JPH09280086A - Engine combustion controller - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce NOx emitted from an engine as well as prevent its misfire especially under low load when an ignition point is set near the range causing misfire. SOLUTION: Calculating means 41 and 42 calculate an air-fuel ratio and ignition timing in lean-burn drive respectively, and a detecting means 43 detects the roughness of an engine. If the roughness is in a level below an allowance value, a correcting means 44 corrects the air-fuel ratio in lean-burn drive larger and retards the ignition timing in lean-burn drive to bring the roughness up to the maximum level in its stability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine combustion control device.

[0002]

2. Description of the Related Art Lean burn system (a system capable of switching between operation at a theoretical air-fuel ratio and operation at a lean air-fuel ratio)
In the engine of, the air-fuel ratio and the ignition timing are set on the rich side where the air-fuel ratio margin is taken from the stability limit,
When operating with lean air-fuel ratio, the air-fuel ratio is leaned to the stability limit based on the detection result of the roughness sensor (lean limit control), and the ignition timing is advanced at the same time when the air-fuel ratio is leaned in this lean limit control. There has been proposed a method in which the angle is corrected (the ignition timing is corrected at the same time when the fuel is made rich) (Japanese Patent Laid-Open No. 60-626).
No. 61).

[0003]

However, when the air-fuel ratio is made lean by the lean limit control, the ignition timing is corrected by advancing regardless of the load condition of the engine as in the conventional device. Misfires will occur due to variations, and the drivability will deteriorate significantly.

Explaining this, at high load, as shown in FIG. 24 (A), when the detected roughness is less than or equal to the allowable value (the stability is better than the allowable value), the air-fuel ratio is controlled by the lean limit control. (Abbreviated as A / F in the figure,
(Same in other figures) is made leaner than the set point, and if the ignition timing is advanced to a point after which the ignition timing is moved closer to the MBT, the fuel consumption is further improved while reducing NOx. is there.

On the other hand, when the load is low, as shown in FIG.
As shown in, the stability limit is closer to the retard side than MBT when compared with high load, when compared with equal stability.
The ignition timing at the set point with a margin of air-fuel ratio (ΔA / F shown in the figure) is ahead of the high load. Also, the air-fuel ratio at the set point is leaner than at high load. On the lean side, if the ignition timing is advanced, the ignitability deteriorates and misfire occurs, resulting in poor drivability. If the ignition timing is advanced at the same time when the air-fuel ratio is made lean by the lean limit control even at a low load, the point in FIG. 24 (B) becomes the control point. Point is very close to the misfire zone,
If there was a variation in the control of the air-fuel ratio at that control point, it jumped into the misfire zone and the drivability deteriorated due to misfire.

Therefore, in the present invention, when the air-fuel ratio is made lean by the lean limit control, the ignition timing is retarded particularly when the load is low, so that the NOx is reduced while the NOx is reduced at the time when the load is close to the set point and the misfire zone. The purpose is to prevent.

[0007]

In the first invention, FIG.
As shown in, means 4 for calculating the air-fuel ratio during lean operation
1 and means 42 for calculating the ignition timing during lean operation,
Means 43 for detecting the roughness of the engine, and correcting the air-fuel ratio in the lean operation to the lean side so that the roughness becomes the stability limit when the roughness is below the allowable value, and setting the ignition timing in the lean operation. Means 44 for correcting to the retard side, means 45 for controlling the air-fuel ratio of the engine so as to obtain the corrected air-fuel ratio, and means 46 for igniting at the corrected ignition timing are provided.

In the second aspect of the invention, as shown in FIG. 26, means 41 for calculating the air-fuel ratio during lean operation, means 42 for calculating the ignition timing during lean operation, and means 43 for detecting the roughness of the engine. And means for correcting the air-fuel ratio during the lean operation to the rich side and correcting the ignition timing during the lean operation to the advance side so that the roughness becomes the stability limit when the roughness is equal to or more than the allowable value. Further, means 45 for controlling the air-fuel ratio of the engine so as to obtain the corrected air-fuel ratio, and means 46 for igniting at the corrected ignition timing are provided.

According to a third aspect of the present invention, in the first or second aspect of the invention, the correction means reduces the load by reducing the reference air-fuel ratio correction amount and the reference ignition timing correction amount with respect to the maximum load during lean operation. It further comprises means for correcting the reference air-fuel ratio correction amount to the smaller side and the reference ignition timing correction amount to the larger side.

According to a fourth aspect of the present invention, in the first or second aspect of the invention, the correction means has the same rotation as the means for calculating the reference air-fuel ratio correction amount and the reference ignition timing correction amount for the maximum load during lean operation. It comprises means for correcting the reference air-fuel ratio correction amount to a smaller side and the reference ignition timing correction amount to a larger side as the load becomes lower under a few conditions and as the engine speed becomes lower at the same load condition.

According to a fifth aspect of the invention, in the third or fourth aspect of the invention, a catalyst capable of purifying NOx during lean operation is provided in the exhaust pipe, while the temperature of the NOx purifying catalyst during lean operation is below the activation temperature. In the above, the corrected reference air-fuel ratio correction amount is decreased and the corrected reference ignition timing correction amount is increased.

[0012]

In the conventional example, the control direction from the set point is determined by the leanness of the air-fuel ratio and the advance correction amount of the ignition timing. In the conventional example in which the control direction is the same regardless of the load, the control point comes to a position very close to the misfire zone when the load is low, so misfire occurs if there is control variation in the air-fuel ratio. On the other hand, in the first aspect of the present invention, the control direction from the set point is determined by the lean degree of the air-fuel ratio and the retard correction amount of the ignition timing, and the control point is set at the low load when the misfire zone approaches the stability limit. Since it is located at a position far from the misfire zone, no misfire will occur. Further, since the control point comes to the limit of stability, the amount of NOx generated is smaller than that of the conventional example, although the fuel consumption is slightly worse than that of the conventional example.

The relationship between the set point and the misfire zone changes depending on the load, and as the load becomes lower, the misfire zone approaches the set point. Therefore, even if it is appropriate for the load state when the control directions are matched, The control direction becomes improper when the load state is other than the above, but in the third invention,
When the roughness is below the allowable value, the leaner the air-fuel ratio becomes, the smaller the load becomes, and the larger the retard correction amount of the ignition timing becomes, so that the control direction is appropriate for any load. And when the load is low, NO
It is possible to prevent misfire while reducing x, and it is possible to improve fuel efficiency while reducing NOx when the load is high.

According to the fourth aspect of the present invention, considering that the misfire zone approaches the stability limit at low speed even under the same load condition as compared to high speed, when the roughness is equal to or less than the allowable value, low load is performed under the same speed condition. And the lower the rotation speed under the same load condition, the smaller the lean air-fuel ratio and the larger the retard correction amount for the ignition timing, so the lean limit control makes the lean correction amount for the air-fuel ratio and the retard correction amount for the ignition timing. Each correction ratio of N is optimized according to the load and the number of revolutions.
Misfire can be prevented while reducing Ox.

In the fifth aspect of the present invention, the NOx purifying catalyst is provided in the exhaust pipe, while the lean air-fuel ratio is reduced when the roughness is less than the allowable value in the region where the temperature of the NOx purifying catalyst during lean operation is below the activation temperature. Since the ignition timing retard correction amount is increased while the ignition timing retard amount is increased, the exhaust temperature rises as much as the ignition timing retard amount increases, and the NOx purification catalyst temperature is quickly raised to the active temperature due to the increase in the exhaust temperature. Therefore, the NOx purification catalyst can be effectively used during lean operation.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, intake air is temporarily stored in a collector 4 of an intake manifold from an air cleaner 2 through a throttle portion 3 and is then supplied to a combustion chamber of each cylinder through a branch pipe 5 of the intake manifold. . Fuel is injected from the fuel injection valve 6 toward the intake port of the engine 1 based on an injection signal from a control unit (abbreviated as C / U in the figure) 11. This injected fuel is mixed with air flowing into the combustion chamber to form a mixture, and the mixture is burned in the combustion chamber by spark ignition by the spark plug 7. The gas burned in the combustion chamber is discharged from the exhaust pipe 8.

The control unit 11 includes a Ref signal from the crank angle sensor 12 (generated every 180 ° in four cylinders and every 120 ° in six cylinders) and a 1 ° signal, an intake air amount signal from the air flow meter 13, and a throttle sensor. The throttle opening signal from 14 and the cooling water temperature signal from the water temperature sensor 16 are input, and the lean air-fuel ratio and the stoichiometric air-fuel ratio are controlled according to the conditions while judging the operating state based on these.

A three-way catalyst 9 is installed in the exhaust pipe 8 to maximize the conversion efficiency when operating at the stoichiometric air-fuel ratio,
It reduces Ox and oxidizes HC and CO. This three-way catalyst oxidizes HC and CO at a lean air-fuel ratio,
The conversion rate of NOx is low. However, as the air-fuel ratio shifts to the lean side, the amount of NOx generated decreases,
Above a predetermined air-fuel ratio, the fuel consumption can be reduced to the same level as the purification by the three-way catalyst 9, and at the same time, the leaner the operation, the better the fuel consumption. Therefore, in a predetermined operation region where the load is not so large, the operation based on the stoichiometric air-fuel ratio is switched to the operation based on the lean air-fuel ratio.

The relationship between the MBT, the stability limit, NOx and the fuel consumption with respect to the air-fuel ratio and the ignition timing when the air-fuel ratio is made lean has the characteristics shown in FIG. 2, so that it is shown under each operating condition at the lean air-fuel ratio. After understanding the characteristics, the air-fuel ratio and the ignition timing will be set on the rich side with a margin of air-fuel ratio margin from the stability limit, taking into consideration the production variations, parts variations, and air-fuel ratio variations due to climate change. .

However, since the set point at this time is rich in the air-fuel ratio, the fuel consumption and NOx are disadvantageous. Therefore, the stability of the air-fuel ratio based on the detection result of the roughness sensor during operation at the lean air-fuel ratio is set. This disadvantage can be eliminated by leaning to the limit (lean limit control).

There is one that corrects the ignition timing regardless of the load condition of the engine when the air-fuel ratio is made lean by this lean limit control. In this, misfire occurs due to the control variation of the air-fuel ratio at low load. Occurs and the drivability deteriorates remarkably.

To deal with this, the present invention focuses on the fact that the relationship between the set point and the misfire limit changes depending on the operating conditions (particularly the load), and changes the control direction from the set point according to the load during lean limit control. . This will be described with reference to FIG. 3. In FIG. 3, it is roughly divided into three loads (from the top, low load, medium load, high load), and the control direction from the set point at each load ( The ratio of the lean amount of the air-fuel ratio and the retard amount of the ignition timing) is indicated by an arrow.

First, when the load is low, the difference in the retard direction between the MBT and the stability limit becomes large as shown in FIG. 3 (A), and the set point is lean and on the advance side, so it is close to the misfire zone. Become. In this case, although the fuel consumption is slightly deteriorated, in order to reduce NOx (see FIG. 2) and prevent misfire, the air-fuel ratio is made lean and the ignition timing is retarded to control the stability limit. The control direction at this time goes to the lower right as shown in the figure (that is, the control point is separated from the misfire zone), and even if there is a control variation of the air-fuel ratio at the control point, it will not jump into the misfire zone. is there.

On the other hand, when the load is high, the set point is rich and on the retard side as shown in FIG. 3C, and there is a large margin from the misfire zone. Therefore, only the air-fuel ratio is lean to improve fuel efficiency and reduce NOx. (See FIG. 2). Here, the ignition timing is fixed, although there is a margin from the misfire zone at the time of high load, but if the ignition timing is advanced in the same direction as in the conventional example, there is a possibility of approaching the misfire zone. Because there is. Therefore, when comparing only when the load is high, the fuel efficiency is better in the conventional example, but the NOx is better in the present application.

When the load is medium, the control direction is intermediate between the low load and the high load (see FIG. 3B).

The contents of the control executed by the control unit 2 will be described with reference to the following flowchart.

The flow charts of FIGS. 4 and 5 are main routines for determining the fuel injection pulse width Ti (determining the air-fuel ratio) Ti given to the fuel injection valve 6 and the ignition advance value (ignition timing) ADV given to the ignition device. If it is a 4-cylinder engine, it is executed every engine revolution.

In step 1 of FIG. 4, operating conditions are calculated,
In step 2, the basic fuel injection amount during operation at the stoichiometric air-fuel ratio (hereinafter referred to as stoichiometric operation) is calculated from the calculated operating conditions. The contents of steps 1 and 2 will be described with reference to the subroutine of FIG. 6. In steps 21 and 22, the intake air amount Qa is calculated from the output of the air flow meter, and the engine speed N is calculated based on the signal from the crank angle sensor. From these values, in step 23 Tp = (Qa / N) × k (1) However, the basic injection pulse width amount Tp at the time of stoichiometric operation is calculated by the equation of k: constant. With this Tp, an air-fuel mixture having a stoichiometric air-fuel ratio is obtained.

In step 3 of FIG. 4, the basic ignition timing during stoichiometric operation is calculated. This will be explained with reference to the subroutine of FIG. 7. In step 31, Tp and N are read, and in step 32, a map having the contents of FIG. 8 is searched to find the basic ignition advance value B during stoichiometric operation.
Find the ADV. Here, since the ignition timing is a crank angle measured on the advance side with respect to the compression top dead center, the basic ignition advance value BADV is a unit of [° BTDC].

In step 4 of FIG. 4, it is determined whether or not the lean operation condition is satisfied. When the lean operation condition is not satisfied, the process proceeds to steps 5 and 6, and Ti = Tp × Co × α × 2 + Ts (2) where Co: various corrections The fuel injection pulse width Ti is calculated by the equation: coefficient α: air-fuel ratio feedback correction coefficient Ts: invalid pulse width, and the basic ignition advance value BADV is transferred to the ignition advance value ADV. In step 7, Ti is output to the output register for controlling the fuel injection valve, and AD
V is transferred to each output register for controlling the ignition device.

Here, Co in the equation (2) is a value for ensuring smooth operation under conditions such as engine startup, warm-up, and high load, and α is the value of the three-way catalyst 9 during stoichiometric operation. A value for correcting the deviation between the basic air-fuel ratio and the stoichiometric air-fuel ratio, which is determined by Tp so that the conversion action can be efficiently extracted, T
s is a value for compensating the operation delay from when the injection valve 6 receives the injection signal to when the valve is actually opened.

Equation (2) is an equation for sequential injection (injection for each cylinder once every two revolutions of the engine).

On the other hand, when the lean operation condition is satisfied, the routine proceeds from step 4 to step 8 to calculate the basic fuel injection amount and the basic ignition timing during lean operation. The contents of steps 4 and 8 will be described with reference to the subroutine of FIG. 9. In step 41, Tp and throttle opening TVO are read,
In steps 42 and 43, Tp and a predetermined value δ (for example, 5.0
ms), the throttle opening change amount ΔTVO and a predetermined value β (for example, 20 ° / s) are compared.

When Tp ≦ δ and ΔTVO ≦ β, it is determined that the lean operation condition is satisfied, and at step 44, a coefficient KLTP for calculating the basic fuel injection amount during lean operation.
From N and Tp to the map (KLTP
This coefficient KLTP is obtained by searching the map)
In step 45, Tp is multiplied to calculate the basic injection pulse width LTP during lean operation. Here, KLTP is a value smaller than 1.0, and Tp is reduced by this value so that an air-fuel ratio leaner than the theoretical air-fuel ratio can be obtained. In step 46, a map (LADV map) having the contents shown in FIG. 11 is retrieved from N and Tp, and the basic ignition advance value LADV during lean operation is searched.
Ask for. The unit of this LADV is also [° BTDC]. On the other hand, when Tp> δ (high load) and ΔTVO> β
At this time (during rapid acceleration), stoichiometric operation is performed, and therefore, steps 44, 45, and 46 are not advanced.

In step 9 of FIG. 5, it is checked whether or not it is stationary. The contents of step 9 will be described with reference to the subroutine shown in FIG.
In step 52, the variation ΔTVO of O and a predetermined value γ (for example, 5 ° / s) are compared. When ΔTVO ≦ γ (steady state), the lean limit control is permitted (set the flag to “1”) in step 53, and when ΔTVO> γ, the lean limit control is prohibited (the flag is reset to “0”) in step 54. ) Do.

Returning to step 9 in FIG. 5, since it is difficult to detect the roughness accurately when it is not steady, the routine proceeds to steps 10 and 11, where Ti = LTP × Co × α × 2 + Ts (3) After calculating the pulse width Ti and shifting LADV to the ignition advance value ADV, the operation of step 12 is executed. Note that α in equation (3) is α = during lean operation.
It is fixed at 1.0.

In the steady state, the routine proceeds from step 9 of FIG. 5 to step 13 and thereafter to perform lean limit control. If the roughness is less than the allowable value in step 13, the basic fuel injection amount is reduced and the basic ignition timing is retarded in step 14 to approach the stability limit. When the roughness is equal to or more than the allowable value, the routine proceeds from step 13 to step 15 to improve the stability by performing the increase correction of the basic fuel injection amount and the advance angle correction of the ignition timing.

Here, the contents of steps 13, 14 and 15 will be described in detail with reference to the subroutine of FIG.

In step 61, the rotation speed N is read, and from this N, in step 62

[Equation 1] The rotation variation rate dN as the roughness is calculated by the following equation.

Here, Nave in equation (4) is a value that changes every cycle because new data is taken in every cycle and the oldest data is discarded while performing calculations.

The roughness is not limited to this. For example, the indicated mean effective pressure Pi may be calculated and the fluctuation rate of this Pi may be used as the roughness. Further, it is also possible to use the surge torque obtained by extracting the Pi variation rate of 3 to 7 Hz.

In steps 63 and 64, a table (dL is a table) having the contents of FIG. 14 is searched from the rotational fluctuation rate dN to obtain the lean limit control correction amount dL, and a table having the contents of FIG. 15 (RAA from Tp). Table) to obtain the lean limit control correction ratio RAA, and in steps 65 and 66, LLCTP = LTP + (dL × kTP) × RAA (5) where kTP: dL is converted to the injection amount correction amount. Of the basic injection pulse width LLCTP at the time of lean limit control by the equation of the constant (for example, 0.3 ms), and LLCADV = LADV + (dL × kADV) × (1-RAA) (6) where kADV: dL is the ignition timing. The ignition advance value L during lean limit control is calculated using a constant (for example, 3 ° CA) formula for conversion into a correction amount. To calculate the CADV [degrees BTDC].

Here, the correction amount dL for lean limit control
As shown in FIG. 14, is a value that is negative when the rotation fluctuation rate dN is equal to or less than an allowable value (for example, near 3%) (small rotation fluctuation), and is positive when it is equal to or higher than the allowable value (large rotation fluctuation). Yes, dL × kTP in equation (5) is the fuel injection amount correction amount for the maximum load during lean operation, and dL × kAD
V is the ignition timing correction amount for the minimum load during lean operation. When the rotation fluctuation is below the allowable value, dL × kT
The basic injection pulse width LTP is corrected (made lean) by the amount of P × RAA and the basic ignition advance value LADV is set to dL.
× kADV × (1-RAA) is corrected to the retard side, and when the rotational fluctuation exceeds the allowable value, dL
× LTP increase by LTP (rich) and LADV is dL × kADV × (1-RAA)
By controlling the advance angle by the amount, the stability limit is controlled. When the rotational fluctuation rate dN is within the allowable range, dL = 0 is set as shown in FIG.

Further, the lean limit control correction ratio RA
As shown in FIG. 15, A is 1.0 at the maximum Tp during lean operation, and is a value that becomes smaller as Tp becomes smaller than this. As the load becomes lower due to the correction ratio RAA, the fuel injection amount correction amount (dL × kTP) becomes smaller (the air-fuel ratio becomes leaner) and the ignition timing correction amount (dL × kADV) becomes larger.

Returning to FIG. 5, in steps 16 and 17, the fuel injection pulse width Ti is calculated by the following formula: Ti = LLCTP × Co × α × 2 + Ts (7)
After shifting V to the ignition advance value ADV, the operation of step 12 is executed. Also at this time, α is fixed at 1.0.

Here, the operation of this embodiment will be described with reference to FIG. 3 when the roughness is equal to or less than the allowable value.

In the conventional example, if the roughness is the same value, the control direction from the set point is the same regardless of the load (see FIGS. 24 (A) and 24 (B)). As described above, since the control point comes to a position very close to the misfire zone as shown in B), misfire is caused by the control variation of the air-fuel ratio.

On the other hand, in the present embodiment, even in the roughness state of the same value (that is, when dL is constant), the control direction from the set point differs depending on the load. First, when the load is low, the correction ratio RAA is high. Since it becomes smaller, the lean degree of the air-fuel ratio (determined by dL × kTp × RAA) becomes smaller, and the retard correction amount (dL ×
kADV x (1-RAA)) becomes large. At this time, the control direction from the set point is shown in FIG.
As shown in (A), the control point comes to a position separated from the misfire zone with a margin, so that no misfire occurs. Further, since the control point comes to the limit of stability, the amount of NOx generated is smaller than that of the conventional example, although the fuel consumption is slightly worse than that of the conventional example.

On the other hand, at the time of the maximum load during lean operation, the correction ratio RAA becomes 1.0 which is the maximum value, so the degree of leaning of the air-fuel ratio is the value for the maximum load during lean operation and the retard correction amount. Becomes zero, and the control direction from the set point moves to the right as shown in FIG. 3 (C). At this time, since the MBT and the stability limit are close to each other, fuel consumption is improved without advancing the ignition timing as in the conventional example, and the control point reaches the stability limit, so that the amount of NOx generated is small. .

As described above, in this embodiment, the correction ratio RAA of the degree of leaning of the air-fuel ratio and the retard correction amount of the ignition timing depending on the load even in the roughness state of the same value.
Was changed (when the load is low, the lean degree is small and the ignition timing retard correction amount is large, and when the load is high, the lean degree is large and the ignition timing retard correction amount is small). It is possible to improve fuel efficiency and NOx performance during lean operation while preventing misfire regardless of load conditions.

FIG. 16 is a flowchart of the second embodiment and corresponds to FIG. Step 7 is different from FIG.
With only 1, here, a map having the contents of FIG. 17 is searched from N and Tp to obtain the lean limit control correction ratio RAA. The other controls are the same as those in the first embodiment (that is, FIGS. 4, 5, 6, 7, 9 and 12 are also used in the second embodiment).

Even under the same load condition, combustion becomes more unstable at low engine speeds than at high engine speeds. Therefore, especially at low engine speeds near idle, combustion is unstable and misfires easily occur. For example, as shown in FIG. 18 (A), the control direction from the set point is almost rightward at the time of high rotation in a high load state,
Even if the control point has an adequate allowance from the misfire zone, at the same high load and low speed, the misfire zone approaches the stability limit as compared to the high speed as shown in FIG. 18 (B). For this reason, if the control direction is set to the right almost the same as at the time of high rotation, the margin from the misfire zone will be small (inappropriate), and misfire will occur if there is a variation in the air-fuel ratio control. become. Therefore, in order to prevent the occurrence of misfire regardless of the engine speed during lean limit control, it is necessary to change the control direction from the set point not only according to the engine load but also according to the engine speed. If so, at low speeds, the control direction is directed to the lower right to increase the margin from the misfire zone.

Specifically, as shown in FIG. 17, the RAA value is made smaller as the rotation speed becomes smaller even at the same value of Tp, so that the lean degree at the lean limit control becomes small at the low rotation speed and That is, the retard correction amount becomes large (that is, the control direction at the time of low rotation becomes the lower right direction as shown in FIG. 18B).

As described above, in the second embodiment, the correction ratio RAA is set to T in consideration of the fact that even under the same load condition, the misfire zone approaches the stability limit at low speed as compared to high speed.
Since not only p but also N is obtained as a parameter, each correction ratio of the lean air-fuel ratio by the lean limit control and the retard correction amount of the ignition timing is optimized according to the load and the rotational speed. In particular, it is possible to prevent misfire while reducing NOx even at a low rotation speed close to idle.

FIG. 19 corresponds to FIG. 1 in the third embodiment.
Since the three-way catalyst 9 has a low NOx conversion rate during lean operation, as shown in FIG. 19, NOx conversion occurs during lean operation.
There is a catalyst (publicly known) 31 that can purify the above is provided downstream of the three-way catalyst 9.

As shown in FIG. 20, the conversion rate of the NOx purifying catalyst 31 becomes significantly worse when the catalyst temperature becomes T50 (temperature at which the conversion rate becomes 50%) or lower. During warm-up (for example, water temperature is 60 ° C)
(Above) and when the temperature of the NOx purification catalyst 31 is lowered during a low load period, it is important to raise the temperature of the NOx purification catalyst to T50 or higher. Here, the retard amount of the ignition timing when the air-fuel ratio is made lean from the set point to the stability limit by the lean limit control (step 66 in FIG. 13).
21 and the exhaust temperature are shown in FIG. 21, the exhaust temperature rises as the retard amount of the ignition timing increases, so when the NOx purification catalyst 31 is at a low temperature, It is not possible to increase the retard amount of ignition timing by changing the control direction during lean limit control.
It is effective in purifying NOx by the x purification catalyst.

The control flow is almost the same as in the first embodiment (or the second embodiment), except that FIG. 13 (or FIG. 16).
22 is used instead of the above. The part different from FIG. 13 (that is, steps 81, 82 and 83 in FIG. 22) will be mainly described. In step 81, the temperature sensor 32 (see FIG.
23), the temperature TCAT of the NOx purification catalyst 31 is read, and in step 82, the temperature correction coefficient RCAT is obtained by searching a table (RCAT table) having the contents of FIG. 23 from this catalyst temperature TCAT. In step 83, this is read in step 64. The value obtained by multiplying the calculated correction ratio RAA is set again as the correction ratio RAA.

The temperature correction coefficient RCAT is set to a value close to 0 in the region where the catalyst temperature TCAT is lower than T50 as shown in FIG. 23, whereby the correction ratio RAA becomes small.
At this time, the degree of leaning of the air-fuel ratio by the lean limit control becomes a value close to 0, and the retard correction amount of the ignition timing becomes large. When the control direction at this time is overlapped with FIG. 3A of the first embodiment and shown by a broken line arrow, the exhaust temperature rises as much as the retard amount of the ignition timing increases. Due to this rise in exhaust temperature, the temperature of the NOx purification catalyst 31 can be promptly raised to the activation temperature, and N
The Ox purification catalyst 31 can be effectively used.

[0059]

The control direction from the set point is determined by the lean degree of the air-fuel ratio and the advance correction amount of the ignition timing. In the conventional example in which the control direction is the same regardless of the load, the control point is set at a low load. Since it comes to a position very close to the misfire zone, misfire occurs if there is a variation in the air-fuel ratio control.
In the first aspect of the invention, the control direction from the set point is determined by the lean air-fuel ratio and the retard correction amount of the ignition timing, and when the misfire zone approaches the stability limit at low load, the control point has a margin over the misfire zone. Since they come to the separated position, there is no accidental fire. Further, since the control point comes to the limit of stability, the amount of NOx generated is smaller than that of the conventional example, although the fuel consumption is slightly worse than that of the conventional example.

The relationship between the set point and the misfire zone changes depending on the load, and as the load becomes lower, the misfire zone approaches the set point. Therefore, even if it is appropriate for the load state when the control directions are matched, The control direction becomes improper when the load state is other than the above, but in the third invention,
When the roughness is below the allowable value, the leaner the air-fuel ratio becomes, the smaller the load becomes, and the larger the retard correction amount of the ignition timing becomes, so that the control direction is appropriate for any load. And when the load is low, NO
It is possible to prevent misfire while reducing x, and it is possible to improve fuel efficiency while reducing NOx when the load is high.

In the fourth aspect of the present invention, considering that the misfire zone approaches the stability limit at low speed even under the same load condition as compared to high speed, when the roughness is equal to or less than the allowable value, low load is performed under the same speed condition. And the lower the rotation speed under the same load condition, the smaller the lean air-fuel ratio and the larger the retard correction amount for the ignition timing, so the lean limit control makes the lean correction amount for the air-fuel ratio and the retard correction amount for the ignition timing. Each correction ratio of N is optimized according to the load and the number of revolutions.
Misfire can be prevented while reducing Ox.

In the fifth aspect of the present invention, while the NOx purification catalyst is provided in the exhaust pipe, when the temperature of the NOx purification catalyst during lean operation is below the activation temperature and the roughness is below the allowable value, the degree of leaning of the air-fuel ratio is reduced. Since the ignition timing retard correction amount is increased while the ignition timing retard amount is increased, the exhaust temperature rises as much as the ignition timing retard amount increases, and the NOx purification catalyst temperature is quickly raised to the active temperature due to the increase in the exhaust temperature. Therefore, the NOx purification catalyst can be effectively used during lean operation.

[Brief description of drawings]

FIG. 1 is a control system diagram of a first embodiment.

FIG. 2 is a diagram for explaining the relationship between various performances with respect to the air-fuel ratio and the ignition timing during lean operation.

FIG. 3 is a diagram for explaining a difference in control direction during lean limit control for each load under low load, medium load, and high load.

FIG. 4 is a flowchart for explaining determination of a fuel injection pulse width Ti and an ignition advance value ADV.

FIG. 5 is a flowchart for explaining determination of a fuel injection pulse width Ti and an ignition advance value ADV.

FIG. 6 is a flowchart for explaining a subroutine of steps 1 and 2 in FIG.

FIG. 7 is a flowchart for explaining a subroutine of step 3 of FIG.

FIG. 8 is a characteristic diagram of a basic ignition advance value BADV.

9 is a flowchart for explaining a subroutine of steps 4 and 8 in FIG.

FIG. 10 is a characteristic diagram of a coefficient KLTP.

FIG. 11 is a characteristic diagram of a basic ignition advance value LADV during lean operation.

FIG. 12 is a flowchart for explaining a subroutine of step 9 of FIG.

13 is a flowchart for explaining a subroutine of steps 13, 14, and 15 of FIG.

FIG. 14 is a characteristic diagram of a lean limit control correction amount dL.

FIG. 15 is a characteristic diagram of a correction ratio RAA.

FIG. 16 is a flowchart of the second embodiment.

FIG. 17 is a characteristic diagram of a correction ratio RAA according to the second embodiment.

FIG. 18 is a diagram for explaining a difference in control direction during lean limit control in each rotation at high rotation and low rotation.

FIG. 19 is a control system diagram of the third embodiment.

FIG. 20 is a characteristic diagram of conversion rate of the NOx purification catalyst according to the third embodiment.

FIG. 21 is a characteristic diagram of exhaust temperature with respect to retard amount of ignition timing during lean limit control of the third embodiment.

FIG. 22 is a flowchart of the third embodiment.

FIG. 23 is a characteristic diagram of a temperature correction coefficient RCAT according to the third embodiment.

FIG. 24 is a diagram for explaining the operation of the conventional example.

FIG. 25 is a diagram corresponding to the claims of the first invention.

FIG. 26 is a diagram corresponding to claims of the second invention.

[Explanation of symbols]

 6 Fuel Injection Valve 7 Spark Plug 11 Control Unit 12 Crank Angle Sensor 13 Air Flow Meter 31 NOx Purification Catalyst

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display F02P 5/153

Claims (5)

[Claims] 1. A means for calculating an air-fuel ratio during lean operation, a means for calculating an ignition timing during lean operation, a means for detecting engine roughness, and the roughness being stable when the roughness is below an allowable value. The air-fuel ratio during lean operation to the lean side and the ignition timing during the lean operation to the retard side so that the lean air-fuel ratio of the engine is adjusted to the corrected air-fuel ratio. A combustion control device for an engine, characterized by comprising: a means for controlling the ignition timing and a means for performing ignition at the corrected ignition timing. 2. A means for calculating the air-fuel ratio during lean operation, a means for calculating the ignition timing during lean operation, a means for detecting the roughness of the engine, and the roughness being stable when the roughness is above an allowable value. The air-fuel ratio during lean operation to the rich side and the ignition timing during the lean operation to the advanced side so that the lean air-fuel ratio is reached, and the engine air A combustion control device for an engine, comprising: a means for controlling a fuel ratio; and a means for igniting at the corrected ignition timing. 3. The correcting means calculates the reference air-fuel ratio correction amount and the reference ignition timing correction amount for the maximum load during lean operation, and the reference air-fuel ratio correction amount decreases as the load decreases. The combustion control device for the engine according to claim 1 or 2, further comprising means for correcting the reference ignition timing correction amount to a larger side. 4. The correcting means calculates the reference air-fuel ratio correction amount and the reference ignition timing correction amount with respect to the maximum load during lean operation, and the lower the load under the same rotational speed condition, and the same load condition. 3. The engine according to claim 1 or 2, further comprising: means for correcting the reference air-fuel ratio correction amount to a smaller side and the reference ignition timing correction amount to a larger side as the rotation speed becomes lower. Combustion control device. 5. A catalyst capable of purifying NOx during lean operation is provided in the exhaust pipe, while the NOx during lean operation is provided.
5. The engine according to claim 3, wherein the corrected reference air-fuel ratio correction amount is reduced and the corrected reference ignition timing correction amount is increased in a region where the temperature of the purification catalyst is equal to or lower than the activation temperature. Combustion control device.