Hybrid drive system wherein electric motor or engine is selectively used for rearward driving of vehicle

A hybrid drive system for a motor vehicle having a drive wheel, comprising:

• an engine operated by combustion of a fuel;
• an electric energy storage device for storing an electric energy;
• an electric motor operated with the electric energy stored in said electric energy storage device;
• a power transmission device having at least one forward-drive position for driving said motor vehicle in a forward direction and at least one rear-drive position for driving said motor vehicle in a rearward direction, and disposed in a power transmission path between said drive wheel and a drive power source consisting of said engine and said electric motor;
• manually operated selecting means for selecting said at least one rear-drive position of said power transmission device;
• rear-drive shifting means for placing said power transmission device in one of said at least one rear-drive position when said manually operated selecting means is operated to a reverse position for selecting said at least one rear-drive position;
• energy determining means for determining whether an amount of the electric energy stored in said electric energy storage device is smaller than a predetermined lower limit;
• and
• rear-drive mode control means, operable when said manually operated selecting means is operated to said reverse position, for operating said electric motor to drive said motor vehicle in said rearward direction if the amount of said electric energy stored in said electric energy storage device is not smaller than said lower limit, and for operating said engine to drive said motor vehicle in said rearward direction if the amount of said electric energy stored in said storage device is smaller than said lower limit.