Control device for hybrid vehicle

A control device for a hybrid vehicle is equipped with an engine, an electric motor, and a damper that is interposed in a motive power transmission path between the engine and the electric motor. The control device is equipped with a hysteresis mechanism and a controller. The hysteresis mechanism is provided in the damper, is configured to have a characteristic that a hysteresis torque that is generated due to a twist of the damper, which transmits a driving force from the electric motor toward the engine, in a negative direction is larger than a hysteresis torque that is generated due to a twist of the damper, which transmits a driving force from the engine toward the electric motor, in a positive direction, and is configured to reduce an engine rotational speed by the electric motor in stopping the engine. The controller is configured to cause a torque, of the electric motor to be output such that the damper assumes a state of being twisted in the negative direction when a torque in such a direction as to drive the engine is applied from the electric motor in stopping the engine.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2012-268185 filed on Dec. 7, 2012 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a control device for a hybrid vehicle that is configured to include an engine, an electric motor, and a damper that is interposed in a motive power transmission path between the engine and the electric motor.

2. Description of Related Art

A hybrid vehicle that is configured to include an engine, an electric motor, and a damper that is equipped with a hysteresis mechanism in a motive power transmission path between the engine and the electric motor is well known. For example, a motive power transmission device described in Japanese Patent Application Publication No. 2006-29363 (JP-2006-29363 A) is also one such example. In a damper of Japanese Patent Application Publication No. 2006-29363 (JP-2006-29363 A), a large hysteresis torque is generated upon fluctuations of a twist angle in a negative-side twist range where a torque is transmitted from driving wheel sides. Thus, abrupt torque fluctuations that are caused in starting and stopping the engine are effectively damped. Besides, torque fluctuations during engine steady operation are effectively damped by generating a small hysteresis torque upon fluctuations in the twist angle in a positive-side twist range where a torque is transmitted from the engine side.

Besides, in a hybrid vehicle in general, the start and stop of an engine are frequently repeated. A plurality of control methods regarding the control of starting and stopping this engine have been proposed. For example, in a vehicular motive power transmission device described in Japanese Patent Application No. 2010-167921, a negative torque in such a direction as to reduce the rotational speed of an engine is output from an electric motor in stopping the engine, the torque is removed immediately before stop of the engine, and a positive torque in such a direction as to increase the rotational speed of the engine is slightly output to prevent reverse rotation of the engine.

By the way, it is known that if the negative torque of the electric motor that is output to reduce the rotational speed of the engine is removed immediately before the stop of rotation as in Japanese Patent Application No. 2010-167921 during stop control of the engine, the reactive force resulting from compression in a combustion chamber of the engine cannot be suppressed, the magnitude of torque fluctuations increases, and gear rattle noise is generated as a result of the torque fluctuations. Besides, the damper for the hybrid vehicle of Japanese Patent Application Publication No. 2006-29363 (JP-2006-29363 A) has a characteristic that a small hysteresis torque is generated if the relative twist angle is equal to or smaller than a predetermined value in a twist in a positive direction in which a torque is transmitted from the engine side. In a damper having a twist characteristic (a hysteresis characteristic) as in this Japanese . Patent Application Publication No. 2006-29363 (JP-2006-29363 A), in the case where engine stop control as in Japanese Patent Application No. 2010-167921 is performed, if the removal of the torque is started from a state where the torque of the electric motor is a negative torque, the range where the small hysteresis torque is generated is utilized. Besides, even if a positive torque is output from the electric motor, the value thereof is small. Therefore, the damper may be twisted in the positive direction upon vibrations thereof, and the small hysteresis torque is generated at that time. Accordingly, the torque fluctuations that are caused in stopping the engine cannot be effectively damped by the large hysteresis torque, and it is difficult to suppress gear rattle noise resulting from the torque fluctuations.

SUMMARY OF THE INVENTION

The invention has been made in view of the foregoing circumstances. In a hybrid vehicle that is configured to include an engine, an electric motor, and a damper that is interposed between the engine and the electric motor, there is provided a control device for the hybrid vehicle that can suppress gear rattle noise in stopping the engine.

Thus, according to one aspect of the invention, there is provided a control device for a hybrid vehicle that is equipped with an engine, an electric motor, and a damper that is interposed on a power transmission path between the engine and the electric motor. The control device is equipped with a hysteresis mechanism and a controller. The hysteresis mechanism is provided in the damper, is configured to have a characteristic that a hysteresis torque generated due to a twist of the damper in a negative direction in a case where the damper transmits a driving force from the electric motor toward the engine, is larger than a hysteresis torque generated due to a twist of the damper in a positive direction in a case where the damper transmits a driving force from the engine toward the electric motor, and the hysteresis mechanism is configured to reduce an engine rotational speed by the electric motor in stopping the engine. The controller is configured to cause a torque of the electric motor to be output such that the damper is twisted in the negative direction when a torque is supplied from the electric motor to the engine in a direction for driving the engine in stopping the engine.

In stopping the engine, if a negative torque is output from the electric motor to reduce the engine rotational speed and then is removed to prevent reverse rotation of the engine, a reactive force resulting from compression of the engine cannot be suppressed, and the magnitude of torque fluctuations increases. As a measure against this phenomenon, according to the control device for the hybrid vehicle as described above, a torque is output from the electric motor until the damper assumes a state of being twisted in the negative direction. Therefore, a range where the hysteresis torque is large can be utilized, and the magnitude of torque fluctuations can be effectively reduced by the hysteresis torque. Accordingly, the magnitude of torque fluctuations caused in stopping the engine can be reduced. Therefore, gear rattle noise generated at that time can be suppressed.

Besides, in the control device for the hybrid vehicle, a torque of the electric motor that causes the damper in a state of being twisted in the negative direction may be set to a value that ensures a twist greater than a twist resulting from torque fluctuations caused in stopping the engine. In this manner, in stopping the engine, the damper always assumes a state of being twisted in the negative direction. Therefore, the range where the hysteresis torque is large can be reliably utilized. Accordingly, torque fluctuations can be effectively damped by this hysteresis torque.

Besides, a magnitude of a torque at a time when the engine rotational speed is reduced by the electric motor may be changed in accordance with a magnitude of a torque output from the electric motor in a direction for driving the engine, in stopping the engine. In stopping the engine, if the magnitude of a torque that is output from the electric motor in a direction for driving the engine increases, it becomes difficult to stop the engine within a predetermined time. Thus, the torque output from the electric motor to reduce the engine rotational speed is changed in accordance with the torque output from the electric motor in a direction for driving the engine, whereby the engine rotational speed is swiftly reduced, and the engine can be stopped within a predetermined time.

DETAILED DESCRIPTION OF EMBODIMENT

The embodiment of the invention will be described hereinafter in detail with reference to the drawings. In the following embodiment of the invention, the drawings are simplified or modified when appropriate, and the dimensional ratios, shapes and the like of respective portions are not necessarily depicted with accuracy. It should be noted herein that in the embodiment of the invention that will be described below, a twist of a damper, which transmits a driving force from an engine toward an electric motor, in a positive direction is treated as a twist at the time when a torque of the engine is transmitted to the electric motor side via the damper. The same twist also occurs in the case where the engine is stopped from the electric motor side, namely, a torque in such a direction as to reduce the engine rotational speed has been transmitted via the damper.

Besides, a twist of the damper, which transmits a driving force from the electric motor toward the engine, in a negative direction is treated as a twist at the time when the engine is driven from the electric motor, namely, a torque in such a direction as to increase the engine rotational speed is transmitted via the damper.

FIG. 1is a schematic configurational view illustrating a vehicular drive unit10for a hybrid vehicle8to which the invention is applied. The vehicular drive unit10is configured to include an engine24, a motive power transmission device12, and a later-described damper38that is provided between the engine24and the motive power transmission device12. Referring toFIG. 1, this vehicular drive unit10is designed such that in the vehicle, a torque of the engine24as a main driving source is transmitted to a wheel-side output shaft14via the damper38and a planetary gear train26, which will be described later, and then is transmitted from the wheel-side output shaft14to a pair of right and left driving wheels18via a differential gear mechanism16. Besides, this vehicular drive unit10is provided with a second electric motor MG2that can selectively perform power running control for outputting a driving force for running and regeneration control for recovering energy. This second electric motor MG2is coupled to the aforementioned wheel-side output shaft via an automatic transmission22. Accordingly, an output torque that is transmitted from the second electric motor MG2to the wheel-side output shaft is increased or reduced in accordance with a speed ratio γs that is set in the automatic transmission22(=a rotational speed Nmg2of the second electric motor MG2/a rotational speed Nout of the wheel-side output shaft).

The automatic transmission22that is interposed in a motive power transmission path between the second electric motor MG2and the driving wheels18is configured such that a plurality of gear stages with the speed ratio γs larger than “1” can be established. During power running in which a torque is output from the second electric motor MG2, the torque can be increased and transmitted to the wheel-side output shaft. Therefore, the second electric motor MG2is configured with an even lower capacity or in an even smaller size. Thus, if the rotational speed Nout of the wheel-side output shaft increases as a result of, for example, a high vehicle speed, the speed ratio γs is reduced to reduce the rotational speed of the second electric motor MG2(hereinafter referred to as a second electric motor rotational speed) Nmg2, with a view to holding the operation efficiency of the second electric motor MG2in a good condition. Besides, if the rotational speed Nout of the wheel-side output shaft decreases, the speed ratio γs is increased to increase the second electric motor rotational speed Nmg2.

The aforementioned motive power transmission device12is configured to be equipped with the first electric motor MG1and the second electric motor MG2, and transmits a torque of the engine24to the driving wheels18. The aforementioned engine24is a known internal combustion engine that burns fuel for a gasoline engine, a diesel engine or the like and outputs a motive power, and is configured such that the operation state such as a throttle valve opening degree, an intake air amount, a fuel supply amount, an ignition timing and the like is electrically controlled by an electronic control unit (an E-ECU) for engine control (not shown), which is mainly constituted of a microcomputer. Detection signals from an accelerator operation amount sensor AS that detects an operation amount of an accelerator pedal, a brake sensor BS for detecting whether or not a brake pedal has been operated, and the like are supplied to the aforementioned electronic control unit.

The aforementioned first electric motor MG1(the electric motor) is, for example, a synchronous electric motor, and is configured to selectively create a function as an electric motor that generates a drive torque Tm1and a function as an electric generator. The first electric motor MG1is connected to an electric storage device32such as a battery, a capacitor or the like, via an inverter30. In addition, the inverter30is controlled by an electronic control unit (an MG-ECU) for motor-generator control (not shown), which is mainly constituted of a microcomputer, whereby an output torque Tm1or a regenerative torque Tm1of the first electric motor MG1is adjusted or set. Incidentally, the first electric motor MG1corresponds to the electric motor of the invention.

The planetary gear train26is a single pinion-type planetary gear mechanism that is equipped with a sun gear S0, a ring gear R0and a carrier CA0as three rotary elements to cause a known differential effect. The ring gear R0is arranged concentrically with the sun gear S0. The, carrier CA0supports a pinion gear P0that meshes with this sun gear S0and this ring gear R0, such that the pinion gear P0can rotate around its own axis and around the carrier CA0. The planetary gear train26is provided concentrically with the engine24and the automatic transmission22. Each of the planetary gear train26and the automatic transmission22is configured symmetrically with respect to a centerline, and hence the lower half thereof is omitted inFIG. 1.

In the embodiment of the invention, a crankshaft36of the engine24is coupled to the carrier CAO of the planetary gear train26via the damper38and the motive power transmission shaft39. On the other hand, the first electric motor MG1is coupled to the sun gear S0, and the wheel-side output shaft is coupled to the ring gear R0. This carrier CA0functions as an input element, the sun gear S0functions as a reactive force element, and the ring gear R0functions as an output element.

In the aforementioned planetary gear train26, if a reactive torque Tm1generated by the first electric motor MG1is input to the sun gear S0in response to an output torque of the engine24that is input to the carrier CA0, a direct delivery torque appears in the ring gear R0as an output element. Therefore, the first electric motor MG1functions as an electric generator. Besides, when the rotational speed of the ring gear R0, namely, the rotational speed of the wheel-side output shaft14(an output shaft rotational speed) Nout is constant, the rotational speed Nmg1of the first electric motor MG1is changed to be increased or reduced, whereby a rotational speed of the engine24(the engine rotational speed) Ne can be continuously (steplessly) changed.

The automatic transmission22according to the embodiment of the invention is constituted of a pair of Ravigneaux-type planetary gear mechanisms. That is, the automatic transmission22is provided with a first sun gear Si and a second sun gear S2, a large-diameter portion of a stepped pinion P1meshes with the first sun gear S1, a small-diameter portion of the stepped pinion P1meshes with a pinion P2, and the pinion P2meshes with a ring gear R1(R2) that is arranged concentrically with each of the sun gears Si and S2. Each of the aforementioned pinions P1and P2is retained by a common carrier CA1(CA2) in a manner rotatable around its own axis and around the carrier CA1(CA2). Besides, the second sun gear S2meshes with the pinion P2.

The second electric motor MG2(an electric motor) is caused to function as an electric motor or an electric generator by being controlled by the electronic control unit for motor-generator control (an MG-ECU) via an inverter40, so that an assist output torque or a regenerative torque is adjusted or set. The second electric motor MG2is coupled to the second sun gear S2, and the aforementioned carrier CA1is coupled to the wheel-side output shaft. The first sun gear S1and the ring gear R1constitute, together with the respective pinions P1and P2, a mechanism equivalent to a double pinion-type planetary gear train. Besides, the second sun gear S2and the ring gear R1constitute, together with the pinion P2, a mechanism equivalent to a single pinion-type planetary gear train.

In addition, the automatic transmission22is provided with a first brake

B1that is provided between the first sun gear S1and a housing42as a non-rotary member to selectively fix the first sun gear S1, and a second brake B2that is provided between the ring gear R1and the housing42to selectively fix the ring gear R1. Each of these brakes B1and B2is a so-called frictional engagement device that generates a braking force through the use of a frictional force, and can adopt a multidisc-type engagement device or a band-type engagement device. In addition, each of these brakes B1and B2is configured such that the torque capacity thereof continuously changes in accordance with an engagement pressure that is generated by a corresponding one of a brake B1hydraulic actuator and a brake B2hydraulic actuator, which are hydraulic cylinders or the like respectively.

The automatic transmission22configured as described above is configured such that the second sun gear S2functions as an input element, that the carrier CA1functions as an output element, that a high shift speed H with a speed ratio γsh larger than “1” is established if the brake B1is engaged, and that a low shift speed L with a speed ratio γs1larger than the speed ratio γsh of the high shift speed H is established if the second brake B2is engaged instead of the first brake B1. That is, the automatic transmission22is a two-staged transmission, and shifting between these shift speeds H and L is carried out on the basis of a running state such as a vehicle speed V, a required driving force (or an accelerator operation amount) or the like. More specifically, shift speed ranges are determined in advance as a map (a shift diagram), and control is performed in such a manner as to set one of the shift speeds in accordance with a detected operation state.

FIG. 2is a cross-sectional view for illustrating the configuration of the damper38shown inFIG. 1in detail. The damper38is provided between the engine24and the planetary gear train26around an axis of rotation C, such that a motive power can be transmitted. Incidentally, a motive power transmission shaft39shown inFIG. 1is spline-fitted in an inner peripheral portion of the damper38. Incidentally, the first electric motor MG1is coupled to the damper38via the planetary gear train26, such that a motive power can be transmitted. Therefore, the damper38is interposed in a motive power transmission path between the engine24and the first electric motor MG1.

The damper38is configured to include a pair of disc plates56, a hub58, coil springs62, cushions63, a first hysteresis mechanism64, a second hysteresis mechanism65, and a torque limiter mechanism68. The disc plates56can rotate around the axis of rotation C. The hub58can rotate around the axis of rotation C relatively to the disc plates56. The coil springs62are interposed between the disc plates56and the hub58, couples the disc plates56and the hub58to each other such that a motive power can be transmitted, and is made of spring steel. The cushions63are incorporated in the coil springs62respectively. The first hysteresis mechanism64generates a small hysteresis torque H1between the disc plates56and the hub58. The second hysteresis mechanism65is provided at an outer peripheral end of the hub58, and generates a hysteresis torque H2larger than the small hysteresis torque H1between the disc plates56and the hub58. The torque limiter mechanism68is provided on an outer peripheral side of the disc plates56. Incidentally, the first hysteresis mechanism64and the second hysteresis mechanism65constitute the hysteresis mechanism of the invention.

The disc plates56are constituted by a pair of right and left discoid plates, namely, a first, disc plate70(hereinafter the first plate70) and a second disc plate72(hereinafter the second plate72), and outer peripheral portions thereof are fastened to each other by a rivet66such that relative rotation therebetween is impossible, with the coil springs62and the hub58sandwiched by the plates70and72in an axial direction. Incidentally, the rivet66also functions as a fastening member for a lining plate76as a component of the torque limiter mechanism68, which will be described later. A plurality of first opening holes70afor accommodating the coil springs62are formed through the first plate70in a circumferential direction thereof. Besides, a plurality of opening holes72afor accommodating the coil springs62are formed through the second plate72as well in a circumferential direction thereof; at positions corresponding to the first opening holes70arespectively. In addition, a plurality of the coil springs62are accommodated at equal angular intervals in a space that is formed by the first opening holes70aand the second opening holes72a. Thus, if the disc plates56rotate around the axis of rotation C, the coil springs62are also caused to rotate around the axis of rotation C in a similar mariner.

Besides, the columnar cushions63are incorporated in the coil springs62respectively.

The hub58is constituted of a cylinder portion58a, a circular plate-like flange portion58b, and a plurality of protrusion portions58c. The cylinder portion58ais equipped, in an inner peripheral portion thereof; with inner peripheral teeth to which the motive power transmission shaft39is spline-fitted. The flange portion58bextends radially outward from an outer peripheral face of the cylinder portion58a. The protrusion portions58cprotrude radially further outward from the flange portion58b. In addition, the coil springs62are interposed in spaces that are formed among the respective protrusion portions58cin a rotational direction. Thus, if the hub58rotates around the axis of15. rotation C, the coil springs62are also caused to rotate around the axis of rotation C in a similar manner. By being thus configured, the coil springs62transmit a motive power while being elastically deformed in accordance with an amount of relative rotation between the members of the disc plates56and the hub58. For example, if the disc plates56rotate, one end of each of the coil springs62is pressed, and the other end of each of the coil springs62presses a corresponding one of the protrusion portions58cof the hub58, so that the hub58is rotated. At this time, the coil springs62transmit a motive power while being elastically deformed, whereby a shock resulting from torque fluctuations is absorbed by the coil springs62.

The first hysteresis mechanism64is provided between the disc plates56and the flange portion58bof the hub58in the axial direction, on the inner peripheral side of the coil springs62. In addition, the hysteresis mechanism64is configured to include a first member64a, a second member64b, and a disc spring64c. The first member64ais interposed between the first plate70and the flange portion58b. The second member64bis interposed between the second plate72and the flange portion58b. The disc spring64cis interposed in a preloaded state between the second member64band the second plate72, and presses the second member64btoward the flange portion58bside. Incidentally, part of the first member64ais fitted in a notch that is formed in the first plate70, whereby the first member64aand the first plate70are prevented from rotating relatively to each other. Besides, part of the second member64bis fitted in a notch that is formed in the second plate72, whereby the second member64band the second plate72are prevented from rotating relatively to each other. In the first hysteresis mechanism64configured as described above, when the hub58slides with respect to the disc plates56, a hysteresis torque is generated due to the generation of a frictional force between the flange portion58bon the one hand and the first plate70and the second plate72on the other hand.

Incidentally, the first hysteresis mechanism64is designed such that a relatively small hysteresis torque H1(a small hysteresis) is generated in a positive-side twist angle range and a negative-side twist angle range. This small hysteresis torque H1is advantageous in damping torsional vibrations with a relatively small amplitude, which are caused during idle operation or steady operation of the engine.

The torque limiter mechanism68is provided on the outer peripheral side of the disc plates56, and has a function of preventing the transmission of a torque exceeding a preset limit torque Tlm. The torque limiter mechanism68is configured to include an annular plate-like lining plate76, a support plate78, an annular plate-like pressure plate80, a first frictional material81, a second frictional material82, and a conical disc spring83. The lining plate76rotates together with the disc plates56by being fastened by the rivet66together with the disc plates56. The support plate78is arranged on the outer peripheral side, and can rotate around the axis of rotation C. The pressure plate80is arranged on the inner peripheral side of the support plate78, and can rotate around the axis of rotation C. The first frictional material81is interposed between the pressure plate80and the lining plate76. The second frictional material82is interposed between the lining plate76and the support plate78. The disc spring83is interposed in a preloaded state between the pressure plate80and the support plate78.

The support plate78is constituted of a discoid first support plate78aand a circular plate-like second support plate78b. Bolt holes for bolt fastening (not shown) for fixing a flywheel (not shown) to the support plates78aand78bare formed through an outer peripheral portion of the support plate78respectively. An inner peripheral portion of the first support plate78ais axially flexed, so that a space is formed between the first support plate78aand the second support plate78b. In this space, the disc spring83, the pressure plate80, the first frictional material81, the lining plate76, and the second frictional material82are accommodated in this order in the axial direction from the first support plate78atoward the second support plate78b.

The lining plate76is an annular plate-like member whose inner peripheral portion is fixed together with the first plate70and the second plate72by the rivet66.

Besides, the pressure plate80is also formed in a similar manner in the shape of an annular plate. The first frictional material81is interposed between this pressure plate80and the lining plate76. The first frictional material81is formed in the shape of, for example, an annular plate. Alternatively, the first frictional material81may be formed in the shape of circular arcs (in the shape of pieces), and these pieces may be arranged at equal angular intervals in the circumferential direction. Incidentally, this first frictional material81is stuck to the lining plate76side, but may be stuck to the pressure plate80side.

Besides, the second frictional material82is interposed between the inner peripheral portion of the second support plate78band the lining plate76. The second frictional material82is formed in the shape of, for example, an annular plate, as is the case with the first frictional material81. Alternatively, the second frictional material82may be formed in the shape of circular arcs (in the shape of pieces), and these pieces may be arranged at equal angular intervals in the circumferential direction. Incidentally, this second frictional material82is stuck to the lining plate76side, but may be stuck to the second support plate78bside.

The disc spring83is interposed in a preloaded state between the first support plate78aand the pressure plate80. The disc spring83is conically formed, has an inner peripheral end abutting on the pressure plate80and an outer peripheral end abutting on the first support plate78a, and is interposed after being deformed to such a flexure amount as to cause the preload (a disc spring load W). Accordingly, the disc spring83axially presses the pressure plate80toward the lining plate76side with the disc spring load W. Then, the limit torque Tlm is set to a target value by adjusting a friction coefficient p. of a friction surface between the pressure plate80and the first frictional material81and a friction surface between the second support plate78band the second frictional material82, an operation radius r of the frictional materials81and82, and the disc spring load W of the disc spring83. Then, if a torque exceeding the limit torque Tlm is input to the torque limiter mechanism68, slippage occurs on the frictional surface between the pressure plate80and the first frictional material81, and on the frictional surface between the second support plate78band the second frictional material82. As a result, the transmission of a torque exceeding the limit torque Tlm is prevented.

The second hysteresis mechanism65is a mechanism that is provided at outer peripheral portions of the hub58and the disc plates56, and generates sliding resistance (a frictional force) therebetween to generate the hysteresis torque H2larger than the small hysteresis torque H1generated by the first hysteresis mechanism64. Besides,

FIG. 3is a partially cut-away A-arrow view of the damper38ofFIG. 2as viewed from the direction of an arrow A. Besides, part ofFIG. 3is represented as a perspective view. As shown inFIGS. 2 and 3, on both faces substantially parallel to the disc plates56on the outer peripheral sides of the protrusion portions58cof the hub58, rectangular (piece-shaped) friction plates90that are made of, for example, a resin material or the like are fixed by rivets92respectively.

Besides, as shown inFIG. 3, an L-shaped notch94is formed in the second plate72. The notch94extends from an outer peripheral end of the second plate72toward an inner peripheral side, and is further formed from the inner peripheral portion along the circumferential direction (the rotational direction). Due to the formation of this notch94, a fan-shaped cantilever portion96that is parallel to the rotational direction is formed on the second plate72. The cantilever portion96is formed at the same radial position as a region where the friction plates90of the protrusion portions58care fixed. Furthermore, the cantilever portion96is formed in a tapered manner with a predetermined gradient S toward the hub58side (the friction plate90sides) along the rotational direction. Accordingly, if the hub58and the second plate72rotate relatively to each other, the friction plates90and the cantilever portion96abut on each other and start to slide, as the coil springs62are compressed. Incidentally, although not shown inFIG. 3, a cantilever portion98that is formed in a shape similar to that of the second plate72is formed on the first plate70shown inFIG. 2as well.

FIG. 4is a view showing, in a more simplified manner, especially the periphery of the cantilever portion96of the second plate72in the damper38ofFIG. 3. Incidentally, the second plate72actually has a discoid shape, butFIG. 4is a view in which the second plate72is rectilinearly deployed. Accordingly, by the same token, the protrusion portions58cof the hub58, which are indicated by broken lines, also actually rotate around the axis of rotation C, but move rectilinearly (in the lateral direction inFIG. 4) inFIG. 4. Besides, a view shown in the upper portion ofFIG. 4is a lateral view of the cantilever portion96and the protrusion portions58c, which are shown below. Incidentally, the friction plates90that are fixed to the protrusion portions58care omitted inFIG. 4.

As is also apparent from the lateral view ofFIG. 4, the cantilever portion96is inclined with a predetermined gradient S. Accordingly, if the, protrusion portions58c(the hub58) and the second plate72are rotated relatively to each other and the protrusion portions58ccome into abutment on the cantilever portion96, the protrusion portions58cand the second plate72are slid with respect to each other. Specifically, if the protrusion portions58cmove leftward relatively to the second plate72inFIG. 4, the protrusion portions58cand the cantilever portion96come into abutment on each other in association with the formation of the cantilever portion96in a tapered manner, and are slid with respect to each other while the hub58presses the cantilever portion96as the twist angle θ changes. Incidentally, although the cantilever portion96of the second plate72is shown inFIGS. 3 and 4, the cantilever portion98of the first plate70is also slid in a similar manner.

In this manner, if the protrusion portions58cand the cantilever portions96and98are slid with respect to each other respectively, a frictional force is generated between the friction plates90, which are fixed to the protrusion portions58crespectively, and the cantilever portions96and98respectively, and the hysteresis torque H2corresponding thereto is generated. That is, the cantilever portions96and98have the functions of both a disc spring and a sliding member in a conventional hysteresis mechanism. This hysteresis torque H2is set to the target hysteresis torque H2by adjusting the board thicknesses of the friction plates90and the hub58, the clearance between the first plate70and the second plate72, the shape of the notches formed in the first plate70and the second plate72, the gradient S (a tapered angle) of the cantilever portions96and98of the first plate70and the second plate72, and the like to adjust the pressing load applied to the friction plates90. Besides, the second hysteresis mechanism65is arranged on the outer peripheral side in the radial direction with respect to the first hysteresis mechanism64. Therefore, the hysteresis torque H2that is larger than the small hysteresis torque H1can be generated. Incidentally, the twist angle θ at which the generation of the hysteresis torque H2begins can also be appropriately adjusted by adjusting the shape of the notches and the gradient S of the cantilever portions96and98.

It should be noted herein that the second hysteresis mechanism65of this embodiment of the invention is set such that the hysteresis torque H2is generated in the case where a torque (a driving force) in such a direction as to drive the engine24(in such a direction as to increase the rotational speed of the engine) has been transmitted from the driving wheel sides (the first electric motor side) toward the engine24, namely, the damper38has been twisted in the negative direction (toward the negative side). That is, the friction plates90are set in such a manner as to slide with respect to the cantilever portions96and98when a torque in such a direction as to drive the engine has been transmitted from the driving wheel sides (the first electric motor side). On the other hand, the friction plates90and the cantilever portions96and98are set in such a manner as not to slide with respect to each other respectively in the case where the damper38, to which a torque (a driving force) is transmitted from the engine side, has been twisted in the positive direction (toward the positive side).

For example, inFIG. 3, if the hub58is set in such a manner as to rotate counterclockwise (such that the protrusion portions58cmove leftward inFIG. 4) when a torque is transmitted from the driving wheel sides (in the negative-side twist angle range), the cantilever portion96and the friction plates90are slid with respect to each other as the twist angle0changes. On the other hand, if the hub58is set in such a manner as to rotate clockwise inFIG. 3(such that the protrusion portions58cmove rightward inFIG. 4) when a torque is transmitted from the engine side (in the positive-side twist angle range), the friction plates90move away from the cantilever portion96. Therefore, even if the twist angle θ changes, the cantilever portion96and the friction plates90do not slide with respect to each other. Accordingly, in the twist angle range in the positive direction (on the positive side) of the damper38where a torque (a driving force) is transmitted from the engine side toward the driving wheel sides, the hysteresis torque H2is not generated by the second hysteresis mechanism65. In the twist angle range in the negative direction (on the negative side) of the damper38where a torque in a driving direction (a driving force) is transmitted from the driving wheel sides toward the engine24, the hysteresis torque H2is generated by the second hysteresis mechanism65.

FIG. 5shows a twist characteristic of the damper38according to this embodiment of the invention. Incidentally, the axis of abscissa represents the twist angle θ (rad), and the axis of ordinate represents a torque (Nm). As shown inFIG. 5, in a twist angle range where the twist angle θ is in the positive direction (on the positive side), namely, a torque (a driving force) is transmitted from the engine side, the small hysteresis torque H1is generated. As described above, this is because only the first hysteresis mechanism64operates and the second hysteresis mechanism65does not operate. On the other hand, in the twist angle range where the twist angle θ is on the negative side, namely, a torque applied to the engine drive side is transmitted from the driving wheel sides, the second hysteresis mechanism65operates. Therefore, a large hysteresis torque (H1+H2) as a sum of the small hysteresis torque H1and the hysteresis torque H2is generated.

In the hybrid vehicle8that is equipped with the damper38configured as described above, when the engine24is stopped, engine stop control by the first electric motor MG1is performed. Specifically, if it is determined that the engine24should be stopped, a negative torque Tm1is output from the first electric motor MG1, whereby a torque for stopping the engine24is transmitted via the damper38due to the differential effect of the planetary gear train26. Accordingly, the engine rotational speed Ne decreases. Then, immediately before the stop of the engine24, control for removing the negative torque Tm1that is output from the first electric motor MG1is performed to prevent reverse rotation of the engine24.FIG. 6includes time charts showing an operation state of the engine rotational speed Ne and the torque Tm1of the first electric motor MG1(the MG1torque) at the time when conventionally performed engine stop control is performed. Referring toFIG. 6, if it is determined that the engine24should be stopped at a time point t1, the negative torque Tm1is output from the first electric motor MG1, whereby a torque in such a direction as to stop the engine24is transmitted to the engine24due to a differential effect of the planetary gear train26, and the engine rotational speed Ne decreases. Then, at a time point t2when the engine rotational speed Ne becomes equal to or lower than a predetermined value, the removal of the negative torque Tm1of the first electric motor MG1is started, and the torque Tm1of the first electric motor MG1is increased to a positive value in the vicinity of zero. If the torque is thus removed, the reactive force resulting from compression in the combustion chamber of the engine24cannot be suppressed, the magnitude of torque fluctuations increases, and gear rattle noise is generated in the motive power transmission device12. Incidentally, in this embodiment of the invention, the negative torque Tm1of the first electric motor MG1is a torque that is applied reversely to the engine rotational direction, and the positive torque Tm1of the first electric motor MG1is a torque that is applied in the same direction as engine rotation.

It should be noted herein that when the first electric motor MG1outputs the negative torque Tm1, a torque that reduces the engine rotational speed Ne is transmitted from the driving wheel sides (the first electric motor side) to the damper38, so that the damper38assumes a state of being twisted in the same positive direction (on the positive side) as in the state in which a driving force in a driving direction is transmitted from the engine24toward the driving wheel sides. That is, the twist angle θ of the damper38is in the positive range. Accordingly, the twist angle θ of the damper38is in the range where the small hysteresis torque H1shown inFIG. 5is generated. On the other hand, when the first electric motor MG1outputs the positive torque Tm1, a torque that increases the engine rotational speed Ne from the driving wheel sides (the first electric motor side) toward the engine24is transmitted to the damper38, and hence the damper38assumes a state of being twisted in the negative direction (on the negative side). That is, the twist angle θ of the damper38assumes a negative value. Accordingly, the twist angle θ of the damper38is in the range where the large hysteresis torque (H1+H2) shown inFIG. 5is generated.

Thus, at and after the time point t2ofFIG. 6, the removal of the negative torque of the first electric motor MG1starts to make a changeover to a positive torque. At this time, the operation range of the hysteresis torque is a spot A shown inFIG. 5. Incidentally, even if the torque Tm1of the first electric motor MG1is changed over to a positive torque, the value thereof is small. Therefore, if the damper38is greatly twisted due to torque fluctuations, the damper38may assume a state of being twisted in the positive direction (on the positive side), and the small hysteresis torque may be generated in that case. Thus, if the removal of the torque Tm1of the first electric motor MG1is started during stop of the engine, the magnitude of torque fluctuations increases, but the large hysteresis torque (H1+H2) cannot be generated at this time, so that it is difficult to damp these torque fluctuations by the large hysteresis torque. Accordingly, there is a problem in that it is difficult to suppress gear rattle noise as shown at and after the time point t2ofFIG. 6, which is generated as a result of these torque fluctuations.

Thus, in this embodiment of the invention, the torque Tm1of the first electric motor MG1is controlled in stopping the engine, and the range of a spot B shown inFIG. 5is used. Thus, the large hysteresis torque (H1+H2) is reliably generated to reduce the magnitude of torque fluctuations and suppress gear rattle noise.

Referring back toFIG. 1, the electronic control unit100is configured to include, for example, a so-called microcomputer that is equipped with a CPU, a RAM, a ROM, input/output interfaces and the like. The CPU performs a signal processing according to a program stored in advance in the ROM while utilizing a temporary storage function of the RAM, thereby performing various kinds of control of the vehicle8. For example, the electronic control unit100performs output control of the engine24, drive control and regenerative control of the first electric motor MG1and the second electric motor MG2, shift control of the automatic transmission22, and the like, and is configured separately for engine control, electric motor control, hydraulic control (shift control) and the like according to need. Besides, the electronic control unit100is functionally equipped with an engine stop control unit102that performs stop control of the engine24as an essential part of the invention.

The engine stop control unit102is activated if it is determined that the engine24should be stopped, for example, if a changeover to motor running is made during engine running. If it is determined that the engine24should be stopped, the engine stop control unit102stops the supply of fuel to the engine24, and outputs to the inverter30a command to output the negative torque Tm1from the first electric motor MG1. Thus, a torque in such a direction as to stop the engine24is transmitted from the carrier CAO to the engine24via the damper38due to a differential effect of the planetary gear train26, and hence the engine rotational speed Ne decreases. Then, if the engine rotational speed Ne becomes equal to or lower than a predetermined value that is set in advance or if a determined time that is set in advance elapses since the start of engine stop control, the engine stop control unit102starts removing the negative torque Tm1of the first electric motor MG1, and further increases the torque Tm1in the positive direction (toward the positive side) to a predetermined value c at which the large hysteresis torque (H1+H2) is generated. This predetermined value α of the first electric motor MG1is set to a torque at which the damper38is twisted in the negative direction (toward the negative side) to a range where the large hysteresis torque can be sufficiently used, for example, a range where the damper38operates at the spot B ofFIG. 5, or the like.

FIG. 7includes time charts illustrating a result of operation performed by the engine stop control unit102. Incidentally, solid lines indicate the operation result of control according to this embodiment of the invention, and an alternate long and short dash line shows the operation result of conventional control. If it is determined that the engine24should be stopped at the time point t1ofFIG. 7, the negative torque Tm1is output from the first electric motor MG1, whereby the engine rotational speed Ne decreases. Then, if the engine rotational speed Ne becomes equal to or lower than a predetermined value or the elapsed time from t1exceeds a predetermined value time at the time point t2, the removal of the negative torque of the first electric motor MG1is started. Then, if the torque Tm1of the first electric motor MG1becomes equal to zero at a time point t3, the torque Tm1of the first electric motor MG1is changed over to a positive torque, and is increased to the predetermined value α that is set in advance as indicated by the solid line. This predetermined value α is set to a value at which the large hysteresis can be sufficiently used, and is set, for example, to a value in the range of the spot B inFIG. 5. Specifically, the predetermined value α is set to a value which is obtained in advance through an experiment or an analysis and at which a twist greater than a twist resulting from torque fluctuations generated in stopping the engine24is obtained, namely, a value at which the damper38is always held at the negative twist angle θ even if a twist results from torque fluctuations. Accordingly, even if torque fluctuations occur in stopping the engine, the large hysteresis torque can be generated, and the torque fluctuations can be effectively damped by the large hysteresis torque. Besides, the predetermined value α need not always be set to a constant value. For example, the predetermined value α may be appropriately changed in accordance with the electric motor temperature of the first electric motor MG1or the like. Besides, the predetermined value α that is set in stopping the engine may be appropriately changed through, for example, learning control.

It should be noted herein that since the positive torque Tm1of the first electric motor MG1is larger than before, the engine rotational speed Ne may not be reduced within a predetermined time as indicated by a solid line ofFIG. 8. In such a case, as indicated by a solid line ofFIG. 9, the magnitude of the negative torque Tm1of the first electric motor MG1that is output to reduce the engine rotational speed Ne is set still larger than a value of conventional control that is indicated by an alternate long and short dash line. Thus, the engine rotational speed Ne that is indicated by the solid line decreases more swiftly than in the case of conventional control indicated by an alternate long and short dash line. Even if the positive torque Tm1is thereafter controlled to a value larger than before, the engine24can be stopped within a determined time.

The magnitude of the negative torque Tm1at the time when the engine rotational speed Ne is reduced by this electric motor MG1is preferably changed in accordance with, for example, the predetermined value α as the torque Tm1that is output from the electric motor MG1in stopping the engine24and applied in such a direction as to drive the engine24. For example, in the case where the predetermined value α that is output from the first electric motor MG1changes in accordance with the electric motor temperature of the first electric motor MG1or the like, the negative torque Tm1that is output from the first electric motor MG1increases in proportion to the predetermined value α at that time.

FIG. 10is a flowchart for illustrating an essential part of control operation of the electronic control unit100, namely, control operation that makes it possible to reduce the generation of gear rattle noise in stopping the engine24. This flowchart is repeatedly executed at intervals of an extremely short cycle time, for example, about several milliseconds to several dozens of milliseconds. Incidentally, all steps S1to S4ofFIG. 10correspond to the engine stop control unit102.

First of all, it is determined in step S1(“step” will be omitted hereinafter) whether or not a determination to stop the engine24has been made. Specifically, this corresponds to, for example, a case where the running state of the vehicle8changes over from engine running to motor running, or the like. If the result of S1is negative, the present routine is terminated. If the result of S1is positive, the negative torque Tm1that is set in advance is output from the first electric motor MG1in S2. Thus, a torque for stopping the engine24is transmitted to the engine24via the damper38, and hence the engine rotational speed Ne decreases. Then, it is determined in S3whether or not a condition for starting so-called torque removal to reduce the negative torque Tm1that is output from the first electric motor MG1is fulfilled. For example, if the engine rotational speed Ne becomes equal to or lower than the predetermined value that is set in advance or if the predetermined time that is set in advance elapses since the start of decrease in the engine rotational speed Ne by the first electric motor MG1, the result of this step is positive. If the result of S3is negative, a return to S2is made, and the negative torque Tm1is continuously output from the first electric motor MG1. If the result of S3is positive, the removal of the negative torque of the first electric motor MG1is started, and the torque Tm1of the first electric motor MG1is increased to the predetermined value αthat is set in advance. Accordingly, the twist angle θ of the damper38is in the range of the spot B shown inFIG. 5, and the large hysteresis torque can be generated. Large torque fluctuations that are generated in stopping the engine can be effectively damped by this large hysteresis torque, and gear rattle noise can be suppressed.

As described above, according to this embodiment of the invention, if the negative torque Tm1is output from the first electric motor MG1to reduce the engine rotational speed Ne and then the negative torque Tm1is removed to prevent reverse rotation of the engine24in stopping the engine, the reactive force resulting from compression of the engine24cannot be suppressed, and the magnitude of torque fluctuations increases. As a measure against this phenomenon, the torque Tm1is output from the first electric motor MG1until the damper38assumes a state of being twisted in the negative direction (toward the negative side). Therefore, the range where the hysteresis torque is large can be utilized, and the magnitude of torque fluctuations can be effectively reduced by the hysteresis torque. Accordingly, the magnitude of torque fluctuations generated in stopping the engine can be reduced, and hence gear rattle noise that is generated at that time can be suppressed.

Besides, according to this embodiment of the invention, the torque Tm1of the first electric motor MG1that renders the damper38in a state of being twisted in the negative direction (toward the negative side) is set to such a value that a twist greater than a twist resulting from torque fluctuations generated in stopping the engine is obtained. In this manner, when the engine is stopped, the damper38is always in a state of being twisted in the negative direction (toward the negative side). Therefore, the range where the hysteresis torque is large can be utilized. Accordingly, the torque fluctuations can be effectively damped by this large hysteresis torque (H1+H2).

Although the embodiment of the invention has been described hereinabove in detail on the basis of the drawings, the invention is also applicable to the following aspects as well.

For example, in the foregoing embodiment of the invention, the hybrid vehicle8is structured such that the first electric motor MG1is coupled to the damper38and the engine24via the planetary gear train26, but may also be structured such that the engine24and the electric motor MG1are coupled to each other via a clutch or directly coupled to each other. That is, the invention is appropriately applicable to a hybrid vehicle that is configured such that the damper38is interposed in a motive power transmission path between the engine24and the electric motor MG1.

Besides, in the foregoing embodiment of the invention, the damper38is equipped with the first hysteresis mechanism64and the second hysteresis mechanism65, thereby realizing a twist characteristic as shown inFIG. 5. However, the specific structure of the hysteresis mechanisms is not thus limited. That is, the specific mechanism of a damper is not limited in particular if the damper has the twist characteristic shown inFIG. 5.

Besides, in the foregoing embodiment of the invention, the large hysteresis torque is uniformly generated as soon as the damper38assumes a negative twist angle.

However, it is also acceptable to adopt a configuration in which a small hysteresis torque is generated in, for example, a minor twist angle range.

Besides, in the foregoing embodiment of the invention, the automatic transmission22is provided. However, the specific structure of the transmission is not limited only to the automatic transmission22, but can be appropriately changed into, for example, a further multiple-staged transmission, a belt-type continuously variable transmission, or the like. Furthermore, the transmission may be omitted.

While the invention has been described with reference to the example embodiment thereof, it is to be understood that the invention is not limited to the described example embodiment or construction. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiment are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the invention.