Happy?

When the engine is running, it burns ~1/5th a gallon of gas per hour (engine overhead.) What the Prius does in 'D' is automatically turn off the engine every chance it can. But 'B' (engine Braking) defeats that logic, it keeps the engine running and burning gas. In ordinary traffic, the engine goes off and braking converts part of the vehicle energy into battery charge to use later . . . keeping the engine off and saving gas.

I grew up with manual transmissions and there are a few, rare times when 'N' makes sense. In the first 1-2 miles, the engine is trying to warm-up. Shifting into 'N' lets the engine turn over in idle at 1/2 to 2/3d the 'D' mode. But you need instrumentation, like a Scangauge to see this effect. In contrast, the PIP let the engine warm-up in minimum fuel burn mode.

Let me suggest what John1701a recommends,'Just drive the car', and not worry about fuel efficiency for now. The engineering term is 'benchmark' the stock performance. Order a Scangauge (PriusChat store) and when it arrives you'll have instrumentation to figure out how to optimize performance.

GOOD LUCK!
Bob Wilson

 

When you apply the brake pedal lightly to regenerate, you're not using the friction brakes at all, but instead (partially) recovering energy and storing it in the battery for later use (assuming the battery isn't already full from a large downhill). If you avoid hard braking, normal moderate-speed "stop and go" makes little use of the friction brakes, so isn't "harder on the brakes." By using "B" needlessly, you defeat the objective of the car's regenerative braking capability.

Nobody said air conditioning can offset MOST of the need for "B" on a major downgrade, but if it offsets a significant portion, that's still worthwhile, and it's free cooling.

We have hills like that in the East, too. Not exactly the same, but amply steep, long, and curvy to fully justify using "B."

Otherwise, I agree with several very good points you've made in this thread, which turns out to be an interesting one.

 

That isn't how I still read the quote to which I was replying:

 

I'll guess that full blast A/C cooling is at least 3 kW. That is a lot of braking if applied over the entire downhill. On the other hand it would not be effective e.g., as a way to quickly slow down before a hairpin turn.

What is a steep descent ? 10% grade ?
Some quick and dirty math says that at 30 mph potential energy adds about 3.5 kW in a 2% grade

 

By my quick and dirty figuring, 3 kW is not much braking at all, not even normal air and mechanical system drag at highway speed.

2% is not much of a grade, it will give just a good moderate speed glide with no braking whatsoever. I doubt that the car would roll up to 60 mph. Add another 1% increment of slope, and the AC ought to provide all the braking needed to keep speed under control.

I consider steep to be 5 - 7%. The former covers many pre-Interstate highways where I grew up, the later is the maximum legal slope of a modern interstate highway. And both figures are well represented by many examples.

A Prius liftback descending Vail Pass (I-70, Colorado) at speed limit (70 when I was last there) with full rated load will be converting potential to kinetic energy at a rate of about 38 kW. I'm guessing that ordinary mechanical friction, tire rolling resistance, and air drag will consume no more than a third of that, other braking measures must take the rest.

That hairpin turn (or boulder just dropped off the hillside) demands friction brakepads, regardless of the situation. When the aforementioned Prius hits the brakes at full force at highway speed, the brake pads and discs will briefly be receiving about 500 kW. Nothing else in the system can handle anything even remotely close to that.

 

It's not the hills around Flagstaff, it's the ones closer to Phoenix after Camp Verde and through Bumble Bee Pass north of Black Canyon City, 6% grades. And they're not really annoying. It's just that if I turn off cruise control, I will speed up too much and need to hit the brakes. I don't mind going 80-85 through Bumble Bee pass, but it does take some work, especially if traffic is heavy. So rather that hit the brakes, I shift in/out of "B" mode. I don't know what's better; cruise, "B" mode or the brakes. I never thought about the impact on mpg, etc., and was under the impression that using the shifter simply put the car in "B" mode sooner on long downhill runs. I knew that once the battery is filled, it's wasted recharge energy, but I thought is saved the brakes at the expense of running the engine. It never dawned on me that it affected mpg because the engine gets turned on.

I also shift to "B" mode around town when traffic slows abruptly and there isn't enough space to glide to slow. Using "B" often slows me down enough so I don't have to use the brakes. In those cases, the engine is already running, but I suppose "B" increases rpm and uses more gas, something I didn't think about. I haven't been using "B" mode as much, but maybe I need to avoid it even more. I'm still not really sure when the friction brakes kick in and how to minimize that while still being able to stop safely. I do watch traffic and slow as much as possible as soon as possible.

 

Right, I was trying to put a practical unit on how much "braking" full AC can consume. If 2% grade is gentle, 4% is moderate and 6% is going down a graded mountain, then heavy AC use turns a fairly steep grade into a "moderate" one. Air, road and mechanical friction eat up another ~ 2% grade, so the driver has to decide what to do with the 2% grade remainder.

Some goes into the battery, and some goes to kinetic speed. Pass those sinks, and it is friction brake time.
This is why I try to start my descents at a slow speed.

"Annoying," as in require friction brake use

I remember the road you are thinking of. Loonnnng descents.

 

So what do you do on a freeway where the speed limit is 65 through the hills, slow down to 55 as you reach the top so you won't speed up as much going downhill? I just leave it on cruise and shift to "B" if cruise can't keep up. If I turn cruise off, I have to hit the brakes and I assume at that point it's the friction brakes doing the work.

 

Exactly. I have been known to drop to 50 mph if traffic allows. The right hand lane and trucks help.
In the Colorado and NM mountains where most of my driving takes place, my speeds wander between 50 - 75 mph. Mostly the speed variation is due to "driving with load (DWL)" for better fuel economy, but I very rarely use friction brakes. Well, except for once a month or so in the winter to keep my rotors from accumulating rust.

For the arithmetic inclined, kinetic energy is equal to m*v*v*0.5
75 mph is 33.5 m/s
50 mph is 23.3 m/s
And my Prius (v)agon weighs about 1500 Kg

So in joules I reserve 750(33.5*33.5 - 23.3*23.3) = 434520 joules
That takes a drop of 434520/mg = 434520/15000 = 29 meters = 95 feet

 

and take it out of eco mode. the a/c use will go up a little.

 

If the battery is filling up, then there is no MPG difference between the three methods. The engine spinning is being driving by the gravitational power of the descents, not by gasoline. The only MPG difference comes if you pick a method that prevents the battery from filling up, (or fills less, on shorter hills that can't fill it up) when another method could have topped it.

While slowing, B doesn't use any more gas to spin the engine. It spins from energy diverted from regeneration, wasting some power that could otherwise have gone into the battery. The extra gas gets burned later, when the battery drops sooner than it would have if that energy had been saved.

The simplest guideline is to watch the CHG portion of the HSI indicator bar, when it moves left during braking.

A reasonable approximation is that if it isn't maxed out, then you are using regeneration only, not friction. Once it reaches the maximum, then any extra braking force is from the friction pads. Note that the regen limit varies with speed, with lower speeds taking more braking force to reach the regen limit.

 

A very good explainer of B-mode can be found here http://techno-fandom.org/~hobbit/cars/b-mode.html

In summary:

 

Filled the battery several times between Phoenix and Houston, only used "B" mode once this time though. I think I just got used to using "B" mode driving through the hills in the Kalispell area and got in the habit on any downhill run.

FWIW, we averaged 38.1 mpg from Fort Stockton to San Antonio this time vs the 34.8 mpg back in February. Nothing scientific, but the differences this time: 37 lbs in tires vs 40/38, wheel covers off, ECO mode, speed limit vs 3-5 mph over and ambient temperature (AC vs heat). Still not sure what any of that really means, just find it interesting.

 

I read somewhere on here that "B" mode spins the engine with no gas. I've been all over the west and have seen several big rigs with hot brakes on some of the hills, never seen an auto on the side of the road with brakes smoking. For me, "B" mode works when it needs to automatically, the Prius is not complicated to drive, works fine on its own. If I had read this thread before buying a Prius, I would have bought something else, sounds too complicated to drive.

 

This thread? Good thing you didn't read all the hyper-miler tips before you bought?

Reminds me that my 15-yr old grandson said he didn't want to drive it because he was watching me use "B" and didn't understand it.

 

If you want to see or smell car brakes smoking, hang out a bit at the gift shop next to the mandatory brake checkpoint on the Pikes Peak road in Colorado.

 

Are YOU sure you bought a Prius?
Because since Gen 2 The Prius hasn't had a trunk.

 

Been there, done that, interesting how little some people know about how to drive their cars.

 

Just use "B" when you are going down a steep grade to keep from "riding" your brakes. That is what it is for. Don't use it on level roadways.

 

Vail pass isn't much of the slope. With 4 people in and all camping gear our 2010 was comfortably maintaining 70mph up on cruise control. You have to get off the beaten path in Colorado to see double digits. The OP ventured in Sierra Nevadas, and there are sustain grades in low teens (even 26%+) in some places.