JP3937487B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an NOx absorbent to an NOx absorbent in an exhaust system of an internal combustion engine having a NOx absorbent that absorbs NOx when the exhaust air-fuel ratio is lean and releases the absorbed NOx when the exhaust oxygen concentration decreases. Estimate the amount absorbed,Estimated NOxThe present invention relates to an exhaust gas purification apparatus for an internal combustion engine that reduces and purifies NOx by reducing exhaust oxygen concentration by so-called rich control of the air-fuel ratio when it is determined that the amount of absorption exceeds the allowable NOx absorption amount in the NOx absorbent.
[0002]
[Prior art]
For example, WO93 / 25806 is known as an exhaust emission control device for this type of internal combustion engine.
[0003]
In this specification, NOx absorbent that absorbs NOx when the air-fuel ratio of the inflowing exhaust gas is lean and releases the absorbed NOx when the oxygen concentration in the inflowing exhaust gas is reduced is described in the engine exhaust passage. NOx absorbed in the NOx absorbentabsorptionNOx to estimate quantityabsorptionQuantity estimation means and NOxabsorptionNOx estimated to be absorbed by the NOx absorbent by the amount estimating meansabsorptionQuantity is predeterminedNOx absorptionAn exhaust purification device is disclosed that includes NOx releasing means for releasing NOx from the NOx absorbent by reducing the oxygen concentration in the exhaust gas flowing into the NOx absorbent when the allowable amount is exceeded.
[0004]
And in this device, the NOx absorption allowable amount of the NOx absorbent is fixed,exhaustThe timing of the decrease in oxygen concentration is taken.
[0005]
[Problems to be solved by the invention]
However, the NOx absorption capacity in the NOx absorbent (the maximum amount of NOx that can be absorbed by the NOx absorbent, also referred to as the NOx saturation amount) is the NOx absorbent capacity in the atmosphere of the NOx absorbent.exhaustAir-fuel ratio orexhaustIt was found to depend on the oxygen concentration.
As a result, when the rich control is performed with the NOx absorption capacity fixed as in the prior art, for example, when the lean air-fuel ratio changes due to a change in external conditions or a failure of sensors, the timing of the rich control is reduced. The optimum time will be missed, resulting in an increase in NOx emissions from the exhaust pipe to the atmosphere and a deterioration in fuel consumption.
The present invention has been made in view of the above points, and provides an exhaust gas purification apparatus for an internal combustion engine capable of performing rich control at an appropriate timing according to a changing NOx absorption capacity. It is an object to prevent an increase in NOx emission to the atmosphere and deterioration of fuel consumption.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, the present invention has the following configuration in an exhaust gas purification apparatus for an internal combustion engine.
[0009]
That is, the present invention is provided in an exhaust system of an internal combustion engine that changes the degree of lean combustion according to the operating state, and absorbs NOx when the exhaust air-fuel ratio is lean and absorbs when the exhaust oxygen concentration decreases. When the NOx absorbent that releases NOx and the NOx absorption amount absorbed by the NOx absorbent during lean combustion are estimated, and the estimated NOx absorption amount is determined to have exceeded the allowable NOx absorption amount in the NOx absorbent, In an exhaust gas purification apparatus for an internal combustion engine, comprising an exhaust oxygen concentration lowering control means for lowering an exhaust oxygen concentration,The exhaust oxygen concentration lowering control means determines that the estimated NOx absorption amount exceeds the NOx absorption allowable amount when the estimated NOx absorption amount exceeds a predetermined determination value, and the determination value Is set to a larger value as the exhaust air-fuel ratio increases.It is characterized by that.
[0010]
When the lean air-fuel ratio changes due to changes in external conditions, sensor failures, etc., exhaust oxygen concentration lowering control (rich control) is performed at a later timing as the degree of lean combustion (lean) increases.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
<Outline of device>The embodiment described below isThis is an example in which the exhaust emission control device of the present invention is applied to a gasoline engine.
[0012]
BookAccording to the embodimentGasoline engine, engine bookthe body'sFixie in cylinderTheHave and burnRoomHave. further,SaidInstitutional bookTo the bodyIsSaidIgnition to ignite gasoline in the combustion chamberPlugIntake and intakeTo the valveTherefore, the intake port is opened and closed and introduces a mixture of gasoline and air into the cylinder.GAnd exhaustTo the valveTherefore, an exhaust port that opens and closes and exhausts the burned exhaust gas in the cylinder.GIs provided.
[0013]
The intake portGThe corresponding branchTubeVia Surge TanToConcatenated,SaidEach branchOn the tubeRespectivelySaidIntake portInsideFuel injection to inject fuel towardValveInstalled. The surge tongueIs, Intake ductTheAir cree throughTo naConcatenated,SaidIntake duckInsideNo throttleValveHas been placed. on the other hand,ExhaustPauG, Exhaust manifoldDoExhaustTubeNOx absorption throughAgentBuilt-in caseToIt is connected.
[0014]
This gasoline engine is an electronic control unit consisting of a digital computer.ToTherefore, this electronic control unit is controlled.GBi-directionalToTherefore, ROM (read-only memory) connected to each other),RAM (random access memory)),CPU (microprocessor),Input portGAnd output portTheI have.
[0015]
SaidInput portToIsSaidSurge TanInsidePressure sensorSa, saidthrottleTo the valveIdle switch providedHCrankshaftToGear position detection provided on linked automatic transmissionInstrumentAnd vehicle speedSa, saidexhaustOn the tubeTemperature sensor providedSa, saidEngine speed center provided on the crankshaftSaIt is connected.
[0016]
SaidPressure sensorSa,SaidSurge TanInsideAn output voltage proportional to the absolute pressure is generated.SaidIdle switchThe aboveDetect that the throttle valve is at idling opening.SaidGear position detectionThe vessel isDetect the gear state of the automatic transmission.SaidVehicle speedSaDetect body speed.SaidTemperature sensorSa,SaidCaseinAboveCurrentSaidDetect the exhaust gas temperature in the exhaust pipe.SaidEngine speed sensorSa is an institutionDetect the number of rotations. In addition,SaidPressure sensorSa, saidTemperature sensorSaOutput voltage is A / D conversionvesselThrough eachSaidInput portToEntered.
[0017]
SaidThe output port has a corresponding driveThe roadThrough,SaidignitionPlugAndSaidFuel injectionTo the valveConnected. CPIn UIs calculated based on the detection results from the sensors and the like, for example, the fuel injection time TAU according to the following equation.
[0018]
TAU = TP · K Here, TP represents the basic fuel injection time, and K represents a correction coefficient. The basic fuel injection time TP indicates the fuel injection time required to make the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder the stoichiometric air-fuel ratio. The stoichiometric air-fuel ratio refers to a mixing ratio in which the amount of fuel in an air-fuel mixture is such that the amount of fuel is completely combusted with respect to the amount of oxygen in the air.
[0019]
The basic fuel injection time TP is obtained in advance by experiments,SaidSurge TanInsideAs a function of absolute pressure PM and engine speed NFIG.In advance in the form of a map as shown inSaidROWithin MIs remembered.
[0020]
The correction coefficient K is a coefficient for adjusting the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder. When K = 1.0, the air-fuel mixture supplied into the engine cylinder becomes the stoichiometric air-fuel ratio. On the other hand, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder when K <1.0 is larger than the stoichiometric air-fuel ratio, that is, lean, and when K> 1.0, the air-fuel ratio supplied into the engine cylinder is mixed. The air-fuel ratio of the gas becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.
[0021]
The value of this correction coefficient K isSaidSurge TanInsideIs determined in advance with respect to the absolute value PM and the engine speed N,FIG.An example is shown in. In this example,SaidSurge TanInsideIn the region where the absolute pressure PM of the engine is relatively low, that is, in the engine low and medium load operation region where the vehicle runs at a constant speed, the value of the correction coefficient K is set to a value smaller than 1.0, and the mixture supplied in the engine cylinder The air-fuel ratio is made lean.
[0022]
on the other hand,SaidSurge TanInsideIn a region where the absolute pressure PM of the engine is relatively high, that is, in an engine high load operation region such as during acceleration operation, the value of the correction coefficient K is 1.0, and the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is theoretically The air-fuel ratio is assumed. Also,SaidSurge TanInsideIn the region where the absolute pressure PM is highest, that is, in the engine full load operation region, the value of the correction coefficient K is greater than 1.0, and the air-fuel ratio of the air-fuel mixture supplied into the engine cylinder is made rich.
[0023]
In an internal combustion engine, the frequency of low-medium load operation is usually the highest, and the lean air-fuel mixture is combusted over most of the operation period. next,SaidAn exhaust emission control device using a NOx absorbent will be described.
[0024]
FIG.IsSaidcombustionRoom1 schematically shows the concentration of typical components in the exhaust gas discharged from the exhaust gas.FIG.As you can see,SaidThe concentration of unburned HC and CO in the exhaust gas discharged from the combustion chamber isSaidcombustionIndoorIncreases as the air-fuel ratio of the air-fuel mixture supplied to the engine becomes richer,SaidcombustionRoomOxygen in exhaust gas discharged from₂The concentration ofSaidcombustionIndoorThe air-fuel ratio of the air-fuel mixture supplied to the engine increases as the air-fuel ratio becomes leaner.
[0025]
SaidCaseinWithinHoused inSaidNOx absorptionAgentNOx occlusion reduction type catalyst, for example, using alumina as a carrier, and on this carrier, for example, alkali metal such as potassium K, sodium Na, lithium Li, cesium Cs, alkaline earth such as barium Ba, calcium Ca, lanthanum At least one selected from rare earths such as La and yttrium Y and a noble metal such as platinum Pt are supported. Engine intake passage andSaidNOx absorptionTopThe ratio of air and fuel (hydrocarbon) supplied in the exhaust passage in the flowSaidNOx absorptionTo the agentThe air-fuel ratio of the inflow exhaust gasSaidNOx absorptionAgentWhen the air-fuel ratio of the inflowing exhaust gas is lean, NOx is absorbed, and when the oxygen concentration in the inflowing exhaust gas decreases, the absorbed NOx is released.
[0026]
In addition,SaidNOx absorptionTopWhen fuel (hydrocarbon) or air is not supplied into the exhaust passage of the flow, the air-fuel ratio of the inflowing exhaust gas isSaidcombustionIndoorIn accordance with the air / fuel ratio of the mixture supplied to theSaidNOx absorptionThe agent is the abovecombustionIndoorWhen the air-fuel ratio of the air-fuel mixture supplied to the engine is lean, it absorbs NOx,SaidcombustionIndoorWhen the oxygen concentration in the air-fuel mixture supplied to the fuel cell decreases, the absorbed NOx is released and reduced.
<NOx absorption / release / reduction mechanism>SaidNOx absorbent (NOx storage reduction catalyst))soNOx absorption and reduction ofFIG.It is thought that it is performed by the mechanism as shown in. This mechanism is the case where platinum Pt and barium Ba are supported on the support, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
[0027]
First, when the exhaust gas becomes considerably leaner, the oxygen concentration in the exhaust gas greatly increases.FIG.Oxygen O as shown in (A)₂Is O₂ ^-Or O^2-It adheres to the surface of platinum Pt. Next, NO contained in the exhaust gas is O on the surface of platinum Pt.₂ ^-Or O^2-Reacts with NO₂(2NO + O₂→ 2NO₂).
[0028]
Then generated NO₂IsSaidAs long as the NOx absorption capacity of the NOx absorbent is not saturated, it is oxidized on platinum Pt.NOxAbsorbed into the absorbent and combined with barium oxide BaO,FIG.Nitrate ion NO as shown in (A)_Three ^-In the form ofSaidNOx absorptionInside the agentTo spread. In this way NOx isSaidNOx absorptionInside the agentTo be absorbed.
[0029]
On the other hand, when the oxygen concentration in the exhaust gas decreases, NO₂The amount of produced decreases, and by the reaction opposite to the above reaction,SaidNOx absorptionInside the agentNitrate ion NO_Three ^-Is NO₂Or in the form of NOSaidNOx absorptionMedicineReleased.
[0030]
In other words, NOx decreases when the oxygen concentration in the exhaust gas decreases.SaidNOx absorptionMedicineWill be released.FIG.As shown in FIG. 4, if the lean degree of the inflowing exhaust gas is lowered, the oxygen concentration in the inflowing exhaust gas is lowered. Therefore, if the lean degree of the inflowing exhaust gas is lowered, the air-fuel ratio of the inflowing exhaust gas is reduced. Even leanSaidNOx will be released from the NOx absorbent.
[0031]
On the other hand,SaidcombustionIndoorWhen the air-fuel mixture supplied to is made rich and the air-fuel ratio of the exhaust gas becomes rich,FIG.A large amount of unburned HC and COorganDischarged from. These unburned HC and CO are oxygen O on platinum Pt.₂ ^-Or O^2-As soon as it reacts and is oxidized.
[0032]
In addition, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the exhaust gas extremely decreases,SaidNOx absorptionAgent, NO₂Or release NO. This NO₂Or NOFIG.As shown in (B), it reacts with unburned HC and CO to be reduced. In this way, NO on platinum Pt₂Or when NO no longer exists,SaidNOx from NOx absorbent to next₂Or NO is released. Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, it will be within a short time.SaidNOx absorptionMedicineNOx is released. O on platinum Pt₂ ^-Or O^2-If unburned HC and CO remain after consumingSaidNOx absorptionMedicineNOx released from theorganNOx discharged from the fuel is also reduced by the unburned HC and CO.
[0033]
Therefore, if the air-fuel ratio of the inflowing exhaust gas is made rich, it will take less time.SaidNOx absorptionTo the agentAbsorbed NOx is released, and the released NOx is reduced, so that NOx can be prevented from being discharged into the atmosphere.
[0034]
Also,SaidNOx absorptionAgentSince it has the function of a reduction catalyst, even if the air-fuel ratio of the inflowing exhaust gas is made the stoichiometric air-fuel ratioSaidNOx released from the NOx absorbent is reduced. However, when the air-fuel ratio of the inflowing exhaust gas is set to the stoichiometric air-fuel ratio,SaidNOx absorptionMedicineHave released NOx only gradually,SaidNOx absorptionTo the agentIt takes a long time to release all the absorbed NOx.
[0035]
Even if the air-fuel ratio of the inflowing exhaust gas is lean, if the leanness of the air-fuel ratio of the inflowing exhaust gas is lowered,SaidNOx absorptionMedicineNOx is released. Therefore,SaidNOx absorptionMedicineIn order to release NOx, the oxygen concentration in the inflowing exhaust gas may be reduced.
[0036]
However,SaidNOx absorptionMedicineEven if NOx is released, if the air-fuel ratio of the inflowing exhaust gas is lean,SaidNOx absorptionTo the agentNOx is not reduced. So in this case,SaidNOx absorptionMedicinalIt is necessary to arrange a reducing agent downstream. Considering this point, inflow and outflowspiritNOx with rich air-fuel ratio of gasThereleaseTo letThis is more economical without providing a separate reducing agent.
<NOx release control device> During warm-up operation at low temperatures and full load operation,FIG.As shown inSaidcombustionIndoorThe air-fuel ratio of the air-fuel mixture supplied to the engine becomes rich, and the air-fuel mixture becomes the stoichiometric air-fuel ratio during acceleration operation. In most of the other operating areas, the lean mixture isSaidcombustionIndoorIs burning.
[0037]
But,SaidNOx absorptionAgentEven if NOx is released during full load operation and acceleration operation, if the frequency of full load operation or acceleration operation is low, NOx gradually accumulates while the lean air-fuel mixture is burning, and NOx Absorption capacity decreases.
[0038]
Then, after a whileSaidNOx absorptionAgent, NOx cannot be absorbed. Therefore, NOxSaidNOx absorptionTo the agentIn order to continuously absorb, even when the lean air-fuel mixture is continuously burning, control to periodically reduce the oxygen concentration of the inflowing exhaust gas, that is,InflowThe exhaust gas air-fuel ratio is periodically made rich, orInflowMake the air-fuel ratio of the exhaust gas periodically the stoichiometric air-fuel ratio, etc.SaidNOx absorptionMedicineTherefore, it is necessary to periodically release NOx.
[0039]
For convenience, this control is hereinafter simply referred to as rich control, but if this rich control cycle is long, the lean air-fuel mixture is being burned.SaidNOx absorptionMedicinalThe NOx absorption capacity is reduced, and NOx cannot be absorbed and released to the atmosphere. On the other hand, if the cycle of the rich control is made too short, the fuel consumption will increase this time. Accordingly, the rich control needs to be performed at an optimal timing in consideration of these.
[0040]
In the NOx release control device in this embodiment,SaidNOx absorptionTo the agentAbsorbed NOxabsorptionRich control is performed when the amount exceeds a predetermined reference, and the reference is changed in accordance with the degree of lean combustion.
[0041]
In this example,SaidNOx absorptionMedicineA program for controlling release periodically is stored in advance in the ROM, and the CP is used by the program.On UIn addition, the following means are realized as the NOx release control device.
[0042]
That is,FIG.As shown in FIG. 4, this apparatus includes NOx emission amount estimation means 51 for estimating the NOx emission amount from the engine based on the detected conditions such as the engine speed, the load information, and the catalyst temperature inputted from the various sensors. ,SaidNOx absorptionTo the agentNOx absorption amount estimation means 52 for estimating the absorbed NOx absorption amount, and the above various conditionsSaidFrom the air-fuel ratio calculating means 53 for calculating the air-fuel ratio before and after the NOx absorbent, the air-fuel ratio calculated by the air-fuel ratio calculating means 53 and the various conditions.SaidNOx absorption capacity estimation means 54 for estimating the NOx absorption capacity in the NOx absorbent, NOx saturation determination value determination means 55 for determining the NOx saturation determination value from the NOx absorption capacity estimated by the NOx absorption capacity estimation means 54, NOx absorption Rich control execution means 56 that executes rich control when the NOx absorption amount estimated by the quantity estimation means 52 becomes equal to or greater than the NOx saturation determination value.Here, the NOx saturation determination value is a determination reference value for determining whether the NOx absorption amount exceeds the NOx absorption allowable amount of the NOx absorbent, and the NOx absorption amount exceeds the NOx saturation determination value. The NOx absorption amount is determined in advance so that it can be determined that the NOx absorption allowable amount has been exceeded.
[0043]
Therefore, these means estimate the NOx absorption amount absorbed by the NOx absorbent during lean combustion as referred to in the present invention, and determine that the estimated NOx absorption amount exceeds the NOx absorption allowable amount in the NOx absorbent. The exhaust oxygen concentration lowering control means 61 reduces the exhaust oxygen concentration.
[0044]
Here, in this exampleExhaust purificationNOx emission estimation means 51 for estimating NOx emission from the engine in the apparatus;SaidNOx absorptionTo the agentThe NOx absorption amount estimation means 52 for estimating the absorbed NOx absorption amount will be described.
[0045]
SaidNOx absorptionTo the agentAbsorbed NOxabsorptionWhen determining the quantity,SaidNOx absorptionTo the agentTotal NOx absorbedabsorptionIt is difficult to detect the amount directly. Therefore, the NOx emission amount in the exhaust gas discharged from the engine is estimated here, and the NOxDischargeFrom quantitySaidNOx absorptionTo the agentThe amount of absorbed NOx was estimated.
[0046]
That is, as the engine speed N increases, the amount of exhaust gas discharged from the engine per unit time increases, so that the engine speed increases as the engine speed increases.ofNOxDischargeThe amount increases. Also, the higher the engine load, that is,SaidSurge TanInsideThe higher the absolute pressure PM ofSaidcombustionRoomAs the amount of exhaust gas discharged from the engine increases and the combustion temperature increases, the higher the engine load (Said(The higher the absolute pressure in the surge tank)ofPer unit timeofNOxDischargeThe amount increases.
[0047]
FIG.(A) is per unit time determined by experimentofFrom the institutionofNOxDischargeQuantity,SaidSurge TanInsideThe relationship between the absolute pressure PM and the engine speed N is shown, and each curve is the same NOxDischargeIndicates the amount. As shown in this figure,From the institutionPer unit timeofNOxDischargeThe amount isSaidSurge TanInsideIncreases as the absolute pressure PM increases, and increases as the engine speed N increases. In addition,FIG.NOx shown in (A)DischargeAmountFIG.(B) in advance in the form of a mapSaidROWithin MIs remembered. So thisFIG.The NOx emission amount can be estimated from the map (B).
[0048]
Next, the NOx absorption capacity estimation means 54 will be described. First, the NOx absorption capacity estimation means 54FIG.As shown in theSaidInflow exhaust gas to NOx absorbentofFrom the relationship between temperature and NOx absorption capacity,SaidThe NOx absorption capacity of the NOx absorbent is estimated. That is, when the exhaust gas temperature T decreases, the activity of the catalyst (absorbent) decreases, so the NOx absorption capacity NoxCAP decreases and the exhaust gas temperature increases.SaidWhen the temperature of the NOx absorbent increases,SaidSince NOx contained in the NOx absorbent is decomposed and spontaneously released, the NOx absorption capacity becomes small. Therefore, the NOx absorption capacity increases when the exhaust gas temperature is between approximately 350 ° C. and 500 ° C.
[0049]
It has been found that the NOx absorption capacity is basically determined by the exhaust gas temperature, but also changes depending on the degree of lean combustion, that is, lean. In the present invention, the relationship between the air-fuel ratio and the NOx absorption capacity isFIG.As shown in Fig. 2, the higher the air-fuel ratio, that is, the leaner the oxygen concentration is,SaidNOx absorptionWith agentExperiments have revealed that the NOx absorption capacity of NO increases.
[0050]
Therefore, the NOx absorption capacity estimating means 54FIG.In the NOx absorption capacity obtained from the relationship inFIG.Make corrections in consideration of the relationship shown in. The relationship between air-fuel ratio and NOx absorption capacitySaidProvided in the ROM,SaidThe NOx absorption capacity corresponding to the air-fuel ratio calculated by the air-fuel ratio calculating means 53 isSaidRead from the ROM map. The degree of change in the NOx absorption capacity is determined according to the change in the air-fuel ratio.FIG.By multiplying the NOx absorption capacity obtained in step 1, the corrected NOx absorption capacity can be obtained.
[0051]
Further, a NOx saturation determination value corresponding to the NOx absorption capacity is set in the map. The NOx saturation determination value determining means 55 reads the NOx saturation determination value corresponding to the NOx absorption capacity from this map.
[0052]
The rich control execution means 56SaidWhen the amount of NOx absorbed by the NOx absorbent exceeds this NOx saturation judgment value,NOx absorption isIt is determined that the allowable amount of NOx absorption has been exceeded, and rich control is started.
[0053]
Note that the NOx saturation determination value is X% of the value of the NOx absorption capacity. Here, X = 100, that is, the same value as the NOx absorption capacity itself may be set, but it is set lower than the value of the NOx absorption capacity, for example, 50% of the NOx absorption capacity.May do.SaidWhen the NOx absorbent deteriorates, before the actual NOx absorption reaches the NOx absorption capacitySaidThe NOx absorbent will saturate. At this time, if the NOx saturation determination value is the same value as the NOx absorption capacity, the rich control has already been started.SaidThe NOx absorbent is saturated, and NOx that cannot be absorbed is released to the atmosphere. By making the NOx saturation judgment value lower than the NOx absorption capacity as described above,SaidIn preparation for deterioration of the NOx absorbent, NOx release control can be reliably performed.
[0054]
A characteristic point of the present invention is that it includes delay means 62 that delays the start timing of exhaust oxygen concentration reduction control by the exhaust oxygen concentration reduction control means 61 as the degree of lean (lean combustion) increases. In the present embodiment, it can be said that the air-fuel ratio detection means 53, the NOx absorption capacity estimation means 54, the NOx saturation determination value determination means 55, and the rich control execution means 56 realize the delay means 62.
[0055]
Note that the cause of the change in the lean burn (lean) may be that the air-fuel ratio changes due to a change in external conditions or a failure of sensors. In addition, the degree of lean changes for each operation region. That is, since the torque is not so required during steady operation, the air-fuel ratio is controlled to be largely lean, and during operation that requires acceleration, control is performed with lean that is close to the ideal air-fuel ratio (stoichiometry).
<Rich Control Timing> As described above, the NOx saturation determination value also fluctuates in accordance with the fluctuation of the NOx absorption capacity of the NOx absorbent estimated by the NOx absorption capacity estimation means 54. Therefore, the execution timing of the rich control also changes. fluctuate. Rich control timing chart showing this situationFIG.Shown in
[0056]
(a) When the lean degree is large When the lean degree is large,FIG.As shown by the thick dashed line,organDischarged from theSaidNOx absorptionTo the agentThe inflow exhaust gas has a relatively low NOx concentration. And as you can see from the figure,SaidNOx absorptionWith agentNOx absorption capacity ofFIG.(A)), the resultNOxSince the saturation determination value is also high, NOx can be absorbed for a long time.
[0057]
Therefore, the cycle of rich control isFIG.It becomes longer as shown in. At this timeSaidNOx absorptionMedicinalExit sideofThe NOx concentration isFIG.It changes as shown by the thick broken line at the bottom. For your reference, if rich control is not executed,SaidNOx absorbentofOutmouth~ sideofThe NOx concentration changes as indicated by a two-dot chain line and approaches the NOx concentration of the inflowing exhaust gas.
[0058]
(b) When the lean degree is small When the lean degree is small,FIG.As shown by the thick solid line,organDischarged from theSaidNOx absorptionTo the agentThe NOx concentration of the inflowing exhaust gas is higher than that in the case (a) where the lean degree is large. Also,FIG.As it is clear from the case of (a) where the degree of leanness is large,SaidNOx absorptionWith agentNOx absorption capacity ofFIG.(B)), the resultNOxSince the saturation judgment value is also low, NOx cannot be absorbed for a long time.
[0059]
Therefore, the cycle of rich control isFIG.As shown in FIG. 5, the lean degree is shortened and the rich control is frequently performed as compared with the case where the lean degree is large (a). At this timeSaidNOx absorptionMedicinalExit sideofThe NOx concentration isFIG.It changes as shown by the thick solid line at the bottom.
[0060]
For your reference, if rich control is not executed,SaidNOx absorbentofOutmouth~ sideofThe NOx concentration changes as indicated by the one-dot chain line and approaches the NOx concentration of the inflowing exhaust gas. Also, here, compared to the case where the lean degree is large (a),Of the NOx catalystThe reason for the large slope of the NOx concentration on the outlet side isFIG.As shown in the middle row, (b) is the inflowing exhaust gas.ofThis is because the NOx concentration is high.
[0061]
in this way,The NOx saturation determination value is set to a variable value corresponding to the air-fuel ratio of the exhaust gas flowing into the NOx absorbent, and the NOx saturation determination value is set to a larger value as the air-fuel ratio of the exhaust gas flowing into the NOx absorbent increases. By doing soAs the degree of lean (lean combustion) increases, the cycle of rich control (exhaust oxygen concentration reduction control) by rich control execution means 56 (exhaust oxygen concentration reduction control means)ButlongBecome, Delay the start of rich controlCan.
<Rich control flow> The NOx release control is performed by periodically enriching the air-fuel mixture as described above.FIG.This will be described with reference to the flowchart of FIG.FIG.Indicates a time interruption routine executed by interruption every predetermined time.
[0062]
First, in step 101, it is determined whether the lean combustion or the rich combustion is performed by determining whether the correction coefficient K is smaller than 1.0 at the fuel injection time TAU = TP · K.
[0063]
When the correction coefficient K <1.0, that is, in the operating state in which the air-fuel mixture should be made lean, the routine proceeds to step 102, the count value D is made zero, and then the routine proceeds to step 103.
[0064]
In step 103,SaidPressure sensorToDetectedSaidSurge TanInsideBased on absolute pressure PM and engine speed NFIG.From the map shown in (B)From the institutionNOx per unit timeDischargeThe amount Nij is calculated by the NOx emission amount estimating means 51.
[0065]
Next, at step 104, the NOx absorption amount estimating means 52Per unit timeNOxDischargeThe quantity Nij is multiplied by the interrupt time interval Δt, and these products (Nij · Δt) are added to ΣNOx. The product (Nij · Δt) is the interrupt time interval ΔtofFrom the institutionofNOxDischargeRepresents the quantity, which isSaidΣNOx is absorbed by NOx absorbentSaidTotal NOx absorbed by NOx absorbentabsorptionRepresents an estimate of the quantity.
[0066]
Next, in step 105,SaidBased on the exhaust gas temperature T detected by the temperature sensor,FIG.The NOx absorption capacity NoxCAP is calculated by the NOx absorption capacity estimation means 54 from the relationship shown in FIG. When the NOx absorption capacity NoxCAP is calculated, in step 106,SaidDetected by oxygen concentration sensorSaidBased on the oxygen concentration in the exhaust gas flowing into the NOx absorbentSaidCalculate the air-fuel ratio of the NOx absorbent atmosphere,FIG.As shown inThis calculatedThe NOx absorption capacity is corrected according to the air-fuel ratio. Next, at step 107, the NOx saturation determination value determining means 55 determines the NOx saturation determination value.
[0067]
In step 108,SaidNOx absorbed in NOx absorbentabsorptionThe amount (ΣNOx) isNOxIt is determined whether or not the saturation determination value has been exceeded. If not, the processing cycle is terminated. At this time, the lean air-fuel mixture is burned and NOx discharged from the engine is reduced.SaidIt is sequentially absorbed by the NOx absorbent.
[0068]
In step 108, ΣNOx isNOxIf it is determined that the saturation determination value has been exceeded, the routine proceeds to step 109, where rich processing is performed to release NOx. Since the rich processing amount is determined by the richness of the air-fuel ratio and the processing time, the processing amount is controlled by increasing / decreasing the richness and the length of the processing time. The rich process is performed by changing the correction coefficient K and changing the fuel injection time (fuel injection amount) TAU in TAU = TP · K described above. When the rich process is completed, the processing cycle ends.
[0069]
On the other hand, when it is determined in step 101 that K ≧ 1.0, that is, in an engine operating state where the air-fuel mixture should be rich or the stoichiometric air-fuel ratio, the routine proceeds to step 118 where the count value D is incremented by 1. Is done. Next, at step 119, the count value D is a constant value D.₀It is determined whether or not it has become larger. D> D₀When it becomes, it progresses to step 120 and (SIGMA) NOx is made zero. That is, when combustion of a rich mixture or a stoichiometric air-fuel mixture continues for a certain period of timeSaidNOx absorptionMedicineNOx is considered to have been released, so NOxabsorptionThe quantity estimated value ΣNOx is set to zero.
[0070]
Finally, as described above, the characteristic point of the present invention is that the delay means for delaying the start timing of the exhaust oxygen concentration reduction control by the exhaust oxygen concentration reduction control means as the degree of lean (lean combustion) increases. Specifically,FIG.As shown in Fig. 2, focusing on the point that the NOx absorption capacity fluctuates according to the air-fuel ratio, it becomes a rich control start trigger for NOx release.NOxChange the saturation judgment value,By setting the NOx saturation determination value to a larger value as the air-fuel ratio of the exhaust gas flowing into the NOx absorbent increases, FIG.As shown in Fig. 3, the richer the degree of leaning, the rich control start timeButlateTo beOr the cycle of rich controlButlongTo beIt was. Except for this point, the basic technique is the same as the technique described in International Publication No. WO93 / 25806, and this publication is referred to as much as possible for understanding the present invention.
[0071]
【The invention's effect】
The present inventionA determination value that is a criterion for determining whether or not to execute exhaust oxygen concentration lowering control (rich control) is a variable value corresponding to the air-fuel ratio of the exhaust gas flowing into the NOx absorbent, and the determination value increases as the exhaust air-fuel ratio increases. By setting to a large value,As the degree of lean combustion (lean) increases, the start timing of exhaust oxygen concentration reduction control by the exhaust oxygen concentration reduction control means is delayed.Will be possible,as a result,When the lean air-fuel ratio changes due to changes in external conditions or sensor failuresEvenFurther, exhaust oxygen concentration lowering control (rich control) can be performed at an optimal timing according to the degree of lean, and an increase in NOx emission amount from the exhaust pipe to the atmosphere and deterioration of fuel consumption can be prevented.
[Brief description of the drawings]
FIG. 1 is a diagram showing a map of basic fuel injection time.
FIG. 2 is a diagram showing correction coefficients.
FIG. 3 is a graph schematically showing the concentrations of unburned HC, CO and oxygen in exhaust gas discharged from the engine.
FIG. 4 is a diagram showing an NOx absorption / release action.
FIG. 5 is a block diagram showing a NOx release control device.
[Figure 6] From the engineofNOxDischargeIt is a figure which shows quantity.
[Fig. 7] NOx absorbentNOxIt is a figure which shows the relationship between absorption capacity and exhaust gas temperature.
FIG. 8 is a graph showing the relationship between the air-fuel ratio and NOx absorption capacity.
FIG. 9 is a timing chart showing air-fuel ratio control and NOx behavior during steady running.
FIG. 10 is a flowchart showing a rich control procedure.
[Explanation of symbols]
51 ... NOx emission estimation means
52 ... NOx absorption amount estimating means
53. Air-fuel ratio calculating means
54 ... NOx absorption capacity estimating means
55 ... NOx saturation determination value determining means
56: Rich control execution means
61. Exhaust oxygen concentration lowering control means
62 ... Delay means

Claims (1)

NOx absorption provided in an exhaust system of an internal combustion engine that changes the degree of lean combustion according to the operating state, and absorbs NOx when the exhaust air-fuel ratio is lean, and releases the absorbed NOx when the exhaust oxygen concentration decreases. And the NOx absorption amount absorbed by the NOx absorbent during lean combustion, and when it is determined that the estimated NOx absorption amount exceeds the allowable NOx absorption amount in the NOx absorbent, the exhaust oxygen concentration is decreased. An exhaust gas purification apparatus for an internal combustion engine, comprising: an exhaust oxygen concentration lowering control means , wherein the exhaust oxygen concentration lowering control means includes the estimated NOx absorption when the estimated NOx absorption amount becomes larger than a predetermined determination value. is intended to determine the amount exceeds the NOx absorption permissible amount, the determination value is an internal combustion machine, characterized in that it is set to a larger value as the exhaust air-fuel ratio becomes larger Exhaust gas purification device of.

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