JP3992004B2 - Engine air-fuel ratio control device - Google Patents

Description

  The present invention includes air-fuel ratio switching means for switching the air-fuel ratio of an air-fuel mixture supplied to an engine to a stoichiometric air-fuel ratio and a leaner air-fuel ratio depending on operating conditions, and an exhaust system at a lean-side air-fuel ratio. The present invention relates to an air-fuel ratio control apparatus for an engine having an exhaust treatment device that adsorbs NOx (nitrogen oxide) during operation and processes the adsorbed NOx during operation at a stoichiometric air-fuel ratio.

  Conventionally, for the purpose of improving fuel efficiency, the air-fuel ratio of an air-fuel mixture supplied to an engine is set to a leaner air-fuel ratio (hereinafter referred to as “stoichi”; A / F = 14.6) under a predetermined operating condition (hereinafter “Stoichi”). There has been proposed a lean control engine which is controlled to “lean”; for example, A / F = 22).

Further, in such a lean control engine, a NOx adsorption three-way catalyst is provided as an exhaust treatment device in the exhaust system so that NOx is adsorbed during operation in lean and the adsorbed NOx is treated during operation in stoichiometry. There is one in which a certain restriction is provided for switching the air-fuel ratio in relation to the above (Patent Document 1).
Japanese Patent Application No. 5-511556; Special Reissue (Republication) No. 5-12863

In view of such a situation, the present invention relates to the NOx adsorption capacity of the exhaust treatment device when the operating condition shifts from the stoichiometric region to the lean region, or when the operating condition shifts from the lean region to the stoichiometric region. Therefore, it is an object to improve the exhaust performance by switching the air-fuel ratio more appropriately.

Therefore, according to the first aspect of the present invention, in an engine provided with an exhaust treatment device that adsorbs NOx during the lean operation and processes the adsorbed NOx during the stoichiometric operation, the engine speed and the engine Means for determining whether an operating condition determined by a load is in a lean region or a stoichiometric region, a means for calculating a cumulative NOx processing amount during a stoichiometric operation (a stoichiometric cumulative NOx processing amount calculating unit), and the cumulative Means for calculating the hysteresis time based on the NOx processing amount, and switching to lean when the operating condition shifts from the stoichiometric region to the lean region until the hysteresis time elapses from the transition of the operating condition maintaining operation at stoichiometric prohibits switches for operation in lean after the hysteresis time unit (strike · The → lean delay means) and the provided, constituting the air-fuel ratio control apparatus for an engine (see Figure 1).

In the invention according to claim 2, in an engine provided with an exhaust treatment device that adsorbs NOx in the exhaust system during the lean operation and processes the adsorbed NOx during the stoichiometric operation, the engine speed and the engine load Means for determining whether the operating condition determined in the lean region or the stoichiometric region, a means for calculating the cumulative NOx adsorption amount during the lean operation (lean cumulative NOx adsorption amount calculating means), and the cumulative NOx Means for calculating a hysteresis time based on the amount of adsorption, and when the operating condition has shifted from the lean region to the stoichiometric region, switching to the stoichiometric period until the hysteresis time has elapsed since the transition of the operating condition. to maintain the operation of the lean is prohibited, means for switching to the operation in the stoichiometric air-fuel ratio after the lapse of the hysteresis time (lean → scan It provided the alive delay means), and constitutes the air-fuel ratio control apparatus for an engine (see Figure 1).

According to the first aspect of the present invention, the accumulated NOx processing amount is calculated during operation at stoichiometry, and based on this, a hysteresis time for delaying the switching at the time of switching from stoichiometric to lean is set. That is, the smaller the cumulative NOx processing amount during stoichiometric operation, the longer the hysteresis time at the time of switching to lean operation, and the adsorption NOx processing is continued by continuing the stoichiometric operation before switching to lean operation. As the cumulative NOx processing amount during stoichiometric operation increases, the hysteresis time at the time of switching to lean operation is shortened, and the operation is switched to lean operation earlier.
Therefore, according to the first aspect of the present invention, the hysteresis time is set when the cumulative NOx processing amount is small by setting the hysteresis time when switching to the lean operation in correspondence with the cumulative NOx processing amount during the stoichiometric operation. The adsorption NOx is processed by stoichiometric operation, and when the accumulated NOx processing amount is large, the hysteresis time is shortened, and the fuel consumption can be improved by early switching to lean operation. It is done.

In the invention according to claim 2, the accumulated NOx adsorption amount is calculated during the lean operation, and based on this, the hysteresis time for delaying the switching at the time of switching from lean to stoichiometric is set. That is, the larger the accumulated NOx adsorption amount during lean operation, the shorter the hysteresis time when switching to stoichiometric operation, and early switching to stoichiometric operation.
Therefore, according to the invention according to claim 2 , by setting the hysteresis time when switching to the stoichiometric operation corresponding to the accumulated NOx adsorption amount during the lean operation, the hysteresis time increases as the accumulated NOx adsorption amount increases. As a result, the exhaust performance can be improved by switching to the stoichiometric operation at an early stage.

Embodiments of the present invention will be described below.
An embodiment of the present invention will be described with reference to FIGS.
FIG. 2 shows a system configuration.
Air is drawn into the combustion chamber of each cylinder of the engine 1 from the air cleaner 2 through the throttle valve 3 and the intake manifold 4. Each branch portion of the intake manifold 4 is provided with an electromagnetic fuel injection valve 5, and an air-fuel mixture is generated by the fuel injected from each fuel injection valve 5. The air-fuel mixture is ignited by the spark plug 6 and burned in the combustion chamber.

The fuel injection valve 5 is energized by a drive pulse signal output at a predetermined timing in synchronization with engine rotation from a control unit 12 described later, and opens the fuel adjusted to a predetermined pressure. Therefore, the fuel injection amount is controlled by the pulse width of the drive pulse signal.
The exhaust from the engine 1 reaches the exhaust pipe 8 through the exhaust manifold 7.

  In the middle of the exhaust pipe 8, a NOx adsorption three-way catalyst 10 is interposed as an exhaust treatment device. The NOx adsorption three-way catalyst 10 not only has a three-way catalyst function of oxidizing HC and CO under stoichiometric conditions and reducing NOx, but also adsorbs NOx under lean conditions, and adsorbs the NOx and the adsorbed NOx under stoichiometric conditions. It has a function to process newly flowing NOx. The exhaust gas passes through the NOx adsorption three-way catalyst 10 and is then exhausted through the muffler 11.

The control unit 12 for controlling the operation of the fuel injection valve 5 has a built-in microcomputer and receives signals from various sensors.
As the various sensors, an air flow meter 13 for detecting the intake air flow rate Q of the engine 1 on the upstream side of the throttle valve 3, and a reference crank angle signal and a unit crank angle signal are output indirectly from the cam shaft rotation of the engine 1. Crank angle sensor 14 that can detect the engine speed N, water temperature sensor 15 that detects the coolant temperature Tw in the water jacket of the engine 1, and the air-fuel ratio of the air-fuel mixture that is attached to the exhaust manifold 7 and sucked into the engine 1 An O ₂ sensor 16 that outputs a voltage signal corresponding to the oxygen concentration in the exhaust gas is provided.

Here, the control unit 12 controls the operation of the fuel injection valve 5 by performing arithmetic processing as described later based on signals from the various sensors.
Next, the calculation processing contents of the control unit 12 will be described with reference to the flowcharts of FIGS. This flow is executed every predetermined time Δt (for example, 20 ms).

In particular, this embodiment has different hysteresis time delays at the time of switching from stoichiometric to lean and at the time of switching from lean to stoichiometric, and the hysteresis time when switching from stoichiometric to lean is changed from lean to stoichiometric. The hysteresis time at the time of switching from stoichiometric to lean is set based on the accumulated NOx processing amount during the stoichiometric operation, while making the stoichiometric operation sufficiently performed prior to the lean operation. Further, the hysteresis time at the time of switching from lean to stoichiometry is set based on the accumulated NOx adsorption amount during lean operation.

In step 1 (indicated as S1 in the figure, the same applies hereinafter), the intake air flow rate Q is detected based on the signal from the air flow meter 13.
In step 2, the engine speed N is detected based on the signal from the crank angle sensor 14.
In step 3, a basic fuel injection amount Tp = K × Q / N (K is a constant) corresponding to stoichiometric (A / F = 14.6) is calculated from the intake air flow rate Q and the engine speed N.

In step 4, the coolant temperature Tw is detected based on the signal from the water temperature sensor 15.
In step 5, it is determined whether or not the cooling water temperature Tw is, for example, 70 ° C. or higher. When the temperature is lower than 70 ° C., the operation proceeds to step 6 because the operation is performed by stoichiometry.
In step 6, from the basic fuel injection amount Tp, the air-fuel ratio feedback correction coefficient α set based on the signal from the O ₂ sensor 16, and the voltage correction amount Ts set based on the battery voltage, the following equation is obtained. Then, the fuel injection amount Ti is calculated, and this routine is finished.

Ti = Tp × α + Ts
When the fuel injection amount Ti is calculated, this is set in a predetermined register, and at a predetermined timing in synchronism with the engine rotation, a drive pulse signal having a pulse width of Ti is output to the fuel injection valve 5 to perform fuel injection. Done. At this time, the air-fuel ratio is controlled to stoichiometric.

When the cooling water temperature Tw is 70 ° C. or higher as determined in step 5, the routine proceeds to step 7 in order to realize the air-fuel ratio switching control corresponding to the operation region.
In step 7, an area (stoichiometric area / lean area) on the map of FIG. 5 is detected based on the engine speed N and the basic fuel injection amount (load) Tp, and the process proceeds to step 8.

In step 8, it is determined whether or not it is a lean region. If it is a lean region, the process proceeds to step 31. If it is a stoichiometric region, the process proceeds to step 40.
In step 31, the value of the lean flag FL (FL = 0 during stoichiometric operation, FL = 1 during lean operation) is determined.
When FL = 0, this is the time when the stoichiometric / lean switching command is issued during the stoichiometric operation. At this time, the routine proceeds to step 32.

In step 32, the timer TL is increased by the execution time interval Δt of this routine in order to measure the hysteresis time from the stoichiometric to lean switching command to the actual switching, and the routine proceeds to step 33.
In step 33, it is determined whether or not the timer TL has become _{equal to} or longer than a hysteresis time T _HL determined in step 47 described later. When TL ≧ T _HL , lean switching is permitted and the process proceeds to step 34. This meaning will be described later.

In step 34, the lean flag FL is set (FL = 1), the stoichiometric flag FS is reset (FS = 0), and the process proceeds to step 35.
In step 35, the timer TL is reset (TL = 0), and the process proceeds to step 36.
In step 36, the accumulated NOx adsorption amount ΣC _NO during lean operation is initialized (ΣC _NO = 0), and the routine proceeds to step 39.

In step 39, the basic fuel injection amount Tp is corrected to the lean (A / F = 22) equivalent as shown in the following equation, the fuel injection amount Ti is calculated, and this routine is terminated.
Ti = Tp × (14.6 / 22) + Ts
When the fuel injection amount Ti is calculated, this is set in a predetermined register, and at a predetermined timing in synchronism with the engine rotation, a drive pulse signal having a pulse width of Ti is output to the fuel injection valve 5 to perform fuel injection. Done. At this time, the air-fuel ratio is controlled to be lean.

Thereafter, as long as the region is a lean region, FL = 1 in the determination in step 31, so the process proceeds to steps 37 and 38.
In step 37, to calculate the cumulative NOx adsorption amount .SIGMA.C _NO during lean operation. That is, as shown in the following equation, the NOx adsorption amount is obtained by the product of the intake air flow rate Q and the adsorption NOx concentration P _1, and this is added to the previous cumulative NOx adsorption amount ΣC _NO to obtain the cumulative NOx adsorption amount ΣC _NO. Update.

ΣC _NO = ΣC _NO + Q × P ₁ where P ₁ = f ₁ (Q)
The adsorbed NOx concentration P ₁ is a function of the intake air flow rate Q (Q large → P ₁ large), and is obtained by searching from the map shown in FIG.
In step 38, the hysteresis time T _HS at the time of switching from lean to stoichiometry is calculated according to the following equation in correspondence with the accumulated NOx adsorption amount ΣC _NO during the lean operation.

T _HS = m ₁ / ΣC _NO However, m ₁ is a constant. That is, the larger the cumulative NOx adsorption amount ΣC _NO during lean operation, the faster the switch to stoichiometric and the more the NOx processing is performed, so the switch from lean to stoichiometric. The hysteresis time T _HS is shortened.
After Steps 37 and 38, the process proceeds to Step 39 and the lean operation is continued.

If it is determined in step 8 that the area is a stoichiometric area, the process proceeds to step 40.
In step 40, the value of the stoichiometric flag FS (FS = 0 during lean operation, FS = 1 during stoichiometric operation) is determined.
When FS = 0, the lean-to-stoichi change command is issued during the lean operation. In this case, the process proceeds to step 41.

In step 41, the timer TS is increased by the execution time interval Δt of this routine in order to measure the hysteresis time from the lean-to-stoichi switching command to the actual switching, and the routine proceeds to step 42.
In step 42, the timer TS is determined whether it is above the hysteresis time T _HS defined at step 38 as described above, when the TS ≧ T _HS, the process proceeds to step 43 to allow stoichiometric switching. This meaning will be described later.
In step 43, the stoichiometric flag FS is set (FS = 1), the lean flag FL is reset (FL = 0), and the process proceeds to step 44.

In step 44, the timer TS is reset (TS = 0), and the process proceeds to step 45.
In step 45, the accumulated NOx processing amount ΣP _NO during stoichiometric operation is initialized (ΣP _NO = 0), and the process proceeds to step 48.
In step 48, based on the basic fuel injection amount Tp equivalent to stoichiometry, the fuel injection amount Ti is calculated according to the following equation, and this routine ends.

Ti = Tp × α + Ts
When the fuel injection amount Ti is calculated, this is set in a predetermined register, and at a predetermined timing in synchronism with the engine rotation, a drive pulse signal having a pulse width of Ti is output to the fuel injection valve 5 to perform fuel injection. Done. At this time, the air-fuel ratio is controlled to stoichiometric.

Thereafter, as long as it is the stoichiometric region, FS = 1 in the determination in step 40, so the process proceeds to steps 46 and 47.
In step 46, to calculate the cumulative NOx throughput .SIGMA.P _NO at stoichiometric operation. That is, as the following equation, and the intake air flow rate Q, obtains the NOx processing amount by the product of the process NOx concentration P _2, which was added to the accumulated NOx throughput .SIGMA.P _NO up to the previous time, the accumulated NOx amount of processing .SIGMA.P _NO Update.

ΣP _NO = ΣP _NO + Q × P ₂ where P ₂ = f ₂ (Q)
The processing NOx concentration P ₂ is a function of the intake air flow rate Q (Q large → P ₂ large), and is obtained by searching from the map shown in FIG. The reason why P ₂ is small when Q is small is that the amount of NOx desorbed is small when Q is small.
In step 47, corresponding to the accumulated NOx amount of processing .SIGMA.P _NO at stoichiometric operation, the hysteresis time T _HL during stoichiometric → lean switching is calculated by the following equation.

T _HL = m ₀ / ΣP _NO where m ₀ is a constant. That is, as the cumulative NOx processing amount ΣP _NO during stoichiometric operation increases, the hysteresis time T _HL when switching from stoichiometric to lean so that the switching to lean becomes faster. To shorten.
Further, assuming that m ₀ > m ₁ and under the same conditions, the hysteresis time T _HL at the time of switching from stoichiometric to lean is set longer than the hysteresis time T _HS at the time of switching from lean to stoichiometric.

After steps 46 and 47, the routine proceeds to step 48 and the stoichiometric operation is continued.
As described above, the cumulative NOx processing amount ΣP _NO is calculated during the stoichiometric operation, and the hysteresis time T _HL at the time of switching from stoichiometric to lean is set correspondingly (steps 46 and 47), and the lean operation is in progress. cumulative NOx adsorption amount .SIGMA.C _NO is calculated, in response to this hysteresis time T _HS during the lean → stoichiometric switching has been set (step 37, 38).

Here, when the operating condition shifts from the stoichiometric region to the lean region, the process proceeds to step 33 through steps 31 and 32, and the elapsed time TL from the switching command is a hysteresis time based on the accumulated NOx processing amount ΣP _NO during the stoichiometric operation. It is determined whether or not T _HL (= m ₀ / ΣP _NO ) or more. If TL ≧ T _HL , lean switching is permitted and the process proceeds to step 34. If TL <T _HL , lean switching is performed. Prohibit and go to step 40.

That is, as the cumulative NOx throughput .SIGMA.P _NO during the current stoichiometric operation is small, the hysteresis time T _HL during stoichiometric → lean switching to longer, before switching to lean operation, the suction NOx by continuing stoichiometric operation On the contrary, as the cumulative NOx processing amount ΣP _NO at the current stoichiometric operation is larger, the hysteresis time T _HL at the time of switching from stoichiometric to lean is shortened, and the operation is switched to the lean operation earlier.

Further, when the operating condition shifts from the lean region to the stoichiometric region, the process proceeds to step 42 through steps 40 and 41, and the elapsed time TS from the switching command is a hysteresis time T based on the accumulated NOx adsorption amount ΣC _NO in the lean operation. _{HS (= m 1 / ΣC NO} ) or whether to determine, in the case of TS ≧ T _HS, although advances allow stoichiometric switched to step 43, in the case of TS <T _HS, prohibits stoichiometric switching Then go to step 31.

That is, as the cumulative NOx adsorption amount ΣC _NO during the current lean operation increases, the hysteresis time T _HS when switching from lean to stoichiometric is shortened, and the operation is switched to stoichiometric operation earlier.

In this embodiment, steps 7, 8, 39 and 48 correspond to air-fuel ratio switching means (particularly, steps 7 and 8 are lean region / stoichiometric region determination means) , and steps 45 and 46 are stoichiometric. The time cumulative NOx processing amount calculation means corresponds to step 47, the hysteresis time calculation means based on the cumulative NOx processing amount, and the steps 32 and 33 correspond to stoichiometric to lean delay means . The portion of the step 36 and 37 corresponds to a lean time of the accumulated NOx adsorption amount calculating means, the portion of the step 38 corresponds to the hysteresis time calculating unit based on the cumulative NOx adsorption amount, part of the step 41 and the lean → stoichiometric It corresponds to a delay means.

Note that cumulative NOx processing amount ΣC during stoichiometric operation _NO (Step 46) and cumulative NOx adsorption amount ΣP during lean operation _NO (Step 37) may be obtained by simply accumulating the intake air flow rate Q as a value corresponding thereto. This is because the cumulative NOx processing amount and the cumulative NOx adsorption amount are substantially proportional to the cumulative intake air flow rate.

That is, the cumulative NOx processing amount (cumulative intake air flow rate) ΣP during stoichiometric operation _NO , The intake air flow rate Q is read as a value related to the NOx processing amount as shown in the following equation, and this is the cumulative NOx processing amount ΣP up to the previous time. _NO Cumulative NOx processing amount ΣP _NO May be updated.
ΣP _NO = ΣP _NO + Q

Also, the accumulated NOx adsorption amount (cumulative intake air flow rate) ΣC during lean operation _NO Is calculated, the intake air flow rate Q is read as a value related to the NOx processing amount as in the following equation, and this is the cumulative NOx adsorption amount ΣC up to the previous time. _NO Cumulative NOx adsorption amount ΣC _NO May be updated.
ΣC _NO = ΣC _NO + Q

Further, in the above embodiment, the cumulative NOx processing amount ΣC during the stoichiometric operation. _NO And cumulative NOx adsorption amount ΣP during lean operation _NO Is calculated based on the intake air amount, and the following effects are obtained.

In the apparatus described in Patent Document 1, the accumulated NOx adsorption amount is estimated by accumulating the engine speed during lean operation, and based on this, the lean operation is limited or stoichiometric. However, since there is no load term in the accumulation of the engine speed, the accuracy when estimating the adsorption limit is poor, and if it is limited only by time, the accuracy is still worse. There was a problem that improvement in exhaust performance could not be expected.

In this regard, in the above-described embodiment, the accumulated NOx processing amount is accurately calculated based on the intake air flow rate during the operation at the stoichiometry, and the accumulated NOx adsorption amount is calculated based on the intake air flow amount during the lean operation. Accurately calculated, and based on these, it is possible to improve exhaust performance by providing a certain limit for switching the air-fuel ratio accurately in relation to the NOx adsorption capacity of the exhaust treatment device and the like.

Functional block diagram showing the configuration of the present invention System configuration diagram of one embodiment of the present invention Flowchart (1) of one embodiment of the present invention Flowchart (2) of one embodiment of the present invention A diagram showing a stoichiometric / lean switching map Diagram showing adsorption NOx concentration characteristics The figure which shows processing NOx concentration characteristic

Explanation of symbols

1 engine
5 Fuel injection valve
10 NOx adsorption three way catalyst
12 Control unit
13 Air flow meter
14 Crank angle sensor
15 Water temperature sensor
17 O ₂ sensor

Claims (2)

In an air-fuel ratio control apparatus for an engine, the exhaust system includes an exhaust treatment device that adsorbs NOx during operation at a lean air-fuel ratio and processes the adsorbed NOx during operation at a stoichiometric air-fuel ratio.
Means for determining whether the operating condition determined by the engine speed and the engine load is in a lean region or a theoretical air-fuel ratio region;
Means for calculating a cumulative NOx processing amount during operation at the theoretical air-fuel ratio;
Means for calculating a hysteresis time based on the cumulative NOx processing amount;
When the operating condition shifts from the stoichiometric air-fuel ratio region to the lean region, switching to the lean-side air-fuel ratio is prohibited until the hysteresis time elapses from the transition of the operating condition. Means for maintaining operation and switching to operation at a lean air-fuel ratio after the hysteresis time has elapsed ;
An air-fuel ratio control apparatus for an engine comprising: In an air-fuel ratio control apparatus for an engine, the exhaust system includes an exhaust treatment device that adsorbs NOx during operation at a lean air-fuel ratio and processes the adsorbed NOx during operation at a stoichiometric air-fuel ratio.
Means for determining whether the operating condition determined by the engine speed and the engine load is in a lean region or a theoretical air-fuel ratio region;
Means for calculating a cumulative NOx adsorption amount during operation at a lean air-fuel ratio;
Means for calculating a hysteresis time based on the accumulated NOx adsorption amount;
When the operating condition shifts from the lean region to the stoichiometric air-fuel ratio region, switching to the stoichiometric air-fuel ratio is prohibited during the period from the transition of the operating condition until the hysteresis time elapses. Means for maintaining operation and switching to operation at the stoichiometric air-fuel ratio after elapse of the hysteresis time ;
An air-fuel ratio control apparatus for an engine comprising:

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