Evidence that most regen is due to braking

Regeneration is an extremely inefficient process, though it's better than the 100% loss when friction braking converts kinetic energy into heat and dissipates it into the passing air.

Whenever your foot touches the brakes, you're losing. There is no more efficient use of that built-up kinetic energy than to convert it into distance by coasting.

As I improve the instrumentation of the car, that becomes more and more clear. The "bars and cars" are a worse descriptor of what's going on than the instantaneous MPG in the other screen. When it shows "99.9mpg" the ICE is usually OFF - there isn't much better MPG than that.

Increasing coasting requires planning, strategy. Of course, the same strategy works to improve MPG in all cars, but in the Prius we talk about it more.

 

If you really want to see a lot of green regen leaves (or cars), stomp on the gas at every stop sign, get it up fast, and then stomp on the brake hard at every stop. You'll get lousy mileage from all the extra gas you burn, but you'll see a lot of regen leaves.

Remember that those low-mpg segments represent a lot of gas burned, and, as Eisenson points out, you only get part of it back in regen. Reneration is a fantastic feature. But what you want to maximize is your mpg.

What I think is cool is when you get a segment with so much electric mode that it shows 99.9 mpg for the 5-minute segment. (Of course it's meaningless, because you've just used electricity that will have to be replaced by the ICE later, but it's still cool.)

 

For true "coasting" you have to put the slightest amount of pressure on the gas pedal. If you don't, the system will start regenerating and the car will slow down. This is an intentional design feature to make the car behave like a conventional car.

 

actually the brake petal pressure controls the level of regen.

here is a graph of the overall braking force and the ratio of friction braking verses regen braking graphed over time.

as you can see, even in emergency braking regen is still working provided the vehicle speed is at least 8 mph. at speeds below 8 mph, it is all friction braking.

to feel a demo of this. get in your Prius go somewhere you have room and accelerate to at least 20-30 mph. then brake lightly. concentrate on maintaining even braking pressure (this is why you need room!!) you will feel something similiar to an automatic transmission downshifting right at 8mph. that is when regen shuts down and friction takes over. this will give you an indication of how effective regen slows the vehicle.

and yes, i love those 99.9 mpg segments too!!

http://priuschat.com/forums/album_pic.php?pic_id=718

 

I have tried and tried and tried, and I cannot feel the transition.

That graph makes no sense to me. Nor does the description at the left.

 

I notice lately when ever exit the freeway, & brake hard before toll booth gain alot of kinetic energy. But no energy gain when driving on freeway, City driving is more mpg gain than driving on freeway.

 

when looking at the graph the total stopping power of the braking effect is shown by the red line.

the gray area is how much of the stopping force is from the hydraulic brakes and the blue area is the amount of braking force that is the result of regen.

as you can see, with light braking force it is all regen.

the graph is a bit confusing because it is graphed verses time and its not clear here, but the farther to the right you go the longer the duration of the foot on the brake.

iow, if you were going 40 mph and brake to a stop in 3 sec. you would be mostly on the right side of the graph and most of the braking force will be hydraulic in nature. at the point of maximum braking force (where the red line levels off) you can see that hydraulic braking force is 60-70% of the total.

but if you were to brake evenly and come to a stop in say 10 secs there is a point where both the red line (total braking force) and the blue line (regen braking force) are parallel. this happens at relatively low speed as you can see it comes rather late in the braking time frame. but you can also see that this results in regen accounting for near 90% of the braking force.

the reason for this is that regen is nearly as effective in stopping the car as friction brakes but regen doesnt work as fast as friction brakes. so when you first jump on the brakes say in a panic stop, most of the initial stopping force will be friction. but as you remain on the brakes and slow down over time, the force shifts over to regen power until its nearly all regen power until you get below 8 mph.

that is why you get much better regen and mpg when you do light gradual braking.

the one thing that took me a bit of getting used to was the fact that the brake pedal goes nearly to the floor because of the travel required in the pedal to gauge braking urgency. but i never really noticed it at first because i knew about the regen braking and knew that the lighter i braked the more it would be regen and not friction.

but since the pedal isnt connect to a pivot arm that controls linkage connected directly to the master cylinder, the brake pedal can act this way giving the driver better control over their braking.

 

I have a prototype of the E-Coastal Brake Display, which shows status of braking with relative friction and regen. The same nifty little module has a red LED that tells you when the engine's running, which is not so obvious at 60-70mph.

Bottom line, keep your foot OFF the brake pedal unless you have no choice, and your MPG will improve. Get up to speed quickly, and use your kinetic energy to coast pretty much to a stop. Within limits, of course - you have to consider the potential road rage of that low-rider behind you.

 

It is true that the amount of brake pedal travel does determine the amount of regen. What I was referring to was that when you take your foot off the accelerator the vehicle immediately goes into a slight regen mode. This was done to better simulate the engine braking in a conventional car. If you don't believe me, give it a try with the energy monitor on and you will see energy going from the wheels to the generator and back to the battery. The trick for true coasting is to apply the slightest pressure on the accelerator pedal, which will turn off this regen mode.

 

You're right.

So the question is whether the net fuel used is lower when you coast without regen (use the kinetic energy to coast further), or coast with regen (put some of the kinetic energy back into the battery for later use).

I suspect that coasting without regen is more efficient.

In either case, when friction braking occurs it's a nonrecoverable loss.

 

Dave:

I understand what you are saying. But I don't see it in the graph. My problem is that graphing time vs pressure makes no sense. I want to see velocity vs pressure, and then, for each point on the graph, the ratio of friction to regen. Of course, you'd need a 3-dimensional graph. Or a grey-scale graph. Velocity on one axis, pressure on the other axis, and the friction:regen ratio displayed as a greyscale from white to black, with black being all friction and white being all regen (or vice versa). Or divide the graph into discrete squares and print the ratio in numbers within each square.

 

it is more efficient to coast with no brake pedal pressure. the reason is that regen isnt a 100% recovery system. realize that regen is "free" energy in that it would normally be wasted. but in saying that, it isnt a lot of energy.

what you need to judge is

1)can you coast without having to use your brakes? if so, this will provide you the most mpg in the long run.

OR

2) if braking is neccesary, the lighter the braking, the less momentum lost, so less energy will be needed to get the car back to cruising speed.

the Prius gas mileage comes from a huge combination of things including the CVT, low drag, efficient use of the hybrid system, computer control, etc.

so regen doesnt come no where near recovering the amount of energy used even when taking into account friction, wind resistance, etc.

you are familiar with the car symbols on the comsumption screen. each represents 25 watt hours or regen. that is what it takes to light a 25 watt light bulb for an hour.

now if anyone has any idea of how much EV only energy is needed to travel say a mile at 35 mph, that would be nice to know.

i tried to get a basic idea of what that would be, but that calculation proved to be more complicated than i realized.

 

On my coast down the mountain See post, "interesting results", on the second page in the main forum, in pure regen mode I was doing about 12-14 amps max, step on the brake and I could get to a max of 57amps charge, once I got the car to 80% BSOC I couldn't regen at all.

 

I've just returned from a trip where I attempted to drive for economy, but doing my usual mix of motorway and mountain driving, and I got 64.5mpg, which is very close to the book value of 65.7mpg (UK gallons, obviously). To do this, I tried to avoid regeneration bars as much as possible, meaning lifting off the throttle as much as a mile in advance of each roundabout or other junction, and "travelling with the land" - slowing right down near the crest of each hill so as to be able to use the "free" acceleration on the downhill side. Route choice can also help - short, fairly steep uphills are better for Prius economy than long drags, but on downhills the reverse is the case - long, gentle downslopes allow the ICE to tick over at >100mpg.

Anyway, on that trip, I had hardly any regeneration symbols - quite often entire half-hour screens with none at all. As others have said, if you're getting regeneration, it means you're recovering a portion of the energy an ordinary car would be throwing it away, but only a portion - if you can reduce the amount available to regenerate, you're using the energy more effectively first time around.

For me, I tend to see most regeneration when I see lots of >100mpg bars - in the mountains. I have seen up to 3½ leaves in one five-minute segment (out of 12 leaves and 4 >100mpg bars on the display), but I've never seen a leaf above the top of the bar.

 

sort of off the topic but sort of on, i have noticed as we have gotten a little cooler weather here and i have the a/c off that i am able to get a much better start-up in electric only mode, regularly getting to about 20 mph before the ICE kicks in.

with my heavy foot, still staying in the 40's but inching up

 

This topic was addressed here. Maybe you can drive 1/4 mile in EV mode on one regen leaf??? Less if you are stopping and accelerating.

I think i get very roughly 1/4 mile on one battery bar, which the above link says is 1.3 leaf. But that's with a few stop signs.

Note please, however, that the Prius does not have a CVT, though they (inexplicably) use that term, probably because they don't want to have to explain it in the promotional materials. The power-split device is as unlike a transmission as Tang is unlike orange juice.

 

ooops!! just read my post... that should be 50 Watt-hours per car symbol not 25...