There are two electric motors in a Camry Hybrid, a torque and a counter torque motor. That's because the gas engine, and the two motors are directly connected to the same planetary gear that drives the wheels there are no gears in the traditional sense of transmissions and no shifting of gears. The proportion of which motor is delivering power to the wheels and by what gear ratio is controlled by the respective motors. That means pretty much all of them need to be working or locked (braked) into position for power to go to the wheels. Even when the gas engine is running, the electric motors have to hold their end of the planetary gear from turning or the power doesn't get to the wheels likewise the gas engine has to provide some resistance for the power to go from the electric motors to the wheels, how much the electric motors hold their end with respect to each other controls the gear ratio and the power regeneration, depending on the gear ratio, the gas engine may actually be turning one electric motor to generate electricity so that the other electric motor can run faster so that the gas engine doesn't need to run faster. It's by varying the controls on the two electric motors and the gas engine that they control the effective gear ratios of the Synergy transmission. When the car is running on electricity only, the gas engine is locked down to provide the resistance. It's really a very clever design that GM is planning on copying.
If the control system is smart enough to lock down the gas engine when it's out of gas then the two electric motors could still move the car but if not, they would be cranking the gas engine which is actually how the gas engine gets started. I would suggest that you ask Toyota what happens when you run out of gas or just make sure you have a jerry can of gas with you when you try.
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