I’ve never seen an ICE car need two engines to drive all four wheels. Why do EVs need 2 motors? Wouldn’t a transmission be cheaper than another motor?
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Efficiency is paramount in an EV for range. Multiple motors is much more efficient than using a sort of transmission or extra driveshafts/differentials to transmit power. More spinning mass reduces efficiency any time the system is accelerated or decelerated. “Drivetrain losses”. Since batteries are expensive and range is important, it’s better to have a motor for each wheel or each pair of wheels. Gas motors and much larger and heavier making this very difficult so they use transfer cases and extra differentials and driveshafts to achieve 4wd or AWD
Not an expert but my understanding is that electric motors are pretty cheap. In fact, all of the key components of an EV are cheaper than ICE – except the battery and HV systems.
So it’s relatively easy to add another motor, something that’d be much harder an more expensive in an ICE.
No, a transmission would cost more and the packaging issues would be way worse.
I don’t know the real prices of the electric motors used in electric cars but going off by eBay listings, they seem like they are the same price as transmissions.
But a transmission isn’t the only part needed for AWD/4WD. You also need a transfer case, drive shafts and differentials.
But ultimately, most likely the main reason for using a second electric motor instead is reliability. Less moving parts mean less failure points, so less chances of a breakdown. Thus, increasing reliability and decreasing maintenance costs.
Tesla has said they are aiming for sub $1000 cost for their motors. Not saying they are there yet but it’s a good benchmark of roughly what they cost.
In an ICE car with AWD, the power can be transmitted through the driveshaft. In most EVs, the battery pack sits where the driveshaft would go. This would not apply in all cases, but in most.
Battery cells are small, so you can design the pack with empty space for the driveshaft. Probably not a huge point in doing that.
Individual cells are small but in total you need every bit of space that you can possibly spare.
Yes, a drive shaft and transfer cases/gear boxes hugely complicate what can be a very simple EV platform. Also adds maintenance.
Yes, a drive shaft and transfer cases/gear boxes hugely complicate what can be a very simple EV platform. Also adds maintenance.
…that sound you’re hearing is all the dealers collectively orgasming
While you could have an all-wheel drive system with a transfer-case/center-differential in an EV, it would have worse performance in most scenarios coupled with much higher losses. By minimizing the amount of parts the electric motor(s) have to spin, you reduce losses and improve efficiency.
Needing to package drive shafts throughout the length of the car eats into passenger room too, that’s why so many ICE cars have rear transmission-tunnels.
The one scenario I can think of where a center-differential/transfer-case would fare better is with lockers in an off-road situation.
The one scenario I can think of where a center-differential/transfer-case would fare better is with lockers in an off-road situation.
But you can also put a locking diff or LSD on a 2-motor electric drive, with a virtual “transfer case” handling the F/R split electronically and many times per second. Or you can go even further like Rivian with their 4-motor electric drive and let each wheel be directly and independently controlled which is far superior to an old locked diff and transfer case.
A virtual locker doesn’t work as well as a physical locker offroad because there’s no leverage limiting wheelslip. In a mechanical locker you only get wheelspin when grip is low enough for both wheels to spin, while a virtual locker will get wheelspin when the grip of one tire is surpassed. Per-wheel motors lack this leverage too, making them not as good as a physical locker. A system with mechanical lockers on both axles with a locked center-differential does even better because you’ll only get wheelspin if all four wheels lose traction.
Pretty sure you can direct these motors to behave however you want them to behave with the right sensors and software. I also don’t see why locking is inherently important - the important thing is putting power to wheels that have traction and not spinning the wheels that don’t. The quad motor configuration would use the computer to accomplish that rather than mechanical linkages which would be much less intelligent about dispatching torque.
While software can be used to manage traction it will always be reactive in a way mechanical lockers aren’t. The reason a locker is so good for offroading is because the wheels spin at the same speeds, preventing power “leaking out of the system” via wheelslip because the grip of every other locked wheel prevents a single low-traction wheel from slipping.
Independently powered wheels don’t have the proactive traction-management of mechanical lockers and thus are limited to reactive traction-management. That’s not to say independent motors can’t be extremely effective off-road, but mechanical-lockers will be more capable in serious offroading. The physical connection of mechanical lockers work in real-time.
Independently powered wheels don’t have the proactive traction-management of mechanical lockers and thus are limited to reactive traction-management.
This just isn’t very accurate as far as I can tell. The physical strain characteristics of a steel transfer case and drivetrain respond to forces in approximately 1ms (the speed of sound in steel over a 14 ft linkage). That’s the same speed resolution as a good quality computer controlled system. They’re exactly as “reactive”.
There’s nothing “delayed” about a good computer drive system, even compared to mechanical linkages. A well programmed computer system would be superior in every possible way. And you could just program it to only ever allow wheels to spin at exactly the same speed, exactly duplicating a mechanical linkage, with zero disadvantages (and much less complexity).
virtual locker will get wheelspin when the grip of one tire is surpassed. Per-wheel motors lack this leverage too, making them not as good as a physical locker.
This just isn’t true.
Let’s imagine a scenario where you have 0% traction on one wheel, 5% on a different wheel, 30% on one wheel and 60% on the final one.
A lockout would basically add torque evenly, which doesn’t necessarily help, especially in dynamic situations (not a “stuck in the mud” but a “driving in the mud” scenario).
A car with dynamic 4-motor drive could do microsecond-resolution changes to torque to prevent any wheel from ever slipping. It’s rather amazing how much better than a traditional locking diff this would be.
There is absolutely zero reason you couldn’t program a 4-wheel electric setup to behave exactly like a locked diff. Electric motors are flexible and can add torque and direction in any amount at any time, but you probably wouldn’t ever do that because it’s not as good.
Because it’s a lot simpler and less prone to breaking. Tough break for automakers that pride themselves on their transmissions but great for the consumers.
There are several advantages with two motors -no long drive shaft with separate dif necessary -in a low power cruise one motor can be disengaged almost completely. Reduced drag = more range. -The handling of the vehicle can be improved by controlling the torque on each motor independently. -no need to change all tires all at once like it is the case with AWD vehicles with a driveshaft (tires have to be exactly the same diameter bc they are mechanical coupled and would otherwise start to rub and wear out fast) -Redundancy. If one motor fails, you can often still drive with the other one.
Eclectic drive is MUCH easier to implement and two systems at 1/2 Power or capacity is much easier - - and the need to have the “tunnel” through the floor plan is a big design negative.
Basically ICE suck - aside from all of the efficiency issues -
It’s pretty easy to make a small, high power electric motor and they can be easily synchronized or adjusted for traction control. It frees up space where a drive shaft would go to instead put the battery there.
There is an EV company that may produce a car with 4 motors. I think It is from Sabb.
Rivian sells both 2-motor and 4-motor versions of their trucks.
I think the Saabs that you’re talking about were motor-in-wheel technology which is a very different thing: it is great for efficiency and packaging, but all the unsprung weight is a challenge.
In order to have a single motor AWD EV you would need a transfer case and drive shaft between the axels. this would all take up space, space which is mainly used to house the battery. It’s probably more expensive to have two motors than to try and design a single motor AWD system for an EV, and it really doesn’t make any sense.
Engines are complex large expensive components so putting to of them in a car would be about the worst thing ever. Electric motors are small simple and cheap. You can hold a 500hp electric motor in your hands and all you need to make it run is plug in a wire and maybe some low pressure cooling pipes. Some EVs have 3 or even 4 of them and there is little downsides to doing this. To make the car AWD you would have to at least have a tunnel and jump inside the car for a drive shaft which is terrible.
LOL. Not close. Go watch some YouTube videos on modern EV tech. They explain the why better than most of us.
Driveshafts, differentials, and transfer cases have left the chat.