Question on Regenerative braking

[wdolson]

An ice does preserve rotational kinetic energy in the flywheel, the rotating gears in the transmission and other rotating parts of the drivetrain. The flywheel energy is what causes a chirp in the wheels when the clutch is dumped. Engine braking occurs when the throttle is closed and the engine becomes a vacuum pump which extracts kinetic energy from the car.

The inverter uses about half a kilowatt to keep the magnetic fields turning at the same speed as the rotor to produce zero torque, so it's better to coast in neutral. After about 2 seconds in neutral, the inverter shuts down and consumes no energy.

I've experimented going down our hill both putting the car in neutral and keeping the car in gear, but trying to keep the energy meter as close to 0 as possible. Putting it in neutral, the car coasts terribly compared to ICEs I've driven in the past. I suspect it has something to do with the motors embedded in the axles. I get much better Wh/Mi at the bottom of the hill keeping the car in gear and trying to keep it as close to 0 as posssible.

Yep, when I'm really trying to conserve power, I go for your first point. I'll usually turn on autosteering so I can focus on speed adjustments and gradually reduce speed going uphill(toggling speed down by 1mph just as the orange bar starts getting high until I reach what I consider a minimum safe speed) and increase speed going downhill(basically reverse that procedure, increasing speed by 1mph as the green bar gets high until I reach a maximum safe speed). It's remarkable how much you can increase range doing that.

Another thing I have found from experience is getting up over 25 mph as fast as possible (without doing a high energy launch) makes a big difference. Leaving the house, I need to go uphill for about 0.2 miles to a crest, then downhill for another 1.5 miles. I get much better energy usage if I get the car up to 25 mph on the first leg as quickly as reasonable, and again from the stop sign t the crest of the hill, if I accelerate moderately hard to 25. On the first leg, if I poke along at 20 mph up the hill, my Wh/Mi at the top will be around 1500 Wh/Mi. If I goose it to 25 mph by 2-3 houses up the hill, it will be around 800-900 Wh/Mi at the top of the hill.

Getting up to 25 as quickly as reasonable at the start and at the first stop sign can get my Wh/Mi at the bottom of the hill down to 75 Wh/Mi if I hit everything right. It doesn't seem to have much connection with whether the A/C is on or not. Heat makes a big difference, but I've done it many times in the spring with no A/C and again in the summer with A/C on (windows closed both times) and I actually got the best Wh/Mi with the A/C on, but I figure that was just hitting the sweet spot for max regen on the way down.

 

[SageBrush]

Another thing I have found from experience is getting up over 25 mph as fast as possible (without doing a high energy launch) makes a big difference. Leaving the house, I need to go uphill for about 0.2 miles to a crest, then downhill for another 1.5 miles. I get much better energy usage if I get the car up to 25 mph on the first leg as quickly as reasonable, and again from the stop sign t the crest of the hill, if I accelerate moderately hard to 25. On the first leg, if I poke along at 20 mph up the hill, my Wh/Mi at the top will be around 1500 Wh/Mi. If I goose it to 25 mph by 2-3 houses up the hill, it will be around 800-900 Wh/Mi at the top of the hill.

Granularity ?

 

[mongo]

I've experimented going down our hill both putting the car in neutral and keeping the car in gear, but trying to keep the energy meter as close to 0 as possible. Putting it in neutral, the car coasts terribly compared to ICEs I've driven in the past. I suspect it has something to do with the motors embedded in the axles. I get much better Wh/Mi at the bottom of the hill keeping the car in gear and trying to keep it as close to 0 as posssible.



Another thing I have found from experience is getting up over 25 mph as fast as possible (without doing a high energy launch) makes a big difference. Leaving the house, I need to go uphill for about 0.2 miles to a crest, then downhill for another 1.5 miles. I get much better energy usage if I get the car up to 25 mph on the first leg as quickly as reasonable, and again from the stop sign t the crest of the hill, if I accelerate moderately hard to 25. On the first leg, if I poke along at 20 mph up the hill, my Wh/Mi at the top will be around 1500 Wh/Mi. If I goose it to 25 mph by 2-3 houses up the hill, it will be around 800-900 Wh/Mi at the top of the hill.

Getting up to 25 as quickly as reasonable at the start and at the first stop sign can get my Wh/Mi at the bottom of the hill down to 75 Wh/Mi if I hit everything right. It doesn't seem to have much connection with whether the A/C is on or not. Heat makes a big difference, but I've done it many times in the spring with no A/C and again in the summer with A/C on (windows closed both times) and I actually got the best Wh/Mi with the A/C on, but I figure that was just hitting the sweet spot for max regen on the way down.

I don't think the 25 is important. On a slope, unless you have brakes on, you are doing non net work to oppose gravity. Ignoring friction and air resistance for the moment, there is a set amount of energy needed to oppose gravity while going up the hill per time unit. The potential energy added by climbing the hill is fixed. The longer you take to climb the hill, the more energy you use.
Extreme case, sit on the hill holding the car in place with the accelerator. You are using energy, but getting no distance.

Same thing is why Falcon 9s don't light off their engines till the last second when landing. The less time fighting gravity, the more efficient you are. (That and the engines would relaunch the 1st stage if they were on any longer)

 

[wdolson]

Granularity ?

Sorry, I don't understand what you're asking for.

I don't think the 25 is important. On a slope, unless you have brakes on, you are doing non net work to oppose gravity. Ignoring friction and air resistance for the moment, there is a set amount of energy needed to oppose gravity while going up the hill per time unit. The potential energy added by climbing the hill is fixed. The longer you take to climb the hill, the more energy you use.
Extreme case, sit on the hill holding the car in place with the accelerator. You are using energy, but getting no distance.

Same thing is why Falcon 9s don't light off their engines till the last second when landing. The less time fighting gravity, the more efficient you are. (That and the engines would relaunch the 1st stage if they were on any longer)

At lower speeds, friction in the drive train reduces efficiency. That's why the hypermilers try to drive over 25. By 25 you have fully overcome the low speed friction losses.

 

[SageBrush]

Sorry, I don't understand what you're asking for.

No question, I was wondering if the system had time to update.

 

[TIppy]

Putting it in neutral, the car coasts terribly compared to ICEs I've driven in the past. I suspect it has something to do with the motors embedded in the axles

The motors and differential are mounted on the chassis like on an ICE car. If I put my car in neutral at 60 mph, it seems like it'll never stop.

If you try to do it manually, you are going back and forth between regen and power, wasting energy. In fact if you try to make sure the power consumptiion is zero by looking at the power meter, you are actually doing some regen to provide the ancillary power the car constantly consumes. That creates drag that is not there when you are in neutral.

 

[TIppy]

At lower speeds, friction in the drive train reduces efficiency. That's why the hypermilers try to drive over 25. By 25 you have fully overcome the low speed friction losses.

The friction is not more below 22 mph. It's just that the car's other systems are constantly discharging the battery no matter how fast you go. If you go below 22 mph that constant power drain becomes significant compared to the drivetrain power. It takes so long to cover a mile that a significant amount of energy is used by the other systems over that time. If you stop, the car will still drain the battery and you will accumulate no miles. The car will consume infinite whrs / mile

 

[voyager]

How effective (in terms of energy % won back) is regenerative braking in the typical Stop & Go traffic we see every day during the commute?

 

[wdolson]

There are a number of variables, but a rough guess would be between 25 and 50%.

 

[SDRick]

Perhaps one way to envision the question of coasting efficiency versus regeneration is with 2 Teslas at the top of a 6%, 1 mile hill with no charge. If one car is in neutral and the other in gear, which car will travel the farthest?

 

[voyager]

Ugh, Edison2... Yes, they made a pretty low drag vehicle. But every design decision they made in the process I disagree with, giving them an ugly, impractical vehicle.

There are a number of variables, but a rough guess would be between 25 and 50%.

I believe to have read that regenerative braking under rather ideal circumstances (already have some speed, or energy put into it ) is 50% at best. Can't see how stop & go could be close to 50%. But perhaps someone can give a more precise estimate...

 

[voyager]

Here's a thought.
One might argue that during stop & go traffic, friction is already (way) higher than when momentum has been built up during a steady speed (of let's say 50 mph).

This is actually another way of saying that during 10 miles of stop & go traffic, the amount of meters that coasting yields, is practically nothing... compared to 10 miles with a steady speed of 50 mph.

If the amount of coasting meters is nothing, so can't regenerative braking yield much won back energy...

 

[whttiger25]

Here's a thought.
One might argue that during stop & go traffic, friction is already (way) higher than when momentum has been built up during a steady speed (of let's say 50 mph).

This is actually another way of saying that during 10 miles of stop & go traffic, the amount of meters that coasting yields, is practically nothing... compared to 10 miles with a steady speed of 50 mph.

If the amount of coasting meters is nothing, so can't regenerative braking yield much won back energy...

I think it's really more about how much you have to use the brakes. In stop and go it's significant, as you cannot come toa full stop with regen.