Why do EV have low max speeds?

[Filibuster]

In Germany, they talk about introducing a speed limit with the main goal to offset the advantage of ICE cars over EV at speeds above 120/130 km/h regarding range....

Not having read all the post and probably it has been said already, but all cars (ICE and EV) use exponentially more energy at high speeds. I'm just back from a road trip in my PHEV (on the autoroute always in ice mode) which involved longer periods of above 100 mph speeds as opposed to my usual 80 mph trips I do normally. Fuel consumption went up by almost 50%.

[Forester1965]

Considering drag squares with speed, I'd imagine the range of EVs drops off pretty quickly when you're into 3 figures, in the same way MPG decreases in an ICE.

Edited by Forester1965 on Monday 15th April 11:26

[CheesecakeRunner]

Pistonheadsdicoverer said:

The max speed on the Nissan Ariya is 99MPH and I've noticed that mainstream EVs (except for Tesla) seem to have similarly low max speeds.
Is it because efficiency nosedives beyond a certain speed?

Whilst there are engineering reasons, the main one is that a bog standard family car doesn't need to go more than 100mph, so manufacturers don't bother dealing with those engineering reasons to make them go faster.

Outright speed is simply not a selling point in those car segments.

Where it is a selling point, such as Porsche with the Taycan, they have much higher top speeds.

[Nomme de Plum]

J4CKO said:

Pan Pan Pan said:

J4CKO said:

Some do have two speed gearboxes like the Taycan does on the rear axle, think the Lucid might as well.

Due tot he way electric motors produce their torque, you dont need a super low gear for setting off or a super high one for cruising, the power band is as wide as the rev range so you dont need to mitigate the deficiencies in power delivery with a gearbox.

That said, you can augment the characteristics to allow more flexibility by having two ratios, but little or no point having any more than that.

Had someone developed a decent battery 100 years ago, nobody would have bothered much with developing the IC engine the electric motor is perfect, elegantly simple, its just that the energy storage is still comparatively crap.

The battery is still the problem, because they are very heavy, very expensive, and in many EVs, still only give poor range, (unless a person buys a `very' expensive EV)
The battery is also just another type of fuel tank, and there is still the cost of the fuel which must be put into it to take into account.
If car buyers were told that the new car they were buying, had a very expensive fuel tank ,which does not even give the range of an equivalent ICEV. and which weighs the same (Heavy) regardless of whether it is completely full, or completely empty, and (like with ICEVs) a range goes down rapidly the more power is used for heavy acceleration, and higher speeds, at the start of a journey, they would be less inclined to buy an EV.
When the manufactures can produce an EV, that costs the same as its ICEV equivalent, has the same range as its ICEV equivalent, and weighs the same as its ICEV equivalent, and has re-sale prospects that are the same as its ICEV equivalent, they will switch to them in droves. The problem is, that to date NO such EV exists.

Indeed, but like anything the rough edges get smoothed off over time, ICE is pretty much at the top of its evolution and there is an overlap in capabilities, I believe that at some point in the next few years it will switch.

As EV range goes up, charging time and cost come down the bar will lower, they will work for more people. They arent for everyone right now, and ICE has downsides, it's just we have lived with those for decades.

This is slightly off topic isn't it.

My EV weighs 1350kg so not dissimilar to and ICE. It's range is however circa 150mile, may even be a bit less but as 99.9% of my journeys are local like most other people why do I need a greater range. I live on the South coast so can get across to Brighton or Gatwick or maybe Southampton and back easily without recharging. On the extremely rare occasion i go further If its central London it will always be on the train and if further afield a stop after 2.5 hours or so is not that inconvenient, in fact at that point I probably need a convenience.

I can recall my Triumph Herald back in 1971. The range was very similar and petrol stations were not plentiful and not 24/7. Yet we never gave a second thought to doing long drive.

I would agree initial cost is currently a major drawback along with a severe lack of used availability.

In terms of top speed I'd much prefer an EV that accelerates rapidly to overtake at normal legal speeds than sacrifice that for a fast top speed.

[ChocolateFrog]

Forester1965 said:

Considering drag squares with speed, I'd imagine the range of EVs drops off pretty quickly when you're into 3 figures, in the same way MPG decreases in an ICE.

Edited by Forester1965 on Monday 15th April 11:26

But worse due to ICE inherent inefficiencies.

If it's already bad at converting energy to motion you don't notice it as much when it gets even worse.

By comparison it feels like EVs fall off a cliff. I doubt I could maintain 110 for more than about 10 minutes in mine.

[ChocolateFrog]

Even with a gearbox on the EV it wouldn't make much difference to how many kw's you're chewing through.

[Maxym]

For the same work, would a high geared EV use less energy than a lower geared one?

[Nomme de Plum]

Maxym said:

For the same work, would a high geared EV use less energy than a lower geared one?

I think in an electric motor the greater the revs the greater the efficiency of the motor.

PS

Just found this very interesting article. Basically It's complicated.


https://chargedevs.com/features/how-to-improve-ev-...

Edited by Nomme de Plum on Monday 15th April 12:04

[Forester1965]

I had wondered if hp increases with rpm in electric motors in a similar way to ICEs (outside of efficiency gains as mentioned above). Or do they generally increase in RPM primarily to match road/gearbox speed rather than to gain power?

[Lester H]

I’m not on here to denigrate EVs though they are still not fully developed, and others have answered the O.P. However I do wonder , provided that a car isn’t sluggish at ‘ real world’ speeds, whether the ultimate top speed really matters.

[Nomme de Plum]

Lester H said:

I’m not on here to denigrate EVs though they are still not fully developed, and others have answered the O.P. However I do wonder , provided that a car isn’t sluggish at ‘ real world’ speeds, whether the ultimate top speed really matters.

If you look at the four top most popular new cars you will see they are modest family cars with top speeds from 100 to 120mph or so. The people buying these won't give a fig about top speed. It is an irrelevance to the vast majority of motorists who in the main will not be car enthusiasts. I don't suppose they are that bothered about acceleration either as most will rarely overtake unless on multi carriageway roads.

[DaveCWK]

ChocolateFrog said:

Forester1965 said:

Considering drag squares with speed, I'd imagine the range of EVs drops off pretty quickly when you're into 3 figures, in the same way MPG decreases in an ICE.

Edited by Forester1965 on Monday 15th April 11:26

But worse due to ICE inherent inefficiencies.

If it's already bad at converting energy to motion you don't notice it as much when it gets even worse.

By comparison it feels like EVs fall off a cliff. I doubt I could maintain 110 for more than about 10 minutes in mine.

Yup. It's also why the most cost efficient way to cruise at 150mph is probably an old LPG-converted BMW.

[GT9]

Pan Pan Pan said:

The battery is still the problem, because they are very heavy, very expensive, and in many EVs, still only give poor range, (unless a person buys a `very' expensive EV)
The battery is also just another type of fuel tank, and there is still the cost of the fuel which must be put into it to take into account.
If car buyers were told that the new car they were buying, had a very expensive fuel tank ,which does not even give the range of an equivalent ICEV. and which weighs the same (Heavy) regardless of whether it is completely full, or completely empty, and (like with ICEVs) a range goes down rapidly the more power is used for heavy acceleration, and higher speeds, at the start of a journey, they would be less inclined to buy an EV.
When the manufactures can produce an EV, that costs the same as its ICEV equivalent, has the same range as its ICEV equivalent, and weighs the same as its ICEV equivalent, and has re-sale prospects that are the same as its ICEV equivalent, they will switch to them in droves. The problem is, that to date NO such EV exists.

Here we go again, the concept of efficiency is totally alien to you isn't it.
Your 600 mile ICE is holding 600 kWh of fuel.
A 200 mile EV holds 60 kWh in the battery.
It is vastly more frugal than the ICE.
One car can travel three 3 times further from the same amount of energy than the other.
And because only 1/3 of today's electricity in the UK comes from fossil fuel, the amount of fossil fuel you consume over the same journey is about one fifth of what the ICE requires.
That's today, in 20-25 years it would be zero fossil fuel.
But yeah, it's totally only about weight.

[GT9]

Maxym said:

For the same work, would a high geared EV use less energy than a lower geared one?

General speaking, an electric traction motor likes to rev to get best efficiency and it also delivers it at partial torque.
Conversely, ICEs deliver best efficiency at close to maximum torque but lower rpm, hence the need for multiple gears to try to keep it near that regime.
The reasons behind limiting the top speed of an EV:
1. Optimising the normal use speeds to the motor's efficiency map without the need for a second gear (except Taycan, etc).
2. Limiting the need for lift-reducing aero features that then increase drag at normal use speed (I'm siue you are aware drag performance is crucial to an EV's overall energy efficiency).
3. Tyres can also be optimised for low rolling resistance at normal use speeds without the need to survive higher wheel rpms.
4. Voltage matching between the battery and the motor's back-emf.
5. Above a certain speed, the range drops substantially due to the aero drag square law kicking in, and at some point it's way too low to justify engineering the car for above that speed.

The motor system usually operate in two regimes:
1, Fixed torque, linearly increasing power
2. Fixed power, linearly decreasing torque

The first regime is to maximise the acceleration available in the 0-60 mph speed range, and the second is to allow the top speed to be reached, but with lower acceleration. There are usually some electro-trickery things going on in the second regime to weaken the back emf of the motor so it doesn't exceed the battery voltage. This weaker field also means less torque is available.

This motor /drive efficiency map explains the desire to keep the rpm relatively high, you can see regime 2 starts at about 4500 rpm, after which the maximum torque available decays.

Edited by GT9 on Monday 15th April 17:42

[SWoll]

It's for the same reason that in the majority of EV's performance drops off after 70-100MPH. They're optimized for the best performance and efficiency in the real world situations 99.9% of drivers will find themselves in.

Makes a lot more sense that Porsche massively over-engineering the Taycan for the sake of a tiny number of autobahn driving owners in Germany.

[JustGetATesla]

delta0 said:

Probably tyres

I’ll bite. Why tyres?

[delta0]

JustGetATesla said:

I’ll bite. Why tyres?

What the manufacturer chose to fit them with. Likely to save cost and efficiency.

I expect it’s more likely the manufacturer don’t want people to abuse the motors. I’ve seen tyres limit top speeds before in other vehicles but it may not be what’s going on with some EVs. It’s clear not all have this limitation.

Edited by delta0 on Monday 15th April 22:39

[Bobley]

Rolling resistance increases with speed but as a proportion its pretty small once you're at high speed. Aero drag power is proportional to speed cubed so no matter how good your CdA is, once you're above 80mph its getting considerable.

As folks have said, motor speed can be an issue. Typically motor efficiency is really good at high power but when you extend the speed range you need to use field weakening to keep the motor efficient, you fight the motors inherent back EMF all the way but if the back EMF (rising with speed) gets crazy then you have really safety issues, i.e. in a fault the motor could drive a massive voltage into the inverter and eventually the battery... anyway, at sustained high speed and high power you get losses in the rotor start to become significant and that cant be cooled without investing in a lot of tech. You can reduce these losses by adding cobalt and other exotic bits but none of this is of interest to the likes of VW (apart from the ID-R!).

Finally the battery. The immense currents and the cells/ busbar/tab resistance lead to lots of heat (heating power = current² x resistance, plus other stuff) so at high load the battery can waste a huge amount of energy just heating itself up ( maybe 20% converted to heat?). When you consider that a Tesla, Ioniq 5 or Taycan will heat up several hundred kilos of cells and ancillaries from 25 to 55°C in a 20 minute fast charge (at maybe 150kW electric input power) then you consider that on the road you dont normally hold 150kW (200HP) output power very long...

Finally, where do you loose all this heat from the motor and battery? The motor radiator doesn't get hot like an ICE (70°C instead of 110°C) so thats always tricky but then the AC system has to deal with the battery which doesn't want to be any hotter than the driver really. A big AC system can probably deal with 10-12kW of heat from the battery... but it needs a lot of investment.

Solid state batteries will be a game changer ... when they arrive...

[SWoll]

delta0 said:

JustGetATesla said:

I’ll bite. Why tyres?

What the manufacturer chose to fit them with. Likely to save cost and efficiency.

I expect it’s more likely the manufacturer don’t want people to abuse the motors. I’ve seen tyres limit top speeds before in other vehicles but it may not be what’s going on with some EVs. It’s clear not all have this limitation.

Edited by delta0 on Monday 15th April 22:39

It's all to do with power and gearing as has been mentioned multiple times. Lower powered EV's don't have the shove or gearing to propel them to high speeds if they want to maintain more useful low speed performance, and the manufacturers realize this doesn't matter for 99.99% of owners as long as they are capable of sensible motorway speeds when needed. Of the millions of ICE cars capable of doing 120mph+ only a tiny % will ever have seen the other side of 100mph.

They then fit tyres fit for the job required.

[Downward]

Pan Pan Pan said:

J4CKO said:

Some do have two speed gearboxes like the Taycan does on the rear axle, think the Lucid might as well.

Due tot he way electric motors produce their torque, you dont need a super low gear for setting off or a super high one for cruising, the power band is as wide as the rev range so you dont need to mitigate the deficiencies in power delivery with a gearbox.

That said, you can augment the characteristics to allow more flexibility by having two ratios, but little or no point having any more than that.

Had someone developed a decent battery 100 years ago, nobody would have bothered much with developing the IC engine the electric motor is perfect, elegantly simple, its just that the energy storage is still comparatively crap.

The battery is still the problem, because they are very heavy, very expensive, and in many EVs, still only give poor range, (unless a person buys a `very' expensive EV)
The battery is also just another type of fuel tank, and there is still the cost of the fuel which must be put into it to take into account.
If car buyers were told that the new car they were buying, had a very expensive fuel tank ,which does not even give the range of an equivalent ICEV. and which weighs the same (Heavy) regardless of whether it is completely full, or completely empty, and (like with ICEVs) a range goes down rapidly the more power is used for heavy acceleration, and higher speeds, at the start of a journey, they would be less inclined to buy an EV.
When the manufactures can produce an EV, that costs the same as its ICEV equivalent, has the same range as its ICEV equivalent, and weighs the same as its ICEV equivalent, and has re-sale prospects that are the same as its ICEV equivalent, they will switch to them in droves. The problem is, that to date NO such EV exists.

Nurse !