Control method and control device of internal combustion engine

Abstract

To solve the problem that 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, 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, low exhaust temperature stoichiometric operation-combustion operation is performed in which an air-fuel ratio is set to be a stoichiometric ratio on an operation point where at least an exhaust gas temperature is lower than a threshold temperature B. At a time t5 when the LNT temperature becomes less than the 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 4 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] A configuration in which a heat exchanger is provided in the exhaust system as in Patent Document 1 is undesirable because it complicates the configuration of the exhaust system and requires a large heat exchanger and associated refrigerant piping. [Means for solving the problem]

[0008] The present invention relates to a control method for an internal combustion engine of a hybrid vehicle that drives a motor generator to generate electricity, 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 an exhaust system, comprising: When a transition from stoichiometric operation to lean operation is required, low exhaust temperature stoichiometric operation is performed at an operating point where the exhaust temperature is lower than a predetermined threshold temperature for avoiding a decrease in NOx adsorption capacity, and then lean operation is started.

[0009] Low exhaust temperature stoichiometric operation introduces low exhaust gas temperature into the lean NOx trap catalyst, lowering the temperature of the lean NOx trap catalyst, which had been high in temperature. [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] A characteristic diagram showing the correlation between the temperature of the lean NOx trap catalyst and NOx emissions. [Figure 5] FIG. 4 is a characteristic diagram showing the correlation between the load on an internal combustion engine and the exhaust temperature. [Figure 6] FIG. 4 is a characteristic diagram showing the correlation between the rotation speed and the exhaust temperature of an internal combustion engine. [Figure 7] A characteristic diagram showing the correlation between EGR rate and exhaust temperature. [Figure 8] 10 is a flowchart showing the flow of processing in a second embodiment. [Figure 9] 6 is a time chart showing the operation of the second embodiment. 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.

[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 may exceed temperature T1, at which NOx treatment performance deteriorates, as shown in Figure 4. 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, 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 low, and low temperature gas is passed through lean NOx trap catalyst 12 to cool lean NOx trap catalyst 12.

[0021] FIG. 2 is a flowchart showing the flow of processing in the first embodiment, which will be described 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. "fL" is a flag indicating that low exhaust temperature stoichiometric operation is being performed.

[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 fL ​​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 fL ​​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 fL ​​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 4, 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 performing low exhaust temperature stoichiometric operation (step 18).

[0024] If the LNT temperature is equal to or higher than the threshold temperature A in step 9, the flag fL ​​is set to 1 (step 10), and low exhaust temperature stoichiometric operation is started (step 11).

[0025] If the LNT temperature drops due to low exhaust temperature stoichiometric operation and becomes less than threshold temperature B, the process proceeds from step 7 to step 13, where it is determined whether flag fL ​​is 1. Since flag fL ​​is 1 after low exhaust temperature stoichiometric operation starts, the process proceeds from step 13 to step 14, where low exhaust temperature stoichiometric operation ends. At the same time, lean operation starts (step 15), and flag fL ​​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 performing low exhaust temperature stoichiometric operation (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) actual rotation speed of the internal combustion engine 2, (k) LNT temperature, (m) flag fL ​​indicating that low exhaust temperature stoichiometric operation is in progress, and (n) 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, low exhaust temperature stoichiometric pressure operation 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, low exhaust temperature stoichiometric operation is performed at time t4, as shown in line (m). This low exhaust temperature stoichiometric operation causes the LNT temperature to drop quickly. At time t5, the LNT temperature falls below threshold temperature B, thereby ending low exhaust temperature stoichiometric operation. At the same time, lean operation begins.

[0031] The operating point of low exhaust temperature stoichiometric operation is originally an operating point on the low-speed, low-load side included in the lean region. In the above embodiment, the operating point of low exhaust temperature stoichiometric operation is set to a lower speed, low-load side than the first operating point for charging battery 5. That is, as shown in (i), low exhaust temperature stoichiometric operation is set to an operating point with a lower load than the first operating point, which is the operating point of lean operation from time t5 onwards, and at the same time, as shown in (j), low exhaust temperature stoichiometric operation is set to an operating point with a lower rotational speed than the first operating point. This ensures that the exhaust temperature is sufficiently low even during stoichiometric operation, thereby cooling lean NOx trap catalyst 12. The operating point of low exhaust temperature stoichiometric operation needs to be an operating point at which the exhaust temperature is at least lower than threshold temperature B, and it is desirable that the exhaust temperature be as low as possible.

[0032] Figure 5 shows the correlation between the load of the internal combustion engine 2 and the exhaust temperature, with the lower the load, the lower the exhaust temperature. Figure 6 shows the correlation between the rotation speed of the internal combustion engine 2 and the exhaust temperature, with the lower the rotation speed, the lower the exhaust temperature.

[0033] Although not shown in Figure 3, during low exhaust temperature stoichiometric operation, it is desirable to recirculate exhaust gas at a relatively high EGR rate so that the exhaust gas temperature is lower. In one embodiment, exhaust gas is recirculated so that the exhaust gas temperature is lower than that at the first operating point, which is the operating point for lean operation at least from time t5 onwards. Figure 7 shows the correlation between the EGR rate and exhaust gas temperature of the internal combustion engine 2, and within the EGR rate range where stable combustion is possible, the higher the EGR rate, the lower the exhaust gas temperature. Therefore, during low exhaust temperature stoichiometric operation, it is desirable to have as high an EGR rate as possible within the range where stable combustion can be maintained.

[0034] As described above, according to the above embodiment, when transitioning from stoichiometric operation to lean operation, the temperature of lean NOx trap catalyst 12 (LNT temperature) can be quickly lowered by low exhaust temperature stoichiometric operation 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.

[0035] 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.

[0036] Furthermore, in the above embodiment, the need for low exhaust temperature stoichiometric operation 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 performing low exhaust temperature stoichiometric operation 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 stage of lean operation.

[0037] Next, a second embodiment of the present invention will be described. In the second embodiment, when transitioning from stoichiometric operation to lean operation, if the current SOC is lower than the SOC level required to execute and complete low exhaust temperature stoichiometric operation, battery 5 is charged prior to initiating low exhaust temperature stoichiometric operation. In other words, in low exhaust temperature stoichiometric operation, the amount of power obtained by power generation is minimized, so the SOC of battery 5 may decrease during low exhaust temperature stoichiometric operation. Therefore, in the second embodiment, if the SOC is insufficient, stoichiometric charging operation is performed at an operating point where the air-fuel ratio is stoichiometric and where battery 5 can be charged before initiating low exhaust temperature stoichiometric operation. The operating point for this stoichiometric charging operation is selected to have the lowest possible exhaust gas temperature within the range where battery 5 can be charged. In a preferred embodiment, the operating point is set to a higher speed and higher load side than a first operating point, which is the best fuel economy point for normal battery 5 charging, and a lower speed and lower load side than a second operating point used during vehicle acceleration. Of course, this is an operating point on the high-speed, high-load side of the operating point of the low exhaust temperature stoichiometric operation.

[0038] 8 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, "fC" is a flag indicating the execution of stoichiometric charging operation, and "SOC" is the SOC of the battery 5.

[0039] 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 fL and fC 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 fL and fC are set to 0 (step 26).

[0040] If the temperature is within the lean region, in step 27, it is determined whether flag fC is 0. Since it is 0 the first time, the process proceeds to step 28, where it is determined whether the LNT temperature is equal to or higher than threshold temperature B. If the result is YES, in step 29, it is determined whether flag fL ​​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 higher than threshold temperature A. If the LNT temperature is lower than threshold temperature A, lean operation is permitted without performing low exhaust temperature stoichiometric operation (step 40).

[0041] If the LNT temperature is equal to or higher than threshold temperature A in step 30, the process proceeds to step 31, 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 low exhaust temperature stoichiometric operation can be continued until the LNT temperature drops to threshold temperature B. If the SOC is higher than threshold temperature D, flag fL ​​is set to 1 (step 32), and low exhaust temperature stoichiometric operation is started (step 33).

[0042] If the LNT temperature drops due to low exhaust temperature stoichiometric operation and becomes less than threshold temperature B, the process proceeds from step 28 to step 35, where it is determined whether flag fL ​​is 1. Since flag fL ​​is 1 after low exhaust temperature stoichiometric operation starts, the process proceeds from step 35 to step 36, where low exhaust temperature stoichiometric operation ends. At the same time, lean operation starts (step 37), and flag fL ​​is set to 0 (step 38). 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 performing low exhaust temperature stoichiometric operation (step 39).

[0043] Therefore, if the SOC is higher than the threshold D when a shift from stoichiometric operation to lean operation is requested (NO in step 31), the control is the same as in the first embodiment.

[0044] On the other hand, if the SOC is equal to or lower than the threshold value D in step 31, the process proceeds to step 41, where the flag fC is set to 1, and stoichiometric charging operation is started (step 42). 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.

[0045] When stoichiometric charging operation starts, flag fC is set to 1, so the determination in step 27 of the next routine is NO, and processing proceeds from step 27 to step 43. In step 43, the SOC is again compared with threshold value D, and if it is equal to or lower than threshold value D, stoichiometric charging operation continues (step 44). If it is higher than threshold value D, flag fC is set to 0 and flag fL ​​is also set to 0 in step 45, and then stoichiometric charging operation ends and low exhaust temperature stoichiometric operation starts in step 46. After low exhaust temperature stoichiometric operation starts, the determination of flag fC in step 27 is YES, and processing proceeds to step 28 and subsequent steps described above.

[0046] 9 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, (j) actual rotation speed of the internal combustion engine 2, (k) LNT temperature, (p) flag fC indicating that stoichiometric charging operation is in progress, (m) flag fL ​​indicating that low exhaust temperature stoichiometric operation is in progress, and (n) lean operation permission flag.

[0047] 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 low exhaust temperature stoichiometric operation is performed to lower the LNT temperature.

[0048] 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.

[0049] 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 (p). This causes the SOC to tend to increase, as indicated in section (h). Thereafter, at time t11, the SOC becomes higher than threshold temperature D, and the stoichiometric charging operation ends, as indicated in section (p).

[0050] The LNT temperature at time t11 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, low exhaust temperature stoichiometric operation is performed at time t11, as shown in line (m). This low exhaust temperature stoichiometric operation quickly reduces the LNT temperature. At time t5, the LNT temperature falls below threshold temperature B, thereby ending low exhaust temperature stoichiometric operation. At the same time, lean operation begins.

[0051] As shown in columns (i) and (j), the stoichiometric charging operation is set to a higher speed and load side than the first operating point, which is the operating point for lean operation from time t5 onwards. 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 the start of low exhaust gas temperature stoichiometric operation.

[0052] As described above, according to the above embodiment, when transitioning from stoichiometric operation to lean operation, if the SOC of battery 5 is insufficient to complete cooling through low exhaust temperature stoichiometric operation, the SOC is increased in advance through stoichiometric charging operation before starting low exhaust temperature stoichiometric operation. This makes it possible to avoid a situation in which the SOC drops before the LNT temperature is cooled to below threshold temperature B through low exhaust temperature stoichiometric operation, making it impossible to complete cooling through low exhaust temperature stoichiometric operation.

[0053] 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. Regardless of the type, the present invention can be widely applied to hybrid vehicles in which the operating point of the internal combustion engine 2 can be set to a low speed, low load while maintaining the required vehicle driving force. Furthermore, in the above embodiment, after low exhaust temperature stoichiometric operation is initiated, the LNT temperature is compared with the threshold temperature B to determine whether to terminate the low exhaust temperature stoichiometric operation. However, low exhaust temperature stoichiometric operation may also be performed for an appropriate period of time using a timer. Similarly, the stoichiometric charging operation of the second embodiment may also be configured to be performed for an appropriate period of time using a timer. [Explanation of symbols]

[0054] 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, that performs lean operation when required torque is low, and that has 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, low exhaust temperature stoichiometric operation is performed at an operating point where the exhaust temperature is lower than a predetermined threshold temperature for avoiding a decrease in NOx adsorption capacity, and the lean operation is then 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. The internal combustion engine is operated at a predetermined operating point as a lean operation when the battery is being charged due to a decrease in the SOC of the battery, The operating point of the low exhaust temperature stoichiometric operation is set to a lower load side than the operating point during battery charging.

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

4. The internal combustion engine is operated at a predetermined operating point as a lean operation when the battery is being charged due to a decrease in the SOC of the battery, The operating point of the low exhaust temperature stoichiometric operation is set to a lower rotation speed side than the operating point during battery charging.

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

5. The internal combustion engine is operated at a predetermined operating point as a lean operation when the battery is being charged due to a decrease in the SOC of the battery, During the low exhaust temperature stoichiometric operation, exhaust gas recirculation is performed so that the exhaust gas temperature is lower than at least the operating point during the battery charging.

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

6. When a transition from stoichiometric operation to lean operation is requested, if the SOC of the battery is at an insufficient level for performing the low exhaust temperature stoichiometric operation, a stoichiometric charging operation is performed at an operating point where the battery can be charged, before starting the low exhaust temperature stoichiometric operation.

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

7. The stoichiometric charging operation continues until the SOC of the battery reaches a predetermined SOC. When the predetermined SOC is reached, the engine shifts to the low exhaust temperature stoichiometric operation.

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

8. A motor generator; an internal combustion engine that drives the motor generator to generate electricity and 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, low exhaust temperature stoichiometric operation is performed at an operating point where the exhaust temperature is lower than a predetermined threshold temperature for avoiding a decrease in NOx adsorption capacity, and the lean operation is then started. Control device for internal combustion engines.

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