Would a 2 wheel drive mode be more efficient?

[ChrisCon]

I saw this post on Facebook , and even though Facebook groups are mainly filled with junk or reposts from here , I thought this would be fun to share because we have many knowledgeable people here .

His question posted below . I’d follow up with , would this be possible with a software upgrade ? I’m assuming it wouldn’t because of the magnet technology used in the motors ?

Don’t shoot the reposter !

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[GarageMahal]

Basically no as there is no way to disengage the fixed magnet motors. They don't freewheel.

 

[sotek2345]

Basically no as there is no way to disengage the fixed magnet motors. They don't freewheel.

Yup, exactly this. To be effective, either Ford would have to add in a clutch system (like Rivian did) which adds cost and weight, or switch to an induction motor for at least one of the axles. Induction motors have some advantages, but tend to be more expensive, larger, and heavier.

Edit: the benefit is also rather small. You still need the same amount of energy to push through the air and overcome rolling resistance, so you are really just saving the internal losses of the motor.

 

[cvalue13]

Admittedly out of my bailiwick here, but given that these electric motors aren’t geared I don’t understand why this person thinks this works even theoretically.

It takes a certain amount of power to move the vehicle at a given speed in given conditions. To move the vehicle at that speed, the motors require a certain amount of power from the battery. That same certain amount of power from the battery would be required by a single motor moving the vehicle, or by multiple motors “sharing” that power to move the vehicle.

ICE vehicles accomplish more efficient highway cruising with gearing, absent on most BEV motors to date.

this different gearing is already accomplished in some models by the multiple motors having different “sweet spots” in their rpm ranges, with the computer then allocating power to the motor with more appropriate sweet spot for the given speed/acceleration. and, where models purport to disengage an axel for greater efficiency, I understand that too is merely a method to direct power to a motor with more appropriate rpm efficiency for the given speed/acceleration

all that said, probably the best and most practical use of such gearing in an EV is to produce greater acceleration and lower speeds, rather than to create meaningful efficiency at higher speeds. The vast majority of resistance at higher speeds is the air, which at a given speed will far outweigh the negligible efficiency improvements of an EV with variable gearing (or a disconnecting axel to accomplish variable gearing).

which leads to the real conclusion for your FB folks: if you want markedly greater efficiency at highway cruising speeds, you need only slow down

as recently pointed out to me, given the nature of air resistance/drag on energy requirements, traveling at 70mph requires as much as 50% more power than traveling at 50mph. (And meanwhile, slowing down also likely placing the EV motor nearer to its “sweet spot” of rpm, adding to the gains in efficiency.)

so: merely disconnecting an axel on one of two equivalent motors i don’t understand to gain any meaningful efficiency; disconnecting an excel on one of two differently “geared” motors may provide nominal efficiency increase (5%? Bit more? Total guess); whereas merely slowing down can, in effect, exponentially increase efficiency in highway “cruising.”

(BtW, I wrote all this out for my own benefit and thought organization, more than to posture as being any kind of expert or knowledgeable person on this - I’m sure the more knowledgeable will be along to correct several errors!)

 

[COrocket]

RWD EVs have potential to be more efficient than AWD. Best example I am familiar with is in 2018 when Tesla offered its long range Model 3 with both RWD and AWD. The RWD was rated at 130 MPGe and the AWD was rated at 116 MPGe, or around a 10% difference in efficiency. This is primarily due to the weight savings of several hundred pounds in the front of the vehicle, and the elimination of the front motor drivetrain heat/friction losses that are present with any mechanical system. So in that case, it was slightly more efficient to run the rear motor at 100% instead of running a front and rear motor at 50% each comparatively.

I could see Ford eventually offering something like this since it could be a lighter/cheaper solution for people who don’t need AWD, but for now it seems like they want to keep the builds and options as simple as possible to scale rapidly, hence the crew cab short bed since I bet that’s their best selling configuration.

 

[sotek2345]

RWD EVs have potential to be more efficient than AWD. Best example I am familiar with is in 2018 when Tesla offered its long range Model 3 with both RWD and AWD. The RWD was rated at 130 MPGe and the AWD was rated at 116 MPGe, or around a 10% difference in efficiency. This is primarily due to the weight savings of several hundred pounds in the front of the vehicle, and the elimination of the front motor drivetrain heat/friction losses that are present with any mechanical system. So in that case, it was slightly more efficient to run the rear motor at 100% instead of running a front and rear motor at 50% each comparatively.

I could see Ford eventually offering something like this since it could be a lighter/cheaper solution for people who don’t need AWD, but for now it seems like they want to keep the builds and options as simple as possible to scale rapidly, hence the crew cab short bed since I bet that’s their best selling configuration.

Ford said they tested a RWD setup but couldn't get the performance and reliability they needs. It was implied this was towing related which makes sense. You would be running that rear motor hard when trying to tow uphill in the heat.

 

[Halbach]

I saw this post on Facebook , and even though Facebook groups are mainly filled with junk or reposts from here , I thought this would be fun to share because we have many knowledgeable people here .

His question posted below . I’d follow up with , would this be possible with a software upgrade ? I’m assuming it wouldn’t because of the magnet technology used in the motors ?

Don’t shoot the reposter !

As an engineer those sorts of posts infuriate me. Innocuous questions posed as a "Those dumb engineers probably never thought of this! Pat my back world!" I'm aerospace, not automotive, but I assume they do at least basic trade studies! I'm glad you didn't take it at face value though!

(Edit. Removed my first half thought that never got fully deleted. Was paying more attention to the game than my phone lol)

 

[cvalue13]

RWD EVs have potential to be more efficient than AWD. Best example I am familiar with is in 2018 when Tesla offered its long range Model 3 with both RWD and AWD. The RWD was rated at 130 MPGe and the AWD was rated at 116 MPGe, or around a 10% difference in efficiency. This is primarily due to the weight savings of several hundred pounds in the front of the vehicle, and the elimination of the front motor drivetrain heat/friction losses that are present with any mechanical system. So in that case, it was slightly more efficient to run the rear motor at 100% instead of running a front and rear motor at 50% each comparatively.

I could see Ford eventually offering something like this since it could be a lighter/cheaper solution for people who don’t need AWD, but for now it seems like they want to keep the builds and options as simple as possible to scale rapidly, hence the crew cab short bed since I bet that’s their best selling configuration.

Weight savings (accomplished by any number of weight deletions) being a separate matter from the FB post asking if (basically) disengaging one of two motors would create >1/3 increased efficiencies.

After-all, here we are like a bunch of suckers using both legs to walk, when of course we’d save a ton of energy using only one leg. And as for the unused leg, we could create some weight savings if…

 

[Roy2001]

Given current volume, it does not make sense to produce too many variations, but we will see RWD eventually.

 

[VTbuckeye]

If only rwd we would probably need a larger motor for the rear to have adequate towing power. Are the rivian motors induction? My xc40 EV has two permanent magnet motors. It would likely be more efficient within induction motor at one end or the other, but the AWD system on it works great, it has gobs of power whenever you want... Highway cruising might be more efficient with a rwd drivetrain, but there would be other compromises.

 

[sotek2345]

If only rwd we would probably need a larger motor for the rear to have adequate towing power. Are the rivian motors induction? My xc40 EV has two permanent magnet motors. It would likely be more efficient within induction motor at one end or the other, but the AWD system on it works great, it has gobs of power whenever you want... Highway cruising might be more efficient with a rwd drivetrain, but there would be other compromises.

As far as I know, the Rivian uses 4 permanent magnet motors and uses a clutch system to disconnect the front 2 for eco mode driving.

 

[AZT9]

So interestingly last week after supercharging my truck something weird happened and the truck “put itself into 2WD mode” as others have said it actually made my kw/mi worse as it was not dragging the front motor not to mention a SIGNIFICANT loss in power. Pulled off the highway and cycled the ignition. All
Better!

 

[FlasherZ]

As an engineer those sorts of posts infuriate me. Innocuous questions posed as a "Those dumb engineers probably never thought of this! Pat my back world!" I'm aerospace, not automotive, but I assume they do at least basic trade studies! I'm glad you didn't take it at face value though!

As a founding member of the St. Louis Tesla group, we started showing our cars at the St. Louis Auto Show annually, and would spend the auto show answering the same questions (how fast does it charge? what range does it have?) along with showing people some of the features and functionality (and letting them crawl in and out of our own personal cars).

Every once in a while, we would get the people - clearly not engineers or even those who may have passed HS Physics I - who offer a great plan. "You know, they should put alternators on each wheel and the car would never have to be charged up!" Initially, I responded that first you had to burn power to make the car go, and that regenerative braking essentially does that but it makes you slow down, and that there are always losses in the conversion. At that point, many would realize that and move on. But some would press it further, "nuh-uh! Think about it, all you have to do is use car alternators, they don't slow the engine down at all!" And at that point, I suggest to them that they should submit their idea to Elon and he could consider it.

Now, in dual motor Tesla vehicles, the rear drive unit has a higher gear reduction than the front drive unit, and I recall that in the initial dual-motor vehicles they did some testing that found diverting more power to the front at speed produced at least some savings. The "range mode" checkbox (not to be confused with the "range mode" charging setting of the very very early days) was supposed to change that balance to use less power, but in practice it didn't turn out to be much. The savings mostly came from limiting HVAC fan and compressor speeds to smooth out the climate load.

 

[Halbach]

Every once in a while, we would get the people - clearly not engineers or even those who may have passed HS Physics I - who offer a great plan. "You know, they should put alternators on each wheel and the car would never have to be charged up!" Initially, I responded that first you had to burn power to make the car go, and that regenerative braking essentially does that but it makes you slow down, and that there are always losses in the conversion. At that point, many would realize that and move on. But some would press it further, "nuh-uh! Think about it, all you have to do is use car alternators, they don't slow the engine down at all!" And at that point, I suggest to them that they should submit their idea to Elon and he could consider it.

You are a Saint for putting up with it lol. I have done my fair share of explaining why you can't just drive the truck with a solar roof and call it a day. The phrase "orders of magnitude" gets thrown around a lot!

I'm all for education, learning and understanding is how civilization grows but I'm so tired of people wanting to "learn" when all they want to do is speak. Hoping that the population in general can start accepting EVs (or other future tech) will be around whether or not they understand it or care to understand it.

 

[RickLightning]

As an engineer those sorts of posts infuriate me. Innocuous questions posed as a "Those dumb engineers probably never thought of this! Pat my back world!" I'm aerospace, not automotive, but I assume they do at least basic trade studies! I'm glad you didn't take it at face value though!

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