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TickTock
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Difference between Eco and D

Sat Mar 03, 2012 2:08 pm

While examine the "Full power with light throttle" issue, I have come to the conclusion that not only is the acceleration profile different between "D" and "Eco", but the units are different. It appears when you select "D" the throttle position is used to set the Torque (or acceleration if you prefer) but when you select "Eco" the throttle position sets the Power (= Torque * RPM ~= Acceleration * Speed). I have two graphs comparing the two modes when I acceleration with roughly half throttle. I kept the throttle position as steady as I could until reaching top speed.

I noticed during my tests that when in Eco mode, the energy screen traction power (kW) stayed constant for constant throttle position. Since Power = Torque*RPM (or Acceleration*Speed, if you prefer), to keep constant power, the torque must be reduced as speed increases. Since (to the first order) Torque is proportional to Current (Amps) for DC motors, we would expect Motor Amps,then, to reduce as speed increases. Indeed, this is what we find when we look at the Motor Amps profile during the Eco mode steady throttle test. I also plotted the Battery Amps to show that physics isn't violated ;) . Since the battery voltage is relatively constant, and Power = Volts * Amps, we would expect the Battery Amps to be constant. Looking at the plot, that is what we see (except for the ramp up at the very beginning)

Now D is another story. During a test with constant throttle position in D, the Motor Amps stays constant so Torque is constant. This explains why the power steadily increases. Power is Torque*RPM so if Torque is constant and RPMs increases then Power must also increase. I plotted the battery amps again to show once more that physics isn't violated. Since the battery Voltage is relatively constant, and Power is Volts * Amps the battery Amps must ramp with speed (which, indeed it does).

You can confirm this without a Can bus logger. Just next time you have the pole position at a light, press the throttle to the desired position and hold it constant while watching the Power in the Energy Screen. If you are in D, power will steadily increase with speed. If you are in ECO, power will ramp up and then stay constant.

Note: that the two graphs are not to scale with each other (Eco mode didn't really reach top speed faster than D mode :))
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turbo2ltr
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Re: Difference between Eco and D

Sat Mar 03, 2012 2:15 pm

Now do the D mode test starting at 5MPH.
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adric22
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Re: Difference between Eco and D

Sat Mar 03, 2012 2:40 pm

I'm confused by the graphs. How can motor amps remain constant while battery amps are different?
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TickTock
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Re: Difference between Eco and D

Sat Mar 03, 2012 3:21 pm

adric22 wrote:I'm confused by the graphs. How can motor amps remain constant while battery amps are different?
The motor controller, among other things, is a DC-AC converter. There are a number of ways to do it. As long as V*A at the output is lower than V*A at the input, conservation is satisfied. So in the cases where the Motor Amps is higher than the Battery Amps, it means the Motor Voltage must be lower.
Last edited by TickTock on Sun Mar 04, 2012 4:43 am, edited 1 time in total.

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garygid
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Re: Difference between Eco and D

Sat Mar 03, 2012 11:39 pm

Right.
And, the Motor Volts is usually proportional to speed.
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TonyWilliams
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Re: Difference between Eco and D

Sat Mar 03, 2012 11:53 pm

I don't follow the timeline exactly. Where does the 3 second programmed ramp up play into this?

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DaveEV
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Re: Difference between Eco and D

Sun Mar 04, 2012 12:18 am

TonyWilliams wrote:I don't follow the timeline exactly. Where does the 3 second programmed ramp up play into this?
Looking at the D graph, the "3 second ramp up" the area where motor amps (and thus torque) is flat while battery amps is climbing.

The ramping is done to limit current sent to the battery, not to preserve the battery. The battery is capable of supply a lot more power under 30 mph.

BTW - my original hypothesis still stands for his bug or quirk in D-mode:

When you quickly depress the accelerator from a stop even a small amount, you are requesting 100% of available motor power due to the max motor current limit. And until you release the accelerator, the car continues to supply 100% of available motor power, even though available motor power increases from a very low level to 80 kW as you pass 30 mph.

I would think that even in Eco mode if you could depress the accelerator quick enough you might be able to trigger this, but you'd probably have to push the pedal a lot further (maybe 75%?) and harder so the effect would not be as noticeable. I haven't tried it, myself.

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TonyWilliams
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Re: Difference between Eco and D

Sun Mar 04, 2012 12:42 am

Ok, T+3 seconds is where motor amps goes flat.
Last edited by TonyWilliams on Sun Mar 04, 2012 12:58 am, edited 1 time in total.

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DaveEV
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Re: Difference between Eco and D

Sun Mar 04, 2012 12:54 am

TonyWilliams wrote:So, T+3 seconds is where battery amps goes flat?
On the D chart, yes.

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TonyWilliams
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Re: Difference between Eco and D

Sun Mar 04, 2012 1:00 am

Crap, I just edited my post! Ok, motor or battery amps?

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