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Discussion Starter #1
Can someone explain why EVs don't tend to have gears in the traditional sense? What is going on inside that makes them drive at "maximum efficiency" across all speeds? I assume it's related the flat torque curve, but really I don't quite understand how a car can be efficient at 60 as well as 20 without shifting cogs somewhere?

Googled, still confused. :)
 

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Just the physics of AC electric motors I suppose...

Only one moving part so the accumulation of friction still increases with speed but it is so low to start with that even at max revs there still isn't much at all.

Similarly, one moving part, low inertia.

No or very little heat generated and thereby lost.

perrhaps someone has a more accurate/scientific answer?
 

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There is no need for the transmission because full torque is available from zero to 50 mph (for the Model S motor), and even from there there is only minimal loss. Trying to introduce any kind of transmission at that kind of torque and at the RPMs of the electric motor is extremely complicated as little slips quickly ruin the gears.

"Tesla tried a 2-speed transmission with the Roadster, but the immense instant torque kept grinding the gears."

http://www.teslamotors.com/it_CH/forum/forums/will-there-ever-be-manual-transmission-offered
 

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There was a "gearbox" presented at the TSB event I was at, at Millbrook last month. It wasn't a traditional multi-ratio effort, but was a single step specifically designed to kick in at higher speeds only so acts more like an overdrive. Designers claiming 15% energy saving at motorway speeds. Would not be used from rest, so different to the Tesla device. Wondering if that effort was a UK designed 'box, as I'm sure Xtrac could take care of such a problem.
 
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Discussion Starter #5
I guess an "overdrive" of sorts was what I had in mind. I know it's more than just gears (drag is a big deal) but I couldn't help thinking that some sort of "shift" in ratios above a certain speed for motorways and such couldn't hurt?
 

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There is some speculation that the front motor in the Model X will be geared to allow higher cruising speeds. To me this seems like a better solution than adding a second gearbox - more regen potential, rear biased AWD at lower speeds, safer front bias at high speed...

Even so, if an EV drivetrain is ~95% efficient up to 50mph, then how fast would you need to be going before the added weight of a gearbox/second motor started to make sense? You still need to 'spend' a certain amount of energy to counter wind resistance, regardless of gearing, so the best you can hope for is to keep the motor in the efficiency band.
 

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I've no idea why some EVs have gears and others don't, but the Formula E cars have 4 speed sequential gearboxes with motors that rev at 17,500rpm.

McLaren obviously have managed to do something that Tesla couldn't:

http://www.theengineer.co.uk/in-depth/analysis/electric-formula-inside-the-formula-e-racing-car/1017296.article
Could be because it's sequential. They're very strong but very noisy and expensive.
Ideal for a racing car where they want some noise. I bet most of it comes from the gearbox.
It's what gives touring cars and the like their distinctive sound.
Also the endurance doesn't have to be anywhere near as long.
I love engineering, it's all about the compromise :)
 

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The BMW i8 has a two-speed transmission on the electric motor.

In the production version low gear is for pure EV mode only. The car shifts into high in all other modes.

Prototypes shifted from low to high somewhere above 50mph. BMW changed this because shifting upset the balance of the car.
 

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In the production version low gear is for pure EV mode only. The car shifts into high in all other modes.
I didn't realise they'd changed it. I remember watching an early EVO video where they walked round an i8 prototype and explained about the 2 stage gearbox for faster low speed acceleration (11:1 in 1st!).
 

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I guess the ultimate answer is, because they aren't as inefficient as petrol engines.

Think about second gear. You can pull away in second in most cars, with a bit of clutch wear, and you can probably get to just over 60mph on the redline. We don't "need" anything other than second gear. Except the economy would be terrible, the clutch would wear out quickly (especially if you need to do hill starts) and so would the engine. So maybe two gears is good, a first-and-a-half gear and a fourth gear. Reliability solved, and most of the fuel economy problem is solved. Except drivability would be awful, imagine on a roundabout you had a choice of fourth or first-and-a-half....
Another issue is going backwards. Until someone invents a petrol engine which can go backwards, the petrol car will need a physical reverse gear. So the petrol car has to have a gear box anyway, might as well put in two or three cogs to solve the reliability, fuel economy and drivability problems.

For the EV, which I tend to think of as "stuck in second", no clutch so no hill start problems. Some torque is available at any speed so no drivability problems. And no physical wear/contact even at high RPM so no engine wear problems. Reverse is handled just running the motor in reverse, no physical changes are required. A traditional muiltispeed gearbox in an EV is trying to fix problems that just don't exist.

Having said all that, a twin motor setup going into a planetary gearset could help a little bit with ultimate efficiency. If motor 1 is turning but motor 2 is not, torque goes to the wheels. Once motor1 is dropping in efficiency you start to bring up the speed of motor2 (in "reverse"). The two motors are working "against" each other so the extra torque is applied to the wheels. There's no clutches to slip or cogs to change so the driver won't feel any changes and it could be programmed nicely to ramp the speed at the right time to provide completely linear acceleration until motor1 is at say 9,000rpm and motor2 is at -9,000 rpm. You get a double benefit that if you want "sport" mode instead of efficiency mode you can get motor1 going forwards and motor2 going backwards from the offset, giving double the torque off the line but falling off very noticeably as rpm increases.
 

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I guess the ultimate answer is, because they aren't as inefficient as petrol engines.
Until someone invents a petrol engine which can go backwards, the petrol car will need a physical reverse gear.
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And that they did a while ago in the little blue single seater invalid carriages. The engine was a two-stroke and to get the car to reverse you simply stopped the engine and restarted it in reverse (not sure exactly how this was managed.
They were banned form UK roads in 2003 due to concerns over safety.
 

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Yes it has been done before. I had a Messerschmitt three wheeler for a short time which also started the engine backwards when you turned the ignition switch the other way.
 

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And that they did a while ago in the little blue single seater invalid carriages. The engine was a two-stroke and to get the car to reverse you simply stopped the engine and restarted it in reverse (not sure exactly how this was managed.
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I just did a little reading about the beloved Invacar. I suddenly realised that I hadn't seen one for years!
Apparently they did have a transmission to select forward and reverse.

I learned that, as well as the two-stroke, there was a 72V electric version by the way.
 

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Yes it has been done before. I had a Messerschmitt three wheeler for a short time which also started the engine backwards when you turned the ignition switch the other way.
Oh yes, and didn't you get four reverse gears as well :) now there's a thought - anyone tried there EV for top speed in reverse ?
 

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Discussion Starter #15
It looks like Bosch are planning to introduce gears to the EV-world. Or maybe not?!

Headline here is: Electric vehicles to get gears, says Bosch

Quote from Bosch however is: “Electric motors have no difficulty in running up to 20,000 revolutions per minute (rpm) – compared to petrol or diesel engines, which usually only run up to a maximum speed of around 9,000 rpm – and so can run in a single gear, without the need for a transmission.”

So not sure if the article isn't written brilliantly or they just ran away with the idea of gears in pure EVs despite it not actually being said by Bosch? It seems more to be about PHEVs and the like, not BEVs.

http://www.electronicsweekly.com/news/design/power/electric-vehicles-get-gears-says-bosch-2014-07/
 

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It looks like Bosch are planning to introduce gears to the EV-world. Or maybe not?!

Headline here is: Electric vehicles to get gears, says Bosch

Quote from Bosch however is: “Electric motors have no difficulty in running up to 20,000 revolutions per minute (rpm) – compared to petrol or diesel engines, which usually only run up to a maximum speed of around 9,000 rpm – and so can run in a single gear, without the need for a transmission.”

So not sure if the article isn't written brilliantly or they just ran away with the idea of gears in pure EVs despite it not actually being said by Bosch? It seems more to be about PHEVs and the like, not BEVs.

http://www.electronicsweekly.com/news/design/power/electric-vehicles-get-gears-says-bosch-2014-07/
DO NOT WANT!
 

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Sure, in an hybrid since there's an ICE but apart from super high performance models (Formula e cars have gears) It's hard to imagine what the point would be
 

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Sure, in an hybrid since there's an ICE but apart from super high performance models (Formula e cars have gears) It's hard to imagine what the point would be
Even Formula E is bizarre, not sure why they have gears as 200kW is hardly high performance.
Tesla Roadster is 225kW and Lord Drayson's LMP1 electric conversion that put out more than 600bhp are both single geared and perform amazingly.
Can't see any reason to lug around 50 or more kgs of a gearbox for the slight efficiency gain at motorway speeds. In the Tesla you can see Motor temperatures and the heat build up is very slow. Normal driving takes more than half an hour to get up to 70/80 degrees on an air cooled motor... Compare that to a couple of minutes for a water cooled ICE!
 
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Even Formula E is bizarre, not sure why they have gears as 200kW is hardly high performance.
Tesla Roadster is 225kW and Lord Drayson's LMP1 electric conversion that put out more than 600bhp are both single geared and perform amazingly.
Can't see any reason to lug around 50 or more kgs of a gearbox for the slight efficiency gain at motorway speeds. In the Tesla you can see Motor temperatures and the heat build up is very slow. Normal driving takes more than half an hour to get up to 70/80 degrees on an air cooled motor... Compare that to a couple of minutes for a water cooled ICE!
Makes a cool noise, gives drivers something to do.
 

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Exactly do you think they have added the use of gears so that there is an element of difficulty in driving the cars?
 
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