[jlsoaz]

I've been wondering for a long time when we might see one or more of the mass-market BEVs offer multiple gears. I realize there are a number of real advantages to single-gear BEVs (smoothness, lower weight, complexity, cost, probable better reliability and durability, little sacrifice of acceleration due to low rpm high torque, synergy with multiple-motor BEV setups, etc.), but I can't help but wonder how multi-speed gear boxes might do in some aspects of some BEVs, such as in high-end acceleration and top speed, and whether range and miles-per-kWh could be improved or not.

 

[metastable]

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).

 

[ScottC]

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.

 

[JWilde]

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.

 

[metastable]

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...

 

[freddym]

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

 

[dk6780]

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 rednecks that want to do serious towing.

 

[EdH]

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

 

[mikegs]

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.

 

[dk6780]

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.

 

[metastable]

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

doubling the the gear ratio when towing cuts the joule heating losses to 1/4th compared to single gear at the same constant speed. that’s because you need half the motor current with double the gear ratio, and the heating drops at the square of the current. if you double the gear ratio in a model 3, top speed drops from ~140mph down to ~70mph...

 

[dk6780]

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.

 

[proddick]

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?.

 

[metastable]

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.

 

[dk6780]

i.e. exactly the same situation.

 

[metastable]

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.

 

[jlsoaz]

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.]
  
  

 

[dk6780]

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.

 

[jlsoaz]

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?).

 

[SueH]

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.