Exhaust gas purification system

Abstract

To reduce NOx emission before SCR catalyst activation.SOLUTION: The exhaust gas purification system (1) includes a zeolite catalytic converter (31) for purifying exhaust gas from the engine (2), a temperature increasing device (33) for increasing a temperature of the zeolite catalytic converter (31), and a ECU5 for controlling the temperature increasing device (33), wherein the ECU5 is configured to estimate an amount of moisture adhering to the zeolite catalytic converter (31) when the engine (2) is stopped or idling, and to operate the temperature increasing device (33) so that the temperature of the zeolite catalytic converter (31) becomes equal to or higher than a moisture evaporation temperature when the estimated amount of adhering moisture exceeds a predetermined first amount of moisture.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification system. [Background technology]

[0002] Internal combustion engines such as diesel and gasoline engines produce exhaust gases containing NOx, which causes air pollution, so they are also equipped with purification devices to remove NOx from the exhaust gases.Purification devices include zeolite catalysts that use zeolite as a catalyst, and zeolite catalysts include so-called urea SCR systems in which an SCR catalyst (selective catalytic reduction catalyst) is placed in the exhaust passage and urea water is injected into the exhaust gases upstream of the SCR catalyst.

[0003] However, the above-described purification device cannot achieve sufficient exhaust gas purification efficiency unless it reaches a predetermined catalyst activation temperature. Possible methods for raising the temperature of the purification device to the catalyst activation temperature include increasing the fuel injection amount or retarding the fuel injection timing or ignition timing, but these methods increase CO2 emissions and are not necessarily desirable in terms of environmental impact. Therefore, for example, the exhaust gas purification device disclosed in Patent Document 1 employs a configuration in which the exhaust gas temperature is raised by a heater located upstream of the SCR catalyst. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-113540 Summary of the Invention [Problem to be solved by the invention]

[0005] However, regulations on emissions of harmful exhaust components such as NOx are becoming stricter every year due to their adverse effects on the human body and the natural environment. For this reason, the above-mentioned conventional technology may not be able to sufficiently reduce NOx emissions in the low temperature range before the purification device reaches the catalyst activation temperature.

[0006] The inventors discovered that even before the zeolite catalyst is activated, NOx is also adsorbed when moisture is adsorbed onto the zeolite catalyst. They thought that by utilizing this property and maintaining a state in which moisture is easily adsorbed onto the zeolite catalyst, it would be possible to further reduce NOx emissions.

[0007] The present invention has been made in view of the above problems, and an object of the present invention is to provide an exhaust gas purification system that can reduce NOx emissions even before the activation of the zeolite catalyst. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the exhaust gas purification system of the present invention comprises a zeolite catalyst that purifies the exhaust gas of an internal combustion engine, a heating device that can raise the temperature of the zeolite catalyst, and a control device that controls the heating device, wherein the control device estimates the amount of moisture attached to the zeolite catalyst when the internal combustion engine is stopped or idling, and when the estimated amount of moisture exceeds a predetermined first moisture amount, operates the heating device so that the temperature of the zeolite catalyst is above the moisture evaporation temperature. [Effects of the Invention]

[0009] The exhaust gas purification system of the present invention estimates the amount of moisture adhering to the zeolite catalyst when the internal combustion engine is stopped or idling, and when the amount of moisture exceeds a certain value, raises the temperature of the zeolite catalyst to reduce the amount of moisture adhering to the zeolite catalyst. This allows the zeolite catalyst to absorb additional moisture from the surrounding area, allowing it to adsorb the NOx along with the moisture. This allows the zeolite catalyst to more effectively remove NOx from exhaust gas. Therefore, the exhaust gas purification system of the present invention can reduce NOx emissions even before the zeolite catalyst is activated. [Brief explanation of the drawings]

[0010] [Figure 1]1 is a schematic diagram of an intake and exhaust system of an internal combustion engine to which an exhaust gas purification system according to the present invention is applied. [Figure 2] 3 is a flowchart showing a control procedure of the exhaust gas purification system. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, the embodiments will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the contents described below, and can be implemented with any modifications within the scope that does not change the gist of the disclosure. Furthermore, all drawings used to explain the embodiments are schematic representations of components, and may be partially emphasized, enlarged, reduced, or omitted to facilitate understanding, and may not accurately represent the scale, shape, etc. of the components.

[0012] 1 is a schematic diagram of the intake and exhaust system of an internal combustion engine 2 to which an exhaust gas purification system 1 according to the present invention is applied. The exhaust gas purification system 1 is a purification mechanism for removing NOx contained in exhaust gas emitted from the internal combustion engine 2, and in this embodiment, it mainly comprises a zeolite catalyst 31, a temperature raising device 33, a catalyst temperature acquisition means 35, a battery 4, and an ECU 5. Here, the vehicle on which the exhaust gas purification system 1 is mounted is an engine vehicle driven by the power of the internal combustion engine 2, but it may also be a hybrid vehicle that obtains driving force by combining the internal combustion engine 2 with a traction electric motor.

[0013] The internal combustion engine 2 is, for example, a multi-cylinder gasoline engine, and FIG. 1 shows only one cylinder for simplicity. An intake passage 21 of the internal combustion engine 2 is provided with a throttle valve 22 for adjusting the amount of intake air. The internal combustion engine 2 is also configured to inject fuel into the intake port 23 of each cylinder from a fuel injection valve 24 provided in the intake port 23 of each cylinder at any injection timing and injection amount. An intake valve is opened to draw a mixture of air and fuel (gasoline) from the intake passage 21 into a combustion chamber 25. The compressed mixture is then ignited in the combustion chamber 25 by a spark plug 26, causing it to burn. The burned mixture (exhaust gas) is exhausted from the internal combustion engine 2 through an exhaust passage 27, purified by a zeolite catalyst 31, and then released into the atmosphere.

[0014] The internal combustion engine 2 is not limited to a gasoline engine, but may be any other type of engine whose exhaust gas may contain NOx, such as a diesel engine or a hydrogen engine.

[0015] The zeolite catalyst 31 is made of zeolite crystals that have been ion-exchanged with Cu, Fe, etc., and promotes NOx purification reactions at an activation temperature of approximately 180°C or higher. Although not described here, other catalysts, such as three-way catalysts, are also used as exhaust gas purification components. The carrier 32 is, for example, a honeycomb-shaped component that supports the zeolite catalyst 31 on the exhaust gas flow path.

[0016] The temperature raising device 33 is a heating mechanism capable of raising the temperature of the zeolite catalyst 31, and is, for example, an electric heater capable of adjusting the output according to the supplied power. In this embodiment, the temperature raising device 33 is provided on the support 32. That is, the zeolite catalyst 31, the support 32, and the temperature raising device 33 constitute a so-called electrically heated catalyst (EHC). Note that the temperature raising device 33 is not limited to an electric heater as long as it can raise the temperature of the zeolite catalyst 31 to at least the water evaporation temperature (e.g., 100°C) or higher. Furthermore, the temperature raising device 33 does not have to be provided on the support 32, and may be, for example, an electric heater that covers the zeolite catalyst 31 from the outside.

[0017] The catalyst temperature acquisition means 35 is, for example, a catalyst temperature sensor that measures the temperature of the zeolite catalyst 31, and is used when controlling the output of the temperature raising device 33, as will be described later. Note that the catalyst temperature acquisition means 35 is not limited to a catalyst temperature sensor, and may be one that estimates the temperature of the zeolite catalyst 31 by measuring the temperature of the exhaust gas.

[0018] In this embodiment, the zeolite catalyst 31 is used as a catalyst that constitutes the urea SCR system 3. The urea SCR system 3 is a purification mechanism that reduces NOx from exhaust gas emitted from the internal combustion engine 2 and decomposes it into harmless nitrogen (N2) and water (H2O), and includes the above-mentioned zeolite catalyst 31, carrier 32, temperature raising device 33, catalyst temperature acquisition means 35, and in addition, a urea water supply device 34.

[0019] The urea water supply device 34 is provided upstream of the zeolite catalyst 31 in the exhaust gas flow passage, and injects urea water that acts as a reducing agent for the purification reaction in the zeolite catalyst 31.

[0020] The battery 4 is an electricity storage device capable of supplying power to various electrical components mounted on the vehicle, and in this embodiment in particular is used to supply power to the temperature raising device 33. The battery 4 can be charged with power generated by the power of the internal combustion engine 2, or, if the vehicle is equipped with a regenerative braking system, can be charged with regenerative energy during vehicle deceleration.

[0021] The ECU 5 is a well-known electronic control unit that includes input / output devices, a memory device, a timer, a central processing unit (CPU), etc., and is a control device that monitors the status of various components of the vehicle and controls the entire vehicle by outputting control signals to the various components.

[0022] More specifically, the ECU 5 controls the internal combustion engine 2 by appropriately sending control signals to the throttle valve 22, the fuel injectors 24, and the spark plugs 26. The ECU 5 also detects the humidity of the outside air using the humidity sensor Sh, acquires temperature information from a catalyst temperature acquisition means 35 as needed, and monitors the SOC (State Of Charge) of the battery 4. As will be described in detail later, if moisture is attached to the zeolite catalyst 31, the ECU 5 drives the temperature raising device 33 to remove the moisture and allows new moisture to attach, thereby attaching NOx together with the moisture and improving the efficiency of NOx purification.

[0023] Next, we will explain the operation of the exhaust gas purification system 1. Figure 2 is a flowchart showing the control procedure of the exhaust gas purification system 1. The exhaust gas purification system 1 periodically repeats the control procedure shown in Figure 2 regardless of a request to start the vehicle engine, thereby controlling the amount of moisture adhering to the zeolite catalyst 31 so that it does not increase too much.

[0024] When the control procedure of the exhaust gas purification system 1 is started, the ECU 5 checks the operating state of the vehicle, that is, whether the internal combustion engine 2 is stopped or, if it is operating, whether it is idling (step S1). Here, the stopped state of the internal combustion engine 2 includes not only the vehicle being parked, but also, in the case of a hybrid vehicle, the state in which the engine is stopped and the vehicle is running on the motor.

[0025] Furthermore, when the internal combustion engine 2 is stopped or idling (Yes in step S2), the ECU 5 estimates the amount of moisture adhering to the zeolite catalyst 31 (step S3). Here, when the internal combustion engine 2 is stopped, the amount of moisture adhering to the zeolite catalyst 31 can be estimated as, for example, an accumulated value over time relative to the humidity acquired by the humidity sensor Sh. When the internal combustion engine 2 is idling, the estimated value can be an amount of moisture calculated based on the accumulated value of the fuel injection amount, taking into account the amount of CH in the fuel, and further adding the amount of moisture produced by the combustion of the fuel injected by the fuel injection valve 24.

[0026] When the amount of moisture adhering to the zeolite catalyst 31 is estimated, the ECU 5 determines whether the amount of moisture exceeds a predetermined first moisture amount (step S4). Here, the predetermined first moisture amount is a value arbitrarily set in advance as the amount of moisture adhering to the zeolite catalyst 31 approaches saturation, thereby slowing down further adhesion of moisture.

[0027] If the ECU 5 determines that the estimated amount of adhered moisture is less than the predetermined first moisture amount (No in step S4), the ECU 5 temporarily ends the control procedure. That is, while the internal combustion engine 2 is stopped or idling, the ECU 5 periodically repeats the procedure of steps S1 to S4 to monitor whether the estimated amount of adhered moisture exceeds the predetermined first moisture amount.

[0028] On the other hand, if it is determined that the estimated amount of adhering moisture exceeds the predetermined first moisture amount (Yes in step S4), the ECU 5 checks the SOC of the battery 4 to confirm that there is sufficient power in preparation for evaporating the moisture adhering to the zeolite catalyst 31 using the temperature raising device 33 (step S5).

[0029] Furthermore, the ECU 5 determines the predetermined second water amount based on the SOC of the battery 4. Here, the predetermined second water amount is a target water amount that is arbitrarily set in advance and indicates to what extent the amount of water adhering to the zeolite catalyst 31 is to be reduced by the temperature raising device 33. When the SOC of the battery 4 has a margin, the ECU 5 sets the second water amount as small as possible, thereby making it possible to evaporate almost all of the water adhering to the zeolite catalyst 31.

[0030] Furthermore, when the SOC of the battery 4 is not sufficient, the second water amount can be set to a larger amount, thereby suppressing power consumption by the temperature raising device 33. That is, the ECU 5 adjusts the second water amount to increase in accordance with a decrease in the SOC of the battery 4, thereby achieving a balance between increasing the efficiency of NOx purification through evaporation of the attached water amount and suppressing power consumption. Note that when the SOC of the battery 4 drops too much, the second water amount may be set equal to the first water amount, thereby effectively prohibiting operation of the temperature raising device 33 and prioritizing preservation of the SOC.

[0031] When the second moisture content is determined in step S6, the ECU 5 supplies power from the battery 4 to the temperature raising device 33 to raise the temperature of the zeolite catalyst 31 (step S7). As a result, the temperature raising device 33 raises the temperature of the zeolite catalyst 31 to a moisture evaporation temperature, for example, about 100°C, and vaporizes and desorbs moisture adhering to the zeolite catalyst 31. This control mode in which the temperature of the zeolite catalyst 31 is raised to vaporize and desorb moisture adhering to the zeolite catalyst 31 is referred to as a moisture evaporation mode.

[0032] Furthermore, the ECU 5 monitors the amount of moisture attached to the zeolite catalyst 31 while the heater is operating, and continues to raise the temperature until the amount of moisture decreases to the second moisture amount determined in step S6 (step S8). Here, the amount of decrease in the amount of attached moisture can be estimated from, for example, the amount of power consumed by the temperature raising device 33, the temperature and heating time of the zeolite catalyst 31, etc.

[0033] If it is determined that the amount of adhered moisture has decreased to the second moisture amount (Yes in step S8), the ECU 5 stops the supply of power from the battery 4 to the temperature raising device 33 and stops the temperature raising by the heater (step S9).

[0034] By periodically repeating steps S1 to S9, the zeolite catalyst 31 is maintained in a state where the amount of adsorbed moisture is relatively small, creating room for new adsorption of surrounding moisture. If NOx is present in the gas when adsorbing moisture, the zeolite catalyst 31 can adsorb NOx together with the moisture even in a temperature range below the catalyst activation temperature. Therefore, when the amount of adsorbed moisture is small, the zeolite catalyst 31 can remove NOx from exhaust gas even during periods when the catalyst temperature is relatively low.

[0035] On the other hand, when the internal combustion engine 2 starts and transitions from an idling state to a vehicle running state (No in step S2), the ECU 5 measures the catalyst temperature of the zeolite catalyst 31 via the catalyst temperature acquisition means 35 and determines whether the catalyst temperature is lower than a predetermined first temperature (step S10). Here, the predetermined first temperature is a temperature value that is set in advance as the lower limit of the catalyst activation temperature of the zeolite catalyst 31 or a temperature slightly lower than the lower limit, and is set to 150°C, for example, in this case.

[0036] If the catalyst temperature of the zeolite catalyst 31 is higher than the first temperature (No in step S10), the ECU 5 determines that the catalyst temperature of the zeolite catalyst 31 is at the catalyst activation temperature, or that the zeolite catalyst 31 will reach the catalyst activation temperature in a relatively short time without operating the temperature-raising device 33, and temporarily terminates the control procedure without operating the temperature-raising device 33.

[0037] Furthermore, when the ECU 5 determines that the catalyst temperature is lower than a predetermined first temperature (Yes in step S10), it supplies power from the battery 4 to the temperature raising device 33 to raise the temperature of the zeolite catalyst 31 toward its activation temperature (step S11). This promotes the reduction of NOx in the exhaust gas in the zeolite catalyst 31, thereby enabling efficient purification of exhaust gas. This control mode for raising the temperature of the zeolite catalyst 31 to its activation temperature is referred to as the catalyst temperature raising mode. Note that the catalyst temperature raising mode may not only involve temperature raising by the temperature raising device 33, but may also involve temperature raising by controlling the fuel injection amount, injection timing, and ignition timing.

[0038] After starting the catalyst temperature raising mode, the ECU 5 determines whether the catalyst temperature of the zeolite catalyst 31 is equal to or higher than a second temperature (step S12). The ECU 5 continues raising the temperature by the temperature raising device 33 until the catalyst temperature reaches or exceeds the second temperature (No in step S12). If the ECU 5 determines that the catalyst temperature is equal to or higher than the second temperature (Yes in step S12), the ECU 5 stops the temperature raising device 33 (step S13). Here, the second temperature is a temperature higher than the first temperature and is a lower limit value of the catalyst activation temperature of the zeolite catalyst 31 or a temperature value that is preset as a temperature higher than the lower limit value, and is set to, for example, 200°C in this example. That is, when the zeolite catalyst 31 reaches the catalyst activation temperature and is in a state where it can exhibit sufficient exhaust purification efficiency, the ECU 5 stops the temperature raising device 33.

[0039] Here, the higher the temperature of the zeolite catalyst 31, the more effectively it can remove moisture, and the higher the NOx purification efficiency in the moisture evaporation mode. On the other hand, in a configuration in which the urea water supply device 34 is provided upstream of the zeolite catalyst 31, such as when the zeolite catalyst 31 is used in the urea SCR system 3 as in this embodiment, if the temperature of the zeolite catalyst 31 is raised to a reduction inhibition temperature of approximately 300°C or higher, ammonia may be oxidized or released from the zeolite catalyst 31, thereby hindering NOx reduction.

[0040] Therefore, in the water evaporation mode, the ECU 5 controls the temperature raising device 33 so that the temperature of the zeolite catalyst 31 does not exceed the reduction inhibition temperature. That is, when the catalyst temperature reaches the reduction inhibition temperature during the water evaporation mode, the ECU 5 stops raising the temperature by the temperature raising device 33. This makes it possible to raise the temperature of the zeolite catalyst 31 to a temperature range where NOx in the exhaust gas can be highly efficiently purified in the water evaporation mode.

[0041] Furthermore, when it is determined that the catalyst temperature is equal to or higher than the reduction inhibition temperature, the ECU 5 stops the temperature raising device 33 to quickly lower the temperature of the zeolite catalyst 31 to below the reduction inhibition temperature.

[0042] In the above example, when the temperature raising device 33 operates in step S7, the mode is exemplified as functioning as the moisture evaporation mode until the amount of moisture adhering to the zeolite catalyst 31 falls below the second moisture amount (step S9), but the mode may be switched to the catalyst temperature raising mode (step S10) when the internal combustion engine 2 is started while the moisture evaporation mode is operating (during step S8).

[0043] In this case, since the temperature of the zeolite catalyst 31 is also raised by the high-temperature exhaust gas supplied from the internal combustion engine 2, the output of the temperature raising device 33 may be reduced in accordance with the temperature rise of the zeolite catalyst 31. This makes it possible to suppress the power consumption of the battery 4. Furthermore, since the temperature of the zeolite catalyst 31 has already been raised before the catalyst temperature raising mode, the catalyst activation temperature is reached earlier, thereby improving the NOx purification efficiency.

[0044] As described above, the exhaust gas purification system 1 according to the present invention estimates the amount of moisture adhering to the zeolite catalyst 31 due to the humidity of the atmosphere when the internal combustion engine 2 is stopped or idling. If the moisture amount exceeds a certain value, the zeolite catalyst 31 is heated to reduce the amount of moisture adhering to the zeolite catalyst 31. This allows the zeolite catalyst 31 to absorb additional moisture from the surrounding area, and if NOx is present, it can adsorb the NOx along with the moisture. This allows the zeolite catalyst 31 to more effectively remove NOx from the exhaust gas. Therefore, the exhaust gas purification system 1 according to the present invention can reduce NOx emissions before the SCR catalyst is activated. Furthermore, when the internal combustion engine 2 is stopped or not idling, the exhaust gas temperature is high, making it highly likely that the moisture on the zeolite catalyst 31 can be removed by the exhaust. Therefore, the moisture evaporation mode is not executed, and unnecessary operation of the temperature raising device 33 can be suppressed. [Explanation of symbols]

[0045] 1 Exhaust gas purification system 2. Internal combustion engine 3 Urea SCR system 4 Battery 5 ECU 21 Intake passage 22 Throttle valve 23 Intake port 24 Fuel injection valve 25 Combustion chamber 26 Spark plug 31 Zeolite catalyst 32 Carrier 33 Heating device 34 Urea water supply equipment 35 Catalyst temperature acquisition means Sh humidity sensor

Claims

1. a zeolite catalyst for purifying exhaust gas from an internal combustion engine; a temperature raising device capable of raising the temperature of the zeolite catalyst; a control device for controlling the heating device, The control device estimates the amount of moisture adhering to the zeolite catalyst when the internal combustion engine is stopped or idling, and when the estimated amount of moisture exceeds a predetermined first moisture amount, activates the heating device so that the temperature of the zeolite catalyst is equal to or higher than the moisture evaporation temperature.

2. The exhaust gas purification system according to claim 1 , wherein the control device stops the temperature raising device when the amount of attached moisture falls to a predetermined second moisture amount or less.

3. the temperature raising device is an electric heater driven by battery power, The exhaust gas purification system according to claim 2 , wherein the control device increases the second water amount in response to a decrease in the SOC of the battery.

4. the temperature raising device has a moisture evaporation mode in which the temperature of the zeolite catalyst is raised to a moisture evaporation temperature or higher, and a catalyst temperature raising mode in which the temperature of the zeolite catalyst is raised to an activation temperature, 3. The exhaust gas purification system according to claim 1, wherein when the internal combustion engine is started during operation in the water evaporation mode, the control device switches to the catalyst temperature increase mode and reduces the output of the temperature increase device.

5. a urea water supply device is provided upstream of the zeolite catalyst; 3. The exhaust gas purification system according to claim 1, wherein the control device sets an upper limit temperature of the temperature raising device that is lower than a reduction inhibition temperature of ammonia in the zeolite catalyst.

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