JP3508301B2 - Engine air-fuel ratio control device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine air-fuel ratio control device for switching the air-fuel ratio of an air-fuel mixture supplied to an engine between a stoichiometric air-fuel ratio and a lean air-fuel ratio according to operating conditions.

[0002]

2. Description of the Related Art Conventionally, for the purpose of improving fuel economy, the air-fuel ratio of an air-fuel mixture supplied to an engine is a theoretical air-fuel ratio (hereinafter also referred to as "stoichi"; A / F = 14.6) under predetermined operating conditions.
Has proposed an air-fuel ratio control device for switching to a lean air-fuel ratio (for example, A / F = 20 to 23).

Further, in order to solve the problem that the output torque suddenly changes due to the sudden change in the air-fuel ratio, which causes a shock to the vehicle, the air-fuel ratio is continuously changed at the time of switching from the theoretical air-fuel ratio to the lean air-fuel ratio. I am trying to let you. On the other hand, in order to reduce the NOx emission amount during operation at a lean air-fuel ratio, NOx can be adsorbed in the exhaust system during operation at a lean air-fuel ratio, and the adsorbed NOx can be reduced during operation at a stoichiometric air-fuel ratio. A NOx adsorption catalyst or a lean NOx catalyst capable of adsorbing HC during operation at a stoichiometric air-fuel ratio and reducing NOx by the adsorbed HC and HC discharged from the engine during operation at a lean air-fuel ratio is provided. .

In particular, when the stoichiometric air-fuel ratio is switched to the lean air-fuel ratio, the NOx emission amount increases at the intermediate air-fuel ratio in the switching process, so the air-fuel ratio is switched even if the drivability is deteriorated due to the shock. Although it is necessary to increase the speed, it is possible to reduce the air-fuel ratio switching speed to some extent by adopting the NOx adsorption catalyst or the lean NOx catalyst.

However, since it is desirable that the air-fuel ratio switching speed depends on the NOx adsorption capacity of the NOx adsorption catalyst or the NOx conversion efficiency of the lean NOx catalyst, for example, in Japanese Patent Application No. 5-250245, the lean NOx catalyst is used. When using, lean NOx is calculated from the temperature and space velocity of the catalyst.
The steady conversion efficiency of the catalyst is estimated, and the switching speed from the stoichiometric air-fuel ratio to the lean air-fuel ratio is determined according to the estimated conversion efficiency.

[0006]

However, the experiments conducted by the inventors of the present invention show that the conversion efficiency of a lean NOx catalyst is not constantly determined by the temperature and space velocity of the catalyst, but is significantly changed by the operation history. Confirmed by. That is, in the lean NOx catalyst, the conversion efficiency is higher immediately after switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio, and the longer the operating time at the immediately preceding stoichiometric air-fuel ratio, the higher the conversion efficiency. This is because HC is adsorbed during operation at the stoichiometric air-fuel ratio, and the greater the amount of adsorbed HC, the higher the conversion efficiency immediately after switching to the lean air-fuel ratio.

Similarly, with respect to the NOx adsorbing catalyst, the NOx adsorbing ability also largely changes depending on the operation history.
That is, in the NOx adsorption catalyst, immediately after switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio, the adsorbing capacity is high, and as the operating time at the immediately preceding stoichiometric air-fuel ratio is longer, the adsorbing capacity is also higher. This is because the longer the operating time at the theoretical air-fuel ratio,
NOx immediately after desorption of NOx and switching to lean air-fuel ratio
This is because the x adsorption capacity becomes high.

Therefore, NOx based on such operation history
NOx adsorption capacity of adsorption catalyst or NO of lean NOx catalyst
If the switching speed at the time of switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio is constant irrespective of the change in the x conversion efficiency, the switching speed is too fast and the shock is generated even though the NOx adsorption capacity or the NOx conversion efficiency is high. However, there is a problem that the switching speed is too slow and the NOx emission amount increases although the NOx adsorption capacity or the NOx conversion efficiency is low.

In view of the above conventional problems, the present invention provides an air-fuel ratio control for an engine that can achieve both a high level of avoidance of a shock due to a sudden change in torque during air-fuel ratio switching and a suppression of NOx emissions. The purpose is to provide a device.

[0010]

Therefore, in the invention according to claim 1, there is provided an air-fuel ratio switching means for switching the air-fuel ratio of the air-fuel mixture supplied to the engine between the theoretical air-fuel ratio and the lean air-fuel ratio according to the operating conditions. In an engine that is provided with an exhaust system that includes a catalyst that adsorbs or reduces NOx during operation at a lean air-fuel ratio, as shown in FIG. 1, the operation history of the engine includes air-fuel ratio, intake air flow rate, and exhaust temperature history. And a switching speed determination means for determining the switching speed to the target lean air-fuel ratio according to the operation history up to that time when switching from the stoichiometric air-fuel ratio to the lean air-fuel ratio. The engine air-fuel ratio control device.

As the catalyst, a NOx adsorption catalyst capable of adsorbing NOx during operation at a lean air-fuel ratio and reducing the adsorbed NOx during operation at a stoichiometric air-fuel ratio, or during operation at a stoichiometric air-fuel ratio HC that adsorbs HC and that is adsorbed by the engine during operation at a lean air-fuel ratio and H that is discharged from the engine
Any lean NOx catalyst capable of reducing NOx with C may be used.

Further, in the invention according to claim 2 , the air-fuel ratio,
As a history of the intake air flow rate and the exhaust gas temperature, the cumulative intake air flow rate and the average exhaust gas temperature during the operation at the stoichiometric air-fuel ratio are stored.

[0013]

In the invention according to claim 1, the history of the air-fuel ratio, the intake air flow rate, and the exhaust temperature is stored as the engine operation history relating to the NOx adsorption capacity of the catalyst or the NOx conversion efficiency, and the stoichiometric to lean state is stored. At the time of switching, the switching speed to the target lean air-fuel ratio is determined according to the operation history up to that point.

This is because the engine operating history, especially air-fuel
Since the NOx adsorption capacity or NOx conversion efficiency of the catalyst changes depending on the ratio, the intake air flow rate, and the history of exhaust temperature , when the NOx adsorption capacity or NOx conversion efficiency is high, the switching speed is slowed to prevent the occurrence of shock, This is because when the NOx adsorption capacity or NOx conversion efficiency is low, the switching speed is increased to suppress an increase in NOx emission amount.

In the invention according to claim 2, more specifically,
The switching speed is changed according to the cumulative intake air flow rate and the average exhaust gas temperature during the continuous stoichiometric operation. As the cumulative intake air flow rate during the continuous stoichiometric operation increases and the average exhaust gas temperature increases, the desorption amount of NOx increases with the NOx adsorption catalyst and the HC adsorption amount increases with the lean NOx catalyst. → N of catalyst immediately after lean switching
This is because the Ox adsorption capacity or the NOx conversion efficiency is improved.

[0016]

EXAMPLES Examples of the present invention will be described below. First, a first embodiment of the present invention will be described with reference to FIGS. FIG. 2 shows the 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. An electromagnetic fuel injection valve 5 is provided in each branch portion of the intake manifold 4, and a fuel-air mixture is generated by the fuel injected from each fuel injection valve 5. Then, the air-fuel mixture is ignited by the spark plug 6 and burned in the combustion chamber.

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

A NOx adsorption catalyst 9 is provided in the middle of the exhaust pipe 8. This NOx adsorbing catalyst 9 has a function as a three-way catalyst, adsorbs NOx during lean operation, desorbs this NOx during stoichiometric operation, and reduces it by the three-way catalyst. Then, the exhaust gas passes through the NOx adsorption catalyst 9 and is then exhausted through the muffler 11. The control unit 12 that controls 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, a reference crank angle signal and a unit crank angle signal are output from the cam shaft rotation of the engine 1. A crank angle sensor 14 that can indirectly detect the engine speed N, a water temperature sensor 15 that detects the cooling water temperature Tw in the water jacket of the engine 1, an air-fuel mixture that is attached to the exhaust manifold 7 and is drawn into the engine 1. O ₂ sensor that outputs a voltage signal corresponding to oxygen concentration in exhaust gas related to fuel ratio
16, an exhaust temperature sensor 17 for detecting the exhaust temperature (catalyst inlet temperature or outlet temperature) Te, and the like are provided.

Here, the control unit 12
Controls the operation of the fuel injection valve 5 by performing arithmetic processing as described below based on signals from the various sensors. Next, the contents of arithmetic processing of the control unit 12 will be described with reference to the flowcharts of FIGS. It should be noted that this flow is executed every predetermined time Δt (for example, 10 ms).

In step 1 (denoted as S1 in the drawing; 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 stoichiometry (A / F = 14.6) is calculated from the intake air flow rate Q and the engine speed N.

In step 4, the cooling water temperature Tw is detected based on the signal from the water temperature sensor 15. In step 5, it is determined whether the cooling water temperature Tw is, for example, 75 ° C. or higher,
When the temperature is lower than 75 ° C., the operation is stoichiometric, so the process proceeds to step 8. When the cooling water temperature Tw is 75 ° C. or higher, the process proceeds to step 6 in order to realize the air-fuel ratio switching control according to the operating region.

In step 6, the region (stoichiometric region / lean region) 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 7. In step 7, it is determined whether or not it is in the lean region. If NO (stoichiometric region), the process proceeds to step 8 for stoichiometric operation, and if YES (lean region), the process proceeds to step 14 for lean operation.

[In the case of stoichiometric operation] In step 8, the value of the lean flag FL (FL = 0 during stoichiometric operation, FL = 1 during lean operation) is determined.
When FL = 1, the lean operation is currently in progress, and a lean-to-stoichi switching instruction is issued. At this time, the routine proceeds to step 9, where the lean flag FL = 0 is set. Also the following
In step 10 ', clear the cumulative intake air flow rate ΣQS
(ΣQS = 0). When FL = 0, the stoichiometric operation is continuing, so the routine proceeds to step 11 ', and the cumulative intake air flow
The amount ΣQS is calculated (ΣQS = ΣQS + Q), and
In step 11 ", based on the signal from the exhaust temperature sensor 17 ,
The exhaust gas temperature Te is detected, and the moving average is sequentially calculated,
The average exhaust temperature MTe is calculated.

After that, the routine proceeds to step 12, where the air-fuel ratio feedback correction coefficient α is calculated based on the signal from the O ₂ sensor 16. In the next step 13, from the basic fuel injection amount Tp equivalent to stoichiometry, the air-fuel ratio feedback correction coefficient α, and the voltage correction amount (ineffective injection time) Ts set based on the battery voltage, the fuel injection is performed according to the following equation. The amount Ti is calculated, and this routine ends. Ti = Tp × α + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and a drive pulse signal having a pulse width of Ti is output to the fuel injection valve 5 at a predetermined timing in synchronization with the engine rotation. Then, fuel injection is performed. At this time, the air-fuel ratio is controlled stoichiometrically.

[In the case of lean operation] In step 14, the value of the lean flag FL (FL = 0 during stoichiometric operation, FL = 1 during lean operation) is determined.
When FL = 0, it means that the stoichiometric-to-lean switching command is being issued during the current stoichiometric operation. At this time, the routine proceeds to step 15, where the lean flag FL = 1 is set. Further, in the next step 16, the timer T for measuring the time after lean switching is set.
Clear ML (TML = 0).

Further, in the next step 17 " , as shown in FIG. 6, the cumulative intake air flow rate ΣQS during the continuation of the stoichiometric operation.
Also , refer to the map that defines the switching time (time from switching start to completion) Tc related to the switching speed to the final target lean air-fuel ratio (for example, A / F = 22) at the time of lean switching according to the average exhaust gas temperature MTe . Then, the actual cumulative intake air flow rate ΣQ
The switching time Tc is retrieved from S and the average exhaust gas temperature MTe .

Here, the cumulative intake air flow rate ΣQS is large.
The switching time Tc is set longer so that the switching speed becomes slower as the average exhaust gas temperature MTe becomes higher . The NOx adsorption catalyst 9 of the present embodiment adsorbs NOx under lean conditions and desorbs NOx under stoichiometric conditions thereafter.
Since it is to be returned, cumulative intake air under stoichiometric conditions
The more air flow rate ΣQS, also a high average exhaust temperature MTe
This is because the NOx desorption / reduction process proceeds, so that the NOx adsorption capacity increases.

When FL = 1, the lean operation is continuing, so the routine proceeds to step 18, where the timer TML for measuring the time after lean switching is counted up by the execution time interval Δt of this routine. After this, the routine proceeds to step 19, where the current lean post-switching time TML is compared with the switching time Tc, and TM
If L ≦ Tc (within the switching time), the process proceeds to step 20.

In step 20, the basic fuel injection amount Tp is corrected and the fuel injection amount Ti is calculated according to the following equation:
This routine ends. Ti = Tp × [1- (1-14.6 / 22) · (TML / Tc)] + Ts That is, within the switching time Tc, the target air-fuel ratio is changed from stoichiometric (A / F = 14.6) as time (TML) elapses. The fuel injection amount Ti is calculated after correcting the basic fuel injection amount Tp to a value corresponding to the target air-fuel ratio while changing the final target lean air-fuel ratio (A / F = 22).

If TML> Tc (after the switching time has elapsed), the process proceeds to step 21. In step 21, the basic fuel injection amount Tp is set to the final target lean air-fuel ratio (A /
F = 22), the fuel injection amount Ti is calculated, and this routine is finished. Ti = Tp × (14.6 / 22) + Ts When the fuel injection amount Ti is calculated, this is set in a predetermined register, and the drive pulse signal having the pulse width of Ti is fueled at a predetermined timing in synchronization with the engine rotation. The fuel is injected by being output to the injection valve 5. At this time, the air-fuel ratio is controlled lean.

In the present embodiment, mainly step 6,
7, 13, 19, 20, and 21 correspond to the air-fuel ratio switching means,
Steps 8, 9, 10 ', 11 ' and 11 " correspond to the operation history recording means, and step 17 " correspond to the switching speed determining means. As described above, in the present embodiment, when the stoichiometric operation is switched to the lean state and the stoichiometric operation immediately before that is continued.
The switching time Tc is set according to the cumulative intake air flow rate ΣQS and the average exhaust gas temperature Mte . That is, stoichi
The higher the cumulative intake air flow rate ΣQS, the higher the average exhaust
The switching time T is set so that the switching speed becomes slower as the temperature becomes higher.
Make c longer. The NOx adsorption catalyst 9 adsorbs NOx under lean conditions, and NO under the stoichiometric conditions thereafter.
x is desorbed and reduced, so under stoichiometric conditions
The larger the cumulative intake air flow rate ΣQS, the more the average exhaust gas temperature M
The higher the Te, the more the NOx desorption / reduction process proceeds.
This is because the Ox adsorption capacity becomes high.

FIG. 7 shows changes in the NOx adsorption capacity (catalyst outlet NOx emission amount) with respect to the air-fuel ratio history of the NOx adsorption catalyst. When switching the air-fuel ratio from stoichiometric to lean as shown in Fig. 7, comparing the history of short stoichiometry as shown in (1) and the history of long stoichiometric as shown in (2) , immediately after the switching from stoichiometric to lean. , Compared with the case where the stoichiometry of (1) is short, the case where the stoichiometry of (2) is long is more advanced in the desorption / reduction process of NOx.
It can be seen that the Ox adsorption capacity is high and the NOx emission amount at the catalyst outlet can be reduced.

Since the NOx adsorption catalyst 9 of this embodiment exhibits the NOx adsorption capacity even with a lean air-fuel ratio, even if the air-fuel ratio is switched at a relatively slow speed, the NOx emission amount greatly increases. However, since the adsorption capacity is reduced depending on the operation history and the NOx emission amount is increased at the switching speed adapted to the high adsorption capacity, the switching speed is changed according to the change of the adsorption capacity. The control of NOx emissions and the improvement of drivability are made compatible at a high level.

Next, a second embodiment of the present invention will be described with reference to FIG . FIG. 8 is a system configuration diagram of the second embodiment,
The difference from FIG. 2 is that a lean NOx catalyst 10A (upstream side) and a three-way catalyst 10B (downstream side) are provided in the exhaust pipe 8 instead of the NOx adsorption catalyst. The lean NOx reduction catalyst 10A adsorbs HC during stoichiometric operation, and adsorbs HC during lean operation and H emitted from the engine.
C and NO reduce NOx.

Therefore, the longer the stoichiometric condition, the greater the amount of HC adsorbed, and the higher the NOx conversion efficiency immediately after lean switching. Therefore, similarly to the first embodiment, the switching time Tc may be set according to the cumulative intake air flow rate ΣQS and the average exhaust gas temperature MTe during the continuous stoichiometric operation. Therefore, the control flow charts shown in FIGS. 3 to 4 can be used as they are.

[0037]

As described above, according to the invention of claim 1, the engine operating history relating to the NOx adsorption capacity of the catalyst or the NOx conversion efficiency is the air-fuel ratio, the intake air
It stores the history of air flow rate and exhaust temperature, and when switching from stoichiometric to lean, according to the operation history up to that point,
By determining the switching speed to the target lean air-fuel ratio, it is possible to avoid the shock occurrence at the time of switching the air-fuel ratio and to reduce the NO.
The effect that the suppression of the x emission amount can be achieved at a high level at the same time is obtained.

According to the second aspect of the present invention, more specifically, by storing the cumulative intake air flow rate and the average exhaust temperature during the continuation of the stoichiometric operation, the NOx adsorption capacity or NOx conversion efficiency of the catalyst can be more accurately measured. The effect is that it is possible to obtain better control.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing the configuration of the present invention.

FIG. 2 is a system configuration diagram of the first embodiment.

FIG. 3 is a flowchart of the first embodiment (part 1).

FIG. 4 is a flowchart of the first embodiment (part 2).

FIG. 5 is a diagram showing a map for determining a stoichio-lean area.

FIG. 6 is a diagram showing a switching time setting map.

FIG. 7 is an explanatory diagram of NOx adsorption capacity when switching from stoichio to lean.

FIG. 8 is a system configuration diagram of the second embodiment.

[Explanation of symbols]

1 Engine 5 Fuel Injection Valve 9 NOx Adsorption Catalyst 10A Lean NOx Catalyst 10B Three-way Catalyst 12 Control Unit 13 Air Flow Meter 14 Crank Angle Sensor 15 Water Temperature Sensor 16 O ₂ Sensor 17 Exhaust Temperature Sensor

Front page continuation (58) Fields surveyed (Int.Cl. ⁷ , DB name) F02D 41/14 310 F01N 3/08 F02D 45/00 314 F02D 45/00 340

Claims (2)

(57) [Claims] 1. An air-fuel ratio switching means for switching an air-fuel ratio of an air-fuel mixture supplied to an engine between a stoichiometric air-fuel ratio and a lean air-fuel ratio in accordance with operating conditions, and an exhaust system having NOx during operation at a lean air-fuel ratio. In an engine equipped with a catalyst that adsorbs or reduces, the operation history of the engine includes air-fuel ratio, intake air flow rate and
Operation history storage means that stores the history of exhaust temperature, and switching speed determination that determines the switching speed to the target lean air-fuel ratio when switching from the theoretical air-fuel ratio to the lean air-fuel ratio according to the operation history up to that point An air-fuel ratio control device for an engine, characterized by comprising: 2. The operation history storage means stores, as the history of the air-fuel ratio, the intake air flow rate, and the exhaust temperature, the cumulative intake air flow rate and the average exhaust temperature when the operation is continued at the theoretical air-fuel ratio. The air-fuel ratio control device for an engine according to claim 1 .