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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?

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?

lars.thomsen | 14 September 2013

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).

Gluaisrothai | 14 September 2013

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.

Benz | 15 September 2013

@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."

SunCoulombs | 15 September 2013

@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!

chrisdl | 15 September 2013

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/

PBEndo | 15 September 2013

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...

Benz | 15 September 2013

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.

jat | 15 September 2013

@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.

theapple | 15 September 2013

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.

Benz | 16 September 2013

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?

chrisdl | 16 September 2013

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).

jackhub | 16 September 2013

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.

jackhub | 16 September 2013

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

Benz | 16 September 2013

@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).

ChristianG | 16 September 2013

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

jat | 16 September 2013

@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.

Benz | 16 September 2013

@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

mrspaghetti | 16 September 2013

@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.

Brian H | 16 September 2013

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!

Benz | 16 September 2013

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.

soma | 16 September 2013

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...

jbunn | 16 September 2013

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.

soma | 16 September 2013

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

Benz | 16 September 2013

@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.

bent | 17 September 2013

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.

Benz | 17 September 2013

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.

Brian H | 17 September 2013

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

chrisdl | 17 September 2013

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! ;-)

Benz | 17 September 2013

@chrisdl

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

Benz | 17 September 2013

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

jbunn | 17 September 2013

Benz,

I'd like to make a small correct to your comment where you stated "Easy driving and fast driving result in different energy usages. Actually just like in ICE's. Fast acceleration will cost you more energy."

In truth, it's not just like an ICE. Let's say I'm going from stoplight to stoplight in both cars. Acceleration to from 0 to 40 MPH is the same for both. Acceleration from 40 to 0 at the red light is the same for both. Both cars are neck and neck at exactly point down the course for the entire duration.

It's just like the ICE, but the difference between the two is the ICE accelerates from 40 to 0 using brake power. The Tesla accelerates from 40 to 0 using regen which takes 60% to 80% of the kinetic energy and stores it back in the battery pack as electrochemical energy.

This brings me great joy because I can drive like a maniac and if I stay off my mechanical brakes, I can recover much of the energy! Go Speed Racer! Also part of the reason a car this heavy can get over 100 miles to the gallon MGPe

Benz | 17 September 2013

In fact you are right, an EV has the advantage of regen braking. But I thought that the question was limited to acceleration of the Model S. Meaning only the few seconds that the speed rises to a certain level. What happens after that was not taken into account (I thought). Only the amount of energy that is being used during acceleration.

Brian H | 17 September 2013

An ICE is very inefficient at low RPM; hence the revving, and launch controlling, tire smoking, etc. The EV is better, but still has some physics penalty for high accel (rather than mechanical).

tomkist | 17 September 2013

@Bonz - My son has a collection of almost 100 hot wheels cars, and the Aston Martin One 77 is the fastest of the bunch. Hoping they come out with a Model S version soon.

STARR X | 17 September 2013

I feel the Model S does not need a gearbox because the car's software acts as one. When I drive with cruise control on the highway, I feel I get better range as the car knows how to power the car properly.

nickjhowe | 17 September 2013

To go back to the original question, looking at Wh/m vs speed and comparing it to rolling resistance and aero drag vs speed suggests that efficiency doesn't vary that much over the rpm range (but I would like to see a motor efficiency curve to be sure).

Since the electric motor has 100% torque at zero RPM and can go to 15,000 RPM and seems to be fairly constant efficiency means a second gear is absolutely not necessary except for raising the top speed, and will decrease efficiency due to mechanical losses.

Benz | 18 September 2013

@jbunn

Actually you have pointed out another interesting question.

Suppose we do the following test with a Performance Plus 85 kWh Model S (the regen is set to standard), on a flat and straight road with no traffic, and with good weather conditions (sunny day, no wind):

We start from still and accelerate fast to a speed of 100 miles/hour, and as soon as we will have reached that speed of 100 miles/hour we then just simply take off our right foot of the pedal and put our right foot on the floor, and simply let the car go slower and slower until the car comes to a complete stop.

My question is about the energy that is used to go to a speed of 100 miles/hour on the one hand (= part 1), and the energy that has been regenerated by slowing down on the other hand (= part 2).

It's Obvious that part 1 will be more than part 2. But what % will part 2 be (if we take part 1 as 100%)?

Brian H | 18 September 2013

Regen likely to return about 80%, by some accounts. YRMV

Benz | 18 September 2013

That's amazing. I thought that it would be something in the range of 10% to 20%.

Benz | 18 September 2013

Nobody else has an opinion or any idea on this matter?

Benz | 18 September 2013

Apparently not.

DTsea | 18 September 2013

A transmission will meed more power conversions- clutch or fluid clutch- that will not likely IMPROVE efficiency. Think how automatics are 5-10% worse than manual transmissions for a useful analogy.

Simpler, fewer gears- less waste.

jbunn | 18 September 2013

Benz,

Brian is correct. Higher up on this page I had also stated a range of 60 to 80%.

It's a pretty damn fine automobile.

Brian H | 19 September 2013

The VW Yup claims 90%, likely because it can afford to apply higher traction drag on the rears without breaking loose (weight).

chrisdl | 19 September 2013

DTsea:
Modern automatics are averagely more economical than modern manual gearboxes. Also, more gears means higher efficiency for an ICE vehicle.
On a BEV the jury is still out, although I'm all for simple myself (simple = single gear).

Mark K | 19 September 2013

When you've got a jet, you don't add a propeller.

Transmissions are very last century. Direct drive is superior on so many levels.

Model S has a single reduction gear and no clutch. That is massively better than the 7 speed gearbox on my MB SL.

nickjhowe | 19 September 2013

Check out this video - 0-113-0 showing energy use and regen...

nickjhowe | 19 September 2013

Helps if I include the URL: http://www.youtube.com/watch?v=d82NP89O_ZQ

Benz | 19 September 2013

Wow, now that is impressive. More than 50% of the energy is returned. This is much more than I had expected. Fantastic. Everybody should see this video.

Nick, thanks for posting the video.

Dr. Bob Reinke | 19 September 2013

There are several reasons not to add a multi reduction gear box to an electric car.

1)The Tesla provides greater torque at starting RPMs than would required to develope wheel spin at start-up.

2) The RPM that the 4 pole 3 phase motor is turning at over 100 MPH adds no friction or drag that would be measurable between starting and driving at high speed. ICEs reqire a transmission because at low RPM the available torque is horrible. And at speed, the V8 has to change the direction and velocity of roughly 200 LBS of pistons, connecting rods, and valve train everytime the crank turns. So a reduction in engine RPM reduces fuel consumption. At highway speed, roughly 20% of the fuel in the cylinder goes out the exhause as unburned gasoline. (the main reason a catalytic converter is required) Overdrive reduces the lost unburned fuel and friction of reversing velocity

3)In an ICE, a manual transmission operation can cost as much as 1 mile per gallon. An automatic transmission can cost as much a 2 MPG. A VTR transmission can cost as much as 4 MPG in friction. That doesn't even take into account the un-nessary added weight. All while providing neither increased torque or reduction of the non-friction for an electric motor.

4) The transmission adds the weakest link in the drive-train.

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