A power transmitting device for four-wheel drive hybrid-vehicle has been known, which includes a drive source, a power distributing mechanism through which a drive power is distributed from the drive source to a first electric motor and a transmitting shaft, a central differential device connected to the transmitting shaft for distributing the drive power from the transmitting shaft to the first drive-wheel-side output shaft and the second drive-wheel-side output shaft, and a second electric motor connected to second drive wheels to transmit power thereto.
FIG. 11 is a skeleton view illustrating a structure of a conventional power transmitting device 200 for four-wheel drive hybrid-vehicle (hereinafter referred to as a “power transmitting device 200”). The power transmitting device 200 includes an engine 202 functioning as a main drive source, a power distributing mechanism 206 for distributing a drive power from the engine 202 to a first electric motor MG1 and a transmitting shaft 204, a central differential device 212 connected to the transmitting shaft 204 for mechanically distributing the drive power from the transmitting shaft 204 to a first drive-wheel-side output shaft 208 for delivery to front wheels 207 and a second drive-wheel-side output shaft 210 for delivery to rear wheels 209, and a second electric motor MG2 connected to the transmitting shaft 204 via a subsidiary transmission 214 to transmit power thereto.
A drive force of the first drive-wheel-side output shaft 208 is transmitted through a gear pair 216, a front propeller shaft 218, a front differential 220 and a pair of left and right front drive shafts 222 to a pair of left and right front wheels 207. Further, a drive force of the second drive-wheel-side output shaft 210 is transmitted through a rear propeller shaft 224, a rear differential 226 and a pair of left and right rear drive shafts 228 to a pair of left and right rear wheels 209. With the power transmitting device 200, the second electric motor MG2 is interposed between the power distributing mechanism 206 and the central differential device 212 to transmit a drive force from the second electric motor MG2 to the subsidiary transmission 214 through the transmitting shaft 204. Thus, no torque distribution ratio between the front and rear wheels is dependent on the output of the second electric motor MG2. That is, a drive force distribution between the front and rear wheels is mechanically determined in terms of a distribution ratio of the central differential device 212. Accordingly, the drive force distribution between the front and rear wheels has a low degree of freedom, which results in difficulty in obtaining an appropriate drive force distribution depending on a running state of a vehicle.
On the contrary, a hybrid-vehicle drive apparatus, disclosed in Patent Publication 1 (Japanese Patent Publication No. 2004-114944), has substantially the same structure as that of a power transmitting device 300 shown in FIG. 12, to be described in more detail. The power transmitting device 200 has the second electric motor MG2 connected through the subsidiary transmission 214 to the transmitting shaft 204 to transmit power thereto. In contrast, in the power transmitting device 300, the second electric motor MG2 is connected through the subsidiary transmission 214 to the second drive-wheel-side output shaft 210 to transmit power thereto. Thus, the output power of the second electric motor MG2 is transmitted only to the second drive-wheel-side output shaft 210, and therefore, controlling the second electric motor MG2 renders a greater degree of freedom of the drive force distribution than that of the power transmitting device 200.
Further, although not disclosed in Patent Publication 1, as shown in the power transmitting device 300, there has been devised a power transmitting device of the type having a clutch C1 disposed between the first drive-wheel-side output shaft 208 and the second drive-wheel-side output shaft 210. With such a structure, for instance, if the clutch C1 is caused to completely engage, then, the first drive-wheel-side output shaft 208 and the second drive-wheel-side output shaft 210 are caused to unitarily rotate such that the drive force is equally distributed to the front wheels 207 and the rear wheels 209. In addition, if such an engaging device C1 is brought into half-engaged state, the drive force is appropriately distributed depending on engagement torque of the clutch C1. Consequently, a drive force distribution is further increased in degree of freedom.
Here, if a drop occurs in a storage capacity of for instance an electric storage device (battery) from which electric power is supplied to the first and second electric motors MG1 and MG2, for increasing charge capacity of the battery, the first and second electric motors MG1 and MG2 execute power generation controls. First, the power transmitting device 200 in FIG. 11 will be described below with reference to the power generation controls of the first and second electric motors MG1 and MG2. Suppose that a drive force (torque) of the engine 202 is 100, and a drive force distribution ratio between the first electric motor MG1 and the transmitting shaft 204 of the power distributing mechanism 206 is 3:7. Then, torque (Tg) of 30 is transmitted to the first electric motor MG1 and torque (Tr) of 70 is transmitted to the transmitting shaft 204.
Here, suppose that power generation torque (Tm), transmitted via the subsidiary transmission 214 to the second electric motor MG2 is 50, and then torque (Tp=Tr−Tm) of 20 is transmitted to the central differential device 212. Moreover, suppose that a drive force distribution ratio between the first drive-wheel-side output shaft 208 and the second drive-wheel-side output shaft 210 of the central differential device 212 is 4:6. Torque (Tpf) of 8 is transmitted to the first drive-wheel-side output shaft 208, and torque (Tpr) of 12 is transmitted to the second drive-wheel-side output shaft 210. Thus, for the drive force (Te=100) of the engine 202, if torque (Tg) of 30 is transmitted to the first electric motor MG1 and torque (Tm) of 50 is transmitted to the second electric motor MG2, then, a total drive force (Tall) transmitted to the front wheels 207 and the rear wheels 209 becomes 20 (=Tpf+Tpr).
Meanwhile, the power transmitting device 300 shown in FIG. 12 will be described below in a case in which the power generation controls are executed under the same condition as that described above. Suppose that a drive power (torque) of the engine 202 is 100, and a drive force distribution ratio between the first electric motor MG1 and the transmitting shaft 204 by the power distributing mechanism 206 is 3:7. Like the above power transmitting device 200, torque (Tg) of 30 is transmitted to the first electric motor MG1 and torque (Tr) of 70 is transmitted to the transmitting shaft 204. Further, suppose that a front-to-rear-wheel drive force distribution ratio between the first and second drive-wheel-side output shafts 208 and 210 by the central differential device 212 is 7:3. Torque (Tdf) of 49 is transmitted to the first drive-wheel-side output shaft 208, and torque (Tcr) of 21 is transmitted to the second drive-wheel-side output shaft 210. Thus, torque (Tpf) of 49 is transmitted to the front wheels 207.
Meanwhile, if torque (Tm) of 50 is transmitted to the second electric motor MG2 like the power transmitting device 200, then torque (Tpr=Tcr−Tm) of −29 is transmitted to the rear wheels. That is, a total drive force (Tall=Tpf+Tpr) of the front and rear wheels 207 and 209 becomes 20 like that attained with the power transmitting device 200. However, if torque (Tm) transmitted to the second electric motor MG2 exceeds torque (Tcr) of the second drive-wheel-side output shaft 210, negative torque occurs on the rear wheels 209. Thus, the drive force distribution between the front and rear wheels is irregularly achieved, giving discomfort to a driver during for instance a turning running or the like.
The present invention has been completed with the above view in mind, and has an object to provide a control device for power transmitting device for a four-wheel drive hybrid-vehicle having a drive source, a power distributing mechanism for distributing a drive force from the drive source to a first electric motor and a transmitting shaft, a central differential device connected to the transmitting shaft to distribute the drive force from the transmitting shaft to first and second drive-wheel-side output shafts, and a second electric motor connected to the second drive-wheel-side output shaft to transmit power thereto. The control device can prevent the occurrence of negative torque caused by power generation control of the second electric motor.