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Discussion in 'Gen II Prius Fuel Economy' started by beckerman, May 26, 2010.
Is it ok to shift into neutral when the car is moving?
You won't hurt anything, but it is not good practice and it won't give you better MPGs.
OK. Thanks. At least I know how to do it if the car suddenly accelerates.
Actually, if used during deceleration instead of coasting, it will save fuel, assuming the shift occurs when the ICE (internal combustion engine) is already off. (The ICE remains in whatever state it was in upon shifting to N until shifting back to D.) The idea is to make full use of kinetic energy to move the vehicle instead of the considerably less efficient regeneration from coasting.
I routinely use it for the glide segment of pulse & glide, with fuel economy results at least as good as conventional gliding (i.e., by feathering the go-pedal just enough to remove regenerative drag).
Be aware that in many states, neutral is illegal (though not inherently unsafe, IMHO) on downhills.
Also remember that the car will suddenly accelerate if you step on the gas pedal.
Well, that's a great answer. I might add: if you do keep on stepping on the gas pedal, it will only accelerate up to a point. So there's a drawback...but would be nice information to someone trying to keep a Prius accelerating.
It won't accelerate if the car is in Neutral when you step on the gas pedal!
Or if you are in park and stopped...
If your not getting high 50's mpg, then your probably not at the level of car control, or in the driving enviorment that Prius N gliding will actually improve your fuel economy.
N gliding gives improved mileage when there are long glides between 20 and 40 mph. The faster MG2 turns, the more current is drawn from the battery, in a D glide. In a N glide the current draw is close to zero at all speeds. The N glide saves this current draw. In a Pulse and D Glide pattern, the battery will not stay at a steady charge level, but drain down. Then the engine will not shut down reliably going into a glide, as the car works to keep the battery with in charge limits. With a Pulse and N Glide pattern, its much easier to avoid running the battery down. If there are long uphills in the right places of the route it becomes possible to avoid running the battery down even though one is using very short pulses. This then avoids the recharge mode the car will get in, where it will not go into a engine-off glide.
Neutral down hill coasting in a manual or traditional automatic transmission vehicle can be very dangerous in long downhill terrain. Above a certain engine/transmission speed mismatch, the mechanical gearing wont reingage. An extreme amount of skill is needed to rematch the two speeds when vehilce speed is high. This is why de-geared downhill coasting is illegal most states. An example of the consequences is detailed in the Harry Chapin song "Thirty Thosand Pounds of Bananas".
Right on jimbo...I use it frequently on a daily basis and the proof is in the pudding. If you want the highest mpg you will use a mix of all techniques including pulse and glide, neutral coast, warp stealth, ev, brake regen...the list goes on and on.
Most drivers are scared to use neutral coast..but its legit. I only use it under 60 mpg...but that's my own preference.
Another place where n-gliding is handy is on a downhill when you slip into "n" just before you hit 42MPH so that you can go over 42 MPH without the ICE spinning. I've got one downhill I travel (speed limit 50 MPH) where I regularly do this. I easily can coast in "n" at 50 to 55 MPH whereas in drive it will either start the ICE or drain the battery to run that speed.
Remember that this is not really recommended driving practice. It's safer in the prius than a regular car because there is no fear of loss of power braking or steering (For example if costing in neutral in a manual your ICE could stall and you never even know about it. Not until you found you had no brakes and perhaps no steering). Also remember that you've only got friction brakes and no regen in "n". Hypermilers using n-coasting will usually flick between "n" and "d" to control the speed on a decent.
As I say it's not really recommended safe driving, but trying it will sure as hell will get you familiar and confident with using the shifter.
As people have already pointed out, there are practical times to slip into [N] for fuel economy. Alone, it will not provide huge fuel savings, but when done properly and coupled with other techniques, they will all add up.
One place to definitely not shift into [N] is when decelerating at speeds below 40. Let the regenerative brakes capture energy and slow you down. Sans emergency situations, if you find yourself using the brakes with the car in [N] you're doing it wrong.
Doesn't the ICE have to spin so as to protect the MGs from over revving?
Here's a whole thread on it:
Yep so you shouldn't use this trick if you want to go over 65 MPH or MG1 might exceed 10,000 RPM. I've read that many people say that MG1 is rated to 10,000 RPM despite the fact that the electronics usually limits it to 6,500 RPM. Personally though I'm not really comfortable doing this over 55 MPH.
I'm not sure what happens (if the ICE does eventually spin if you go fast enough) but I do know that when I do this at 50 to 55 MPH the ICE definitely will stay off provided that I make the shift to "N" prior to hitting 42 MPH. If however I make the shift to "N" above 42MPH (even if it was in warp stealth mode prior to the shift) then the ICE starts up and idles all the way and I can see it using fuel on the iMPG display.
Thanks! And here it is:
I won't risk my car but some adventurous drivers may.
About 3 years ago I was emailed a mathematical formula fleshing out the Speed/RPM/MG Spinning threshold (it was 3 years ago so I can't provide it for you). The threshold as provided to me indicated 63 mph was the point at which engine overs-pinning "could" occur.
I've also spoken with Hobbit on this issue and he claims it is a fallacy (and I'm sure he can provide volumes of information to support this claim).
Taking both opinions into account I do not use neutral coast above 60 mph...my own choice. Depending on any specific driving condition when you consider using neutral coast...very rarely will neutral coast at that high of speed be of great help to mpg in a Prius (in an ICE-only vehicle I would recommend neutral coast at these speeds).
Often during highway driving when a long downhill is approaching and the driver may want to use neutral coast it may be more beneficial to use the downhill as a "pulse" section so any impending uphill sections can be glided up rather than engaging the ICE. Additionally, depending on battery SOC...the downhill coast in ICE may result in very positive results to gain energy for upcoming sections of driving when battery-only mode (not EV) can be used from a full charge for a great distance.
FYI - when shifting back from Neutral to Drive...be sure to have your foot off the GO pedal or you'll get a bump when the ICE re-engages.
Hi Matt. According to eahart's simulator (see Toyota Prius - Power Split Device ) MG1 hits 10,000RPM at 65 MPH and the relation is approximately MG1_RPM = 154 times MPH
Uart...thanks for the follow up post.
65 mph...the reason I don't do it over 60 mph since a downhill neutral coast will inevitably take the vehicle past 65 mph.
Also, assuming most highway drivers are operating their vehicle in the 50-65 mph range on 55-65 mph speed limit roads (serious hypermilers anyway)...a long down hill stretch of neutral coast puts you in danger of driving at excessive speeds and in the "speeding ticket" zone.
What is more contradictory than a hypermiler getting a speeding ticket?