low for the high exhaust flow
velocity Vex and for the low exhaust flow velocity Vex, similar to
FIG. 9.
[0065] After S181, the ECU 11, at S191, controls the driving
operation of the addition valve 2 so that the urea aqueous solution
is added to the exhaust pipe 13 by the amount Qu calculated in S17.
In particular, the ECU 11 provides the addition valve 2 with a
driving pulse of the driving duty D that is calculated in S181.
FIGS. 13A and 13B, which are similar graphs as the ones in FIGS.
10A and B, illustrate a time change of the NH3 storage amount after
the addition of the urea aqueous solution. FIG. 13A is a graph of a
high exhaust flow velocity case and FIG. 13B is a graph of a low
exhaust flow velocity case. A pulse series 26, 27, represent a
series of the driving pulses that are provided for the addition
valve 2. A period T (i.e., an addition period) of the pulse series
26 of FIG. 13A is same as a period T of the pulse series 27 of FIG.
13B. A drive time .tau.2 of FIG. 13B is smaller than a drive time
.tau.1 of FIG. 13A. In other words, a driving duty of FIG. 13B is
smaller than a driving duty of FIG. 13A. Further, a total amount of
the urea aqueous solution added from the addition valve 2 is the
same for both cases, i.e., in FIG. 13A and FIG. 13B.
[0066] Therefore, when the exhaust flow velocity is high, as shown
in FIG. 13A, the driving duty is high in comparison to the low
exhaust flow velocity in FIG. 13B, thereby resulting in an increase
of the amount added per unit time, which enables a quick approach
of the estimated NH3 storage amount 22 to the target NH3 storage
amount 21. In contrast, when the exhaust flow velocity is low as
shown in FIG. 13B, the driving duty is low in comparison to the
high exhaust flow velocity in FIG. 13A, thereby resulting in a
decrease of the amount added per unit time and efficient addition
of the urea aqueous solution that reaches the SCR catalyst 1 by
preventing the urea aqueous solution from landing or condensing on
the wall of the pipe 13. However, the time required for the
estimated NH3 storage amount 24 to approach the target NH3 storage
amount 21 in FIG. 13B is longer than the required time in FIG. 13A.
After S191, the process of the flowchart in FIG. 11 is
finished.
[0067] As described above, the present disclosure is applicable to
a case where the addition valve is duty-controlled, for achieving
the same effects as the first embodiment.
Third Embodiment
[0068] The third embodiment of the exhaust purifier of an internal
combustion engine regarding the present disclosure is described in
the following, and focuses on differences from the first and second
embodiment. In the third embodiment, the addition method for adding
the urea aqueous solution from the urea aqueous solution addition
valve is different from the first and second embodiments, in which
the addition valve is frequency-controlled for adding the urea
aqueous solution periodically from the valve by driving the valve
according to a driving period (i.e., at a driving frequency). In
such frequency-controlled valve driving, an amount of the urea
aqueous solution added at each drive is same, regardless of the
driving period (i.e., the driving frequency).
[0069] The configuration of the exhaust gas purification system of
the present embodiment is same as the configuration of the first
embodiment as shown in FIG. 1. FIG. 14 shows a flowchart of the
process performed by the ECU 11. In FIG. 14, like numbers show like
steps of the process in FIG. 2. In FIG. 14, S182 and S192 in the
process are different from the process of FIG. 2.
[0070] After S17, the ECU 11 calculates a driving period F at S182,
when the ECU 11 drives the addition valve 2. More practically, the
ECU 11 calculates a longer driving period F when the exhaust flow
velocity Vex (i.e., the space velocity SV) calculated in S12 is
lower. If the driving period F is converted (i.e., inverted) to a
driving frequency (i.e., a reciprocal number of the driving period
F), the lower the exhaust flow velocity Vex (i.e., the space
velocity SV), the lower the calculated driving frequency should
be.
[0071] More specifically, as shown in FIG. 15, the ECU 11
calculates the driving period F based on a map 501, which is
pre-stored in the memory 111. The map 501 defines a relationship
between the exhaust flow velocity Vex and the driving period F. The
map 501 defines a continuous increase of the driving period F when
the exhaust flow velocity Vex decreases. Further, at S182, the
driving period may be increased in a stepwise manner as the exhaust
flow velocity Vex decreases, similar to FIG. 8, or the driving
period may be selectively switched between long and short for the
high exhaust flow velocity Vex and for the low exhaust flow
velocity Vex, similar to FIG. 9.
[0072] After S182, the ECU 11, at S192, controls the driving
operation of the addition valve 2 so that the urea aqueous solution
is added to the exhaust pipe 13 by the amount Qu calculated in S17.
More practically, the ECU 11 provides the addition valve 2 with a
driving pulse of the driving period F that is calculated at S182.
FIGS. 16A and 16B, which are similar to FIGS. 10A and 10B,
illustrate a time change of the NH3 storage amount after the
addition of the urea aqueous solution. FIG. 16A is a graph of a
high exhaust flow velocity case, and FIG. 16B is a graph of a low
exhaust flow velocity case. A pulse series 28, 29 represent a
series of driving pulses that are provided for the addition valve
2. Further, a pulse width of each of the driving pulses in the
pulse series 28, 29 is the same (i.e., a constant pulse width). In
other words, an amount of the urea aqueous solution added at each
driving pulse is constant. Further, a driving period F2 in FIG. 16B
is longer than a driving period F1 in FIG. 16A.
[0073] Therefore, when the exhaust flow velocity is high as shown
in FIG. 16A, the driving period is shorter in comparison to the low
exhaust flow velocity in FIG. 16B. Thus a quick approach of the
estimated NH3 storage amount 22 to the target NH3 storage amount 21
is enabled. In contrast, when the exhaust flow velocity is low as
shown in FIG. 16B, the driving period is longer in comparison to
the high exhaust flow velocity in FIG. 16A, thereby resulting in a
decrease of the amount added per unit time and efficient addition
of the urea aqueous solution that reaches the SCR catalyst 1 by
preventing the urea aqueous solution from landing or condensing on
the wall of the exhaust pipe 13. However, the time required for the
estimated NH3 storage amount 24 to approach the target NH3 storage
amount 21 in FIG. 16B is longer than the required time in FIG. 16A.
After S192, the process of the flowchart in FIG. 14 is
finished.
[0074] As described above, the present disclosure is applicable to
a case where the addition valve is frequency-controlled, for
achieving the same effects as the first and/or second
embodiments.
[0075] Also, in the first, second, and third embodiments the amount
of the urea aqueous solution added per injection or per addition
may be referred to as a "per-unit-time addition amount".
[0076] Although the present disclosure has been fully described in
connection with the preferred embodiment with reference to the
accompanying drawings, it is to be noted that various changes and
modifications will become apparent to those skilled in the art. For
example, the exhaust purifier of the present disclosure is
applicable to a system that uses both of the duty control in the
second embodiment and the frequency control in the third
embodiment. In such a case, for example, when the exhaust flow
velocity is low, the driving duty is decreased and the driving
period is increased for driving the addition valve in comparison to
the high exhaust flow velocity case. Further, the present
disclosure is applicable to the urea SCR system for a gasoline
engine, or, more specifically, for a lean-burn gasoline engine.
Further, the present disclosure is applicable to the exhaust gas
purification system that uses a reduction agent other than the urea
aqueous solution (e.g., a water solution containing ammonia).
[0077] Such changes and modifications are to be understood as being
within the scope of the present disclosure as defined by the
appended claims.
* * * * *