EV transmissions are coming

motors have an efficient operating rpm range, but i don't know if adding a tranny would be able to take advantage of it. maybe something like an overdrive for high speed driving. but i agree with myth, seeing is believing.

 

AC motors are very different beasts than engines. There isn't that much efficiency loss to be recovered.
The added weight, mechanical linkages, etc loose efficiency.

Here is an old blog discussing Tesla's change from the two gear transmission and the efficiency gains made by simplifying the drive train (as well as making some other changes).
An Engineering Update on Powertrain 1.5 | Tesla Motors

 

This discussion was framed wrong. The real discussion is how to increase EV efficiency in the most cost effective way. Mechanical transmissions may or may not have a viable use depending on vehicle function. While a mechanical transmission may provide an viable efficiency improvement, then it is extremely likely a pure electric "transmission" is also viable. Like Bob Wilson indicated, different motor winding arrangements or different motors may end up replacing mechanical failure possibilities with electrical redundancy possibilities....while improving efficiency more than a mechanical transmission .... all at lower cost.

 

ZF or Zahnrad Frabrik Fredrichshaffen am Bodensee was founded by Dr. Hugo Eckener of the Zeppelin Company (Deutsche Luftschiffe Zeppelin) in 1915 to produce gearing and transmission for the world famous Zeppelin airships.

The are currently the most innovative and premier supplier t transmissions to the automotive industry. They supply Chrysler, BMW, Mercedes, Rolls Royce, Jaguar, Land Rover, Ferrari and others.

I would NOT sell them short. They know what thy are doing to corner the market.

My home is less than five miles from BMW US Headquarters and 20 miles from the new US ZF plant in Fountain Inn. If they say that they are developing an EV transmission, I know they will make a world class product, which would likely corner the market.

 

Where is the motor's peak efficiency?
I know a basic rule of thumb I once came across for DC ones is peak torque is at 0rpm with steady decrease as rpm increase, peak power is around 50% of the motor's rated rpm, and peak efficiency at 75% of rpm. I don't know how well that holds for the AC motors in BEVs.

An electric motor is very efficient compared to an ICE, but it still has points in its output where it is best and where power output is better. That's why the AWD S gets a slightly better range and efficiency without changing the battery. The question is will the gains at higher speeds off set the overall losses of a 2 speed transmission.

Yes, a transmission will add cost, but it won't be as costly as the beast of a transmission put into an ICE car. But that has to be balanced against the costs of a larger battery. The transmission will probably win out on the packaging front.

Or someone will finally say, "screw the gear bo", and just use hub motors.

On an unrelated note, how do BEVs coast? They are always in gear to speak. Does the motor just freespin?

 

Motors are always running. Coasting means no power flow (in or out).

 

Efficiency of the 85d is less than 2% better than the regular 85. A transmission would drop that much or more. The 85d's efficiency also stems from better regen braking, while a transmission would not help at all. What the d gives you is better accelleration and top speed, a transmission can do that, but really isn't the model S fast enough, and you go to the P85D if you really want improvement. IMHO a transmission would only slow that car down.

Say you are right, and IMHO you are far off on efficiency, but pretend instead of hurting efficiency a transmission increased range on the 60kwh by 5% (more than double what the duel motors do). You could increase the pack size 5% then anouther 10% to overcome inefficiencies from extra weight, or 3.3 kwh which with the gigafactory would cost less than $700 or you can spend R&D and create a transmission, install, maintain it, for much more.

+1

Hub motors have greater unsprung mass, which requires work on steering and suspension.

Think about it this way, normal axle motors are fixed gears, with most of the motor mass attached to the car body. The gear allows a higher speed motor to be used in automotive applications. This lets you balance performance with cost in a good engineering sense. A hub motor likely will be more expensive for the same job as it must be built for lower rpm. The added unsprung weight hurts suspension and handling. Michelan is working on all these problems with an "active wheel" that attempts to include suspension and breaking optimized for a light weight hub motor. Bottom line today hub motors add cost if other things like handling and suspension are taken inot account Porsch in the 918 first went for hub motors for the front wheels, but later decided it wasn't worth the complication of wheel suspension design.
A motor coasts when you remove power from its windings

 

Starting a motor from zero results in high current draw. If it is stopped and restarted, power is lost from the battery.

If the motor can be kept spinning at a very low speed, there is very little current draw.

While this can be done with capacitors, a transmission can be helpful.

Like ICE's electric motors have optimum run speeds that provide the optimum torque and speed, a transmission, even with a small power drain, can be offset by the gain in efficiency.

 

But how exactly is it done? When the motor is spinning, the coils are rotating in the magnetic field and create a current. Does the system simply not draw on that current for 'neutral' coasting, and then draw on it at increasing loads for regen braking?

And there would be no reason to not allow the driver to select keeping it primary gear at all times. Or even in the high one if it isn't extremely tall.

 

Umm whatever.

To point one, yes its easier to design a higher reving motor with a reduction gear. So absolutely We are talking fixed gear transmissions/transaxles depending on the car. Now that is going to be part of the mechanical connection between the motor and the tires, and there will be about 2% more losses mechancally here than in a hub motor. That hub motor will be less efficient at many speeds along with other problems and higher costs, so you eat the 2%.

2) Show me someone that actually has a multigear transmission that makes a car more efficient in the real world. You have additional frictional losses and the transmission has to be fairly beefy to take the torque of the motor. Here is a tesla roadster motor curve that I think is correct, but may be bogus as this is the internet.

Changing torque curves - Page 2

It looks like best efficiency of motor/converter is 92%, 7000rpm=60mph, and we should be in the 85% efficiency range or lower at 65mph. Say the transmission adds 3% more mechanical losses and could jump 65mph from 84% to 91%, range would increase 5.1% at 65 with the cruise on. I of course don't think the gain would be this big, and you trade off 3% lower range at all speeds bellow say 35 and do worse in the epa city and combined tests. For the range it would be less expensive to just make the battery a little bigger. That is why no one is really clamoring for this.

The volt does have a psd transmission that it uses and in gen I form, no engine involved, just psd with two motors is only as efficient as the heavier (4464 lbs) much more powerful tesla 60kwh. If the transmission helped I would think we would see better numbers from the volt.

 

AC motors have a much flatter efficiency. Power reaches peak quickly and the maintains it. Torque is virtually instant and then maintains it, with a slow decrease at high speeds.

They are much more efficient over the power range EVs are typically using.

 

Yes, you are getting 92% at two speeds. But you are getting 92% of a lower number in both cases because you added extra overhead (weight) to get it.

Mike

 

Yes, there is increased weight. The one speed gear box of the Tesla Roadster is 8kg lighter than the original two speed one. Let's say it's 20 pounds difference since that two speed had durability problems, and a reliable one will likely be beefed up. It is said that a hundred pounds of stuff in a car will reduce its efficiency by 1%.

So efficiency may be reduced by 0.2% from weight. It then becomes a question of the efficiency gains being high enough that the new range is greater than putting a like amount in weight of batteries.

PS: The Volt isn't a like comparison. It's a hybrid, and the transmission has other considerations and compromises going into its design that a BEV or ICE doesn't. The PPI is lighter than it, and less efficient in EV mode. That is partly due to the larger battery for better capture of regen.

 

I just have never seen a multi-speed transmission that didn't add losses compared to a single speed one. I don't even know if it is technically possible. Now some on this forum are doing a thought experiment, but does anyone have figures on losses through a 4 speed zf that can take the torque of an electric motor. A transmission would likely allow you to use a cheaper motor that did not have as flat of an efficiency or torque curve, but ... then you need to pay for the transmission and maintenance.

 

If you are asking for one for an ICE to compare, I suggest using a manual or maybe double clutch. I doubt you'll find info for a step transmission without the torque convertor, which an electric motor won't need.

I don't think if this came to be that the multi-speed transmission would just be slapped into a BEV with a single speed without pairing it to the right motor.

 

The early Tesla Blogs went into great details about this since so many posters were thinking that hub motors are the ultimate answer. Why was Tesla taking the past solution instead of the future solution? The Tesla answer over and over was that automotive design is a whole vehicle balancing act. The disadvantages of high stress cabling, cost, safety, vibration, shock, repair and suspension impacts certainly made Tesla engineers avoid this approach.

Depending on whether you are an optimist or pessimist, it is when the M/G is either a motor with no power input or a generator with no power output. (There may still be currents or voltages present, so power flow is the determining factor of when "free spinning" is happening.) What I have noticed is EV designs are minimizing "coasting". Specifically, the accelerator is either used to accelerate or slow down, leaving true coasting to be only possible with one exact accelerator position.

 

That answer depends greatly on the type of motor. In most PHEV applications, the rotor is embedded with a set of permanent magnets, so the rotor magnetic field is always present at the same strength. (Look Mom....no commutator brushes!) The stator is a computer controlled rotating magnetic field generated from a bunch of interlaced coil windings that can generate a spinning magnetic field at what ever speed the computer directs. The computer directs a bunch of switch transistors to sequentially send current pulses through different coil windings....creating a rotating magnetic field from a bunch of stationary windings. Either the computer is making the stator field push the rotor field to change electric power into rotational power...or...the computer is letting the shaft driving the rotor to amplify the electric power out of the rotating stator field. Coasting is achieved when the MG computer ensures the rotating stator field is neither pushing, nor being pushed by the rotor field. It is the computer determining all aspects of the MG functioning.

 

Apparently few realize:

• Energy recirculate and PWM - gives the NHW11 +2 e-gears.
• Voltage doubler - gives the NHW20 +3 e-gears
• Voltage tripler - gives the ZVW30 +4-6 e-gears

I know this is a hard concept to wrap one's mind around but this is what voltage doublers do. They allow adequate power into a motor at high motor rpm when they are nearly at maximum rpm and 'back EMF.' It is basic electronics.

Bob Wilson

ps. A mechanical engineer by training, electrical for fun, these are concepts are trivial. In effect, the advanced, power electronics means the EV and/or smart hybrid has the functional equivalent of mechanical gears without the mass.