ICE vs Electric Motor Transmissions

General Discussion about electric vehicles.
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ICE vs Electric Motor Transmissions

Post by speedyebikenoob » Feb 16 2019 11:17pm

Okay I'm aware at how many times transmissions on electric motors have been discussed, and how it always boils down to "they aren't necessary because of the high rpms and instant torque" BUT, taking a look at two I guess somewhat similar in power cars still confuses me about this. A bmw i3 weighs 3000 lbs, and a mini cooper, let's go with the s version just to even out the power differences, also weighs about 3000 lbs. Both cars produce roughly the same horsepower (around 180), but the mini has an internal combustion engine and the bmw i3 is a fixed ratio single speed. The 0-60 acceleration times of both cars are basically exactly the same at 6.5 seconds, but the mini needs six gears to do it and the bmw needs one. Now I'm just wondering how much better that acceleration time would be for the bmw if it also had six gears rather than just one, I mean there must be a significant improvement, right? What are your thoughts on this? (and yeah obviously it would be more complicated to manufacture and it would add more weight and take away efficiency but this is purely theoretical, it's probably not practical)

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Re: ICE vs Electric Motor Transmissions

Post by spinningmagnets » Feb 17 2019 1:50am

The biggest problem for most electric vehicles is the price, and a transmission would take up space and cost more. The motor could arguably be slightly smaller, but not significantly cheaper.

The Ford Model-T (1910's) had a very simple and robust planetary 2-speed transmission (with reverse), and that might be interesting to see a rolling example of an EV with a similar 2-speed.

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Re: ICE vs Electric Motor Transmissions

Post by Hillhater » Feb 17 2019 2:35am

Arguably, Ebikes are a classic example of the effects of gearing on equivalent wattage motors
Hub motors are cheap, reliable, simple and efficient..
But the mid drive Ebikes, which benefit from both higher rpm motors and multiple gear ratios, can give better accelleration and top speeds, but generally at the expence of efficiency and cost.
These results are difficult to see as ebikes are really "Hybrid" vehicles with variable human input,
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Re: ICE vs Electric Motor Transmissions

Post by jonescg » Feb 17 2019 8:53am

The mini needs a 6 speed box because the motor only produces sufficient torque at a very specific rev range. You can get there by planting your foot in first, but you run out of top speed real quick, so you grab second - sweet, now we're doing 40 km/h and the engine is making an awful racket. Hmm, better grab third. There's the torque sweet spot again - woo now we're going 70 km/h and damn - it's revving its tits off again. Sigh, I'll grab 4th. OK sweet now we hit 100 km/h and the engine still seems to be complaining. I guess we don't need as much torque to cruise at 100, so I might just grab 5th and let it rev a bit slower...

Meanwhile the i3 just delivers it's max rated torque all the way up to 8000 rpm, at which point it tapers off as it trades torque for speed at cruise.

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Re: ICE vs Electric Motor Transmissions

Post by Grantmac » Feb 17 2019 12:52pm

The mini can hit 60mph just barely into 3rd. Those other gears are for efficient cruise and "autobahn" speeds.

A 2spd speed (or really reduction then direct) planetary would be an excellent match for an electric.

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Re: ICE vs Electric Motor Transmissions

Post by speedyebikenoob » Feb 17 2019 2:22pm

spinningmagnets wrote:
Feb 17 2019 1:50am
The biggest problem for most electric vehicles is the price, and a transmission would take up space and cost more. The motor could arguably be slightly smaller, but not significantly cheaper.

The Ford Model-T (1910's) had a very simple and robust planetary 2-speed transmission (with reverse), and that might be interesting to see a rolling example of an EV with a similar 2-speed.
Yup, it would be more costly and take up more space, but if the i3's motor had a six speed transmission, could they reduce the output to let's say, 130 hp or so and get the same performance?

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Re: ICE vs Electric Motor Transmissions

Post by speedyebikenoob » Feb 17 2019 2:25pm

Grantmac wrote:
Feb 17 2019 12:52pm
The mini can hit 60mph just barely into 3rd. Those other gears are for efficient cruise and "autobahn" speeds.

A 2spd speed (or really reduction then direct) planetary would be an excellent match for an electric.
Most electric vehicles don't even reach past 90 mph, and that's a problem for highways where the speeds are much higher (like in texas where the limit can go upto 85 mph) and yeah the autobahn. If they added an overdrive gear too so instead it could reach something like 120 mph, that would be great, although it would use a lot more current at that speed.

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Re: ICE vs Electric Motor Transmissions

Post by speedyebikenoob » Feb 17 2019 2:28pm

jonescg wrote:
Feb 17 2019 8:53am
The mini needs a 6 speed box because the motor only produces sufficient torque at a very specific rev range. You can get there by planting your foot in first, but you run out of top speed real quick, so you grab second - sweet, now we're doing 40 km/h and the engine is making an awful racket. Hmm, better grab third. There's the torque sweet spot again - woo now we're going 70 km/h and damn - it's revving its tits off again. Sigh, I'll grab 4th. OK sweet now we hit 100 km/h and the engine still seems to be complaining. I guess we don't need as much torque to cruise at 100, so I might just grab 5th and let it rev a bit slower...

Meanwhile the i3 just delivers it's max rated torque all the way up to 8000 rpm, at which point it tapers off as it trades torque for speed at cruise.
I mean even the i3 has a powerband, it's just a lot larger than the mini's. Just like you said, after 8000 rpm you start losing power, so if you had another gear or two it could still keep accelerating, and it should improve acceleration off the line if the gearing is lowered.

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Re: ICE vs Electric Motor Transmissions

Post by speedyebikenoob » Feb 17 2019 2:29pm

Hillhater wrote:
Feb 17 2019 2:35am
Arguably, Ebikes are a classic example of the effects of gearing on equivalent wattage motors
Hub motors are cheap, reliable, simple and efficient..
But the mid drive Ebikes, which benefit from both higher rpm motors and multiple gear ratios, can give better accelleration and top speeds, but generally at the expence of efficiency and cost.
These results are difficult to see as ebikes are really "Hybrid" vehicles with variable human input,
Aren't middrive's more efficient than hub drives though through hills and mountains and from a standstill?

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Re: ICE vs Electric Motor Transmissions

Post by Hillhater » Feb 17 2019 4:54pm

speedyebikenoob wrote:
Feb 17 2019 2:29pm
........
Aren't middrive's more efficient than hub drives though through hills and mountains and from a standstill?
They can be in some situations depending on the terrain and riding style.
But ultimately, they will "waste" some power due to extra friction in the chain and gear train compared to the direct application of power from the hub motor to the road.
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Re: ICE vs Electric Motor Transmissions

Post by jonescg » Feb 17 2019 8:20pm

speedyebikenoob wrote:
Feb 17 2019 2:22pm

Yup, it would be more costly and take up more space, but if the i3's motor had a six speed transmission, could they reduce the output to let's say, 130 hp or so and get the same performance?
No, because one will have 130 hp and the other will have more. Power is the product of torque and speed. If you opt for a less powerful motor you're opting for a motor with either less torque, or less speed. So it will be less capable no matter what gearbox you bolt it to.

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Re: ICE vs Electric Motor Transmissions

Post by Sunder » Feb 17 2019 10:45pm

speedyebikenoob wrote:
Feb 17 2019 2:29pm
Aren't middrive's more efficient than hub drives though through hills and mountains and from a standstill?
That's for a different reason. Electric motors are less efficient when the actual speed starts deviating from no load speed at the desired effective voltage.

This is really hard to explain, because of multi-dimensional graphs, but bear with me.

Lets say you have for arguments sake, a 100V electric bike. At 100v, the winding and wheel sized hits a no load speed of 100km/h

If you open your throttle up, so that the "effective voltage" seen by the motor is 100v, and you are traveling at 100km/h, your bike will operate pretty close to max efficiency.

If you open your throttle 50%, so that the "effective voltage" seen by the motor is 50v, and you are traveling at 50km/h, your bike will also be pretty close to max efficiency.

However, if you are towing a large trailer, and climbing up a hill, and you have your throttle as high as it goes, so that the motor sees 100v, but you are only traveling 50km/h, then your motor will be inefficient.

Mid-drive bikes try to get around that, by decoupling the motor from the wheel - Your motor can spin at the 100v speed, while the wheels are only traveling at the 50km/hspeed, so the motor is happy, and you've doubled your torque at the wheel at the expense of speed, maintaining the whole power = Speed x torque equation.

Make sense so far?

The thing is, it makes sense for low powered bikes, because you would frequently be using 100% throttle, but going less than 100% speed. That's usually because electric bikes are 250-3kw.

Despite being larger, a BMW I3 has 127kw, meaning you'd almost never be on full throttle for more than a few seconds. A gear box introduces losses, requires maintenance, is additional weight, additional failure point, additional cost, etc. All for a bit of efficiency gain a few seconds every take off? Not a good trade off.

With regards to your original question, torque curves for I3s are hard to come by, but Tesla published the below graphic to indicate why they don't really need a gearbox:

Image

Basically, by the time the torque starts falling off enough to make it worth gearing up to return to the higher part of the torque curve, you'd be well over 100km/h, and most people care how fast a car does 0-100, not 150-200km/h.
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Re: ICE vs Electric Motor Transmissions

Post by Hillhater » Feb 18 2019 2:07am

Actually its obvious the i3 would be a faster car ..better accereration and higher top speed, even with a 2 speed gearbox.
Every EV has a reduction transmission between the motor and the roadwheels, and that reduction ratio is carefully chosen to suit the motor characteristics (torque, rpm , etc) , and the target vehicle performance (acceleration, top spaad , range, etc)
But that ratio is always a compromise to give "acceptable" acceleration with "accepable" max speed.
A simple 2 speed gear system would allow an inproved initial accelleration AND a higher top speed.
As has been said, even the mighty Tesla S starts to get beaten after the 1/8 mile mark, because it has run pasts its Torque/power peaks...(at only 6-7000 of its 18,000 rpm range !).. So an extra ratio in its box would also help it regain som of its accelleration over the rest of the 1/4 mile.
But, as Was said, multiple ratio gearing is not done due to the extra cost and complexity, ( weight would be a minor factor )
Of course there are exceptions, ...
...notable when maximum performance is the objective , as with pro drag racing, the custom built EV drag race cars do use multiple gear ratio transmissions, because they have shown it gives better accelleration and higher top speeds (lower ETs)
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Re: ICE vs Electric Motor Transmissions

Post by SlowCo » Feb 18 2019 4:26am

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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Re: ICE vs Electric Motor Transmissions

Post by Hillhater » Feb 18 2019 6:39am

SlowCo wrote:
Feb 18 2019 4:26am
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.
Go tell the EV drag racers.
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Re: ICE vs Electric Motor Transmissions

Post by SlowCo » Feb 18 2019 6:49am

Hillhater wrote:
Feb 18 2019 6:39am
SlowCo wrote:
Feb 18 2019 4:26am
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.
Go tell the EV drag racers.
Should I really have added "for road legal" EV's...?

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Re: ICE vs Electric Motor Transmissions

Post by speedyebikenoob » Feb 18 2019 1:19pm

jonescg wrote:
Feb 17 2019 8:20pm
speedyebikenoob wrote:
Feb 17 2019 2:22pm

Yup, it would be more costly and take up more space, but if the i3's motor had a six speed transmission, could they reduce the output to let's say, 130 hp or so and get the same performance?
No, because one will have 130 hp and the other will have more. Power is the product of torque and speed. If you opt for a less powerful motor you're opting for a motor with either less torque, or less speed. So it will be less capable no matter what gearbox you bolt it to.
But if it had less torque and you had a multi-speed transmission you could gear it lower in the first couple gears, or if it had a lower top speed you could gear it higher with a transmission (provided of course that the motor has the power to reach those higher speeds)

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Re: ICE vs Electric Motor Transmissions

Post by speedyebikenoob » Feb 18 2019 1:33pm

Sunder wrote:
Feb 17 2019 10:45pm
speedyebikenoob wrote:
Feb 17 2019 2:29pm
Aren't middrive's more efficient than hub drives though through hills and mountains and from a standstill?
That's for a different reason. Electric motors are less efficient when the actual speed starts deviating from no load speed at the desired effective voltage.

This is really hard to explain, because of multi-dimensional graphs, but bear with me.

Lets say you have for arguments sake, a 100V electric bike. At 100v, the winding and wheel sized hits a no load speed of 100km/h

If you open your throttle up, so that the "effective voltage" seen by the motor is 100v, and you are traveling at 100km/h, your bike will operate pretty close to max efficiency.

If you open your throttle 50%, so that the "effective voltage" seen by the motor is 50v, and you are traveling at 50km/h, your bike will also be pretty close to max efficiency.

However, if you are towing a large trailer, and climbing up a hill, and you have your throttle as high as it goes, so that the motor sees 100v, but you are only traveling 50km/h, then your motor will be inefficient.

Mid-drive bikes try to get around that, by decoupling the motor from the wheel - Your motor can spin at the 100v speed, while the wheels are only traveling at the 50km/hspeed, so the motor is happy, and you've doubled your torque at the wheel at the expense of speed, maintaining the whole power = Speed x torque equation.

Make sense so far?

The thing is, it makes sense for low powered bikes, because you would frequently be using 100% throttle, but going less than 100% speed. That's usually because electric bikes are 250-3kw.

Despite being larger, a BMW I3 has 127kw, meaning you'd almost never be on full throttle for more than a few seconds. A gear box introduces losses, requires maintenance, is additional weight, additional failure point, additional cost, etc. All for a bit of efficiency gain a few seconds every take off? Not a good trade off.

With regards to your original question, torque curves for I3s are hard to come by, but Tesla published the below graphic to indicate why they don't really need a gearbox:

Image

Basically, by the time the torque starts falling off enough to make it worth gearing up to return to the higher part of the torque curve, you'd be well over 100km/h, and most people care how fast a car does 0-100, not 150-200km/h.
Yeah it's probably just easier to add more power, but I mean it would be fun to have a manual transmission in an electric car, and it would probably feel quicker too. And I'm sure they could make it two speeds though with not that much difficulty or added complexity, the i3 maxes out at 150 kph which isn't enough for a lot of places. They could have a lower gear that goes from 0-100 kph and maybe one from 100-200 kph.

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Re: ICE vs Electric Motor Transmissions

Post by speedyebikenoob » Feb 18 2019 1:36pm

Hillhater wrote:
Feb 18 2019 2:07am
Actually its obvious the i3 would be a faster car ..better accereration and higher top speed, even with a 2 speed gearbox.
Every EV has a reduction transmission between the motor and the roadwheels, and that reduction ratio is carefully chosen to suit the motor characteristics (torque, rpm , etc) , and the target vehicle performance (acceleration, top spaad , range, etc)
But that ratio is always a compromise to give "acceptable" acceleration with "accepable" max speed.
A simple 2 speed gear system would allow an inproved initial accelleration AND a higher top speed.
As has been said, even the mighty Tesla S starts to get beaten after the 1/8 mile mark, because it has run pasts its Torque/power peaks...(at only 6-7000 of its 18,000 rpm range !).. So an extra ratio in its box would also help it regain som of its accelleration over the rest of the 1/4 mile.
But, as Was said, multiple ratio gearing is not done due to the extra cost and complexity, ( weight would be a minor factor )
Of course there are exceptions, ...
...notable when maximum performance is the objective , as with pro drag racing, the custom built EV drag race cars do use multiple gear ratio transmissions, because they have shown it gives better accelleration and higher top speeds (lower ETs)
That's what I thought, I'm sure my 2.4 kw middrive e bike pulls a lot harder and overheats a lot less than the hub drive power equivalent does. I believe Tesla tried a two speed gearbox with the original roadster, but it kept breaking due to the torque, so they just stuck to a single speed and added more power, so it does and should make it quicker I think.

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Re: ICE vs Electric Motor Transmissions

Post by spinningmagnets » Feb 18 2019 3:42pm

Years ago, all that was available were low amp batteries. If you spec put a small 2-speed Transmission (or 3-sp), it might have allowed you to use a slightly smaller battery and a slightly smaller motor, but...these days I have to agree with Luke.

Perhaps the build budget is better spent on higher-amp batteries and a slightly larger motor.

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Re: ICE vs Electric Motor Transmissions

Post by Hillhater » Feb 18 2019 4:45pm

spinningmagnets wrote:
Feb 18 2019 3:42pm
......
Perhaps the build budget is better spent on higher-amp batteries and a slightly larger motor.
Possibly for an Ebike,..... though even there, more and more are going with the geared mid drive ?
...but not so practical for a mainstream EV, where i suspect those items have already been the subject of a lot of thought and cost /benefit analysis, and "Range" is likely one of the main considerations.
"High amp", and " high capacity" do not mix easy with low cost .!
Even Teslas solution with a powerful motor and battery, shows its limitations as the speed goes beyond the motor power band (80-100 mph)
But as we have said, for most EV users, multiple gears just are not needed and manufacturers certainly do not want any additional costs in either gearboxes, motors or batteries, to make marketing even more difficult.
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Re: ICE vs Electric Motor Transmissions

Post by jonescg » Feb 18 2019 8:05pm

speedyebikenoob wrote:
Feb 18 2019 1:19pm
jonescg wrote:
Feb 17 2019 8:20pm
speedyebikenoob wrote:
Feb 17 2019 2:22pm

Yup, it would be more costly and take up more space, but if the i3's motor had a six speed transmission, could they reduce the output to let's say, 130 hp or so and get the same performance?
No, because one will have 130 hp and the other will have more. Power is the product of torque and speed. If you opt for a less powerful motor you're opting for a motor with either less torque, or less speed. So it will be less capable no matter what gearbox you bolt it to.
But if it had less torque and you had a multi-speed transmission you could gear it lower in the first couple gears, or if it had a lower top speed you could gear it higher with a transmission (provided of course that the motor has the power to reach those higher speeds)
Sure - you will have better acceleration (briefly) but your ultimate top speed will be limited by the lack of power. That's one way to determine the outright horsepower of a road going machine - a top speed run. Who ever goes fastest, has the most power. Regardless of the transmission.

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Re: ICE vs Electric Motor Transmissions

Post by Punx0r » Feb 18 2019 8:07pm

speedyebikenoob wrote:
Feb 18 2019 1:33pm
the i3 maxes out at 150 kph which isn't enough for a lot of places. They could have a lower gear that goes from 0-100 kph and maybe one from 100-200 kph.
High-speed driving is uniquely demanding of any vehicle in that it's the only way to demand high- or peak-power levels for a sustained periods, which is hard on the battery/motor/controller (or ICE): much more so than a 0-60 mph traffic light GP or a 1/4 mile drag race. Air resistance be a bitch. An extreme example of this is caravan-towing speed record attempts. Nothing like a modified car with hundreds of HP blowing its guts struggling to reach 130mph. Runways are long at those speeds, so plenty of time to find the weakest link in the engine :D

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Re: ICE vs Electric Motor Transmissions

Post by liveforphysics » Feb 18 2019 9:13pm

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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Re: ICE vs Electric Motor Transmissions

Post by spinningmagnets » Feb 18 2019 10:47pm

Possibly for an Ebike,..... though even there, more and more are going with the geared mid drive ?
Your point is well-taken, My most-ridden ebike is a BBSHD at 52V with a 7-speed derailleur system. And yet my ebike also reflects on the OP's point, since I have found that I only use two of the available gears (with 1500W on tap).

When it comes to the popularity of mid drives, I do feel compelled to point out that the vast majority of them in the world are used in locations where 250W-500W is the law. If I was truly limiting myself to 250W, I would absolutely want an Alfine 11-speed.

However, I would never be happy with 250W. In fact, I plan to take my 1500W "2-speed" ebike, and bumping it up to 2500W. I suspect I may only need one gear when that happens (for the types of jobs I give it).

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