JP2000356150A - LEAN NOx TRAP CONTROL METHOD AND SYSTEM BASED ON DEPLETION OF NOx OCCLUSION CAPACITY OF LEAN NOx TRAP - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize a discharge amount of CO and NOx to an atmosphere while maximizing a fuel economics of a vehicle, by continuously operating trap under an optimum condition in an NOx conversion efficiency. SOLUTION: A trap optimizes a regeneration cycle of an NOx trap 34 in which a trap is filled to reach a predetermined rate of present capacity and fully emptied during a trap purging. When the trap capacity is substantially reduced and a real filling time becomes equal to or less than a predetermined minimum filling time, trap desulfurization is carried out in order to recover the trap capacity. A programmed computer 10 controls filling and purge time based on a voltage amplitude of a switching type O2 sensor 38 and a time response of the sensor 38. A purge frequency which is ideal that the frequency is directly associated with a reduction rate of a trap NOx occlusion capacity is controlled such that the trap does not exceed an upper limit of the NOx occlusion capacity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a method for controlling the regeneration cycle of a downstream NO _x trap emission control system of a lean-burn vehicle.

[0002]

BACKGROUND OF THE INVENTION Conventional lean burn engine control systems are proportional to the measured air mass to periodically obtain an air / fuel ratio leaner than stoichiometric, thereby improving fuel economy. Includes an air-fuel ratio controller that supplies fuel to the engine. A typical three-way catalytic converter located in the exhaust passage of an engine is engine-generated in the presence of excess oxygen, such as during lean-burn engine operation
Because of the inefficiency of NOx conversion, some prior art systems include lean NOx in the exhaust passage downstream of the three-way catalytic converter.
x Trap is installed, residual N generated during lean operation
Some disclose that Ox is chemically absorbed.

It is important that the trap have a finite NOx storage capacity, for example, an air-fuel mixture richer in its air-fuel ratio than the stoichiometric air-fuel ratio of about 14.65, such as exposing the trap to an air-fuel ratio of less than about 13. By exposing the trap to the trap, the trap is periodically "purged" for stored NOx. During this purge event, excess HC and CO pass through the three-way catalytic converter and through the trap
By reacting with NOx to HC and CO are occluded reduced to harmless N ₂ and O _2. The amount of excess That purging fuel required to release the occluded NO _x is a period in which the purge fuel in the form of a rich air-fuel mixture is supplied at a substantially constant state mass airflow "Purge Often expressed as a function of time.

[0004] NOx passes through the trap and N
The timing of each purge event must be controlled so that the trap does not exceed its NOx storage capacity, as it will increase Ox emissions. Preferably, the frequency of the purge is controlled so as to avoid purging the partially filled trap due to excess fuel of the air-fuel mixture enriched for the purge event.

[0005] It has been recognized by the prior art that the NOx storage capacity of a trap is itself a function of many variables. These variables include trap temperature, trap history, degree of sulfidation, and the degree of thermal damage or damage to the trap's NOx absorbent due to excessive heat. For example, referring to U.S. Patent No. 5,437,153, it states that when a trap approaches its maximum capacity,
That the speed of absorbing NOx may start to fall
It has been described. Further, U.S. Patent No. States that the trap will absorb any NOx produced by the engine as long as it remains below its nominal capacity. A purge event is planned so that the trap is regenerated whenever the accumulated NOx produced by the engine reaches the nominal trapping capacity.

[0006]

The actual amount of NOx stored in the trap will vary depending on the NOx concentration of the gas supplied to the engine, exhaust flow rate, atmospheric humidity, trap temperature, and other variables. Thus, both the capacity of the trap and the actual amount of NOx stored in the trap are complex functions of many variables. It is desirable to diagnose and control trap purge and fill events to ensure that the trap always operates at its optimum conditions.

When the engine is operated with a fuel containing sulfur, the sulfur is absorbed in the trap and
This leads to a decrease in both the absolute NOx storage capacity of the trap and the instantaneous NOx absorption efficiency of the trap. If the sulfation of such traps exceeds a certain threshold, the absorbed SOx must be "burned off" or desorbed during the desulfurization event. The trap temperature is then raised to about 650 ° C. or higher in the presence of excess HC and CO. By way of example, U.S. Pat.No. 5,746,049 is directed to increasing the trap temperature to at least 650.degree. C. by introducing secondary air to the exhaust upstream of the NOx trap when operating the engine with a rich air-fuel mixture; and Utilizing the resulting exothermic reaction to raise the trap temperature to the extent desired for purging the trap for SOx,
It describes a desulfurization method for traps.

[0008]

According to the lean NOx control method of the present invention, the trap status, that is, "soundness" is diagnosed,
The NOx stored in the trap is then adjusted so that the parameters for purging the trap, the impairment rate of the NOx storage capacity, the purge time and the purge strength can be adjusted online during engine operation in the vehicle. Use information related to volume and maximum NOx storage capacity of the trap. Where NOx
The storage capacity impairment rate is the speed at which the NOx storage capacity of the trap decreases during the NOx filling process. In addition, the decision to purge the trap for sulfur, i.e., to schedule a desulfurization event, is determined by the NO of the diagnosed trap.
This is done based on the decrease in x storage capacity and the associated increase in impairment of NOx absorption capacity. Thus, the trap is operated continuously at optimal conditions for NOx conversion efficiency, thereby minimizing CO and NOx emissions to the atmosphere while maximizing the fuel economy of the vehicle. Intelligent desulfurization of the trap ensures that the NOx conversion efficiency of the trap is always maintained above a given minimum.

More specifically, according to the present invention, the NOx storage capacity of the trap is monitored for impairment and closed loop control of desulfurization of the trap as well as feedback control of the trap purge frequency and depth are performed. Purge frequency trap is inversely proportional to the rate at which NOx is occluded in the trap, the depth of the purge is related to the amount of NOx released from the conversion has been trapped as N ₂ and O _2. A programmed computer controls the trap filling and purging times based on the switching oxygen sensor voltage amplitude and sensor response time. The frequency of the purge, ideally directly related to the NOx storage capacity impairment rate of the trap, is controlled such that the trap is not filled beyond its NOx storage capacity limit. When the NOx storage capacity limit is exceeded, that is, when the trap is overfilled, NOx passes through the trap without being stored. If the trap is not completely filled, frequent purging will result in an unnecessary increase in fuel consumption. Also, excess air
CO and HC emissions will occur.

Further, according to the present invention, based on the NOx storage capacity impairment rate, the trap is filled to a predetermined percentage of its current storage capacity, and then completely emptied during the purge. As the trap's storage capacity decreases due to degradation, the closed loop purge optimization routine uses a NOx storage capacity impairment rate to store enough NOx to fill the trap to the desired percentage of its storage capacity. Generate an adjustment multiplier that is used to adjust. If the actual NOx storage capacity impairment rate is substantially equal to or greater than the predetermined maximum NOx storage capacity impairment rate, the trap storage capacity is restored to its original value when the trap storage capacity is significantly reduced. For this purpose, desulfurization of the trap is performed. If a predetermined number of trap desulfurization operations are performed without increasing the NOx storage capacity loss rate, the trap must be replaced,
The indicator is then indicated to the driver by an indicator.

The default or initial value of the impairment rate of the NOx absorption capacity of the trap is determined by a function of the engine system and the trap. The functionalization gives the values of the trap filling rate and the optimal NOx storage capacity impairment rate as a function of engine load or mass air flow. The operating air-fuel ratio where the trap is used does not change significantly, and changes in engine speed do not significantly affect the NOx storage capacity impairment rate of the trap. Thus, the primary variable for the trap NOx absorption capacity impairment rate is the root of the engine (eg, the square root) where the index of the engine load or mass flow is small.

The objects, features and advantages of the present invention, such as those set forth above, will be readily understood from the following detailed description of the best mode for carrying out the invention, when taken in conjunction with the accompanying drawings.

[0013]

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, and first to FIG. 1, a powertrain control module, generally designated PCM, indicated generally by the numeral 10 is shown.
Is an electronic engine controller, including ROM, RAM and CPU as shown. The PCM controls a set of injectors 12, 14, 16 and 18 that inject fuel into the four cylinders of the internal combustion engine 20. The fuel injector is of a general construction, and the controller 10
Are arranged to inject the correct amount of fuel into the corresponding cylinder as determined by The controller 10 sends a fuel injector signal to the injector in order to maintain the air-fuel ratio (AFR) determined by the controller 10. An air gauge or mass flow sensor 22 is located at the air inlet of the engine manifold 24 and provides a signal regarding the mass air flow determined by the position of the throttle valve 26. The air mass signal is used by controller 10 to calculate an air mass value indicative of the mass of air entering the intake system per unit time. Heated exhaust gas oxygen
A HEGO (abbreviated for short) sensor 28 detects the oxygen content of the exhaust generated by the engine and sends a signal to the controller 10. HE
The GO sensor 28 is used for controlling the engine air-fuel ratio particularly during the stoichiometric air-fuel ratio operation.

The exhaust system has one or more exhaust pipes, and converts exhaust gas generated by combustion of the air-fuel mixture in the engine into a general direct-coupled three-way catalytic converter (three-way catalyst 30). ). The three-way catalyst 30 contains a catalytic substance that chemically changes exhaust generated by the engine in order to generate catalytically reacted exhaust. The exhaust gas after the catalytic reaction passes through an exhaust pipe 32, is sent to a downstream NOx trap made of the above-mentioned substance, and is then discharged to the atmosphere through a tail pipe.

Downstream of the trap 34 is a second HEGO sensor 38, which supplies signals to the controller 10 for diagnosis and control according to the present invention. The second HEGO sensor 38 operates during the normal stoichiometric air-fuel ratio closed loop limit cycle operation.
It is used to monitor the HC conversion efficiency of the three-way catalyst 30 using a well-known method of comparing the signal amplitude of the second HEGO sensor 38 with that of the first HEGO sensor 28. The trap internal temperature sensor 42 generates an output signal representing the instantaneous temperature T of the trap 34. Further, other information regarding the engine performance, such as the camshaft position, the crankshaft position, the angular velocity, the throttle valve position, the temperature, and the like, are also sent to the controller from other sensors (not shown). Information from these sensors is used by controller 10 to control the operation of the engine.

A typical response of voltage to air / fuel ratio for a switching oxygen sensor, such as the second HEGO sensor 38, is shown in FIG. The output voltage of the second HEGO sensor 38 switches between a low level and a high level when the exhaust gas mixture switches from lean to rich at a stoichiometric air-fuel ratio of approximately 14.65. Since the air-fuel ratio is lean during the charging time, NOx generated by the engine is sent to the trap 34 through the three-way catalyst 30 and the exhaust pipe 32, where it is stored.

A typical operation of a trap purge cycle
The movement is shown in FIG. The top waveform (Figure 3 (a))
Lean charge for three different purge times 1, 2 and 3
Filling time t _FAnd rich purge time t_PShows a relationship with
You. The second waveform (Fig. 3 (b)) shows three purge times.
The response of the second HEGO sensor 38 is shown. G
CO passing through the wrap and affecting the downstream sensor 38
The amount of HC shall indicate the efficiency of the trap purge event.
Used. Peak voltage level of the downstream oxygen sensor
Are trapped in NOx and O_TwoThe amount of
It is shown. For short purge time 1, trap is N
Oxygen sensor is not completely purged for Ox
Response is extremely small, resulting in CO emissions to the atmosphere
And closely related response spikes of the second HEGO sensor
Is small. In this case, the sensor peak voltage V_P
Is the reference voltage V_refHas not reached. Moderate, or optimal
In the simple purge time 2, the output V of the second HEGO sensor is_PIs the standard
Voltage V_refAnd this is an extremely small amount of air that is acceptable
Traps have been purged by the required amount
Indicates that the Long purge time 3
About the peak voltage V of the second HEGO sensor_PIs the reference voltage
V_refThe trap has been completely purged,
3 (d).
Which results in excessive CO emissions to the atmosphere as shown in
Absent.

Data on the output voltage of the second HEGO sensor
The data capture window shows the waveform shown in Fig. 3 (c).
It works like During this window, PCM is the second HEG
The response data of the O sensor 38 is obtained. FIG. 4 shows FIG.
Sensors 38 for three levels of indicated purge times
FIG. Δt_{twenty one} Is based on the sensor output voltage
Reference voltage V_refIs the length of time that was exceeded. Reference voltage
V_refSensor peak voltage V less than_Pabout,
PCM 10 is the sensor saturation time t_satTo t_sat = t_{sat ref}From
t_sat = 0 by linear extrapolation until
Make the values smoothly continuous. PCM 10 has the relationship shown in FIG.
Used, and as indicated there, the sensor saturation time t_satThe
Sensor peak voltage V_PProportional to

FIG. 5 shows the normalized oxygen sensor saturation time.
4 shows the relationship between t _sat and the purge time t _P. Sensor saturation time
t _sat is the normalized amount of time when the signal of the second HEGO sensor exceeds V _ref , and Δt ₂₁ / Δt
_It is equal to _{21 norm} (where Δt _{21 norm} is a normalization factor).
The sensor saturation time t _sat is normalized by the desired value t _{sat desired} . For a given fill time t _F and trap state, there is an optimal purge time value t _{P sat #desired} . The value is a normalized optimal saturation time t _sat = 1 that does not result in excessive HC and CO emissions to the atmosphere and maintains acceptable NOx trap efficiency. When the sensor saturation time t _sat is t _sat > 1, the purge time is too long and needs to be shortened. Sensor saturation time t
_{If sat.} is t _sat. <1, the purge time is too short and needs to be increased. In that way, the second HEGO
Based on the output of the sensor 38, closed loop control of trap purge can be realized.

FIG. 7 shows a given operating condition of the engine and
Purge time t for given conditions_PWhen
Filling time t_FIndicates the nominal relationship between Two suboptimal pars
Di time t_{P subopt1}And t_{P subopt2}Is the fixed filling time
t_FTUnder or over purge of trap 34 for
Corresponding to NOx trapped in the trap
Purge time t for optimal purging_PIs t_PTIndicated
I have. This point is the target, that is, the desired purge time t._sat = t
_{sat desired}Corresponding to This purge time is fixed filling
Time t_FTMinimize CO emissions to the atmosphere during This hand
As a result of the order, it is related to the amount of oxygen stored directly in the trap.
Storage oxygen purge time t_{P osc}Is determined. Oxygen is
For example, it can be stored directly in the form of cerium oxide. Sucking
Storage oxygen purge time t_{P osc}Is two or more
T_F Extrapolates to point = 0 or has
I t_F = 0 near the point at 0_POptimization
You can decide. The operating point T2 is intentionally t
_{F T2} <t_FTAnd through optimization t _{P T2}Find this
And is obtained by

[0021] Figure 7a illustrates the optimization of the fill time t _F. For a given filling time _tFT , an optimal purge time _tPT is determined as shown in FIG. Then fill time, initial value t _FT step is changed to a value slightly smaller t _FB from, and by a step change in the initial value t _FT than slightly larger value t _FA, swung. t _F
The optimization of the purge time is applied to all three points T, A and B to determine the change in t _P due to The change in t _P from A to T and from B to T is evaluated. In FIG. 7a, the change from B to T is greater than the change from A to T. The absolute values of these differences are controlled to fall within a predetermined tolerance DELTA # MIN, as described more fully in connection with FIG. The absolute value of the difference is proportional to the slope of the curve showing the relationship between t _P and t _F. Optimization process defines the operating point T as the shoulder of the curve showing the relationship between t _P and t _F. Time t
_{P sat} represents the saturation value of the purge time for an infinitely long fill time.

Four trucks having different NOx and oxygen storage amounts
Purge time t_PAnd filling time t_F Most
The result of the adaptation routine is shown in FIG. Purge time
t_PAnd filling time t_FBoth were shown in FIGS. 7 and 7a
Optimized using techniques. Points determined by FIG.
Is shown as the optimal operating point T1 and purge for that point
Time is t_{P T1}And the filling time is t_{F T1}It is. "1" is
Indicates that the trap has not deteriorated, and this is referred to as state A.
I do. Trap is sulfur poisoning, thermal damage, or other factors
, The trap state becomes B, C and
You will reach D. Nearly steady engine condition exists
The purge and fill optimization routine runs continuously.
Is performed. Optimal operation according to trap conditions B, C and D
The rolling states T2, T3 and T4 will be reached. t_{P T1},
 t_{P T2}, t_{P T3}And t_{P T4}NOx saturation level reflected in
And the oxygen storage related purge time t_{P osc T1}, t_{P osc T2}, t
_{P osc T Three}And t_{P osc T4}Are both trapped
Depending on the trap, and typically
Value will be reduced. About the NOx part of the purge
The purge fuel is t_{P NOx}= t_PT -t_{P osc}With a value equal to
is there. Purge fuel should be available for a given operating condition.
It is expected to be equivalent to the time. The controller 10
The gin 20 is allowed to operate at a predetermined rich air-fuel ratio
Adjusting the time limits the actual purge fuel.
In order to simplify the explanation here, the purge time
To purge fuel at the operating conditions assumed for, etc.
Value. NOx thus occluded
And the purge time required for the stored oxygen is directly determined.
It will be used for diagnosis and control.

FIG. 9 illustrates the relationship between the NOx purge time t _{P NOx} and the absorption capacity of the NOx trap. States A, B and
C is determined that the NOx absorption efficiency, trapping capacity, and fuel economy are within allowable ranges, and State D is determined to be outside the allowable ranges. Thus, as State D is approached, a trap desulfurization event is planned to regenerate the NOx trapping capacity and reduce fuel consumption associated with high NOx purge frequencies. The change in t _{P osc is based on} the change in oxygen storage
It can provide more information about the aging of the trap.

FIG. 10 shows the purge time t_PAbout optimization of
It is a flowchart. The purpose of this routine is given
Filling time t_FAbout rich purge of air-fuel ratio
Optimizing the spikes. This routine will be
The software for system optimization described with reference to FIG.
Software. At decision block 46, the password
The status of the storage flag is checked and the flag is set.
If so, lean as shown in block 48.
NOx purge is performed. Purge flag is trapped
Is set when the filling of is completed. For example, the hula
Is the time when that purge planning method is used
In block 136 of FIG.
You. In block 50, the peak voltage V_PAnd if you have
Is the transition time t₁And t_TwoOxygen sensor (E
GO) voltage is specified in the capture window
Is sampled through. As shown in FIG. 3 (c),
A window captures changes in the EGO sensor waveform. Judgment
It is V determined by block 52_P > V_refIf it is
Has a sensor saturation as shown in blocks 54 and 56
Time t_satIs Δt_{twenty one}, That is, the EGO sensor voltage is V_refOver
It is said that it is in proportion to the time that was required. V_P <V_refIn
The linear extrapolation function, as shown in block 58.
Using t_satIs determined. For this function, see FIG.
And t_satThe peak voltage V_PProportional to
By t_satIs required. This is,
 V_P > V_refFrom V_P <V_refSmooth transition to the case
Line and return control as shown in block 60
Suitable continuous positive and negative deviation function t_{sat error}(k)
I can. Where the deviation function t_{sat error}(k) is sensor saturation
Desired value of time t_{sat desired}From the actual sensor saturation time t_sat
Equivalent to subtracting And the deviation function t
_{sat error}(K) is a block 62 showing the desired sensor saturation.
Sum time t_{sat de sired}Is normalized by dividing by.

As a result, the normalized deviation function t
_{sat error # norm}(k) is like a PID (proportional-integral-derivative) controller
It is used as an input to a simple feedback controller. In block 64
As shown, the output of the PID controller is during the trap purge
Multiplicative correction factor PURGE # MUL for the interval. t
_{sat error # norm}Direct monotonic between (k) and PURGE # MUL
Have a relationship. t_{sat error # norm}If (k)> 0,
Pump is under purge and NOx purge
PURGE # MUL is its reference value to provide more CO
Must be increased from. t_{sat error # norm}(k) <
 If 0, the trap is overpurge and the NOx
PURGE # MUL to reduce CO supply for
It must be reduced from that reference value. This result
As a result, a new value for the purge time is
 Is t_P(K + 1) = t_P(K) × PURGE # MUL. At the time of purging
Optimization between the old and the new, as shown in blocks 68 and 70
The absolute value of the deviation of the purge time value is smaller than the allowable tolerance
It continues until it becomes. If | t_P(k + 1) -t_P(k) |> = ε
In some cases, the PID feedback control loop is_P
Is not within the allowable range ε. Therefore, as shown in block 70
The new par calculated in block 66 is
The delay time is delayed until the condition in block 68 is satisfied.
Used in the storage cycle. Filling time t_FIs the best par
Di time t_PT until_PDuring the optimization routine, Equation 2
And adjusted as needed. | t_P(k + 1)-t_P
If (k) | <ε, the purge time optimization converges
The purge time, as shown in block 72.
The new value is stored, and the optimization process is performed as shown in FIG._FOptimal
It will be shifted to a routine for conversion. Purge time t_POnly
Instead of being used during the purge event (see Figure 3)
Adjust the relative richness of the air-fuel ratio in a similar manner.
You can also.

FIG. 11 is a flowchart for system optimization, including optimization of both purge and fill times. Optimization of the fill time is performed only when the engine is operating at approximately steady state, as indicated by block 74. In this regard, the substantially steady state means that the rate of change of certain engine operating variables, such as engine speed, load, intake air amount, ignition timing, EGR amount, is maintained below a predetermined level, Characterized. In block 76, one fill step increment FILL # ST
EP is equal to STEP #SIZE, if FILL # STEP> 0, increase the fill time. STEP_SIZE is adjusted with _respect to the storage capacity utilization rate Rij as shown in FIG.

At block 78, the purge time optimization routine described above in connection with FIG. 10 is executed. This will optimize the purge time t _P for a given fill time. PURGE # MUL at the end of the purge optimization routine executed in block 78 is CTRL # START
And the fill time multiplier FILL # MUL is incremented by FILL # STEP, as shown in block 80. The filling time step change FILL # MUL is multiplied t _F at block 82 is promoted. In block 84, the purge optimization routine of FIG. 10 is executed for a new filling time t _F (k + 1). The PURGE_MUL at the end of the purge optimization routine performed in FIG. 10 is stored at block 86 as CTRL_END. The amount of change in the purge multiplier CTRL # DIFF = ABS (CTRL # END CTRL # START) is also stored in block 86, and in block 88 the reference value DELTA #
Compared to MIN. DELTA # MIN corresponds to the tolerance stated in FIG. 7a, and CTRL # END and CTRL # START
corresponding to two values of t _P obtained at a of A and T or B and T. If the change in the purge multiplier is greater than DELTA # MIN, the sign of FILL # STEP will be
In order to be able to search for the optimal filling time in the opposite direction,
Can be changed. If the change in the purge multiplier is smaller than DELTA # MIN, search the optimum fill time t _F is, as indicated in block 92, is continued in the same direction. In block 94, FILL # MUL is incremented by the selected FILL # STEP. Then, at block 96, the filling time t _F (k
+1) is corrected by multiplying FILL # MUL. As a result, selection of the optimum t _P as the operating point and continuous destabilization at this point are performed. If the engine does not experience near steady state during this procedure, the fill time optimization process is aborted and the fill time from Equation 2 (discussed below) is used, as shown at block 74.

FIG. 12 shows the desulfurization of a trap according to the present invention.
Is shown. At block 100,
For a trap that does not degrade under given operating conditions
Reference value t representing purge time_{P NOxref}Looks up
Read from the table. t_{P NOxref}Is the intake air volume,
Can be a function of air-fuel ratio and other parameters.
come. In block 102, the latest purge time t_P(K)
Is called and t_{P NOxref}Minus the specified tolerance TOL
Is compared to So if t_P(k) <t_{P NOxref}− At TOL
In some cases, trap desulfurization events are planned. Desulfurization
Has an air-fuel ratio slightly less than the theoretical value of, for example, 0.98λ.
Set the trap to rich, approximately 650 ° C for about 10 minutes
Heating. Desulfurization counter D
Reset in block 104 and shown in block 106.
Each time the desulfurization process is performed as
Is done. After the desulfurization process has been completed,
As indicated, at block 108
The interval is determined. New purge time t_P(k + 1) is block 1
At 10, the reference time t_{P NOxref}From the specified tolerance TOL
Is compared to the value_P(k + 1) <t_{P NOxref} − At TOL
In some cases, as determined by the decision in block 112
Then, at least two additional desulfurization events are performed. It
However, if the trap still does not fall within the acceptable range,
Fault indicator lamp (malfunction indicator la
mp for MIL) is lit and as shown in block 114
The trap should be replaced with a new one. t_P(k)
> = t_{P NOxref} − If TOL, trap 34 immediately
Service has not deteriorated to the extent necessary and normal operation has been restored.
Is opened.

The trap 34 has a lower storage capacity than the actual storage capacity.
The obtained storage capacity is filled with the stored NOx,
When used, a NOx purge event is planned. oxygen
Is oxygen or NOx in the form of cerium oxide
And the sum of those two substances is
This is the amount of oxidant stored. FIG. 13 shows the state of the trap 34
Oxidizer and time the trap 34 is exposed to incoming NOx
Is shown in the drawing. NOx storage is oxygen
It happens at a slower rate. Optimal for NOx generation time
The driving conditions are shown in this diagram as the "shoulders" of the curve.
About 60-70% relative NO _xCorresponds to the generation time. 100% of horizontal axis
Corresponds to the saturated NOx storage capacity of the trap 34. Occlusion
The values for stored NOx and stored oxygen are also shown.
You. Storage capacity utilization speed R_ijIs the initial slope of this curve
The ratio of the stored oxidizer is divided by the ratio of the NOx generation time.
Value.

[0030] Figure 14, except that a graph of the relative purged fuel amount with respect to the relative fill time t _F, is similar to FIG. The storage capacity utilization speed R _ij (purge fuel ratio / fill time ratio) is expressed as the initial slope of this curve.
For a given calibration value of air-fuel ratio, EGR and ignition timing (SPK) at a given speed and load point, the relative N
Ox generation amount is linearly related to the relative fill time t _F.
FIG. 14 illustrates the relationship between the amount of purge fuel containing HC and CO added to the trap and the time the trap is exposed to the incoming NOx. Purge fuel is divided into those required for purging stored oxygen and those required for purging stored NOx.

The reduction in the NOx absorption capacity of the trap is as follows:
It is represented by Equations 1 and 2.

(Equation 1)

(Equation 2)The storage capacity utilization rate RS before modification, ie, before modification (%)
Is given by Equation 1. This is the engine speed and
As a function of load, all actions in which the trap filling operation occurs
For a moving cell, the cell filling rate R_ij (% / s) time
Represents the total found. Relative cell filling speed R_ij
(Purge fuel ratio / fill time ratio)
Filling time t corresponding to 100% filling_FChange in purge time
Obtained by dividing the quantity. Where Equation 1 is the reference value
Equation 2 is a real-acting formula with modifiers
It is. The modifier in Equation 2 is
M ₁M about air-fuel ratio_Two, M about EGR amount_Three, That
Then the ignition advance about M_FourIt is. Individual R_ijBut tiger
Yes, but not fully utilized
The points are summed up to a value less than 100%. this
For storage capacity, the sum of the time spent in all cells
Total t_FIs the trap filling time. The result of this calculation
Is the effective trap capacity utilization RSM given by Equation 2.
(%). Reference filling rate for a given area
Is the time t spent in that area_KMultiplied by_Two,
M_ThreeAnd M_Four, And continuously summed.
The sum is the trap temperature modifier M₁Modified by (T)
It is. Trap when modified sum RSM reaches 100%
Is almost completely filled with NOx, and a purge event is planned
Is done.

FIG. 15 shows a map of the data stored for the reference trap filling rate _Rij . The entire system, consisting of the engine and the exhaust gas purification system including the three-way catalyst and the trap, is functioned on a matrix map of engine speed and load. As typical calculations, typical calibration values for the air-fuel ratio (AFR), EGR, and spark advance are used. The trap temperature T _ij is recorded for each area represented by the rotation speed and the load. Figure 16a
To 16d are the air-fuel ratio, EGR, ignition advance SPK, and trap temperature T for which the trap filling speed _Rij was determined in FIG.
_{For ij} , typical functionalization conditions are listed.

If the actual driving condition of the vehicle is different from the functioning condition shown in FIG.
₁ (T), M ₂ (air-fuel ratio), M ₃ (EGR amount), M ₄ (ignition advance)
Is corrected. Figure 1 shows the correction for M ₁ (T)
Shown in Figure 7. Trap of NOx absorption capacity is the optimum temperature T _o
Reaches a maximum value under (350 ° C. in this embodiment), as shown, when the trap temperature T falls below it or whether more than the optimal temperature T _o is the trap of the NO _x
Modifications are made to reduce storage capacity.

Modifications to the modifiers M ₂ , M ₃ and M ₄ are shown in FIG.
To 18c. These corrections are applied when the actual air-fuel ratio, the actual EGR amount, and the actual ignition advance differ from the values used in the functionalization of FIG.

FIG. 19 is a flowchart showing a procedure for determining the reference filling time of the trap 34 when the trap 34 should be purged. If the purge event is complete (determined at block 120) and the engine is running lean (determined at block 122), block 1
As indicated by 24, the trap is filled. The charging time is based on the prediction of the NOx storage capacity impairment _Rij , which is appropriately modified for the air-fuel ratio, EGR amount, ignition advance and trap temperature. At block 126 the engine speed and load are read, and at block 128 the reference fill rate R _ij is obtained from a look-up table (FIG. 15) using engine speed and load as a starting point. Trap temperature, engine air-fuel ratio, EGR quantity, spark advance and the time t _K is the block 1
16a-16d and used at block 132 to calculate the time weighted sum RSM based on the time spent in the region determined by the given engine speed and load. If the RSM is close to 100%,
As shown in blocks 134 and 136, a purge event is scheduled. Otherwise, the trap filling process continues at block 122. The filling time determined in FIG. 19 is the reference filling time. This will change when the trap is sulphided or thermally damaged. However, in the foregoing procedure (FIGS. 7a, 8 and 11), the optimal filling time was determined by the destabilization process, the need for desulfurization was determined, and a determination was made whether the trap had been thermally damaged. .

The planned value of the purge time t _P is the oxygen purge time
Both components of t _{P osc} and NOx purge time t _{P NOx} must be included. Thus, t _P = t _Posc + t _{P NOx} .
Controller 10 has a look-up table that provides t _{P osc} , a function that is strongly dependent on temperature. For traps containing cerium oxide, t _{P osc} is Arrheniu
s) according to the formula t _{P osc} = C _exp (−E / kT). Where C is a constant determined by the type and state of the trap, E is the activation energy, and T is the absolute temperature.

Having described in detail the best mode for carrying out the invention, those skilled in the art to which the invention pertains will appreciate
Various alternatives and embodiments will be envisaged for implementing the invention as defined in the appended claims.

[0038]

According to the present invention, the trap is operated continuously at optimal conditions for NOx conversion efficiency, thereby maximizing vehicle fuel economy and reducing CO and atmospheric emissions.
NOx emissions can be minimized. Furthermore, it can be ensured that the high-performance desulfurization of the trap is constantly maintained above the given minimum value of the NOx conversion efficiency of the trap.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an engine control system embodying the present invention.

FIG. 2 is a graph showing a voltage response of an oxygen sensor with respect to an air-fuel ratio.

FIG. 3 shows (a) engine air-fuel ratio, (1) short purge time, (2) intermediate purge time, and (3) long purge time.
FIG. 6 is a graph comparing (b) the response of the downstream oxygen sensor, (c) the data capture window of the EGO sensor, and (d) the amount of CO emissions to the atmosphere.

FIG. 4 is a more detailed graph showing the response of the oxygen sensor to time for (1) short purge time, (2) intermediate purge time, and (3) long purge time.

FIG. 5 is a graph of normalized oxygen sensor saturation time t _sat as a function of purge time t _P.

[6] if the peak voltage V _P of the oxygen sensor is smaller than the reference voltage V _ref, the graphs of the normalized oxygen sensor saturation time t _sat as a function of the peak voltage V _P.

FIG. 7 shows the relationship between trap purge time t _P and trap fill time t _F, and shows the optimal purge time t _PT for a given fill time t _FT and two suboptimal purge points 1 and 2; It is a graph.

FIG. 7a shows the relationship between trap purge time t _P and fill time t _F when the purge time is optimized for all fill times, with the optimal purge time t _PT and fill time t _FT being the preferred system. Indicates the operating point T and is located on the response curve 2
The two suboptimal points A and B are also the graphs shown.

FIG. 8 shows the relationship between purge time t _P and fill time t _F for four different trap operating conditions as the degradation of NOx trap performance progresses gradually, and the total purge time
shows the outer挿化been purge time t _{P osc} for oxygen storage portion in t _P, a graph.

FIG. 9 is a graph showing the relationship between NOx trapping capability and purge time for four different trap operating condition states as degradation progressively progresses due to sulfation, thermal damage, or both.

FIG. 10 is a flowchart for optimizing a trap purge time t _P.

FIG. 11 is a flowchart for system optimization.

FIG. 12 is a flowchart for determining whether or not desulfurization of a trap is necessary.

FIG. 13 is a graph showing the relationship between the relative amount of oxidant stored in the trap and the relative time of exposure of the trap to NOx flowing into the trap.

FIG. 14 is a graph of a relative purge fuel with respect to a relative charging time.

FIG. 15 is a map diagram of a reference trap filling speed R _ij (NOx storage capacity impairment rate) for various rotation speeds and loads at given map values of temperature, air-fuel ratio, EGR amount, and ignition advance angle. is there.

FIG. 16a shows the relationship between the trap filling speed R _{ij and} FIG.
FIG. 7 is a map diagram showing a function state of an air-fuel ratio determined in FIG.

FIG. 16b shows the trap filling speed R _ij in FIG.
FIG. 4 is a map diagram showing a functioning state of an EGR amount determined in FIG.

FIG. 16c shows the trap filling speed R _ij in FIG.
FIG. 7 is a map diagram showing a function state of an ignition advance angle determined in FIG.

FIG. 16D shows the trap filling speed R _ij in FIG.
FIG. 6 is a map diagram showing a function state with respect to a trap temperature determined in FIG.

FIG. 17 is a diagram showing how the modifier of the trap capacity impairment ratio changes with temperature.

FIG. 18a is a diagram showing how the air-fuel ratio modifier changes when the value of the air-fuel ratio changes from the value functionalized in FIG. 16;

FIG. 18B is a diagram showing how the EGR amount modifier changes when the value of the EGR amount changes from the value functionalized in FIG. 16;

18c is a diagram showing how the ignition advance modifier changes when the value of the ignition advance changes from the value functionalized in FIG. 16; FIG.

FIG. 19 is a flowchart for determining a time for planning trap purging.

[Explanation of symbols]

 20 Engine 34 Lean NOx trap 38 Oxygen sensor

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme Court II (Reference) B01D 53/36 101B (72) Inventor Garth Michael Meyer 48128, Dearborn N. Surbury Lane, Michigan USA 124 (72) Inventor Joseph Richard Aislook United States 48304, Michigan Bloomfield Hills Great Oaks Drive 776

Claims (6)

[Claims] 1. A lean NOx trap disposed in an exhaust passage of an engine upstream of an oxygen sensor is charged to almost storage capacity during a charging period, and during a subsequent purging period,
In a method of filling and purging a lean NOx trap so as to be substantially emptied, based on a calibrated trap filling rate multiplied by the time spent in that region in the region of engine speed and load,
Calculating the impairment of the NOx storage capacity, continuously adding the calculated impairment, and if the added impairment exceeds a predetermined value that is less than 100% of the storage capacity impairment, a purge event is performed. A planning step. 2. The method of claim 1 wherein said calibrated trap fill rate is modified as a function of air-fuel ratio, EGR and spark advance. 3. The method of claim 2 wherein said calculated impairment is modified as a function of trap temperature. 4. functionalizing the trap fill rate over the operating range for engine speed and load using calibration values representing trap temperature, air-fuel ratio, EGR, and spark advance, and The impairment of the NOx storage capacity is corrected by taking into account the difference between the actual and the calibrated operating conditions. 4. The method of claim 3, wherein the sum is a weighted sum of time based on the time spent in. Purging between the trap 5. A purge time t _P (k) and, based on the output voltage of the sensor, to determine the purge time for the next purge cycle t _P (k + 1) 5. The method of claim 4, further comprising the step of: monitoring the output signal of the oxygen sensor. 6. Adjusting the fill time by a predetermined increment, each greater or less than the original fill time, until the change in purge time for the corresponding fill time equals a predetermined target value. 6. The method of claim 5, further comprising: optimizing the filling time.