JP7848615B2 - Method and apparatus for controlling the warm-up of a three-way catalytic converter - Google Patents

Description

This invention relates to a warm-up control method and apparatus for an internal combustion engine, which promotes the warm-up of a three-way catalytic converter by periodically varying the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalytic converter between rich and lean.

To quickly warm up the three-way catalytic converter installed in the exhaust passage of an internal combustion engine to near its activation temperature, it has been proposed to control the combustion by periodically alternating between rich and lean combustion. For example, Patent Document 1 discloses a method for periodically fluctuating the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalytic converter in a four-cylinder in-line internal combustion engine, such as setting cylinder #1 to rich combustion, cylinders #3 and #4 to lean combustion, and then cylinder #2 to rich combustion.

Japanese Patent Publication No. 2004-353552

Under the same intake air volume, rich combustion (where the equivalent ratio is greater than stoichiometric pressure) and lean combustion (where the equivalent ratio is less than stoichiometric pressure) result in different combustion pressures. Therefore, the above control method produces large variations in combustion pressure, causing discomfort to the driver. In particular, when the combustion sequence changes from rich to lean combustion in two consecutive cylinders, or vice versa, the combustion pressure changes in a stepwise manner, becoming a factor in rotational fluctuations.

Furthermore, if the range of variation in the equivalent ratio of stoichiometric or lean combustion is reduced in order to minimize combustion pressure fluctuations, the catalyst warm-up effect will decrease.

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.
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.
The stoichiometric combustion between these rich and lean combustion phases takes place in a relatively small number of cylinders .

According to this invention, rich combustion and lean combustion do not occur consecutively; instead, stoichiometric combustion occurs between them. Therefore, the change in combustion pressure between two cylinders with consecutive combustion sequences is reduced, and combustion pressure fluctuations during catalyst warm-up are suppressed.

A diagram illustrating the configuration of an internal combustion engine in one embodiment to which this invention is applied. A time chart showing an example of a repeating pattern along with changes in the air-fuel ratio. A time chart showing several examples of repeating patterns.

The following describes in detail an embodiment of this invention based on the drawings. Figure 1 is an explanatory diagram showing the schematic configuration of an internal combustion engine 1 in one 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 this invention, a port injection configuration in which fuel is injected towards the intake port 7 of each cylinder is also possible.

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.

The exhaust ports 12 of each cylinder are connected to the branch sections 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.

An air-fuel ratio sensor 16 for detecting the exhaust air-fuel ratio is positioned upstream of the three-way catalytic converter 15 in 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 the deterioration of the three-way catalytic converter 15.

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 for detecting the intake air volume upstream of the throttle valve 10, a crank angle sensor 21 for detecting engine rotational speed and crank angle position, a water temperature sensor 22 for detecting coolant temperature, and an accelerator pedal position sensor 23 for detecting the amount of accelerator pedal depression by the driver. Based on these input signals, the engine controller 9 optimally controls the fuel injection amount and timing by the fuel injector 6, the ignition timing by the spark plug 4, the opening degree of the throttle valve 10, and other parameters.

The engine controller 9, as one of the various controls for the internal combustion engine 1, performs air-fuel ratio control to optimize the exhaust gas 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 then 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, such as 50%). This maintains the exhaust air-fuel ratio near the stoichiometric air-fuel ratio.

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 relatively large, periodic fluctuations in 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.

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, rich combustion with an equivalence ratio greater than 1 is performed twice in a row, followed by one stoichiometric combustion with an equivalence ratio of 1, and then two consecutive lean combustions with an equivalence ratio less than 1. After one stoichiometric combustion, rich combustion is performed again twice in a row. This repeating pattern repeats the cycle of rich combustion/stoichiometric combustion/lean combustion/stoichiometric combustion/rich combustion...

As is clear from Figure 2, by performing stoichiometric combustion with an equivalence ratio of 1 during the transition from rich to lean combustion and from lean to rich combustion, the combustion sequence (ignition sequence) between two consecutive cylinders does not change immediately from rich to lean (or vice versa). For example, the change occurs in two stages: from rich to stoichiometric combustion, and then from stoichiometric combustion to lean combustion, resulting in a smaller change in combustion pressure between two consecutive cylinders. Therefore, variations in combustion pressure and rotational fluctuations during catalyst warm-up with perturbation control are suppressed. While variations in combustion pressure can be quantitatively evaluated, for example, by the magnitude of the standard deviation σPi of the indicated mean effective pressure Pi, the intervention of stoichiometric combustion, as shown in Figure 2, makes it possible to suppress σPi to nearly half compared to the case without stoichiometric combustion.

In other words, by introducing stoichiometric combustion during the transition between rich and lean combustion, it is possible to suppress fluctuations in combustion pressure while ensuring a sufficiently large range of equivalence ratio changes between rich and lean combustion in perturbation control.

In one example, rich or lean combustion is performed sequentially in multiple cylinders according to the combustion sequence, while stoichiometric combustion between these rich/lean combustion phases is performed in a relatively small number of cylinders. In another example, rich or lean combustion is performed sequentially in multiple cylinders according to the combustion sequence, while stoichiometric combustion between these rich/lean combustion phases is performed sequentially in the same number of cylinders.

Furthermore, for catalytic warm-up through perturbation control, it is desirable that the number of stoichiometric combustion cycles is not excessively high. Therefore, when there are multiple repeating patterns with different numbers of cylinders, such as rich combustion/stoichiometric combustion/lean combustion/stoichiometric combustion/rich combustion, it is desirable to select a repeating pattern during warm-up that keeps the combustion pressure variation between cylinders below an acceptable level and minimizes the number of stoichiometric combustion cycles.

In another example, when the number of cylinders in an internal combustion engine is N (3 in the illustrated example), at least N rich or lean combustion cycles are performed consecutively according to the combustion sequence, and at least one stoichiometric combustion cycle is performed between these rich/lean combustion cycles. In other words, rich and lean combustion are performed consecutively multiple times, at least once in each cylinder of the multi-cylinder system. This ensures that rich and lean exhaust gases alternately pass through the entire cross-section of the three-way catalytic converter 15 located at the outlet of the exhaust manifold 13, thereby reliably achieving the effects of perturbation control (oxidation of HC, etc., and recovery of primary degradation) throughout the entire surface of the three-way catalytic converter 15.

The time chart in Figure 3 shows four repeating patterns for a three-cylinder engine as an example. In a three-cylinder engine, as shown in the diagram, the combustion sequence is #1 cylinder → #3 cylinder → #2 cylinder. In the diagram, "R" indicates rich combustion, "L" indicates lean combustion, and "S" indicates stoichiometric combustion.

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.

The repeating patterns are not limited to these examples. Preferably, even during perturbation control, the average exhaust air-fuel ratio is controlled to be near the stoichiometric air-fuel ratio. In patterns 1 to 4 of Figure 3, the number of rich combustion cycles and lean combustion cycles are equal. By making the range of equivalence ratio changes towards the rich side and the lean side (relative to stoichiometric pressure) equal, the average exhaust air-fuel ratio is essentially kept near the stoichiometric air-fuel ratio.

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. The 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 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 and lean combustion cylinders as a stoichiometric combustion cylinder.

The above describes one embodiment of this invention applied to a three-cylinder in-line internal combustion engine. However, this invention is not limited to three-cylinder in-line internal combustion engines and can be similarly applied to other types of multi-cylinder internal combustion engines.

1...Internal combustion engine 6...Fuel injector 9...Engine controller 15...Three-way catalytic converter 16...Air-fuel ratio sensor

Claims (3)

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. 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. 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.