No idea if reducing the motor RPM would reduce consumption.
But I do know top speed is dictated by motor RPM.
So top speed could certainly be improved by adding gears, if needed.

 

You have a "wild hypothesis": what you would need are rotation speed versus load and efficiency curves for the motor. You also need Cd and frontal area plus rolling resistance for your Leaf so that you could investigate if there was ANY theoretical advantage at different speeds. Even if there was, the implications of mechanical complication might well outweigh any benefits from reduction of power.

Probably, there are better methods to reduce energy consumption at speed (aerodynamic tweaks, drive slower, improved motor control software etc)

 

Probably, there are better methods to reduce energy consumption at speed (aerodynamic tweaks, drive slower, improved motor control software etc)

Er....

"How do I improve the efficiency of my four bed house?"
"Move to a three bed"

 

Electric motors do have a very flat efficiency line against revs that will reduce after a point but ICE's are far worse.

Tesla tried a two speed gearbox in the early days but dropped the idea for production. As no other companies have a gearbox to my knowledge then I guess it simply isn't cost effective.

The GM Volt/Ampera does limit the maximum motor revs on its Voltec drive module by using planetary gear sets with two motors (one is the generator when in hybrid mode) operating in sync with each other depending on the road speed to keep the motor revs at optimal.

 

In a theoretically ideal electric motor, adding an additional gear could increase top speed (by staying below the motor's RPM redline) but would not help with efficiency/range or acceleration. In fact if you gear it too high power output will fall.

In a theoretical motor you have a maximum torque and a maximum power limit. From zero RPM up to some specific RPM you have maximum torque available, and power climbs linearly with speed since power is torque x rpm.

From that RPM onwards you have maximum power up to the top speed of the car and torque falls linearly, again because power is torque x rpm. This is what an ideal motor would look like:

So if you're near the redline of the motor and you switch to a higher gear, as long as that higher gear keeps you in the flat peak power region you won't actually gain anything - the motor will spin slower but put out more torque - same total power, so same torque at the wheels for the same road speed after passing through the different gear ratio.

If you make the gearing too high you'll end up down the power curve (below 3500 here) and actually lose power output.

So apart from increasing top speed if it was previously limited by the motor RPM, adding a higher gear doesn't do you any good with this theoretical motor, you certainly don't gain any efficiency.

Adding a lower gear however can provide increased acceleration at lower speeds, which is why a very few very high performance EV's have two gears - like the Rimac concept one. They're not really gears as such since they vastly overlap each other - think of them more as "high range" and "low range" as you would get in some 4x4's or off road vehicles. You can do a wide overlapping range of speeds in the two different gears.

Real motors are not perfect though, many types of motors suffer from a modest fall off in horsepower and efficiency near their upper RPM limit due to issues such as "field weakening". This causes that flat red power line to start to droop above say 5000 in the graph above, and is part of what causes a real EV to start to run out of puff near it's top speed.

In theory an additional gear could bring that road speed back down the power curve to say 4000 where it is still at maximum and give slightly more power and efficiency. In practice, the additional power losses added by the more complex gearbox can exceed the small gain that the change in ratio would give. And those additional losses caused by the more complex gearbox are dragging performance down at all speeds.

Not to mention you now have a gearbox that will wear out, as changing gears can never be wear free, whether it's meshing gears in a manual gearbox or friction clutches in some sort of auto box. So 9 times out of 10 a higher ratio for high speeds just isn't worth the hassle, and will reduce performance at slower speeds through increased losses in the gearbox.

There is also another approach - which is to design motors that come a lot closer to the theoretically ideal motor that maintains maximum power right to maximum RPM, and a lot of research is being done on that right now, and there are some examples of this already such as the motor in the BMW i3, and possibly the one in the Tesla Model 3, both of which aim to improve the performance of the motor near it's maximum RPM to minimise any droop in power output or loss of efficiency.

Electric motors are already pretty darn amazing, but there is still more that can be done to increase maximum operating RPM and maximise power output and efficiency at high RPM without compromising low RPM performance and efficiency, effectively widening the already very wide useful power band even more.

Once you have that there is really no need for multiple gear ratios at all, even in a very high performance car. As far as we know, the Roadster 2.0 is going to be a single gear ratio for example.

Rimac (both the old concept one, and the new not yet released model) is the only high performance road legal production EV I can think of which has two gear ratios.

 

Not to mention you now have a gearbox that will wear out.

Oh come on....that's a bit of a stretch....

 

The original poster wanted more RANGE: so the obvious solution is to drive SLOWER DUH!

 

In a thread about improving efficiency at speed, saying "slow down" is about as DUUUH as it gets....

 

I guessed as much otherwise the manufacturers would of already taken the idea more seriously, thanks for the responses, bigger battery it is then!

 

I read that the Tesla dual-motor setup has (in some variants at least) different ratios for each motor, but I'm not clear if that was to achieve the effect of having two possible 'gears'...

 

My theory goes that the motor wouldn't have to rotate as fast which could reduce consumption but perhaps electric motors don't work like that?

The BMW i8 has a two-speed gearbox on the electric machine. The car uses low gear when it is in EV only mode and high gear when the petrol engine is running. Originally it was going to shift at speed but BMW couldn't stop the gear shift from unsettling the car in a high speed turn.

On the i8 the gear box has more to do with performance than consumption. A single gear tall enough to reach 155mph did not work well for low speed EV only driving.

AFAIK this the only production car with a gearbox for the electric drivetrain. Of course other PHEVs have gear boxes but they don't have a separate one for the electric motor.

There is a cost consideration too. A new i8 sells for over £100,000

 

AFAIK this the only production car with a gearbox for the electric drivetrain.

Rimac Concept one has two gears. Although whether it counts as a "production car" when only 88 (?) were made I'm not sure.

Were more than 88 BMW i8's ever sold ?

 

Science!

It's quite hard for us to understand why EVs don't have gears, when for years we've been riding bicycles with several gears and then driving petrol/diesel cars with gears. Add to that our basic understanding of gears and pulleys and their relationship with rotary motion from science lessons and it gets harder to work out.

Then there are steam trains that have/had no gears, but pistons directly attached to cranks on the driving wheels. So maybe we don't need gears after all

I can only conclude that EV makers are employing steam train technology. Well, one in particular springs to mind

 

Science!

It's quite hard for us to understand why EVs don't have gears, when for years we've been riding bicycles with several gears and then driving petrol/diesel cars with gears. Add to that our basic understanding of gears and pulleys and their relationship with rotary motion from science lessons and it gets harder to work out.

Then there are steam trains that have/had no gears, but pistons directly attached to cranks on the driving wheels. So maybe we don't need gears after all

Internal combustion engines and human legs both have a critical weakness - very narrow power band.

In the case of legs there is only so much force you can push your foot with regardless of whether you push it on a slowly moving pedal or a quickly moving pedal. Once translated into rotational energy that sets your peak torque.

Muscles also have a maximum frequency at which they can expand and contract at - and it's a relatively slow speed.

No matter how hard you try you just can't pedal faster than a certain rpm on the pedals. So to get useful power over a wide range of speeds you need a lot of gears on a bike - more than an ICE car.

Cyclists will have noticed that while ICE gearboxes are always a step down ratio (including the final drive ratio) bicycles gear the pedals up on all but the lowest gear - because our legs can produce a lot of force (torque) but can't move up and down very fast.

Compared to that the power band of an electric motor (and apparently a steam engine!) is exceptionally wide. The maximum RPM an electric motor can do is primarily limited by how much centrifugal force the rotor can take before it either expands enough to touch the stator or literally flies apart!

BEV motor red lines vary all the way from about 8000 rpm in something like my Ion to (I believe) 18,000 rpm in a Tesla, with something like a Leaf being around 10000-12000 rpm.

Gearbox step down ratio is somewhere between about 7 to 1 and 9 to 1, which is roughly comparable to 3rd gear in an ICE. Now imagine driving from a standstill in 3rd gear in an ICE and compare that to how quickly a BEV with the same gear ratio pulls away.

 

As I recall reading elsewhere (this could be BS so I stand to be corrected) the leaf simply doesnt need a higher gear.
Its top speed is already artificially limited before maximum revs are reached.
Wind resistance goes up with the square of speed so drain on the battery goes up signficantly at higher speeds.
As I understand it the limit is in place to stop too much current from being drawn which could otherwise cause overheating problems in various places

 

Most EV's will have "artificially" limited top speeds, where the top speed is set by the ECU programming rather than the car running out of power to push itself through the air any faster, which was the case with older ICE cars before electronic 155mph limiters started being used...

It's certainly the case even on my 66hp Ion - top speed is 80mph, and above that the power gauge will progressively throttle back to keep you going much past 80. However it will just as happily accelerate up to 80mph up a relatively steep hill as it will on the flat! So clearly it has enough power to go faster than the programmed in speed limit on the level. Purely from a power vs wind resistance perspective it should theoretically be able to reach about 100mph on the level.

However electric motors do have a maximum rpm limit - as I mentioned, if you keep spinning something faster and faster a point will be reached where it can't hold itself together anymore and it will literally disintegrate. So there has to be an rpm limit in the motor and one with a hefty safety margin.

Power dissipation is probably a factor but may not be as much of a factor, and is more likely to be a factor for the battery than the motor. Not going faster than the chassis/suspension/tyres of the car can reasonably handle is also a factor in choosing the top speed of a car! Just because it has the power to theoretically go faster doesn't mean it would be safe or wise to do so...

 

Purely from a power vs wind resistance perspective it should theoretically be able to reach about 100mph on the level.

100mph on 66bhp is a tad optimistic....

 

Hence why I went Golf GTE. 6 speed gearbox with electric only or hybrid.

In last 1000 miles I've only used ~ £15 of petrol too with 95% of my journeys being short im nearly always in electic range only mode(~22mile range). Most of the petrol use was also down to short bursts of GTE mode with performace that only expensive/high end electric only can beat.

 

Sorry, but I don't see much/any justification in you getting a hydrid at all.
You admit most of your journeys are done in electric mode, so why not get a full EV?
Your justification for getting a hybrid would only make sense if you were doing longer journeys where relaxed motorway cruising is an advantage (and which is the whole point of the thread, that EVs aren't great at motorways).

 

Did one round trip 180 mile journey recently involving a mix of motorway and A roads with some stop-start traffic. Driving in Hydrid (sat-nav planned) mode the journey worked out a 105mpg with the car choosing when to use battery or ICE. Would likely have had a 50 minute charge if I'd gone eGolf yet for most of my journeys I'm benefitting driving as pure electric.

If I need to, in GTE mode when both ICE and motor are used its even quicker off the mark than a GTI upto ~40mph. Only The DSG Golf R I had was quicker but that was AWD 0-60 in 4.6s

I will likey go full electric next car when the VW ID is out if it has a real use range of +250 mile and hopefully better charging infrastructure in 2-3 years.

PS your thread is about extra gear for Motorway so GTE having a gearbox is a solution.

 

I remember us discussing this at length years ago for the ZOE 22kWh. We came to the conclusion that there is a real reduction in its motor efficiency above about 60mph.

Maximising Range in a ZOE 2 - My Renault ZOE electric car

This would imply the ZOE would benefit from another gear above this speed.

 

When I got my Golf GTE I was surprised to find that the electric motor drives through the DSG gearbox just the same as the engine does.

If your fixed gear in a Leaf, for example, is equivalent to third gear in an ICE, and the GTE starts in first gear, then it should produce much more torque and acceleration from a standstill. I think it does that.

Equally sixth gear in the GTE should produce relatively low RPM in the motor at high speeds. Perhaps the electic motor has been designed to run more slowly and impersonate the ICE. I think they are on the same shaft and running at the same speed.

I suppose if you have to have the geared drivetrain for the ICE, it would actually be more complex to incorporate a hybrid electric motor without gears! (unless you do the Volvo thing and have a separate axle).

 

If your fixed gear in a Leaf, for example, is equivalent to third gear in an ICE, and the GTE starts in first gear, then it should produce much more torque and acceleration from a standstill. I think it does that.

Gearing clearly can't be relatable between EV and ICE.
There's no comparible ICE gear can take a car from rest to 90+mph and still give decent acceleration.
The Leaf does that, it's very quick to 30mph and still does 90+

 

I've had this idea recently that if my leaf had a gear for motorways etc. I'd get more range. I haven't done any calculations at all so I've got no idea if it would make any difference.

My theory goes that the motor wouldn't have to rotate as fast which could reduce consumption but perhaps electric motors don't work like that?

If I might offer a more definitive answer, reflecting on previous history.

It 'was' well known that an electric motor was ideally suited to be matched to a two or three speed gearbox. If you read any of the Bosch engineer's guides prior to 2000 you will see it discusses these things and guides the designer to such gearboxes.

It was not for clear insight and innovation that Tesla originally chose their two speed box, it was because that was the 'known wisdom' of the day.

So the question is not as folks here think it actually is. If you have a long enough memory and knowledge, the real question is why we have ended up with EVs with only one gear?

First I should explain why two gears is good, and 3 gears possibly better; the OP is correct that more gears would be more efficient, but perhaps not for the reasons you might suspect, efficiency at high speed. It is not the high speed efficiencies that are most benefited, it is the lower speeds.

Two gears means you can pick the ideal ratio for drive-off traction, but also choose the optimum ratio for maximum speed. Two gears means you do not need to pick a compromise ratio in the middle somewhere, which is neither optimum for traction nor for higher speeds, but one which has a wider efficiency peak in combination with two gears.

There are multiple problems with high ratios at zero speeds, mainly related to motor control and cogging. These weren't simply troublesome in days gone by, they were showstoppers.

However, with the advent of digital PWM control, massive Moore's-law type improvements in power semiconductors, and finally building EVs this decade with significant real battery capacity, once people started making cars with this new tech they realised that actually a multi-speed box added such small advantages that it was not worth the cost and reliability complexities.

If you were building an EV in the 1980s, a 3 speed transfer box would look like a good idea, not only benefiting from improved take-off and highspeed ratios, but something in the middle to cover the cross-over points where the benefits are fairly balanced, i.e. pottering around the higher speed suburbs.

So we have actually evolved backwards because the technology got ahead of what we thought we knew. The low speed performance was entirely adequate and more than the tyres could actually grip, while exceeding the speed limit at the top end.

The other thing is that we could not practically build EVs to do 80mph, but now we do, it is clear that getting a few extra % at 30mph when you can already get 6mi/kWh is immaterial when most of your energy drain happens at higher speeds. A 2 speed box will improve your NEDC figures by 10 to 15% but in reality will achieve precious little.

So you might now be thinking to ask me if 2 speed boxes have any future? Well, I think so, personally. This is because we can make even lighter, smaller and more efficient motors than we do already, good for the environment both for energy and material use. But that's only really possible by some performance-envelope compromises and that will mean looking at 2 speed boxes again to keep the whole system efficiency and performance to the required level. For now, the industry as a whole is still coming to grips with the current technology, which is sufficient for now and there is no real motivation to add complexities to EV drive-trains whose long term reliability is still somewhat of a speculation.

 

So you might now be thinking to ask me if 2 speed boxes have any future? Well, I think so, personally. This is because we can make even lighter, smaller and more efficient motors than we do already, good for the environment both for energy and material use. But that's only really possible by some performance-envelope compromises and that will mean looking at 2 speed boxes again to keep the whole system efficiency and performance to the required level. For now, the industry as a whole is still coming to grips with the current technology, which is sufficient for now and there is no real motivation to add complexities to EV drive-trains whose long term reliability is still somewhat of a speculation.

I can see the argument for 2 speed boxes vs 1 motor with a 1 speed box. However I wonder if Tesla's trick of having 2 motors each with a 1 speed box with different ratios actually gives you most of the same benefit?

 

I imagined that we might still end up with a hybrid of sorts anyway, possibly due to consumer/enthusiast demand. There may be a place in the future for fully geared electric cars with de-rated motors, perhaps to replicate the experience of driving an old fashioned car without the dirty oily bit. It wouldn't be at all efficient but may be desirable for some people. I do miss gears some times although my original question was really about efficiency than top speed.

 

Thanks @donald. That provokes another thought. My wife's Jazz uses the CVT box brought into use by DAF some years ago. Whilst it is unsettling as it simulates a slipping clutch it does provide a seamless auto progress. Whilst I appreciate that it would introduce some loss in transmission friction etc would it work using an electric motor to drive such a box to enable the motor to run at peak efficiency at any one cruise velocity?

 

No, it would be quite pointless. I will draw up a diagram to explain why and I am confident it'll all make sense then.

However, the world of engineering is ahead of this thinking already, in a way, stranger than you can imagine, so to speak. There is now a class of rotary machines called electromagnetic gearboxes. By processes best explained if you go find a youtube clip of how it actually works, an electromagnetic gearbox just just what it sounds like and instead of teeth there are magnetic poles working in such a way that they synchronise at different speeds.

That might not be news or even surprising that that is possible, but there is a current move towards embedding such gearboxes within the topology of motors. This is not merely a motor with an EM gearbox, but it produces a new class of hybrid motor that has a different efficiency/torque output to conventional motors. These things are in very early development stages at this point so there is no detail I can add but only to mention there are new and fascinating motor topologies due to arrive at this party. Fun times!

 

Data from the web:

GTE 0-30 e-Mode 2.8 sec, GTEmode 2.5 sec
Leaf 0-30 in 3.1 sec
eGolf 0-30 in 3.44 sec
Golf R 0-30 in 1.7 sec
Golf GTI 0-30 in 3.2 sec
Gilf GTD 0-30 in 3.8 sec

So only GolfR faster than the GTE 0-30 with above cars.

Leaf 0-60 in 8.3 sec
Golf GTE 0-60 in 7.6 sec
eGolf 0-60 in 9.6 sec

 

Back to basics, an ICE needs gears because it is a reciprocating engine, pistons go up and down, these restrict the ability of the engine to increase the revs as needed for driving.

An EV is similar to a jet turbine, the power components can just spin at high revolutions.

A restricting factor is the ability of the motor controller to switch fast enough, but not aware that those speeds occur.

So, an EV has removed the restrictive reciprocating parts which required the addition of a gearbox.

 

Back to basics, an ICE needs gears because it is a reciprocating engine, pistons go up and down, these restrict the ability of the engine to increase the revs as needed for driving.

Not really. The issue is that an engine cannot produce torque at zero speed. Nor could a turboprop. Both would need a gearbox to be able to set the required slip-torque (through some form of slipping device) when the vehicle's demand is to pull away at zero speed. Slip torque needs to be as slow a relative slip speed as possible to maximise the lifetime of the slip device, but at the same time you need to go a lot faster than that later on in the drive cycle.

 

Ok, Ioniq EV does 103mph, with 16" wheels and a reduction of 7.412 so electric motor goes to about 7400rpm. 1.6 hybrid delivers maximum of 147Nm at 4,000rpm, while EV does 295Nm which is delivered over wide rpm range.

So power is better and consistent due to more torque at lower revs?

 

Yamaha R6, 14,500rpm, peak torque at 11,500rpm.

... if you are trying to say that the ratio between lowest operating speed to highest is bigger for motors, then yes it is infinite (zero speed torque). But the ability to rev to high rotational speeds is little to do with reciprocating pistons in regards automotive applications.

 

I thought that speeds at that level were typical of turbines in practice.

However for a traction application you don't need huge RPMs. Except for steam engines, all power sources are geared down to the final drive speed. That depends on the diameter of the wheels for a particular vehicle speed but you must be talking the low thousands of rpm. I haven't done the sums.

Ideally you want an even delivery of torque throughout the speed range. An electric motor comes close to the ideal without gears but any ICE needs a good few to achieve the same.

A second gear on an electric motor should reduce bearing wear and reliability at high speeds.

 

Check these slides out, see if this makes sense of it for you all;-

© 2018 (copyright donald, c/o speakev)

 

Just to emphasise why too few gears can't work for an ICE;-
View attachment 102550

 

Lovely graphs!

I think this is pretty much what we're all saying.