JPH09105345A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] During warm-up of a three-way catalyst, exhaust gas recirculation is performed only in a rich cylinder to balance the average effective pressure between the cylinders, thereby making the structure complicated and the air-fuel ratio after warm-up. The engine vibration is suppressed during warm-up without causing the fluctuation of. SOLUTION: Means 53 for controlling the fuel supply amount so that the air-fuel ratio of some cylinders is rich and the air-fuel ratios of other cylinders are lean while the engine is warming up after starting, and rich during this rich / lean control. A means 54 for circulating the exhaust gas only to the cylinders and a means 52 for circulating the exhaust gas to all the cylinders after warming up are provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an air-fuel ratio control system for an internal combustion engine.

[0002]

2. Description of the Related Art In order to activate a three-way catalyst installed in an exhaust system of an internal combustion engine as early as possible after starting the engine, during warming up, the air-fuel ratio of some cylinders is made rich and the air-fuel ratios of other cylinders are made rich. It is known to control leanly (Actual Kaihei 3-10).
2038).

Immediately after starting the engine, the exhaust gas temperature is low, and it takes a considerable time for the three-way catalyst to be activated. In this case, if the oxidation reaction in the three-way catalyst is promoted, the temperature of the three-way catalyst rises due to the heat generated by the oxidation, and the activation is promoted.

Therefore, by controlling the air-fuel ratios of some of the cylinders to be rich and the air-fuel ratios of the other cylinders to be lean, the exhaust air-fuel ratio flowing into the three-way catalyst is periodically changed to rich and lean. By forming an oxidizing atmosphere in the original catalyst, we are trying to activate it early.

By controlling the air-fuel ratios different among the cylinders in this way, the in-cylinder pressure in the lean cylinder becomes relatively lower than that in the rich cylinder, and due to the output torque difference between the cylinders, warming up occurs. The engine vibration of becomes large.

Therefore, in the invention described in the above-mentioned publication, the intake system is divided into two systems, the intake air amount of the lean cylinder is relatively increased compared to the rich cylinder, and the difference in the average effective pressure between the cylinders. It is balanced so that there is no engine vibration.

[0007]

However, in this way, the intake system is divided into two systems, each of which is provided with an intake throttle valve,
In addition, since the feedback control system of the air-fuel ratio is also two systems, the structure is correspondingly complicated, and after activation of the three-way catalyst, the air-fuel ratio between the cylinders is rich or lean,
Both control are performed in the vicinity of the theoretical air-fuel ratio, which is the required air-fuel ratio of the three-way catalyst. However, since there are two footback control systems, the air-fuel ratio tends to fluctuate and exhaust emissions deteriorate. there were.

According to the present invention, the exhaust gas recirculation is performed only in the rich cylinder during warm-up of the three-way catalyst to balance the average effective pressure between the cylinders, and after warm-up, the exhaust gas recirculates uniformly in all cylinders. The purpose of the present invention is to suppress engine vibration during warm-up without complicating the structure or causing variations in the air-fuel ratio after warm-up.

[0009]

The first invention, as shown in FIG. 13, is an internal combustion engine equipped with a catalyst in the exhaust system,
A means 51 for detecting the operating state of the engine, a means 52 for circulating a part of the exhaust gas to the intake system, a rich air-fuel ratio for some cylinders, and a lean air-fuel ratio for the other cylinders during warm-up after the engine is started. A means 53 for controlling the fuel supply amount so as to satisfy the above condition and a means 54 for circulating the exhaust gas only in the rich cylinder during the rich / lean control are provided.

According to a second aspect of the invention, in the first aspect of the invention, the exhaust gas discharged from the rich cylinder during the rich / lean control is circulated to the rich cylinder.

According to a third aspect of the present invention, in the first or second aspect, a means 55 for detecting an average effective pressure of each cylinder is provided.
During rich / lean control, the exhaust gas circulation amount of the rich cylinder is controlled so that the average effective pressures of the rich cylinder and the lean cylinder match.

In a fourth aspect based on the third aspect, the average effective pressure detecting means calculates the average effective pressure from the engine intake air amount, the engine speed, the air-fuel ratio, and the exhaust gas circulation amount.

In a fifth aspect based on the third aspect, the fuel supply amount control means controls the air-fuel ratio of the lean cylinder to a lean stability limit.

[0014]

In the first aspect of the invention, when the air-fuel ratio is rich / lean controlled while the engine is warming up, exhaust gas is circulated in the rich cylinder.

The torque of the rich cylinder is larger than that of the lean cylinder, and if it is left as it is, a torque difference is generated between the cylinders, which causes engine vibration. However, if exhaust gas is circulated only in the rich cylinder in this manner, The torque of the cylinder decreases,
Torque fluctuation between cylinders is suppressed. In this case, the deterioration rate of the engine stability with respect to the change of the exhaust gas circulation rate is low, and even if there is some error in the exhaust gas circulation rate, the influence on the stability is small and the stability is good. Can be maintained near the lean limit, which enhances the oxidizing atmosphere and accelerates the oxidation reaction of the three-way catalyst.
Early activation can be achieved.

When the engine warm-up is completed, fuel is uniformly supplied to all cylinders, the air-fuel ratio is controlled to the same, and the exhaust gas circulation amount is also controlled uniformly.

In this way, the exhaust torque is circulated only in the rich cylinders only during the air-fuel ratio rich / lean control during engine warm-up, whereby the torque generated between the cylinders can be balanced and the engine vibration can be suppressed. In addition, compared to the case where the intake air amount between the cylinders is made different, the structure is simpler if exhaust circulation is performed only for some cylinders, and the air-fuel ratio is feedback-controlled for all cylinders after warm-up. However, since there is only one control system, the control system of the air-fuel ratio is good, and the structure can be prevented from becoming complicated.

In the second aspect of the invention, since the exhaust gas from the rich cylinder is circulated to the rich cylinder during the rich / lean control, the exhaust circulation cylinder has no influence of the air-fuel ratio of the lean cylinder and depends only on the exhaust gas circulation rate. Thus, the average effective pressure can be controlled, and the variation in the average effective pressure between the lean cylinder and the rich cylinder can be prevented.

In the third aspect of the present invention, while detecting the average effective pressure of each cylinder during the rich / lean control, the exhaust gas circulation amount of the rich cylinder is controlled so that the average effective pressures of the rich cylinder and the lean cylinder match. The torque deviation between the cylinders can be reliably eliminated, and engine vibration can be effectively reduced.

In the fourth invention, since the average effective pressure is calculated from the engine intake air amount, the rotation speed, the air-fuel ratio, and the exhaust gas circulation amount, no special in-cylinder pressure detecting means or the like is required and the structure is simplified. Can be achieved.

In the fifth aspect of the invention, the air-fuel ratio of the lean cylinder is controlled so as to reach the lean stability limit, so that a good oxidizing atmosphere can be formed for the three-way catalyst, and the early activation of the three-way catalyst is promoted. .

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the present invention, in which an intake passage 19 of an internal combustion engine 11 is provided with an intake throttle valve 20 located downstream of an air flow meter 12.
Fuel is injected from the fuel injection injector 17 installed in the intake port of each cylinder (four cylinders) according to the intake air amount controlled according to the opening degree of the intake throttle valve 20.

This fuel injection amount is controlled by a signal from the controller 14, so that the controller 14 is
An intake air amount signal from the air flow meter 12, a rotation speed signal from the crank angle sensor 13, and a signal from an air-fuel ratio sensor 23 installed in the exhaust passage 21 are input. The fuel amount injected from each fuel injection injector 17 is controlled so that the target air-fuel ratio will be described later.

An exhaust gas circulation passage 18 is formed which branches from the exhaust gas passage 21 and circulates a part of the exhaust gas to an intake air passage 19. In the exhaust gas circulation passage 18, an exhaust gas circulation amount control valve 15 for controlling the exhaust gas circulation amount is interposed. To be dressed. Also, the exhaust gas circulation control valve 1
5 is branched off from the exhaust circulation passage 18 and
A branch passage 18a connected to the intake branch 19a of the cylinder is provided, and at this branch point, a three-way switching valve 16 for passage switching is provided.
Is interposed.

As will be described later, the controller 14 controls the opening degree of the exhaust gas circulation amount control valve 15 and the passage switching of the three-way switching valve 16 according to operating conditions.

A three-way catalyst 22 is provided in the exhaust passage 21 at a position downstream of the branch point of the exhaust circulation passage 18 to oxidize HC and CO in the exhaust and reduce NOx. Purify.

While the engine is warming up, the controller 14 controls the injection amount of fuel to the fuel injection injector 17 of each cylinder to be different among the cylinders. In this example, the air-fuel ratio of the first cylinder is rich, Other than the above, the air-fuel ratios of the second to fourth cylinders are made lean, thereby maintaining the exhaust air-fuel ratio in an oxidizing atmosphere and promoting the oxidation reaction in the three-way catalyst 22,
The three-way catalyst 22 is quickly warmed up. On the other hand, after warming up, the injection amount of fuel to each cylinder is made the same, and the required air-fuel ratio of the three-way catalyst 22, that is, the theoretical air-fuel ratio, is obtained.
The fuel injection amount is feedback-controlled based on the output of the air-fuel ratio sensor 23.

At the same time, the controller 14
During the air-fuel ratio rich / lean control during warm-up, the opening degree of the exhaust gas circulation control valve 15 is controlled to the exhaust gas circulation rate (EGR rate) described later, and the three-way switching valve 16 is switched to change the exhaust gas circulation passage 18 Exhaust gas is guided to the branch passage 18a so that the exhaust gas is circulated only in the first cylinder whose air-fuel ratio is controlled to be rich, and the generated torque between the rich cylinder and the lean cylinder is made uniform to prevent engine vibration. Suppress.

After warming up, the three-way switching valve 16
The exhaust gas is introduced into the intake passage 19 on the downstream side of the intake throttle valve 20, and the exhaust gas is uniformly circulated in the first to fourth cylinders.

FIG. 2 is a flow chart showing the control operation of the rich / lean air-fuel ratio and the exhaust gas circulation, which will be described in detail based on this flow chart.

First, at step 11, the intake air amount Q and the engine speed N are detected, and the basic fuel injection amount TP is calculated based on these as TP = k * Q / N (step 12). Next, the basic effective pressure Pi of each cylinder is calculated from TP and N.
Is read with reference to the map in FIG. However, this P
For i, attach a pressure sensor directly to each cylinder,
You may make it respectively detect a basic effective pressure.

In step 14, in order to obtain the stability of the engine, the rotational fluctuation rate dN is calculated by dN = Σ (Ni-Nave) / by statistical processing of the engine speed N for 50 cycles.
Calculate as i (however, i: 50 cycles, Nav
e: average value of 50 cycles).

In steps 15 to 17, the fuel injection amount TPL of the lean cylinder is calculated so that the air-fuel ratio of the lean cylinder becomes lean best (note that this TPL is smaller than the basic injection amount TP by a predetermined value, (Preset to have a predetermined lean air-fuel ratio (lean vest)).
Therefore, the rotation fluctuation rate dN is compared with the predetermined lean stability limit judgment values A and B. If dN> B, the stability is poor, so that the fuel injection amount is increased by setting TPL = TPL + Δtp in step 16, whereas if dN <A, the stability is better than the stability limit, and thus step 17 Then, the fuel injection amount is reduced by setting TPL = TPL-Δtp. Note that, for example, the judgment value A = 10 rpm, B = 20 rpm,
Δtp = 0.1 ms is set.

On the other hand, when A≤dN≤B, it is determined that the lean air-fuel ratio is within the stability limit, and the routine proceeds to step 18, where the lean amount AbyFL of the lean cylinder is entered.
Is calculated as AbyFL = 14 × (TP−TPL) / TP.

Then, in step 19, the decrease rate ΔPi of the average effective pressure Pi is looked up from the map shown in FIG. 4 based on AbyFL, and this decrease rate ΔP.
i, the reduction margin dPi of the average effective pressure is expressed as dPi = Pi
It is calculated as × ΔPi. Next, at step 20, the fuel injection amount TPR of the rich cylinder is calculated as TPR = TP × K1 so as to obtain a predetermined rich air-fuel ratio.

Then, at step 21, the EGR (exhaust gas circulation) rate of the rich cylinder is looked up from the above-described decrease margin dPi of the average effective pressure according to the map shown in FIG. With this EGR rate, the average effective pressure of the rich cylinder and the average effective pressure of the lean cylinder are controlled to match.

The operation based on such control operation will be described. FIG. 6 shows the relationship among the oxygen (O ₂ ) concentration, the stability, and the average effective pressure when the air-fuel ratio is changed to the lean side.

In order to activate the three-way catalyst 22 early after the engine is started, the higher the O ₂ concentration in the exhaust gas discharged from the lean cylinder is, the better so that the reaction in the catalyst proceeds. It is preferable to set the fuel ratio as lean as possible. However, if the air-fuel ratio is made lean, the stability of the cylinder deteriorates, so control is performed so that the air-fuel ratio becomes the leanest air-fuel ratio that can maintain the air-fuel ratio at the stability limit.

By the way, when the air-fuel ratio is set to the lean best in this way, the average effective pressure Pi also decreases, and a difference of Pi between the rich cylinder and the rich cylinder appears, and the balance of the output torque between the cylinders is lost. Vibration increases.

As a method of eliminating this torque difference, as shown in FIG. 7, it is conceivable to make the ignition timing ADV between the cylinders different and retard the ignition timing of the rich cylinder. In order to match the Pi of the lean cylinder, it is necessary to retard it considerably. Further, in the vicinity of the retard angle limit, the sensitivity of the stability to a slight change in ADV is high (the deterioration rate is high), and therefore the stability is greatly adversely affected by the variation in ADV.

On the other hand, exhaust gas circulation (E
GR), and when the average effective pressure Pi is controlled by changing the EGR rate at this time, as shown in FIG. 8, the rate at which the stability decreases with respect to the increase in the EGR rate is
Since it is not as large as ADV, the variation in stability due to the error in the EGR rate is small.

Therefore, as shown in FIG. 9 and FIG. 10, in order to correspond to Pi which is lowered by making the air-fuel ratio lean, the reduction amount of Pi calculated from the lean amount AbyFL (ΔA / F) of the air-fuel ratio is taken. By determining the EGR rate of the rich cylinder so as to meet (ΔPi), the Pi of the rich cylinder and the lean cylinder are made to match, thereby eliminating the torque step due to the deviation of Pi between the cylinders and reducing the engine vibration. It becomes possible to do.

In particular, in the control for balancing the average effective pressure between the cylinders by circulating the exhaust gas to the rich cylinders in this way, the stability of the rich cylinders is extremely deteriorated even if there is some error in the control of the EGR rate. Therefore, the stability of the air-fuel ratio during the rich / lean control is enhanced, and on the other hand, the air-fuel ratio of the lean cylinder can be made lean without impairing the engine stability.

As shown in FIG. 11, when the Pi of the lean cylinder is reduced by retarding the ignition timing ADV, the stability fluctuation rate is high with respect to the ADV change. Is large, and considering the margin to the stability limit, the air-fuel ratio has to be set to a slightly richer side than the leanest state. However, as in the present invention,
When the Pi of the rich cylinder is changed according to the EGR rate, the air-fuel ratio of the lean cylinder can be surely set to the lean best because the variation in stability is small.

Next, another embodiment of the present invention shown in FIG. 12 will be described.

This is the exhaust passage 21 of the exhaust circulation passage 18.
Is connected to the exhaust branch 21a of the first cylinder, which is a rich cylinder. By circulating only the exhaust of the rich cylinder during rich / lean control, the circulation of the exhaust containing the excess oxygen of the lean cylinder is blocked. However, the correlation between the EGR rate and the decrease amount of the average effective pressure Pi is made closer.

When excess oxygen enters from the lean cylinder into the circulation exhaust, the air-fuel ratio of the rich cylinder slightly changes and Pi changes slightly by this amount. However, when exhaust gas from the rich cylinder is circulated in this manner, there is no excess oxygen,
Pi depends only on the exhaust gas circulation rate, and therefore Pi of the lean cylinder and that of the rich cylinder can be accurately matched. This makes it possible to further improve the balance of the output torque between the cylinders and further reduce the engine vibration during the rich / lean control.

[0048]

According to the first aspect of the present invention, the exhaust gas is circulated only in the rich cylinders only during the air-fuel ratio rich / lean control during engine warm-up to balance the average effective pressure between the cylinders and to warm the engine. After completion, the exhaust gas is circulated evenly in all cylinders, so engine vibration during warm-up is effectively suppressed, and the exhaust gas circulates in the rich cylinders to balance the average effective pressure between the cylinders. Even if there is some error in the control of the engine, the stability of the rich cylinder will not be extremely deteriorated, the stability at the time of rich / lean control of the air-fuel ratio will increase, and at the same time the engine stability will be impaired. Since the air-fuel ratio of the lean cylinder can be made lean, the oxidation reaction of the three-way catalyst can be promoted to that extent, and the warm-up of the three-way catalyst can be completed within a short time. Further, compared to the case where the intake air amount between the cylinders is made different for the purpose of torque balance, the structure of the exhaust circulation cylinder is simpler, and the air-fuel ratio of all cylinders is feedback-controlled after warming up. In addition, since there is only one control system, the control system is good and it is possible to avoid complication of the structure.

According to the second aspect of the invention, since the exhaust gas of the rich cylinder is circulated to the rich cylinder, the influence of the air-fuel ratio of the lean cylinder is eliminated for the exhaust gas circulation cylinder, and the average effective pressure depends only on the exhaust gas circulation rate. It is possible to control, and it is possible to prevent the variation in the average effective pressure between the lean cylinder and the rich cylinder.

According to the third invention, while detecting the average effective pressure of each cylinder during the rich / lean control, the exhaust gas circulation amount of the rich cylinder is controlled so that the average effective pressures of the rich cylinder and the lean cylinder coincide with each other. Therefore, it is possible to accurately match the output torques of the respective cylinders and effectively reduce the engine vibration due to the output fluctuation between the cylinders.

According to the fourth aspect of the invention, the average effective pressure is calculated from the engine intake air amount, the engine speed, the air-fuel ratio, and the exhaust gas circulation amount, so that no special cylinder pressure detecting means or the like is required.
The structure can be simplified.

According to the fifth aspect of the invention, the air-fuel ratio of the lean cylinder is controlled so as to reach the lean stability limit, so that a good oxidizing atmosphere can be formed for the three-way catalyst, and the early activation of the three-way catalyst is promoted. To be done.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention.

FIG. 2 is a flowchart showing the control operation.

FIG. 3 is an explanatory diagram showing a relationship between an engine speed and an average effective pressure with respect to a load.

FIG. 4 is an explanatory diagram showing the relationship between the lean change amount of the air-fuel ratio and the reduction margin of the average effective pressure.

FIG. 5 is an explanatory diagram showing a relationship between an exhaust gas circulation rate and a reduction margin of an average effective pressure.

FIG. 6 is an explanatory diagram showing the relationship among oxygen concentration, stability, and average effective pressure with respect to changes in the air-fuel ratio.

FIG. 7 is an explanatory diagram showing the relationship between stability and average effective pressure with respect to changes in ignition timing.

FIG. 8 is an explanatory diagram showing the relationship between stability and average effective pressure with respect to changes in exhaust gas circulation rate.

FIG. 9 is an explanatory diagram showing the relationship between the change amount of the air-fuel ratio and the decrease amount of the average effective pressure.

FIG. 10 is an explanatory diagram showing a relationship between a change in the exhaust gas circulation rate and a decrease margin of the average effective pressure.

FIG. 11 is a characteristic diagram showing the relationship between air-fuel ratio and stability during rich / lean control.

FIG. 12 is a schematic configuration diagram showing another embodiment.

FIG. 13 is a configuration diagram of the present invention.

[Explanation of symbols]

 51 Operating State Detection Means 52 Exhaust Circulation Means 53 Rich / Lean Control Means 54 Exhaust Circulation Means During Rich / Lean Control 55 Average Effective Pressure Detection Means

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koji Hiratani 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd.

Claims (5)

[Claims] 1. An internal combustion engine having a catalyst in an exhaust system, means for detecting an operating state of the engine, means for circulating a part of exhaust gas to an intake system, and some cylinders during warm-up after starting the engine. And a means for controlling the fuel supply amount so that the air-fuel ratio of the other cylinders becomes lean and the air-fuel ratios of the other cylinders lean, and means for circulating the exhaust gas only to the rich cylinders during the rich / lean control. Air-fuel ratio control device for internal combustion engine. 2. The air-fuel ratio control system for an internal combustion engine according to claim 1, wherein the exhaust gas discharged from the rich cylinder is circulated to the rich cylinder during the rich / lean control. 3. A means for detecting an average effective pressure of each cylinder, wherein the exhaust gas circulation amount of the rich cylinder is controlled so that the average effective pressures of the rich cylinder and the lean cylinder coincide with each other during the rich / lean control. 2. The air-fuel ratio control device for an internal combustion engine according to item 2. 4. The air-fuel ratio control device for an internal combustion engine according to claim 3, wherein the average effective pressure detecting means calculates the average effective pressure from the engine intake air amount, the rotational speed, the air-fuel ratio, and the exhaust gas circulation amount. 5. The air-fuel ratio control device for an internal combustion engine according to claim 3, wherein the fuel supply amount control means controls the air-fuel ratio of the lean cylinder so as to reach the lean stability limit.