Better to coast or brake?

[HPRifleman]

A lot of times I find myself approaching a stop light (or sign) with plenty of advanced warning that I will have to stop. If I take my foot off the accelerator the upward chevron will appear over the battery display. I take this to mean that I am charging the battery without touching the brake.

 

If I lightly apply the brake then obviously I will be recharging the battery. In this case, not only does the upward chevron appear but the regeneration spinner will appear as well.

 

My questions are, do both of these scenarios recharge the battery in the same amount? Am I better off coasting, letting the speed drop, and brake later or to begin light braking as early as possible?

[salsaguy]

for the best brake score on the coach i think it wants one long show steady brak so you regen the most energy back.but it's good to coast you just have to decide at what point do you start the braking to stop in time and do it without slamming on the brakes. of course it depends on the current battery level since if you are already topped off or near full on batty level you will lose the recharge.

just my thoughts, i could be wrong since i don't own a C-Max yet.

[fotomoto]

Those with a scanguage which can display both current flow and SOC can answer best.  My money is firmly on light braking providing more charge being that the longer the coast, the more energy is consumed by tire and air rolling resistance that never makes it back to the batts.  I get 100% brake score doing both so that doesn't reveal an answer.

 

But your title asks which is better?  Depends on what you're wanting.  Hypermilers preach to never touch the brakes if you're looking for maximum mpg's.  

Edited June 6, 2013 by fotomoto

[plus 3 golfer]

.....

 

 Hypermilers preach to never touch the brakes if you're looking for maximum mpg's.  

Because there are conversion losses (whether during regeneration during braking or coasting), one wants to coast to slow down in the hopes one never has to touch the brakes since braking increases the rate of regeneration over coasting and thus conversion losses are higher than over coasting - thus, the reason why hypermilers never want to touch the brakes. They'll go so far as cutting through corner businesses rather than stop at an intersection to turn and of course start their coasting well in advance of most other drivers to minimize potential braking (likely irritating drivers following them).

 

I agree with fotomoto, if one knows that one will have to apply the brakes at some point in time as one is slowing. Reducing speed (by lightly applying the brakes, don't want to use the friction brakes) sooner rather than later will reduce overall drag on the car as one continues to slow down.  Thus, more energy from the reduction in drag losses will be available for regeneration later in time as one continues to slow down.  This could be significant for example if one has to slow down from 70 mph to zero (like exiting a freeway to a stop sign).

  

[HPRifleman]

I will describe a typical scenario that caused me to ask the question.

 

I am traveling at about 45MPH and notice that the stoplight 300-500 yards ahead of me has just turned yellow. At that point I can do one of two things:

 

1) If no one is behind me, I can take my foot off the accelerator and coast to the light only applying the brake in the final few yards.

 

2) Apply light braking immediately and begin the regeneration process. The problem with this is that even if I use a very light touch on the brake, I will come to a stop well short of the stoplight.

 

I guess another option might be to brake early, and then when the speed has reduced greatly, let the car coast to a stop. This would require experience knowing braking/coasting distance.

 

My intent is to determine which method recaptures the most energy back to the car.

[grggwlkr]

What I have observed.  On my way to work, there is a section where i am going from 55mph to either a stop at a light or turning right that requires a near stop to make. There is a gradual down slope to the light of roughly a 10th of a mile. If traffic allows, I can tap off the cruise at the top of the slope and coast nearly to a stop by the time i reach to light with no significant increase in battery charge level. Or I continue in cruise to about the half-way point and aggressively brake and nearly fully charge the battery. The choice between to two is always dictated by traffic.  If I have the choice, I will aggressively brake even if I take a slight hit on braking score because of the recharge bonus. This is primarily why I would like an indicator the shows when friction brakes start to be applied. Yes, I know where you stand on this issue Plus 3 Golfer.

[SnowStorm]

My bet is that it will make no practical difference as long as you get a 100% braking score.  Even if your score is less than 100, I don't think it will matter very much so long as you pressed the friction brakes into service below, say, 10 mph.  Your car has 20 times as much kinetic energy at 45 mph then at 10.

 

The simple formula would be 1) "coast" when at higher speeds, 2) use regen brakes at moderate speeds, and 3) use friction brakes (if necessary) only at very low speeds.  I think we tend to do that naturally.

[plus 3 golfer]

....Your car has 20 times as much kinetic energy at 45 mph then at 10.

 

.,,

And also about 2 1/2 times more drag at 45 mph than at 10.  Can't ignore drag. :)  

 

The goal (albeit not easy to achieve) assuming one will ultimately have to stop would be to reduce speed earlier rather than later.  The best way would likely be to virtually coast up to the stop using no regeneration / friction braking but for the last few feet.  But we always can't do that.  An example would be traveling at 55 mph towards a traffic light say less than 1/4 mile away that changes to red and one knows one can't simply coast to the stop without braking.  Applying the right amount of regen braking quickly to reduce speed early will likely allow for the capture of more of the energy that otherwise would be consumed by the higher drag, rotational losses at higher speeds and then coast to the stop vs coasting first and expending the energy on the higher losses and then braking later when the energy has been reduced considerably.  This certainly is not the intuitive way but seems logical to me (unless I'm missing something which could easily be the case :confused: ) and would seem to result in more energy being captured.

 

Several times I've recorded the SOC while achieving 100% brake score.  I thought I reported this before in posts but can't locate it.  IIRC, a rough calculation showed that the change in SOC was capturing only 50 - 60 % of the initial KE + PE at the start of the regen braking until the stop. There's drag + drivetrain losses + conversion losses + load on the HV battery that affects the estimate.   Suffice it to say that 100% braking might capture 90% of the available energy at the generator but a lot less of the KE + PE of the car when I ran the tests.

 

But I agree that under normal circumstances where one can anticipate their stops, the "instinctive" way (coast, regen, friction) will not be very different from optimal provided 100% brake score is reached.

Edited June 6, 2013 by Plus 3 Golfer

[BIG ROCCO]

I think engaging hill descent mode also increases the regen rate similar to lightly applying the brakes - right?

[Sparky]

I think engaging hill descent mode also increases the regen rate similar to lightly applying the brakes - right?

 

No, Grade Assist only prevents you from increasing speed while going down hills.  As you coast to a stop, Grade Assist has no effect.  Regen is from coasting, braking or "low gear".

Edited June 6, 2013 by Sparky

[homestead]

From the manual p209 Grade Assist...

Provides additional grade braking with a combination of engine motoring and high-voltage battery charging to help maintain vehicle speed when descending a grade.

[Recumpence]

I am a hypermiler and my experience is, if all else is equal, you should coast because the added distance coasting is not equaled by the energy gained in regeneration because there is a loss of energy in the conversion process during regen (as has already been stated). However, that being said, at high speed you may see so much drag that it is a bit better braking a bit to gain some regen down to a speed that drag is not so enormouse (say 45mph for sake of arguement). I do not know what that point is, however.

 

My methodology is to use one or the other technique depending on my situation. I do mostly 45mph and slower driving, so I like to coast. However, if I know I will need the added battery for a soon upcoming section of road, I may sacrifice the coasting (that would give me better efficiency right then) for added efficiency later with a higher battery charge. One example of that is when I come home. There is a section of road that my car likes to stay in EV until the battery is very low. However, at that point, EV+ wants to kick in, but there is not enough battery to use EV+ effectively. So, I will tend to use regen to gain some added battery capacity before that section and gain the added .1mpg average by the time I enter my driveway. I also will purposely force the engin to start when in EV+ mode because I know I need a touch more voltage so I do not roll into my driveway with a dead battery. This is done by accellerating for a bit, then braking for a bit to charge then regen into the battery for 25% more power in that road section. I can normally do that without dropping much (nothing indicated) on my daily MPG average. However, the small added battery range will give me an extra .1mpg at the end of the drive.

 

Little tricks like this (and many others) are what make this car such an adventure to drive.   :)

 

Matt

[SnitGTS]

I look at this a little differently.  If I see a light changing to red from a distance I will coast until the point where I can lightly press the brake to initiate the brake regeneration process to keep regenerating for as long as possible.  If the light is already red and I think there is a possibility it will be green before I get there I will brake a little initially to slow myself down to a speed that I can maintain the rest of the distance until the light turns green.  (I don't jump the light because I saw someone get into an accident doing that but I do watch the cross-light)

 

Basically I try to conserve the energy the car already has because any amount of regeneration will not recoup everything that is lost.

Edited June 6, 2013 by SnitGTS

[charlesd]

Hi, guys !

I'm just having a "beef" with my friend, and I'd like your input about it :)

I'm saying that if I drive the C-MAX Energi (his car) or any other hybrid at night, when there's the lowest amount of traffic on the road and the highest chances to catch most of the traffic lights on green, I get the best mileage.

He says that driving at rush hour and stopping at red lights (on the same route that I'd be driving at night) gives him a better mileage over me.

I say I'm right because at night I can coast a lot, he says that he's right because he uses regenerative braking a lot. I'm trying to explain him that no matter how "perfect" the regenerative braking process is there will be losses involved than by just coasting and letting the car use its kinetic energy.

Who do you think is right ?

Thank you !

Edited August 15, 2013 by charlesd

[salsaguy]

always better to coast and not stop. there was a recent discussion about this here in another forum recently. remember there is the 2 part brakes (the regen part meaning no friction, and the friction part).having to stop and then use fuel to get back up to that speed is always less efficient.you said he has a Energi which is not the hybrid but the plugin so this will be slightly different than just the hybrid since the plugin can run a long way in EV now mode at the lower city speeds where the hybrid only will only be able to go so far in EV mode (and can't force it to stay there) before the battery runs out.

Edited August 15, 2013 by salsaguy

[Jus-A-CMax]

You win charlesd. On days where the lights are sync and little to no traffic, it's the best...you have more control of the car & FE, and no pressure to kick in ICE. In heavy peak and busy roads you face 2 things bad for FE: start & stop which you have to use ICE to get up to traffic speed (someone is always behind you) and continued pressure to go 5 to 10 above speed limit to keep up with traffic. Plus you will never get any decent regen in true bumper to bumper traffic since you use more to EV and eventually your SOC is so low, you'll ICE it light all the other drivers...this is speaking from driving the highways from hell in LA....

Edited August 15, 2013 by Jus-A-CMax

[charlesd]

you said he has a Energi which is not the hybrid but the plugin so this will be slightly different than just the hybrid since the plugin can run a long way in EV now mode

IMHO this is irrelevant, the plugin and the non-plugin C-MAX have the same technology behind it. As I said, the losses involved in the regenerative breaking process are always there versus coasting in a mimimun traffic environment, it does not matter that you drive in EV mode only or in EV-ICE mode, you will never recoup the same amount of energy lost during the regenerative breaking process as opposed to just coasting. It's common sense, if you ask me  ...

Thanks again for your input, I have a feeling I'll actually have to hit the road in order to prove it to him. :)

Edited August 15, 2013 by charlesd

[HPRifleman]

I'm still not sure that we have answered my original question. Certainly if you feel the light will turn green before a complete stop is made, it is better to retain as much speed as possible so coasting is best. But if you know that you will have to completely stop, what is the best way to recover the most energy?

 

I guess what would really help is knowing how the C-Max recovers kinetic energy by just coasting with no brakes applied. At least I assume it recovers energy because the upward chevron appears above the battery gauge when coasting.

[val1ca]

what my friend Charles , forget to mention , is that  entire dispute was for driving just in EV mode situation, and it starts from my trip I did yesterday with my C-MAx Energi. Well I jumped in the car and the battery showed 36km EV driving. My trip to destination , about 10 km was in a flat, long line with few stops,  one way I did it coasting ( against the traffic ) and when I parked it showed that 26 km left of EV, and I stopped only ones . Returning was a different story , all traffic made me to stop every single stop, with a flow of 60km/hour, By the time I arrived home my gauge showed 21km left . All driving was only EV mode, and on my last stop I got for the first time 100% on brakes, and my gauge increase from 22 to 24 km when I fully stopped. My average score is above 90% on brakes. Almost every time I stopped my gauge increases 1 km , which I get very excited every time. I tried also to see if coasting doing a same thing , but no luck.

[plus 3 golfer]

I'm still not sure that we have answered my original question. Certainly if you feel the light will turn green before a complete stop is made, it is better to retain as much speed as possible so coasting is best. But if you know that you will have to completely stop, what is the best way to recover the most energy?

 

I guess what would really help is knowing how the C-Max recovers kinetic energy by just coasting with no brakes applied. At least I assume it recovers energy because the upward chevron appears above the battery gauge when coasting.

The real issue is the amount of KE stored depends on the time one wants to expend on maximizing FE.  The goal should be maximizing FE not storing energy.

 

If time is immaterial, coast in neutral all the way to the stop (all KE is used to overcome drag and final drive losses).  Of course this results in a significant amount of time to coast to a stop, maximizes FE, and stores no energy in the HV battery.

 

If time is somewhat material, then I believe that 1) coasting in gear applying more regen braking earlier rather than later and  2) at the appropriate time coast in neutral (or in gear) to the stop. In essence, one wants to time the scrubbing of higher speed using regen earlier to slow the car down where the drag is higher on the car rather than later where there is less KE to capture via regen.  I have simulated this in my drag model and this appears to capture more of the initial KE although there are many assumptions including do we keep time constant for the various cases. I'm trying to maximize FE including energy stored to be used later.

 

If there's traffic and one doesn't want to hold up the cars behind them (time is material to at least them), then simply brake to achieve 100% braking coach score while not irritating the drivers behind you.  IMO, this mimics how I've always applied my brakes and seems consistent with "normal" braking (whatever that is). :)  

[viajero]

It's probably not too difficult to construct a mathematical model of the energy loss/storage while decelarating to a stop, but the trick is finding the constants to plug in to the equations.  And some of those "constants", such as regen efficiency are probably functions of speed, state of charge, etc.

 

If you really want a precise answer, it might be easier to measure the end result by taking an Energi out and doing repeated test runs each way, and seeing how many kWh you used.  But I don't want a precise answer badly enough to do that.  I'll just not drive like a maniac and call it good.