Regen ... which is more efficient?

Time the traffic so that you don't have to come to a stop. Momentum is king.

 

If you have to brake, brake as early and softly as you can. If you are really clever and a little lucky you should be able to do a no arrows glide to the stop line and either brake from 12km/h down, when creep starts to work or flip into neutral at 12km/h and coast to a stop with no brakes. If it's a red light you are stopping for it will have gone green so no need to stop.

 

Exactly! The more energy you can save by not braking the more you have on the restart. Starting from a dead stop uses the most energy...continuing from a roll uses less and less.

For argument sake, if you have some distance between you and the light (and ideally no cars in front of you), I use neutral coast when I see a red light and have some distance...it will give you a little more "roll" and you will not lose energy from engine braking.

 

+1 on the earlier postings.

I've been playing with this business of trying to eek out the last few
percentage of MPGs in my daily urban driving commute for about a
year.

It's a complicated business and wholly dependent on constantly
changing variables like:
* SOC - I's best to have high 50-low 60%s -- you need a ScanGauge to
see the numbers and react to small changes
* topography - taking advantage of even the smallest declines to
maintain forward motion and mabe using a little SOC if you're much
over 60%
* traffic load and aggressiveness
* highly selective use of my EV mode switch to suppress ICE on before
reaching Stage 4 (~156 degF coolant temps) to get into an ICE-off no
arrow glide approaching a potential full stop situation
* occasional use of ICE-off glides in Neutral (this to save SOC as in the
iconic "no-arrows glide" there is power flow from the HV battery although
it is not shown on the battery mimic)
* maintaining largeish following distances ahead to have
speed/distance flexibilities for "driving without brakes," including
ballistic stops, (roll to a stop without using the brake pedal) and
exquisitely timed intersection roll-throughs.

In my experience, its best to avoid braking originated regen whenever
possible. This requires considerable foresight and prior planning in a
constantly changing traffic environment -- for sure, somebody is going
to cut into your lane and reduce your following-distance "cushion"
ahead.

I'm not even sure I could explain how I mix-and-match all the possible
responses for a general case response. I know that in my daily
commute, something that works best early in the trip before Stage 4
is perfectly the wrong thing to do later fully warmed-up, with High
SOC and sitting at the top of a hill with a longish no arrows/regen
glide coming up.

Don't get me wrong, although the savings are small, playing around
and finding out what works best is a worthwhile effort. But one that I
find is difficult to work-up into a one-size-fits-all recommendation;
"One mans fish (Fr: poisson) is another man's poison."

No real hard data here, but maybe some help nonetheless.

 

To reinforce others' comments, avoiding regeneration as much as possible is most efficient.

These are from a recent morning commute:





This shows the impact of avoiding regen on this particular commute. Some regeneration is inevitable here -- I have 13 lights and 3 stop signs in 13 miles -- and an MFD completely free of regen icons is very unusual. Even here, there was some regen, apparently insufficient to register on the screen. I choose a route to maximize pulse and glide opportunities (and minimize regen), I've learned the timing of the lights, and I leave home a few minutes earlier than necessary when traffic is lighter.

 

If I was looking for max regen over 600 feet I'd drag the brakes down to 7 mph (when regen braking ends)...although it pains me to give you this advice.

 

Max regen = Max conversion loss. That's why it pains him. :madgrin:

 

It pains me because braking, if you're driving most efficiently, can be avoided most times under normal driving circumstances (I'm not talking about rush hour, bumper to bumper...I'm talking about looking ahead to time stop lights or in avoidable braking situations).

Everybody's route is different and some drivers are limited. It's really a test of your own to determine what works best for you. Monitoring instrumentation (SGII or other) if you have it is the only real way to get it as close to 100% efficient as possible.

Without driving your route with you it's like teaching Driver's Ed. in the classroom with no actual road experience to compliment it. Telling you to drag the brakes for 600 feet might produce the best regen for you but it will NEVER produce the best fuel economy (ever).

If I knew I had to stop at a light in 600 feet I would crawl (without brakes) as long as I could in hopes that the light would change during my crawl and, at the very least, I did not have to use any brakes before traffic started moving again OR I would coast with no brakes until I had to lightly use the brakes or stop. Any time you can continue moving (even 1 mph) you are in a better FE setup than braking to a stop.

 

I know what you are talking about, its a stop sign

 

What he said. The only time you should come to a stop is to comply with the law or to avoid hitting something.

 

If I knew I HAD to stop I would drag the brakes vs. coasting without brakes and then hard braking (leaving enough distance to gently coast to a stop). There's a break even point with regen capture vs. damaging/overusing your brakes over time with hard breaking.

If in your scenario you are estimating you need regen braking to avoid the ICE kicking on to regen the battery for you...I see what you're going for here.

If your goal is to waste as little energy as possible and to try and recapture every little bit off the regen braking as possible...you're really splitting hairs as most of the energy you are trying to capture is lost when braking anyway. By most, I mean the difference in energy capture between dragging the brakes vs. hard braking is minimal at best (again, not caring what effect braking has on your vehicle parts).

The regen braking feature on the Prius is extraordinary...incredible really. But, sometimes this "cool" feature muddles the mind and makes it seem more important than it is in the overall operation of the car. In very few circumstances does the average driver need to be concerned with being perfect at braking to keep their battery at a sustainable level (if this is you...I'd go with a light brake to stop). Unless you are in extreme driving conditions where your battery is being taxed beyond it's capabilities...your focus should be on what's coming up after your stop (EV coast out of the stop, traffic, etc).

Good luck...I'll be looking for the glow of your brake lights!

 

Well, it's also helpful to stop when you're ready to park and get out.

 

bac,

Here is what I do if I know I will have to stop:

Gentle brake to get down to below 42 mph, then
glide with no arrows, letting the road slow me down, then
gentle brakes if I need to slow down quicker.

The mechanical reasoning is as follows:
1) turn off engine spin which eats 2 kw
2) avoid conversion losses of about 15%
3) avoid friction braking, power bleed > 20 kw

You can estimate 20 kw by accelerating from a stop with a full battery SOC in EV. The speed rate going up is the maximum rate you want to slow down.

Don't let any of this distract you from your *real* payback: Anticipating road conditions as ahead of time as you can.

Have fun, and happy hypermiling

 

You *stop* at "stop" signs ?

 

Agreed! I get the idea of trying to maximize regen that he's looking for...but without actually measuring with testing it's really impossible to give him any sound advice other than to not unduly wear out his brakes. And, I'm not sure I would want to maximize regen anyway...too much regen trips the ICE and forces it to burn off the extra engery anway.

I avoid regen as much as possible because under normal operation (for me) the car regens the battery enough to keep it above 3 bars and below max (so as to not trip the ICE). I have no stop signs on my commute (only a few lights) and I often have 0 regen cars on my MFD at the end of my commute (and that's a goal of mine every trip to work).

I get the question of maximizing regen but unless it's a dire battery circumstance it's really splitting hairs for very little result...maybe that's the case...who knows?