Control method and control device of internal combustion engine

Abstract

To solve a problem in which when a temperature of a lean NOx trap catalyst is high at the time when stoichiometric operation is transferred to lean operation, an NOx adsorption capability deteriorates, which deteriorates NOx until the temperature decreases.SOLUTION: As a vehicle is accelerated, an internal combustion engine for power generation is stoichiometrically operated, at a time t2, on an operation point at a high-speed high-load side. As the acceleration of the vehicle is ended at a time t4, the stoichiometric operation of the internal combustion engine is required to be transferred to lean operation on an operation point at a low-speed low-load side. When an LNT temperature is equal to or above a threshold temperature A at the time t4, combustion operation is stopped before starting the lean operation and motoring of the internal combustion engine is performed by power running by a motor generator for power generation. At a time t5 when the LNT temperature becomes less than a threshold temperature B, the lean operation is started.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to control of an internal combustion engine that is provided with a lean NOx trap catalyst in addition to a three-way catalyst in its exhaust system and that operates under specific operating conditions with a lean target air-fuel ratio. [Background technology]

[0002] Lean operation is advantageous for improving fuel economy of internal combustion engines, and lean NOx trap catalysts are sometimes used to treat the NOx emitted during lean combustion. The NOx emitted during lean combustion is temporarily adsorbed by the lean NOx trap catalyst and purified when the exhaust reaches the stoichiometric air-fuel ratio or a rich atmosphere.

[0003] Such lean NOx trap catalysts have a relatively low catalytic activation temperature suitable for NOx treatment, lower than the appropriate temperature range for, for example, a three-way catalyst. Figure 8 shows an example of the correlation between NOx emitted through the lean NOx trap catalyst and the temperature of the lean NOx trap catalyst. When the temperature exceeds a certain temperature T1 (for example, around 400°C), the NOx adsorption ability decreases and NOx emissions increase.

[0004] Due to the temperature characteristics of the lean NOx trap catalyst, for example, when switching from stoichiometric operation, in which the target air-fuel ratio is the theoretical air-fuel ratio, to lean operation, if the lean NOx trap catalyst is at a high temperature during stoichiometric operation, NOx will deteriorate until the temperature of the lean NOx trap catalyst drops.

[0005] Patent Document 1 describes a configuration in which, in order to avoid a decrease in the performance of the lean NOx trap catalyst at high temperatures, a heat exchanger is provided upstream of the lean NOx trap catalyst to recover exhaust heat, and the exhaust gas flowing into the lean NOx trap catalyst is cooled by vaporizing a liquid-phase refrigerant within the heat exchanger. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-216044 Summary of the Invention [Problem to be solved by the invention]

[0007] In a configuration in which a heat exchanger is provided in the exhaust system as in Patent Document 1, the configuration of the exhaust system becomes complicated, and a large heat exchanger and associated refrigerant piping and the like are required, which is not preferable. [Means for solving the problem]

[0008] The present invention relates to an internal combustion engine for a hybrid vehicle that drives a motor generator to generate electricity and is capable of motoring by powering the motor generator, that performs lean operation when the required torque is low, and that is equipped with a three-way catalyst and a lean NOx trap catalyst in the exhaust system, and a control method for the internal combustion engine, When a transition from stoichiometric operation to lean operation is required, the combustion operation is stopped and motoring is performed until the temperature of the lean NOx trap catalyst becomes lower than a predetermined threshold temperature for avoiding a decrease in NOx adsorption capacity, and then lean operation is started.

[0009] Motoring introduces cool air into the lean NOx trap catalyst, quickly lowering the temperature of the lean NOx trap catalyst. [Effects of the Invention]

[0010] According to this invention, the lean NOx trap catalyst can be cooled without the need for an additional device such as a heat exchanger, thereby preventing deterioration of NOx due to the high temperature of the lean NOx trap catalyst immediately after switching to lean operation. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating the configuration of a series hybrid vehicle to which the present invention is applied; [Figure 2] 4 is a flowchart showing the flow of processing in the first embodiment. [Figure 3] 4 is a time chart showing the operation of the first embodiment. [Figure 4] 10 is a flowchart showing the flow of processing in a second embodiment. [Figure 5] 6 is a time chart showing the operation of the second embodiment. [Figure 6] 10 is a flowchart showing the flow of processing according to a third embodiment. [Figure 7] 10 is a time chart showing the operation of the third embodiment. [Figure 8] A characteristic diagram showing the correlation between the temperature of the lean NOx trap catalyst and NOx emissions. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment in which the present invention is applied to an internal combustion engine for generating electricity in a series hybrid vehicle will be described below. FIG. 1 shows a schematic configuration of a series hybrid vehicle. The series hybrid vehicle includes a power-generating motor-generator 1 that operates primarily as a generator, an internal combustion engine 2 used as a power-generating internal combustion engine that drives the power-generating motor-generator 1 in response to power demands, a traction motor-generator 4 that operates primarily as a motor to drive drive wheels 3, and a battery 5 that temporarily stores the generated electricity. The drive wheels 3 are driven by the traction motor-generator 4 via a reduction gear 6. Electric 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 7. The drive of the traction motor-generator 4 is controlled using the power from the battery 5. The regenerated electricity from the traction motor-generator 4 is also stored in the battery 5 via the inverter device 7.

[0013] In the figure, the internal combustion engine 2 is arranged in series with the power-generating motor-generator 1, and the two are depicted as rotating at the same speed, but a gear mechanism may be provided between the internal combustion engine 2 and the power-generating motor-generator 1. In either case, the internal combustion engine 2 can be motored by the power generation of the power-generating motor-generator 1.

[0014] The operation of the motor generators 1 and 4, the charging and discharging of the battery 5, and the operation of the inverter device 7 are controlled by a vehicle controller 8. The internal combustion engine 2 is controlled by an engine controller 9. The engine controller 9 and the vehicle controller 8 are connected to each other so as to be able to communicate with each other via an in-vehicle network such as CAN communication, and exchange control signals and detection signals from sensors with each other. Information such as the accelerator pedal position and vehicle speed (not shown) is input to the vehicle controller 8. The vehicle controller 8 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 9 to generate electricity, thereby charging the battery 5. Even if the SOC is above the lower limit, if the vehicle's required driving force is relatively large, the internal combustion engine 2 is driven to generate electricity, and the generated electricity is supplied to the traction motor generator 4 to supplement the power supplied from the battery 5 to the traction motor generator 4. In addition, as the driving modes of such a series hybrid vehicle, the state in which the vehicle runs on power from the battery 5 without combustion operation of the internal combustion engine 2 is called EV mode, and the state in which the vehicle runs while generating electricity through combustion operation of the internal combustion engine 2 is called HEV mode.

[0015] The internal combustion engine 2 is a four-stroke cycle spark ignition internal combustion engine, a so-called gasoline engine. This internal combustion engine 2 is configured to be switchable between stoichiometric operation (stoichiometric combustion mode) in which combustion is performed with the theoretical air-fuel ratio as the target air-fuel ratio, and lean operation (lean combustion mode) in which combustion is performed with a lean air-fuel ratio (for example, an air-fuel ratio of around 30) as the target air-fuel ratio. Note that the internal combustion engine 2 may be equipped with a supercharger such as a turbocharger to ensure the amount of intake air required during lean operation.

[0016] In the HEV mode described above, the internal combustion engine 2 is basically operated at two different operating points (combinations of load (or torque) and engine speed). The first operating point is an operating point when the internal combustion engine 2 is operated to charge the battery 5 after the SOC of the battery 5 has dropped to a predetermined lower limit level, and is set with the best fuel economy point of the internal combustion engine 2, including power generation efficiency, as a target. The first operating point is not strictly fixed to one point, but is generally located at one point and is located on the lower speed / lower load side than the second operating point. At this first operating point, lean operation is performed to suppress fuel economy. In other words, the first operating point is included in a lean operation region on the lower speed / lower load side determined by the load and engine speed. The second operating point is an operating point used when the required driving force of the vehicle is relatively large, such as during vehicle acceleration, and power is generated to supplement the power supplied from the battery 5. The second operating point is an operating point on the higher speed / higher load side than the first operating point in order to actively generate power. The second operating point varies slightly depending on the required driving force of the vehicle. For example, the greater the accelerator pedal depression, the higher the operating point becomes on the high-speed, high-load side. At this second operating point, stoichiometric operation is performed to ensure torque. In other words, the second operating point is included in the stoichiometric operation region on the relatively high-speed, high-load side.

[0017] To accommodate both stoichiometric and lean-burn operation, a lean NOx trap catalyst 12 is disposed in an exhaust passage 11 of the internal combustion engine 2. In the illustrated example, an upstream lean NOx trap catalyst 13 and a three-way catalyst 14 are disposed upstream of the lean NOx trap catalyst 12. The upstream lean NOx trap catalyst 13 and the three-way catalyst 14 are housed in the same casing, with the three-way catalyst 14 positioned relatively upstream. The upstream lean NOx trap catalyst 13 and the three-way catalyst 14 are disposed directly below the exhaust manifold of the internal combustion engine 2 as so-called manifold catalytic converters with relatively small capacities. In contrast, the lean NOx trap catalyst 12 is disposed under the vehicle floor as a so-called main catalytic converter with a large capacity. An exhaust temperature sensor 15 is disposed at the inlet of the lean NOx trap catalyst 12 in the exhaust passage 11 to obtain temperature information about the lean NOx trap catalyst 12. The temperature of the lean NOx trap catalyst 12 is calculated based on the temperature detected by the exhaust temperature sensor 15. A temperature sensor may be attached to the catalyst carrier so as to directly detect the catalyst carrier temperature (so-called bed temperature) of lean NOx trap catalyst 12. Alternatively, the temperature of lean NOx trap catalyst 12 may be estimated by estimation calculation based on the amount of fuel introduced, etc., without using exhaust gas temperature sensor 15.

[0018] In one embodiment, the temperature of the three-way catalyst 14 located upstream is estimated by an estimation calculation based on the amount of fuel introduced, etc. A temperature sensor may be provided to detect the temperature at the inlet of the three-way catalyst 14 or the catalyst carrier temperature.

[0019] In the above-described configuration, for example, when internal combustion engine 2 is operating at stoichiometric combustion at the second operating point as the vehicle accelerates, the high exhaust gas temperature causes the temperature of lean NOx trap catalyst 12 to rise and exceed temperature T1, shown in Figure 8, at which NOx treatment performance deteriorates. In this state, if the operating point of internal combustion engine 2 changes from the second operating point to the first operating point as the vehicle stops accelerating, transitioning from stoichiometric operation to lean operation, the NOx associated with lean combustion will be released without being sufficiently treated by lean NOx trap catalyst 12.

[0020] Therefore, in the present invention, when transitioning from stoichiometric operation to lean operation, the power generating motor generator 1 is used to motor the internal combustion engine 2, and air is passed through the lean NOx trap catalyst 12 to cool the lean NOx trap catalyst 12.

[0021] FIG. 2 is a flowchart showing the flow of processing in the first embodiment, which will be explained below. The processing shown in the flowchart is repeatedly executed while the vehicle is in operation. "LNT temperature" in the figure refers to the temperature of the lean NOx trap catalyst 12, which is the large-capacity main catalytic converter. "fM" is a flag indicating motoring execution.

[0022] In the first step 1, it is determined whether the vehicle operating conditions are in the engine operating region, that is, whether the vehicle operating conditions require operation of the internal combustion engine 2 (whether the vehicle is in HEV mode). If the result is NO, the internal combustion engine 2 is stopped (step 2), and flag fM is set to 0 (step 3). If the internal combustion engine 2 is in the engine operating region where it should be operated, it is determined in step 4 whether the requested operating point of the internal combustion engine 2 is within the lean region. If it is not within the lean region, stoichiometric operation is performed (step 5), and flag fM is set to 0 (step 6).

[0023] If the LNT temperature is within the lean region, in step 7, it is determined whether the LNT temperature is equal to or higher than threshold temperature B. If the answer is YES, in step 8, it is determined whether flag fM is 1. The answer is NO the first time (immediately after the requested operating point changes from the stoichiometric region to the lean region), so the process proceeds to step 9, where it is determined whether the LNT temperature is equal to or higher than threshold temperature A. Threshold temperatures A and B are both set near temperature T1 in Figure 8, and are set such that there is an appropriate temperature difference for control hysteresis, such that A>B. If the LNT temperature is below threshold temperature A, lean operation is permitted without motoring (step 18).

[0024] If the LNT temperature is equal to or higher than the threshold temperature A in step 9, the flag fM is set to 1 (step 10), the combustion operation is stopped, and motoring is started (step 11).

[0025] If the LNT temperature drops due to motoring and becomes less than threshold temperature B, the process proceeds from step 7 to step 13, where it is determined whether flag fM is 1. After motoring starts, flag fM is 1, so the process proceeds from step 13 to step 14, where motoring is terminated. At the same time, lean operation is initiated (step 15), and flag fM is set to 0 (step 16).

[0026] If the LNT temperature is below threshold temperature B when a transition from stoichiometric operation to lean operation is requested, lean operation is permitted without motoring (step 17).

[0027] 3 is a time chart for explaining the operation of Example 1. From top to bottom, the chart shows (a) vehicle speed, (b) whether the engine is in the operating range, (c) vehicle required driving force, (d) output required for the internal combustion engine 2, (e) torque required for the internal combustion engine 2, (f) whether the engine is in the lean range, (g) output of the battery 5, (h) SOC of the battery 5, (i) actual torque of the internal combustion engine 2, (j) LNT temperature, (k) flag fM indicating that motoring operation is in progress, and (m) lean operation permission flag.

[0028] The illustrated example corresponds to a situation in which the driver depresses the accelerator pedal to accelerate, then releases the accelerator pedal to an appropriate depression, and transitions to constant speed driving. During acceleration, the internal combustion engine 2 operates at stoichiometric pressure at the second operating point, and after transitioning to constant speed driving, it operates at lean pressure at the first operating point until the SOC recovers. During this transition, motoring is performed to reduce the LNT temperature.

[0029] At time t1, the accelerator pedal is fully depressed, and the vehicle begins to accelerate using the output of the battery 5. Then, a short time later at time t2, the internal combustion engine 2 starts. At this time, the torque required of the internal combustion engine 2 is high, and the engine operates at stoichiometric pressure at the second operating point. As the engine operates at stoichiometric pressure, the LNT temperature rises. Then, at time t3, the accelerator pedal is released, and a short time later at time t4, the engine operating point of the internal combustion engine 2 changes to the first operating point on the low-speed, low-load side, requesting a transition from stoichiometric pressure to lean pressure.

[0030] At this time (time t4), the LNT temperature is compared with threshold temperatures A and B as described above. In the illustrated example, because the LNT temperature is equal to or higher than threshold temperature A, motoring is performed at time t4, stopping combustion operation, as shown in line (k). This motoring quickly reduces the LNT temperature. At time t5, the LNT temperature falls below threshold temperature B, thereby ending motoring. At the same time, lean operation begins.

[0031] As described above, according to the above embodiment, when transitioning from stoichiometric operation to lean operation, the temperature (LNT temperature) of lean NOx trap catalyst 12 can be quickly lowered by motoring without the need for an additional device such as a heat exchanger. Therefore, NOx can be treated by lean NOx trap catalyst 12 immediately after the start of lean operation, and a deterioration in NOx immediately after transitioning to lean operation can be avoided.

[0032] In the illustrated time chart, lean operation starts at time t5 when the LNT temperature reaches threshold temperature B, but the actual start of lean operation may be delayed depending on other lean operation permission conditions. At the very least, it is sufficient that the LNT temperature is below threshold temperature B when lean operation starts.

[0033] Furthermore, in the above embodiment, the need for motoring and the start of lean operation are controlled by focusing on the temperature (LNT temperature) of lean NOx trap catalyst 12, which is the main catalytic converter, but motoring also simultaneously reduces the temperature of upstream lean NOx trap catalyst 13. Therefore, upstream lean NOx trap catalyst 13 can also achieve good NOx purification action from the early stages of lean operation.

[0034] Next, a second embodiment of the present invention will be described. In the second embodiment, motoring is not performed when the three-way catalyst 14 is at a high temperature during a transition from stoichiometric operation to lean operation. During motoring, oxygen-rich air passes through the three-way catalyst 14. If oxygen is supplied to the three-way catalyst when the catalyst is at a high temperature (e.g., 600°C or higher), Pt in the catalyst, for example, combines with the oxygen, causing so-called oxygen poisoning, which reduces the performance of the three-way catalyst. Therefore, in the second embodiment, low exhaust temperature stoichiometric operation is performed, in which the internal combustion engine 2 is operated at an operating point where the target air-fuel ratio is the stoichiometric air-fuel ratio and the exhaust temperature is low, until the temperature of the three-way catalyst 14 drops to a certain extent. This operating point for low exhaust temperature stoichiometric operation is originally a low-speed, low-load operating point included in the lean region. In a preferred embodiment, the operating point for low exhaust temperature stoichiometric operation is set lower in speed and lower in load than the first operating point for charging the battery 5.

[0035] 4 is a flowchart showing the processing flow of the second embodiment, and the following mainly describes the differences from the first embodiment. In the figure, "fL" is a flag indicating that low exhaust temperature stoichiometric operation is being performed, and "TWC temperature" is the temperature of the three-way catalyst 14.

[0036] First, it is determined whether the vehicle operating conditions are in the engine operating range (step 21), and if NO, the internal combustion engine 2 is stopped (step 22), and flags fM and fL are set to 0 (step 23). If the internal combustion engine 2 is in the engine operating range where it should be operated, it is determined in step 24 whether the requested operating point of the internal combustion engine 2 is in the lean range. If it is not in the lean range, stoichiometric operation is performed (step 25), and flags fM and fL are set to 0 (step 26).

[0037] If the TWC temperature is within the lean region, in step 27, it is determined whether the TWC temperature is equal to or higher than threshold temperature C. Threshold temperature C is set corresponding to the catalyst temperature at which oxygen poisoning of the three-way catalyst 14 described above becomes a problem. If the TWC temperature is equal to or higher than threshold temperature C, it is determined in step 40 whether flag fL ​​is 1. Since the result is NO the first time (immediately after the requested operating point has changed from the stoichiometric region to the lean region), the process proceeds to step 41, where flag fL ​​is set to 1 and low exhaust temperature stoichiometric operation is started (step 42).

[0038] In low exhaust temperature stoichiometric operation, the exhaust temperature is lower than in stoichiometric operation at the second operating point, so the three-way catalyst 14 is cooled and its temperature decreases. While the TWC temperature is equal to or higher than the threshold temperature C, the process proceeds from step 27 to step 40, and since flag fL ​​is 1 from the second time onwards, the process proceeds to step 43, and low exhaust temperature stoichiometric operation continues.

[0039] If the TWC temperature becomes less than threshold temperature C due to low exhaust temperature stoichiometric operation, the process proceeds from step 27 to step 28. The subsequent processing of steps 28 to 39 is the same as the processing of steps 7 to 18 in FIG. 2 in the first embodiment. That is, in step 28, it is determined whether the LNT temperature is equal to or greater than threshold temperature B, and if the result is YES, it is determined in step 29 whether flag fM is 1. Since the result is NO the first time, the process proceeds to step 30, where it is determined whether the LNT temperature is equal to or greater than threshold temperature A. If the LNT temperature is less than threshold temperature A, lean operation is permitted without motoring (step 39).

[0040] If the LNT temperature is equal to or higher than the threshold temperature A in step 30, flags fM and fL are set to 1 (step 31), combustion operation is stopped, and motoring is started (step 32).

[0041] If the LNT temperature drops due to motoring and becomes less than threshold temperature B, the process proceeds from step 28 to step 34, where it is determined whether flag fM is 1. Since flag fM is 1 after motoring starts, the process proceeds from step 34 to step 35, where motoring ends. At the same time, lean operation is started (step 36), and flag fM is set to 0 (step 37). If the LNT temperature is less than threshold temperature B when a transition from stoichiometric operation to lean operation is requested, lean operation is permitted without motoring (step 38).

[0042] 5 is a time chart for explaining the operation of Example 2. From top to bottom, the chart shows (a) vehicle speed, (b) whether the engine is in the operating range, (c) vehicle required driving force, (d) output required for the internal combustion engine 2, (e) torque required for the internal combustion engine 2, (f) whether the engine is in the lean range, (g) output of the battery 5, (h) SOC of the battery 5, (i) actual torque of the internal combustion engine 2, (n) actual rotation speed of the internal combustion engine 2, (j) LNT temperature, (p) flag fL ​​indicating that low exhaust temperature stoichiometric operation is in progress, (k) flag fM indicating that motoring operation is in progress, and (m) lean operation permission flag.

[0043] 3, the illustrated example corresponds to a situation in which the driver depresses the accelerator pedal to accelerate, then releases the accelerator pedal to an appropriate depression, and transitions to constant speed driving. During acceleration, the internal combustion engine 2 operates at stoichiometric pressure at the second operating point, and after transitioning to constant speed driving, it operates at lean pressure at the first operating point until the SOC recovers. During this transition, low exhaust temperature stoichiometric pressure operation is performed to lower the temperature of the three-way catalyst 14, and then motoring is performed to lower the LNT temperature.

[0044] At time t1, the accelerator pedal is fully depressed, and the vehicle begins to accelerate using the output of the battery 5. Then, a short time later at time t2, the internal combustion engine 2 starts. At this time, the torque required of the internal combustion engine 2 is high, and the engine operates at stoichiometric pressure at a second operating point. As the engine operates at stoichiometric pressure, the LNT temperature rises. The TWC temperature (not shown) also rises. Thereafter, at time t3, the accelerator pedal is released, and a short time later at time t4, the engine operating point of the internal combustion engine 2 changes to a first operating point on the low-speed, low-load side, requesting a transition from stoichiometric pressure to lean pressure.

[0045] At time t4, the TWC temperature (not shown) is compared with a threshold temperature C. In the illustrated example, when the TWC temperature is equal to or higher than the threshold temperature C, low exhaust temperature stoichiometric operation is initiated, as indicated by flag fL ​​in column (p). Thereafter, at time t11, the TWC temperature becomes lower than the threshold temperature C, and low exhaust temperature stoichiometric operation ends, as indicated in column (p).

[0046] The LNT temperature at time t11 is compared with threshold temperatures A and B. In the illustrated example, because the LNT temperature is equal to or higher than threshold temperature A, motoring is performed at time t11, stopping combustion operation, as shown in line (k). This motoring quickly reduces the LNT temperature. At time t5, the LNT temperature falls below threshold temperature B, thereby ending motoring. At the same time, lean operation begins.

[0047] As shown in columns (i) and (n), the low exhaust temperature stoichiometric operation is set to a lower speed and lower load side than the first operating point, which is the operating point for lean operation after time t5. As a result, even during stoichiometric operation, the exhaust temperature is sufficiently low, and the three-way catalyst 14 can be cooled.

[0048] As described above, according to the above embodiment, if the three-way catalyst 14 is at a high temperature when transitioning from stoichiometric operation to lean operation, the three-way catalyst 14 is cooled by low exhaust temperature stoichiometric operation before the start of motoring, in which air flows through the three-way catalyst 14. Therefore, it is possible to avoid a decrease in performance of the three-way catalyst 14 due to oxygen poisoning that occurs during motoring.

[0049] It should be noted that motoring does not necessarily have to start exactly at the same time that the TWC temperature falls below the threshold temperature C, and motoring may start with a slight delay. It is sufficient that the TWC temperature is at least below the threshold temperature C when motoring starts.

[0050] The operating point for the low exhaust temperature stoichiometric operation may be any operating point at which the exhaust temperature is at least lower than the threshold temperature C.

[0051] Next, a third embodiment of the present invention will be described. In the third embodiment, when transitioning from stoichiometric operation to lean operation occurs, if the current SOC is lower than the SOC level required to execute and complete motoring, the battery 5 is charged prior to the start of motoring. Specifically, stoichiometric charging operation is performed at an operating point where the air-fuel ratio is stoichiometric at a point where the battery 5 can be charged. The operating point for this stoichiometric charging operation is selected to have the lowest possible exhaust gas temperature within the range where the battery 5 can be charged. In a preferred embodiment, the operating point is set on the high-speed, high-load side of the first operating point, which is the best fuel economy point for normal battery 5 charging, and on the low-speed, low-load side of the second operating point used during vehicle acceleration.

[0052] 6 is a flowchart showing the processing flow of the third embodiment, and the following mainly describes the differences from the first embodiment. In the figure, "fC" is a flag indicating the execution of stoichiometric charging operation, and "SOC" is the SOC of the battery 5.

[0053] First, it is determined whether the vehicle operating conditions are in the engine operating range (step 41), and if NO, the internal combustion engine 2 is stopped (step 42), and flags fM and fC are set to 0 (step 43). If the internal combustion engine 2 is in the engine operating range where it should be operated, it is determined in step 44 whether the requested operating point of the internal combustion engine 2 is in the lean range. If it is not in the lean range, stoichiometric operation is performed (step 45), and flags fM and fC are set to 0 (step 46).

[0054] If the temperature is within the lean region, in step 47, it is determined whether flag fC is 1. Since it is 0 the first time, the process proceeds to step 48, where it is determined whether the LNT temperature is equal to or higher than threshold temperature B. If the result is YES, in step 49, it is determined whether flag fM is 1. Since the result is NO the first time, the process proceeds to step 50, where it is determined whether the LNT temperature is equal to or higher than threshold temperature A. If the LNT temperature is less than threshold temperature A, lean operation is permitted without motoring (step 60).

[0055] If the LNT temperature is equal to or higher than threshold temperature A in step 50, the process proceeds to step 51, where it is determined whether the SOC is equal to or lower than a predetermined threshold temperature D. Threshold temperature D is set to correspond to an SOC value at which motoring can continue until the LNT temperature drops to threshold temperature B. If the SOC is higher than threshold temperature D, flag fM is set to 1 (step 52), combustion operation is stopped, and motoring is started (step 53).

[0056] If motoring causes the LNT temperature to drop below threshold temperature B, the process proceeds from step 48 to step 55, where it is determined whether flag fM is 1. Since flag fM is 1 after motoring starts, the process proceeds from step 55 to step 56, where motoring is terminated. At the same time, lean operation is initiated (step 57), and flag fM is set to 0 (step 58). If the LNT temperature is below threshold temperature B when a transition from stoichiometric operation to lean operation is requested, lean operation is permitted without motoring (step 59).

[0057] Therefore, if the SOC is higher than the threshold D when a shift from stoichiometric operation to lean operation is requested (NO in step 51), the control is the same as in the first embodiment.

[0058] On the other hand, if the SOC is equal to or lower than the threshold value D in step 51, the process proceeds to step 61, where the flag fC is set to 1, and stoichiometric charging operation is initiated (step 62). This stoichiometric charging operation increases the SOC. This stoichiometric charging operation is performed at an operating point that is lower in speed and load than the second operating point and where the exhaust gas temperature is relatively low, thereby suppressing an increase in the LNT temperature.

[0059] When stoichiometric charging operation starts, flag fC is set to 1, so the determination in step 47 of the next routine is NO, and processing proceeds from step 47 to step 63. In step 63, the SOC is again compared with threshold D, and if it is equal to or lower than threshold D, stoichiometric charging operation continues (step 64). If it is higher than threshold D, flag fC is set to 0 and flag fM is also set to 0 in step 65, and then stoichiometric charging operation ends and motoring starts in step 66. After motoring starts, the determination of flag fC in step 47 is YES, and processing proceeds to step 48 and subsequent steps described above.

[0060] 7 is a time chart for explaining the operation of Example 3. From top to bottom, the chart shows (a) vehicle speed, (b) whether the engine is in the operating range, (c) vehicle required driving force, (d) output required for the internal combustion engine 2, (e) torque required for the internal combustion engine 2, (f) whether the engine is in the lean range, (g) output of the battery 5, (h) SOC of the battery 5, (i) actual torque of the internal combustion engine 2, (n) actual rotation speed of the internal combustion engine 2, (j) LNT temperature, (q) flag fC indicating that the engine is in stoichiometric charging operation, (k) flag fM indicating that the engine is in motoring operation, and (m) lean operation permission flag.

[0061] 3, the illustrated example corresponds to a situation in which the driver depresses the accelerator pedal to accelerate, then releases the accelerator pedal to an appropriate depression, and transitions to constant speed driving. During acceleration, the internal combustion engine 2 operates at stoichiometric pressure at the second operating point, and after transitioning to constant speed driving, it operates at lean pressure at the first operating point until the SOC recovers. Then, since the SOC of the battery 5 is below threshold D at the time of transition, stoichiometric charging operation is performed, and then motoring is performed to reduce the LNT temperature.

[0062] At time t1, the accelerator pedal is fully depressed, and the vehicle begins to accelerate using the output of the battery 5. Then, a short time later at time t2, the internal combustion engine 2 starts. At this time, the torque required of the internal combustion engine 2 is high, and the engine operates at stoichiometric pressure at the second operating point. As the engine operates at stoichiometric pressure, the LNT temperature rises. Then, at time t3, the accelerator pedal is released, and a short time later at time t4, the engine operating point of the internal combustion engine 2 changes to the first operating point on the low-speed, low-load side, requesting a transition from stoichiometric pressure to lean pressure.

[0063] At time t4, the LNT temperature is compared with threshold temperatures A and B, and if it is equal to or higher than threshold temperature A, the SOC of battery 5 is compared with threshold temperature D. In the illustrated example, when the LNT temperature is equal to or higher than threshold temperature A and the SOC is equal to or lower than threshold temperature D, stoichiometric charging operation is initiated, as indicated by flag fC in section (q). This causes the SOC to tend to increase, as indicated in section (h). Thereafter, at time t21, the SOC becomes higher than threshold temperature D, and the stoichiometric charging operation ends, as indicated in section (q).

[0064] The LNT temperature at time t21 is again compared with threshold temperatures A and B. In the illustrated example, because the LNT temperature is equal to or higher than threshold temperature A, motoring is performed at time t21, stopping combustion operation, as shown in line (k). This motoring quickly reduces the LNT temperature. At time t5, the LNT temperature falls below threshold temperature B, thereby ending motoring. At the same time, lean operation begins.

[0065] As shown in columns (i) and (n), the operating point for stoichiometric charging operation is set on the higher-speed, higher-load side than the first operating point, which is the operating point for lean operation from time t5 onward. However, an operating point is selected that provides the lowest possible exhaust gas temperature within the range that allows charging of the battery 5. This allows the SOC to increase while suppressing an increase in LNT temperature before motoring begins.

[0066] As described above, according to the above embodiment, when transitioning from stoichiometric operation to lean operation, if the SOC of battery 5 is insufficient by the time motoring is completed, the SOC is increased in advance by stoichiometric charging operation before motoring starts. This makes it possible to avoid a situation in which the SOC drops before motoring can cool the LNT temperature below threshold temperature B, preventing motoring from completing cooling.

[0067] While the present invention has been described above as being applied to a series hybrid vehicle, it is not limited to the above embodiment and various modifications are possible. For example, the present invention can be applied to hybrid vehicles other than series hybrid vehicles, and can be widely applied to hybrid vehicles in which motoring of the internal combustion engine can be performed by powering the motor generator, regardless of the type. Furthermore, in the above embodiment, after motoring begins, the LNT temperature is compared with threshold temperature B to determine whether to terminate motoring. However, motoring may be performed for an appropriate period of time using a timer. The low exhaust temperature stoichiometric operation of the second embodiment and the stoichiometric charging operation of the third embodiment may also be configured to be performed for an appropriate period of time using a timer. [Explanation of symbols]

[0068] 1...Power generating motor generator 2...Internal combustion engine 3...Drive wheels 4...Traction motor generator 5. Battery 6…Reduction device 7...Inverter device 8...Vehicle controller 9...Engine controller 11...Exhaust passage 12...Lean NOx trap catalyst 13...Upstream lean NOx trap catalyst 14...Three-way catalyst 15...Exhaust gas temperature sensor

Claims

1. A control method for an internal combustion engine of a hybrid vehicle that drives a motor generator to generate electricity and is capable of motoring by powering the motor generator, that performs lean operation when required torque is low, and that is equipped with a three-way catalyst and a lean NOx trap catalyst in an exhaust system, comprising: When a transition from stoichiometric operation to lean operation is required, the combustion operation is stopped and motoring is performed until the temperature of the lean NOx trap catalyst becomes lower than a predetermined threshold temperature for avoiding a decrease in NOx adsorption capacity, and then the lean operation is started. A method for controlling an internal combustion engine.

2. The internal combustion engine is an internal combustion engine for generating electricity in a series hybrid vehicle, In this series hybrid vehicle, the motor generator generates electricity and the vehicle is driven by a separate motor generator for driving.

2. The method for controlling an internal combustion engine according to claim 1.

3. When a transition from stoichiometric operation to lean operation is requested, if the temperature of the three-way catalyst is equal to or higher than a predetermined second threshold temperature that is higher than the threshold temperature for avoiding oxygen poisoning, before the start of motoring, low exhaust temperature stoichiometric operation is performed with the air-fuel ratio at a stoichiometric value at an operating point where the exhaust temperature is lower than the second threshold temperature.

2. The method for controlling an internal combustion engine according to claim 1.

4. the low exhaust temperature stoichiometric operation continues until the temperature of the three-way catalyst becomes lower than the second threshold temperature; starting the motoring after the temperature falls below the second threshold temperature; 4. The method for controlling an internal combustion engine according to claim 3.

5. When a transition from stoichiometric operation to lean operation is requested, if the SOC of the battery is at an insufficient level for executing the motoring, a stoichiometric charging operation is performed at an operating point where the battery can be charged, before the motoring is started.

2. The method for controlling an internal combustion engine according to claim 1.

6. The stoichiometric charging operation continues until the SOC of the battery reaches a predetermined SOC. After the predetermined SOC is reached, the motoring is started.

6. The method for controlling an internal combustion engine according to claim 5.

7. A motor generator; an internal combustion engine that drives the motor generator to generate electricity and is capable of motoring by powering the motor generator, and that performs lean operation when the required torque is low; a three-way catalyst and a lean NOx trap catalyst provided in an exhaust system of an internal combustion engine; a controller for controlling the motor generator and the internal combustion engine; Equipped with The above controller is When a transition from stoichiometric operation to lean operation is required, the combustion operation is stopped and motoring is performed until the temperature of the lean NOx trap catalyst becomes lower than a predetermined threshold temperature for avoiding a decrease in NOx adsorption capacity, and then the lean operation is started. Control device for internal combustion engines.

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