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Driving an EV in slow moving traffic?

Posted: Sat, 30 Dec 2017, 19:31
by HuffnPuff
I don't actually have an EV yet as I'm still in the early planning phase for a vintage vehicle conversion and my mind wanders while waiting for traffic to move ahead of me. When in heavy traffic how do you manage the throttle in an EV. In a manual ICE vehicle you can creep along in 1st or sometimes 2nd and if your vehicle has suitable torque just keep moving/stopping using the clutch. In an auto ICE simply releasing the brake pedal is enough to keep moving in traffic. My morning commute can have a couple of sections where this creep motion is kinda handy.

My understanding is that no throttle = no motion in an EV and I'd guess that feathering the pedal to idle along could be annoying/difficult. How do you actual EV owners manage it?

Re: Driving an EV in slow moving traffic?

Posted: Sat, 30 Dec 2017, 20:18
by Johny
Mine does not have creep mode and I find it no concern at all. With Regen braking it's pretty much single pedal with just moving to the brake pedal to hold still. On hills I two foot it. I don't miss creep mode and find driving normal autos a bit annoying now.

Re: Driving an EV in slow moving traffic?

Posted: Sat, 30 Dec 2017, 22:42
by mikedufty
i-mievs creep with no throttle, but I don't think it is that important. They are very nice to drive in traffic compared to ICE, range is very good in slow traffic too.

Re: Driving an EV in slow moving traffic?

Posted: Sun, 31 Dec 2017, 00:24
by antiscab
My previous dc car would roll for ages. You take your foot off the pedal and you just stop accelerating. It takes ages to actually slow down without using the brakes, especially at low speed.

My current ac car does Regen a 0 throttle (which I absolutely hate BTW). So you have to hold a specific throttle position to coast without power going in either direction.

In both cases, it's far better than driving a manual ice car.

Re: Driving an EV in slow moving traffic?

Posted: Sun, 31 Dec 2017, 09:52
by weber
Hi HuffnPuff. Driving an EV at a crawl is not a problem at all, with or without creep. I'm not even sure I understand why you think it might be difficult.

Regarding creep, I have it set for just enough positive torque when stationary with zero pedal, to take the backlash out of the drivetrain so it doesn't go clunk when I take off. But it isn't enough to overcome the static rolling resistance on a level road. So it doesn't actually creep.

Pedal mapping is tricky because you want to control both torque and rpm with one pedal. The trick is to map pedal position directly to requested speed, but map it only indirectly to the torque that the drive is allowed to use to try to achieve that speed. The torque should be proportional to the difference between the requested speed and the actual speed, with a carefully chosen (e.g. by trial and error) constant of proportionality.

Also, you want a progressive mapping from pedal motion to both torque and rpm. This is what makes it easy to crawl in slow traffic. In other words, near zero pedal, a small movement has a much smaller effect than the same degree of movement near maximum pedal. In mine, when stationary, half pedal gives a quarter of the torque.

Antiscab, it sounds like the pedal mapping in your AC car is not well designed. Mine does regen at zero pedal, with the strength of the regen depending on how close the actual speed is to zero. i.e. the regen torque reduces gradually to zero (or rather the tiny anti-backlash "creep" value) as the speed goes to zero. And yes, strictly speaking, to coast with absolutely no power going in either direction I would have to hold a specific pedal position, but because the torque only increases gradually with deviation from the ideal pedal position, there is a wide enough region where the power either way is negligible.

It's all explained here: viewtopic.php?p=30613#p30613 and on the following page.
I note that by trial and error Coulomb and I gradually increased the torque proportionality constant ("k" in the formulae) to 0.6 and that's where it has stayed for many years now. It is an absolute joy to be able to drive the MX-5 on winding mountain roads using only the accelerator pedal.

This scheme also provides a kind of traction control in regen, because if the wheels lock up due to excessive regen torque, due to me having yanked my foot off the accelerator at max rpm (e.g. at 100 km/h in 2nd gear), the difference between actual and requested rpm goes to zero (because both are zero), and so the regen torque goes to zero and frees it up again. The calculation is done at 25 Hz, so there is a 40 ms delay and you typically get a single loud (but brief) chirp from the tyres.

Re: Driving an EV in slow moving traffic?

Posted: Mon, 01 Jan 2018, 05:02
by HuffnPuff
Thanks all. I guess, I was just curious as to the differences between ICE and EV. Shows I have a lot to learn!

Re: Driving an EV in slow moving traffic?

Posted: Mon, 01 Jan 2018, 08:30
by Johny
Didn't (don't) we all. It was a good question and I congratulate you on remembering to ask. My driving musings get forgotten 30 seconds out of the car.

Re: Driving an EV in slow moving traffic?

Posted: Mon, 01 Jan 2018, 16:46
by 4Springs
I go between our DC conversion (Brumby) and our Outlander.

The Brumby has no creep and no regenerative braking. It has a 'clunk' as described by weber, where if you start out from a standstill the motor gets up a bit of movement before all the play in the drivetrain is taken up. I can stop this clunk by touching the accelerator pedal while the handbrake is on, or more recently I've been experimenting with using my left foot on the brake. This takes up the slack before the car actually moves, so when the brake is released the car moves forward smoothly. Because of this clunk the Brumby is slightly irritating in slow moving stop/start traffic. But not as irritating as an ICE.

The Outlander has creep and works just like an ICE automatic. The creep means that in slow stop/start traffic you drive with your foot on the brake, releasing it slightly to move slowly. It also means that the Outlander uses electricity when stopped - one of the differences often cited between ICE and EV. I've been sitting motionless in traffic with my foot on the brake and had the petrol engine start up!
Creep is annoying when coming to a stop. Our garage is at the top of a slight slope, so when parking we come in with the foot on the accelerator (just slightly). Then we get to the flat garage floor and have to quickly switch from accelerator to brake. This always seems to result in a jerky stop for me, although the wife is better at it. In the Brumby it is smooth as silk, going from slight accelerator to none, then the foot moves to the brake.

Of the two I prefer to have no creep. weber's set-up sounds ideal, but unfortunately my controller isn't as configurable as his. The Outlander also is not able to be changed. I hear that in a Tesla you can turn the creep off.

Writing this has made me realise that both our cars are more suited to using the right foot for accelerator and left for brake. I'm not sure I want to go down that path though, there is potential for bad things to happen if my brain mixes things up...

Re: Driving an EV in slow moving traffic?

Posted: Mon, 01 Jan 2018, 17:48
by MDK
4Springs wrote:
Mon, 01 Jan 2018, 16:46
I hear that in a Tesla you can turn the creep off.
Yes in a Tesla "creep" is optional. I've always driven manual cars & find creep annoying in automatics, so I've never turned it on.

And the mapping on the accelerator pedal is awesome. It's easy to crawl along at very slow speeds, which is useful because my driveway also slopes upwards.

And in slow moving traffic I use the adaptive cruise control (or autopilot) to perfectly follow the cars in front...

Re: Driving an EV in slow moving traffic?

Posted: Sat, 27 Jan 2018, 12:38
by Peter C in Canberra
No problem at all having fine control while going slowly, both in my converted car and commercial cars, iMiEV and Volt. The converted car has a Kelly controller that can plug into a PC to use an application to set the motor controller parameters, including various aspects of throttle response.