Engine system

Abstract

To provide an engine system in which exhaust emission is improved.SOLUTION: An engine system includes: an engine; a catalyst for purifying exhaust of the engine, and having an oxygen storage capacity; and a control device for executing return rich control of controlling the engine so that an air-fuel ratio of exhaust flowing into the catalyst after return of the engine from fuel cut becomes a rich air-fuel ratio. The control device includes: a calculation part for calculating a maximum oxygen storage possible amount of the catalyst; and an execution period control part for shortening an execution period of the return rich control as the maximum oxygen storage possible amount is smaller.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an engine system. [Background technology]

[0002] When the engine is in a fuel cut, air is supplied to the catalyst, increasing the catalyst's oxygen storage capacity. Therefore, after the engine returns from fuel cut, a return rich control is executed to control the engine so that the air-fuel ratio of the exhaust gas flowing into the catalyst becomes rich. For example, there is a technology that reduces the fuel injection amount during return rich control as the catalyst's maximum oxygen storage capacity decreases (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-121593 Summary of the Invention [Problem to be solved by the invention]

[0004] When the fuel injection amount is reduced as in the above technology, the air-fuel ratio of the exhaust gas flowing into the catalyst approaches the stoichiometric air-fuel ratio from a rich air-fuel ratio. If the air-fuel ratio of the exhaust gas flowing into a catalyst with a large oxygen storage capacity is close to the stoichiometric air-fuel ratio, the amount of NOx emitted from the catalyst increases, which may worsen exhaust emissions.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an engine system with improved exhaust emissions. [Means for solving the problem]

[0006] The above object can be achieved by an engine system comprising an engine, a catalyst that purifies the exhaust of the engine and has oxygen storage capacity, and a control device that executes recovery rich control to control the engine so that the air-fuel ratio of the exhaust flowing into the catalyst becomes a rich air-fuel ratio after the engine returns from a fuel cut, wherein the control device includes a calculation unit that calculates the maximum oxygen storable amount of the catalyst, and an execution period control unit that shortens the execution period of the recovery rich control as the maximum oxygen storable amount becomes smaller. [Effects of the Invention]

[0007] According to the present invention, an engine system with improved exhaust emissions can be provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic configuration diagram of an engine system. [Figure 2] 4 is a flowchart illustrating a return rich control. [Figure 3] FIG. 10 is a diagram illustrating a map defining thresholds α and β according to OSC. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Engine system overview] 1 is a schematic diagram of an engine system 1. The engine system 1 has an engine 10, an intake passage 20, and an exhaust passage 30. The engine system 1 is mounted on a vehicle. The vehicle may be, for example, an engine vehicle equipped with only the engine 10 as a power source for running, or a hybrid vehicle equipped with both the engine 10 and a motor as a power source for running.

[0010] The engine 10 is a multi-cylinder engine having multiple cylinders. The engine 10 is provided with an in-cylinder injection valve 12 and a spark plug 14. The in-cylinder injection valve 12 directly injects fuel into the combustion chamber of the engine 10. Note that a port injection valve may be provided instead of or in addition to the in-cylinder injection valve 12. The spark plug 14 ignites the mixture of fuel and air. A throttle valve 22 is provided in the intake passage 20. The throttle valve 22 is driven, for example, by an actuator (not shown) to adjust the amount of intake air.

[0011] A catalyst 32 is provided in the exhaust passage 30. The catalyst 32 is a three-way catalyst that contains catalytic metals, platinum (Pt), palladium (Pd), and rhodium (Rh), and has oxygen storage capacity. The three-way catalyst has catalytic action and oxygen storage capacity, and therefore has the effect of purifying NOx and HC according to the amount of oxygen stored.

[0012] An air-fuel ratio sensor 40 is provided upstream of the catalyst 32. The air-fuel ratio sensor 40 outputs a signal corresponding to the air-fuel ratio of the exhaust gas flowing into the catalyst 32. An air-fuel ratio sensor 42 is provided downstream of the catalyst 32. The air-fuel ratio sensor 42 outputs a signal corresponding to the air-fuel ratio of the exhaust gas discharged from the catalyst 32.

[0013] The engine system 1 includes an ECU (Electronic Control Unit) 50. The ECU 50 is mainly composed of a computer including a CPU (Central Processing Unit) and volatile and non-volatile memories such as RAM (Random Access Memory) and ROM (Read Only Memory). The ECU 50 executes programs installed in the memory on the CPU to realize various control processes related to the engine 10. The ECU 50 is an example of a control device, and functionally realizes a calculation unit and an execution period control unit, which will be described in detail later.

[0014] The ECU 50 calculates the air-fuel ratio of the exhaust gas flowing into the catalyst 32 based on the output value of the air-fuel ratio sensor 40. Based on the calculated air-fuel ratio, the ECU 50 performs feedback control of the intake air amount and fuel injection amount of the engine 10 so that the equivalence ratio in the engine 10 becomes a target equivalence ratio. The target equivalence ratio is set by the ECU 50 according to the operating state of the engine 10. Here, the equivalence ratio is an index value representing the fuel concentration in the mixture, and is a value obtained by dividing the fuel amount that results in the stoichiometric air-fuel ratio by the actual fuel amount. The equivalence ratio is "1" when the air-fuel ratio of the mixture is the stoichiometric air-fuel ratio, a value greater than "1" when the air-fuel ratio is a rich air-fuel ratio, and a value less than "1" when the air-fuel ratio is a lean air-fuel ratio. A rich air-fuel ratio is a value smaller than the stoichiometric air-fuel ratio. A lean air-fuel ratio is a value greater than the stoichiometric air-fuel ratio.

[0015] The ECU 50 selectively executes return rich control and active control. In return rich control, the engine 10 is controlled so that the air-fuel ratio of the exhaust gas flowing into the catalyst 32 becomes a rich air-fuel ratio after the engine 10 returns from a fuel cut. In return rich control, oxygen stored in the catalyst 32 during fuel cut is released, preventing a decrease in the purification capacity of the catalyst 32. In active control, the engine 10 is controlled so that the air-fuel ratio of the exhaust gas flowing into the catalyst 32 alternates between a rich air-fuel ratio and a lean air-fuel ratio. In active control, oxygen is repeatedly released and rapidly increased from the catalyst 32, maintaining the purification capacity of the catalyst 32. In both return rich control and active control, a target equivalence ratio is calculated according to the respective control.

[0016] [Return rich control] 2 is a flowchart illustrating the return rich control. The ECU 50 calculates the OSC (step S1). The OSC is the maximum oxygen storage capacity of the catalyst 32. The OSC decreases as the use time of the catalyst 32 passes. The OSC is calculated, for example, based on the exhaust flow rate from the timing when the air-fuel ratio detected by the air-fuel ratio sensor 42 switches from one of the rich air-fuel ratio and the lean air-fuel ratio to the other until the timing when the air-fuel ratio switches from the other of the rich air-fuel ratio and the lean air-fuel ratio to the other. Note that the OSC may be calculated by any other known method. Step S1 is an example of processing executed by the calculation unit.

[0017] Next, the ECU 50 calculates thresholds α and β according to the OSC by referring to the map of FIG. 3 (step S2). FIG. 3 is an example of a map that defines thresholds α and β according to the OSC. Both thresholds α and β are defined so that they become lower as the OSC decreases. Threshold β is set to a lower value than threshold α. The degree of decrease in OSC is greater for threshold α than for threshold β. Therefore, as the OSC decreases, the magnitude of the difference ΔD between threshold α and threshold β also decreases. The thresholds α and β will be described in detail later.

[0018] Next, the ECU 50 determines whether a fuel cut is being performed (step S3). If the answer is Yes in step S3, the ECU 50 calculates OSA (step S4). OSA is the amount of oxygen stored in the catalyst 32. Since the calculation of OSA here is performed while the fuel cut is continuing, OSA is calculated to increase. The amount of increase in OSA is calculated based on, for example, the amount of air flowing into the catalyst 32 and the oxygen concentration of the air flowing into the catalyst 32. The air amount is, for example, the amount of intake air detected by an air flow meter. The oxygen concentration is calculated based on, for example, the detection result of the air-fuel ratio sensor 40.

[0019] If the answer is No in step S3, the ECU 50 calculates the OSA (step S5). The calculation of the OSA here is such that the OSA decreases when the return rich control is being executed, and the OSA is calculated according to the air-fuel ratio of the exhaust gas flowing into the catalyst 32 when the active control is being executed. The calculation method of the OSA is the same as that in step S4.

[0020] Next, the ECU 50 determines whether or not OSA is greater than a threshold value α (step S6). The threshold value α is set to the amount of oxygen stored in the catalyst 32 at which oxygen needs to be released from the catalyst 32 by the return rich control.

[0021] If the answer is Yes in step S6, the ECU 50 switches on the return rich control flag for executing the return rich control (step S7). After executing step S7, or if the answer is No in step S6, the ECU 50 determines whether the OSA is less than the threshold value β (step S8). As described above, the threshold value β is a value smaller than the threshold value α. If the answer is Yes in step S8, the ECU 50 switches off the return rich control flag (step S9). After executing step S9, or if the answer is No in step S8, the ECU 50 determines whether the return rich control flag is on (step S10).

[0022] If the answer is Yes in step S10, the ECU 50 executes the return rich control (step S11). The target equivalence ratio in the return rich control is calculated by the following equation (1). Target equivalence ratio = (1 + (base equivalence ratio - 1) x correction coefficient) ... (1) The base equivalence ratio is an equivalence ratio that serves as a reference in the return rich control and is a value greater than 1. The base equivalence ratio may be a fixed value or a variable value. For example, the base equivalence ratio may be a variable value that decreases as the air-fuel ratio detected by the air-fuel ratio sensor 42 is a rich air-fuel ratio and the difference between the rich air-fuel ratio and the stoichiometric air-fuel ratio increases. The correction coefficient is calculated to be a smaller value as the OSC decreases. This is because the purification ability of the catalyst 32 decreases when the air-fuel ratio of the exhaust gas flowing into the catalyst 32 deviates from the stoichiometric air-fuel ratio as the OSC decreases. The ECU 50 feedback-controls the fuel injection amount and intake air amount of the engine 10 according to the target equivalence ratio calculated in this manner.

[0023] If the answer is No in step S10, the ECU 50 executes active control (step S12). Therefore, when OSA is greater than the threshold value α, rich return control is executed, and when OSA falls below the threshold value β, rich return control is stopped and active control is executed. As described above, the difference ΔD between the threshold value α and the threshold value β decreases as the OSC decreases. Therefore, the execution period of the return rich control becomes shorter as the OSC decreases. In this way, the execution period of the return rich control is controlled according to the OSC. Steps S2, S6, S8, and S11 are examples of processing executed by the execution period control unit.

[0024] Here, it is conceivable to make the execution period of the return rich control uniform and reduce the fuel injection amount as the OSC decreases. In this case, the decrease in OSC is reflected only in the reduction in the fuel injection amount. Therefore, the air-fuel ratio of the exhaust gas flowing into the catalyst 32 approaches the stoichiometric air-fuel ratio as the OSC decreases, which may increase the amount of NOx emissions. In this embodiment, the decrease in OSC is also reflected in the execution period of the return rich control. Therefore, the air-fuel ratio of the exhaust gas flowing into the catalyst 32 during execution of the return rich control is prevented from approaching the stoichiometric air-fuel ratio, the amount of NOx emissions is suppressed, and exhaust emissions are improved.

[0025] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as defined in the claims. [Explanation of symbols]

[0026] 1 Engine System 10 Engine 32 Catalyst 50 ECU (calculation unit, execution period control unit)

Claims

[Claim 1] The engine and a catalyst that purifies exhaust gas from the engine and has oxygen storage capacity; a control device that executes a recovery rich control to control the engine so that the air-fuel ratio of the exhaust gas flowing into the catalyst becomes a rich air-fuel ratio after the engine returns from a fuel cut, The control device a calculation unit that calculates the maximum oxygen storage capacity of the catalyst; an execution period control unit that shortens the execution period of the return rich control as the maximum oxygen storable amount decreases.

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