goldbrick
Posts: 443
Joined: Tue Aug 01, 2017 3:33 pm
Delivery Date: 01 Aug 2017
Leaf Number: 311806
Location: Boulder, CO

Re: Help me understand my Leaf's acceleration...

Mon Jan 14, 2019 10:29 am

Nubo wrote:Be wary of using a "butt-dynomometer" to determine acceleration rates. :)

There is a slight lull when accelerating from a dead-stop. Imho this (initial slew rate) is done to protect the drivetrain.


Agree with both your points. I'd also add that depending on how the motor is synchronized, there may be a limit to the initial acceleration so that the motor can stay in sync. At least with some permanent magnet DC motors, the drive must be turned off during a small time window so the synchronization circuitry can operate. These windows are controlled by computer and during initial acceleration from a dead stop it can be hard to accurately time them while keeping them sufficiently short.

Finally, the derivative of acceleration is jerk and minimizing it will reduce the stress on the mechanical drivetrain.

goldbrick
Posts: 443
Joined: Tue Aug 01, 2017 3:33 pm
Delivery Date: 01 Aug 2017
Leaf Number: 311806
Location: Boulder, CO

Re: Help me understand my Leaf's acceleration...

Mon Jan 14, 2019 12:59 pm

No need for calculus here: F = MA so acceleration A = F/M.

Unless the car nears the speed of light and experiences relativistic effects, the mass will be constant and the F (force) required for any amount of A (acceleration) will be constant.

powersurge
Posts: 1334
Joined: Wed Oct 21, 2015 10:24 am
Delivery Date: 06 Dec 2014
Location: Long Island, NY

Re: Help me understand my Leaf's acceleration...

Mon Jan 14, 2019 1:32 pm

SageBrush wrote:ROFL !
powersurge wrote:What the original poster asks is basic physics.

No, but understanding basic physics is a good thing.
So [w]hen the car is moving, it takes less force for it to accelerate at a faster rate (pick up).

I know you are a Trumper (and thus by definition the bottom of the gene pool and an abject moron) but try anyway: energy is conserved. Kinetic energy is proportional to the velocity squared. I hesitate to use the "calculus" word so just google it: power is proportional to the cube of velocity


You know... Your ignorance and short sightedness is really showing.... I talk about physics and you start throwing out insults about my gene pool.... and Trump...??? #$%^^ WTF??

I think you really enjoy the protection you have from the anonymity of the internet.... You would never talk like that if you were looking right at me.... You are quickly adding names to the list of enemies, and there will be a breaking point on this and other sites you may be trolling. I will just ignore you ,,,

SageBrush
Posts: 4217
Joined: Sun Mar 06, 2011 2:28 am
Delivery Date: 13 Feb 2017
Location: Colorado

Re: Help me understand my Leaf's acceleration...

Mon Jan 14, 2019 2:04 pm

goldbrick wrote:No need for calculus here: F = MA so acceleration A = F/M.

Unless the car nears the speed of light and experiences relativistic effects, the mass will be constant and the F (force) required for any amount of A (acceleration) will be constant.

One way to explain the error is to realize that a car has constant power, not constant force.

So let's try some 7th grade algebra now:
W: work
t: time
A: acceleration
V: velocity
P: power
F: force
d: distance
M: mass


W = F * d
Now divide both sides by t:
W/t = F * (d/t). This is
P = F * V
F = M * A
Substitute:
P = M * A * V

P and M are constant, so A decreases as V increases
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Car is now enjoying an easy life in Colorado
03/2018: 58 Ahr, 28k miles
11/2018: 56.16 Ahr, 30k miles
-----
2018 Tesla Model 3 LR, Delivered 6/2018

golfcart
Posts: 382
Joined: Mon Jan 04, 2016 6:08 am
Delivery Date: 21 Nov 2015
Location: Virginia Beach, VA

Re: Help me understand my Leaf's acceleration...

Mon Jan 14, 2019 3:54 pm

I don't see how effective mass or power is constant for the leaf but y'all are entertaining arguing about high school physics.

While actual mass of the vehicle is constant, the effective mass is not when you consider air resistance, friction, and other forces that all increase with speed. You'd be surprised how big a factor this plays above 50mph or so.

While the power is "relatively" constant across a range of speeds for the leaf it is not completely flat either. I've only seen rolling Dyno charts so I really don't know what happens from a stop but if you search Dyno charts for the leaf there is a peak somewhere around 30-40mph then it slowly drops off until the end of the chart around 90 mph or whenever the thing is speed limited...

https://www.flickr.com/photos/tuanies/15103792190/

Generally speaking Sage is right though, some of y'all should check this out for a decent explanation of the work-energy relationship. Power is work per unit time...

https://courses.lumenlearning.com/boundless-physics/chapter/work-energy-theorem/

I've felt what the OP is talking about and suspect there is something in the software that limits the amps in certain situations near 0mph but I can't really say for sure without monitoring current to the motor across a range of speeds, scenarios and throttle positions. It'd be an interesting experiment, which the link the OP posted sort of does for a couple of specific situations, but it is beyond my capabilities.
2015 S with Charge Package

SageBrush
Posts: 4217
Joined: Sun Mar 06, 2011 2:28 am
Delivery Date: 13 Feb 2017
Location: Colorado

Re: Help me understand my Leaf's acceleration...

Mon Jan 14, 2019 7:28 pm

I'm not certain, but perhaps this torque curve explains OP's observation:

Image
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Car is now enjoying an easy life in Colorado
03/2018: 58 Ahr, 28k miles
11/2018: 56.16 Ahr, 30k miles
-----
2018 Tesla Model 3 LR, Delivered 6/2018

SageBrush
Posts: 4217
Joined: Sun Mar 06, 2011 2:28 am
Delivery Date: 13 Feb 2017
Location: Colorado

Re: Help me understand my Leaf's acceleration...

Tue Jan 15, 2019 2:34 am

golfcart wrote:While the power is "relatively" constant across a range of speeds for the leaf it is not completely flat either.

Certainly true, though we can pretty easily set up an experiment where the power flowing from the battery is constant, as is the heat coming out of the motor. dW/dt going to the wheels is constant. In fact that situation is a pretty good description of an EV.

Over a time interval the work added is constant and is equal to F*d
Since velocity is increasing the distance traveled over the same time interval increases ... and the force must decrease.

Roh roh
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Car is now enjoying an easy life in Colorado
03/2018: 58 Ahr, 28k miles
11/2018: 56.16 Ahr, 30k miles
-----
2018 Tesla Model 3 LR, Delivered 6/2018

golfcart
Posts: 382
Joined: Mon Jan 04, 2016 6:08 am
Delivery Date: 21 Nov 2015
Location: Virginia Beach, VA

Re: Help me understand my Leaf's acceleration...

Tue Jan 15, 2019 4:19 am

SageBrush wrote:Certainly true, though we can pretty easily set up an experiment where the power flowing from the battery is constant, as is the heat coming out of the motor. dW/dt going to the wheels is constant. In fact that situation is a pretty good description of an EV.

Over a time interval the work added is constant and is equal to F*d
Since velocity is increasing the distance traveled over the same time interval increases ... and the force must decrease.

Roh roh


Meh, l give u one Scooby snack but I posted the actual Dyno power curve and the work-energy theorem already so I don't know why we're designing idealized experiments.
2015 S with Charge Package

SageBrush
Posts: 4217
Joined: Sun Mar 06, 2011 2:28 am
Delivery Date: 13 Feb 2017
Location: Colorado

Re: Help me understand my Leaf's acceleration...

Tue Jan 15, 2019 2:58 pm

golfcart wrote:
SageBrush wrote:
Roh roh


Meh, l give u one Scooby snack but I posted the actual Dyno power curve and the work-energy theorem already so I don't know why we're designing idealized experiments.

To point out two things:

1. That it is easy to see that force is not constant
2. That one does not have to talk about non-conserved forces (e.g., air friction) to realize that acceleration drops in a car as velocity increases.

I agree, it would be nice to just post: P = F * V and have the gallery respond with an "Oh, yeah ..."
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Car is now enjoying an easy life in Colorado
03/2018: 58 Ahr, 28k miles
11/2018: 56.16 Ahr, 30k miles
-----
2018 Tesla Model 3 LR, Delivered 6/2018

WetEV
Posts: 2912
Joined: Fri May 04, 2012 8:25 am
Delivery Date: 16 Feb 2014
Location: Near Seattle, WA

Re: Help me understand my Leaf's acceleration...

Tue Jan 15, 2019 3:15 pm

SageBrush wrote:One way to explain the error is to realize that a car has constant power, not constant force.


Electric cars have a torque and a power limit.

At lower RPMs, torque or force is constant. Above some RPM, the torque starts to decrease to keep the power the motor and inverters are handling below the power limit. As the RPMs continue to rise, various stray losses cause the allowable power to fall. The graph below is idealized, but shows the general picture.

Image

(source https://insideevs.com/why-buy-tesla-model-3-long-range/ )
WetEV
#49
Most everything around here is wet during the rainy season. And the rainy season is long.
2012 Leaf SL Red (Totaled)
2014 Leaf SL Red

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