JP2012145054A - Apparatus for detecting fluctuation abnormality of air-fuel ratios among cylinders of multi-cylinder internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for detecting a fluctuation abnormality of air-fuel ratios among cylinders of a multi-cylinder internal combustion engine, which increases a detecting opportunity.SOLUTION: The apparatus includes: a changing means for changing an operation angle of an intake valve according to an operating condition of the internal combustion engine; a fuel cutting means for executing a fuel cut when a predetermined condition is established; a rich control means for controlling an air-fuel ratio to be richer than a predetermined standard value; and a detecting means for detecting the fluctuation abnormality of the air-fuel ratio among the cylinders, when the operation angle S is not more than a predetermined value Sx, and when the rich control is being made by the rich control means just after return from the fuel cut.

Description

  The present invention relates to an apparatus for detecting an abnormal variation in the air-fuel ratio between cylinders of a multi-cylinder internal combustion engine, and more particularly to an apparatus for detecting that the air-fuel ratio between cylinders in a multi-cylinder internal combustion engine is relatively large. .

  In general, in an internal combustion engine equipped with an exhaust gas purification system using a catalyst, a mixture ratio of air and fuel in an air-fuel mixture burned in the internal combustion engine, that is, an air-fuel ratio, is used to efficiently remove harmful components in exhaust gas with a catalyst. Control is essential. In order to perform such air-fuel ratio control, an air-fuel ratio sensor is provided in the exhaust passage of the internal combustion engine, and feedback control is performed so that the air-fuel ratio detected thereby coincides with a predetermined target air-fuel ratio.

  On the other hand, in a multi-cylinder internal combustion engine, air-fuel ratio control is normally performed using the same control amount for all cylinders. Therefore, even if air-fuel ratio control is executed, the actual air-fuel ratio may vary between cylinders. If the degree of variation is small at this time, it can be absorbed by air-fuel ratio feedback control, and harmful components in the exhaust gas can be purified by the catalyst, so that exhaust emissions are not affected and there is no particular problem.

  However, for example, if the fuel injection system of some cylinders breaks down and the air-fuel ratio between the cylinders varies greatly, exhaust emission deteriorates, causing a problem. It is desirable to detect such a large air-fuel ratio variation that deteriorates the exhaust emission as an abnormality. In particular, in the case of an internal combustion engine for automobiles, it is required to detect an abnormal variation in air-fuel ratio between cylinders in an in-vehicle state in order to prevent the vehicle from running with deteriorated exhaust emissions (so-called OBD; On-Board Diagnostics). Recently, there is a movement to make it legally regulated.

JP 2010-001861 A

  When the air-fuel ratio variation abnormality occurs, the output fluctuation of the air-fuel ratio sensor increases. Therefore, by monitoring this output fluctuation, it is possible to detect an abnormality in the air-fuel ratio variation.

  In general, abnormality detection is performed during high-load operation of the engine so that the difference in output fluctuations according to the presence or absence of variation abnormality increases. However, this would limit the detection timing during high-load operation, leading to a reduction in detection opportunities.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an inter-cylinder air-fuel ratio variation abnormality detection device for a multi-cylinder internal combustion engine that can increase detection opportunities.

According to one aspect of the invention,
Changing means for changing the operating angle of the intake valve according to the operating state of the multi-cylinder internal combustion engine;
Fuel cut means for performing fuel cut when a predetermined condition is satisfied;
Rich control means for controlling the air-fuel ratio to be richer than a predetermined reference value;
Detecting means for detecting a variation in air-fuel ratio between cylinders when the operating angle is not more than a predetermined value and rich control is performed by the rich control means immediately after returning from the fuel cut;
An inter-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine is provided.

  Preferably, the detection means detects the variation abnormality even when the operating angle is larger than the predetermined value and the load of the internal combustion engine is equal to or larger than a predetermined value.

  Alternatively, it is preferable that the detection unit has the variation even when the operating angle is larger than the predetermined value, the load of the internal combustion engine is equal to or larger than a predetermined value, and the rich control is performed by the rich control unit. Detect anomalies.

  Preferably, the changing means increases the operating angle as the load of the internal combustion engine increases.

  Preferably, the reference value is stoichiometric.

  Preferably, the detection means detects the variation abnormality based on an output fluctuation of an air-fuel ratio sensor provided in the exhaust passage.

  According to the present invention, an excellent effect that detection opportunities can be increased is exhibited.

1 is a schematic view of an internal combustion engine according to an embodiment of the present invention. It is a graph which shows the output characteristic of a pre-catalyst sensor and a post-catalyst sensor. It is a graph which shows the fluctuation | variation of the exhaust air fuel ratio according to the air-fuel ratio variation degree between cylinders. FIG. 4 is an enlarged view corresponding to a U portion in FIG. 3. It is a time chart which shows the pre-catalyst sensor output fluctuation | variation at the time of high load driving | running | working in a general engine. The valve lift diagram of an intake valve is shown. It is a graph which shows the relationship between a working angle and an air fuel ratio. It is a time chart which shows transition of each value at the time of a vehicle run test. It is a time chart which shows the pre-catalyst sensor output fluctuation | variation in F / C return rich control in the engine of this embodiment. It is a flowchart of a variation abnormality detection process.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic view of an internal combustion engine according to the present embodiment. As shown in the figure, an internal combustion engine (engine) 1 is powered by burning a mixture of fuel and air inside a combustion chamber 3 formed in a cylinder block 2 and reciprocating a piston in the combustion chamber 3. Is generated. The internal combustion engine 1 of the present embodiment is a multi-cylinder internal combustion engine mounted on an automobile, and more specifically, an in-line 4-cylinder spark ignition internal combustion engine. The internal combustion engine 1 includes # 1 to # 4 cylinders. However, the number of cylinders, the type, etc. are not particularly limited.

  Although not shown, the cylinder head of the internal combustion engine 1 is provided with an intake valve for opening and closing an intake port and an exhaust valve for opening and closing an exhaust port for each cylinder. Each intake valve and each exhaust valve is connected to a camshaft. It is driven to open and close by a valve mechanism including A spark plug 7 for igniting the air-fuel mixture in the combustion chamber 3 is attached to the top of the cylinder head for each cylinder.

  A variable valve mechanism 21 for changing the opening / closing characteristics of the intake valve is employed in the valve operating mechanism for the intake valve. Specifically, there is provided a variable valve timing mechanism that uniformly changes the opening / closing timing of the intake valves of all the cylinders by changing the rotational phase of the camshaft with respect to the crankshaft. A variable valve lift that changes the lift and operating angles of all cylinders uniformly and continuously by changing the swing amount of the swing mechanism provided between the intake cam and intake valve of the camshaft. The variable valve mechanism 21 is configured by the variable valve timing mechanism and the variable valve lift mechanism provided with the mechanism. A known structure can be employed for the variable valve timing mechanism and the variable valve lift mechanism.

  Although not shown, a variable valve mechanism for changing the opening / closing characteristics of the exhaust valve is also adopted in the valve operating mechanism for the exhaust valve. This variable valve mechanism can be composed of only a variable valve timing mechanism. However, the variable valve lift mechanism may be included.

  The intake port of each cylinder is connected to a surge tank 8 which is an intake air collecting chamber via a branch pipe 4 for each cylinder. An intake pipe 13 is connected to the upstream side of the surge tank 8, and an air cleaner 9 is provided at the upstream end of the intake pipe 13. An air flow meter 5 for detecting the intake air amount and an electronically controlled throttle valve 10 are incorporated in the intake pipe 13 in order from the upstream side. An intake passage is formed by the intake port, the branch pipe, the surge tank 8 and the intake pipe 13.

  An injector (fuel injection valve) 12 for injecting fuel into an intake passage, particularly an intake port, is provided for each cylinder. The fuel injected from the injector 12 is mixed with intake air to form an air-fuel mixture. The air-fuel mixture is sucked into the combustion chamber 3 when the intake valve is opened, compressed by the piston, and ignited and burned by the spark plug 7.

  On the other hand, the exhaust port of each cylinder is connected to the exhaust manifold 14. The exhaust manifold 14 includes a branch pipe 14a for each cylinder forming an upstream portion thereof and an exhaust collecting portion 14b forming a downstream portion thereof. An exhaust pipe 6 is connected to the downstream side of the exhaust collecting portion 14b. An exhaust passage is formed by the exhaust port, the exhaust manifold 14 and the exhaust pipe 6.

  A catalyst composed of a three-way catalyst, that is, an upstream catalyst 11 and a downstream catalyst 19 are attached in series to the upstream side and the downstream side of the exhaust pipe 6, respectively. First and second air-fuel ratio sensors for detecting the air-fuel ratio of the exhaust gas, that is, a pre-catalyst sensor 17 and a post-catalyst sensor 18 are installed on the upstream side and the downstream side of the upstream catalyst 11, respectively. The pre-catalyst sensor 17 and the post-catalyst sensor 18 are installed at positions immediately before and immediately after the upstream catalyst 11, and detect the air-fuel ratio based on the oxygen concentration in the exhaust gas. In this way, the single pre-catalyst sensor 17 is installed at the exhaust merging portion on the upstream side of the upstream catalyst 11.

  The above-described spark plug 7, throttle valve 10, variable valve mechanism 21, injector 12 and the like are electrically connected to an electronic control unit (hereinafter referred to as ECU) 20 as control means. The ECU 20 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like, all not shown. In addition to the air flow meter 5, the pre-catalyst sensor 17, and the post-catalyst sensor 18, the ECU 20 includes a crank angle sensor 16 that detects the crank angle of the internal combustion engine 1 and an accelerator that detects the accelerator opening, as shown in the figure. The opening sensor 15 and other various sensors are electrically connected via an A / D converter or the like (not shown). The ECU 20 controls the ignition plug 7, the throttle valve 10, the variable valve mechanism 21, the injector 12, etc. so as to obtain a desired output based on the detection values of various sensors, etc., and the ignition timing, fuel injection amount, fuel The injection timing, throttle opening, intake valve opening / closing timing, lift, operating angle, and the like are controlled. The throttle opening is normally controlled to an opening corresponding to the accelerator opening.

  The pre-catalyst sensor 17 is a so-called wide-range air-fuel ratio sensor, and can continuously detect an air-fuel ratio over a relatively wide range. FIG. 2 shows the output characteristics of the pre-catalyst sensor 17. As shown in the figure, the pre-catalyst sensor 17 outputs a voltage signal Vf having a magnitude proportional to the exhaust air-fuel ratio. The output voltage when the exhaust air-fuel ratio is stoichiometric (theoretical air-fuel ratio, for example, A / F = 14.6) is Vreff (for example, about 3.3 V).

On the other hand, the post-catalyst sensor 18 is a so-called O ₂ sensor, and has a characteristic that the output value changes suddenly with the stoichiometric boundary. FIG. 2 shows the output characteristics of the post-catalyst sensor 18. As shown in the figure, the output voltage when the exhaust air-fuel ratio is stoichiometric, that is, the stoichiometric equivalent value is Vrefr (for example, 0.45 V). The output voltage of the post-catalyst sensor 18 changes within a predetermined range (for example, 0 to 1 V). When the exhaust air-fuel ratio is leaner than stoichiometric, the output voltage of the post-catalyst sensor becomes lower than the stoichiometric equivalent value Vrefr, and when the exhaust air-fuel ratio is richer than stoichiometric, the output voltage of the post-catalyst sensor becomes higher than the stoichiometric equivalent value Vrefr.

  The upstream catalyst 11 and the downstream catalyst 19 simultaneously purify NOx, HC and CO, which are harmful components in the exhaust gas, when the air-fuel ratio A / F of the exhaust gas flowing into each of them is close to the stoichiometric range. The air-fuel ratio width (window) that can simultaneously purify these three with high efficiency is relatively narrow.

  Therefore, air-fuel ratio control (stoichiometric control) is executed by the ECU 20 so that the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 11 is controlled in the vicinity of stoichiometric. This air-fuel ratio control is detected by a main air-fuel ratio control (main air-fuel ratio feedback control) that makes the exhaust air-fuel ratio detected by the pre-catalyst sensor 17 coincide with a stoichiometry that is a predetermined target air-fuel ratio, and detected by the post-catalyst sensor 18. The auxiliary air-fuel ratio control (auxiliary air-fuel ratio feedback control) is performed so that the exhaust air-fuel ratio thus made coincides with the stoichiometry.

  Now, for example, it is assumed that the injectors 12 of some cylinders out of all the cylinders have failed and air-fuel ratio variations (imbalance) occur between the cylinders. For example, the # 1 cylinder has a larger fuel injection amount than the other # 2, # 3, and # 4 cylinders, and its air-fuel ratio is greatly shifted to the rich side. Even at this time, if a relatively large correction amount is given by the above-described main air-fuel ratio feedback control, the air-fuel ratio of the total gas supplied to the pre-catalyst sensor 17 may sometimes be stoichiometrically controlled. However, looking at each cylinder, # 1 cylinder is larger and richer than stoichiometric, and # 2, # 3 and # 4 cylinders are leaner than stoichiometric. Is clear. In view of this, the present embodiment is equipped with a device that detects such a variation in air-fuel ratio between cylinders.

  As shown in FIG. 3, when the variation in air-fuel ratio between cylinders occurs, the variation in the exhaust air-fuel ratio during one engine cycle (= 720 ° CA) increases. The air-fuel ratio diagrams a, b, and c in (B) are not varied, and the pre-catalyst in the case of a rich shift at an imbalance ratio of 20% for only one cylinder and a rich shift at an imbalance ratio of 50% for only one cylinder. An air-fuel ratio A / F detected by the sensor 17 is shown. As can be seen, the greater the degree of variation, the greater the amplitude of the air-fuel ratio fluctuation.

  Here, the imbalance ratio (%) is a parameter related to the degree of variation in the air-fuel ratio between cylinders. In other words, the imbalance ratio is the amount of fuel injection in a cylinder (imbalance cylinder) causing the fuel injection amount deviation when only one of the cylinders has caused the fuel injection amount deviation. The ratio is a value indicating whether the fuel injection amount is not deviated from the fuel injection amount of the cylinder (balance cylinder) that has not caused the fuel injection amount deviation, that is, the reference injection amount. When the imbalance ratio is IB, the fuel injection amount of the imbalance cylinder is Qib, and the fuel injection amount of the balance cylinder, that is, the reference injection amount is Qs, IB = (Qib−Qs) / Qs. The greater the imbalance ratio IB, the greater the fuel injection amount deviation between the imbalance cylinder and the balance cylinder, and the greater the air-fuel ratio variation.

  When the air-fuel ratio variation abnormality occurs, the output variation of the pre-catalyst sensor 17 becomes large. Therefore, it is possible to detect the variation abnormality based on the output variation using this characteristic.

  In the present embodiment, an output fluctuation parameter that is a parameter correlated with the degree of fluctuation of the air-fuel ratio sensor output is calculated, and an abnormality is detected based on the output fluctuation parameter and a predetermined determination value.

  A method for calculating the output fluctuation parameter will be described below. FIG. 4 is an enlarged view corresponding to the U portion of FIG. 3, and particularly shows a fluctuation of the sensor output before the catalyst within one engine cycle. As the pre-catalyst sensor output, a value obtained by converting the output voltage Vf of the pre-catalyst sensor 17 into an air-fuel ratio A / F is used. However, the output voltage Vf of the pre-catalyst sensor 17 can be directly used.

As shown in FIG. 4B, the ECU 20 acquires the value of the pre-catalyst sensor output A / F every predetermined sample period τ (unit time, for example, 4 ms) within one engine cycle. The value A / F _n obtained in this timing (second timing), the absolute value of the difference .DELTA.A / F _n between the value A / F _n-1 obtained at the previous timing (first timing) It calculates | requires by following Formula (1). This difference ΔA / F _n can be rephrased as a differential value or inclination at the current timing.

Most simply, this difference ΔA / F _n represents the fluctuation of the sensor output before the catalyst. This is because the slope of the air-fuel ratio diagram increases as the degree of fluctuation increases, and the difference ΔA / F _n increases. Therefore, the value of the difference ΔA / F _{n at} a predetermined timing can be used as the output fluctuation parameter.

However, in this embodiment, in order to improve accuracy, an average value of a plurality of differences ΔA / F _{n is used} as an output fluctuation parameter. In the present embodiment, the difference ΔA / F _n is integrated at each timing within one engine cycle, and the final integrated value is divided by the number of samples N to obtain the average value of the differences ΔA / F _n within one engine cycle. . Further, the average value of the difference ΔA / F _n is integrated for M engine cycles (for example, M = 100), the final integrated value is divided by the number of cycles M, and the average value of the difference ΔA / F _n within the M engine cycle is obtained. Ask for. The final average value thus obtained is used as an output fluctuation parameter, and is displayed as “X” hereinafter. The output fluctuation parameter X increases as the fluctuation degree of the pre-catalyst sensor output increases.

Since the pre-catalyst sensor output A / F may increase or decrease, the difference ΔA / F _n or the average value thereof may be obtained for only one of these cases, and this may be used as the output fluctuation parameter. .

  Also, any value that correlates with the degree of fluctuation in the pre-catalyst sensor output can be used as the output fluctuation parameter. For example, output fluctuation based on the difference between the maximum peak and minimum peak of the sensor output before the catalyst within one engine cycle (so-called peak to peak), or the absolute value of the maximum peak or minimum peak of the second derivative. Parameters can also be calculated. This is because the difference between the maximum peak and the minimum peak of the pre-catalyst sensor output increases as the degree of fluctuation of the pre-catalyst sensor output increases, and the absolute value of the maximum peak or minimum peak of the second-order differential value also increases.

  If the calculated output variation parameter X is equal to or greater than a predetermined determination value, it is determined that there is a variation abnormality, and if the calculated output variation parameter X is smaller than the determination value, it is determined that there is no variation abnormality, that is, normal.

  As described above, generally, variation abnormality detection is performed during high-load operation of the engine. This is because the intake air amount, that is, the exhaust gas flow rate is relatively large during high load operation, and the gas contact with the pre-catalyst sensor 17 is relatively strong, so that the difference in output fluctuations depending on the presence or absence of variation abnormality becomes large.

  FIG. 5 shows the test results obtained by examining the pre-catalyst sensor output fluctuation during high load operation in a more general engine without the intake valve and exhaust valve variable valve mechanisms. In this case, the opening / closing timing, lift, and operating angle of the intake valve and the exhaust valve are constant. (A) is the engine speed Ne (rpm), (B) is the load KL, (C) is the output voltage Vf of the pre-catalyst sensor, and (D) is the air-fuel ratio A / F obtained by converting the output voltage Vf. Indicates. The output voltage Vf and the air-fuel ratio A / F are substantially the same. For (B) to (D), normal and abnormal data are shown. The engine speed Ne (rpm) is synonymous with the engine speed.

  As shown in (C) and (D), no fluctuation is observed in the normal waveform, but there is a fluctuation in the abnormal waveform. Therefore, the presence / absence of variation abnormality can be determined based on the difference between the normal waveform and the abnormal waveform, and the variation abnormality can be detected based on the abnormal waveform.

  However, with this method, the detection timing is limited during high-load operation, leading to a reduction in detection opportunities. From the viewpoint of securing a detection opportunity, it is preferable to enable detection other than during high-load operation.

  Therefore, in this embodiment, a variation abnormality is detected during low load operation, particularly during rich control immediately after the end of fuel cut. As a result, the number of detection opportunities can be increased and more detection opportunities can be secured.

  The fuel cut is a control for stopping the fuel injection from the injector 12. The ECU 20 performs fuel cut when a predetermined fuel cut condition is satisfied. The fuel cut conditions are, for example, 1) the accelerator opening Ac detected by the accelerator opening sensor 15 is substantially fully closed, and 2) the engine speed Ne calculated based on the output of the crank angle sensor 16 is predetermined. This is established when the two conditions of a predetermined return rotational speed (for example, 1200 rpm) that is slightly higher than the idle rotational speed (for example, 800 rpm) are satisfied.

  When the accelerator opening degree Ac is substantially fully closed at the return rotational speed or higher, the fuel cut is immediately executed, and the engine and the vehicle are decelerated (execution of the deceleration fuel cut). When the engine speed Ne falls below the return speed, fuel cut is completed and fuel injection is restarted (return from deceleration fuel cut), and at the same time rich control (rich control at F / C return) is started. The

  The rich control is control that makes the air-fuel ratio richer (eg, 14.1) than a predetermined reference value, specifically, stoichiometric (eg, 14.6). At this time, the ECU 20 sets the target air-fuel ratio to a value richer than stoichiometric, increases the fuel injection amount beyond the reference injection amount determined by the engine operating state, and the reference air determined by the engine operating state. Do at least one of decreasing than the amount. Feedback control is performed on at least one of the fuel injection amount and the intake air amount so that the air-fuel ratio detected by the pre-catalyst sensor 17 matches the target air-fuel ratio.

  The reason for performing the rich control is mainly to restore the performance of the upstream catalyst 11. That is, the upstream catalyst 11 has an oxygen storage capacity, stores excess oxygen when the atmosphere gas in the catalyst is leaner than stoichiometric, reduces and purifies NOx, and stores oxygen when the atmosphere gas in the catalyst is richer than stoichiometric. Release and oxidize and purify HC and CO.

  During the fuel cut, oxygen continues to be stored in the catalyst. At this time, if the catalyst occludes oxygen to its full occlusion capacity, oxygen cannot be occluded any more immediately after returning from the fuel cut, and NOx may not be purified. Therefore, rich control is performed to forcibly release the stored oxygen.

  During rich control, the engine is often operated in an idle or low load state. Therefore, it is possible to increase detection opportunities by performing detection in such an operating state.

  By the way, the operating angle of the intake valve is changed according to the engine operating state, particularly the engine load. This change is performed by the ECU 20 controlling the variable valve mechanism 21. The operating angle is increased as the engine load increases.

  On the other hand, the cause of the air-fuel ratio variation abnormality is not only the fuel system abnormality in which fuel is not supplied as scheduled, but also the air system abnormality in which air is not supplied as scheduled. The air system abnormality includes a working angle deviation in which the working angle of the intake valve is not as planned.

  Further, according to the research results of the present inventors, when the operating angle of the intake valve is relatively small, the air-fuel ratio is a reference value (here, stoichiometric) due to the deviation of the air amount corresponding to the deviation of the operating angle from the reference value. ) Was found to be significantly different from

  FIG. 6 shows a valve lift diagram of the intake valve. In the figure, the diagram a shows a case where the operating angle is a reference value S0 determined by the engine operating state. On the other hand, the diagram b shows a case where the operating angle is larger than the reference value S0 by α (S0 + α). The diagram c shows a case where the operating angle is smaller than the reference value S0 by α (S0−α). α is, for example, 2.4 ° CA.

  FIG. 7 shows the relationship between the operating angle and the air-fuel ratio. In particular, as shown in the diagrams b and c of FIG. 6, the air-fuel ratio shift when the operating angle is shifted from the reference value S0 by α on the increasing side and the decreasing side. The state of is shown.

  In the figure, Si indicates the minimum operating angle, particularly the operating angle during idling. As the load increases, the operating angle S increases from Si and moves to the right side of the figure. In a small working angle region equal to or less than Sx, a diagram b when the working angle increases and a diagram c when the working angle decreases and shifts, respectively, are stoichiometric (= 14) that is the reference air-fuel ratio. 6), it can be seen that there is a large air-fuel ratio deviation on the increasing side (lean side) and decreasing side (rich side). It can also be seen that the smaller the operating angle, the larger the air-fuel ratio deviation.

  In the case of the diagram b, the air-fuel ratio at the time of idling is about 15.3, which is a maximum deviation of about +0.7 with respect to the stoichiometry. In the case of the diagram c, the air-fuel ratio at idling is about 13.9, which is a maximum deviation of about −0.7 with respect to the stoichiometry.

  On the other hand, it can be seen that the air-fuel ratio deviation decreases as the operating angle increases. In the operating angle region larger than Sx, both the diagram b and the diagram c only cause a slight air-fuel ratio shift with respect to the stoichiometry.

  Because of such characteristics, when a working angle shift occurs in some cylinders (particularly one cylinder), variations in the air-fuel ratio among all cylinders occur at a small working angle, resulting in fluctuations in sensor output before the catalyst. Further, it has been found that the pre-catalyst sensor output fluctuation is more noticeable when the air-fuel ratio is richer than stoichiometric. That is, it has been found from the characteristics of the air-fuel ratio sensor that an output change with respect to an air-fuel ratio change hardly occurs near the stoichiometric range.

  Therefore, in this embodiment, the operating angle S is equal to or smaller than the predetermined value Sx, and a variation abnormality is detected during the rich control at the time of F / C return. During rich control at the time of F / C return, in many cases, idling or low load operation is being performed, and the operating angle S is equal to or less than a predetermined value Sx. Therefore, by performing detection at this opportunity, it is possible to increase the detection opportunity while ensuring the detection accuracy.

  In addition, since variation abnormality detection is performed using the opportunity of rich control at the time of F / C return, detection accuracy can be improved without performing useless rich control. Rich control should be avoided as much as possible because it degrades fuel consumption and CO2 emissions.

  In terms of control, the feedforward control is executed so that the operating angles of all the cylinders are uniformly matched to the reference operating angle determined by the engine operating state regardless of the actual operating angle deviation. The condition is that the reference working angle is not more than the predetermined value Sx.

  In addition, in the present embodiment, the above-described abnormality detection during general high-load operation is also performed. Since the normal operating angle is larger than the predetermined value Sx during this high load operation, the conditions for detecting abnormality are that the reference operating angle is larger than the predetermined value Sx and the engine load is equal to or higher than the predetermined value. Here, the condition that the engine load is not less than a predetermined value is that the intake air amount detected by the air flow meter 5 is not less than a predetermined value.

  Further, since the pre-catalyst sensor output fluctuation is more noticeable when the air-fuel ratio is richer than stoichiometric, rich control is also executed at the time of abnormality detection during this high load operation. Note that the rich control at this time can be omitted.

  In the present embodiment, abnormality detection can be performed during idling or low-load operation. Therefore, if there is other diagnostic control that is performed during high-load operation, it is possible to perform abnormality detection while avoiding interference with this. That is, if only abnormality detection during high-load operation is performed, there is a possibility that interference with other diagnostic controls may occur and abnormality detection cannot be performed, but this embodiment can prevent this. It is also possible to detect an abnormality in the same manner as during high load operation during large working angle operation such as before engine warm-up is completed or before each learning is completed.

  By the way, the abnormality detection is performed by comparing the output fluctuation parameter X and the determination value. When the abnormality is determined, it is caused by at least one of an air system abnormality (especially a working angle deviation abnormality) and a fuel system abnormality (especially an injector abnormality). There is a possibility. Abnormality detection during F / C return rich control is particularly advantageous for detecting air system abnormalities. This is because the operating angle deviation at the small operating angle causes a large air-fuel ratio deviation. However, when the operating angle is small, the sensitivity of the operating angle deviation to the air-fuel ratio deviation is large, so that even a slight abnormality in the fuel system may be superimposed and lead to a large sensor output fluctuation. Therefore, it is considered that the abnormality detection during the rich control at the time of F / C return is also effective in detecting the fuel system abnormality.

  In addition, since the sensitivity to the air-fuel ratio deviation of the working angle deviation becomes small at the large working angle, it is considered that the abnormality detection during the high load operation is mainly advantageous for the detection of the fuel system abnormality.

  FIG. 8 shows changes in values during a running test of a vehicle equipped with the engine of the present embodiment. (A) is the vehicle speed Vv, (B) is the operating angle S, and (C) is the air-fuel ratio A / F. In (C), a broken line indicates a target value, and a solid line indicates a value detected by the pre-catalyst sensor 17.

  Abnormality detection during high-load operation is performed when, for example, the operating angle is larger than a predetermined value Sx and the engine load is equal to or greater than a predetermined value (that is, the intake air amount is equal to or greater than a predetermined value). To be done.

  On the other hand, the abnormality detection during the rich control at the time of F / C return is, for example, during the rich control at the time of the F / C return when the operating angle S is equal to or less than the predetermined value Sx in the period as indicated by an ellipse g in the figure. It is performed in the period Δt1. During this period Δt1, the target air-fuel ratio is set to a predetermined value that is richer than the stoichiometric value, and rich control is performed. Further, the deceleration fuel cut is performed immediately before the period Δt1, and the detected value of the air-fuel ratio A / F is stuck to the upper limit value on the lean side.

  FIG. 9 shows the test results obtained by examining the pre-catalyst sensor output fluctuation during the rich control at the time of F / C return when the operating angle shift occurs in only one cylinder in the engine of the present embodiment. (A) is the engine speed Ne (rpm), (B) is the load KL, (C) is the output voltage Vf of the pre-catalyst sensor, and (D) is the air-fuel ratio A / F obtained by converting the output voltage Vf. , (E) shows the operating angle S, and (F) shows the vehicle speed Vv. For (B) to (D), normal and abnormal data are shown.

  In the figure, Δt1 corresponds to the F / C return rich control period. As shown in (C) and (D), no fluctuation is observed in the normal waveform, but there is a fluctuation in the abnormal waveform. Therefore, the presence / absence of variation abnormality can be determined based on the difference between the normal waveform and the abnormal waveform, and the variation abnormality can be detected based on the abnormal waveform.

  FIG. 10 shows a flowchart of a variation abnormality detection process executed by the ECU 20.

First, in step S101, it is determined whether or not the intake valve operating angle (especially the reference operating angle) S is greater than a predetermined value Sx.
When the operating angle S is larger than the predetermined value Sx, the process proceeds to step S102, and abnormality detection is performed during high load operation.

  On the other hand, when the operating angle S is equal to or smaller than the predetermined value Sx, the process proceeds to step S103, and abnormality detection is performed during the rich control at the time of F / C return.

  The preferred embodiment of the present invention has been described in detail above, but various other embodiments of the present invention are conceivable. For example, the intake valve operating mechanism may be any mechanism that can change at least the operating angle, and it is not essential to change the opening / closing timing and the lift. Moreover, you may change a working angle in steps. The fuel cut execution condition can be changed as appropriate. In the above embodiment, the air-fuel ratio control is performed using stoichiometry as a reference value, but this reference value may be other than the stoichiometry.

  The embodiment of the present invention is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

1 Internal combustion engine
2 Injector 11 Upstream catalyst 17 Pre-catalyst sensor 20 Electronic control unit (ECU)
21 Variable valve mechanism

Claims (6)

Changing means for changing the operating angle of the intake valve according to the operating state of the multi-cylinder internal combustion engine;
Fuel cut means for performing fuel cut when a predetermined condition is satisfied;
Rich control means for controlling the air-fuel ratio to be richer than a predetermined reference value;
Detecting means for detecting a variation in air-fuel ratio between cylinders when the operating angle is not more than a predetermined value and rich control is performed by the rich control means immediately after returning from the fuel cut;
An inter-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine, comprising: 2. The multi-cylinder according to claim 1, wherein the detection unit detects the variation abnormality even when the operating angle is larger than the predetermined value and a load of the internal combustion engine is equal to or larger than a predetermined value. An inter-cylinder air-fuel ratio variation abnormality detection device for an internal combustion engine. The detection means detects the variation abnormality even when the operating angle is larger than the predetermined value, the load of the internal combustion engine is equal to or larger than a predetermined value, and the rich control is performed by the rich control means. The inter-cylinder air-fuel ratio variation abnormality detection device for a multi-cylinder internal combustion engine according to claim 1. The inter-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine according to any one of claims 1 to 3, wherein the change unit increases the operating angle as the load on the internal combustion engine increases. . 5. The inter-cylinder air-fuel ratio variation abnormality detecting device for a multi-cylinder internal combustion engine according to any one of claims 1 to 4, wherein the reference value is stoichiometric. The cylinder of the multi-cylinder internal combustion engine according to any one of claims 1 to 5, wherein the detection unit detects the variation abnormality based on an output fluctuation of an air-fuel ratio sensor provided in an exhaust passage. Inter-air-fuel ratio variation abnormality detection device.

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