EV's and multiple gears musings

[mik]

So for some reason the question of multiple gears with EVs was stuck in my head last night, so I did a little digging and found some interesting things (although you may be your own judge of that).

I watched this vid of a Tesla P100D in the ludicrously named ludicrous (plus?) mode on a dyno. Video included for reference only.


I didn't watch it all - was most interested in the output. See screenshot here


So it's a peak of 920 lbs.ft (blue line - torque is the only thing you feel on your arse-dyno remember) and 589bhp (black line).

Electric motors create their peak torque at 0rpm (so in theory if they had started the dyno run from 0mph it would have been an even higher peak torque figure - but they suffered wheelspin) and torque tails off as motor speed rises. This is clearly visible in the torque trace, and rather interesting as it ends up looking not-dissimilar to a smoothed version of the output of an IC engine with gears.

The EV dyno trace is from "slow" mph to VMax, which you don't usually see in a dyno trace. Usually you'd use 4th on a dyno (or whichever gear in the gearbox is closest to 1:1 to minimise losses), so the dyno trace will be measured from "moderate" mph to whatever the VMax is in that specific gear. If you ran the dyno over the full speed range of the car, using the gears, then you'd see something like this. Yes I did just draw this in powerpoint and then take a photo of my screen with my phone.



Hence the way a single-geared EV accelerates from slow to high speed feels very familiar to us.

And whilst the near- 1000lb.ft headline torque figure from the Tesla sounds "whaaa??" , gears multiply torque. So an IC car has engine torque significantly multiplied in the lower speed range too, hence the torque that reaches the wheels is way more than the torque the engine is developing.

So could you use multiple gears in an EV?

Obviously yes, but more gears means more weight, complexity, and mechanical losses. In theory you could fit a 2-speed box running a higher gear at higher road speeds, which would drop the motor rpm back down into a "fatter torque" range, but the increased gearing (and additional losses) would probably make it feel little different in terms of "shove".

Some interesting points/comments in this vid (which I found after I'd been musing this for a while - doh )

[Jobbo]

I know the Taycan and Audi eGT use two-speed gearboxes, so single gear EVs is not universal - just near-ubiquitous.

[mik]

@Jobbo - true - I'd be interested to drive one to see how it feels.

Not sure it would help on that Tesla with the torque dropping fairly gently - if I look at that trace (with it's stoopid axes) then it's making about 440lbs.ft at 102,000. If you doubled the gearing you'd drop right back into the fat of the torque at 55,000 and around 880lbs.ft.

But 880lbs.ft with the gearing doubled would feel exactly like 440lbs.ft without the gearing doubled.....

As per the second vid I posted, you could use a second lower gear to make the lower-speed performance more chunky?

[Mito Man]

I've said it before but the Model 3 just feels dead above 120mph, the motor is clearly optimised for 0-100mph which makes sense. You'd need a really long runway to hit its top speed.

[integrale_evo]

I’m sure the crazy power Evs have a very gentle throttle curve and modulated power supply at low rpm so they don’t destroy the tyres and become 2T pinballs as soon as someone not really into cars steps from a 2.0d into one.

But yeah, no reason they can’t have gearboxes, a lot of the classic ev conversions retain the original gearbox. Would still be better than ic in traffic as you could roll to a stop in 1st, and off mapped properly still just drive it like an auto at low speeds in stop/ start traffic.

[Mito Man]

When Formula E was first inaugurated the cars had a 5 speed gearbox (I think), just because they were all built to spec and that's what the rules said. Then they opened up the rules after a couple years so manufacturers could make their own powertrains. Every team switched to single speed apart from the few who used a Taycan style 2 speed. But then the year after everyone went to single. What limited advantage the 2 speed yielded in terms of gearing was negated by efficiency losses, complexity and weight.

I'd have thought the main advantage with a 2 speed could be for towing, where it may be noticeably more efficient. Looking at the Tesla Semi though it's still single speed. The Volvo Electric lorry still has a 12 speed gearbox inherited from the regular version - but it could just be lazy engineering like a classic car EV change rather than due to efficiency.

[RobYob]

I’m sure the crazy power Evs have a very gentle throttle curve and modulated power supply at low rpm so they don’t destroy the tyres and become 2T pinballs as soon as someone not really into cars steps from a 2.0d into one.

Not even crazy power EVs, the proto build Ipaces were absolutely savage before being calmed with calibration to make them more user friendly. An interesting bit of calibration is just how carefully and precisely the pitching resonant freqeuncy has to be tuned and damped out, otherwise you can get a spectacular feedback loop shaking the motor very nastily.

@mik The torque reduction is just simply the power limit coming in, the battery/inverter/motor system can only throw so many electrons/second around and the torque reduction at speed is just it being unable to keep up.

[integrale_evo]

I’d love to see a Zoe de-restricted and turned into a torque steering tyre shredding beast off the line

I guess that’s part of it too, not just the controlabillity but also not snapping driveshafts and still being able to use relatively soft bushings etc

[nuttinnew]