Engine system

Abstract

This prevents the deterioration of exhaust emissions when the engine restarts after intermittent shutdown. [Solution] An engine system comprising an upstream catalyst and a downstream catalyst, which are three-way catalysts having oxygen storage capacity; an air-fuel ratio sensor provided between the two catalysts; a NOx sensor provided downstream of the downstream catalyst; and a control device that controls the engine so that the air-fuel ratio becomes a target air-fuel ratio, wherein the control device includes a first candidate calculation unit that calculates a first candidate for a return target air-fuel ratio used for return control from intermittent stop based on the air-fuel ratio detected by the air-fuel ratio sensor during intermittent stop of the engine; an oxygen concentration calculation unit that calculates an oxygen concentration based on the pump cell current of the NOx sensor during the intermittent stop; a second candidate calculation unit that calculates a second candidate for the return target air-fuel ratio based on the oxygen concentration; and a setting unit that sets a final return target air-fuel ratio used for the return control, taking into consideration the first candidate and the second candidate.

Description

Technical Field

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

Background Art

[0002] An upstream catalyst and a downstream catalyst, which are three-way catalysts, are provided in the exhaust passage of an engine. There is a technique for setting a return target air-fuel ratio used in return control from an intermittent stop based on the detected air-fuel ratio of an air-fuel ratio sensor disposed between the upstream catalyst and the downstream catalyst while the engine is in the intermittent stop. Thereby, deterioration of exhaust emissions at the time of return from the intermittent stop is suppressed (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the above technique, the return target air-fuel ratio is set in consideration of the atmosphere of the upstream catalyst during the intermittent stop of the engine, but the atmosphere of the downstream catalyst is not considered. For this reason, there is a possibility that exhaust emissions at the time of return from the intermittent stop deteriorate.

[0005] Therefore, an object is to provide an engine system in which deterioration of exhaust emissions at the time of return from an intermittent stop of an engine is suppressed.

Means for Solving the Problems

[0006] The above objective can be achieved by an engine system comprising: an engine; an upstream catalyst, which is a three-way catalyst having oxygen storage capacity and provided in the exhaust passage of the engine; a downstream catalyst, which is a three-way catalyst having oxygen storage capacity and provided in the exhaust passage of the engine and provided downstream of the upstream catalyst; an air-fuel ratio sensor provided between the upstream catalyst and the downstream catalyst in the exhaust passage; a NOx sensor provided downstream of the downstream catalyst in the exhaust passage; and a control device that controls the engine so that the air-fuel ratio of the engine becomes a target air-fuel ratio, wherein the control device includes: a first candidate calculation unit that calculates a first candidate for a recovery target air-fuel ratio used for recovery control from the intermittent stop of the engine based on the air-fuel ratio detected by the air-fuel ratio sensor during the intermittent stop of the engine; an oxygen concentration calculation unit that calculates an oxygen concentration calculated based on the pump cell current of the NOx sensor during the intermittent stop of the engine; a second candidate calculation unit that calculates a second candidate for the recovery target air-fuel ratio based on the calculated oxygen concentration; and a setting unit that sets a final recovery target air-fuel ratio used for recovery control, taking into consideration the first candidate and the second candidate. [Effects of the Invention]

[0007] This provides an engine system that suppresses the deterioration of exhaust emissions when the engine restarts from intermittent shutdown. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic diagram of the engine system. [Figure 2] This flowchart illustrates the control mechanism for setting the target air-fuel ratio for recovery. [Modes for carrying out the invention]

[0009] [Overall Configuration of the Engine System] Figure 1 is a schematic diagram of the engine system 1. The engine system 1 is mounted on a vehicle, for example, but is not limited to that and may be mounted on a ship or other vessel. The engine system 1 has an engine 10 and an exhaust passage 20. The engine 10 is a multi-cylinder engine having multiple cylinders. The engine 10 is equipped with a spark plug and an in-cylinder injection valve. An intake passage (not shown) is also connected to the engine 10. The engine 10 is equipped with a crank angle sensor 40 for detecting the rotation angle of the crankshaft.

[0010] The exhaust passage 20 includes an exhaust manifold 21 connected to the engine 10, and an exhaust pipe 22 downstream of the exhaust manifold 21. An upstream catalytic converter 31 is positioned between the exhaust manifold 21 and the exhaust pipe 22. A downstream catalytic converter 32 is positioned in the exhaust pipe 22. An upstream air-fuel ratio sensor 41 is provided at the confluence point of the branch sections connected to each cylinder of the exhaust manifold 21. A downstream air-fuel ratio sensor 42 is provided in the exhaust pipe 22 downstream of the upstream catalytic converter 31. A NOx sensor 43 is provided in the exhaust pipe 22 downstream of the downstream catalytic converter 32. The upstream air-fuel ratio sensor 41 detects the air-fuel ratio of the exhaust flowing into the upstream catalytic converter 31. The downstream air-fuel ratio sensor 42 detects the air-fuel ratio of the exhaust discharged from the upstream catalytic converter 31 and flowing into the downstream catalytic converter 32. The NOx sensor 43 detects the NOx concentration in the exhaust.

[0011] The upstream catalyst 31 and the downstream catalyst 32 are three-way catalysts containing catalytic metals such as platinum (Pt), palladium (Pd), and rhodium (Rh), and possessing oxygen storage capacity. Due to their catalytic action and oxygen storage capacity, the three-way catalysts purify NOx and HC depending on the amount of oxygen stored. When the air-fuel ratio of the exhaust flowing into the three-way catalyst is a lean air-fuel ratio, oxygen is absorbed from the exhaust by the three-way catalyst when the oxygen storage capacity of the three-way catalyst is low. Consequently, NOx in the exhaust is reduced and purified. When the oxygen storage capacity of the three-way catalyst increases, the concentrations of oxygen and NOx in the exhaust flowing out from the three-way catalyst increase. When the air-fuel ratio of the exhaust flowing into the three-way catalyst is a rich air-fuel ratio, when the oxygen storage capacity of the three-way catalyst is high, the oxygen stored in the three-way catalyst is released, and HC in the exhaust is oxidized and purified. When the oxygen storage capacity of the three-way catalyst decreases, the concentration of HC in the exhaust flowing out from the three-way catalyst increases.

[0012] The ECU (Electric Control Unit) 50 is equipped with a CPU (Central Processing Unit), RAM (Random Access Memory), ROM (Read Only Memory), and memory devices such as flash memory, and performs various controls by executing programs stored in the ROM and memory devices. The ECU 50 controls the engine 10 based on the amount of operation of the accelerator pedal and brake pedal operated by the driver, as well as the engine speed and load of the engine 10. The crank angle sensor 40, upstream air-fuel ratio sensor 41, downstream air-fuel ratio sensor 42, and NOx sensor 43 are electrically connected to the ECU 50. The ECU 50 is an example of a control device. The ECU 50 functionally implements the first candidate calculation unit, oxygen concentration calculation unit, second candidate calculation unit, and setting unit, which will be described in detail later.

[0013] The ECU 50 controls the target air-fuel ratio, which is the target value of the air-fuel ratio of the exhaust gas emitted from the engine 10, so that it alternates between a rich air-fuel ratio smaller than the stoichiometric air-fuel ratio (e.g., 14.6) and a lean air-fuel ratio larger than the stoichiometric air-fuel ratio. Specifically, the ECU 50 controls the air-fuel ratio of the exhaust gas emitted from the engine 10 so that the air-fuel ratio detected by the upstream air-fuel ratio sensor 41 becomes the target air-fuel ratio. Specifically, the ECU 50 controls the air-fuel ratio of the exhaust gas emitted from the engine 10 by feedback control of the fuel injection amount, mainly based on the air-fuel ratio detected by the upstream air-fuel ratio sensor 41 and the downstream air-fuel ratio sensor 42.

[0014] [Return Target Air-Fuel Ratio Setting Control] This flowchart illustrates the recovery target air-fuel ratio setting control performed by the ECU 50. The recovery target air-fuel ratio setting control sets the recovery target air-fuel ratio used for recovery control of the engine 10 from intermittent stop while the engine 10 is intermittently stopped. Intermittent stops include, for example, stops due to idle stop, stops while driving with the motor in a hybrid vehicle, and stops due to fuel cut. The air-fuel ratio of the engine 10 during recovery control is controlled to a recovery target air-fuel ratio, which is richer than the stoichiometric air-fuel ratio, unlike the target air-fuel ratio during normal operation. This is because the oxygen storage capacity of the upstream catalyst 31 and the downstream catalyst 32 increases during intermittent stop. First, the ECU 50 determines whether or not the engine 10 is in an intermittent stop (step S1). If the answer in step S1 is No, this control is terminated.

[0015] If the answer in step S1 is Yes, the ECU 50 obtains the air-fuel ratio detected by the downstream air-fuel ratio sensor 42 (step S2). This allows the system to determine the air-fuel ratio of the atmosphere at the upstream catalyst 31 during intermittent shutdown of the engine 10.

[0016] Next, the ECU 50 refers to the first candidate map and calculates a first candidate for the recovery target air-fuel ratio used for recovery control from intermittent stop of the engine 10, based on the acquired detected air-fuel ratio (step S3). The first candidate map is a map that defines the relationship between the detected air-fuel ratio of the downstream air-fuel ratio sensor 42 and the first candidate for the recovery target air-fuel ratio during intermittent stop of the engine 10, and is stored in the ROM of the ECU 50. The first candidate map is defined such that the leaner the detected air-fuel ratio, the richer the first candidate for the recovery target air-fuel ratio. This is because a leaner detected air-fuel ratio indicates an atmosphere with a larger oxygen storage capacity in the upstream catalyst 31, and the emission of NOx from the upstream catalyst 31 is likely to increase when the engine 10 is restarted. Alternatively, the first candidate may be calculated using a calculation formula with the detected air-fuel ratio as an argument, instead of the first candidate map. Step S3 is an example of the processing performed by the first candidate calculation unit.

[0017] The ECU 50 calculates the oxygen concentration in the exhaust gas discharged from the downstream catalyst 32 based on the pump cell current of the NOx sensor 43 (step S4). The calculation of the oxygen concentration based on the pump cell current is performed using a known method, for example, as described in Japanese Patent Application Publication No. 2015-215334. This makes it possible to determine the oxygen concentration in the atmosphere of the downstream catalyst 32 during intermittent stopping of the engine 10. Step S4 is an example of the process performed by the oxygen concentration calculation unit.

[0018] Next, the ECU 50 calculates a second candidate for the return target air-fuel ratio used in the return control from the intermittent stop of the engine 10 based on the calculated oxygen concentration by referring to the second candidate map (step S5). The second candidate map is a map that defines the relationship between the oxygen concentration in the exhaust gas discharged from the downstream catalyst 32 during the intermittent stop of the engine 10 and the second candidate for the return target air-fuel ratio, and is stored in the ROM of the ECU 50. In the second candidate map, it is defined such that the second candidate for the return target air-fuel ratio becomes richer as the oxygen concentration is higher. This is because as the oxygen concentration is higher, the atmosphere has a larger oxygen storage amount in the downstream catalyst 32, and the NOx emission amount from the downstream catalyst 32 tends to increase when the engine 10 is restarted. Note that instead of the second candidate map, the second candidate may be calculated by an arithmetic expression with the oxygen concentration as an argument. Step S5 is an example of the process executed by the second candidate calculation unit.

[0019] Next, the ECU 50 sets the final return target air-fuel ratio in consideration of the calculated first and second candidates (step S6). For example, among the first and second candidates, the candidate on the rich side may be set as the final return target air-fuel ratio. Thereby, the NOx emission amount during the return control can be suppressed. Also, the average value of the first and second candidates may be set as the final return target air-fuel ratio. Thereby, both the NOx emission amount and the HC emission amount during the return control can be suppressed in a well-balanced manner. Step S6 is an example of the process executed by the setting unit.

[0020] As described above, the return target air-fuel ratio is set in consideration of the air-fuel ratio of the atmosphere of the upstream catalyst 31 and the oxygen concentration of the atmosphere of the downstream catalyst 32 during the intermittent stop of the engine 10. Thereby, the deterioration of the exhaust emission is suppressed.

[0021] Although the preferred 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 changes are possible within the scope of the gist of the present invention described in the claims.

Explanation of Reference Numerals

[0022] 1 Engine system 10 Engine 31 Upstream catalyst 32 Downstream catalyst 42 Downstream air-fuel ratio sensor 43 NOx sensor 50 ECU (control device, first candidate calculation unit, oxygen concentration calculation unit, second candidate calculation unit, setting unit)

Claims

[Claim 1] The engine and An upstream catalyst, which is a three-way catalyst having oxygen storage capacity, is provided in the exhaust passage of the aforementioned engine. A downstream catalyst, which is a three-way catalyst provided in the exhaust passage of the engine, has oxygen storage capacity, and is located downstream of the upstream catalyst, An air-fuel ratio sensor is provided between the upstream catalyst and the downstream catalyst of the exhaust passage, A NOx sensor is provided downstream of the downstream catalyst in the exhaust passage, The system includes a control device that controls the engine so that the air-fuel ratio of the engine becomes a target air-fuel ratio, The control device is A first candidate calculation unit calculates a first candidate for a target air-fuel ratio used in the engine's recovery control from intermittent shutdown, based on the air-fuel ratio detected by the air-fuel ratio sensor during the engine's intermittent shutdown. An oxygen concentration calculation unit that calculates the oxygen concentration based on the pump cell current of the NOx sensor during the intermittent shutdown of the engine, A second candidate calculation unit calculates a second candidate for the target air-fuel ratio to be restored based on the calculated oxygen concentration, A setting unit that sets the final target air-fuel ratio for the return control, taking into consideration the first candidate and the second candidate, Engine system.

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