Anything wrong with this technique....?

Someone with more knowledge can answer definitively. However when in neutral no charging of the hybrid battery is hapening from the regen braking or inverter. If you let that run down too low with the AC, radio, etc running you will have issues. This is why the engine will start & run anytime you get down to about 2 bars. This issue most commonly happens when people have the car in neutral for an automatic car wash for too long.

BTW... welcome!

 

Above 46 mph, the ICE is supposed to spin in order to protect MG1 from overspeed. However, the Neutral position locks out the car's self protection mechanism, preventing it from being able to start up the ICE to keep MG1 rpm below its redline.

The Owner's Manual specifically prohibits driving in 'N' in order to prevent damage, but doesn't say why. This corner case, going into 'N' at low speed where the ICE is stopped then coasting up to higher speed where it would normally spin even without fuel, is the only case we are aware of that risks drive train damage.

There is some safety margin built into that 46 mpg ICE-off maximum, but we don't know just how much margin is really there. Without such knowledge, you are risking damage. And any such damage is likely excluded from warranty coverage, the ECUs can log your activity.

I believe the Plug-In Prii have different limits, suggesting that they also have some different hardware. If you are lucky, Toyota is putting that better hardware in all of them. If you are unlucky, ...

 

^ For a definitive answer about the MG1 safety margin, you simply are not going to get a definitive answer without being a Toyota insider. Some hard core hypermilers here and at CleanMPG.com have hinted at having better answers from their inside contacts, but it wasn't meant for public consumption so wasn't shared out in the open. Too much liability involved, and even they were taking on more risk by using that inside information.

 

Proper Prius glides do require light pressure on the gas pedal to make the HSI bar disappear, getting rid of that slight regenerative synthetic 'engine drag'.

You can still do Neutral glides without known (to the PC community) risk of damage, as long as you start with the ICE spinning. The slight fuel burn (probably 0.2 gallon per hour in idle mode) may not be satisfying, but does provide protection against redlining MG1.

 

No, no, no! I've never heard that interpretation before.

When ICE RPM = 0, MG1 RPM is directly proportional to road speed, the higher the speed, the higher its RPM. In this condition, MG1's official redline occurs at 46 mph (or 42 on a Gen 2, or slightly different speeds on the 'v' and 'c')

Any time the ICE is spinning, MG1 can spin slower. In Drive, the Gen3 Prius normal no-fuel spin rate is 992 RPM, allowing a significantly slower MG1 RPM. At really high speeds, the ICE can be (electrically) forced to spin faster to keep MG1 RPM safe.

But in Neutral, when ICE RPM = 0, the car has no way to get it spinning, so MG1 will be forced (mechanically through the HSD's planetary gears, not by any electrical drive) to spin as the car moves. Coast up above 46 mph, and MG1 will be forcibly overspun. But if the ICE is idling and spinning, its speed can be adjusted (by the normal throttle controls) to keep MG1 speed down to safe RPMs.

At highway speed, with ICE spinning, you should be able to shift between D and N at will, without hurting anything. Even attempts to shift to R and P are 'safe', as the car will recognize the error and instead go into N with a double beep warning. I've done the R shift many times, as a faster way to get to N (without the normal delay), as practice in case I'm ever struck with an alleged 'Sudden Unintended Acceleration'. (I have first hand experience with this in a past car, saved in part by hitting the clutch in a manual transmission. The Prius lacks that specific recovery mechanism.)

Now you are catching on.

 

So, the safe solution is to press the accelerator slightly to hold the HSI bar graph at the zero (disappeared) position all the way down your long downhill. Even though the engine is spinning in that circumstance, it's normally (with occasional exceptions such as if it's not warm enough) not burning any fuel. It is mechanically spun by the wheels.

Engine speed, MG1 speed, and the speed of the car are interrelated through a simple equation, based on the relevant gear ratios and tire size. If you know any two of those three speeds, you should be able to compute the other one.

 

Interesting! If my algebra (based on gear published gear tooth counts and tire size) is correct, the car speed above which the engine must spin faster than 992 RPM to avoid spinning MG1 faster than -6500 RPM should be about 85 mph. Similarly, engine speed must be limited at low car speeds to avoid spinning MG1 above +10,000 RPM.

MG2 is limited to 13,500 RPM, which corresponds to about 113 mph.

 

The MG2 limitation is from various earlier threads, which I'm sorry I can't find fast. There was a lot of discussion of such technical data around the time the 3rd-generation Prius was introduced. That speed also matches perfectly with the 113 mph limit on forward speed, considering relevant gear ratios. As for MG1, I do not remember its official speed limits, and was using your tentative numbers as examples. They appear plausible. Someone might jump in to tell us the actual speed limits for MG1.

 

Damage? No. Wise? No.

The point for having the Prius in gear is so IF something happens, all systems are ON.

I find that if you "glide" (press the accelerator enough to cancel out regenerative braking), you get a similar benefit, and when the ICE kicks in and stays on, it uses so little gas it is as if it's not even on in the first place.

I'd suggest driving as it's designed and not fret over the few pennies in gas you save by running downhill in neutral.