1. Field of the Invention
The present invention relates to a control apparatus for a hybrid vehicle drive system, and more particularly to system starting of a hybrid vehicle.
2. Discussion of Prior Art
There is known a control apparatus for a hybrid vehicle drive system including (a) an engine, and (b) an electrically controlled differential portion having an input shaft connected to the engine, an output shaft, a differential mechanism, and an electric motor which is connected to a rotary element of the differential mechanism and the operating state of which is controlled to control a differential state of rotating speeds of the input and output shafts. JP-2003-63258 A discloses an example of such a hybrid vehicle drive system. In hybrid drive systems as disclosed in the above-identified publication, a hybrid vehicle is generally driven by the vehicle drive electric motor with the engine held at rest in a running condition of the hybrid vehicle in which the operating efficiency of the engine is relatively low. In a running condition of the hybrid vehicle in which the engine is operated at a comparatively low speed to produce a comparatively small torque, for example, during starting of the hybrid vehicle, the vehicle drive electric motor is operated to drive the vehicle with the engine held at rest in view of a comparatively low operating efficiency of the engine
The rotary elements of the differential mechanism or other rotary members are required to be lubricated even while the hybrid vehicle can be driven by a drive force produced by the vehicle drive electric motor with the engine held at rest. The above-identified publication proposes the use of a mechanical oil pump which is operated by the engine to lubricate the rotary members. When it is determined that the rotary members is required to be lubricated, an electricity generating electric motor provided as an electric generator is operated to operate the engine for a predetermined time for operating the mechanical oil pump, so that a lubricant is supplied to the rotary members.
In recently developed hybrid vehicle drive systems, the above-indicated electrically controlled differential portion is connected to a hydraulically operated automatic transmission portion. To start the hybrid vehicle equipped with such a hybrid vehicle drive system including such an automatic transmission portion, hydraulically operated coupling devices incorporated in the automatic transmission portion to establish a desired operating position thereof must be selectively supplied with a pressurized working fluid. To this end, the mechanical oil pump is operated by the engine driven by the electricity generating electric motor, to supply the pressurized working fluid to the relevant hydraulically operated coupling devices of the automatic transmission portion, upon starting of the hybrid vehicle with the vehicle drive electric motor with the engine held at rest. In this condition, however, the engine is operated with a low operating efficiency, leading to a risk of deterioration of fuel economy of the vehicle. In view of this drawback, it is considered to use an electric oil pump which is provided in addition to the mechanical oil pump and which is electrically driven to actuate the desired hydraulically operated coupling devices of the automatic transmission portion while the engine is at rest.
The use of those two oil pumps makes it possible to reduce the risk of deterioration of the fuel economy of the hybrid vehicle, owing to the operation of the electric oil pump rather than the engine in the running condition of the vehicle in which the operating efficiency of the engine is relatively low. It is generally required to start the hybrid vehicle drive system before starting of the hybrid vehicle. The starting of the hybrid vehicle drive system requires a predetermined length of time before the drive system becomes ready to start. In this respect, it is noted, for example, that the hybrid vehicle drive system is provided with a high-voltage electric system for driving the vehicle drive electric motor. To start this high-voltage electric system, it is required to effect a diagnosis of the high-voltage electric system for circuit protection, which requires a predetermined length of time. Where the hybrid vehicle drive system includes the automatic transmission portion operated under the control of a hydraulic control unit, this hydraulic control unit is required to be supplied with the pressurized working fluid to permit running of the hybrid vehicle, as a part of the starting procedure of the hybrid vehicle drive system. Where the supply of the pressurized working fluid to the hydraulic control unit is achieved by the electric oil pump, the supply of the fluid requires a further time since the electric oil pump becomes operable only after a high voltage of the high-voltage electric system has been lowered to a suitable level by a DCDC converter after the high-voltage electric system is started. Thus, the hybrid vehicle drive system provided with the automatic transmission portion requires a relatively long time for starting thereof, which may give the operator of the hybrid vehicle a discomfort associated with slow or delayed starting of the hybrid vehicle.