If you want to go faster....just spin the motor faster. You want to go backwards? Just spin the motor in reverse. No need from a transmission.

For the majority of real-world use, the speeds at which efficiency becomes a real issue is just not attainable by most (due to legal limits, or just weight of traffic). Introducing more parts to go wrong is probably not a good idea, and goes against the EV grain - the less to go wrong, the better.

 

If you want to go faster....just spin the motor faster. You want to go backwards? Just spin the motor in reverse. No need from a transmission.

Thanks for the physics 101

 

but given EVs tend to be less efficient at higher speeds (compared to ICE being more efficient at higher speeds and the opposite at lower speeds)

I think this is a bit of an oversimplification; EVs are less efficient at high speeds because of aerodynamic drag. ICE vehicles have this too, but it is somewhat compensated for by the engine becoming less inefficient because of the efficiency curve that is much steeper for ICE than it is for EV.

EV motors do have an efficiency curve, but it is much shallower than ICE so introducing gears doesn't really make all that much difference and so far has not been worth the added cost and complexity.

 

We know range anxiety is still a big barrier to EV adoption and all new EVs tout their range as a major selling point/feature of their product.
There's got to be some mileage () in having some sort of gearing f you can eek more miles out of the same battery in same conditions.
The question is cost/complexity vs the benefit in additional miles achieved.

It might not seem much if averaged over varying driving styles (Town and urban combined) but if the evidence suggested that it only made sense at higher speeds then perhaps they could offer it as an option for high mileage drivers that do more miles on motorway.

 

The question is cost/complexity vs the benefit in additional miles achieved.

Not just cost/benefit I think; maybe the additional transmission losses or the weight of the additional hardware would actually outweigh the greater efficiency from the motor...

There have been some one-off EVs with two or more gears so it must make sense in some circumstances, but maybe it's not just down to cost.

I quite like the Tesla solution of having two motors with different peak efficiencies and changing from one to the other as the speed changes, but Tesla presumably don't have to worry about cost quite as much as those building smaller runabouts. I wonder what a dual motor Zoe would be like

 

But as stated, the drop in efficiency isn't due to gearing. It's mainly down to aero. What advantage do you gain by changing the gearing?

 

AIUI its not efficiencies that drive the need for two gears its the trade off about how fast a motor can spin.
With a particular motor it can only spin so fast which directly limits how fast it can go. So you could have a bigger heavier motor that can spin faster or heavier gearing &complexity to increase top speed. Thats a reasonable trade off to look at in Formula E.
There are only gears in ICE cars because the engines are very poor at generating torque outside a narrow band of revs. This isn't the case with electric motors which have a much better torque curve that only drops off near the top limit.
With "ordinary" EVs you dont need gearing because the torque is there at most revs, and the top speed derived from a standard electruc motor with fixed gearing* is well above legal limits already, indeed many (most?) are artificially limited to well below the top theoretical speed.
No point having the extra complexity of gearing if it will help a Zoe get from say theoretical 110mph to 180mph if you've limited it to 84mph anyway.
As said the "inefficiency" in an EV at higher speeds is all about drag and nothing to do with the electric motor.

* EVs have a single fixed gear.

 

So are we saying, if an EV was placed on a rolling road or in a wind tunnel (without the wind of course) and we took away the drag element, the energy consumed to produce 30mph at the wheel would be roughly the same if it had to produce 60mph at the wheel?

Obviously we can't take away drag in a real life scenario and it increases with speed. As for the weight of the 'gears/gearbox', is that because we're restricting our thinking to what we know current gearboxes (even a 2 speed) weigh.

I get that it's needed in an ICE to achieve a stepped increase in speed from a given RPM range. I'm not suggesting an EV would need it to increase speed range capability. I'm thinking from point of consuming energy at low motor speed but at a higher road speed

 

So are we saying, if an EV was placed on a rolling road or in a wind tunnel (without the wind of course) and we took away the drag element, the energy consumed to produce 30mph at the wheel would be roughly the same if it had to produce 60mph at the wheel?

Obviously we can't take away drag in a real life scenario and it increases with speed. As for the weight of the 'gears/gearbox', is that because we're restricting our thinking to what we know current gearboxes (even a 2 speed) weigh.

I get that it's needed in an ICE to achieve a stepped increase in speed from a given RPM range. I'm not suggesting an EV would need it to increase speed range capability. I'm thinking from point of consuming energy at low motor speed but at a higher road speed

No it would be much more like 1/2. But you'd need it for 2x as long to do the same distance. Basically its a straight line except at the extremes.
You seem to me to be mixing up power used with inefficiency.
You can use more power without being inefficient. eg if you used X power to go at 30mph and 2X at 60mph that would be perfect efficiency*. Same applies to a petrol car except because the engine has a narrow rev range where its maximally efficient you need gears to keep it in that range. Electric motors dont need that, put 2x as much power in and it revs 2x faster.
Inefficiency would mean, for example it needed 3x or 4X as much to drive the wheels at 60 as at 30 rather than 2X.

Thats AIUI, not being an engineer by training.

* in real life there will always be other stuff gets in the way of course, eg theres no doubt some power losses at low speeds and also high.

 

I think people here have answered your question but I wanted to point out something else... you should have been asking "why do ICE need gears?".

There is an inherent problem with the operation of an ICE... sorry had a look at google, which produced this:

It even has numbers...

 

I think people here have answered your question but I wanted to point out something else... you should have been asking "why do ICE need gears?".

<snip>

Nice.

 

ICE has a narrow power curve (2-4000rpm) :

 

ICE has a narrow power curve (2-4000rpm) :

This is only applicable to majority of diesel engines.
There are many ICE's out there with a usable range of 1-6500rpm but a bit thirsty on petrol.

 

why don't they have some sort of gearing?

Most EVs have a single-speed reduction gear. Why? Adding gears increases cost with little benefit.

A few performance EVs have gears on the electric machine. The BMW i8 has a two-speed gearbox on the front motor. In the original design the gears would shift from low to high at something like 90mph. Shifting at upset the balance of the car however. In the end, the car is in low gear when in EV only mode and high gear in hybrid. There is a separate 11HP electric machine connected to the rear ICE. That one has a six-speed transmission.


FormulaE cars have a 4-speed gearbox.


There are some others too but I don't recall them.

 

FormulaE cars have a 4-speed gearbox.

Actually, while the entirely-spec car used in the first season of Formula E used a 5-speed gearbox, since they opened up the powertrains for development there's been a whole host of different solutions, with everything from 1 to 5 gears. It'll be interesting to see if the solutions converge as development continues!

 

I (and many others) have an EV that has a six speed gearbox. It's called a Golf GTE in electric mode - how I drive mine most of the time.

Certainly the gearbox was originally fitted for the benefit of the ICE, but the motor drives through it too. (Much to my surprise when I got the car).

Presumably that's what allows a pretty brisk acceleration and an 80mph top speed from a fairly small motor pulling a fairly heavy car.

Frightening level of complexity, of course!

 

I (and many others) have an EV that has a six speed gearbox. It's called a Golf GTE in electric mode - how I drive mine most of the time.

Can you pull away in 5th? ;-)

 

I am not into timed acceleration trials, myself, but there's a lot of threads in the GTE forums about acceleration with many claiming 0-60 times better than the VW official figures.

As for 0-30, they say that pure electric is quicker as, using GTE mode, the electric motor has to bump start the ICE which absorbs some power at the wrong moment!

Not belonging to the hot hatch fraternity, I find there's plenty of acceleration without flooring the pedal since the DSG gearbox powers through the changes seamlessly. So most of the traffic is left behind effortlessly at the lights without resort to petrol.

 

HandyAndy said:- "The GTe has, on paper, 105 + 150 hp available = 255 hp, and that would have been enough to leave their GTi in the dust, but we can't have that happen, can we? So they limited the combined power to 204 hp. Supposedly this was so they didn't break their 6-speed DSG gearbox! Max torque is 250 Nm from 1600 to 3500 rpm for the ICE alone, and combined max torque of 350 Nm when electric motor is added in. They claim 0-60 in 7.6 secs (using ICE + electric motor to get this). I'd expect 0-60 in ~14 secs if powered by only a 105 hp electric motor for a town-Golf EV-only version. But that had that covered already.

Ampera has 150 hp electric motor(s), 370 Nm max torque, and does 0-60 in 8.7 secs claimed. With no ICE being fired up at all."

I have looked at the paperwork - my GTE handbook.

Your numbers are roughly right but you missed one. The torque for the electric motor alone is 330Nm (243 LbsFt). So, given the variable gear ratios (which I do not know in detail), I would guess that the torque at the wheels is a whole lot higher than your Ampera at low speeds when accelerating. Hence the license-losing initial acceleration for 0-30 or 0-40 which needs watching until you are used to it. Runs out of puff after that as the aerodynamics cut in and it has probably reached peak power which is only 75kW after all.

Your 350Nm figure is the artificially limited maximum combined torque "to protect the gearbox".

 

Just looked up the Golf R numbers. Maximum torque is quoted as 280 Lb Ft (379Nm). So limiting the hybrid torque to 350 Nm seems quite sensible if the internals of the DSG box are the same. It would add up to 580 Nm otherwise!!

 

Typically EVs have a single-stage reduction ratio of about 10:1 between the motor & the driveshaft. But some motors are designed to have a much slower rotational speed but higher torque, allowing them to be used in direct-drive e.g. hub-centre arrangements, saving the cost of the 2 gears as well. So yes, it's hard to compare these torques, as Ampera has in effect a single-stage reduction in this scenario, and the GTe is swapping cogs.

I rather doubt the GTe torque at the wheels is a whole lot greater than Ampera; if it was, you'ld be getting horrendous wheelspin! Which I do if the road's a bit moist. So I'm sure the VW engineers will have restricted the electric motor output torque, especially in the lowest gears, to make sure the total of that + ICE torque don't cause wheelspin.

The difference in 0-60 times of GTe 7.6 versus Ampera 8.7 is entirely consistent with GTe power of 204 versus Ampera 150. Gross vehicle weights are almost identical, 2020 versus 2105.

So on this basis the torque at the wheels has to be pretty similar, the GTe having just a bit more than Ampera.

My GTe book doesn't have the 330 Nm figure for electric motor, but states the combined max torque of 350 Nm. So that means your electric motor can pretty-much provide all the torque your car "wants", certainly in low gears at low speed. Your ICE will start to provide significant torque once you get the revs up, and I expect my torque will start falling off over 30-40 mph more than yours will. I have some, but not a lot, of spare power at 95 mph, while you can do 138!

Earlier, Proddick replied:
"The GTE is 10.5 seconds 0 to 60 in EV mode. For performance it needs the ICE so needs gears."

True, it does need ICE + gears to get performance, but only because the electric motor is feebler than the ICE. It could perfectly well also have got near-identical performance with a 105 hp electric motor fitted after the ICE+gearbox stage, and not driving through a dsg/auto gearbox. There's no law saying that for electric motors to give you performance you must have it driving through a multi-speed gearbox. Fact is the mfrs can get adequate performance & cheaper cars by sticking with a single reduction stage, even if it means they slightly over-specify the motor in order to maintain decent power at high speeds. Which shows itself as most EVs having huge torque at zero speed with the potential to shred tyres like nothing on earth, if they allowed the motors to output full torque at standstill.

Several people on this forum have said that electric motors have a flat torque output up to some speed, and above that the torque drops off. Actually, the torque curve starts as max at zero speed, and drops pretty-much linearly at the speed rises (for permanent magnet motors anyway). Sadly the current through the windings is similarly huge at low speeds, causing large current-squared heat losses as well as overheating the motor if you were to stall it for more than a few seconds. So the controllers sensibly limit the current at low speeds. Lower current = less heat loss = more efficiency, also cheaper Mosfets & more reliable electronics as not being pushed so hard.

 

So the controllers sensibly limit the current at low speeds.

I notice this more in the Zoe R90 than the Ampera - perhaps a cost decision by Renault?

 

About gearing on EV ... Some formula E cars have two gears, and some have only one ... so ... apparently the difference on having single gear vs having two gears is very small ...

eMotor have a rotation speed proportional to voltage .. I guess the new high voltage eMotor in the latest EV (new Porsche will have 800v battery) should allow flexibility for reach high speed .. most of EV are designed with a relative low high speed for simplify the gearing ...

 

Why no gears, seems to me that both ICE and EV are set up for top speed ok, but the ICE can just not work from stationary up to full speed, with an EV, the motor will pull from 0 rev/m.

For the answer to the question, you need to look at what happens at low speed rather than top speed.

 

the torque of an electric motor comes from the current flow through the windings whilst bathed in a magnetic field.

if I remember my A level physics correctly...

electric motors are effectively a dead short when stationary, so you can get huge currents and huge torque if you didn't limit the current from the battery! As the motor spins up, it also acts as a generator, causing "back EMF" which limits the voltage across the motor and thus limits the current, so as you go faster the motor controller needs to be able to increase the voltage to increase the current to maintain torque.

depending on the type of motor, and the magnetic field changing faster with speed, the efficiency may fall due to the changing magnetic field causing eddy currents, but in a good design these shouldn't be too bad.

fortunately switch mode power supplies can be very efficient at converting one voltage to another, allowing the motor voltage and thus current and thus torque and power to be controlled precisely. It's possible to switch at a fairly high frequency without enduring big losses in the power components.

 

Yes indeed, stall currents can be colossal! But you'll get magnetic saturation in the soft iron poles of the motor, so you won't get near-infinite torque from your near-infinite current! In practice the torque at low speeds is plenty enough to spin the wheels, so the controller will limit the current through the windings as you say. At hgh speeds I rather doubt the controllers can push the voltage above the 400 or thereabouts coming from the battery, it's just not worth the huge cost in circuitry to try any voltage-doubling or other tricks. So you just have to accept the torque dropping off at higher speeds rather more than conventional ICE, where max power happens near max rpms.

So it's a trade-off for the mfrs. If they want the torque to stay up here at higher speeds, they need a motor which will generate less back EMF. One way to get this is fewer turns of thicker wire (well known to anyone who has rewound scalextric motors to get more power!). The problem with this approach is the magnetic flux = N * I where N = number of turns & current. Current may well be limited by how much your semiconductors can pass (more = more expensive Mosfets etc), and more current drains your battery faster. Thicker wire = smaller N as the space for wires is finite.

Result is you get lower torque at low speeds as N is smaller, but the no-load speed of the motor is much higher as less back EMF at high speeds, and the max-power point of the motor is at half max-speed i.e. higher up the car's speed range.
Also thermal losses rise, as that's I * I * R. The I squared effect is greater than the lower R, so the motor tends to run hotter. So many trade-offs to consider!

It's up to the mfr how to size the motor, & how to gear it to get the best balance between torque & speed over the ranges used.