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Title:
ENGINE START CONTROL SYSTEM FOR HYBRID VEHICLE
Document Type and Number:
WIPO Patent Application WO/2009/109831
Kind Code:
A1
Abstract:
An engine start shock reduction is achievable even if an influence of a torque variation of a second clutch is so small that its slip is insufficient to reduce the shock. At a first time point where engine speed Ne approaches motor/generator revolution speed Nm within a predetermined range, the transmission torque capacity of a first clutch is reduced from a cranking torque value. Then, when the Nm=Ne, the torque capacity is set to zero. When the Ne is higher than Nm by a predetermined value, the torque capacity is increased. At a last time point where Nm=Ne, the torque capacity is set to compensate for engagement of the first clutch. The reduction of torque capacity from the first to the last time point reduces engine start shock according to a slip of the first clutch even where the second clutch slip cannot reduce the shock.

Inventors:
KAN SHOJI (JP)
YAMANAKA FUMIHIRO (JP)
NOZAKI YUJI (JP)
Application Number:
PCT/IB2009/000394
Publication Date:
September 11, 2009
Filing Date:
March 02, 2009
Export Citation:
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Assignee:
NISSAN MOTOR (JP)
KAN SHOJI (JP)
YAMANAKA FUMIHIRO (JP)
NOZAKI YUJI (JP)
International Classes:
B60W10/02; B60K6/48; B60K6/547; B60L50/16; B60W10/06; B60W10/08; B60W10/10; B60W20/00; F02D29/00; F02D29/02; F16D48/02; F16H61/02; F16H63/40; F16H59/18; F16H59/68; F16H59/74; F16H61/686
Foreign References:
JP2006306209A2006-11-09
JP2007131070A2007-05-31
JP2002349309A2002-12-04
JP2006188223A2006-07-20
JP2007320550A2007-12-13
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Claims:

What is claimed is:

1. An engine start control apparatus for a hybrid vehicle, comprising: an engine; a motor/generator; a first clutch disposed between the engine and the motor/generator to selectively connect the engine to the motor/generator; a second clutch disposed between the motor/generator and drive wheels to selectively connect the motor/generator to the drive wheels; and a controller configured to: select from an EV mode in which the first clutch is disengaged and the second clutch is engaged to transfer power from only the motor/generator to the drive wheels and an HEV mode in which both of the first clutch and the second clutch are engaged to transfer power from at least the engine to the drive wheels; perform an engine start, in response to selection of the HEV mode, by engaging the first clutch and cranking the engine by means of a drive torque of the motor/generator to start the engine; and reduce a transmission torque capacity of the first clutch when determining that an engine speed rises in response to the engine start and the engine speed has approached a revolution speed of the motor/generator within a predetermined range.

2. The engine start control apparatus for the hybrid vehicle of claim 1 wherein the controller is further configured to perform the engine start by reducing a transmission torque capacity of the second clutch.

3. The engine start control apparatus for the hybrid vehicle of claim 1 , further comprising: an automatic transmission disposed between the motor/generator and the drive wheels; and wherein the second clutch is a clutch element within the automatic transmission.

4. The engine start control apparatus for the hybrid vehicle of claim 1, wherein the controller is further configured to: make a determination of whether the engine speed has approached the revolution speed of the motor/generator within the predetermined range by at least one of:

determining whether the engine has reached a self rotation state; determining whether a fuel injection process in which fuel is provided to the engine has started; and determining whether an abrupt rise in the engine speed has occurred.

5. The engine start control apparatus for the hybrid vehicle of claim 1, wherein the controller is further configured to: make a transmission torque capacity of the first clutch zero responsive to a difference between the engine speed and the revolution speed of the motor/generator becoming zero.

6. The engine start control apparatus for the hybrid vehicle of claim 1 , wherein the controller is further configured to reduce the transmission torque capacity of the first clutch by gradually reducing the transmission torque capacity of the first clutch by a predetermined time variation gradient.

7. The engine start control apparatus for the hybrid vehicle of claim 6, wherein the controller is further configured to: determine the predetermined time variation gradient in accordance with a revolution difference between the engine speed and the revolution speed of the motor/generator.

8. The engine start control apparatus for the hybrid vehicle of claim 6, wherein the controller is further configured to: determine the predetermined time variation gradient in order for the transmission torque capacity of the first clutch to be zero when the revolution difference becomes zero.

9. The engine start control apparatus for the hybrid vehicle of claim I 5 wherein the controller is configured to: increase the transmission torque capacity of the first clutch when the engine speed increases above the revolution speed of the motor/generator by a

predetermined set revolution speed and after the transmission torque capacity of the first clutch is reduced.

10. The engine start control apparatus for the hybrid vehicle of claim 1, wherein the controller is further configured to: increase the transmission torque capacity of the first clutch after a predetermined set time elapses from a time at which the engine speed is coincident with the revolution speed of the motor/generator and after the transmission torque capacity of the first clutch is reduced.

11. The engine start control apparatus for the hybrid vehicle of claim 9 or claim 10, wherein the controller is configured to increase the transmission torque capacity of the first clutch by gradually increasing the transmission torque capacity of the first clutch by a predetermined time variation gradient.

12. The engine start control apparatus for the hybrid vehicle of claim 11, wherein the controller is further configured to: determine the predetermined time variation gradient based on at least one of a requested acceleration feeling, an allowed acceleration shock and an accelerator opening angle corresponding torque.

13. The engine start control apparatus for the hybrid vehicle of claim 11, wherein the controller is further configured to: reduce an engine torque while increasing the transmission torque capacity of the first clutch.

14. The engine start control apparatus for the hybrid vehicle of claim 13, wherein the controller is further configured to: reduce the engine torque by delaying an ignition timing of the engine when an engine load is reduced by a constant quantity independent from an accelerator opening angle.

15. The engine start control apparatus for the hybrid vehicle of claim 13, wherein the controller is further configured to:

determine whether a revolution speed difference between an input revolution speed of the first clutch and an output revolution speed thereof becomes zero after the engine torque is reduced; return the engine torque to a value corresponding to an accelerator opening angle when the revolution speed difference is zero; and increase the transmission torque capacity of the first clutch to allow a complete engagement state of the first clutch when the revolution speed difference is not zero.

16. The engine start control apparatus for the hybrid vehicle of claim 1 , wherein the controller is further configured to: determine whether an effect of shielding an input torque fluctuation responsive to a reduction in the transmission torque capacity of the second clutch is equal to or greater than a set value; and execute a reduction in the transmission torque capacity of the first clutch when determining that the effect of shielding the input torque fluctuation is smaller than the set value.

17. The engine start control apparatus for the hybrid vehicle of claim 16, wherein the controller is further configured to: make the set value of the effect of shielding the input torque fluctuation a value based on an expected engine start shock.

18. The engine start control apparatus for the hybrid vehicle of claim 16, further comprising: an automatic transmission disposed between the motor/generator and the drive wheels; and wherein the controller is further configured to: use select a disengagement-side clutch element of the automatic transmission to be switched from an engagement state to a disengagement state during a down-shift as the second clutch where at least one of a down-shift request of the automatic transmission is issued during the engine start and an accelerator operation is carried out that would issue the down-shift request.

19. The engine start control apparatus for the hybrid vehicle of claim 16, further comprising: an automatic transmission disposed between the motor/generator and the drive wheels; and wherein the controller is further configured to: select one of clutch elements of the automatic transmission used to select a present shift stage whose effect of shielding the input torque fluctuation is highest as the second clutch where at least one of no down-shift request of the automatic transmission is issued during the engine start and where an accelerator operation that has no possibility of issuing the down-shift request is carried out.

20. The engine start control apparatus for the hybrid vehicle of claim 16, further comprising: an automatic transmission disposed between the motor/generator and the drive wheels and having a plurality of clutch elements; and wherein the controller is further configured to: determine the effect of shielding the input torque fluctuation based on a previously determined input torque fluctuation shielding factor corresponding to each of the plurality of clutch elements.

21. The engine start control apparatus for the hybrid vehicle of claim 16, wherein the controller is further configured to: simultaneously start a reduction in a transmission torque capacity of the second clutch and an engagement of the first clutch to be carried out during the engine start; determine whether the second clutch is slipped when the engine speed has approached the revolution speed of the motor/generator within the predetermined range; and carry out a reduction control for the first clutch transmission torque capacity irrespective of whether the effect of shielding the input torque fluctuation is equal to or larger than the set value where the second clutch is not yet slipped when the engine speed has approached the revolution speed of the motor/generator within the predetermined range.

22. The engine start control apparatus for the hybrid vehicle of claim 16, wherein the controller is further configured to: make the determination of whether the engine speed has approached the revolution speed of the motor/generator within the predetermined range by determining whether the engine speed is greater than a set revolution speed.

23. An engine start control method for a hybrid vehicle comprising an engine, a motor/generator, a first clutch disposed between the engine and the motor/generator to selectively connect the engine to the motor/generator and a second clutch disposed between the motor/generator and drive wheels to selectively connect the motor/generator to the drive wheels, the method comprising: selecting from an EV mode in which the first clutch is disengaged and the second clutch is engaged to transfer power from only the motor/generator to the drive wheels and an HEV mode in which both of the first clutch and the second clutch are engaged to transfer power from at least the engine to the drive wheels; performing an engine start, in response to selection of the HEV mode, by engaging the first clutch and cranking the engine by means of a drive torque of the motor/generator to start the engine; and reducing a transmission torque capacity of the first clutch when determining that an engine speed rises in response to the engine start and the engine speed has approached a revolution speed of the motor/generator within a predetermined range.

Description:

ENGINE START CONTROL SYSTEM FOR HYBRID VEHICLE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Japanese Patent Application Serial

Nos. 2008-052307, filed March 3, 2008, and 2008-052309, filed March 3, 2008, each of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The present invention relates to an engine start control system for a hybrid vehicle equipped with an engine and a motor/generator as power sources.

BACKGROUND

[0003] Various types of hybrid drive apparatuses used in hybrid vehicles have been proposed. Japanese Patent Publication No. Heisei 11-082260 (which was patented as No. 3870505) exemplifies one such previously proposed hybrid drive apparatus. Japanese Patent Publication No. 2005-221073 (which corresponds to United States Patent No. 7,360,616) exemplifies another previously proposed hybrid drive apparatus. [0004] In each of these publications, the hybrid drive apparatus is equipped with an engine and a motor/generator connected in tandem via a first clutch and is further connectable and disconnectable from drive wheels via a second clutch. A clutch element within an automatic transmission interposed between the motor/generator and the drive wheels is used as the second clutch.

[0005] In a hybrid vehicle so equipped, an electric traveling mode (an EV mode) can be carried out only by power transmitted from the motor/generator when the first clutch is disengaged and the second clutch is engaged. In addition, the hybrid vehicle can carry out a hybrid traveling mode (an HEV mode) using only the power transmitted from the engine or using the power transmitted from the engine and the motor/generator when both of the first clutch and the second clutch are engaged.

[0006] In the hybrid vehicle described above, the EV mode is used during a small or low load state and a low vehicle speed including a vehicle start due to ease in controlling minute driving forces. The HEV mode is used during a large load and a high

vehicle speed upon a request of a high output due to an insufficient driving force only through the power transferred from the motor/generator. Hence, when an accelerator pedal is depressed during travel under a small or low load state and low vehicle speed, or when a rise in vehicle speed occurs under a small or low load and low vehicle speed, it becomes necessary to start the engine to switch to the HEV mode.

BRIEF SUMMARY

[0007] Taught herein are embodiments of an engine start control system for a hybrid vehicle. The hybrid vehicle comprises power sources including an engine and a motor/generator connected together in a tandem configuration, a first clutch via which the engine is connectable to the motor/generator and a second clutch via which the motor/generator is connectable to drive wheels of the vehicle. The first clutch is engaged, and the engine performs an engine cranking according to a drive torque of the motor/generator to carry out an engine start for HEV drive mode when the engine is started in a state in which the first clutch is disengaged and the second clutch is engaged to perform an electrical run through only the motor/generator in EV mode. [0008] The engine start control system for the hybrid vehicle described above according to one embodiment of the invention is so structured that the transmission torque capacity of the first clutch is reduced from a time when an engine speed has approached a revolution speed of the motor/generator within a predetermined range. [0009] Since the transmission torque capacity of the first clutch is so reduced, the first clutch absorbs the torque fluctuation during the start of the engine due to the slip of the first clutch so that the engine start shock can be reduced even under a driving state in which the transmission torque capacity during slip control for the second clutch that is performed to reduce the engine start shock is insufficient to reduce the engine start shock. [0010] Details of and modifications of this embodiment and others are described in additional detail hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:

[0012] FIG. 1 is a block diagram representing a vehicular power train and control system of a hybrid vehicle of a front-engine-and-rear-wheel drive type incorporating an engine start control apparatus in a first embodiment according to the invention;

[0013] FIG. 2 is an engagement logic diagram representing a relationship of combinations among selected shift stages in an automatic transmission and clutch elements thereof;

[0014] FIG. 3 is a diagram representing a ratio between a degree of contribution of a variation in a transmission torque capacity of the clutch element in the automatic transmission shown in FIG. 1 on a transmission output torque and a degree of contribution of a variation in a transmission input torque on the transmission output torque at a certain shift stage;

[0015] FIG. 4 is a flowchart representing an engine start program executed by the control system of the power train in FIG. 1;

[0016] FIGS. 5 A to 5F are integrally an operation timing chart based on the execution of the program shown in FIG. 4;

[0017] FIG. 6 is a flowchart representing a control program of a first clutch executed at a time of start of an engine by the control system of the power train shown in

FIG. 1;

[0018] FIG. 7 is a flowchart representing a control program of a second clutch executed at the time of start of the engine by the control system of the power train shown in FIG. l; and

[0019] FIGS. 8 A to 8E are integrally an operational timing chart by the clutch control during the start of the engine shown in FIGS. 6 and 7.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION [0020] It is common that, in the hybrid vehicle of the type described above, a starter motor to start the engine is not included. In the engine start during a mode transfer from EV mode to HEV mode, the first clutch, which is in a disengaged state in the EV mode, becomes engaged to crank the engine using the power of the motor/generator to raise an engine speed up to a revolution speed required to start the engine. [0021] A large torque fluctuation is developed during the engine start. This torque fluctuation is transmitted to the drive wheels and can cause an engine start shock that gives an unpleasant feeling to a vehicle occupant. Each of the above-identified Japanese publications describes a technique that reduces such an engine start shock.

Specifically, a transmission capacity of the second clutch is reduced during the engine start, and a slip of the second clutch absorbs the torque fluctuation, which is not transmitted to the drive wheels so as to reduce the engine start shock.

[0022] However, it is difficult to reduce the engine start shock through this technique.

[0023] In addition, the highest priority for the second clutch is to have a transmission torque capacity by which the second clutch can transmit the vehicular requested driving force to the drive wheels. Hence, it is difficult to generate the slip of the second clutch sufficient to address the input torque fluctuation generated during the start of the engine. Thus, most of the torque fluctuation is transmitted to the drive wheels as a transmission output torque, which can also make the reduction of the engine start shock difficult.

[0024] In contrast, embodiments of the invention provide an engine start control technique for a hybrid vehicle that can prevent the torque fluctuation from being transmitted to the drive wheels as transmission output torque during the start of the engine. This transmission is prevented even where the transmission torque capacity of the second clutch during the slip control is insufficient to reduce the engine start shock.

[0025] Reference is, hereafter, made to the drawings in order to facilitate a better understanding of certain embodiments according to the invention.

[0026] A power train for a hybrid vehicle including an engine 1 , an automatic transmission 2 and a motor/generator is shown in FIG. 1. Automatic transmission 2 is arranged at a rearward position in a vehicular longitudinal (forward-and-backward) direction in a tandem configuration with respect to engine 1 in the same way as an ordinarily available rear- wheel drive vehicle, and motor/generator 3 is connected to a shaft 5 to transmit rotation from engine 1 to an input shaft 4 of automatic transmission 2 using crankshaft Ia.

[0027] Motor/generator 3 includes an annular stator 3a fixed and housed within a housing and a rotor 3b coaxially mounted in the stator 3a with a predetermined air gap.

Motor/generator 3 acts as an electrically operated motor and acts as a generator according to a driving state of the hybrid vehicle. Motor/generator 3 is interposed between engine 1 and automatic transmission 2. Motor/generator 3 is coupled to shaft 5 that is connected to a center of rotor 3b, and shaft 5 acts as a shaft of motor/generator 3.

[0028] A first clutch 6 is interposed between motor/generator 3 and engine 1, specifically, between motor/generator shaft 5 and engine crankshaft Ia, to selectively

connect engine 1 and motor/generator 3. It should be noted that first clutch 6 can continuously modify its transmission torque capacity and preferably includes a wet-type multi-plate clutch whose transmission torque capacity is modified by continuously controlling a clutch working oil flow quantity and a clutch working oil pressure through a proportional valve.

[0029] Direct coupling between motor/generator shaft 5 and transmission input shaft 4 causes a mutual direct connection between motor/generator 3 and automatic transmission 2. A transmission mechanism portion of automatic transmission 2 is the same as a well known planetary gear type automatic transmission. Automatic transmission 2 is structured so that a torque converter is eliminated from the planetary gear type automatic transmission, and, in place of the torque converter, motor/generator 3 is directly coupled to transmission input shaft 4.

[0030] Automatic transmission 2 includes an output shaft 7 arranged in a coaxial- and-butting relationship to input shaft 4. A front planetary gear group Gf, a center planetary gear group Gm and a rear planetary gear group Gr are mounted in sequence from the engine 1 (and motor/generator 3) side on these input and output shafts 4, 7. These gear groups Gf, Gm and Gr are main constituents of a planetary gear transmission mechanism in automatic transmission 2.

[0031] Front planetary gear group Gf nearest to engine 1 is a simple planetary gear group including a front sun gear Sf 5 a front ring gear Rf, a front pinion Pf meshed with these gears and a front carrier Cf rotatably supporting front pinion Pf. Next, center planetary gear group Gm is a simple planetary gear group including a center sun gear Sm, a center ring gear Rm, a center pinion Pm meshed with these gears and a center carrier Cm rotatably supporting center pinion Pm. Next, rear planetary gear group Gr farthest from engine 1 is a simple planetary gear group including a rear sun gear Sr, a rear ring gear Rr, a rear pinion Pr meshed with these gears and a rear carrier Cr rotatably supporting rear pinion Pr.

[0032] Clutch elements that determine a transmission path (a shift stage) of the planetary gear shift mechanism include a front brake Fr/B, an input clutch I/C, a high- and-low reverse clutch H&LR/C, a direct clutch D/C, a reverse brake RTB and a forward " brake FWD/B. These brakes and clutches are correlated to the components of planetary gear groups Gf 5 Gm and Gr as described below to form the planetary gear transmission mechanism of automatic transmission 2.

[0033] Front ring gear Rf is coupled to input shaft 4, and center ring gear Rm is selectively connectable to input shaft 4 by means of input clutch I/C. Front sun gear Sf is selectively fixable to a transmission casing 2a via front brake Fr/B. Front carrier Cf and rear ring gear Rr are coupled to each other. Center ring gear Rm and rear carrier Cr are coupled to each other. Center carrier Cm is coupled to output shaft 7. High-and-low reverse clutch H&LR/C serves to selectively couple center sun gear Sm and rear sun gear

Sr.

[0034] Direct clutch D/C serves to selectively couple rear sun gear Sr and rear carrier Cr, and rear carrier Cr is selectively fixable to transmission casing 2a by means of reverse brake R/B. Center sun gear Sm is, furthermore, selectively fixable to transmission casing 2a by means of forward brake FWD/B.

[0035] The planetary gear transmission mechanism can obtain forward shift stages of a forward first-speed, a forward second-speed, a forward third-speed, a forward fourth-speed, a forward fifth-speed and a reverse shift stage by selective engagement denoted by a circle mark o as shown in FIG. 2 for each of six clutch elements Fr/B, I/C,

H&LR/C, D/C, R/B and FWD/B.

[0036] It should be noted that a hybrid vehicle equipped with the power train shown in FIG. 1 and provided with engine 1, motor/generator 3 and automatic transmission 2 described above requires a second clutch that selectively connects motor/generator 3 and the drive wheels coupled to transmission output shaft 7. However, in the first embodiment, one of the clutch elements selected from among six clutch elements Fr/B, I/C, H&LR/C, D/C, R/B and FWD/B is preferably used as the second clutch.

[0037] Hereinafter, functioning of the power train described above with reference to FIG. 1 is explained for each of the selected modes.

[0038] In the power train in FIG. 1, where an electric traveling mode (EV mode) used during a Io w-load-and-low- vehicle-speed including a start of the vehicle from a stopped state is requested, first clutch 6 is disengaged and automatic transmission 2 is in a power transmission state in which a predetermined shift stage is selected.

[0039] When motor/generator 3 is driven in this state, only an output rotation from motor/generator 3 reaches transmission input shaft 4, and automatic transmission 2 shifts the rotation of input shaft 4 in accordance with the shift stage under selection to be output through transmission output shaft 7. The rotation from transmission input shaft 4

thereafter reaches to the left and right drive wheels via a differential gear unit (not shown) so that the vehicle can perform the traveling only through motor/generator 3 (EV mode). [0040] Where a hybrid traveling mode (HEV mode) used during high-speed travel, during large-load travel, or during a request for battery discharge of a small amount of electric power is requested, first clutch 6 is engaged and automatic transmission 2 is in the power transmission state in which a predetermined shift stage is selected. In this state, the output revolution from engine 1 or an output rotation from both engine 1 and motor/generator 3 reaches transmission input shaft 4. Then, automatic transmission 2 shifts this rotation of transmission input shaft 4 in accordance with the shift stage under selection to be output through transmission output shaft 7. The rotation from transmission output shaft 7 reaches the left and right drive wheels via the differential gear unit (not shown) so that the vehicle can perform traveling by means of both of engine 1 and motor/generator 3 (HEV mode).

[0041] Where, during the HEV mode, energy produced by engine 1 exceeds what is required (engine 1 is driven at an optimum fuel economy), this extra energy causes motor/generator 3 to be operated as a generator so the extra energy is converted into electric power. This generated electric power is stored in a battery to be used for motor drive of motor/generator 3. Thus, the fuel economy of engine 1 can be improved. [0042] One of the clutch elements from among six clutch elements Fr/B, I/C,

H&LR/C, D/C, R/B and FWD/B within automatic transmission 2 is selected and used as the second clutch, as described hereinafter. The second clutch performs a reduction control (slip control) for its transmission torque capacity to reduce a start shock when engine 1 is started. In addition, the engine start request is generated along with a mode switch from the EV mode to the HEV mode during the state in which the engine load is increased. Hence, it is often a case that a down-shift of the automatic transmission 2 in response to the increase of the engine load occurs. Thus, a determination is made of whether any one of clutch elements Fr/B, I/C, H&LR/C, D/C, RTB, FWD/B is selected and used as the second clutch in relation to a presence or absence of the down-shift and an accelerator operation of a vehicle driver representing the engine load. [0043] Specifically, where the down-shift of automatic transmission 2 during the mode switch from EV mode to HEV mode (EV mode → HEV mode) is requested, or where an accelerator operation causes a down-shift request to be issued, the transmission torque capacity of a disengagement-side clutch element switching from an engagement state to a disengagement state is reduced during the down-shift. Hence, this

disengagement-side clutch element is selected and used as the second clutch. The disengagement-side clutch element (second clutch) is slipped according to the transmission torque capacity reduction control to reduce the engine start shock. [0044] Where no down-shift request for automatic transmission 2 is issued during the engine start, or where an accelerator operation occurs that has no possibility of causing the down-shift request to be issued, one of the clutch elements constituting the present gear shift stage (that is, the clutch elements marked by circle o in FIG. 2 for each of the gear shift stages) that has a highest input torque fluctuation suppression effect is selected and used as the second clutch. This clutch element (second clutch) is slipped for reducing the engine start shock.

[0045] Therefore, a factor of shielding the input torque fluctuation of each of clutch elements Fr/B, I/C, H&LR/C, D/C, R/B and FWD/B within automatic transmission 2 (that is, a rate which each clutch can shield the transmission input torque fluctuation by means of the slip due to the transmission torque capacity reduction control of each) is predetermined for each of the gear shift stages. The one of the clutch elements used to provide the present gear shift stage that has the highest input torque fluctuation shielding factor is selected and used as the second clutch. Then, the clutch element selected as the second clutch is slipped according to the transmission torque capacity reduction control for reducing the engine start shock.

[0046] With reference to FIG. 3, a ratio between a degree of contribution of affecting a clutch torque variation due to the transmission torque capacity reduction control of each of clutch elements A, B and C on the transmission output torque (heights of bar graphs denoted by oblique lines in FIG. 3) and a degree of contribution of affecting the transmission input torque variation on the transmission output torque (heights of bar graphs denoted by dots in FIG. 3) is predetermined according to a calculation using vehicle specifications (a weight of the vehicle, an inertia, or so forth) or through experiments for a certain gear shift stage provided by the engagements of clutch elements A 5 B and C. Then, for other gear shift stages, the ratios between the degrees of contributions of the clutch torque variation of the corresponding respective clutch elements on the transmission output torque and the degrees of contributions of the transmission input torque variation on the transmission output torque are individually predetermined through calculations using vehicular specifications (vehicle weight, inertia, and so forth), or through experiments.

[0047] The heights of bar graphs denoted by the oblique lines in FIG. 3 (that is, the degree of contribution of the clutch torque variation on the transmission output torque) correspond to the shielding factors (input torque fluctuation shielding effect) of the input torque fluctuation during the transmission torque capacity reduction control of the respective clutch elements A, B and C. Hence, when automatic transmission 2 maintains the gear shift stage provided by the engagements of clutch elements A, B, C, and the mode is switched from EV mode to HEV mode (engine 1 is started) without the down shift of automatic transmission 2, the clutch element that has the highest input torque fluctuation shielding factor is selected and used as the second clutch (clutch B in FIG. 3, for example). Then, the clutch element used as the second clutch is slipped according to the transmission torque capacity reduction control for reducing the engine start shock.

[0048] It should be noted that the transmission torque capacities of clutch elements in automatic transmission 2 used for the second clutch are naturally modifiable continuously in the same way as first clutch 6.

[0049] Automatic transmission 2 described above has been explained as a stepped automatic transmission, but automatic transmission 2 is not limited thereto and may be a continuously variable transmission. In the case of a continuously variable transmission, a forward selection clutch and a reverse selection brake in a forward-backward switch mechanism constitute the second clutch.

[0050] The control system of engine 1, motor/generator 3, first clutch 6 and the second clutch within automatic transmission 2 selected and used in the way described above (denoted hereinafter by CL2) is briefly described with reference to FIG. 1. This control system includes an integrated controller 11 configured to perform integrated control for operation points of the power train, the operation points of the power train being based on a target engine torque tTe, a target motor/generator torque tTm, a target transmission torque capacity tTcl of first clutch 6 and a target transmission torque capacity tTc2 of second clutch CL2

[0051] Integrated controller 11 receives inputs from an engine speed sensor 12 to detect an engine speed Ne of engine 1, motor/generator revolution speed sensor 13 to detect a revolution speed Nm of motor/generator 3, an input revolution sensor 14 to detect a transmission input revolution speed Ni 5 an output revolution sensor 15 to detect a transmission output revolution speed No (a vehicle speed), sensor 16 to detect an

accelerator opening angle APO and a charge state sensor 17 to detect a state-of-charge SOC of a battery that supplies electric power to motor/generator 3. [0052] Integrated controller 11 selects a drive mode (EV mode or HEV mode) that can achieve a vehicular driving force that the vehicle driver desires according to the accelerator opening angle APO, the battery charge state (SOC) and transmission output revolution speed No (vehicle speed). Integrated controller 11 also calculates target engine torque tTe, target motor/generator torque tTm, first clutch target transmission capacity tTcl and second clutch CL2 target transmission torque capacity tTc2. [0053] Target engine torque tTe is supplied to an engine controller 21. Engine controller 21 controls engine 1 to perform throttle opening angle control or fuel injection quantity control to achieve target engine torque tTe based on engine speed Ne detected by engine speed sensor 12 and the target engine torque tTe.

[0054] Target motor/generator torque tTm is supplied to a motor/generator controller 22. This motor/generator controller 22 performs a direct current-to-alternating current conversion (DC- AC conversion) of the electric power of the battery through an inverter, supplies the AC power to stator 3 a of motor/generator 3 under control of the inverter and controls the motor/generator 3 to reach the target motor/generator torque tTm. Where target motor/generator torque tTm is a negative torque value that requires motor/generator 3 to perform a regenerative brake action, a generation load for the motor/generator 3 is limited in accordance with the battery SOC so as not to excessively charge the battery. Electric power generated by motor/generator 3 according to the regenerative brake action is AC-DC converted and then charged into the battery. [0055] First clutch target transmission torque capacity tTcl is supplied to a first clutch controller 23. This first clutch controller 23 compares a first clutch engagement pressure command value corresponding to first clutch target transmission torque capacity tTcl with an actual engagement pressure PCl of first clutch 6, controls the engagement pressure of first clutch 6 in order for the actual engagement pressure of first clutch 6 to be equal to the first clutch engagement pressure command value and executes control in order for the transmission torque capacity of first clutch 3 to become equal to a target value tTcl.

[0056] Second clutch target transmission torque capacity tTc2 is supplied to a transmission controller 24. This transmission controller 24 compares a second clutch engagement pressure command value corresponding to second clutch target transmission torque capacity tTc2 with the actual engagement pressure of second clutch CL2 to control

the engagement pressure of second clutch CL2 in order for actual engagement pressure Pc2 of second clutch CL2 to be equal to second clutch engagement pressure command value tTc2. Responsive thereto, transmission controller 24 executes control for the transmission torque capacity of second clutch CL2 to become a target value tTc2. [0057] It should be noted that transmission controller 24 basically determines a gear shift stage suitable for the present driving state based on transmission output revolution speed No detected by sensor 15 and accelerator opening angle APO detected by sensor 16 and performs automatic shifting of automatic transmission 2 in order for the suitable gear stage to be selected.

[0058] The integrated controller 11 and controllers 21, 22, 23, 24 are each implemented by, for example, a conventional engine control unit such as is known in the art. Each can thus be a microcomputer including a random access memory (RAM), a read-only memory (ROM) and a central processing unit (CPU), along with various input and output connections. Generally, the control functions described herein and associated with the respective controllers and their control sections are performed by execution by the CPU of one or more software programs stored in ROM. Of course, some or all of the functions can be implemented by hardware components. Moreover, although several controllers are shown, the functions could be combined in fewer controllers. [0059] The description is an outline of ordinary control executed by the control system shown in FIG. 1. hi the first embodiment, the engine is started when an accelerator pedal is depressed, and/or a rise of vehicle speed occurs while driving in the electric travel (EV) mode since the vehicle is operating in the large-load-and-high-speed driving state, and/or the mode is requested to switch to the hybrid travel mode (HEV mode) because the state of charge SOC of the battery is below a charge threshold. The engine start is carried out by the control system shown in FIG. 1 using a control program shown in FIG. 4

[0060] In the hybrid vehicle equipped with the power train shown in FIG. 1, a starter motor for the engine start is not provided. Instead, during the engine start at the time of switching the mode from EV mode to HEV mode (EV -→HEV), first clutch 6 becomes engaged, the power of motor/generator 3 causes engine 1 to be cranked in order for this engine 1 to raise the engine speed to a speed at which engine 1 can be started. Simultaneously, second clutch CL2 is slipped to perform transmission torque capacity reduction control to reduce the engine start shock. Hence, the engine start control shown in FIG. 4 shows the transmission torque capacity control of first clutch 6.

[0061 ] On the other hand, where the transmission torque capacity of second clutch

CL2 is reduced, and automatic transmission 2 is in a state that the engine start shock cannot be reduced by the engine start control shown in FIG. 4, the slip of second clutch CL2 can reduce the engine start shock.

[0062] The transmission torque capacity reduction control of second clutch CL2 is described with reference to FIGS. 5 A through 5F.

[0063] FIGS. 5 A through 5E integrally show an operational timing chart where a command to switch the mode from the EV mode to the HEV mode is issued at a time point tl due to an increase of accelerator opening angle APO (as shown in FIG. 5A). That is, at time point tl the engine start request is generated. Transmission torque capacity reduction control of second clutch CL2 starts at time point tl, and the second clutch target transmission torque capacity tTc2 reduces to a value of target driving torque Tin corresponding to accelerator opening angle APO multiplied by 0.7. [0064] Target motor/generator torque tTm increases as shown in FIG. 5E in order to raise motor/generator revolution speed Nm for the engine start as shown in FIG. 5D in synchronization with transmission torque capacity reduction control of second clutch CL2 from time point tl .

[0065] At a time point t2, second clutch CL2 starts to slip in response to the reduction of second clutch target transmission torque capacity tTc2, and first clutch target transmission torque capacity tTcl increases from zero to a cranking torque required for the cranking of engine 1. Second clutch target transmission torque capacity tTc2 increases up to a cranking upper limit torque (tTm — tTcl). Power from motor/generator 3 cranks engine 1 as shown in FIG. 5 C due to the rise in engine speed Ne due to the increase of first clutch target transmission torque capacity tTcl . Target motor/generator torque tTm is used to perform feedback control in order for motor/generator revolution speed Nm to become a target revolution speed tNm for the engine cranking during this time.

[0066] At step SI l of FIG. 4, the control system shown in FIG. 1 determines whether engine 1 has reached a self rotation state as a result of engine cranking. That is one specific way of determining whether engine speed Ne due to the engine start by the engine cranking of engine 1 has approached revolution speed Nm of motor/generator 3 within a predetermined range. Instead of detecting whether engine 1 has reached a self rotation state to assess whether engine speed Ne has approached revolution speed Nm within the predetermined range, the detection may alternatively be carried out by

determining whether fuel injection to engine 1 has started, or by determining whether engine speed Ne has abruptly increased.

[0067] Until engine 1 has been determined to be in a self rotation state at step

Sl I 5 the cranking of engine 1 continues by making first clutch target transmission torque capacity tTcl equal to the engine cranking torque at step S 12.

[0068] At a time point t3 at which the determination is made that engine 1 is in a self rotating state, control advances to step S 13. At step S 13, the control system determines whether an absolute value in a revolution speed difference between engine speed Ne and revolution speed Nm of motor/generator 2 (that is, |Ne — Nm|) is smaller than a minute set value δN1. This step checks whether a speed difference between engine speed Ne and motor/generator revolution speed Nm has become approximately zero (that is, whether engine speed Ne is approximately coincident with revolution speed Nm of the motor/generator 3).

[0069] Before a time point t4 shown in FIGS. 5A through 5F at which the control system determines that the speed difference between engine speed Ne and revolution speed Nm of motor/generator 2 is approximately zero (corresponding to a NO at step S 13), first clutch target transmission torque capacity tTcl is gradually reduced at a predetermined time variation gradient δTca at step S 14. Predetermined time variation gradient δTca of first clutch target transmission torque capacity tTcl is determined in accordance with revolution speed difference between engine speed Ne and revolution speed Nm of motor/generator 3, specifically what is required for this speed difference (Ne — Nm) to become approximately zero at time point t4.

[0070] If, at step Sl 3, the control system determines that the revolution speed difference between engine speed Ne and revolution speed Nm of motor/generator 3 is approximately equal to zero (at time point t4 of FIGS. 5A through 5F), at a step S15, the control system determines whether Ne > Nm + δN2, that is, whether engine speed Ne is equal to or faster (higher) than revolution speed Nm of motor/generator 3 by a second set revolution speed δN2. If engine speed Ne is not higher than revolution speed Nm of motor/generator 2 by second set revolution speed δN2 (NO in step S 15), the routine advances to step S 16. In step S 16, the control system sets first clutch target transmission torque capacity tTcl to zero.

[0071] After a time point t5 shown in FIGS. 5 A through 5F, if engine speed Ne becomes higher than revolution speed Nm by second set revolution speed δN2 (YES at

step S 15), the control system checks to see whether first clutch 6 is not slipped and engine 1 is synchronized with motor/generator 2 such that a revolution difference (Ne -Nm) before and after the occurrence of the slip becomes zero. Before a time point t6 shown in FIGS. 5 A through 5F at which the control system determines that first clutch 6 is synchronized at step S17 (after time point t5), the control system gradually increases first clutch target transmission torque capacity tTcl by a predetermined time variation gradient δTcb at step S18.

[0072] After time point t5 at which first clutch 6 is synchronized with the zeroing of Ne -Nm at step S 17, the control system sets first clutch target transmission torque capacity tTcl to a static torque value derived by multiplying accelerator opening angle APO by a clutch engagement compensation safety factor in step S 19. This is shown occurring after time point t6 in FIGS. 5 A through 5F.

[0073] According to the engine start control apparatus for the hybrid vehicle in the first embodiment, even if the engine start shock cannot be sufficiently reduced by the reduction in the transmission torque capacity of second clutch CL2 because the transmission torque capacity of second clutch CL2 must achieve the requested driving force of the vehicle, or if the reduction in the transmission torque capacity of second clutch CL2 is sufficient but a degree of influence of a clutch torque variation of second clutch CL2 on the transmission output torque is small, the slip of first clutch 6 causes the torque fluctuation during the start of the engine to be absorbed so that the engine start shock can be reduced. This is because the transmission torque capacity tTcl of first clutch 6 is reduced from time point t3 (step SIl) where it is concluded during engine cranking whether the engine 1 reaches a self rotating state based on engine speed Ne approaching motor/generator revolution speed Nm within a predetermined range, based on a start determination of the fuel injection of engine 1, or based on a determination of an abrupt rise in the engine speed.

[0074] In addition, the timing of the determination that engine speed Ne has approached to revolution speed Nm of motor/generator 2 within the predetermined speed range is made so that the reduction of the first clutch transmission capacity tTcl can be adjusted to correspond to the timing at which the engine start torque fluctuation (engine start shock) is truly generated. Accordingly, the described actions and effects can become significant.

[0075] Furthermore, at time point t4 at which the revolution difference between engine speed Ne and motor/generator revolution speed Nm becomes approximately zero

(step S13), transmission torque capacity tTcl of first clutch 6 is made to be approximately zero (step S 16). At time point t4 at which the revolution difference is approximately zero, switching is made from a state in which motor/generator 3 causes engine 1 to be cranked to a state in which engine 1 is in the self rotating state, and engine speed Ne is increased to a higher speed than revolution speed Nm of motor/generator 3. Thus, the direction of the torque is reversed to generate a stepwise torque variation. This stepwise torque variation can be absorbed by the slip of first clutch 6 whose transmission torque capacity tTcl is approximately zero. Thus, the occurrence of shock due to the stepwise torque variation can be avoided.

[0076] In addition, since the reduction of transmission torque capacity tTcl of first clutch 6 is carried out gradually by predetermined time variation gradient δTca (step S 14), no shock occurs due to a sudden large slip of first clutch 6 according to the reduction of transmission torque capacity tTcl.

[0077] It should be noted that, since this reduction gradient δTca of transmission torque capacity tTcl of first clutch 6 is determined in accordance with the revolution difference between engine speed Ne and revolution speed Nm of motor/generator 3 so that transmission torque capacity tTcl of first clutch 6 is approximately zero when this revolution difference is approximately zero, control to make transmission torque capacity tTcl of first clutch 6 approximately zero (step S 16) can be assured. The described actions and advantages can more accurately be assured.

[0078] Transmission torque capacity tTcl of first clutch 6 is increased from time point t5 at which engine speed Ne becomes equal to or higher than revolution speed Nm of motor/generator 3 and set threshold δN2 (step S 15). Hence, no torque shock occurs due to the stepwise torque variation when the transmission torque capacity tTcl during first clutch 6 is increased since transmission torque capacity tTcl of first clutch 6 is increased from approximately zero from a time at which the stepwise torque variation is eliminated due to the reversal of the direction of the torque described above. [0079] The actions and effects of this embodiment can be achieved in the same way by the increase of transmission torque capacity tTcl of first clutch 6 from when a set time has elapsed as the increase from a time at which engine speed Ne becomes coincident with revolution speed Nm of motor/generator 3.

[0080] The gradual increase of transmission torque capacity tTcl of first clutch 6 by predetermined time variation gradient δTcb (step S 18) avoids the generation of shock. The increase gradient δTcb of transmission torque capacity tTcl of first clutch 6 may be

determined in accordance with at least one of the requested acceleration feeling, the allowed acceleration shock, and accelerator opening angle APO corresponding torque. [0081] It is preferable to reduce the engine torque during the increase of transmission torque capacity tTcl of first clutch 6 in order to assure the engagement of first clutch 6 whose transmission torque capacity tTcl is increased. The reduction of the engine torque described above is suitable for achieving a highly responsive reduction of the engine torque by delaying the engine ignition timing, with engine throttle opening angle TVO reduced by a constant quantity α as shown by a time series variation of engine torque tTe at time points t4 through t6 in FIGS. 5A through 5F. [0082] The reduction in the engine torque ends at time point tβ of revolution synchronization, at which input and output revolution speed difference of first clutch 6 is eliminated. After time point tβ, engine torque tTe is returned to a value corresponding to accelerator opening angle APO as shown in FIGS. 5 A through 5F. On the other hand, after time point tβ, transmission torque capacity tTcl of first clutch 6 is deemed to be the static value determined by multiplying the safety factor for clutch engagement compensation with the accelerator opening angle APO corresponding engine torque so that the complete engagement of first clutch 6 is compensated.

[0083] Hereinafter, a second embodiment according to the invention is described.

FIGS. 1 through 3 are common to both of the first and second embodiments. FIG. 6 shows a transmission torque control of first clutch 6 when the switch from EV mode to HEV mode (EV → HEV) is carried out (that is, during the engine start). FIG. 7 shows transmission torque capacity control of second clutch CL2 during the switch of the mode from EV mode to HEV mode. The transmission torque capacity control of first clutch 6 shown in FIG. 6 and the transmission torque capacity control of second clutch CL2 shown in FIG. 7 are simultaneously executed, for example, in response to the increase of accelerator opening angle APO as shown in FIG. 6, at a time point tl at which the EV → HEV mode switch request is issued.

[0084] In more detail, a first step S 11 ' of FIG. 6, in order to raise motor/generator revolution speed Nm starting at time point tl at which the mode switch request from EV mode to HEV mode (engine start request) occurs for starting engine 1, transmission torque capacity tTcl of first clutch 6 is increased to the cranking torque corresponding value in the stepwise manner as shown in FIGS. 8 A through 8E. The motor/generator torque is, at this time, increased up to the cranking torque corresponding value and is feedback controlled in order for revolution speed Nm of motor/generator 3 to reach to

target value tNm for starting engine 1 after a time point t2 at which the determination of the slip in the second clutch is made. Thus, engine 1 is cranked by motor/generator 3 via first clutch 6, and engine speed Ne is raised as shown in FIGS. 8 A through 8F. [0085] On the other hand, when the transmission torque capacity control for second clutch CL2 shown in FIG. 7 is carried out, at first step S21, the control system determines if the shift-down request occurs at time point tl at which the mode switch request from EV mode to HEV mode is made. If no shift-down request is made, the routine goes to step S22, at which the control system determines whether an accelerator pedal depression speed (that is, a rate of change of accelerator opening angle APO) is equal to or greater than a set value to determine whether an abrupt depression down-shift request occurs.

[0086] In a case where no shift-down request is determined to have been issued

(at step S21), and the accelerator pedal depression speed is not such as to generate the abrupt depression of the down-shift request (at step 22), the mode switch request from EV mode to HEV mode occurs without the shift-down of automatic transmission 2. Hence, at a step S23, from among the clutch elements that constitute the present gear shift stage, one of the clutch elements having a largest transmission input torque fluctuation shielding effect as described before with reference to FIG. 3 is selected as second clutch CL2. [0087] However, in a case where the down-shift request is issued at step S21, or in a case where the depression speed of the accelerator pedal indicates such an abrupt depression as to generate shifting down of automatic transmission 2 for the down-shifting at step S22, the control system determines that the mode switch from the EV mode to HEV mode (EV → HEV), along with the shifting down of automatic transmission 2 occurs. The routine goes to step S24 at which the disengagement-side clutch element to be disengaged from the engagement state during the down-shift is selected as second clutch CL2.

[0088] Next, at step S25, if the mode switch from EV mode to HEV mode without the shifting down of automatic transmission 2 occurs, the control system reduces target transmission torque capacity tTc2 of the clutch element selected as second clutch CL2 in S23 up to an engine start shock reducing transmission torque capacity to perform the slip control for second clutch CL2.

[0089] The transmission torque capacity reduction control for second clutch CL2 has as its priority allowing the requested driving force of the vehicle to be transmitted to the drive wheels, and the control is carried out within this range. For example, as denoted

by a solid line αl in FIG. 8C, at time point tl, the transmission torque reduction control is started to reduce second clutch target transmission torque capacity tTc2 to a value multiplied by 0.7 for the target driving torque corresponding to accelerator opening angle APO. At the next step S26, the control system checks to see whether engine speed Ne is equal to or greater than motor/generator revolution speed Nm due to the autonomous driving of engine 1 (that is, due to the engine 1 reaching a self rotating state). Until Ne > Nm, control returns to step S25, and the transmission torque capacity reduction control (slip control) for second clutch CL2 is continued.

[0090] During the execution of steps S26 and S25, the disengagement-side clutch element during the shifting-down of automatic transmission 2 is not slip controlled. Hence, its target transmission torque capacity is maintained at the maximum value as denoted by a solid line β in FIG. 8D, and the disengagement-side clutch is held in a complete engagement state.

[0091] In a case where the mode switch from EV mode to HEV mode along with the shifting down of automatic transmission 2 occurs, at step S25 the control system reduces target transmission torque capacity tTc2 of disengagement-side clutch element that was selected as second clutch CL2 in S24. The transmission torque capacity reduction control for second clutch CL2, in this case, gives priority to allowing the requested driving force of the vehicle to be transmitted to the drive wheels of the vehicle, and control is performed within this range.

[0092] In this case, the transmission torque capacity reduction control for second clutch CL2 first reduces second clutch target transmission torque capacity tTc2 to a value of second clutch target transmission torque capacity tTc2 multiplied by 0.7 for accelerator opening angle APO corresponding target drive torque as shown by line γl in FIG. 8D. The transmission torque capacity reduction control (slip control) for second clutch CL2 is continued until engine speed Ne becomes equal to or greater (faster) than revolution speed Nm of motor/generator 3 due to the autonomous driving of engine 1. [0093] In contrast, during this time (expressed as Ne < Nm) in a case where the mode switch from EV mode to HEV mode along with the shifting down of automatic transmission 2 occurs, the clutch element having the maximum input torque fluctuation shielding effect is not slip controlled. Hence, with target transmission torque capacity maintained at the maximum value as denoted by a line δ in FIG. 8 C, the clutch element

having the maximum input torque fluctuation shielding effect is maintained at the complete engagement state.

[0094] At step S 12' in FIG. 6, the control system checks to see if engine 1 has reached a self rotation state due to the cranking of engine 1 at step SI l'. That is one specific way of determining whether engine speed Ne has approached revolution speed Nm of motor/generator 3 within a predetermined range. Alternatively, whether engine 1 has reached such a state may be carried out by the determination of whether fuel injection into engine 1 has started, or by the determination of whether engine speed Ne reaches a set revolution speed (for example, 500 rpm) of engine speed Ne. [0095] After the determination at step S 12', at step S 13 ' the control system determines whether second clutch CL2 has slipped according to the control at step S25 in FIG. 7. As shown in FIGS. 8A through 8F, where time point t2 at which the slip occurrence of second clutch CL2 is determined is followed by time point t3 at which the engine 1 self rotation state is determined, the routine (control) goes from step S 13' to step S 14'.

[0096] At step S14', the control system checks to see if the input torque fluctuation shielding effect at second clutch CL2 is equal to or larger than a set value for the engine start shock reduction enabling determination at step S25 in FIG. 7 to determine whether the engine start shock can be reduced according to the slip control of second clutch CL2. Hence, step S 14' corresponds to a function performed by a second clutch torque fluctuation shielding effect determining section of integrated controller 11. [0097] A phenomenon can occur such that the input torque fluctuation shielding effect of second clutch CL2 is lower (smaller) than the set value of the engine start shock reducing enabling determination (meaning that even if slip control for second clutch CL2 is carried out at step S25, the engine start shock cannot be reduced) where the degree of influence of the clutch torque variation of second clutch CL2 on the transmission output torque is small as described above with reference to FIG. 3. A phenomenon can also occur whereby the reduction control for transmission torque capacity of second clutch CL2 is carried out giving priority to achieving the requested driving force of the vehicle so that the transmission torque capacity of second clutch CL2 is not sufficiently reduced, and the engine start shock cannot be reduced as is desired due to only the reduction of the transmission torque capacity of second clutch CL2.

[0098] At time point t3, corresponding to the determination at step S12' that the engine 1 has reached a self rotating state, the control system determines that second

clutch CL2 has already been slipped at step S 13', and, at step S 14', the control system determines that the input torque fluctuation absorbing effect of second clutch CL2 has a magnitude such that second clutch CL2 can reduce the engine start shock, hi this case, since the engine start shock reducing effect due to the slip of first clutch 6 is not needed, control goes to step S 15' to raise target transmission torque capacity tTcl of first clutch 6 as denoted by line εl shown in FIG. 8B, and this control is continued until instant time point t5 shown in FIG. 8B, where first clutch 6 is completely engaged. [0099] Even if integrated controller 11 determines that second clutch CL2 has already been slipped at time point t2 in response to the determination at step S 13', the control system determines that the input torque fluctuation absorbing effect of second clutch CL2 lacks sufficient magnitude to reduce the engine start shock. In this case, since the engine start shock reducing action due to the slip of first clutch 6 is needed, control goes to step Sl 7' at which target transmission torque capacity tTcl of first clutch 6 is reduced by according to line ε2 in FIG. 8B to reduce the engine start shock through the slip of first clutch 6. Hence, step S 17' in FIG. 6 corresponds to a function performed by a first clutch target transmission torque capacity reducing section of the integrated controller 11. In addition, step S 14' does not only perform the function of determining a second clutch torque variation shielding effect of the second clutch torque variation shielding effect determining section of integrated controller 11, it also performs the function of determining whether the first clutch transmission capacity should be reduced, corresponding to a first clutch transmission capacity reducing enabled determining section of integrated controller 11.

[00100] During slip control of first clutch 6, at step S 18', the control system determines whether engine speed Ne is raised as shown in FIG. 8E from time point t3 to become approximately equal to motor/generator revolution speed Nm in response to the engine start. Control advances to a loop passing through steps S15' and S16' from time point t4 shown in FIG. 8E at which engine speed Ne is approximately equal to motor/generator revolution speed Nm. Then, the transmission torque capacity tTcl of first clutch 6 is raised as denoted by line ε3 in FIG. 8B to completely engage first clutch 6. [00101 ] Where the control system determines that second clutch CL2 is not yet slipped at time point t3 (in response to the query of step S 13 '), the engine start shock reducing action due to the slip of second clutch CL2 is undesired, and the control system relies only on the engine start shock reducing action according to the slip of first clutch 6. Hence, in this case, control goes to step S 17' at which target transmission torque capacity

tTcl of first clutch 6 is reduced as denoted by line ε2 in FIG. 8B so that the engine start shock is reduced according to the slip of first clutch 6.

[00102] Then, from time point t4 at which engine speed Ne is approximately equal to revolution speed Nm of motor/generator 3 (in response to the query of step Slδtarget transmission torque capacity tTcl of first clutch 6 is raised as denoted by line ε3 shown in FIG. 8B so that the first clutch 6 is completely engaged according to the loop passing through steps S 15' and S 16'.

[00103] Turning again to FIG. 7, when the control system determines that engine speed Ne of engine 1 is equal to or faster than revolution speed Nm of motor/generator 3 in response to the query of step S26, control goes to step S27, which determines whether the disengagement-side clutch element was selected as second clutch CL2 in step S24. If not, in other words, if one of the clutch elements selecting the present gear shift stage whose input torque fluctuation shielding effect is the maximum is selected as second clutch CL2 in step S23, at step S28, target transmission torque capacity tTc2 of second clutch CL2 is raised as denoted by line α2 in FIG. 8C. This rise α2 of target transmission torque capacity tTc2 of second clutch CL2 is executed continuously until the determination that second clutch CL2 is completely engaged. [00104] If the control system determines at step S27 that disengagement-side clutch element during the shift-down of automatic transmission 2 is selected as second clutch CL2, control is advanced to step S30, where it is determined whether or not the down-shift request is still generated. If the down-shift request has vanished and is not generated (No), it is not necessary to carry out the shift. Hence, control advances to step S28 and step S29. Target transmission torque capacity tTc2 of second clutch CL2 (that is, the disengagement-side clutch element during the shift-down) is increased as denoted by line γ2 in FIG. 8D, and second clutch CL2 (disengagement-side clutch element during the shift-down) completely engages.

[00105] Where the control system determines that the disengagement-side clutch element during the down-shift is selected as second clutch element CL2 at step S27, and the control system determines that the down-shift request is continued (Yes) at step S30, the gear shift is needed. Hence, control advances to step S31 at which target transmission torque capacity tTc2 of second clutch CL2 (that is, the disengagement-side clutch element during the down-shift) is reduced as denoted by the line γ3 in FIG. 8D. Reduction of target transmission torque capacity tTc2 according to rate γ3 is executed continuously until the determination that the down-shift is ended at step S32. This makes second

clutch CL2 (that is, the disengagement-side clutch element during the shift-down) completely disengaged.

[00106] According to the engine start control of the hybrid vehicle in the second embodiment described above, transmission torque capacity tTcl of first clutch 6 is reduced as denoted by line ε2 in FIG. 8B from time point t3 at which engine speed Ne approaches motor/generator revolution speed Nm within a predetermined range (step S 17) after the determination that engine 1 has reached a self rotation state, the fuel injection of engine 1 is started, or the engine speed is raised during the engine start (cranking). The engine start shock can be reduced with the torque fluctuation during the engine start absorbed due to the slip of first clutch 6, whose transmission torque capacity TcI is reduced even where the degree of influence on the transmission output torque due to the clutch torque variation of second clutch CL2 is small such that the reduction of the transmission torque capacity in second clutch CL2 cannot reduce the engine start shock and even where the reduction of the transmission torque capacity reduction is not sufficient due to the priority given to achieving the requested driving force of the vehicle. [00107] First clutch 6 is interleaved between engine 1 and motor/generator 3, mainly because it is difficult to create a lubricating system having a superior durability forthe slip control. When slip control of first clutch 6 for reducing the engine start shock is carried out unconditionally, this makes the time for the slip longer so that the durability of the first clutch 6 is reduced due to a heat generation thereon. [00108] hi the second embodiment, only when the input torque fluctuation shielding effect according to the transmission torque capacity reduction control of second clutch CL2 (step S25) is lower (smaller) than the set value (step S 14') is the transmission torque capacity reduction control of first clutch 6 performed (step S 17'). When the input torque fluctuation shielding effect according to the transmission torque capacity reduction control of second clutch CL2 (step S25) is equal to or higher than the set value (step S 14'), the transmission torque capacity reduction control of first clutch 6 is not performed (step S 15'). Therefore, in the latter case, although the engine start shock can be reduced according to the transmission torque capacity reduction control of second clutch CL2, the transmission torque capacity reduction control of first clutch 6 is performed for reducing the engine start shock to avoid a case where the input torque fluctuation shielding effect according to the transmission torque capacity reduction control of second clutch CL2 is large.

[00109] Hence, the transmission torque capacity reduction control of first clutch 6 for reducing the engine start shock is not carried out unconditionally during the engine start. Thus, a slip time duration of first clutch 6 is minimized, and the problem associated with reducing the durability due to the heat generation on first clutch 6 can be eliminated. [00110] As in the first embodiment, in the second embodiment the determination that engine speed Ne has reached revolution speed Nm of motor/generator 3 within the predetermined range determines whether engine 1 has reached a self rotation state (step S 12'). However, the determination of whether fuel injection in engine 1 is started, the determination of whether engine speed Ne is raised, or so forth, can be alternatively used. Hence, the reduction in first clutch transmission torque capacity tTcl can truly be adjusted to a time at which the torque fluctuation (engine start shock) during the start of engine 1 is generated. These actions and advantages can become significant. [00111] Furthermore, the set value related to the input torque fluctuation absorbing effect of second clutch CL2 is the set value for determining whether the engine start shock can be reduced. Hence, when the engine start shock is positively reduced using slip control (step S25) of second clutch CL2, it is assured that the selection of step S 15' a wasteful slip control for first clutch 6 can be avoided by the selection of step si 5'. Thus, the actions and advantages of the described control can more accurately be achieved. [00112] In addition, where the shift-down of automatic transmission 2 during the engine start is requested (step S21), or where an accelerator operation that would generate the shift-down is carried out (step S22), the disengagement-side clutch element to be switched from the engagement state to the disengagement state during the shift-down is used as second clutch CL2 (step S24). Hence, a frequency at which the slip control of first clutch 6 is required can furthermore be reduced by using a disengagement-side clutch element that has the largest input torque fluctuation shielding effect (engine start shock reducing effect) during the down-shift. Thus, the described actions and effects can more become significant.

[00113] Furthermore, where the down-shift of automatic transmission 2 during the start of engine 1 is not requested (step S21), or where the accelerator operation is such that there is no possibility that the down-shift request is generated (step S22), one of the clutch elements to select the present gear shift stage that has the largest input torque fluctuation shielding effect is used for second clutch CL2 (step S23). Since the clutch element having the largest input torque fluctuation shielding effect during non-shift is used as second clutch CL2, the frequency at which the slip control of first clutch 6 is

required can furthermore be reduced. The actions and advantages of the control taught herein can be more significant.

[00114] As described with reference to FIG. 3, with the input torque fluctuation shielding factor of each of the clutch elements within automatic transmission 2 predetermined for each of the gear shift stages, the input torque fluctuation shielding factor of each clutch element for the selected gear shift stage is a criterion for determining the input torque fluctuation shielding effect related to second clutch CL2. Hence, the input torque fluctuation shielding effect of second clutch CL2 can be expressed numerically so that an easy determination can be made at step S 14' as to whether second clutch CL2 can reduce the engine start shock.

[00115] Furthermore, the reduction in the transmission torque capacity of second clutch CL2 carried out during the engine start (step S25) and the engagement of first clutch 6 (step SH') are simultaneously started. When the determination that engine speed Ne has approached the revolution speed of motor/generator 3 within the predetermined range is made (at time point t3), the following action and advantage can be achieved since the transmission torque capacity reduction control of first clutch 6 is carried out irrespective of the effect of determination of the torque fluctuation shielding effect of second clutch CL2.

[00116] In other words, with the robust characteristic taken into consideration in the engine start shock reduction technique to reduce the input torque fluctuation during the engine start, the detection of the slip of second clutch CL2 is followed only by the start of engagement of first clutch 6 to start engine 1. The engine start cannot be performed according to the engagement of first clutch 6 after second clutch CL2 starts slippage. This can result in an uncomfortable feeling at the time of the engine start response (driving force increase response) in the vehicle driver with the generation of engine start request (EV →HEV mode switch request) in response to the depression of the accelerator pedal.

[00117] However, in the second embodiment, where the transmission torque capacity reduction of second clutch CL2 carried out during the engine start (step S25) and the engagement of first clutch 6 (step SH') are simultaneously started, the dissatisfaction with the engine start response (driving force increase response) can be eliminated. At any rate, according to the control described above, it often occurs that second clutch CL2 is not yet slipped at the time point at which engine speed Ne has approached revolution speed Nm of motor/generator 3 within the predetermined range. In this case, since the

input torque fluctuation along with the engine start is largest in the proximity to time point t3 as which the engine 1 is in a self rotating state, it is difficult to reduce the possibility of generating a large engine start shock.

[00118] However, in the second embodiment, if second clutch CL2 is not yet slipped (step S 13') when the determination is made that engine speed Ne has approached revolution speed Nm of motor/generator 3 within the predetermined range, the transmission torque capacity reduction control of first clutch 6 is carried out (step S 17') irrespective of the torque fluctuation shielding effect determination result of second clutch CL2 (step S 14). Hence, even if second clutch CL2 is not slipped, the torque fluctuation during the engine start (engine start shock) can be reduced according to the transmission torque capacity reduction control (step S 17') of first clutch 6. The elimination of dissatisfaction with the engine start response (driving force increase response) and the assured engine start shock reducing action are compatible. [00119] In each of the embodiments described above, one of the generally available clutch elements Fr/B, I/C, H&LR/C, D/C, R/B and FWD/B within automatic transmission 2 is selected as the second clutch CL2. However, the invention is similarly applicable to a hybrid vehicle in which a newly added power train is installed after or before automatic transmission 2 to achieve the desired object.

[00120] Accordingly, the above described embodiments have been described in order to allow easy understanding of the present invention, and do not limit the present invention. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.