There is known a vehicular drive system including a differential mechanism operable to distribute an output of an engine to a first electric motor and an output shaft, and a second electric motor provided in a power transmitting path between the output shaft of the differential mechanism and vehicle drive wheels. Patent Document 1 discloses an example of such a vehicular drive system in the form of a hybrid vehicle drive system. In this hybrid drive system, the differential mechanism is constituted by a planetary gear set, which has a differential function to mechanically transmit a major portion of a vehicle drive force from the engine to the drive wheels, and to electrically transmit the remaining portion of the vehicle drive force of the engine from the first electric motor to the second electric motor through an electric path, so that the planetary gear set functions as a transmission the speed ratio of which is continuously variable, for instance, as an electrically controlled continuously variable transmission. The vehicular drive system is controlled by a control apparatus to drive the vehicle with the engine being held in an optimum operating state so as to improve the fuel economy of the vehicle.
Patent Document 1: JP-2003-301731 A
Generally, a continuously variable transmission is known as a device to improve the vehicle economy of the vehicle, while a gear type power transmitting device such as a step-variable automatic transmission is known as a device having a high power transmitting efficiency. However, there is not available a power transmitting mechanism which has advantages of both of those two devices. For example, the hybrid vehicle drive system as disclosed in the above-identified Patent Document 1 has an electric path through which the electric energy is transmitted from the first electric motor to the second electric motor, that is, a power transmitting path for transmitting a portion of the vehicle drive force as the electric energy, so that the required size of the first electric motor increases with an increase of the required output of the engine, resulting in a consequent increase of the required size of the second electric motor which is operated by the electric energy received from the first electric motor, whereby the required overall size of the drive system is unfavorably increased. Further, the hybrid vehicle drive system has a risk of deterioration of the fuel economy in some running condition of the vehicle such as a high-speed running, due to the conversion of a portion of the engine output into an electric energy subsequently used for driving the drive wheels. A similar problem exists where the above-described power transmitting mechanism is a continuously variable transmission such as a transmission the speed ratio of which is electrically variable, for instance, a so-called “electric CVT”.
In the hybrid vehicle drive system as disclosed in the above-identified publication, the first electric motor is required to receive a reaction torque corresponding to the engine torque so that the power distributing mechanism can function as the electric CVT. Accordingly, during a high-load low-speed running of the vehicle, for instance, during a vehicle running while towing a trailer, the amount of generation of an electric energy by the first electric motor is increased so that the amount of electric energy to be transmitted through the electric path is increased, resulting in an increase of the output of the second electric motor. Such a high-load low-speed running of the vehicle continued for a long time may cause a considerable rise of the temperatures of the first and second electric motors due to an increase of the amount of electric energy, and an increase of loads of components associated with the electric path for transmission of the electric energy generated by the first electric motor to the second electric motor, which converts the electric energy into a mechanical energy. Those temperature rise and increase of the loads give rise to deterioration of the function and durability of the components associated with the electric path which includes the first and second electric motors.
Therefore, it is required to increase the cooling capacity of the vehicular drive system, for the purpose of avoiding a temperature rise of cooling water in the cooling system for cooling the first and second electric motors, for example. It is also required to increase the capacities of the components associated with the electric path, for withstanding the loads due to an increase of the amount of generation of the electric energy. Meeting these requirements may cause increases of the sizes and weights of the components of the cooling system and the components associated with the electric path, unfavorably leading to an increase of the cost of manufacture of the vehicular drive system.
The present invention was made in view of the background art descried above. It is an object of this invention to provide a control apparatus for a vehicular drive system including a differential mechanism operable to distribute an output of an engine to a first electric motor and a power transmitting member, and a second electric motor provided in a power transmitting path between the power transmitting member and a wheel of a vehicle, and to electrically transmit a portion of a vehicle drive force such that an electric energy generated by the first electric motor is transmitted through an electric path to the second electric motor and converted into a mechanical energy by the second electric motor, which control apparatus makes it possible to reduce loads of components associated with the electric path or restrict a temperature rise of those components, for thereby permitting size reduction of a cooling system of the vehicular drive system.