2014 Tesla Model S 'P85D' all-wheel-drive modelEnlarge Photo
All in all, improving acceleration, top speed, and efficiency in a heavier car is an engineering tour de force that Tesla should be very proud of.
And it's one more example of the inherent superiority of the electric motor over combustion engines when it comes to the business of propelling a car.
We can picture engineers at BMW, Mercedes, and Audi pounding their heads on their desks in frustration.
You almost have to feel sorry for the poor guys still stuck working on those old-fashioned cars with combustion engines.
UPDATE: In the comments section, a sharp-eyed reader posted a link to a Tesla patent document describing a “control system for an all-wheel-drive electric vehicle.”
The document describes a system in which the two motors have different torque characteristics and gear ratios matched to different conditions, as we theorized above.
Each motor has what’s called a “base speed,” defined as “the speed at which the torque declines to 95 percent of the flat peak torque, and will continue to drop off....”
In other words, the base speed is the highest motor rpm at which the motor can provide nearly peak torque.
When gearing is taken into consideration, this rpm is called the “drive system base speed”--the highest wheel rpm (and therefore car speed) at which near-peak torque is available.
In one example cited in the document, the primary drive motor has a base speed at 7200 rpm. But the assist motor is designed to provide a “much higher” drive system base speed than that of the primary drive motor--either due to a torque curve that maintains its peak to a higher rpm, or due to higher gearing.
Preferably, the document says, the assist motor should have a drive sytem base speed “at least 50 percent higher” than that of the primary drive motor.
Of course the motors desribed in the document may or may not be the Model S motors. But the example gives a good eidea of Tesla’s general approach to dual-motor AWD systems.