ICE vs Electric Motor Transmissions

liveforphysics said:

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The biggest appeal of EV's in racing is no transmission. Interruptions in torque delivery are not acceptable.
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.....from the chief engineer at one of the FE race teams.... (An old quote but still applicable..)

...“Simply put, using a gearbox helps you to extract the maximum efficiency possible from your chosen electric motor and to stay at full power for as long as possible at all road speeds,” says Vin Patel, chief engineer at Mahindra Racing. The Indian team has elected to use a four-speed ’box in season two, dropping a gear from the first season unit. “Motor efficiency varies greatly depending on its architecture and design philosophy. This mainly translates as variations of efficiency at different motor speeds.”

In the second season, different motors are in use, with a twin-unit configuration found at DS Virgin Racing and NextEV TCR and single motors from varying suppliers employed elsewhere in the pit lane. Mahindra Racing’s M2Electro uses a revised and updated version of the McLaren unit used in season one, developed in partnership with McLaren Applied Technologies. Altering the specification of key components of the motor, improving materials used and revising software controls resulted in a 20% increase in peak torque and a 15% increase in rpm.​

“Some motors might rev up to 25,000rpm while others only reach 9,000rpm,” Patel goes on. “They will be delivering very different torque bands, which need to be translated into extracting maximum permitted power from the battery. Using a gearbox helps you keep the motor in the most efficient operating range possible.”.......

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Also...
Renault, The winning team last year, used a 2 speed only transmission with the lower gear devoted solely to "launch" the car from the start (IE, Torque increase at the wheels)

https://insideevs.com/lucid-ev-torque-is-irrelevant/?fbclid=IwAR19tHaCGHSN1LEo9OhD56s5jXbWGmeCRCr6lHG5UtrcMiKgaPcBDWZD8ZI

Agree completely with his sentiment - A motor with half the torque and twice the speed, with an 8:1 reduction will get a vehicle up to top speed in exactly the same time as a motor with twice the torque and half the speed and a 4:1 reduction.

So...
If the Two motors had the same torque and rpm range, the one with the 8:1 ratio would accellerate faster ( but with a lower top speed.
And if you gave it a 2 speed transmission . 8:1 and say 3:1..
..it would have faster acceleration, AND a higher top speed .. than the 4:1 ratio.. ??

The wheel torque required to maintain this higher top speed may not be achieved with that reduction ratio.

Has anyone tried to put a 2:1 ratio in a conventional ICE diff? I wonder what speed it bogs out at...

A transmission just takes power your motor made and rubs a share of it away like a tax.

I think formula E is a perfect example, they forced them to have 5spd gearboxes initially. As soon as permitted, teams dropped to 4spd, then 3spd at next race, then 2spd, and cars keep getting faster. Half the teams went clear to single speed and just hauling around and empty transmission case, and these teams were successful (though the championship winner Spark-Renault car used a mult-speed gearbox).

Had the initial mass and size of the 5spd gearbox been applied towards a larger diameter motor (AKA, designing the vehicle around being a single speed), then it would have been higher efficiency and higher continous power and higher burst power capable.

If you pick a really poor power band motor for your given application, then a transmission can serve as a crutch to let that setup limp around.

Hillhater said:

.....from the chief engineer at one of the FE race teams.... (An old quote but still applicable..)

...“Simply put, using a gearbox helps you to extract the maximum efficiency possible from your chosen electric motor and to stay at full power for as long as possible at all road speeds,” says Vin Patel, chief engineer at Mahindra Racing. The Indian team has elected to use a four-speed ’box in season two, dropping a gear from the first season unit. “Motor efficiency varies greatly depending on its architecture and design philosophy. This mainly translates as variations of efficiency at different motor speeds.”

In the second season, different motors are in use, with a twin-unit configuration found at DS Virgin Racing and NextEV TCR and single motors from varying suppliers employed elsewhere in the pit lane. Mahindra Racing’s M2Electro uses a revised and updated version of the McLaren unit used in season one, developed in partnership with McLaren Applied Technologies. Altering the specification of key components of the motor, improving materials used and revising software controls resulted in a 20% increase in peak torque and a 15% increase in rpm.​

“Some motors might rev up to 25,000rpm while others only reach 9,000rpm,” Patel goes on. “They will be delivering very different torque bands, which need to be translated into extracting maximum permitted power from the battery. Using a gearbox helps you keep the motor in the most efficient operating range possible.”.......

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Quote from Mahindraracing.com:

"Just like the inverter and the motor, in the inaugural season of the series, there was a standard Formula E specification for the gearbox too.

Zoom ahead to the season just gone by where the M4Electro had a Mahindra-specified reduction gearbox with one fixed speed, no gear shifts, and no bevel gear for greater efficiency."

Yes , Mahindra ( and most other teams). have modified their powertrains with the seasons .
This season (2018/19) has seen a big change in rules and drivetrain specs (more power, etc).
Several teams run single speed , and others have developed refined dual speed, and " multi" speed transmissions.
There is much unknown tech in the series as its very competitive, with rapid changes in technology.
The overriding performance factors are not so much accelleration or speed, but seems to still be "energy management" and "efficiency".
The last race was won/lost as the lead cars ran out of battery on the last lap !.

Hillhater said:

So...
If the Two motors had the same torque and rpm range, the one with the 8:1 ratio would accellerate faster ( but with a lower top speed.
And if you gave it a 2 speed transmission . 8:1 and say 3:1..
..it would have faster acceleration, AND a higher top speed .. than the 4:1 ratio.. ??

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Well, I think so? That does make sense.

Awesome to have this debate continue after a few years off.

Its probably the most interesting one ever for me here, that i have also learnt the most from, and over time and with certain advances in EV motor and controller design i would now generally agree with Luke on this stuff, but not 100%.
So for the sake of quality debate and a potential of a greater understanding for us all -for now ill retain my original position..

liveforphysics said:

Anytime you add the same mass of transmission into the motor/controller, you will be winning the performance gains.

The biggest appeal of EV's in racing is no transmission. Interruptions in torque delivery are not acceptable.

Multi-speed transmissions are a relic crutch of things with pathetic ICE powerband curves that make 0 torque at 0rpm.

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The first sentence i know to be incorrect in many cases.

One of them is my first attempt at an electric race bike that could keep up with ICE dirtbikes 7 or 8 years ago, it has a two stage transmission and the first stage has a second gearset that weighs 160 grams (and have 13w of parasitic drag when that gear is not being used) and there's no way a 7% increase in motor size would have made the same gains of double the torque for steep hillclimbs in 1st and halving the rpm related losses at cruise in second.

Later i used literally double the motor mass (and controller up by 60%ish) but with a fixed ratio and still could not quite match those two abilities but admittedly did gain peak power at high speed, mostly beyond what was needed for the bike and the battery pack could manage.

Also, whenever an electric motor is used for anything in a geared system then the gear ratio finally decided upon is always a compromise. -not necessarily a bad one but clearly in all cases some sort of compromise between various gains that could be made in torque/speed or efficiency as conditions vary.

I don't think its possible to deny that for EV's in real use, that there are potential gains to be made in one area OR another (but not both) by making an adjustment to the existing drive ratio.
And if we accept that, then it has to be at least possible for a variable speed transmission to have the best of both worlds -or have a benefit.

Would the benefits of a super low speed torque gear on your race dirt bike be something you'd find useful on a road going commuter bike?

It seems the utility of a multi-ratio transmission on an EV seems to exist only in extreme use cases, like a farm tractor or an F1 car with a "launch only" gear where saving a half-second puts you into the first corner in front and potentially sets you up for the rest of the case. It's arguably useful, but would be an absurd feature in any other circumstance.

Punx0r said:

Would the benefits of a super low speed torque gear on your race dirt bike be something you'd find useful on a road going commuter bike?

It seems the utility of a multi-ratio transmission on an EV seems to exist only in extreme use cases, like a farm tractor or an F1 car with a "launch only" gear where saving a half-second puts you into the first corner in front and potentially sets you up for the rest of the case. It's arguably useful, but would be an absurd feature in any other circumstance.

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That seems a reasonable requirement for a bicycle however. Needing to start on a steep hill but still rocket down it at 40 all with a limited battery size.

Not really! For one thing, a (typically underpowered) ebike has pedals to provide any extra torque that might be required in difficult circumstances.

Punx0r said:

Would the benefits of a super low speed torque gear on your race dirt bike be something you'd find useful on a road going commuter bike?

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In Hilly areas yes, and at least the momentary pause in torque production wouldn't matter so much.
TBH i don't really use second on that bike in battle, its mainly for cruising.

Punx0r said:

Not really! For one thing, a (typically underpowered) ebike has pedals to provide any extra torque that might be required in difficult circumstances.

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Yep, but that would surely constitute a fail for the electric drive system..

I think the biggest problem with the whole multi-speed tramsmission on an EV argument is not defining the problem.

Define 'better', and what is it better at?
Define 'more efficient' - with regards to Wh/km? Heat in the motor?
Are we comparing an EV with no multispeed transmission to an identical EV, but with a transmission?
What if the same mass budget was allocated to a bigger motor for the single speed EV?

Unless these matters are clearly defined, the answer will always be a long-running argument over nuance.

Punx0r said:

Not really! For one thing, a (typically underpowered) ebike has pedals to provide any extra torque that might be required in difficult circumstances.

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My bike climbs worse when I try to help. The human side simply doesn't matter once you have +1.5kw available from the motor, but that motor still needs a bit of gear range. Once you get to 3-4kw on a bicycle the motor doesn't seem to need gears but your legs still do.

toolman2 said:

TBH i don't really use second on that bike in battle, its mainly for cruising.

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So wouldn't you rather just stick with a 1st gear as a fixed ratio transmission and have a motor with lower core losses at high speed? I generally assume a 90% efficiency per stage for a real-world gear reduction.

toolman2 said:

Punx0r said:

Not really! For one thing, a (typically underpowered) ebike has pedals to provide any extra torque that might be required in difficult circumstances.

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Yep, but that would surely constitute a fail for the electric drive system..

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Not really: it's an electrically-assisted bike so the motor is (typically) under-powered because you're expected to pedal :wink:

Like jonescg has said, the answer depends how you frame the question. For well-designed typical vehicles for typical uses reality appears to demonstrate that a single-ratio reduction offers the best compromise when all factors are taken into account.

Punx0r said:

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....... For well-designed typical vehicles for typical uses reality appears to demonstrate that a single-ratio reduction offers the best compromise when all factors are taken into account.

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Exactly.....its a "compromise" ..
Between performance parameters..accelleration, speed, efficiency, ....and costs.
Single reduction ratios do not result in the "optimum" performance.

Everyone here has missed the point. Sure, a given engine is more optimized with a transmission and the vehicle will probably be more optimized as well assuming low enough penalties on weight efficiency and shift delay.

But, you can get around this on electric motors easily due to their very high potential power density, at a cost well below equivalent reciprocating engines. In other words, add power until your single-gear electric vehicle is as fast as it needs to be. Simple.

Punx0r said:

So wouldn't you rather just stick with a 1st gear as a fixed ratio transmission and have a motor with lower core losses at high speed? I generally assume a 90% efficiency per stage for a real-world gear reduction.

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Yes, definitely.
If the selected motor did not come with comparably larger losses elsewhere that would be awesome.
-Mainly efficient torque production, as this is a prime consideration to reduce drivetrain/extra stage losses.

flat tire said:

.... In other words, add power until your single-gear electric vehicle is as fast as it needs to be. Simple.

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Fast (speed), or quick (accelleration) ?
In most production EVs, that would mean more copper in the motor, more silicone in the controller , and a higher power/capacity battery...all cost and weight additions.
....not quite so simple.

Once economies of scale take off, yes it is that simple. You pick slightly bigger versions of the things that would have been in the bike anyway and avoid adding the weight and notable mechanical complexity of a tranny.

Hillhater said:

SlowCo said:

As far as I understand it, if you want better acceleration and speed it is better to put the extra complexity, cost and weight in a bigger/stronger electric motor, controller and battery then in a gearbox.

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Go tell the EV drag racers.

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'Big Daddy' Don Garlits uses single ratio drivetrains on both his latest electric dragsters.

gogo said:

'Big Daddy' Don Garlits uses single ratio drivetrains on both his latest electric dragsters.

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But... The EV dragster 1/4 mile record is still held by a 2speed drive .

A while back, the record-holder for a certain class of drag racing car used a 2-speed automatic. I seem to remember it was a street-legal 48V Chevy sedan. Super-high amps, of course...

Even if you want to win the holeshot, you still want a direct drive motor, but sized for adequate torque to be breaking the tires loose all the way to corner 1 entry.

Rimacs new supercar dropped having a 2spd to having direct drive.

It does 1.9sec 0-60mph and does 258mph as a single speed.

Its simply not needed to add the losses and failure modes of ICE crutches to EVs.