MUFFLED NOISE

BACKGROUND OF THE INVENTION

1. Field of the Inventio

[0001] This invention relates to a drive control apparatus for a hybrid vehicle provided with an engine and a motor as sources of drive power, and to a method for avoiding muffled noise produced by the driving of a motor in the hybrid vehicle.

2. Description of Related Art

[0002] Conventionally, a hybrid vehicle provided with an engine and a motor as sources of drive power is widely available. In a specific operating state (at a specific rotation speed or a specific torque), vibrations generated by the engine and the motor are amplified and transmitted to the vehicle body, and muffled noise may be generated.

There is a possibility that such muffled noise may cause a sense of discomfort in the occupants of the vehicle.

[0003] Therefore, conventionally, a plurality of technologies for avoiding muffled noise have been proposed. For example, Japanese Patent Application Publication No. 2008-144859 (JP 2008-144859 A) discloses technology wherein, when the operating point of an engine as specified by the engine speed of rotation NE and the engine torque TE is situated in muffled noise generation region which is determined in advance by

experimentation and stored, then the operating point of the engine is varied to change the distribution of load between the engine torque TE and the assist torque TM ₂ of the electric motor, thereby avoiding muffled noise. Furthermore, JP 2008-144859 A indicates that, in changing the operating point of the engine, the engine output may be kept at a

predetermined value by raising the speed of rotation of the engine while reducing the engine torque, in such a manner that the operating point of the engine moves along a constant power curve indicating predetermined engine output. [0004] Furthermore, another document proposes that, in order to avoid muffled noise caused by the driving of the motor, when the operating point of the motor indicated by the motor torque the rotation speed, and the like, is situated in a predetermined muffled noise generating region, then a controller implements control so as to perform an operation for avoiding the muffled noise.

[0005] When the controller begins to implement the operation for avoiding muffled noise after the muffled noise occurrence, the muffled noise cannot be prevented reliably. Consequently, a start line is generally set outside the muffled noise generation region in which muffled noise is generated, and when the motor operating point reaches the start line, then the controller starts an operation for avoiding muffled noise.

Furthermore, when a start threshold value and a stop threshold value for the operation for avoiding muffled noise are set to the same value, then when the motor operating point varies around this threshold value, a chattering phenomenon occurs in which the operation for avoiding muffled noise is started and stopped repeatedly at short time intervals.

Therefore, in general, a hysteresis area having a prescribed hysteresis width is set between the start line and the stop line, to the outside of the muffled noise generation region in which muffled noise is generated. When the motor operating point reaches the start line, the operation for avoiding muffled noise is started, and when the motor operating point reaches the stop line, the operation for avoiding muffled noise is stopped.

SUMMARY OF THE INVENTION

[0006] However, conventionally, the hysteresis region has been set uniformly at all times, irrespective of the driving conditions of the vehicle. Nevertheless, the variation in the motor operating point differs greatly depending on the driving conditions of the vehicle. For example, when an engine output keeping control is being implemented to keep an engine output at a predetermined value, the variation in the motor operating point tends to be smaller, compared to cases where such control is not being implemented. In a driving state where the variation in the motor operating point tends to be small, when the same hysteresis region (start line and stop line) is used as in a driving state where the variation in the motor operating point tends to be large, then the operation for avoiding muffled noise is continued until the motor torque reaches a value that is higher than necessary. When the motor torque is high when the engine output keeping control is being implemented, then the engine rotation speed become higher and fuel consumption deteriorates.

[0007] Therefore, this invention provides a control apparatus for a hybrid vehicle, and a method for avoiding muffled noise, whereby fuel consumption can be further improved.

[0008] A first aspect of this invention is a control apparatus for a hybrid vehicle includes an engine and a motor. The control apparatus includes an electronic control unit configured to store a muffled noise generation region and a hysteresis region, the muffled noise generation region being set in advance as a region in which muffled noise caused by driving of the motor generates, and the hysteresis region being set between a start line and a stop line outside the muffled noise generation region. The electronic control starts to change an engine operating point and a motor operating point such that the motor operating ' point is distanced from the muffled noise generation region, when the motor operating point has reached the start line, the motor operating point being indicated by at least one of a motor torque and a motor rotation speed. The electronic control ends to change the engine operating point and the motor operating point such that the motor operating point is distanced from the muffled noise generation region, when the motor operating point has reached the stop line. The electronic control determines whether or not an engine output keeping control is being implemented to keep an engine output at a predetermined value. The electronic control sets at least one of the start line and the stop line such that at least one of the start line and the stop line, when the engine output keeping control is being implemented, is closer to the muffled noise generation region than that when the engine output keeping control is not being implemented.

In the aspect described above, the electronic control unit determines whether or not the motor operating point is within the muffled noise generation region. The electronic control unit may set at least one of the start line and the stop line such that at least one of W

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the start line and the stop line is further from the muffled noise generation region compared to the currently set. start line and stop line, when the engine output keeping control is being implemented and the motor operating point is within the muffled noise generation region.

A second aspect of this invention is a control method for a hybrid vehicle including an engine, a motor and an electronic control unit. The control method stores a muffled noise generation region and a hysteresis region, by the electronic control unit, the muffled noise generation region being set in advance as a region in which muffled noise caused by driving of the motor generates, and the hysteresis region being between a start line and a stop line outside the muffled noise generation region. The control method starts, by the electronic control unit, to change an engine operating point and a motor operating point such that the motor operating point is distanced from the muffled noise generation region, when the motor operating point has reached the start line, the motor operating point being indicated by at least one of a motor torque and a motor rotation speed. The control method ends, by the electronic control unit, to change the engine operating point and the motor operating point such that the motor operating point is distanced from the muffling noise generation region, when the motor operating point has reached the stop line. The control method determines by the electronic control unit whether or not an engine output keeping control is being implemented to keep a engine output at a predetermined value. The control method sets at least one of the start line and the stop line, by the electronic control unit, such that at least one of the start line arid the stop line, when the engine output keeping control is being implemented, is closer to the muffled noise generation region than that when the engine output keeping control is not being implemented.

In the aspect described above, the control method determines by the electronic control whether or not the motor operating point is within the muffled noise generation region unit. At least one of the start line and the stop line may be set by the electronic control unit such that at least one of the start line and the stop line is further from the muffled noise generation region compared to the currently set start line and stop line, when the engine output keeping control is being implemented and the motor operating point is within the muffled noise generation region. [0009] According to the aspect described above, the ECU narrows the hysteresis region to the muffled noise generation region, when implementing the engine output keeping control. Therefore, the operation for avoiding muffled noise is stopped while the motor torque is relatively small, and hence the fuel consumption is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a drawing showing a composition of a drive mechanism of a hybrid vehicle;

FIG. 2 is a drawing showing one example of change in the motor torque;

FIG. 3 is a drawing showing one example of a constant power curve;

FIGi 4 is a drawing showing one example of change in the motor torque, and the like; FIG. 5 is a flowchart showing a flow of an operation for avoiding muffled noise in the embodiment;

FIG. 6 is a flowchart showing another example of determination of switching of a hysteresis region; and

FIG. 7 is a drawing showing another example of change in the motor torque. DETAILED DESCRIPTION OF EMBODIMENTS

[0011] Embodiments of this invention are described below with reference to the drawings. FIG. 1 is a drawing showing a composition of a drive mechanism of a hybrid vehicle.

[0012] , An internal combustion engine 12 (called "engine 12" below) , a first motor-generator 14 (called "first MG 14" below) and a second motor- generator 16 (called "second MG 16" below) are installed in the hybrid vehicle 10 as prime motors. The motive force of these prime motors 12, 14, 16 is transmitted to the drive wheels 24 via a drive power transmission mechanism 18 including a drive power distribution and combination mechanism 20 and a speed reducing mechanism 22, whereby the vehicle begins to travel.

[0013] The first MG 14 and the second MG and 16 are synchronous

motor-generators, which function as electric generators and also function as electric motors. The first MG 14 and second MG and 1 are electrically connected to a battery 28 via the inverter 26. The battery 28 is constituted by a rechargeable secondary cell, for example, a nickel - hydrogen secondary cell or lithium ion secondary cell, or the like. The electric power stored in the battery 28 is converted from direct current (DC) to alternating current (AC) by the inverter 26, and is then supplied to the first MG 14 and second MG and 16 to drive these MGs 14 and 16. Furthermore, the electric power generated by the first MG 14 and second MG 16 is converted from AC to DC by the inverter 26 and is then sent to the battery 28 and stored therein. In this way, the first MG 14 and second MG 16 can function as an electric motor and as an electric generator.

[0014] The first MG 14 functions as a starter motor when starting up the engine 12. Furthermore, the first MG 14 generates electricity from a portion of the output of the engine 12. On the other hand, the second MG 16 assists the output of the engine 12 and raises the drive power of the vehicle. Furthermore, the second MG 16 performs regenerative power generation from the kinetic energy of the vehicle which is input from the drive wheels 24 during deceleration. In this way, in this embodiment, the first MG 14 and second MG 16 function as an electric motor and as an electric generator. It is also possible for the first MG 14 to function as an electric generator only, and for the second MG J 6 to function as an electric motor only.

[0015] The hybrid ECU 30 functions as a control apparatus which controls the driving of the drive mechanism which is constituted by the first MG 14 and second MG 16 described above, and therefore controls the outputs of the engine 12 and the first MG 14 and second MG 16, and the drive power distribution ratio thereof, in such a manner that the vehicle assumes a desired operating state. Furthermore, as described in detail below, the hybrid ECU 30 determines whether or not an avoidance operation is required to avoid muffled noise caused by the driving of the second MG 16, in accordance with the motor operating point of the second MG 16, for example, the motor torque Tm thereof.

Moreover, a hysteresis region may be set as a reference for defining the start and stop of the operation for avoiding muffled noise. The engine ECU 31 communicates with the hybrid ECU 30, and controls the operation of the engine 12 by means of control signals from the hybrid ECU 30, as well as outputting data relating to the operating state of the , engine 12, as necessary.

[0016] Next, the operation for avoiding muffled noise is described with reference to FIG. 2. FIG. 2 is a diagram showing one example of variation of the motor torque Tm of the second MG 16, in which the vertical axis represents the motor torque Tm and the lateral axis represents the time. The muffled noise is an unpleasant sound which occurs due to vibrations caused by the engine 12 and the first MG 1 and second MGand 16 being amplified and transmitted to the vehicle body in a particular operating state (at a particular number or rotation speed or torque). The hybrid ECU 30 according to this embodiment instructs an operation for avoiding muffled noise, as appropriate, to at least one of the engine ECU 31 , the first MG 14 and the second MG 16, in order to prevent the occurrence of this muffled noise. An operation for avoiding the muffled noise caused by driving of the engine 12 is described in JP 2008.-144859 A, and the like, and detailed description thereof is not given here. Below, an operation for avoiding the muffled noise caused by driving a motor-generator, and in particular, the second MG 16, will be described in detail.

- [0017] In this embodiment, the hybrid ECU 30 determines whether or not an operation for avoiding muffled noise caused by driving of the second MG 16 is necessary, on the basis of the motor torque Tm output from the second MG 16; in other words, the magnitude of the assist torque which supplements the torque of the engine 12. In order to determine whether or not this avoidance operation is necessary, the hybrid ECU 30 stores two regions. The two regions are a muffled noise generation region and a hysteresis region. The muffled noise generation region is the region of the motor torque Tm in which the muffled noise is generated, and this region is determined in advance by experimentation, or the like. In the example in FIG. 2, the region A at and below the value a is the muffled noise generation region. [0018] The hysteresis region is a region set outside the muffled noise generation region and is set between a start line and a stop line. In the example in FIG. 2, the hysteresis region is the region C which is set between the start line Lb indicated by the value b and the stop line Lc indicated by the value c. The start line Lb is a line forming a reference for deciding whether or not to start an operation for avoiding muffled noise, and the values of this line are obtained by adding a predetermined margin region B to the muffled noise generation region A. When an operation for avoiding muffled noise is not being executed, the hybrid ECU 30 decides the start of an operation for avoiding muffled noise, when the motor torque Tm reaches the start line Lb. Furthermore, the end line Lc is a reference for deciding whether or not to stop the operation for avoiding muffled noise, and is obtained by adding a hysteresis region C to the start line Lb. While an operation for avoiding muffled noise is being executed, the hybrid ECU 30 decides to stop the , operation for avoiding muffled noise, when the motor torque Tm reaches the stop line Lc.

[0019] Consequently, in the example in FIG. 2, the hybrid ECU 30 starts an operation for avoiding muffled noise at the time tl where the motor torque Tm reaches the start line Lb. The hybrid ECU 30 stops the operation for avoiding muffled noise at time t2 where the motor torque Tm reaches the stop line Lc.

[0020] In the operation for avoiding muffled noise, the hybrid ECU 30 changes the output torque of the second MG 16 in such that the motor torque Tm exceeds the stop line Lc. Furthermore, when the output torque of the second MG 16, in other words, the assist torque for supplementing the torque of the engine 12 is increased, the engine ECU 31 decreases the torque of the engine 12 in order to cancel out this increase. In other words, the operation for avoiding the muffled noise is a process for changing the distribution ratio of the motor torque Tm and the engine torque Te, so as to move the motor torque Tm outside the hysteresis region, and in performing this operation, a change in the engine torque Te may also occur, in addition to the motor torque Tm of the second MG 16.

[0021] Here, the engine output Pe itself may be changed in accordance with the change in the engine torque Te, during steady operation where there is little change in the accelerator depression amount, it is desirable to change the engine torque Te while keeping 4 000388

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the engine output Pe constant. In other words, during steady operation, since the amount of change required in the engine torque Te is small, then a desired engine output Pe is obtained by changing rotation speed, or the like, without changing the engine output Pe. In such cases, it is beneficial in terms of improving fuel consumption to implement an engine output keeping control to keep the variation of the engine output Pe at a

predetermined level.

[0022] In the case of the engine output keeping control, a constant power curve indicating engine operating points of engine output Pe at a predetermined value, is stored in advance for each output, and the engine ECU 31 moves the engine operating point along the constant power curve. FIG. 3 is a graph showing one example of a constant power curve, in which the vertical axis represents the engine torque Te and the horizontal axis represents the engine rotation speed Ne. When the motor torque Tm (assist torque) of the second MG 16 is increased in order to avoid muffled noise caused by the driving of the second MG 16, then in order to cancel out this increase, the engine ECU 31 reduces the torque of the engine 12 while raising rotation speed of the engine 12, along the constant power curve.

[0023] The operation for avoiding muffled noise described thus far involves control that prioritizes the avoidance of the muffled noise and does not prioritize fuel consumption. Therefore, in order to improve fuel consumption, it is desirable to keep the continuation time of the operation for avoiding muffled noise as short as possible.

Furthermore, when the engine output keeping control is implemented, then the greater the value of the motor torque Tm, the smaller the engine torque Te, the higher the engine rotation speed Ne and the worse the fuel consumption. In other words, when the stop line Lc, in other words, the upper limit value of the hysteresis region is raised and the motor torque Tm is increased to a high value, then the fuel consumption deteriorates accordingly.

[0024] However, in a state where there is a large amount of change in the accelerator depression amount and the _, engine output keeping control is not implemented, such as when accelerating, there is a tendency for the variation in the motor torque Tm of the second MG 16 to become large. For example, FIG. 4 is a diagram showing one example of change in the vehicle speed, accelerator depression amount, engine output Pe and motor torque Tm, when the engine output keeping control is not implemented. When the engine output keeping control is not implemented, as in FIG. 4, the engine output Pe increases sharply in accordance with increase in the accelerator depression amount.

When the engine output Pe increases sharply, the motor torque Tm temporarily falls sharply, and then rises back to a value higher than that prior to the sharp fall. In a situation where the motor torque Tm changes greatly in this way, when the hysteresis region is narrowed to the muffled sound generation region, then due to variation in the motor torque Tm caused by reasons other than the operation for avoiding muffled noise, the motor torque Tm may reach the stop line, the operation for avoiding muffled noise may stop despite the fact that the muffled noise has not been avoided satisfactorily, and chattering may occur in which the avoidance operation is started and stopped repeatedly at short time intervals.

[0025] On the other hand, in a situation where there is a small amount of change in the accelerator depression amount and the engine output keeping control is being performed, then the change in the motor torque Tm of the second MG 16 is also small. For example, when the engine output keeping control is implemented, then the engine output Pe and the motor torque Tm change as shown by the dotted lines LI, L2 in FIG. 4. In this way, in situations where there is little, change in the motor torque Tm, even when the hysteresis region is narrowed to the muffled noise generation region, there is virtually no change in the motor torque Tm due to reasons other than the operation for avoiding muffled noise. Furthermore, since the operation for avoiding muffled noise ends when the value of the motor torque Tm is small, then it is possible to prevent excessive increase in the engine rotation speed Ne and hence excessive deterioration of the fuel consumption.

[0026] Therefore, in this embodiment, the size of the hysteresis region is switched depending on whether or not the engine output keeping control is being implemented, More specifically, ^' in this embodiment, the hybrid ECU 30 stores hysteresis regions of two types, namely, a first hysteresis region which is used when the engine output keeping control is not being implemented, and a second hysteresis region which is used when the engine output keeping control is being implemented. The second hysteresis region is a ^~ region that is narrower towards the muffled noise generation region, than the first

hysteresis region. In the example in FIG. 2, when the region C is set as the first hysteresis region, then a region D, which is narrower on the side approaching the muffled noise generation region than the region C, is set as a second hysteresis region.

[0027] When the engine output keeping control is not implemented in the example in FIG. 2, then the operation for avoiding muffled noise is started at the time tl where the motor torque Tm reaches the start line Lb, and is continued until time t2 where the motor torque Tm reaches the upper limit value (end line Lc) of the first hysteresis region C. On the other hand, when the engine output keeping control is being

implemented, the start timing of the operation for avoiding muffled noise is the same time tl as when this control is not being implemented, the end timing of the operation for avoiding muffled noise is the time t3 where the motor torque Tm reaches the upper limit value (end line Ld) of the second hysteresis region D, and hence the continuation time of the operation for avoiding muffled noise can be shortened. Furthermore, since the operation for avoiding the muffled noise is stopped while the motor torque Tm is relatively small, then the engine rotation speed Ne can be kept relatively low. As a result of this, it is possible to improve fuel consumption.

[0028] Next, the flow of control relating to the avoidance of muffled noise caused by driving of the motor will be described with reference to FIG. 5. In order to avoid muffled noise caused by driving of the motor, the hybrid ECU 30 firstly determines whether or not the motor torque Tm of the second MG 16 has reached the start line (step S 10). When the motor torque Tm has not reached the start line, then since there is little of risk of muffled noise being generated, ah operation for avoiding muffled noise is not ' started and only monitoring of the motor torque Tm is continued. On the other hand, when the motor torque Tm has reached the start line, the hybrid ECU 30 instructs the start of an operation for avoiding muffled noise (step S 12). More specifically, the hybrid ECU 30 changes the torque distribution ratio of the second MG 16 and the engine 12, so as to raise the motor torque Tm of the second MG 16. [0029] When the operation for avoiding muffled noise is started, the hybrid ECU 30 then determines whether or not the engine output keeping control is being implemented (S 14). When the engine output keeping control is not being implemented, then since the variation in motor torque Tm is large, the first hysteresis region, which is relatively large, , is set as the hysteresis region (S 18). On the other hand, when the engine output keeping control is being implemented, then since the variation in motor torque Tm is small, the second hysteresis region, which is narrowed towards the muffled noise generation region, is set as the hysteresis region (SI 6).

[0030] When the hysteresis region has been set, it is then determined whether or not the motor torque Tm has reached the upper limit value of the hysteresis region, in other words, the stop line (S20). When the motor torque Tm has not reached the stop line (No at S20), then the procedure returns to step SI 4. Furthermore, when the motor torque Tm has reached the stop line (Yes at S20), then the operation for avoiding muffled noise is stopped (S22). After stopping the operation for avoiding muffled noise, the procedure returns again to step S10, and similar processing is repeated.

[0031] As is clear from the description given above, in this embodiment, the hysteresis region is narrowed towards the muffled noise generation region when the engine output keeping control is being implemented. As a result of this, an operation for avoiding muffled noise which leads to deterioration of the fuel consumption can be prevented from continuing unnecessarily, and furthermore, the motor torque Tm can be restricted to a small value and therefore the overall fuel consumption can be improved.

[0032] In this embodiment, it is determined whether or not an operation for avoiding muffled noise is necessary on the basis of the motor operating point which is indicated by the motor torque Tm, this determination can be made on the basis of another parameter, provided that the parameter (operating point) indicates the state of operation of the motor torque. Consequently, for example, it is possible to determine whether or not an avoidance operation is necessary on the basis of an operating point indicated by the rotation speed of the motor or an operating point indicated by both the rotation speed of the motor and the torque, for example. [0033] Moreover, in this embodiment, only the avoidance of muffled noise caused by driving of the second MG 16 is described, this embodiment may also be applied to an operation for avoiding muffled noise caused by driving of another motor, for example, the first MG 14, provided that the motor is mounted in a hybrid vehicle. Furthermore, in this embodiment, muffled noise is avoided by changing the torque distribution ratio of the engine and the second MG 16, another method capable of avoiding the muffled noise may also be adopted.

[0034] Furthermore, in addition to considering the fact that the engine output keeping control is being implemented as a condition for switching the hysteresis region, it is also possible to switch the hysteresis region by taking account of the value of the motor torque Tm. For example, the processing from step S 14 to step S20 in FIG. 5 may be substituted with the processing shown in FIG. 6.

[0035] In FIG. 6, in addition to considering the fact that the engine output keeping control is being implemented (SI 4) as a condition for switching the hysteresis region, the fact that the motor torque Tm is not in the muffled noise generation region (S24) and the fact that the motor torque Tm has not reached the stop line (S20) are also provided as conditions. These conditions are provided for the following reason. When the motor torque Tm is within the muffled noise generation region, then this means that muffled noise is actually being generated. In this case, the operation for avoiding muffled noise should not be stopped unless the cause of the muffled noise has been eliminated reliably. However, the motor torque Tm may momentarily undergo great variation due to other causes, such as noise. In a case such as this, when the second hysteresis region, which is relatively narrow, has been set as the hysteresis region, then the motor torque Tm may momentarily reach the stop line, and there is a risk that the operation for avoiding muffled noise may be stopped, despite the fact that the cause of the muffled noise has not been completely eliminated. In order to avoid a problem of this kind, when the motor torque Tm is in the muffled noise generation region, then the first hysteresis region, which is relatively large, may be set as the hysteresis region, whereby the stopping of the operation for avoiding muffled noise may be delayed. [0036] Furthermore, the use of a relatively small second hysteresis region is exceptional processing, and in principle, the relatively large first hysteresis region should be used. Therefore, as shown in FIG. 6, the setting may be returned always to the first hysteresis region, when the motor torque Tm reaches the stop line and the operation for avoiding the muffled noise is stopped.

[0037] Furthermore, the description given thus far relates to a configuration which reduces the hysteresis region, in other words; makes only the stop line close the muffled noise generation region, in the case of Pe keeping control, the start line may also be closer to the muffled noise generation region, instead of the stop line or in addition to the stop line. For example, as shown in FIG. 7, when Pe keeping control is not being implemented, then the start line Lb is set to a line indicating a value b obtained by adding a first margin region B to the muffled noise generation region A, and the stop line Lc is set to a line indicating a value c obtained by adding a first hysteresis region C to the value b.

[0038] On the other hand, when Pe keeping control is being implemented, then the start line Le may be set to a line indicating a value e obtained by adding a second margin region E (E < B) to the muffled noise generation region A. In this case, the stop line which is the reference line for stopping the operation for avoiding muffled noise is desirably set to a stop line Ld which indicates a value d obtained by adding a second hysteresis region D (D < C) to the start line Le. In this case, the operation for avoiding muffled noise is implemented from time t4 to time t5 in FIG. 7. By narrowing the hysteresis region rather than just the margin regions in this way, it is possible to shorten the continuation time of the operation for avoiding muffled noise even more, and keep the motor torque Tm even smaller, which means that fuel consumption can be improved further. When the margin region is changed depending on whether or not the engine output keeping control is being implemented, then prior to step S10 in FIG. 5, it is determined whether or not the engine output keeping control is being implemented; when the engine output keeping control is being implemented, the second start line Le is set as the start line, and when the engine output keeping control is not being implemented, the first start line Lb is set as the start line. [0039] Furthermore, when the start line is made to approach the muffled sound generation region, then the stop line is set to a value g obtained by adding the same first hysteresis region C to the value during normal control. In this case, the operation for avoiding muffled noise is implemented from time t4 to time t6 in FIG. 7. In this case also, since the stop line Lg can be kept to a small value compared to during normal control, then it is possible to keep the motor torque Tm small, and hence the fuel consumption can be improved.

[0040] Furthermore, when the start line is made to approach the muffled noise generation region, then the stop line can be set to the same line Lc as during normal control. In this case, the operation for avoiding muffled noise is implemented from time t4 to time t3 in FIG. 7. In other words, in this case also, it is possible to shorten the continuation time of the operation for avoiding muffled noise, compared to normal control.

Furthermore, when the start line itself is set to a lower value, then even when there is no change in the value of the stop line, the motor torque Tm is not liable to reach the start line Le, and therefore it is possible to make the actual operation for avoiding muffled noise less liable to occur. By reducing the possibility of the occurrence of an operation for avoiding muffled noise, which tends to make the fuel consumption deteriorate, it is possible to improve fuel consumption.

[0041] As is clear from the description give above, when the engine output keeping control is being implemented, compared to when the engine output keeping control is not being implemented, it is possible to shorten the continuation time of the operation for avoiding muffled noise, by making at least one of the start line and the stop line closer to the muffled noise generation region, and as a result of this, it is possible to improve fuel consumption.