jason98
Posts: 85
Joined: Mon May 24, 2010 2:29 pm

Energy waste: Brake heat, Drag, Rolling resistance readings?

Tue Mar 29, 2011 11:36 am

Is there a way to get reading on how much energy is wasted on particular energy consuming component?
In particular I am really interested in brake heat waste. This should be easily calculated by the car computer by subtracting regenerating braking power from a function of deceleration rate.

I understand that drag is not possible to calculate without some sort of air speed sensor.
The rest is the rolling resistance, climate, and the "Other Systems".
Also the "Other Systems" consuming a quarter of the KWH (as much as my plasma TV) is a big mystery to me. What could be possibly powered by it?

Smidge204
Posts: 940
Joined: Wed Nov 24, 2010 1:42 pm

Re: Energy waste: Brake heat, Drag, Rolling resistance readings?

Tue Mar 29, 2011 1:01 pm

"Other systems" would be lights, power steering, brakes, and on-board computer(s).

Calculating braking efficiency is a lot harder than that. You would need to know vehicle mass (for total energy) and a way to measure how much energy is dumped into the rotors. There are a few ways to do that:

1) Figure out the friction coefficient of the brake pad/rotor system. Then you can use brake pressure (measured somehow...) to calculate brake force contributed by friction. Combined with stopping time/distance and initial kinetic energy you can calculate how much energy the friction brake dissipated.

2) Measure braking force on the rotor directly by a strain gauge applied to the brake caliper support and apply material mechanics to figure out the force. The rest is as in option 1.

3) Monitor the temperature of the braking system components and model the heat loss and heat capacity of them to determine total energy dissipated.

=Smidge=

jason98
Posts: 85
Joined: Mon May 24, 2010 2:29 pm

Re: Energy waste: Brake heat, Drag, Rolling resistance readings?

Tue Mar 29, 2011 1:10 pm

Smidge204 wrote:"Other systems" would be lights, power steering, brakes, and on-board computer(s).


So when the lights are off, not even touching the steering wheel and brakes what we have left, just computers? If so those are really inefficient ones.


Smidge204 wrote:Calculating braking efficiency is a lot harder than that.


Well, the net weight of the car is a known constant plus adding an average person weight for a driver should be good enough. Kinetic energy is just directly proportional to a mass. I think it is ok to ignore the rest of the passengers and still come up with very comparable number to whatever 1,2,3 could give you without requiring any temperature measurements/sensors/models whatsoever.

Smidge204
Posts: 940
Joined: Wed Nov 24, 2010 1:42 pm

Re: Energy waste: Brake heat, Drag, Rolling resistance readings?

Tue Mar 29, 2011 6:02 pm

jason98 wrote:So when the lights are off, not even touching the steering wheel and brakes what we have left, just computers? If so those are really inefficient ones.

Is it possible to have the lights completely off while the car is in drive?

Also, you said "a quarter of the KWH (as much as my plasma TV)" - I'm guessing you meant kilowatts, not kilowatt-hours. ie 250 watts. If the car is on and in drive that's entirely possible... there are a lot of electronics in that vehicle. The brake booster and power steering units should always be using SOME power since they need to be immediately available for use.


jason98 wrote:Well, the net weight of the car is a known constant plus adding an average person weight for a driver should be good enough.

Estimated, anyway.

You wanted to know how much energy is "wasted" when braking. The only way to do that is to eliminate all the variables you can and try to measure non-regenerating brake power as directly as possible. Wind is one large factor. Slope of the road (change in elevation) is another. Rolling resistance depends heavily on road surface type and condition. The computer can't account for any of these.

Ultimately, the can will tell you how many KW of regen you are getting. Knowing how much energy is being lost through the brakes is strictly academic since you can't do a damn thing about it either way - especially if you've maxed out regen capacity. Regen power tops out at 30KW so efficiency will be a function of speed and how hard your foot is on the pedal.
=Smidge=

jason98
Posts: 85
Joined: Mon May 24, 2010 2:29 pm

Re: Energy waste: Brake heat, Drag, Rolling resistance readings?

Tue Mar 29, 2011 6:19 pm

Smidge204 wrote:Is it possible to have the lights completely off while the car is in drive?


Yes. There is a light switch on the lower left side of the dash.

Smidge204 wrote:I'm guessing you meant kilowatts, not kilowatt-hours. ie 250 watts.


Correct.

Smidge204 wrote:If the car is on and in drive that's entirely possible...


My point is that it is 250 watts regardless of whether it is in drive or not in drive.

Smidge204 wrote:Regen power tops out at 30KW so efficiency will be a function of speed and how hard your foot is on the pedal.
=Smidge=


So can I assume that before 30KW the brake pads are not used? Is there any reference for it from Nissan? That would be at least something to consider while driving.

User avatar
davewill
Posts: 4962
Joined: Thu Mar 17, 2011 6:04 pm
Location: San Diego, CA, US

Re: Energy waste: Brake heat, Drag, Rolling resistance readings?

Tue Mar 29, 2011 6:30 pm

jason98 wrote:So can I assume that before 30KW the brake pads are not used? Is there any reference for it from Nissan? That would be at least something to consider while driving.

Probably not. Nissan wants the brakes to not only be efficient, but predictable and smooth. I would expect some mix of regen and friction at different pedal pressures, not a sudden application of friction when you reach 30kW. I wouldn't expect them to be used any more than necessary, however.
2014 Rav4 EV, Blizzard Pearl White
2011 LEAF SL w/QC, Blue Ocean, returned at end of lease

Return to “Range / Efficiency / Carwings”