Future options: 2 speed transmission? AWD Model S?

[bacrossman]

I have searched the forum and found little mention of 2 features I would love to see on future EV's from Tesla (Perhaps my forum searching skills are somewhat poor?).

1. An AWD Model S - Why? Because I drive frequently on compact snow and ice and would like the benefit of improved handling without having to go with the larger Model X. IMO an AWD car with an electric motor drive train, in theory, should be able to be tuned to have the best handling of any car in the world.
2. A 2+ Speed Transmission - Why? Possibility of improved efficiency at high speeds, and faster quarter mile time

Obviously I made a couple of assumptions here, but I am still curious if there is any news with regards to and AWD Model S or a 2+ Speed Transmission for future cars. Any thoughts?


Brian
Electrical Engineer and Fan of Tesla(The person and the company).

 

[Zythryn]

Nooooooooooooooooooo!!!!

Please no two speed transmition. It would lower efficiency, decrease responsiveness in some situations add complexity.
Yes, you could get a higher top speed, but how often are people going to take it over 120mph?
If someone wants to race, they should take a Roadster

 

[Kevin Harney]

Brian,

If you go way back in the Roadster files you should find plenty of info on the failed two speed transmission. I am sure TEG has the links at hand

Most people believe that TM will eventually make a AWD Model S. But it will be quite a few years. They have a lot on their plate to digest first.

Kevin

 

[aronth5]

Nooooooooooooooooooo!!!!

Agree completely. One of the great features of the Model S is simplicity and a 2 speed transmission is not needed.

As to AWD, I think the Model S will eventually have this option but Tesla will start with AWD on the Model X only.

 

[CapitalistOppressor]

The only really good reason for a multi-speed transmission on the Model S is to improve driving range at high speeds, while also enabling high(er) speeds. The biggest market for that would be Germany with it's Audobahn. It could also conceivably help at highway speeds of 80+, but that requires even more gears, which would impact the driving experience at speeds that most people use the car.

And it's not even clear that you would see drastically better mileage with a conventional transmission at those medium speeds, thanks to mechanical losses in the system. An advanced CVT which could stand the strain that a Model S could put on it might well need hundreds of millions of dollars in R&D and still might only get marginal benefits on typical urban highways.

Personally, I'd rather see increased range from a larger battery. Electricity is cheap, and there looks to be a clear path to a 100kWh Model S in a few years. I'd take being inefficient at 80mph as long as I could have a range of ~240+ miles at that speed. Spending a lot of money to develop a transmission that you might not want when bigger batteries become available isn't my first choice.

That said, if Tesla gets big enough to drop a half billion dollars on developing a new high performance CVT, that could be pretty awesome for folks who like to drive fast.

 

[ElSupreme]

As others have said a transmission will not help efficiency. Induction motors are essentially equally efficient at all speeds. The amount of losses a transmission would induce would surely negate any of the improvements.

The only thing a transmission would do is change top speed. Right now the Model S (non-performance) makes it to 100 very easily and quickly. For 120+mph speeds you would need a transmission. It could help with acceleration (slightly lower gear for 0-60 runs) but I think at the current gearing tire grip is the limiting factor. So lower gearing isn't going do anything but help spin the tires.

I fully expect an AWD Model S in the future. But I would expect it to be a fairly pricey option. It will probably require a 'performance' package and a large battery. And I wouldn't expect it until at least 2 years after the X is shipped.

 

[bacrossman]

Thanks for the responses. I suspected this was the response I would get regarding the transmission. I was really just curious in the 4-5 years since it was last mentioned if they had made any progress on the idea or if they just dropped the idea completely. But I agree, there is inherent robustness and elegance in keeping it simple.

 

[agentsmith1612]

Electric motors are very efficient but only until the rpm going not to high. Like in all electric cars at to high speeds the rpm for the motors goes to high that the motor get more inefficent. Another aspect is that at high speeds we need much more power to continue the high speed against the wind resistance. Wind redistance rises up to the square of speed.
All in all at high speed, we need high rpm from the motor, that gets more inefficent and we need much more power.

You can make a decission to take a "longer" transmission do decrease the rpm at higher speed, but that decrease accerlation also.

Let me explain on example that GM did in the volt to reduce the energy consumption at higher speeds.

The Volt has a main driving motor, a generator and a combustion engine.

If you have a charged battery and driving other I think it was round about 70 mph, the main motor rotates to high that it gets ineffcient. And now the generator gets connectet to the planet gearbox and rotates with a constant rpm also.
That reduces the rpm of the main driving motor. Together both motors at low rpm are more efficient as one motor at higher rpm.

Now what wil the system do in the Volt if the battery is empty. Ok the combustion engine runs the generator to produce electricity for the main motor to drive. At higher velocities the generator can not connect to the planet gearbox because the combustion engine runs them to make power. But now the combustion engine get connect to the planet gearbox to reduces rotation of the main motor.

This happend only at constant speeds.


But let transfer this to full EVs like Tesla.

I mean you can put in one main motor not so big as in the Model S but a little bit smaller and take a second motor beside the main motor but much more smaller. This smaller one can do two task. The smaller one can be used as a "booster" for a short time go get maximum accerlation.
The second task could be the same as in the Volt. Reduce the rotation at higher speeds for the main motor.
Both can be connected with a planet gearbox.
Thats much more simple as it looks like, ;-)

 

[Raffy.Roma]

What is the max rpm of the Model S electric engine?

 

[agentsmith1612]

It is different, depending on the battery version. The specs shows that the transmission is always 9,73 : 1, but I can not believe that if you look to the rotations of the motor in the different battery versions.

Ok we don't know if the specs here in the data sheet for the motor is the motor alone on a test rank or the motor inside the car, while driving 0-Vmax ?
The other point we don't know is the fact that the rotations are only these at the shown power ?

 

[Raffy.Roma]

From the specs it looks that the Performance version is more efficient than the normal 85 KWh version because max rpm is lower. Right?

 

[agentsmith1612]

No thats would be too easy.

I solved my problem in understanding the specs.

I read somewhere that Tesla always build in the same motor, only the maximum power form the inverter is changed or the whole inverter is another, what so ever.

The named rotations are only these that take the named power out of the motor.

Imagine a diagramm with power on the y-axle and rotation on the x-axle you could understand why the 40 kwh motor take out 175 kw between 4000-10.300 rpm and the performance only 310 wk between 5000-8600 rpm.

The specs there doesn't mean that the rotation are the maximum. All motor can rotate to much higher rotations.

But for example you can see that the motor that is used in all versions only makes 175 kw at 10.000 rpm also in the performance model . At higher rotations than 10.000 rpm the power is less than 175 kw (I would assume). In performance model it can be higher because of bigger inverter, but it don't have to. I don't know if the inverter is the limiter or the motor.

I calculate with 21 wheels (245/35 tires) and gearbox of 9,73:1 we neeed motor rotations of round about 15.000 rpm for 130 mph.

 

[arg]

I mean you can put in one main motor not so big as in the Model S but a little bit smaller and take a second motor beside the main motor but much more smaller. This smaller one can do two task. The smaller one can be used as a "booster" for a short time go get maximum accerlation.
The second task could be the same as in the Volt. Reduce the rotation at higher speeds for the main motor.
Both can be connected with a planet gearbox.
Thats much more simple as it looks like, ;-)

In one of the threads in the Model X section, there was speculation (backed by comments from a Tesla guy in one of the Model X beta testdrive videos) that the two motors in the 4WD Model X have different transmission ratios, thereby getting much of the advantage of a 2-speed transmission without the disadvantages.

 

[ElSupreme]

From the specs it looks that the Performance version is more efficient than the normal 85 KWh version because max rpm is lower. Right?

You can tell nothing about efficiency from these numbers. If the motor starts to drop power (or torque), it doesn't necessarily mean that efficiency is dropping. It means the motor is putting out less power, but also consuming less power. But yes AC induction motors do drop off their efficiency towards the top end, but normally not significantly. The Tesla motor has a very high top speed so there might be some significant drop in efficiency, but at those speeds who really cares your usage is going to be so great that the small efficiency losses will be in the 'float'.

 

[agentsmith1612]

In one of the threads in the Model X section, there was speculation (backed by comments from a Tesla guy in one of the Model X beta testdrive videos) that the two motors in the 4WD Model X have different transmission ratios, thereby getting much of the advantage of a 2-speed transmission without the disadvantages.

thanks arg:
That can be a great idea. I dont reaized that this can be very easy possible for a 4 WD car.


I think the most impact factor of loosing range/consuming more power at higher speeds is the fact that the wind resistance rises up to the square of valocity. Also with an EV you can not cheat at the physics.
Look at ICE are at speeds over 75 mph the fuel consumption get much higher not more linear at at 70 oder 65 mph.

To reduce the wind resistance the Tesla gets lower, in the Model X in first concepts Tesla wanted to use cameras instead of mirrors. But the law don't agree with this, it is a pity.

 

[bacrossman]

thanks arg:
That can be a great idea. I dont reaized that this can be very easy possible for a 4 WD car.


I think the most impact factor of loosing range/consuming more power at higher speeds is the fact that the wind resistance rises up to the square of valocity. Also with an EV you can not cheat at the physics.
Look at ICE are at speeds over 75 mph the fuel consumption get much higher not more linear at at 70 oder 65 mph.

To reduce the wind resistance the Tesla gets lower, in the Model X in first concepts Tesla wanted to use cameras instead of mirrors. But the law don't agree with this, it is a pity.

I was well aware of the different transmission ratios for the front and rear of the model X and think that is a great solution. Also as you stated, wind resistence increase with the square of velocity, thus power to push through the extra drag increases with the cube of velocity, hence the importance of the low drag coefficient. Eliminating something like mirrors, door handles, etc. can make a notable difference in efficiency at high speeds.

 

[agentsmith1612]

Also as you stated, wind resistence increase with the square of velocity, thus power to push through the extra drag increases with the cube of velocity, hence the importance of the low drag coefficient. Eliminating something like mirrors, door handles, etc. can make a notable difference in efficiency at high speeds.

I'm sorry but I don't get the idea what you wanted to say with your post, due to my bad english. (I'm a native german speaker and I have always got bad marks in school in english ;-) )


the significants of reduce the wind resistance take a bigger effect at high speeds.
But the wind resistance has two factors.
The first one is the cw value, that tells us how aerodynamic the car is and the second one is the face surface.

To get a feeling how wind resistant the car is you have to multiply the cw value by the face surface.
Both values have to reduce to take advantage of the low wind resistance.
You will have no advantage if your car has a low cw value but the face surface bigger than a truck.

The model S is very wide, in comparison to other european cars, in comparison to american car it could be normal wide, I don't know.
On the otherside the S ist not so high and it can low down at higher speeds.

I know an example, from a sports bike, that can drive 187 mph with removing the mirrors it can drive 206 mph.

 

[bacrossman]

You are correct in your thinking, but let me clarify.

Wind Resistence, also called Drag Force (Fd).
Fd = 1/2p v^2 Cd A

P is the density of air
V is velocity
Cd is the cars measured drag coefficient
A is the face Area

Reducing either A or Cd will ultimately reduce the drag force. (An example would be removing the mirrors, it will obviously reduce A, and would likely decrease Cd as well).

The point I was making is that the power required to overcome wind resistance is
Pd = Fd*v = 1/2p v^3 Cd A
Thus decreases in Cd or A, when multiplied by v^3 can have huge consequences in the power required to move at high values of v (such as on the autobahn cruising at 160km/h.).

Hopefully this clarified and didn’t just add more confusion. J

 

[agentsmith1612]

Thanks for you more explanations, all is understud by me now. All in all we talked about the same. I didn't know that "drag coefficient" is the same like the cw value.

But I learned more from you with the power requierment. That is a lot more that we needed for higher speeds.

Another disadvantage of a second gear in the Tesla could be that at lower transmissions the motor needs more torque, that generate higher currents, the motor needs to has more cooper inside to hande this higher currents. The motor becomes heavier.


You mention the "Autobahn". I think a lot of people in other country belive that in germany on the autobahn you can drive how fast you want. That is no true. A lot of parts of the autobahn has restrictions in speed sometimes 80 or 120 or 100 or 130 km/H. The other parts with no limitations, there you can drive how fast you want.
But a lot of the track is only in 2 lanes for one direction. That means that on the right line the trucks are driving 80 or 85 km/H and on the left you can try to drive 160 or more but not for a long time because very often somebody will from the the right lane to pass a truck and this guys ofter drive only 110 km/h or 120 km/h.

The fact is in germany that you can not drive your speeds like 160 km/h for a long time, because the roads are to full of over cars and trucks. And this makes no sense, in comparision to the whole driving efficency, because you have to slow down and pull up an the high speed very often and this costs a lot of energy. Also in ICE cars if you try to drive fast but very frequently you have to break and speed up again, you will see this in you fuel consumption very well.

In my driving experience is was more efficent do drive 100 or 120 km/H on the autobahn instead of always jumping between the speeds.

My best values in fuel consumption was in the past, I drove behind a truck with round about 85 km/H (53 mpH) and my consumption about 300 miles were "only" 4,77 L/100 km (with a benzin car not a diesel) that is equal to 49,31 miles per gallon.

Another experience was that i drove with the Volt 88 km (55miles) only with the battery.