Originally published on EV Obsession.
A hot issue of complaint with a number of Tesla P85D customers over the past several months or so has been what they deem to be an inflated horsepower figure. Part of this is that the horsepower figure of the P85D is much higher than some other Tesla vehicles (like the P85+ that many owners upgraded from) while not seeming to provide an equivalent extra amount of actual power.
Tesla has finally responded, with a blog post from CTO JB Straubel. Depending on how you read it, you could either say JB “burned” or “trolled” the complainers. More neutrally, you could say that he provided a logical, cool-headed response that clears up some technical confusion and provides an official word on how Tesla came up with the numbers and why they vary like they do — I personally think this best describes the story.
No matter how you take the blog post, though, there’s still the separate matter as to whether Tesla provided the horsepower numbers (and 0–60 mph numbers, btw) that it should have (ethically speaking). Did Tesla use the right approach for determining the numbers? Did it try to deceive buyers or did it simply not consider that buyers may be confused and misled? Did it inadvertently use methodology that inadequately compares its various Model S options? Or did it know that it was going to make many buyers think the P85D was much more powerful than it is?
I can’t answer those questions, but I think the very last line in JB’s article captures a couple of important points. JB wrote, “The true measures for any performance EV driver are acceleration times and driving performance of the vehicle.” I think buried in this line is an implication that buyers test drove the P85D before buying it, and that they made a decision to buy based on how the car drove, not based on some numbers on the website. I think that’s logical, yet in some cases incorrect. Many buyers did buy the car without test driving it, or did so considering the experience from a short test drive as well as the horsepower and acceleration figures (and how they compared to other Tesla Model S options). I personally don’t think I’d buy a car without test driving it a couple of times, as well as test driving the other cars I’m considering, but that doesn’t mean everyone is the same. Furthermore, it’s hard to get a good sense for some things in a test drive, and relying on provided specs to help make a decision is very reasonable.
Getting back to the blog post, some people who have had an issue with these figures think the post didn’t fully address the complaints people have, skirted around the issue, and wasn’t specific enough when it comes to the P85D figures. I’ll let you decide (or argue about it, if that’s what you’d like to do).
But I’ve just provided a bunch of synopsis and commentary and haven’t even quoted the bulk of JB’s post yet. You can read the full thing + comments over on the Tesla blog, or you can just read the blog reposted below. It’s quite interesting from a technical point of view, imho, as it shows yet again how different ICE vehicles and EVs are, and further explains how an EV works. But, yes, I’m thinking it doesn’t fully provide the critics with what they wanted.
Tesla All Wheel Drive (Dual Motor) Power and Torque Specifications
By JB Straubel, Chief Technical Officer
Attempting to directly correlate horsepower ratings in petroleum burning vehicles to horsepower in an electric vehicle is a difficult challenge. The physics of an electric vehicle propulsion system are very different from a gasoline one. In an EV, electrochemical reactions in the lithium ion cells create electricity. That electricity flows through power electronics that control the voltage and current, then it flows to electromagnets in the motor that create powerful magnetic fields rotating the shaft to turn the wheels. The power required to rotate this shaft has the most correlation to traditional measures of horsepower. However, the chain actually begins in the electrochemical reactions that happen in the battery pack. Depending on the battery’s temperature, state of charge and age, the amount of electricity extracted can vary widely.
There is some confusion about our methodology for specifying “equivalent” horsepower ratings for our all-wheel drive, dual motor vehicles – the “D” versions of Model S. This document will hopefully answer those questions.
Defining electric power in terms of horsepower is not very intuitive. Kilowatts or Megawatts are a much more useful unit. Electricity alone can’t generate physical motion the way a horse or a fuel-burning engine does. An electric motor converts electricity into motion. Think of electric power as flowing much like fuel flows from a tank to an engine. Various situations (low state of charge, cold temperatures, etc.) can reduce this flow of electrons below the ultimate capability of the electric motor. In other cases, the potential flow of electricity may exceed the capability of the electric motor (warm battery, short duration accelerations, etc.). Since the battery electric horsepower rating varies it is not a precise number to use for specifying the physical capability of an EV. The motor shaft horsepower, when operating alone, is a more consistent rating. In fact, it is only this (single or combined) motor shaft horsepower rating that is legally required to be posted in the European Union.
Dual Motor vs. Single Motor (P85 vs. P85D)
The shaft horsepower rating of the rear wheel drive single motor Model S is straightforward and roughly 360-470 hp depending on the variant (60, 85 or P85). Also, it is generally similar, but not the same, as the battery electrical “horsepower” output. The difference is most obvious to drivers when the battery is at a very low SoC. In this state, the chemical reactions generate less voltage and less equivalent horsepower, even though the physical electric motor hasn’t changed. The maximum torque the electric motor(s) are capable of is nearly unchanged as the battery horsepower changes even though the maximum shaft horsepower is reduced as the battery horsepower reduces.
When we launched the all-wheel drive P85D, we took the straightforward and consistent approach of specifying the combined capability of the two electric motors, front + back. The torque from the two motors comes together resulting in a huge boost in acceleration, the “g’s” you feel in a P85D. This is why Insane Mode is so delightful. The vehicle takes off slightly faster than 1g of acceleration delivering the amazing 3.1 second 0-60 mph (96.6 kph) performance. This acceleration was verified by Motor Trend using a base vehicle and medium weight driver. It should be noted that a larger occupant and additional options that increase weight will reduce the acceleration. Also, the Motor Trend standard excludes the first 28 cm of rollout. Including this rollout adds approximately 0.2 seconds to the acceleration.
One additional note is that, while gasoline cars get worse with altitude, electric cars actually get faster. All cars experience reduced air resistance, but gasoline cars become increasingly oxygen-deprived the higher they go. The Motor Trend test was done at approximately sea level, so the Model S will outperform a combustion car of the same nominal acceleration as altitude increases.
With the shaft horsepower coming out of the motors the situation is not always as simple as front + rear. As we have pushed the combined motor horsepower higher and higher, the amount of times where the battery chemical horsepower is lower than the combined motor horsepower has increased.
Also, the all wheel drive system in the dual-motor cars distributes available electrical horsepower to maximize torque (and power) in response to road grip conditions and weight transfer in the vehicle. For instance, during hard acceleration, weight transfers to the rear of the vehicle. The front motor must reduce torque and power in order to prevent the front wheels from spinning. That power is fed to the rear motor where it can be used immediately. The opposite happens when braking, when the front motor can accept more regenerative braking torque and power.
All Wheel Drive 85D and 70D
When developing the 85D and the 70D dual motor vehicles, we maintained a consistent strategy of specifying the motor power using front + back methodology. In both of these vehicle variants the rear motor is of a very similar design to the front motor. The 70D motors deliver roughly 165 horsepower each, the 85D motors roughly 211 horsepower each and the front motor on the P85D delivers 221 horsepower. Although these motors are in the same family, their power electronics and control software are tuned differently.
Where some confusion occurs is that in the 85D and 70D vehicles the combined motor shaft power is very similar to the battery electrical horsepower under many normal conditions. With the P85D the combined motor shaft power can often exceed the battery electrical horsepower available. The dual motors utilize the battery horsepower in the widest variety of real world conditions. The true measures for any performance EV driver are acceleration times and driving performance of the vehicle.