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?
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?
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.
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).
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.