Regenerative Braking

[Dusty]

I've only had Li'l Sparky for a couple weeks, so I'm still a bit confused about the regenerative braking. As I understand it, it works in both D (Drive) and L (Low) modes as is shown on the power flow graphic. However, in the L mode, there is much greater resistance to coasting. Does this mean more electricity is being regenerated? And what's the mechanism involved - are there separate generators used or is the primary source of regeneration the main electric motor? If the latter, does shifting into L change the gear ratio between the motor and drive wheels? But that doesn't make sense in other ways and makes my head hurt just thinking about it. Can anyone explain what's going on?

TIA,
Dusty

 

[ImTedBell]

Think of it this way. We normally pull electricity out of the battery and use that to drive the electric motor. Regen as running the drive motor in "reverse" to generate electricity and put back into the battery.

Shifting into L just changes the amount of regen that will happen at the motor. There are no changes to gearing.

 

[Homer]

There is a display option for the driver that shows exactly how much power is being used or regenerated (instead of the graphic of the green ball) which I like a little better.

Initially I would "downshift" to L when I needed to slow down, but lately I have just started using L as my default for driving around town (when it's hard to go more than a mile without slowing down). I was actually wondering how other people use it.

 

[sTeeve]

There is a display option for the driver that shows exactly how much power is being used or regenerated (instead of the graphic of the green ball) which I like a little better.

Initially I would "downshift" to L when I needed to slow down, but lately I have just started using L as my default for driving around town (when it's hard to go more than a mile without slowing down). I was actually wondering how other people use it.

Homer et al,
I do something similar (in both my Spark EV and my 2013 Leaf SL). I'll use "D" mode mostly in both cars, slapping in and out of "L" (Spark EV) and "B" (Leaf) when I must decelerate...using the foot brakes sparingly.

I've been toying with "L" mode like you said; using it more and more as the default...but I find that I still like to coast (sometimes in "N"), or light regen ("D").

They're just hypermile techniques; some of us take them to an extreme...which isn't a bad idea in these short-range 1st Gen EVs.

As battery capacities grow, there may be less need for extreme hypermile techniques. I find that they are useful to squeeze a few more miles out of the battery (along with judicious use of the throttle).

-Bob K.

 

[Dusty]

Shifting into L just changes the amount of regen that will happen at the motor. There are no changes to gearing.

OK, let's see if I got this right. The motor does the regenerating and not any separate generators connected to the drive wheels. When you shift into L, the connection between the motor and drive wheels is direct, but when in D, the drive wheels are allowed to slip which results in longer coasting? If it isn't due to gearing, is that some sort of clutch mechanism? And does anyone know how much more juice flows back to the battery when in L as opposed to D?

It's only been a short time, but using L seems advantageous for anything but open freeway. I find I almost never need the brakes when in L except at red lights. This might give me 2 or 3 times the normal brake life. I like it!

 

[nikwax]

Maybe this will help, there's a diagram of the drive:


http://sandyblogs.com/techlink/?p=2081


L doesn't change anything mechanically.

 

[Homer]

Maybe this will help, there's a diagram of the drive:

http://sandyblogs.com/techlink/?p=2081

L doesn't change anything mechanically.

Hmm, that source says--

When driving the Spark EV, the transmission should be placed in the Drive (D) position for all normal driving conditions. This provides maximum efficiency and range economy and enables the drive unit to operate in a forward direction by electronically varying the output torque and speed of the electric motor.

The transmission can be placed in Low (L) for the “feel” of engine braking and to slow the vehicle once the throttle is lifted. While in the Low position, the vehicle will slow more quickly and use the motor to more aggressively collect energy during a regenerative braking event.

The Spark EV also features a Pedestrian Friendly Alert Function (PFAF) to alert those nearby of the vehicle’s presence. To activate a friendly horn, pull the turn signal lever toward you like you are flashing the headlights. The alert will only work in Drive.

The PFAF also can be set to activate automatically. It is active in Drive and Reverse at speeds below 18 mph (28 km/h). The sound is intended to be barely audible in the cabin.

Does using L really cause a noticeable range hit (and/or cause other issues for the drive unit)?

What does it mean to say "the PFAF can also be set to activate automatically"?

 

[nozferatu]

Maybe this will help, there's a diagram of the drive:

http://sandyblogs.com/techlink/?p=2081

L doesn't change anything mechanically.

Hmm, that source says--

When driving the Spark EV, the transmission should be placed in the Drive (D) position for all normal driving conditions. This provides maximum efficiency and range economy and enables the drive unit to operate in a forward direction by electronically varying the output torque and speed of the electric motor.

The transmission can be placed in Low (L) for the “feel” of engine braking and to slow the vehicle once the throttle is lifted. While in the Low position, the vehicle will slow more quickly and use the motor to more aggressively collect energy during a regenerative braking event.

The Spark EV also features a Pedestrian Friendly Alert Function (PFAF) to alert those nearby of the vehicle’s presence. To activate a friendly horn, pull the turn signal lever toward you like you are flashing the headlights. The alert will only work in Drive.

The PFAF also can be set to activate automatically. It is active in Drive and Reverse at speeds below 18 mph (28 km/h). The sound is intended to be barely audible in the cabin.

Does using L really cause a noticeable range hit (and/or cause other issues for the drive unit)?

What does it mean to say "the PFAF can also be set to activate automatically"?

After 19500 miles of driving my car, I can honestly tell you that if you learn how to modulate the regen yourself via the accelerator pedal in L, you'll get more miles than using D. You can usually squeeze more regen out of doing it yourself than D. That's been my personal experience.

I personally like using L to slow down...because I can really control how I do without using the brakes.

 

[Dusty]

Maybe this will help, there's a diagram of the drive:


http://sandyblogs.com/techlink/?p=2081


L doesn't change anything mechanically.

Yeah, I see that now. Thanks for the link. It's pretty cool, actually, except the need for the differential escapes me.

So, if there is no mechanical explanation for the increased drag in L, is there an electrical one? Something must account for the rapid slowing of the car. I don't think the brakes are being applied since the rear brake lights don't come on. (I could see that in the grill of a big SUV riding my a**.) Perhaps some of the regenerated juice is used to reverse polarity in an attempt to magnetically slow the motor. The article talks about the “feel” of engine braking implying some sort of simulation. What if the article is correct and D really is the most efficient driving mode, and L was added as a safety measure for steep downhills where a lack of compression braking and overheated brake pads could be disastrous? And if that really is the way it works, will I (and you) still like driving in L so much?

 

[Zoomit]

All cars need a differential because the outside wheel travels a further distance than the inside wheel during a turn. The differential lets the tires travel different distances and is always needed if the two wheel axles are connected in any mechanical fashion. (A car with one motor connected to each front or rear wheel would be the case where a differential is not required because there would be no common axle.)

The full throw of the accelerator pedal and roughly half the throw of the brake pedal doesn't do anything directly mechanical in the car. It only tells the computer how much energy to give or take from the propulsive motor. Similarly, the D or L or Sport button only changes how much the particular position of the accelerator or brake affects the car's motion. For simplicity sake, lets assume the hydraulic brakes, the ones on each wheel hub, are actuated on the last half of the brake pedal travel. When in L, the brake bias is increased on the accelerator pedal. When in Sport, the motor produces more propulsive force earlier in the pedal travel. Independent of which mode you're operating within (D, L, or Sport), a floored accelerator pedal does the same thing. Similarly, independent of which mode you're operating within (D, L, or Sport), a floored brake pedal provides maximum braking.

It's my speculation that the straight efficiency of the car is the same in any mode (D, L, or Sport). However, you're more likely to use faster acceleration in the Sport mode and have a tendency to use more energy. When frequently stopping, when driving around town, the L mode allows you to use only one pedal for all acceleration and braking, without ever using the hydraulic brakes. This maximizes the regen and minimizes energy use. The same braking can be done in D, but because your foot is needed on the brake more often, you're more likely to use the hydraulic brakes and waste energy.

Any efficiency can be achieved in any mode, but the typical users will see more efficient driving in L around town (again due to the bias to avoid the hydraulic brakes). When cruising on the highway, it wouldn't matter at all, except when slowing or speeding up. In that case, the L mode would increase the rate of deceleration and potentially require more acceleration to get back up to normal cruising speed. As such, I suspect the tendency would be to use more energy in L mode on the highway.

One final thought, there's absolutely no reason to use L intermittently to slow down, just use the brake pedal! You can extract the SAME regen energy out of the motor and decelerate the same.

Second final thought, the only reason there's a D and L mode is because everyone is used to having two pedals, one to stop and one to go. The BMW i3 and i8 have "permanent" L modes with heavy regen when letting off the accelerator.

 

[nikwax]

Maybe this will help, there's a diagram of the drive:


http://sandyblogs.com/techlink/?p=2081


L doesn't change anything mechanically.

Yeah, I see that now. Thanks for the link. It's pretty cool, actually, except the need for the differential escapes me.

So, if there is no mechanical explanation for the increased drag in L, is there an electrical one? Something must account for the rapid slowing of the car. I don't think the brakes are being applied since the rear brake lights don't come on. (I could see that in the grill of a big SUV riding my a**.) Perhaps some of the regenerated juice is used to reverse polarity in an attempt to magnetically slow the motor. The article talks about the “feel” of engine braking implying some sort of simulation. What if the article is correct and D really is the most efficient driving mode, and L was added as a safety measure for steep downhills where a lack of compression braking and overheated brake pads could be disastrous? And if that really is the way it works, will I (and you) still like driving in L so much?

When you are generating power via regenerative braking (either via the brake pedal or the L setting), the magnetic field in the motor is what is slowing the car down. Just as the magnetic field makes the car go when you step on the accelerator.

 

[Sparkler]

...

...
The Spark EV also features a Pedestrian Friendly Alert Function (PFAF) to alert those nearby of the vehicle’s presence. To activate a friendly horn, pull the turn signal lever toward you like you are flashing the headlights. The alert will only work in Drive.

The PFAF also can be set to activate automatically. It is active in Drive and Reverse at speeds below 18 mph (28 km/h). The sound is intended to be barely audible in the cabin.

...

What does it mean to say "the PFAF can also be set to activate automatically"?

I believe that this part of the Owner's Manual is in error. The PFAF horn does not in fact stop working above 18 MPH, it is always active, which is very annoying when trying to flash the brights at speed. I've learned to push the stalk forward and then pull back quickly to avoid having the horn sounding. It is something that was done correctly on the Volt, which does stop working above 40 MPH, so I was surprised that it was screwed up on the Spark. Later Volts have the horn function on a small button on the end of the stalk, totally divorcing the flash from the honk. The Spark design is very poor in comparison, especially when you consider that they already had an example of the proper way to build it.

The second part actually refers to the sound made by a speaker, sort of an electric whine, which changes pitch with speed to let people know you are there. That sound is fairly quiet, I did not hear it until I was actively listening for it with my window down. It does seem to stop as you speed up, that's probably where the 18 MPH reference comes from. Oh, and there are no settings that I am aware of to shut the sound off, or even change the volume. Ideally we could get a few different choices of sounds, the whine is so unobtrusive that people really don't notice it. However it is sufficiently Jetson-like that I don't mind it when I have the windows down...

 

[Dusty]

When frequently stopping, when driving around town, the L mode allows you to use only one pedal for all acceleration and braking, without ever using the hydraulic brakes. This maximizes the regen and minimizes energy use. The same braking can be done in D, but because your foot is needed on the brake more often, you're more likely to use the hydraulic brakes and waste energy.

Something slows the car down when you lift your foot off the accelerator while in L, but doesn't when you lift it off in D. Are you saying it's the brakes or not the brakes? I'm more confused than ever.

 

[Dusty]

When you are generating power via regenerative braking (either via the brake pedal or the L setting), the magnetic field in the motor is what is slowing the car down. Just as the magnetic field makes the car go when you step on the accelerator.

OK, if I understand you correctly, you're saying that hitting the brake pedal in D is the same as letting off the accelerator in L, except eventually the hydraulic brakes are activated?

But something activates to slow the car when you let off the accelerator in L but doesn't activate when you let off the accelerator in D. What is it? How does it work? Does it waste energy or generate it? Is it the brakes coming on? If so, why don't the brake lights activate? If it isn't brakes but some magnetic drag on the rotor, where is the source of the power to create that drag?

Sorry to drive anyone crazy with this. I thought there was a simple answer.

 

[kevin]

When you are generating power via regenerative braking (either via the brake pedal or the L setting), the magnetic field in the motor is what is slowing the car down. Just as the magnetic field makes the car go when you step on the accelerator.

OK, if I understand you correctly, you're saying that hitting the brake pedal in D is the same as letting off the accelerator in L, except eventually the hydraulic brakes are activated?

But something activates to slow the car when you let off the accelerator in L but doesn't activate when you let off the accelerator in D. What is it? How does it work? Does it waste energy or generate it? Is it the brakes coming on? If so, why don't the brake lights activate? If it isn't brakes but some magnetic drag on the rotor, where is the source of the power to create that drag?

Sorry to drive anyone crazy with this. I thought there was a simple answer.

When you release the accelerator in L or D there is regenerative braking and you can see the amount of power that is being returned to the battery on the right part of the driver's display. Both L and D do that but the effect is more significant in L. The friction (mechanical) brakes are not used in either case.

When you apply light pressure to the brake the regenerative braking is increased - again you can see this on the display.

Only when you apply higher pressure do the friction brakes come into effect.

Although regeneration is very useful in conserving energy it is not 100% efficient and so it is usually better to coast rather than use regeneration so D can give higher miles per kWh.

kevin

 

[Zoomit]

I'll try to create graphic depicting the simplified energy flow given an accelerator or brake pedal position. It'll take me a few days and I'll likely create a new thread. In the meantime, I suggest using the Driver Information Center display that shows the numerical energy usage as seen in the right of this picture:

http://icdn3.digitaltrends.com/image/2014-chevrolet-sparkev-067b-1500x1000.jpg

It can give you a better sense of how much regen braking is being done in the different modes and pedal positions. I'll add that interpreting the regeneration numbers is challenging because they vary with speed at the same pedal position, and speed is always changing when you're braking.

 

[Dusty]

OK, let me ask this another way. Forget about braking. We're just letting our collective foot off the accelerator. What is the mechanism which slows the car in L, that doesn't activate in D?

 

[nikwax]

The mechanism is a combination of the software that the car's computer runs when you are driving the car, responding to input from the driver, and the magnetic field of the motor, taking direction from the computer.


When you move the selector from L to D, you are selecting a different set of software, a profile if you will, that tells the computer how to run the motor.




Just as digital cameras still look like film cameras and still make clicking sounds, even though neither is necessary, the "shift lever" is a leftover from the bygone era of manual transmissions (joke intended) where moving the lever moved a set of gears around in a transmission. In other words, it is skeuomorphic.

 

[Zoomit]

OK, let me ask this another way. Forget about braking. We're just letting our collective foot off the accelerator. What is the mechanism which slows the car in L, that doesn't activate in D?

With no accelerator deflection, the mechanism is exactly the same in D vs L, it's just a different regen quantity. On an open freeway, with no traffic behind you...
- Get up to 70 mph in D and lift off, you'll briefly see around 11 kW peak regen
- Get up to 70 mph in L and lift off, you'll briefly see around 35 kW peak regen

The only difference is the amount of energy the software tells the motor to extract from the wheels.

(Those numbers are from memory and may be a little off.)

 

[sTeeve]

...

...
The Spark EV also features a Pedestrian Friendly Alert Function (PFAF) to alert those nearby of the vehicle’s presence. To activate a friendly horn, pull the turn signal lever toward you like you are flashing the headlights. The alert will only work in Drive.

The PFAF also can be set to activate automatically. It is active in Drive and Reverse at speeds below 18 mph (28 km/h). The sound is intended to be barely audible in the cabin.

...

What does it mean to say "the PFAF can also be set to activate automatically"?

I believe that this part of the Owner's Manual is in error. The PFAF horn does not in fact stop working above 18 MPH, it is always active, which is very annoying when trying to flash the brights at speed. I've learned to push the stalk forward and then pull back quickly to avoid having the horn sounding. It is something that was done correctly on the Volt, which does stop working above 40 MPH, so I was surprised that it was screwed up on the Spark. Later Volts have the horn function on a small button on the end of the stalk, totally divorcing the flash from the honk. The Spark design is very poor in comparison, especially when you consider that they already had an example of the proper way to build it.

The second part actually refers to the sound made by a speaker, sort of an electric whine, which changes pitch with speed to let people know you are there. That sound is fairly quiet, I did not hear it until I was actively listening for it with my window down. It does seem to stop as you speed up, that's probably where the 18 MPH reference comes from. Oh, and there are no settings that I am aware of to shut the sound off, or even change the volume. Ideally we could get a few different choices of sounds, the whine is so unobtrusive that people really don't notice it. However it is sufficiently Jetson-like that I don't mind it when I have the windows down...

I agree. And I wonder if there is a way in the programming of the CAN-bus communications that the PFAF horn "feature" could be disabled? It's beyond annoying for me, as I use the "flash your brights" method of alerting other vehicles to my presence, rather than to toot the ******* horn...