Evidence that most regen is due to braking

[jamarimutt]

For the past couple of weeks I have been driving with the bars display on, instead of the energy flow diagram. Try it, you may like it.

I've noticed that I usually get more little green (regen) cars during low-mpg 5-minute periods than during high-mpg periods. I frequently see a short bar packed with 1.5 or 2 cars, and yesterday, for the first time, I saw a short bar with 2 cars plus half a car outside and above the bar!

The above strongly suggests, as expected, that most regen is due to braking, and comparatively little is due to coasting in D (which produces tall, mostly empty bars). This is what I've read all along, but it's nice to see the evidence in the display.

[Eisenson]

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.

[daniel]

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.)

[8AA]

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.

[DaveinOlyWA]

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

[daniel]

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.

[Speedracer]

Originally posted by DaveinOlyWA
actually the brake petal pressure controls the level of regen.

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.

[DaveinOlyWA]

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.

[Eisenson]

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.

[8AA]

Originally posted by DaveinOlyWA
actually the brake petal pressure controls the level of regen.

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.