It would improve efficiency driving up mountains and towing... single gear EVs already get great electrical to mechanical conversion efficiency at constant high speed on flat ground (95-96%). They get much lower efficiency while accelerating through the lower rpms at full throttle, or towing very heavy trailers up very steep slopes at low speeds. That’s because those situations require high torque from the motor and torque is directly proportional to the motor current and heat (loss) increases at the square of the current. Doubling the motor torque quadruples the heating, but if you double the gear ratio instead, you still get double the torque and thrust at the wheels without increasing the heat (but roughly half the top speed).

 

Given the fastest roads in the UK are 70mph, and several Tesla will reach that speed in less than 4 seconds, I can't see any demand for more performance in mass market cars. TBH, the P85D is actually uncomfortable to sit in doing a launch, much faster and sick bags will be required.

Top speed is not limited by max motor rpm in most cars, but by where efficiency drops too much, so adding another gear (plus clutches to handle monster torque = weight) could improve top speed, with slower acceleration due to the extra mass involved.

Efficiency for most motors at up to and above 70mph is better than 93%. Adding a gearbox etc to reduce the motor rpm and increase efficiency would almost certainly be worthless.

20kW into motor ~ 18.6kW of useable motive power at 93% efficiency.
20kW into motor ~ 19kW of useable motive power at 95% efficiency.

So if you drive for an hour at 60mph, your gearbox 'enhanced' car will travel a further 2 miles, before factoring the additional drivetrain losses, and the extra weight. These together surely cancel any advantage.

Special applications would obviously benefit from gearing, and those solutions already exist.

 

Seeing as the Porsche has a 2 speed box and it isn't any faster than the S suggests it has almost no benefit but a lot of potential issues in the long term.

If Tesla fit a 2 speed box to their roadster then it's possible there is some kind of advantage but if they pull off the promised performance with just one gear I'd say that's probably the end of the road for the multiple gears in EVs argument.

 

Seeing as the Porsche has a 2 speed box and it isn't any faster than the S suggests it has almost no benefit

One can’t accelerate any faster than 1g without aerodynamic downforce so there’s no real benefit to having more thrust than the weight of the car unless you have fins, or for hillclimbing or towing...

 

Metastable has it nailed. Only case for which gears might possibly be justified is for super heavy duty towing.

 

And you have to have the battery capacity to justify doing it - halving the range of a LEAF24 when towing with lower ratio gears would be a niche product. Only people like Tesla (Model X) and Audi (E-Tron) have that sort of capacity, but like all EVs lack the ability to recharge fully in 5 minutes like an ICE. Also, the efficiency of an ICE at high load is at its best and closer to an EV than at other times.
It'll be interesting to see what capacity Tesla end up putting in the Cybertruck for the ******** that want to do serious towing.

 

Gears offer no advantage in terms of efficiency so why bother? They go fast enough and accelerate adequately anyway.

 

Gears offer no advantage in terms of efficiency so why bother?

Wrong. It depends on the balance between power and efficiency at different speeds. The original LEAF24s from Japan had lower gearing to give more efficiency at low speeds and acceleration vs. the UK version that had higher gearing to be more efficient at high speed. If you wanted to tow a lot the Japan built ones would be better. Or a two speed version if you want to pay the extra for a benefit you might never use.
As a vaguely related anecdote, when Rover adopted the Buick V8 Rover asked Buick to raise the maximum continuous rated RPM which they refused. They did make the mistake of sending one of their chief Engineers over to the UK to continue the negotiations to sell an otherwise redundant product. Spen King the Rover chief Engineer kindly arranged for a driver to pick him up from the redeye in one of the prototypes and take him from Heathrow to Solihull via Edinburgh to "see the sights", but with instructions to be back for a late afternoon meeting. After a trip averaging over 100 mph (no speed limits on Motorways at the time) including stops. The Buick executive arrived at Solihull and immediately agreed that he understood the need to raise the maximum continuous rated RPM. Perversely this ended up creating the biggest weakness of the engine - it needs to be driven hard to avoid failures unlike a classic American V8.

 

when we might see one or more of the mass-market BEVs

such as in high-end acceleration and top speed,

You don't generally see the latter in the former.
Mass-market = people who want transport.
high-end acceleration and top speed = a different type of people

I believe one of the electric supercars does use a 2-speed box ... but that obviously isn't mass-market.

 

And there in lies the issue. Even the LEAF24 is geared for 82 mph which was considered the minimum acceptable in Europe. I'm not sure many M3P owners would accept 70 mph to be able to tow significantly more up an Alp.

 

I agree that multi-speed transmission in a BEV won't be mainstream. Cost and complexity just isn't worth the limited gains. Sure, we might see it on high value, performance cars but not on mainstream. Even Rivian only plan single speed for their pickup ‐ their motor-per-wheel is probably better for off road performance anyway?.

 

Even Rivian only plan single speed for their pickup ‐ their motor per wheel is probably better for off road performance anyway?.

each doubling of motors halves the total joule heating losses and gives 1/4th the heating per motor at constant speed, so 4 motors gives 1/4th the total heating losses and 1/16th the heating per motor at the same constant speed.

 

i.e. exactly the same situation.

 

no— quadrupling the motors gives 1/4th the total heating at constant speed (like halving the gear ratio) (1/16th heating per motor)- but without sacrificing the top speed— you can also get 4x the thrust with the same total joule heating losses with 4 motors compared to 1 motor. multiplying the motors drastically improves efficiency without sacrificing performance. getting 4x the torque/thrust with a single motor at the same gear ratio would give you 16x as much heating loss.

 

fwiw, my further comments on this:

• for vehicles where speed and acceleration are of some interest above 100 or 150 km/hr.... this is not just about supercars, but can also be a good quality differentiator on something as mundane as a mid-level sedan driving on a high speed road in North America or Europe.
• As to the high-end non-mass-market vehicles, both the Rimac Concept-One and the Porsche Taycan appear to employ multiple gears. Supercar conversations are somewhat interesting, and they are somewhat relevant to learning things that may apply in mainstream vehicles, but my own main interest here is not ultra-fast vehicles. When I talk about mass-market, I have in mind anything from a Leaf to an entry level luxury BEV to a bev pickup to an exec class sedan or SUV, though I suppose those are much smaller volume.
• Modern BEVs are a relatively new phenomenon. To me, these are (in some ways) still relatively early days, and we really haven't yet seen that much competition yet. I'd like to keep an open mind as to what will work out and what innovations we may see.
• Multiple gears in theory might help, in some implementations, with net efficiency gains (notwithstanding a bit of argument about that here), and thus might help with range and thus might help with cost. Since we know that there is a lot of concern in today's market about range/cost tradeoffs, this fact takes on some added importance. Even if multiple gears have significant drawbacks (and they do), I'd like to get a sense of whether in some vehicles they might add range, and how much, whether they could help help in subtler ways such as allowing a manufacturer to use a somewhat less powerful somewhat less costly electric motor.
• it's interesting how the EV revolution has transformed this conversation and how multiple motors is now such a common direction and how that in effect does impact the multiple gear conversation.
• it will also be interesting to see how the traditional mechanical transmission manufacturers try to make themselves useful. Yes, that will include heavy trucks, but it seems at least within the realm of possibility that it could include light duty vehicles.
• Most probably already know this, but the pre-production Tesla Roadsters were designed with more than one gear. It's true that Tesla's lack of need to return to that conversation does point to a reduced relevancy of exploring multiple gear BEVs, but for manufacturers putting less costly vehicles on the road with less power and trying to keep overall costs down, if a 2 or 3 gear setup could help with range a bit, it will be interesting to see if they try this. I'm not saying I think this approach will win in the end, but I'm saying I think it would be interesting to see someone try it, and see what the results are.
• I thought the points about towing and heat were worth mulling over. As well, similar points about simply driving uphill. So, this isn't just about whether a pickup can tow this or that. If multiple gears, or other competing strategies, could help flatten the range curve for some vehicles depending on the demands being made of the vehicle, then that would be interesting.
• [edit to add this comment - for right or wrong, BEVs are so good and powerful, and efficient and clean, for such modest money once we work the kinks out, that the very low end cheap efficient econocar side of this conversation kind of goes away for me.... so my mass-market focus here is more on something like a relatively luxurious highway cruiser.]
  
  

 

The issues of cost and weight against the possible benefits needs to be examined. A multiple gearbox and the limited benefits for a lower cost BEV make less sense than for a higher powered/cost one. Hence at present they are limited to the most expensive BEVs.

 

I think there are various things here that could be examined and debated. As well, it would be good if we had some more empirical data points to work from in trying to discuss matters. matters. If in the end there's only a fractional efficiency advantage in many of the cases, or some disadvantage, then I think there's not much point in those cases. On the other hand, part of the possible cost advantage could be substantial if it includes being able to include a more moderate electric motor or a somewhat smaller battery than the direct competition.

I wonder if a multi-speed gearbox has has been tried on any garden-variety mainstream BEVs in China. Perhaps as well a path to see what happens could come from some big-battery PHEV (depending on the PHEV architecture?).

 

You mention manufacturers possibly trying to keep costs down, but as a non-mechanical person I'm really intuiting that adding a different component would not give the returns in overall efficiency that the additional complexity would require. I'm pretty sure that a more powerful motor would be cheaper than additional complexity in turning power into motion.

That's not to say that top end cars wouldn't benefit (in sales terms?!) from the technology, but I'm all for simplicity.

 

You may be right, but:

• multi-speed mechanical transmissions have been made for a long time, .... I'm guessing some of the manufacturers are ok to go on making them at what may seem to be a moderate cost to some, if a use can be found for them.
• AFAIK, some electric motor components can be somewhat high cost and may not just drop in price in a sharp way, even with mass production. This would include I guess the rare-earth-based magnets used in some of the motors.

 

I think what it comes down to is -- comparing the option of adding a second gear vs adding a second motor -- adding a 2nd gear improves efficiency at the sacrifice of performance while using that second gear -- but adding a second motor improves efficiency without sacrificing (and in fact enhancing) performance all around.

But the question becomes why not just use a bigger motor instead of a second motor? Putting twice as much copper in the same motor at first glance gives the same efficiency improvements as using 2 motors.

Upon closer inspection though, if the goal is to increase performance not just improve efficiency, motors are more likely limited by the amount of current at which they become magnetically saturated and motor volume. You can't just simply add more copper to a motor because it's already full of copper if it's well designed, and it's probably already close to magnetic saturation at peak performance so you can't simply add more current for better performance since there are diminishing returns once the motor saturates. Also if you want to make a bigger motor you face the fact that it is harder and more dangerous to manufacture and handle ever larger permanent magnets. So from a business perspective it probably makes more sense to use multiple motors rather than manufacture bigger motors when greater performance is demanded in a larger or higher performance vehicle.

Just look at the tesla semi-- I believe it uses (4) model 3 motors rather than a unitary larger motor. and the cybertruck uses 1, 2, or 3 motors which likely gives performance flexibility at reduced cost rather than having 3 completely different motor sizes based on desired performance.

 

[...] adding a 2nd gear improves efficiency at the sacrifice of performance [....]

I'm not sure where you're getting this.

As well, when one adds motors, I think there are some potential range (efficiency) penalties, not only due to added weight, but the added current draw. The recent example I suppose would be the Taycan.

 

If you add the second gear (doubling the ratio), you can get 1/4 the total heat loss for the same thrust at the same speed (top speed is halved while using the gear).

If you add 3 more motors, you also get 1/4 the total heat loss for the same thrust at the same speed, but the top speed isn't halved (no gear changing necessary).

So if the goal of the second gear is to improve efficiency, the same can be achieved by adding more motors, but with more motors there is also the benefit of the option of increased overall performance assuming the performance of the original motor was magnetic saturation limited.

 

Thanks, I think some of your way of approaching this is over my head, but in any case, notwithstanding your points (from what I can understand of them), as far as I know, my two points stand:

• I think you were trying to say that adding multiple gears does not increase peformance? To the best of my knowledge this is simply wrong.
• I also was trying to point out that use of multiple motors may reduce efficiency. I don't think this is terribly controversial. It is just another way of saying if one is going to go for higher-power and faster acceleration, there is going to be a higher use of energy per distance traveled.

Basically, AFAIK, whether multiple gears or multiple motors, one can in theory achieve higher efficiency, higher performance or some balanced trade-off. The question, as far as I know, is whether multiple gears in a BEV really gets you that much of either.

 

Multi ratio gearboxes were invented to allow an engine with a narrow peak power band to travel at different speeds. An electric motor does not have such a narrow band, nor is it rpm limited to the same extent as an ICE.

The problem with using a smaller power motor is you need a certain amount of power to travel up hills without slowing. If you accept that you will go slower, you can use a smaller motor with gear to increase efficiency, or you could simply drive at the same speed without the gear and the larger motor for similar efficiency.

 

Multi ratio gearboxes were invented to allow an engine with a narrow peak power band to travel at different speeds. An electric motor does not have such a narrow band, nor is it rpm limited to the same extent as an ICE.
[...]

Yes, I know. The thing is, I'm not sure this will mean that such mechanical gearboxes are finished now and for all time as to their usefulness in light duty vehicles, as some might believe. Perhaps. Quite possibly. But I'd still be curious to see if some artful combination of mechanical gearbox and electric powertrain might result in advantages (perhaps known advantages that are under-appreciated, or perhaps even heretofore not-yet-innovated ones) that some in the marketplace could value.

 

Here is a comparison of a vehicle somewhat comparable to a Leaf towing a 2000kg trailer uphill vs:

-adding a second gear which doubles the gear ratio and halving the motor current limit setting

vs

-adding 3 more same size motors and reducing the motor and battery current limit settings per motor to 1/4th...

You can see all options have the same low speed performance & both improvement options improve efficiency and reduce the total heat by the same amount at lower speeds, but adding the 3 additional motors retains the original low speed performance and slightly improves the higher speed performance (because of the reduced heating) unlike adding the doubled gear ratio which cuts the top speed in half...

So adding the 3 motors gives the same efficiency improvement as adding a doubled gear ratio, without reducing (and in fact slightly improving) the top speed performance.

 

For the same low speed performance comparing adding a doubled gear ratio or adding 3 more motors-

-both options give the same efficiency improvement at low speeds (1/4th the joule heating)

-the 3 additional motors slightly improves the high speed performance for the same total battery current limit on account of the reduced overall heating

-the doubled gear ratio eliminates the high speed performance, cutting top speed roughly in half

so given the choice between 3 additional motors or a doubled gear ratio, the 3 motors is a better option all else being equal.

doubled gear ratio performance cons: halves top speed

3 extra motors performance cons: none

doubled gear ratio performance pros: 1/4th low speed heating for the same thrust

3 extra motors performance pros: 1/4th low speed heating for the same thrust & slightly greater thrust and efficiency at higher speeds for the same total battery current on account of the drastically reduced joule heating

 

Gearing something down can increase torque, but reduce rpm.
Gearing something up will reduce torque, but increase rpm.

In both cases the output power is the same and in both cases the output power is less than if the something is a single gear direct drive electric motor.

Adding a gearbox does not increase performance if you are measuring power.

 

But what if you are measuring torque? Torque determines acceleration. I think the role of a second gear would be a lower (not doubled) gear for a faster getaway.

 

We put 3 vehicles in a race, all with the same total battery current limit... 1) a Leaf... 2) a Leaf with an option of a doubled gear ratio (with half the motor current limit while in the doubled gear) for getting off the line with less heat & greater efficiency but the same thrust, and 3) a Leaf with 4 motors but 1/4th the motor current limit per motor but the same total battery limit as the others (1/4th the motor & battery current limit per motor so same initial acceleration off the line as the others)...

Initially they all accelerate at the same speed.... Leaf #2 and Leaf #3 are initially using slightly less battery current than Leaf #1 for the same acceleration...

...but regardless of when Leaf #2 decides to switch gears, Leaf #3 is going to win the race because Leaf #3 has slightly more thrust at the higher speeds than the other 2 Leafs because it wastes less of its battery current limit as heat and that directly translates into more thrust at the higher speeds using the same amount of power from the battery, assuming they all weigh the same.

 

There's the small matter of cost.

I imagine that four small motors (with four single speed gearboxes) are a lot more expensive than one normal motor and a 2- speed gearbox.

 

Gearbox, in an ICE it is as much for low speed transmission as for top speed.
In BEV, do you set the gear ratio for top speed, or for lower speed. In general, what ever ratio you choose the car will still have reasonable low speed performance. An ICE simply cannot start from 0 revs without a gearbox and clutch system.

 

I don’t know but 0-60 in 2.28 + 0.269 seconds is only 1.07g.

I can also mentally picture the body lifting up from the initial 1g acceleration so when it comes back down from gravity it has some downwards inertia above and beyond the aerodynamic downforce and the computer probably reacts fast enough to apply extra torque during that time. That’s probably why it gets its best acceleration 10.3-30mph.

 

So if you don’t know, why are you so sure about your claim?

 

Well for a wheel powered vehicle to accelerate faster than 1g the coefficient of friction on the tire has to be more that 1 which isn’t typically the case. 1 means the friction force equals the normal force.

 

In this video you can pretty clearly see the tesla body lift up... & what goes up must come down...

 

Here we see a vehicle similar to a leaf towing a 2000kg trailer, compared to switching to a doubled gear ratio, compared to switching to 4 motors with 1/2 the motor current limit setting per motor and 1/4 the battery current limit setting per motor.

Both the doubled gear ratio and the 4 motors options double the thrust of the leaf enabling towing the trailer up a 31.5% grade that it wasn't able to climb before without producing extra joule heating and improving electrical to mechanical % efficiency...

But the 4 motors option achieves all the same benefits as the doubled ratio while improving the electrical to mechanical efficiency (reducing joule heating) at higher speeds compared to the 1 motor vehicle, all with a single gear ratio.

So the doubled gear ratio has benefits for towing but the 4 motors option has all the same benefits plus additional benefits once the vehicle reaches ~50mph.

You can see in the top middle chart the 4 motor option is the only option that can reach 50mph on the 10% slope, and it does so without drawing any more power from the battery than both of the 1 motor options, and it does so while producing 27.8% the total heat or 6.95% as much heat per motor.