Method and apparatus for controlling the warm-up of a three-way catalytic converter

Abstract

To suppress variation of combustion pressure based on change of an equivalence ratio, in control of periodically repeating rich combustion with a large equivalence ratio and lean combustion with a small equivalence for catalyst warming-up.SOLUTION: A three-way catalyst is provided at an outlet part of an exhaust manifold of an internal combustion engine. In order to promote warming-up of the three-way catalyst, perturbation control is performed in which rich combustion with a large equivalence ratio and lean combustion with a small equivalence ratio are periodically repeated. Since stoichiometric combustion with an equivalence ratio of 1 is performed during transition from the rich combustion to the lean combustion and from the lean combustion to the rich combustion, it is possible to prevent large combustion pressure change from occurring between two consecutive cylinders, to reduce variation of combustion pressure.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a warm-up control method and apparatus for promoting warm-up of a three-way catalyst by periodically varying the exhaust air-fuel ratio of exhaust gas flowing into the three-way catalyst in a rich / lean manner in an internal combustion engine.

Background Art

[0002] In order to quickly warm up a three-way catalyst provided in an exhaust passage of an internal combustion engine to near its activation temperature, it has been proposed to perform control that periodically repeats rich combustion and lean combustion. For example, Patent Document 1 discloses that in a straight-four cylinder internal combustion engine, the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst is periodically varied rich / lean by setting the #1 cylinder to rich combustion, the next #3 and #4 cylinders to lean combustion, and the next #2 cylinder to rich combustion, and so on.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Under the same intake air amount, the combustion pressures generated by rich combustion with an equivalence ratio greater than stoichiometry and lean combustion with an equivalence ratio less than stoichiometry are different from each other. Therefore, in the above control, large variations in combustion pressure occur, giving the driver a sense of discomfort. In particular, when the combustion changes from rich combustion to lean combustion or vice versa in two cylinders with consecutive combustion orders, the combustion pressure changes stepwise, which becomes a factor in rotational fluctuations.

[0005] Also, if the variation range of the equivalence ratio of stoichiometric combustion or lean combustion is reduced in order to reduce combustion pressure variations, the catalyst warm-up effect will decrease. [Means for solving the problem]

[0006] This invention relates to a warm-up control method for a three-way catalytic converter, which is installed in the exhaust passage of an internal combustion engine, and which controls the warm-up of the three-way catalytic converter by periodically repeating rich combustion with a large equivalence ratio and lean combustion with a small equivalence ratio. Stoichiometric combustion with an equivalent ratio of 1 is performed during the transition from rich combustion to lean combustion and during the transition from lean combustion to rich combustion. stomach, Here, Multiple cylinders are configured to perform either rich or lean combustion sequentially according to the combustion sequence. Stoichiometric combustion, which occurs between rich and lean combustion, takes place in a relatively small number of cylinders. cormorant. [Effects of the Invention]

[0007] According to this invention, rich combustion and lean combustion do not occur consecutively, and stoichiometric combustion occurs between them. As a result, the change in combustion pressure between two cylinders with consecutive combustion sequences is reduced, and fluctuations in combustion pressure during catalyst warm-up are suppressed. [Brief explanation of the drawing]

[0008] [Figure 1] A diagram illustrating the configuration of an internal combustion engine in one embodiment to which this invention is applied. [Figure 2] A time chart showing an example of a repeating pattern along with changes in the air-fuel ratio. [Figure 3] A time chart showing several examples of repeating patterns. [Modes for carrying out the invention]

[0009] Hereinafter, an embodiment of this invention will be described in detail with reference to the drawings. Figure 1 is an explanatory diagram showing the schematic configuration of an internal combustion engine 1 in an embodiment to which this invention is applied. The internal combustion engine 1 in this embodiment is a 3-cylinder, 4-stroke, spark-ignition internal combustion engine (a so-called gasoline engine), in which a pair of intake valves 2 and a pair of exhaust valves 3 are provided in the combustion chamber 5 of each cylinder, and a spark plug 4 is positioned in the center of the combustion chamber 5. In the illustrated example, as a direct injection engine, a fuel injection valve 6 that injects fuel into the cylinder is positioned, for example, on the intake valve 2 side. In the present invention, a port injection configuration in which fuel is injected toward the intake port 7 of each cylinder is also possible.

[0010] An electronically controlled throttle valve 10, whose opening degree is controlled by a control signal from the engine controller 9, is interposed upstream of the collector portion 8a of the intake passage 8 connected to the intake port 7 of each cylinder.

[0011] Each cylinder's exhaust port 12 is connected to a branch section of the exhaust manifold 13, and these are combined into a single exhaust passage by the exhaust manifold 13. A three-way catalytic converter 15 for exhaust purification is provided at the outlet of the exhaust manifold 13. The three-way catalytic converter 15 is a so-called monolithic ceramic catalyst, for example, a monolithic ceramic body with fine passages formed therein, coated with a catalytic layer containing a catalytic metal. In addition to the three-way catalytic converter 15 located at the outlet of the exhaust manifold 13, the three-way catalytic converter may also include another three-way catalytic converter (for example, an underfloor catalytic converter) arranged in series downstream.

[0012] An air-fuel ratio sensor 16 for detecting the exhaust air-fuel ratio is positioned upstream of the three-way catalytic converter 15, on the inlet side of the exhaust passage 14. This air-fuel ratio sensor 16 is a so-called wide-range air-fuel ratio sensor that provides an output corresponding to the exhaust air-fuel ratio. Furthermore, a downstream air-fuel ratio sensor, such as an O2 sensor that responds to the composition of the exhaust gas that has passed through the three-way catalytic converter 15, may be additionally provided downstream of the three-way catalytic converter 15 for purposes such as calibrating the air-fuel ratio feedback control system including the air-fuel ratio sensor 16 and diagnosing deterioration of the three-way catalytic converter 15.

[0013] The detection signal from the air-fuel ratio sensor 16 is input to the engine controller 9. Furthermore, the engine controller 9 receives detection signals from numerous sensors, including an air flow meter 20 that detects the intake air volume upstream of the throttle valve 10, a crank angle sensor 21 that detects the engine rotation speed and crank angle position, a water temperature sensor 22 that detects the coolant temperature, and an accelerator opening sensor 23 that detects the amount the accelerator pedal is pressed by the driver. Based on these input signals, the engine controller 9 optimally controls the fuel injection amount and injection timing by the fuel injector 6, the ignition timing by the spark plug 4, the opening degree of the throttle valve 10, and so on.

[0014] As one of the various controls for the internal combustion engine 1, the engine controller 9 performs air-fuel ratio control to optimize the exhaust purification performance of the three-way catalytic converter 15. The air-fuel ratio control estimates the oxygen storage amount of the three-way catalytic converter 15 based on the exhaust air-fuel ratio detected by the air-fuel ratio sensor 16, and feedback-controls the fuel injection amount (injection pulse width) of the fuel injector 5 so that this oxygen storage amount becomes the target oxygen storage amount (usually set to an intermediate value of the oxygen storage capacity (e.g., 50%)). As a result, the exhaust air-fuel ratio is maintained near the stoichiometric air-fuel ratio.

[0015] For this type of air-fuel ratio feedback control, it is necessary for the three-way catalytic converter 15 to reach its activation temperature. For example, it is desirable that the three-way catalytic converter 15 be warmed up to its activation temperature as soon as possible after the internal combustion engine 1 is started. Therefore, the engine controller 9 performs a control that periodically repeats rich combustion with a large equivalence ratio and lean combustion with a small equivalence ratio (hereinafter, for convenience, this will be called perturbation control) while the three-way catalytic converter 15 is warming up, or more specifically, when the three-way catalytic converter 15 has warmed up to some extent but has not yet reached its sufficient activation temperature. In perturbation control, the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalytic converter 15 fluctuates relatively large between rich and lean. This actively promotes the reaction between HC etc. during rich combustion and oxygen during lean combustion, thereby accelerating the temperature rise of the catalyst. Furthermore, to address the temporary degradation of the catalyst (a phenomenon where oxygen, HC, etc., adhere to the catalyst metal surface, reducing the catalyst metal surface area and thus lowering catalyst performance), perturbation control involves cyclically and significantly varying the air-fuel ratio of the gas in contact with the catalyst. This causes the poisoned substances covering the catalyst metal surface to peel off, expanding the catalyst's reaction area, thus accelerating catalyst warm-up.

[0016] In this invention, stoichiometric combustion with an equivalence ratio of 1 is performed at least once during the transition from rich combustion to lean combustion and during the transition from lean combustion to rich combustion. For example, Figure 2 is a time chart showing an example of a repeating pattern along with the change in air-fuel ratio, where the horizontal axis represents time or crank angle. In this example, as shown in the figure, rich combustion with an equivalence ratio greater than 1 is performed twice in a row, then stoichiometric combustion with an equivalence ratio of 1 is performed once, followed by lean combustion with an equivalence ratio less than 1 being performed twice in a row. After one stoichiometric combustion, rich combustion is performed twice in a row again. Rich combustion / stoichiometric combustion / lean combustion / stoichiometric combustion / rich combustion... is repeated in this repeating pattern.

[0017] During the transition from rich combustion to lean combustion and from lean combustion to rich combustion, by performing stoichiometric combustion with an equivalence ratio of 1, as is clear from FIG. 2, the combustion mode (ignition order) does not immediately change from rich combustion to lean combustion (or vice versa from lean combustion to rich combustion) between two cylinders with consecutive combustion orders. For example, it changes in two steps from rich combustion to stoichiometric combustion and then from stoichiometric combustion to lean combustion, and the combustion pressure change between two cylinders with consecutive combustion orders becomes smaller. Therefore, variations in combustion pressure and rotational fluctuations during the catalyst warm-up operation with perturbation control are suppressed. The variation in combustion pressure can be quantitatively evaluated, for example, by the magnitude of the standard deviation σPi of the indicated mean effective pressure Pi shown in the figure. However, by intervening stoichiometric combustion as shown in FIG. 2, it is possible to suppress σPi to nearly half of that in the case where stoichiometric combustion is not intervened.

[0018] In other words, by intervening stoichiometric combustion during the transition between rich combustion / lean combustion, it is possible to suppress the variation in combustion pressure while ensuring a sufficiently large change range in the equivalence ratio of rich combustion and lean combustion in perturbation control.

[0019] In one example, rich combustion or lean combustion is performed continuously in a plurality of cylinders according to the combustion order, and the stoichiometric combustion between these rich combustion / lean combustion is performed in a relatively small number of cylinders. In another example, rich combustion or lean combustion is performed continuously in a plurality of cylinders according to the combustion order, and the stoichiometric combustion between these rich combustion / lean combustion is performed continuously in the same number of a plurality of cylinders.

[0020] In addition, in the catalyst warm-up operation by perturbation control, it is desirable that the number of stoichiometric combustions does not become excessively large. Therefore, when there are a plurality of repetition patterns having different numbers of cylinders included in each, as a repetition pattern of repeating rich combustion / stoichiometric combustion / lean combustion / stoichiometric combustion / rich combustion ···, during warm-up, it is desirable to select a repetition pattern in which the combustion pressure variation between cylinders is below the allowable level and the number of stoichiometric combustions is minimized.

[0021] Also, in one example, when the number of cylinders of an internal combustion engine is N (3 in the illustrated example), at least N times of rich combustion or lean combustion are continuously performed in accordance with the combustion order, and at least one stoichiometric combustion is performed between these rich combustion / lean combustion. That is, since rich combustion and lean combustion are continuously performed a plurality of times so as to be performed at least once in all cylinders of a plurality of cylinders, rich exhaust gas and lean exhaust gas alternately pass through the entire cross-section of the three-way catalyst 15 provided at the outlet of the exhaust manifold 13, and the action by perturbation control (oxidation action of HC etc. and recovery action of primary deterioration) can be surely obtained throughout the three-way catalyst 15.

[0022] The time chart of FIG. 3 shows four repetition patterns in a three-cylinder engine as an example. In a three-cylinder engine, as shown, the combustion order is in the order of #1 cylinder → #3 cylinder → #2 cylinder. In the figure, "R" indicates rich combustion, "L" indicates lean combustion, and "S" indicates stoichiometric combustion, respectively.

[0023] In Pattern 1, rich combustion and lean combustion occur seven times each consecutively, with one stoichiometric combustion occurring during the transition from rich to lean combustion and one during the transition from lean to rich combustion. In Pattern 2, rich combustion and lean combustion occur seven times each consecutively, with two stoichiometric combustions occurring during the transition from rich to lean combustion and two during the transition from lean to rich combustion. In Pattern 3, rich combustion and lean combustion occur three times each consecutively, with three consecutive stoichiometric combustions occurring during the transition from rich to lean combustion and three during the transition from lean to rich combustion. In other words, each cylinder performs rich / stoichiometric / lean combustion once in sequence. In Pattern 4, rich combustion and lean combustion occur three times each consecutively, with one stoichiometric combustion occurring during the transition from rich to lean combustion and one during the transition from lean to rich combustion.

[0024] The repeating patterns are not limited to these examples. Preferably, the average exhaust air-fuel ratio is controlled to be near the stoichiometric air-fuel ratio even during perturbation control. In patterns 1 to 4 of Figure 3, the number of rich combustion cycles and lean combustion cycles are equal in all cases, so by making the range of change in the equivalent ratio towards the rich side and the range of change in the equivalent ratio towards the lean side relative to stoichiometric pressure equal, the average exhaust air-fuel ratio is basically near the stoichiometric air-fuel ratio.

[0025] Among the four repeating patterns in Figure 3, for example, pattern 2 has the longest rich / lean combustion reversal period, and pattern 4 has the shortest rich / lean combustion reversal period. An appropriate repeating pattern may be selected according to the rotational speed of the internal combustion engine 1 so that the perturbation control period is appropriate. Alternatively, rich combustion and lean combustion may be periodically repeated by designating a specific cylinder as a rich combustion cylinder, the other cylinders as lean combustion cylinders, and the cylinder burning between the rich combustion cylinder and the lean combustion cylinder as a stoichiometric combustion cylinder.

[0026] Although one embodiment of this invention applied to an in-line 3-cylinder internal combustion engine has been described above, this invention is not limited to in-line 3-cylinder internal combustion engines, but can be similarly applied to other types of multi-cylinder internal combustion engines. [Explanation of symbols]

[0027] 1…Internal combustion engine 6…Fuel injector 9…Engine controller 15...Three-way catalyst 16…Air-fuel ratio sensor

Claims

1. In a warm-up control method for a three-way catalytic converter, which is equipped in the exhaust passage of an internal combustion engine, the warm-up of the three-way catalytic converter is controlled to periodically repeat rich combustion with a large equivalence ratio and lean combustion with a small equivalence ratio, During the transition from rich combustion to lean combustion and during the transition from lean combustion to rich combustion, stoichiometric combustion with an equivalent ratio of 1 is performed. Here, Multiple cylinders are configured to perform either rich or lean combustion sequentially according to the combustion sequence. Stoichiometric combustion, which occurs between these rich and lean combustion cycles, takes place in a relatively small number of cylinders. A method for controlling the warm-up of a three-way catalytic converter.

2. When the number of cylinders in an internal combustion engine is N, at least N rich or lean combustion cycles are performed consecutively according to the combustion sequence. Between these rich / lean combustion cycles, perform at least one stoichiometric combustion cycle. A method for controlling the warm-up of a three-way catalytic converter as described in claim 1.

3. In a warm-up control device for a three-way catalytic converter, which is equipped in the exhaust passage of an internal combustion engine, the device controls the warm-up of the three-way catalytic converter by periodically repeating rich combustion with a large equivalence ratio and lean combustion with a small equivalence ratio, During the transition from rich combustion to lean combustion and during the transition from lean combustion to rich combustion, stoichiometric combustion with an equivalent ratio of 1 is performed. Here, Multiple cylinders are configured to perform either rich or lean combustion sequentially according to the combustion sequence. Stoichiometric combustion, which occurs between these rich and lean combustion cycles, takes place in a relatively small number of cylinders. A three-way catalytic converter warm-up control system.

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