2014 Tesla Model S 'P85D' all-wheel-drive model
How can you add all-wheel drive to an electric car, making it hundreds of pounds heavier--and still increase its efficiency and range?
Amid all of last week's hoopla over the blazing performance of the all-wheel-drive Tesla Model S P85D--0 to 60 mph in 3.2 seconds, 155-mph top speed--what really caught my eye was this: Tesla accomplished it all while actually improving the efficiency of the car, despite a weight gain of almost 300 pounds.
[EDITOR'S NOTE: Please see author's update on page 3].
That's unheard of.
Every car powered by an internal-combustion engine loses efficiency when all-wheel drive is added. It's due to friction losses in the extra shafts and gears, as well as the additional weight.
A rear-wheel-drive BMW 328i, for example, drops from 27 mpg combined to 26 if you add the X-drive all-wheel-drive system.
Tesla Model S 'P85D' photo, uploaded to Tesla Motor Club forum by Adelman, October 2014
Standard 85D: even more efficient
In the standard 85-kWh Model S that most buyers will likely choose, Tesla's new AWD system boosts the range by anywhere from 4 to 19 percent--depending on which of Tesla's contradictory numbers you believe.
The official EPA combined range of the standard S-85 is 265 miles. On its website configurator, Tesla says the new AWD version will go 295 miles at 65 mph. (Temperature and climate-control use are not specified.)
Though it's not quite an apples-to-apples comparison, that's an 11-percent improvement.
The range calculator on the Tesla website provides another basis for comparison. At a steady 65 mph, at 70 degrees with climate control on, the range calculator says the standard S-85 will go 242 miles.
By that yardstick, the range bonus for the 85D is even more astounding: 53 miles, or 19 percent.
Photo from Tesla 'D' tweet by Tesla Motors CEO Elon Musk, October 1, 2014
On the other hand, a blog post on the Tesla website says that both the 85D and 60D pick up an additional 10 miles of range over their single-motor counterparts.
That number is suspect on the face of it, however: With its smaller battery and shorter range, the 60D should have a proportionally smaller advantage over the standard 60-kWh model.
The most accurate comparison, however, may be that suggested by a speed vs range curve published in a 2012 blog post by Elon Musk and Tesle CTO JB Straubel on the Tesla website.
That curve shows the 85-kWh Model S to have a range of 262 miles at a steady 65 mph, the same condition ascribed to the 85D's 295-mile range. That's an increase of 33 miles, or 13 percent.
Take your pick: 30 miles of extra range, or 33 miles, or 53 miles...or 10 miles.
Whichever is correct, that's an impressive engineering feat.
Different AWD systems
As it turns out, the AWD system on the standard Model S 85D is markedly different from that of the much-ballyhooed P85D.
On the P85D, Tesla pretty much maintained the existing high-performance rear motor--at 350 kilowatts (470 horsepower)--and added an additional 165-kW (221-hp) motor up front.
Tesla Motors CEO Elon Musk delivers first Tesla Model S electric cars in Tokyo, Sept 2014
The result is a 47-percent increase in total power, to a mind-boggling 515 kW (691 hp). No surprise that performance went off the charts. It's a simple matter of power-to-weight ratio.
But something different is going on with the AWD version of the standard 85-kWh and 60-kWh versions of the Model S when D all-wheel drive is added.
The 85D and 60D replace the previously standard 285-kW (380-hp) rear motor with a pair of 140-kW (188-hp) motors, one front and one rear. That gives a total of 280 kW (376 hp), which is actually a slight decrease in total horsepower.
No details have been released yet about the new, less powerful motor fitted to those vehicles.
Tesla Model S electric motor and drive unit [photo posted by user Tam to Tesla Motors forum]
Is it merely a derated version of the current 285-kW motor, or something all new? A version of the 115-kW (154-hp) motor that Tesla provided to Toyota for the RAV4 EV? Or the 132-kW (177-hp) one provided to Mercedes for its B Class Electric Drive?
Despite the slight power cut, 0-to-60-mph time for the 85D falls from 5.4 to 5.2 seconds. Top speed rockets from 125 mph to 155 mph. And, of course, it's 4 to 19 percent more efficient.
So...how the heck did they do that?
During the D reveal, Tesla Motors CEO Elon Musk gave a general idea of how this was accomplished.