Method and device for controlling electrically heated catalyst

Abstract

In the present invention, disposed in an exhaust passage (11) of an electric-power generating internal combustion engine (2) of a series-hybrid vehicle is an EHC (17) in which a honeycomb monolithic catalyst support (31) comprising a conductive ceramic serves as a heating element. When the internal combustion engine (2) is cold-started while the vehicle is running, current is passed into the monolithic catalyst support (31) ahead of start-up, thereby preheating the catalyst. In a vehicle key-off soak period, in order to remove soot that would impede current flow during next-cycle preheating, conditional upon the monolithic catalyst support (31) being at or above 400℃, current is passed for a predetermined time with the target temperature being 600℃. At the same time, electric motoring of the internal combustion engine (2) by powering a motor generator (1) is carried out, to supply oxygen necessary for combustion of the soot.

Description

Control method and device for electrically heated catalyst

[0001] The present invention relates to a catalyst provided in the exhaust system of an internal combustion engine, and more particularly to the control of an electrically heated catalyst in which a honeycomb monolithic carrier made of conductive ceramic itself serves as a heating element.

[0002] Electrically heated catalysts (EHCs) are known as catalysts used in the exhaust systems of internal combustion engines, and can be heated by passing current through them to promote catalytic activation. Patent Document 1 describes a method for preheating an electrically heated catalyst by starting to pass current through the catalyst prior to starting the internal combustion engine, thereby reducing emissions immediately after starting the internal combustion engine.

[0003] Furthermore, Patent Document 1 discloses an electrically heated catalyst in which a conductive ceramic such as SiC is used as the ceramic material constituting the honeycomb-type monolithic carrier, and electricity is passed through a pair of electrodes, causing the monolithic carrier itself to function as a heating element.

[0004] However, in such an electrically heated catalyst in which the monolithic carrier itself is the heating element, if soot accumulates on the inner peripheral surface of the cells of the monolithic carrier, the electrical resistance of the soot, which is primarily carbon, is lower than the electrical resistance of the ceramic material of the monolithic carrier, and the soot acts as a short-circuit current path, causing a large amount of current to flow. As a result, less current flows through the monolithic carrier itself, which acts as the heating element, and sufficient heat is not generated during preheating prior to startup, or some areas have a low temperature (i.e., areas where catalyst activation is delayed).

[0005] Japanese Patent Application Laid-Open No. 2020-067073

[0006] This invention is a control method for an electrically heated catalyst that is placed in the exhaust system of an internal combustion engine mounted on a hybrid vehicle and uses a honeycomb-shaped monolithic carrier made of conductive ceramic as a heating element by passing electricity through the monolithic carrier, and when the internal combustion engine is started cold while the vehicle is in operation, electricity is passed through the monolithic carrier to preheat it prior to start-up, and when the vehicle is keyed off, electricity is passed through the monolithic carrier so that its temperature reaches 500°C or higher in order to remove adhering soot that could become a short-circuit current path during the next pre-heating.

[0007] The soot adhering to the monolithic carrier is burned off when the monolithic carrier reaches a temperature of 500° C. or higher. By removing the soot adhering to the monolithic carrier when the vehicle is turned off in this manner, the monolithic carrier effectively generates heat when the monolithic carrier is preheated the next time the engine is cold started, and the occurrence of partially low-temperature areas is suppressed.

[0008] The present invention relates to an electric catalyst for a vehicle having an internal combustion engine, an electric heating ...

[0009] An embodiment of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows a schematic configuration of a series hybrid vehicle as an example of a vehicle to which the present invention can be applied. The series hybrid vehicle includes a power-generating motor-generator 1 that operates primarily as a generator, an internal combustion engine 2 that serves as a power-generating internal combustion engine and drives the power-generating motor-generator 1 in response to a power demand, a traction motor-generator 4 that operates primarily as a motor to drive drive wheels 3, and a battery 5 that stores the generated power. The power obtained by the internal combustion engine 2 driving the power-generating motor-generator 1 is stored in the battery 5 via an inverter device (not shown). The traction motor-generator 4 is driven and controlled using the power from the battery 5. The power generated by the traction motor-generator 4 during regeneration is stored in the battery 5 via an inverter device (not shown).

[0010] The operation of the motor generators 1 and 4, the charging and discharging of the battery 5, and the operation of the internal combustion engine 2 are controlled by a controller 6. The controller 6 is composed of multiple controllers connected to each other so that they can communicate with each other, such as a motor controller 7 that controls the motor generators 1 and 4, an engine controller 8 that controls the internal combustion engine 2, and a battery controller 9 that manages the battery 5. Information such as the accelerator pedal position and vehicle speed (not shown) is input to the controller 6. The battery controller 9 also calculates the SOC of the battery 5 based on the voltage and current of the battery 5. When the SOC drops to a predetermined lower limit, the internal combustion engine 2 is started via the engine controller 8 to generate electricity. Such a series hybrid vehicle has two driving modes: an EV mode in which the vehicle runs on power from the battery 5 without combustion operation of the internal combustion engine 2, and an HEV mode in which the vehicle runs while generating electricity through combustion operation of the internal combustion engine 2. Even if the SOC is above the lower limit, the internal combustion engine 2 is driven and the vehicle runs in the HEV mode when the required driving force of the vehicle is relatively large. Therefore, the internal combustion engine 2 repeatedly performs combustion operation and stops of combustion operation while the main switch of the vehicle is on.

[0011] 2 shows the configuration of an intake system and an exhaust system of the internal combustion engine 2. The internal combustion engine 2 of one embodiment is a four-stroke spark-ignition gasoline engine equipped with a turbocharger 13 as a supercharger, and is, for example, a so-called direct injection type internal combustion engine in which fuel is injected directly into the cylinders by a fuel injection valve.

[0012] A compressor 15 of a turbocharger 13 is disposed in the intake passage 12 of the internal combustion engine 2, and an electronically controlled throttle valve 21 that controls the amount of intake air is disposed downstream of the compressor 15. A water-cooled intercooler 22, for example, that cools the supercharged intake air is disposed between the compressor 15 and the throttle valve 21. An air cleaner 28 is disposed upstream of the compressor 15, and a vacuum generating valve 27 that generates the vacuum required for exhaust gas recirculation is provided between the compressor 15 and the air cleaner 28. The throttle valve 21 and the vacuum generating valve 27 are both butterfly valves with circular valve bodies, and their respective opening degrees are controlled by the engine controller 8.

[0013] An exhaust turbine 14 of a turbocharger 13 is disposed in an exhaust passage 11 of the internal combustion engine 2. Downstream of the exhaust turbine 14 are disposed an electrically heated catalyst (hereinafter referred to as EHC) 17 of one embodiment, which serves as an upstream catalytic converter using, for example, a three-way catalyst, and a downstream catalytic converter 18 using, for example, a three-way catalyst. As will be described later, the EHC 17 of one embodiment is configured to function as a filter that captures exhaust particulates in addition to its catalytic function. An exhaust silencer 19 is disposed further downstream of the downstream catalytic converter 18 in the exhaust passage 11, and the exhaust passage 11 is opened to the outside via the exhaust silencer 19.

[0014] An exhaust gas recirculation passage 24 for recirculating a portion of the exhaust gas to the intake system is provided between the exhaust passage 11 and the intake passage 12. A base end 24a of the exhaust gas recirculation passage 24 branches off from the exhaust passage 11 downstream of the exhaust turbine 14, specifically between the EHC 17 and the downstream catalytic converter 18. A tip end 24b is connected to the intake passage 12 at a position upstream of the compressor 15. The exhaust gas recirculation passage 24 includes, for example, a water-cooled EGR gas cooler 26 for cooling the recirculated exhaust gas, and an EGR valve 25 whose opening is variably controlled by the engine controller 8.

[0015] 1 receives detection signals from a variety of sensors, not shown, directly or via other controllers, such as an air flow meter that detects the amount of intake air, an air-fuel ratio sensor that detects the exhaust air-fuel ratio, a crank angle sensor that detects the engine speed, a water temperature sensor that detects the temperature of the cooling water, a boost pressure sensor that detects the boost pressure, an accelerator position sensor that detects the amount of depression of the accelerator pedal, an atmospheric pressure sensor that detects the atmospheric pressure, and an outside air temperature sensor. Based on these detection signals and requests from the other controllers, the engine controller 8 optimally controls the fuel injection amount and injection timing, ignition timing, the opening of the throttle valve 21, the boost pressure, the EGR rate, the opening of the vacuum generating valve 27, and the like.

[0016] 3 and 4 show an example of an EHC 17. The EHC 17 of this example is constructed by housing a cylindrical honeycomb monolithic carrier 31 (shown) in a metal case (not shown) via a buffer material. The monolithic carrier 31 is integrally molded using a porous ceramic material, more specifically, a conductive ceramic material such as SiC. The monolithic carrier 31 has numerous fine passages, or cells 32, along its axial direction, and each cell 32 is separated by a thin wall 33. The inner peripheral wall surface of the cells 32 is coated with a catalyst layer containing a catalytic metal. Some of the numerous cells 32 are configured as closed cells 32A, which are secondarily blocked by first plugging sections 34 at the upstream end of the monolithic carrier 31, while the remaining cells are configured as open cells 32B, which are secondarily blocked by second plugging sections 35 at the downstream end of the monolithic carrier 31. In the illustrated example, the cells 32, each having a substantially square cross section, are arranged in a grid pattern, with closed cells 32A with closed upstream ends and open cells 32B with open upstream ends alternately arranged adjacent to each other. Therefore, exhaust gas flowing into the open cells 32B from the upstream side passes through the porous walls 33 into the adjacent closed cells 32A and then flows out from the open downstream ends of the closed cells 32A. Exhaust particulates contained in the exhaust gas are trapped and removed as the exhaust gas passes through the porous walls 33. In other words, the illustrated EHC 17 also functions as a wall-flow particulate filter. Note that the size, number, and dimensional ratios of the cells 32 in the figure are for illustrative purposes only and are not accurate.

[0017] The plugging portions 34, 35 are made of a ceramic material, just like the monolith carrier 31 itself, and are formed by filling the cells 32 as a slurry and then solidifying. At least the first plugging portion 34 is made of a conductive ceramic material such as SiC. This makes both the monolith carrier 31 and the first plugging portion 34 conductive and serves as a heating element that generates heat when electricity is applied. Note that by appropriately adjusting the components of the ceramic material of the monolith carrier 31 and the ceramic material of the first plugging portion 34, it is possible to adjust the resistance values ​​of each.

[0018] As shown in FIG. 4 , the axial length L1 of the first plugging portion 34 is longer than the axial length L2 of the second plugging portion 35. The axial length L2 of the second plugging portion 35 is basically the minimum length required for sealing the cells 32. In contrast, the first plugging portion 34 is intentionally set longer than the length required for sealing in order to ensure an area with low resistance where soot does not accumulate. In other words, within the range of length L1 where the first plugging portion 34 exists in the closed cell 32A, gas does not flow from the adjacent open cell 32B into the closed cell 32A. Therefore, soot does not accumulate within the open cell 32B (on the inner wall surface of the open cell 32B). In other words, soot accumulates on the inner wall surface of the open cell 32B in the range to the right of length L1 in FIG. 4 , and soot is unlikely to accumulate within the range of length L1.

[0019] A pair of electrodes 37 are provided on the outer peripheral surface of the cylindrical monolithic carrier 31 to pass current through the monolithic carrier 31, which serves as a heating element (resistor). Each electrode 37 is, for example, rectangular. As shown in FIG. 3, the pair of electrodes 37 are arranged at two diametrically opposed locations so that current flows in a direction perpendicular to the axial direction. Furthermore, as shown in FIG. 4, the electrodes 37 are arranged at axial positions that at least partially overlap the length range L1 of the first plugging portions 34. In the illustrated example, the entire axial length of the electrode 37 overlaps with the first plugging portions 34, that is, within the length L1 of the first plugging portions 34.

[0020] In the illustrated EHC 17 configured as described above, when current is applied via the pair of electrodes 37, as shown in Fig. 4, current flows in the vertical direction in Fig. 4 through the upstream end of the monolith carrier 31 where the first plugging portion 34 is present. This generates heat at the upstream end of the monolith carrier 31. Because the heat-generating portion is the most upstream portion of the gas flow in the monolith carrier 31, in a situation where a gas flow is present, the heat spreads downstream along the gas flow, allowing the monolith carrier 31 to be heated efficiently.

[0021] The supply of electricity to the EHC 17 is controlled by the engine controller 8. The supply of electricity to the EHC 17 is performed mainly in two situations. The first is preheating to activate the catalyst when the internal combustion engine 2 is started cold. When a start of the internal combustion engine 2 is requested due to a request for power generation in a cold state, the EHC 17 is supplied with electricity prior to the start of combustion operation (fuel injection and ignition), and the monolithic carrier 31 carrying the catalyst is preheated. This preheating activates the catalyst, resulting in improved exhaust emissions from the beginning of combustion operation. This preheating is performed with a target temperature of approximately 400°C.

[0022] The other heating operation is performed when soot accumulates on the EHC 17 to burn off the soot. When the vehicle ends a trip and the main switch is turned OFF, the EHC 17 is energized to heat and remove the soot, provided that the EHC 17 is at a certain temperature (e.g., 400°C) or higher. Soot is generated and adheres to the monolithic carrier 31 when the internal combustion engine 2 is cold or operated under low exhaust gas temperature conditions. Soot is naturally removed from the monolithic carrier 31 when the exhaust gas temperature increases due to high-load operation. However, when low-load operation is repeated, the soot may not be naturally removed and may accumulate to a certain extent. Because the electrical resistance of such soot deposits is lower than the electrical resistance of the ceramic material of the monolithic carrier 31, they form a short-circuit current path during the preheating operation, reducing the current flowing through the monolithic carrier 31 itself, which serves as a heating element. Therefore, if the amount of soot buildup exceeds a predetermined amount, the soot is forcibly removed by energizing the engine when the key is turned off, i.e., when the main switch is turned off. This heating for soot removal is performed with a target temperature of approximately 500 to 700°C, for example 600°C.

[0023] 5 is a flowchart showing the specific flow of the soot incineration removal process executed by the engine controller 8. In step 1, it is repeatedly determined whether the vehicle's main switch has been switched from ON to OFF. When the main switch is turned OFF as the vehicle trip ends, the process proceeds to step 2, where it is determined whether a predetermined EHC energization permission condition is met.

[0024] The conditions for permitting EHC energization are an AND condition of the following five conditions: "there is no fault such as a broken wire in the EHC 17," "the temperature of the monolithic carrier 31 is 400°C or higher," "the battery SOC is a predetermined value or higher," "the amount of soot deposited (adhesion) at the time of key-off is a predetermined amount or higher," and "the energization time is a predetermined time or less." In other words, if all five conditions are met simultaneously, the determination in step 2 is YES. If any one of the conditions is not met, the determination in step 2 is NO, and the routine ends without energizing the EHC.

[0025] The condition that "the temperature of the monolithic carrier 31 must be 400°C or higher" is intended to reduce the power consumption required for soot removal. A temperature of 500°C or higher is required to remove soot, and heating from a low temperature state using electricity would consume a lot of power. Therefore, if the temperature is below 400°C, soot will not be burned off during the current key-off. The temperature of the monolithic carrier 31 can be estimated from the exhaust temperature, etc. while the internal combustion engine 2 is operating. A temperature sensor may be provided to directly detect the temperature of the monolithic carrier 31.

[0026] The amount of soot accumulation is constantly calculated by the engine controller 8 based on various parameters while the internal combustion engine 2 is operating. If the amount of soot accumulation is less than a predetermined amount, soot incineration removal is not performed in order to reduce power consumption associated with energization. As described above, as the exhaust temperature of the internal combustion engine 2 increases, the accumulated soot naturally decreases.

[0027] The battery used to energize the EHC 17 may be a battery (not shown) for accessories of the internal combustion engine 2 having an appropriate voltage, separate from the vehicle-driving battery 5 shown in FIG. 1, or may be configured to use the power of the vehicle-driving battery 5.

[0028] If the determination in step 2 is YES, the process proceeds to step 3, where the EHC 17 is energized, causing the monolithic carrier 31 to heat up. At the same time, in step 4, the internal combustion engine 2 is motored by powering the power-generating motor-generator 1. This motoring causes air to flow through the exhaust system of the internal combustion engine 2, supplying oxygen necessary for the combustion of soot. In addition, the combustion of soot generated at the upstream end of the monolithic carrier 31 is diffused downstream by the air flow, thereby enabling the soot to be effectively incinerated and removed.

[0029] In the next step 5, it is determined again whether the above-described EHC energization permission condition is satisfied. If the result is YES, energization of the EHC 17 and motoring are continued.

[0030] If the determination in step 5 is NO, for example, because the energization time exceeds a predetermined time, the process proceeds to steps 6 and 7, where energization to the EHC 17 and motoring by the power-generating motor-generator 1 are terminated. The threshold energization time, which is one of the conditions for permitting EHC energization, is set to an appropriate time so that the temperature of the monolith carrier 31 can be raised from 400°C to 500°C or higher, preferably 600°C, and soot can be burned off.

[0031] In this embodiment, if a predetermined amount or more of soot has accumulated on the monolithic carrier 31, the soot is forcibly burned off by energizing the EHC 17 when the key is turned off (when the main switch is turned off). Therefore, when the EHC 17 is preheated by energizing the EHC 17 prior to a cold start of the internal combustion engine 2 during the next vehicle trip, the soot does not act as a short-circuit current path to inhibit the generation of heat from the monolithic carrier 31. Therefore, preheating is completed quickly, and emissions can be reduced immediately after the start of the internal combustion engine 2. Although one embodiment of the present invention has been described above in detail, the present invention is not limited to the above embodiment and various modifications are possible.

[0032] For example, in the above embodiment, the EHC 17 is configured to function as an exhaust particulate filter, but it may also be a simple electrically heated catalyst using a monolithic carrier having a large number of cells with both ends open.

[0033] In addition, in the above embodiment, motoring is performed to supply oxygen during soot combustion, but instead of motoring, air may be supplied upstream of the EHC 17 using a secondary air pump or the like. Alternatively, depending on the layout of the exhaust system, combustion may be ensured by convection within the exhaust pipe without actively supplying air.

[0034] Furthermore, in the above embodiment, a series hybrid vehicle has been described as an example, but other types of hybrid vehicles may also be used.

Claims

1. A method for controlling an electric heating type catalyst that is disposed in an exhaust system of an internal combustion engine mounted on a hybrid vehicle and uses a honeycomb type monolith carrier made of a conductive ceramic as a heating element, the method comprising: when the internal combustion engine is cold-started during vehicle operation, energizing the monolith carrier prior to starting to perform preheating; when the vehicle is turned off, energizing the monolith carrier so that the temperature becomes 500° C. or higher for removing adhered soot that becomes a short-circuit path during the next preheating. A method for controlling an electric heating type catalyst.

2. The method for controlling an electric heating type catalyst according to claim 1, wherein the monolith carrier is configured as a wall flow type filter in which upstream end portions and downstream end portions of a large number of cells are alternately blocked.

3. The method for controlling an electric heating type catalyst according to claim 1, wherein energization is started on the condition that the temperature of the monolith carrier is 400° C. or higher when the key is turned off.

4. The method for controlling an electric heating type catalyst according to claim 1, wherein energization is permitted on the condition that the SOC of the battery is equal to or higher than a predetermined value.

5. The method for controlling an electric heating type catalyst according to claim 1, wherein the amount of adhered soot is estimated during operation, and energization is started on the condition that the amount of adhered soot is equal to or higher than a predetermined amount when the key is turned off.

6. The method for controlling an electric heating type catalyst according to claim 1, wherein the internal combustion engine is a power generation internal combustion engine that drives a generator in a series hybrid vehicle.

7. The method for controlling an electric heating type catalyst according to claim 6, wherein during energization when the key is turned off, motoring of the internal combustion engine is performed by the power generation of the generator.

8. An electric heating type catalyst control device comprising: an internal combustion engine mounted on a hybrid vehicle; an electric heating type catalyst disposed in an exhaust system of the internal combustion engine and using a honeycomb type monolith carrier made of a conductive ceramic as a heating element; and a controller, wherein the controller energizes the monolith carrier prior to starting to perform preheating when the internal combustion engine is cold-started during vehicle operation, and energizes the monolith carrier so that the temperature becomes 500° C. or higher for removing adhered soot that becomes a short-circuit path during the next preheating when the vehicle is turned off.

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