Anything wrong with this technique....?

Coasting that way above about 45 mph, the engine normally doesn't burn any fuel at all, at least not directly. However, because power from the wheels is being used to twirl the engine, it does retard forward progress slightly, much like light regenerative braking.

 

ScanGauge normally shows at least 0.2 GPH (gallon per hour) when the ICE really is burning fuel, and 0.02 GPH or less when that error occurs.

 

I recall reading somewhere that the MG1 RPM limit actually has to do with the electronics. Even in N, you still have a magnet spinning inside the windings, which is going to produce voltage proportional to RPM, but no current because you're in N. Above 6500 RPM MG1 isn't going to physically fly apart, but it will expose the electronics to higher voltages than they were "designed" to handle (ie, for extended periods on a regular basis, as opposed to an infrequent "oops I shifted to N").

In D, MG1 can act as a generator and the resulting current will produce an opposing voltage. This will lower the voltage the electronics are exposed to at a given RPM, and thus allow a higher RPM to be safely achieved assuming a significant amount of power is being drawn.

 

That's correct, although I can't recall ever seeing as low as 0.2 GPH when the engine is running on its own power. When the engine is being pushed by the wheels (i.e., when coasting at speeds over about 45 mph), sometimes my ScanGauge correctly shows 0.00 GPH, and sometimes shows an erroneous leftover figure around 0.32 GPH. The only practical way I've found to confirm fuel is not being injected in that case is to watch the coolant temperature. If it isn't, that temperature slowly drops as the engine cools. You won't see the temperature fall if the coast distance is too short.

 

Yes, and, as I understand, that's why MG1 has a higher speed limit spinning forward as a generator than spinning backward when you're coasting. The maximum speed at which the system allows us to coast in D with the engine stopped, about 45 mph, results in MG1 speed well below 6500 RPM, as a safety margin.

 

I would just use the car as intended by Toyota. Meaning normally. You'll get great fuel mileage downhill regardless. 100+ mpg. What app you guys using? I have an ELM 327 BT device and iPhone 6.

 

Why bother using B(rake) mode? I leave it in D and when the battery is fully charged the engine will automatically use B on long downhill segments.

 

That is just wrong, wrong, wrong.

For a longer description, read this thread:
priuschat.com/threads/3-500-foot-decent-crazy-hot-brakes-and-burning-smells.157731/

 

Regenerative braking, if I'm correct, is 100% electrical, and once the battery is maxed, it stops entirely.

You can press the brake pedal and get more "regeneration" without engaging the pads (about half the CHARGE bar segment).

However, to what I know, the pads never auto-engage on any standard Prius braking system. That's why there's a B mode. Most automatics would have the same issue with brakes if the driver didn't think to shift down into 2 or 1.

Think of it...on ANY adequate downhill run, the Prius goes FASTER if you take your foot off the gas. Regenerative braking is used to capture excess kinetic energy, not to slow you down.

 

I believe some folks are referring to the light engine compression braking that cranks up when the battery is maxed out in 'D' mode. They are confusing this as being the same as 'B' mode compression braking. But there is a huge difference.

Following some disputes on other threads, I recently watched while descending a long grade at 60 mph in Eastern Washington. After my battery topped out, 'B' mode stabilized with strong engine braking at 46xx RPM. (And lots of 'jet engine' or 'vacuum cleaner' roar.) Shifting to 'D' caused the engine to slow to 20xx RPM, with greatly reduced braking force that allowed the car to quickly roll up above 65 mph. This still represented some engine braking, but far far less than 'B' mode, and not nearly enough to control the car's speed on that slope.

The 'D' vs 'B' misunderstanding of some small-mountain drivers has contributed to some newbie confusion, and a least two cases of brake damage for a big-mountain newbie. For details, see the thread I linked above.

 

Well, that's "old school" tech for you.

Low compression motors (which most every vehicle that's not a performance car or motorcycle) DO NOT produce very effective "engine braking" when you let off the gas. By comparison, a high compression motor produces a lot more "engine braking" because of the higher compression. Just letting off the gas results in a natural resistance that will recapture forward momentum.

If I'm correct, "engine braking" increases resistance to existing compression by blocking exhaust flow...creating back pressure. The Prius may do this in addition to finding a way to increase intake air flow. Typical diesel motors draw in the same amount of air whether fuel is applied or not. So, restricting exhaust outflow does the job. Regular ICE motors DO NOT have inflow if there's no gas, so enhanced "engine braking" likely finds a way to maintain air intake so there's the compression but no addition of fuel.

 

Were you responding to someone??

 

My understanding of gasoline engine 'compression braking' is that it is just maximized 'pumping loss'. There is no exhaust restriction.

A strong vacuum is created in the intake manifold, generally by restricting the throttle while the engine is spinning at substantial RPM. The engine then pumps a large volume of that vacuum up to atmospheric pressure, a process that requires significant power. Increased airflow into the intake would actually reduce the braking effect by reducing vacuum level.

 

Most of the energy to compress air in cylinders is recovered during the subsequent power stroke, even in a non-firing engine (in good condition). Thus, "compression braking" is mainly just engine friction, combined with some pumping loss due to the throttle, etc. Obviously increasing the speed at which the engine is forced to turn increases the power it absorbs. I don't know whether Prius software manipulates the throttle to maximize throttling loss in "B" mode.

 

The pumping loss component should be fairly easy to compute, it is simply the volume pumping rate times the pressure differential. For a normal 1.8 liter engine spinning at 4600 RPM (a number I recently witnessed on a downgrade) with 0.8 bar of intake manifold vacuum (from memory on a previous car, I'll have to pull up the Prius figure on the ScanGauge next time):
Pumping loss = [4600 RPM] * [1 minute/60 seconds] * [1 full intake displacement / 2 revolutions] * [1.8 liter] * [1 meter^3 / 1000 liters ] * [0.8 bar] * [100kPa / 1 bar] = 5.5 kW = 7.4 horsepower.

Because of the Atkinson-style late valve closing, the Prius' effective intake displacement is less than 1.8 liters, probably closer to 1.4 L, dropping the pumping loss to about 4.3 kW. Not huge, but quite significant when descending a hill. And it is in addition to all the mechanical friction losses.

My memory of past Toyota descriptions is that the Prius does adjust the valves for maximum braking effect.

 

Easy for you maybe...

 

... and that was only the greatly simplified order-of-magnitude estimate. There's a lot of other stuff going on with air flow that's harder to compute theoretically.

 

I know some Prius owners and drivers are much more attentive to feedback from their vehicles. Much more aggressive about applying techniques to driving to improve efficiency.

Sometimes when I read about the contortions and lengths some drivers go to, to squeeze out an ounce of real or perceived higher efficiency, my reaction is WHY? Really Why?

I'm not Prius expert enough or engineer enough to say whether this application is potentially damaging long term, short term or not at all.

But my approach is just to drive the Prius...as The Prius. I trust the software, hardware, and the whole HSD system.
Which means, I only use "B" when descending long down hill stretches. I don't use it as a downshift, or simply to slow the vehicle.
And it means, I use "Neutral" as "Neutral". I don't drive the vehicle in neutral, even if it is possible.

The Prius does have an electric motor, an e-CVT transmission that is planetary gear based, and regenerative brakes.
I'm going to trust that the system by design knows how to use and protect those systems better than I.

Part of the reason I became interested in Prius, and part of the reason I bought one, is out of respect for the engineering and design of the machine itself. I'm not going to put that at risk either greatly or minimally for what may only be a slight gain in MPG's for a brief period of time.

This personally held opinion I suspect will NOT stop anyone with the desire to experiment.

 

I could not have said it better myself. Wild technics that could put unknown wear and tear on the car may cost thousands in lost reliability years later, while only saving pennies in fuel.

In another thread a Prius owner that lives in the mountains has a car with 188,000 miles and burns 1 qt of oil every 600 miles. It's not running well and he wants advice about if it is worth putting money into to fix. Using high power to climb the mountain, then using "B" to go down the mountain every day for 188,000 miles is high wear and tear. Not that that could be avoided in that environment. He said he didn't abuse the car.

So, I agree. Just drive normally and let Toyota's ECU programming take care of the car.