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Discussion in 'Model S: Driving Dynamics' started by TheAustin, May 28, 2013.
Would anyone be shocked if Tesla announced an Autobahn model?
So another way then to look at this graph, is to say your best efficiency is anything below 7k RPM, as after that you have some voltage not being directed to the propulsion of the car. Obviously Elon has posted the graph for speed vs range, but this dynamic comes in play, behind the scenes... correct?
- 125mph is enough even for Autobahn
- they won't sacrifice lowspeed acceleration
- multigear transmission is in the tesla's past not future
Also, those torque/RPM charts should be tourqe/speed charts now that we have singespeed gearboxes.
Or even better: max acceleration/speed charts considering vehicle weight
Personally, I think Tesla has them dialed down. People don't need to drive as fast as 125, but Tesla has said they expect Germany to be a good market. Without "Autobahn speeds" I don't think it will be. German journalists will be merciless
Not necessarily, no.
The drop off above 7k RPM is not necessarily because of inefficiency. I think it's far more likely that the motor is simply limited in how much power it can handle. Power is torque times RPM [times 2*pi/60 if your torque is in Nm and you want your power in W].
I think the graph might be easier to understand if it were shown as Power vs RPM instead of Torque vs RPM.
In that case, I think you'd see a straight line up to 7k RPM, a relatively flat line from 7k RPM to 15k RPM, and then it dropping off sharply from there.
Nothing in the plot indicates what the input power is doing. If one were to plot Input Power vs RPM and Output Power vs RPM on the same plot, I suspect that they'd track reasonably closely up to 15k RPM. The least efficient RPM as a percentage has to be 0 RPM, where output power is by definition 0, but input power is greater than 0.
I will note that Tesla specs the standard 85 kWh power output at 9500 RPM, so the motor is probably reasonably efficient up to 9500 RPM.
Right there wouldn't be a large efficiency drop. Your motor is probably going to be 95% (probably 98%) efficient at all speeds above stall. You would not be able to put as much electrical power (P=IV) into the motor. So you don't lose power, you use the ability to convert electrical power into kinetic power.
In general inductions motors operate at peak torque at speeds up to their rated speed. Then run peak power above that speed (where torque drops off as RPMs increase). You can do some cool stuff with inverters/VFDs to change this basic statement a little. But in general it is a good way of thinking. Remember that Power is Torque*RPMs.
Not really, electric motor is primarily limited by heat buildup and secondarily by maximum current surge it can take without burning the wires. The third limit is maximum voltage that is rarely a limiting factor i.e. insulation is usually strong enough.
Electric motor is under maximum stress at 0 RPM, it acts almost like a short-circuit. After it manages to start turning it can take some maximum current all the way up to maximum power. After that back-EMF starts to eat away its power output. People doing DIY electric cars often found out that they can climb a hill faster in 4th gear than in third - motor would spin faster and output less power.
I transformed Torque/Rpm chart that EarlyAdopter posted above into a Power/Speed chart for MSP:
We can learn from this chart that:
- From 0 to 60 there is constant torque
- Model S has max power output at 60 mph
- From 60 up to 125 mph (15k rpm) power drops slightly but not much
- At 125 mph power drops sharply (at 16k rpm there is only ~ 60 Nm/100kW of torque/power left)
- Model S drivetrain (reduction gearbox) is optimized for 0 - 60 times
- At 125mph there is still ~250kW of power that should suffice for at least 150 mph with longer gear-ratio. 0-60 times would be worse/slower though.
Autobahn model S won't happen, but one could try "making one at home" - mount rear tires with bigger diameter like say 285/40R24. Such tires would move max speed from 125 up to ~150 mph.
Thanks for the power plot. Although, if your only source was that tiny picture of torque vs RPM, I think there's enough error in the numbers that the power line might actually be flatter than it looks in the new graph.
Also, I'm not sure what the source for the plot was, but it doesn't necessarily match the specs of any production car:
Model S Options Pricing | Tesla Motors
Lastly, I strongly disagree with your conclusion that Model S gearing is optimized for 0-60 times. To achieve the best 0-60 times in a system only limited by a max torque, a max power, and a max RPM, you're going to do best when you gear the motor to hit its max RPM right around 60 mph. If you do that, you'll be putting out maximum power from like 20 - 60 mph. As the car currently is, you only put out the maximum power when you're above 50 mph.
Tesla had to choose between better 0-60 times or better top speed. They struck a somewhat reasonable balance. If Tesla was willing to live with a maximum speed of 70 mph, they could get much better 0-60 times. Instead of 150 kW at 25 mph, it could be putting out 310 kW at 25 mph. They'd probably need to beef up some other things (like the tires) to match the larger forces, but no, just because Model S doesn't output 310 kW until it's close to 60 mph doesn't mean that it's optimized for 60 mph. It actually means nearly the opposite: it doesn't even reach its full potential until close to 60 mph.
Tires are torque-limited as well and most people wouldn't want their Model S to have big/wide drag slicks on the back of a 70mph-limited car, even it it could get to 70mph in a few seconds...
No, it doesn't. It is a plot of preproduction version. I believe Tesla never released more up to date chart.
Here is a chart from a dynorun:
I didn't say it would have been a good idea to change the gear ratio by 2.
I was just saying that when your 310 kW engine barely reaches 310 kW before 60 mph, the gearing is decidedly not optimized for 0-60 runs. (and I did note that they'd need to beef up some things like tires if they wanted to dramatically improve launch times).
Not that it matters, but 130mph is available.
Aehm? Don't think so. My normal cruising speed when I am in Germany is 100 MPH, something the Model S wouldn't handle well with any meaningful range. In addition, there is a reason why many car companies if they limit their cars have that limit at 155MPH. 125 MPH are exceeded frequently on the Autobahn. And again, you wouldn't be able to drive the Model S at this speed for any extended period of time.
Then why should they enable even higher speeds and thus sacrifice low-speed performance? As I see it, you contradict yourself.
This problem, if it is a problem for you, can hardly be fixed by increasing the Model S' top speed (by whatever means).
The point you make is certainly valid albeit you may be in for a pleasant surprise. Two German journalists already tried to bring down range as far as possible by racing the Autobahn, and they could still go 250 km (155 mi) on a single charge. That counts as "meaningful" in my book.
Der ganze Test im Überblick: Drei Tage in Teslas Model S - manager magazin online - Nachrichten - Lifestyle
@ Derekt75, WarpedOne, ElSupreme... You guys are part of what make this forum so awesome/helpful. Thanks!
And so I look forward to testing out this power band curve around town, and passing on the highway.
That's very good range at high speeds! Many gas cars have similar ranges at those kinds of speeds. Heck, I had a Subaru which would only do about 230 miles before I had to fill up at US highway speeds, at Autobahn speeds I'd expect to do no better than 150 miles or so.
They haven't published such a plot but they did publish top speed, max torque, max power and rpm ranges:
MS60MS85MSPTop speed120 mph125 mph130 mphMax torque0-5,000 rpm 317 lb-ft (430 Nm)0-5,800 rpm 325 lb-ft (440 Nm)0-5,100 rpm 443 lb-ft (600 Nm)Max power5,000-8,000 rpm 302 hp (225 kW)6,000-9,500 rpm 362 hp (270 kW)5,000-8,600 rpm 416 hp (310 kW)They have also published the reduction ratio of 9,73:1 and tyre dimensions (both 19" and 21" have almost the same diameter of 2,212m).
This data and a little hint from here (that after constant power range torque drops with 1/f[SUP]2[/SUP]) was enough to produce a chart of Power and Torque at various speeds for 60, 85 kWh and Performance versions of Model S. This plot is not exact but it should offer insight into the power that is available at high speeds and how different Model S versions compare.
Under 45 mph MS60 and MS85 should perform almost the same - MS85 has a bit more torque but it is also a bit heavier. Above 45 mph MS85 pulls ahead with about 80 Nm more torque.
MSP on the other hand has huge performance advantage from 0 to about 50 mph. Around there it starts to diminish and it almost vanishes above 80 mph where MS85 and MSP offer similar amount of power.
MS60 "redlines" at ~14.200 rpm, MS85 at ~14.800 and MSP at ~15.300 rpm.
These limits are set electronically, even MS60 has still enough power (~ 80kW) to climb to at least 130mph, but it would take same time to get there.
MS85 and MSP reach top speed when still having about 110kW of power.
Power in above plot is max motor output power but this is not the power that actually accelerates the vehicle as some of it is consumed to overcome the ever increasing air drag and rolling resistance.
We can deduce required power to maintain a constant speed from energy consumption chart here:
Power kW2554510551565208032Raw max output power at 80 mph is ~280 kW for MS85/MSP and ~200 kW for MS60.
MS85/MSP have ~40% more raw output power at this speed but almost half stronger acceleration.
Interesting. Power consumption at 130 mph looks likely to be over 100 kW.
If I assume that power is proportional to speed cubed plus a constant, then I get a fit of:
Power (W) = .054 * speed(mph)^3 + 5060
which gives me 123 kW at 130 mph.
If I instead do a best fit for the exponent, I get a fit of:
Power (W) = .5 * speed(mph)^2.5 + 3400
which gives me 99 kW at 130 mph.
Anyway, according to your first graph, the 85 kWh battery can supply 175 kW at high speeds, so yeah, they should make it just fine.
The 60 kWh battery would be starting to reach its limit at 130 mph, but would be fine at 120 mph.
Got my standard 85 to 128 before I could feel some electronic limiting starting. At that point I backed off, but believe it would have gone a few mph more.
Solid, stable, smooth, precitable.......only issue was pano wind noise became distracting as speed increased. Fantastic automobile!