Tesla P85D: Top Speed Monster?

Tesla P85D: Top Speed Monster?

Our buddies over at Green Car Reports posted a story suggesting that there's more to the Tesla top speed numbers than meets the eye and the site is calling for a top speed test.  Though we're as thrilled as anyone to see someone get out on the salt and run 'em til they pop, the suggestion that simply removing the speed limits on the car would give you free speed points to some basic confusions that we all wrestle with on this new EV drivetrain thing.

Back in the old days, when we were all riding minibikes with Briggs and Stratton 5-horse lawnmower motors, if our fathers had any sense at all they were equipped with "governors." These were simple devices that, depending on the RPM you were spinning them at, would control the throttle.  Their main intention was to feed the throttle more gas when the RPM dropped, so it would respond to the load automatically.  Your Dad figured out that he could also use it to control the top RPM of the motor, thus extending the probability of his child reaching his next birthday with relatively little brain damage.  But we digress.

Later in the history of automotive development and speed controls we saw Detroit muscle cars with speed-limiters which, legend had it, were installed for insurance and regulation purposes.  There's the notorious German "gentlemen's agreement", and Japan's agreement to limit horsepower.  If memory serves, it was the Buick Grand National that was a particularly loathsome example of that, from where we sat in the driver's seat of the Mustang 5.0.  Are we digressing yet again?  No, point being, this isn't that kind of "speed limiter".

Electric motor

Electric motor

While Green Car Reports' math and physics looks okay, how they're thinking of the speed limiter might be a little wrong.

Here's the theory:

"Both the Dodge Charger Hellcat and the 552-hp Aston Martin Rapide max out at 203 or 204 mph.

With far lower drag, far more power, the P85D ought to be able to top 210 mph--if CEO Elon Musk would let his engineers remove the speed limiter."

Here's what's wrong with that.  Remember about how you rate power in an electric drive system being far different than a gas drive.  The bottom line is, yes, if the power rating of 700 or whatever is good for the several minutes you need to spool up a speed-run to 200+, then you should be able to get there.  Chances are, though, it's not.

First, let's start with the basic principle of reaching your top speed in any vehicle.  It comes down to a simple equation of drag, which increases exponentially, and power - your machine's ability to push against that exponential force.  Given identical methods of rating horsepower, you should be able to say that two cars with equal power and differing drag should show a top speed reflecting the drag coefficient.  Also, let's say that two cars with equal drag, but differing power should show similar results.  It's fair to say that the Tesla with more power and less drag should trump the contenders.

Now, when you horsepower-rate a gas motor, you plug it into a dynamometer, spin it up, and see what it produces for power.  Power is a function of RPM and torque, remember, and gas motors develop peak horsepower at a very narrow RPM range.  An electric motor, in theory, will deliver whatever it's load is, given adequate Watts supplies, until it melts.  Rating an electric motor then must be a rating for a given time period.  Typically it's rated at peak, which may be only ten seconds, as well as continuous, which is, well, continuous for days on end.  Remember, electric motor ratings were developed for industrial applications.

Which brings us to that pesky speed limiter.  In the case of an EV, you may be limiting speed for any number of reasons.  The most likely is the inverter power limits.  As you (exponentially) increase load on the drivetrain from air drag, you increase load on the inverter.  There are very hard-and-fast numbers as to what the inverters are rated at before they fail, and how long they car run at various outputs.  It's not at all likely that Tesla limited the top speed to 155 mph (not coincidentally, that 155 mph limit is the German Gentleman's Agreement figure too) out of safety, insurance or regulatory concerns.  It's far more likely it's limited because of the power limits of the inverters.  Now, you may be running a drive system that can push out 700hp to the wheels, but for how long?  That's the crux of the issue...  and we've already seen the cars cut back after a few seconds on the drag strip.  To pull a 200mph run, we'd have to run that peak power for minutes.

Misspent youths<em> (via <a href=MotoMusings.com)" draggable="false">

Misspent youths (via MotoMusings.com)

So yeah.  Maybe you could go in and reprogram the current limits on the controllers, do a 210 mph speed run or two, and pop a couple of inverters after a few minutes.  But what would that prove?

"So, Mr. Musk: Take off the speed governor completely, at least on the P85D, and let's see what the car can do!"

We may as well ask, "Hay Mr Musk, run a few P85Ds until they blow, and let's see what the crater looks like!"  This ain't like your minibike where you can take the governor off and "go 30!".  (No. Of course not. We never would have done such a thing.  Not at least without getting it back together before our Dad's saw it.)

... and we haven't even mentioned the fact that the tires are limited to a 149-mph top speed.  ...and it appears Tesla would like to keep it that way, since there's been no sign of a P85D tire specifically developed for speeds above 149.  But that's another discussion.

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