Tesla Model S Dual-Motor Is Quicker, Has Higher Range Too: How Do They Do That? (UPDATED) Page 2


Tesla Motors CEO Elon Musk reveals Tesla Model S 'D' all-wheel-drive system, Oct 2014

Tesla Motors CEO Elon Musk reveals Tesla Model S 'D' all-wheel-drive system, Oct 2014

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"Because we have two drive units, where we can shift the power from front to rear, and constantly be at the optimal efficiency point for each motor," Musk said, "we're actually able to overcome the penalty of the increased mass of the motor."

Later he elaborated  for Bloomberg News: "We're able to balance the efficiency of the motors....If you have just one motor, it's always on a particular power-vs-efficiency curve."

"But if you have two motors, you can optimize between them, and have the motors operate in their more efficient regime more of the time."

2014 Tesla Model S

2014 Tesla Model S

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While Musk didn't get further into the details of this system,  it's likely that one motor is optimized for powerful acceleration,  the other for efficient high-speed cruising.

Presumably each has a gear ratio optimized for its primary duty. Power is then shunted to whichever motor is in the more efficient part of its power/efficiency curve and rpm range for the job at hand.

Single-motor compromise

Due to the rotating-speed limits of a single fixed-gear motor, Tesla designers originally had to choose between acceleration and top speed for the standard single-motor Model S.

They clearly chose acceleration, at the expense of top speed.

The single-motor 85-kWh Model S has a top speed of 125 mph--far slower than comparable sport sedans with similar power.

It's a flaw that's largely irrelevant to most U.S. buyers, but it has apparently hurt Model S sales in Germany, where the ability to cruise at 130 to 140 mph on the Autobahns is de rigueur in a car costing $100,000 or more.

2014 Tesla Model S

2014 Tesla Model S

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The new 85D , with two motors, has the best of both worlds.

We would presume the rear motor is geared low for quick acceleration, while the front motor pushes a taller "overdrive" gear for more efficient high-speed cruising.

The fact that top speed has jumped from 125 to 155 without a power increase suggests a radical change in gearing for at least one of the motors.


 
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