Regenerative Brakes question... (silly?)

so that means that the way i am "gliding" is correct then? or should i change something? (sorry but i am still a newb when it comes to prius and driving techniques and all...)

 

Once again you fail to understand basic physics. I've tried to help with some of your posts in the past, but I grow weary with the effort. Despite being an engineer and having taught college level physics, my skills as an educator are insufficient. Hopefully someone else will pick up the baton and try to make some sense.

Tom

 

Sounds great. 50% is better than ZERO. Good generalising, which is the best we can do!

 

No, Dominic, you are not there yet. Your pulses need to be stronger, up near or even into the PWR section of the HSI bar. The ICE doesn't even come on until the HSI bar crosses the center point, and you want to use the ICE at optimal efficiency which requires a decent amount of throttle.

Then, you describe what I call coasting when you are seeking to glide. When you lift off the throttle, the HSI bar goes a little into regen, creating wasteful conversion of kinetic to chemical energy.

edit:
Of course, true P&G is only up to 45 mph. Above that you have warp stealth which draws a little from the pack to turn the ICE, which must spin to protect MG1 from excesssive rpms.

Instead, feather back into the throttle to make the HSI bar dissappear completely. This is akin to being in neutral.

For a nice primer see Hobbit's write-up: HSI

 

I hope so too. What we need is real world physics AND real world math, comprehensible to the average Prius owner. Which one point do I fail to understand? we'll take them one at a time. Not for the sake of argument. but for the benifit of all PC'ers
:rockon:

 

Hello Dominic, You are or seem to be discussing getting into warp stealth for maximum mpg on any surface. I was advocating the use of the brake pedal on downhill or approaching stop situations. Two different subjects.

 

Hello Lonewolf, you need the rest of your pack back to assist. Both of your paragraphs are superfluous and not relating to anything, especially the fine Toy product we are priveledged to own. Why did you initiate a new thread, that was already in timely progress??? This is against the posting rules. AND does not in any way add to the benefit of the subjects progress and cohesiveness.

 

Andy, you are better off (ignoring speed limits for now) letting the car pick up speed which is then used on the next climb, than you are using regen braking. That is qbee's point, as I see it. Putting it into and back out of the pack wastes about 50% of the energy that gravity is offering.

At the speed limit, by all means, fill your pack by regen and obey the law. But don't be deluded into thinking it improved your mpg. It hurt your mpg less than friction braking or even engine braking in a conventional car, but it reduced your Prius mpg below the optimal level of avoiding the pack.

 

oh ok... sorry about that... as i said, i'm really a newby when it comes to the prius... hence why i hang around here to try and absorb as much as i can... but some of the posts are just way too technical for me! lol

 

I'll try.

Gliding is the most efficient use of your momentum. Regen is lossy.
E.g. say a glide decelerates at 1mph/s and braking at 3mph/s.
Then at 30mph going to a stop you can glide for 30s or travel at 30mph for 20s and regen for 10s. Yes you get some charge but you've been consuming power for 20s before that.

Similarly, lighter braking is better than heavy braking.

 

That is a great policy, keep hanging around. There is a wealth of knowledge on PC.:cheer2:

 

I agree with 99% of what you are saying, but coming down the mountain at 90mph in a 65 zone is not in my safety program. So I would judiciously slow it down. ty

 

I totally agree.

 

Exactly, as I said in my post you are replying to. But regen hurt your fuel economy compared to letting it roll. Glad you are still alive. Be safe.

 

Let's say someone gives you a big sack of rice. You have two choices. You can keep the sack, and use a couple cups per day to feed yourself and your family. Then you spend less on buying food.

Or, you can sell the sack of rice to a restaurant and get some money. The problem is, the money you get is taxable, so you have to pay tax. You can use the money to buy rice later, but you'll never be able to buy back the whole sack...the money you paid as tax is lost forever.

So you can use kinetic energy to move the car forward (or up a hill), but if you regen brake, you pay a tax. Some of the kinetic energy is lost as heat during the conversion to battery energy. So, when you try to get it back, there isn't as much there as you started with.

While some kinetic energy is storable and redeliverable, some is always lost. So, your best bet is to regen brake as little as possible, but if you have slow down, some recoverable energy is better than nothing. Friction brakes are like a 100% tax...you lose it all.

However, one little "regen car" on the display (50 Wh of energy) is worth about 1 cent in gas savings, so don't rear-end someone trying to stay off the friction brakes. $10,000 worth of damage is equivalent to 1,000,000 little car symbols.

 

Thankyou for the lengthly explanation, which I agree with. This new thread got cross threaded with my original thread which stated that I was astounded that after rolling down a moderate hill with slight braking ( regenerative). The average mileage on the consumption screen at the bottom of the big grade was 90 mpg! When I got home after an additional 25-30 miles of mixed driving, my average mpg read 58.6 mpg. This was the best reading I ever obtained for a short trip. 47 miles. My aim was not to produce those 50Wh cars but to see how high I could get the average mpg on the consunption screen. Please see my "Brakes =Mileage" thread. Regeneration was a side issue, and because of this LIGHT brake use I believe I boosted the mpg reading. I like your sack of rice analogy tho!

 

My wife commented once on a long drive that I wasn't doing very well mileage wise because there were not very many "little cars" on the economy display.

I had to explain to her that the "little cars" were rather like a booby prize.

Thus began the long discussion of regenerating electric energy. My wife is about as far from a "gearhead" as one can get, she has many other redeeming qualities, but does not know which end of a screwdriver to grab.

I demonstrated to her by speeding up and then immediately slowing down, rather than cruising at a steady speed.

Physics is fun, after that long discussion she now understands.

 

Way back, someone (probably EFusco) found out from Toyota that the wheel to battery to wheel efficiency is 30% for the Gen II. Don't know about Gen III.

 

Attila's study tells us it's around 50% efficiency.

Ken@Japan

 

Another way to think about efficiency is to consider all the different places energy can be in your car. It can be in the gas tank as gas (chemical energy), it can be in your car's momentum (kinetic energy), it can be in your car's elevation (gravitational potential energy), or it can be in the traction battery (also chemical energy).

I recall seeing somewhere that the energy efficiency of the engine is ~35%. This assumes it's being operated in an efficient range. So any time we convert the chemical energy of gasoline to some other form, we lose 65% of it. As noted above, the electrical system has something near 55% efficiency. Any time we convert any form of energy to the chemical energy in the battery and back, we lose 45% of it. So if we convert gasoline to kinetic energy, we get 35% of it, but if we convert gasoline to battery to kinetic energy, we get .35*.55 = 19.25% of it. Ditto for if we convert gasoline to kinetic to battery to kinetic.

So ideally, you'd want to run the engine at the most efficient point for just long enough to gather enough speed to coast to a stop right at the next red light. Now, that's not terribly practical, since we don't know when the next light will turn red, or exactly what speed is needed to coast to that ideal stop, and there are people on the road (including ourselves) that wouldn't be very happy coasting for a long period of time at speeds well under half the speed limit. So we accelerate more than we really need to to get to the next stop; the extra energy can be saved in the battery, just not with perfect efficiency.

Now there are times that using energy from the battery is more efficient than using it from the engine; specifically, at very low and very high power requirements. The Prius is smart enough to take care of that automatically, for as long as it is able; if you just need to creep a bit, you're better off using the 19% efficient path than the engine, which would be far from its 35% peak, and much worse than 19%. Similarly, if you floor it, the engine burns extra gas with little extra power benefit, reducing the efficiency. So to get the best acceleration without hurting economy too much, the Prius will draw some power from the battery. In both cases, the efficiency is better off by running the engine closer to the peak efficiency at some other time to keep the efficiency to no less than ~19%. However, it's also true that it would be even more efficient to not ask for those inefficient conditions, but rather to drive in a way that always uses the engine at its peak efficiency.

The only reason the Prius needs a big battery is because it's not always possible to drive with peak efficiency, given desired acceleration rates, surrounding traffic, stop signs and lights, etc.

The moral of the story: For best economy, try to use only the engine, and use it only in its most efficient range. When that's not possible or reasonable, the Prius will do its best to help you out anyway.