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Could the Model S in the future be equiped with an automatic gearbox? Will driving in a higher gear lead to more km per kWh?

Could the Model S in the future be equiped with an automatic gearbox?
Will driving in a higher gear lead to more km per kWh (for more range)?

The Model S is currently equiped with a 1 gear gearbox (which works fine).

First of all, would it make sense to engineer an EV with a automatic gearbox?

But suppose that it technically indeed would make sense to do that, would it effect the range of the EV? As in ICE cars driving in a higher gear leads to a lower fuel consumption, would that also be the case with EV's (more miles per kWh)?

Somebody with a technical background could answer this question.

When I read about the new Audi A3 E-Tron. This new Plug-In Hybrid has a 1.4 liter TFSI engine and a DSG S-Tronic automatic gearbox. And inbetween those there is the electric motor. When driving in electric mode the transmission goes via the DSG S-Tronic automatic gearbox. And that made me wonder if that could also be done with a pure electric car like the Tesla Model S?

One of benefits of an electric drive is that you dont need a gearbox, because unlike an ICE the motor has tourque and power at almost all rpms. I am so glad, my Model S has not clutch or gearbox: Less is more (and better).

Gearboxes are an accommodation for the limitations of speed/power for IC motors.

Generally speaking electric motors have a speed/power curve sufficiently flexible and broad to avoid the need for multiple gear ratios.

@klevins

Could you be more specific about: "Generally speaking electric motors have a speed/power curve sufficiently flexible and broad to avoid the need for multiple gear ratios."

@Benz
This is related to the torque, which depends on the speed of internal combustion engines, and is therefore not constant. For electric motors, the speed range with sufficient efficiency is much greater. Under these circumstances, a gearbox would be totally counter-productive!

A gearbox could be beneficial to improve the range of an EV. However, the added complexity compared to the MS's current reduction box may also reduce reliability. And since Tesla holds tight to the KISS mantra (except for the doorhandles), I wouldn't expect it anytime soon. Personally, I'd rather not have it, because I think improved battery technology is the way to go to improve range versus a gearbox.

Read this to learn more, for example:
http://www.gizmag.com/antonov-3-speed-transmission-ev/19088/

They tried this in the Roadster with bad results. Transmissions have a hard time handling 100% torque at 0 RPM with a motor as powerful as the one in the MS.
http://green.autoblog.com/2008/01/23/breaking-tesla-has-a-solution-for-t...

Probably only Plug-In Hybrids (like the Audi A3 E-Tron) can be equiped with an automatic gearbox, as these vehicles have both an ICE and an electric motor.

@Benz - you could put them in, but you wouldn't want to. The efficiency curve of electric motors is pretty flat, so as long as you stay below the breakdown torque value it isn't going to change the efficiency much if you change the gear ratio. At high speeds, drag is by far the biggest loss of energy anyway.

Every extra gear you include increases friction losses itself plus weight, so you would have to gain a significant amount to overcome the losses.

It's true that electric motors do have lower efficiency at high torque and low RPM. However, the advantages of a gearbox would be minimal at best and counterproductive at worst. Motor heating could be reduced slightly (by reducing torque requirements during take-off and at low speed), but using a bigger motor would have the same effect, most likely at lower cost and most definitely with higher reliability. Gearing could also give you the benefit of more torque to the wheels at low RPM, but the Tesla doesn't exactly need more power and the tires would just break free. Finally, don't forget that a gearbox has mechanical (friction) losses too that would work to offset many of the gains; in fact, they get worse at speeds where you do most of your cruising while the benefits mostly apply to the transient conditions (accelerating from a stop).

In fact, some electric motors (Tesla's included) can be electrically "geared" through a special control process known as "field-weakening." This enables the use of higher initial gear ratios for improved efficiency. Tesla likely uses it.

I don't know anything about the Audi E-Tron, but it sounds similar to the Prius. A special transmission (planetary gears, in the case of the Prius) allows the motor and ICE to work together to power the wheels. It's not there because the electric motor needs it.

Finally, to contradict myself a little bit, there is one place Tesla might gain 'improvement' from a gear box: top speed. The following is purely speculation. Top speed is most likely limited by max (mechanical) motor RPM, not available horsepower. Tesla made a tradeoff decision that 130MPH was "good enough" for a top speed. A variable gear ratio would allow it to go faster without a bigger motor and without sacrificing performance at lower speeds. Personally, I don't even care that it can go 130, so I don't think the end justifies the means.

I understand that the Model S does not need a gearbox.

But looking at the sales figures, it appears that Plug-In Hybrids are more popular than EV's. Shouldn't that be the other way around?

Benz:
How is this related to the gearbox question?
If it is a completely unrelated question, then maybe it's better to post a new topic. Just my 2c.

Plug-in's are naturally more popular than EV's because: 1) they're (arguably) marketed more, 2) there's more choice, 3) the range is better, 4) they don't fully rely on charging points (of which there are not enough and most of which are too slow), 5) they come from big, known brands, 6) you can operate them like any regular ICE vehicle, should you choose to do so, and still reap the tax benefits.

Not all of these apply to each plug-in hybrid, just as not all rules apply to each EV (the Model S being an exception to EV's on many points, for example).

A new thread on Tesla's application for a hybrid battery pack seems to be their approach to this issue. Change the energy availability rate rather than a gear box.

Sorry should have said 'application for a hybrid battery pack patent."

@chrisdl

What I meant was that as EV's are technically better cars than Plug-In Hybrids therefore in my opinion more EV's should be sold than Plug-In Hybrids. But that is not the case. I think that the main reason for that is that people do not easily abandon what they are used to for years and years. They rather stick to what they are used to. And the Plug-In Hybrid does give these people an option to do a step towards EV's but still have an ICE. Maybe that's more comfortable for them (in their heads).

you probably could drive faster with a gearbox and therefore use more energy :P

@Benz - by Plugin Hybrid, I think you mean basically the Volt, as the PHEV Prius isn't selling that many. And the answer there is GM has been sowing range anxiety FUD and confusion over what an EV is.

@jat

I should have mentioned that I meant the sales figures in The Netherlands.

In the first 8 months of 2013 the sales figures are:
Toyota Prius Plug-In Hybrid: 2,343
Nissan Leaf: 771

@Benz

I agree (as would many on this forum) that all-electric is clearly better, but keep in mind - this was not demonstrated until quite recently. Until the Model S, the best available all-electric cars had serious limitations in terms of range, peformance, storage space, comfort, etc. Tesla has shown that those are no longer inherent limitations of EVs, but it will take time for people to figure this out en masse. Once they do, there will be a stampede to buy EVs.

In the Roadster, they got various companies to try and produce a gear box that could handle even one additional gear. All broke, I think at the point of upshifting. Imagine trying to make a clutch capable of coping with Tesla's motor!

Gearboxes have been invented to compensate the limitations of ICE's. Electric motors do not have these limitations, therefore don't need these gearboxes. Electric motors are superior to the ICE's. That's what I have learned here.

Actually, aside from the simple question posed by the OP, I would be curious to see some curve of the watt-hour efficiency per mile at very low speeds. And not the curve that is always shown of the simulated Wh vs mph curve that takes into account wind resistance. I mean the ideal motor efficiency versus rpm.

Basically, I want to know whether I should bother accelerating slowly from a stop light, or I can just gun it and still be basically as efficient...

Gunning it at the stop light is less efficient if there is a parked cop mid-block.

Ideal motor efficiency is not as relevant because we do not live in an ideal environment. We have inertia, friction, drag, and the occasional police officer.

Under most driving conditions, I enjoy putting my boot into it up to the speed limit. I observe the speed limit, but there is no QUICK limit.... hehehehe.

I assume friction / drag is negligible at low speeds, so that's why I'm interested in the ideal motor efficiency curve!

@soma

"Basically, I want to know whether I should bother accelerating slowly from a stop light, or I can just gun it and still be basically as efficient..."

You can NOT gun it and STILL be basically as efficient.....
Therefore, if you do gun it, then do expect a higher energy usage as well.

Easy driving and fast driving result in different energy usages. Actually just like in ICE's. Fast acceleration will cost you more energy.

Being limited to 200kph is going to hurt Tesla among the Autobahn crowd in Germany and I'd be surprised if they wouldn't love to solve that issue somehow.

The car currently has a fixed reduction gear, and is limited in top speed by max safe RPM of the motor (I think, do correct me if I'm way off base – I don't think it's the heat generation that stops it but it could be power delivery).

The obvious fix would be to change the fixed gear ratio, giving a higher top speed for the same max RPM at the cost of reducing the lower end acceleration. The poor man's Autobahn Tesla. I don't know if the PEM could handle the extra load though.

Possibly they could instead put in a more powerful/durable electric motor (and possibly bigger battery/beefier PEM to get the necessary power) that can run at higher RPM; or on which you could put a different gear ratio while maintaining lower end acceleration.

A multiple gear gear box seems rather unlikely, at least until someone invents a material that can cope with the forces involved.

The current top speed of the Model Ses (60/85/P/P+) is sufficient for most of the buyers. True you can drive fast on the German autobahn (not everywhere though), but that does not make it a race track like the Nurburgring. Those hardcore racers will buy a car that has been built to be real sportscar like a Ferrari (or something like that) anyhow. I am absolutely not saying that the Model S is not a fast car. My point is that it's a different market segment. By they way, the new Tesla Roadster (Model R?) will be faster than any Ferrari.

Hyper-miling in a TMS involves being fairly gentle with acceleration, and limiting top speed. Goosing it costs.

Benz:
The question about Model R (good name) is: will it be faster than the 2015 GT-R, the new 911 Turbo S, and... the Veyron! ;-)

@chrisdl

Tesla Motors have employed Chriss Porritt for more than one reason.

Chriss Porritt was engineer at Aston Martin. He worked on the Aston Martin One 77.


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