6

In the page http://www.teslamotors.com/models if we scroll down we see the several flavors Tesla sells its Model S.

It puzzles me why the 85 version has a shorter range than the 85D. The first can run 265 miles, the second 270. Given that in the former the battery has to feed only one motor and in latter it has to feed two, why the first has a more limited ride distance?

  • In order to get the same total power, the battery only has to provide half as much power to each motor... – Nate Eldredge Feb 17 '15 at 0:53
11

It says right on the webpage:

Conventional all-wheel drive cars employ complex mechanical linkages to distribute power from a single engine to all four wheels. This sacrifices efficiency in favor of all weather traction. In contrast, each Model S motor is lighter, smaller and more efficient than its rear wheel drive counterpart, providing both improved range and faster acceleration.

  • The copy on the web page quoted is pretty bad. It seems to be conflating in the quote given the issues of why the all-wheel-drive Model S is more efficient than the rear-wheel-drive Model S, and why the AWD Model S is more efficient than conventional AWD ICE cars. OP is not asking about the latter. – dodgethesteamroller Sep 29 '15 at 18:16
  • @dodgethesteamroller - As I highlighted in the quote, Tesla was talking about the difference between their 2WD version and the Model S (at that time). I'm not sure what your issue with what I've written is, or exactly what you are trying to say. – Pᴀᴜʟsᴛᴇʀ2 Sep 29 '15 at 22:58
  • If you have an issue with being downvoted, take it up in chat. The text you cited is poorly written and ambiguous; you provide no explanation beyond the quote alone; the latter is in direct violation of SE quidelines for a good answer. – dodgethesteamroller Sep 29 '15 at 23:16
  • 3
    Being downvoted doesn't bother me. Lots of people disagree with me, usually over stupid things. Not an issue. I would like to say, however, in my answer I pointed out an obvious fact which the OP had overlooked, which it appears you overlook as well, which does answer the question at large. It is comparing the 85 to the 85D and telling why the 85D has both improved range and faster acceleration than it's 2WD counterpart. It completely answers the question. I'm sorry you cannot see this fact. Why does anyone need to post more than the answer when Tesla provides it for you? No reason at all. – Pᴀᴜʟsᴛᴇʀ2 Sep 29 '15 at 23:39
5

This is pieced together from articles about this and the talk we had from a Tesla rep at the car leasing company I work for.

The improved range of the dual motor setup arises from the front and rear gearboxes (one per motor, nothing interlinking them) having different ratios. Electric motors have a rpm range in which they operate most efficiently, having the motors geared differently means that the car has 2 road speed ranges (1 for each motor) in which the motors are at peak efficiency, covering much more of the car's total speed range.

Also, each motor can be smaller which may be more efficient than a larger motor as a lot of the time only 1 motor will be propelling the car, though that will be offset to some degree by the increased frictional losses from having double the number of drivetrain components.

On a side note, I don't have any info but my guess is that the shorter geared motor has a 1-way clutch of some sort to prevent it from over speeding when the car is travelling at high speed.

  • Following you reasoning, for the 1 axle traction, I mean the RWD model, if it had two motors, one per wheel, instead of only one, it would have more autonomy, right? – sergiol Dec 22 '15 at 22:37
  • If the motors were the same size with the same gearing, I'm not sure what difference that would make. That setup might even be less efficient than 1 motor. However, if the 2 motors are both driving the whole axle but the motors are geared to cover different speed ranges it should improve the range/autonomy. That's how the dual motor setup works except the motors are driving separate axles instead of sharing one. – squigbobble Dec 23 '15 at 8:55
2

I am not an expert but I read in an article somewhere when Tesla introduced the P85D that the dual-motor version is more efficient at putting power to the wheels since it can use all four wheels instead of just the rear wheels. This even offsets the increased weight of the dual-motor setup and translates to a 2% increase in range.

  • what the source? – sergiol Oct 23 '15 at 15:23
  • 1
    Unfortunately I don't recall what the source was. But I found an article explaining a bit more here: wired.com/2014/10/… From the article:"Musk said the added efficiency is thanks to the electronic system that will shift power between the front and rear motors from one millisecond to the next, so each is always operating at its most efficient point." – lexeter Oct 26 '15 at 10:17
  • 2%? That doesn't impress me much. If it would only be 2%, Tesla would not even think to put the 4WD model on the market. – sergiol Dec 22 '15 at 22:40
  • I think Tesla wanted to have a 4WD car in their model line-up. The 2% increase in range is probably just a nice side effect and not the primary goal. – lexeter Jan 8 '16 at 10:50
-2

Think of how a high-end computer, like a server or hand-built gaming machine, can run just as cool and make less noise doing it with several small fans in the case rather than one big one.

The AWD Model S has two electric motors, one between each pair of wheels in front and back. The RWD Model S has just one, in the back, where the differential would be in a conventional car.

Now imagine if Tesla had designed the P85D to have performance equal to the original RWD Model S. Each motor would only have to produce half the power and torque of the original single motor. The smaller motors stay cooler, though, so they run more efficiently (heat is the enemy of electric motors, and makes them less efficient, unlike internal combustion engines). So the sum of the power requirements of both motors in the AWD Model S would be less than the requirement of the single motor in the RWD car.

Rather than making the AWD and RWD cars have equal performance, Tesla instead chose to make the AWD car more powerful. The result actually is still more efficient than the RWD car, as you've noted; the engineers sacrificed some of the potential greater efficiency of the dual motors in the name of performance, but not all of it.

  • 3
    If you look at the Tesla page, you'll realize the 85D beats the 85 in all performance categories. The reason the 85D has a longer potential range than the 85 is probably because it doesn't have to use both engines all the time, allowing it to run on the more efficient motor than the 85, giving it a longer range. – Pᴀᴜʟsᴛᴇʀ2 Sep 29 '15 at 23:12
  • That is pure speculation, which my answer is not. – dodgethesteamroller Sep 29 '15 at 23:18
  • 2
    I'm sorry ... your answer is pure speculation. – Pᴀᴜʟsᴛᴇʀ2 Sep 29 '15 at 23:19

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