Fundamentally, yes, it's no different from 2WD... because one engine is replaced by one electric motor. Of course most modern AWD systems are managed by a computer, which expects to talk to the engine controller to reduce power as required for traction control... but again, that's the same as 2WD.
Yes, but again it is the same problem as with 2WD: an automatic transmission is unsuitable for use with an electric motor, because it is designed and programmed to keep the engine turning at some minimum speed or higher (by allowing slip in the torque converter), and that is unnecessary and unwanted with an electric motor. If you allow the transmission input (motor output) to stop, the transmission loses hydraulic pressure and so it can't stay in gear. Conversion of an automatic transmission to work well with an electric motor is expensive... and pointless, since the whole mess can be avoided by starting with a manual transmission (or not using the car's original transmission at all).
Actually mounting motors with the wheels on the hubs is terrible for suspension performance (ride and handling), and packaging them in there is quite difficult. This is true whether the motor directly drives the wheel (which means the motor is way too big and heavy, as Duncan explained), or there is both a motor and a reduction gearbox (so the motor is smaller but there is also that gearbox to fit in).
Particularly in a do-it-yourself conversion project, the complication of doing AWD well is just too much to be worthwhile for almost any builder. Doing it by just replacing the engine with an electric motor, and keeping a simple non-electronic 4WD system, has been done by a few builders, typically with vehicles such as old Land Rovers or Suzukis. Most conversions are RWD because most of them are of old cars or sports cars, but front wheel drive conversions are not unusual, and have usually been done the same way as RWD (adapt electric motor to the car's original transmission).
It's not clear to me that you understand how a Tesla is arranged, or what you are concluding from what you believe the Tesla design to be.
I wonder...
If you're talking about brushed DC motors (and I assume that you are)... yes, you can probably do that, but it doesn't work well. If you connect the motors in series with each other they get the same current, and so would apply about the same torque torque to each wheel (which is good) but it's not clear to me what would happen when the two tires have different traction or the speeds are different due to turning a corner. Connected in parallel if one slips it will take off, possibly to excessive speed.
This actually makes a lot of sense to me, to make a hybrid which normally sends the engine power through an efficient mechanical path to the rear axle, but can drive the front when desired and regeneratively brakes with the front axle.
I'm thinking that the outside corner would just rotate faster.
Oh, let me be clearer then.
Yes, but it would be ridiculous in an electric car with a motor at each wheel to waste energy by using a motor to a wheel against turn a brake, instead of just controlling each motor properly. In a vehicle with one engine or motor a differential needs to be used to split the mechanical power between the wheels, and so designers resort to energy-wasting brake and clutch systems; there's no reason to do that with separate motors.
Okay, I wondered about that, which is why I included this:
The clarification also suggests the motor for the rear would be in the original engine location, not in the transmission location or at the rear axle. That means all three motors are "under the hood", which leads to the pure EV version of this:
With three motors under the hood, plus their controllers, plus the radiator for any liquid cooling and likely the charger, there will be little if any room left for battery. That means finding space for all of the battery pack somewhere in the back, in a car which was not designed for that and has a shaft running down the middle of it (from a motor in the front to the rear axle).
Yes, but there is only one rear engine car still in production (the Porsche 911). Old rear-engine cars exist, but have their own issues (such as all of them being piles of rust, equipped with antiquated suspension). Mid-engine cars (with the engine behind the driver) would also work, but they tend to be expensive, even exotic, because it is fundamentally an impractical layout. All that is really needed to make the multi-motor AWD setup more viable is to be able to put the rear drive motor at the rear axle, but Matt may be thinking of his Opel GT plan, and that car has a beam axle in the rear... it would need independent suspension.
Yes, but how would the motor respond to that, in terms of current and voltage, and torque produced?
Same problem, other side. Nothing tends to make the car straighten, but the reaction of the motors to different speeds determines how the car is affected. Ideally, you would want the motor on the outside of the turn to produce more torque (in proportion to the greater traction it has due to dynamic load transfer), but that's not going to happen.
Have a look at this site, which provides a lot of detail about converting a 2006 Subaru WRX.
It's thoroughly and clearly documented, which I certainly appreciate, but that also means some of the undesirable features and false assumptions are clear. Some examples:
Wow, is it ever.
No DIY conversion is perfect, partly because you have to work within the original design of the car. This guy did a nice, high-quality and functioning conversion, documents it so that it may be helpful for others, and you review his site and the best you can do is offer pedantic comments. Nice. I thought about contacting him and pointing out this post so that he could come defend himself, but decided it would be a waste of his time.It's thoroughly and clearly documented, which I certainly appreciate, but that also means some of the undesirable features and false assumptions are clear. Some examples:
- It now has about 50:50 weight distribution... until you add anything to the rear seat, at which point it becomes undesirably rear-heavy. The AWD system is not designed for a rearward bias.
- All of the battery was placed behind the rear axle, and the hefty motor is in the original engine location ahead of the front axle, so the mass is too close to the ends, hurting dynamic response.
- The rear spring rate was not changed (the springs were just shimmed), so the rate is now too low for the mass of the rear of the car.
- "The car neither understeers nor oversteers"... that's completely implausible, so this indicates that the builder is not a sufficiently skilled and experienced driver to be able to tell what the car is doing. It probably does handle acceptably for typical driving.
- 85% motor efficiency might be achievable at one specific speed under ideal conditions, but most of the time it's just a fantasy.
- He thinks that an automatic transmission would be a good idea.
I think it's fascinating that this writer makes the usual nonsense claim that "Electric motors provide maximum torque at all speeds", then shows a graph which clearly shows that this is not true!
I had a question about this, since I'm about to struggle with the same myself.
Yes, obviously. This one has done about as little as possible to modify the vehicle design to accommodate the conversion.
There's no need for him to "defend" himself. He has done a very good conversion by DIY standards, and a wonderful job of documentation.
