Using Neutral?

OK, I'll add a little here from my knowledge of the 2nd gen Prius. With the Toyota HSD design, the drivetrain is always fully engaged-- neutral in any Prius is nothing like neutral in any other car whether manual or automatic transmission. This means that even in neutral, there will be more mechanical drag in a Prius than in a normal car in neutral-- you're not going to coast quite as well as in other cars.

Shifting into neutral in a Prius or other Toyota HSD-equipped hybrid only electrically disconnects the two motor-generators from the voltage inverter which means that it's physically impossible for the electric motors to apply motive force to the wheels, and is also physically impossible for the electric motors to generate a charging current for the HV battery-- so no regenerative braking is possible either, and the system will switch over to mechanical brakes when the car is shifted into neutral.

This is important to know when it's time to brake, because if you plan on recapturing any of the energy you lose when braking, you have to shift back into D before applying any pressure to the brake pedal, because shifting into drive with any pressure applied to the brake pedal won't re-activate regen braking.

The same phenomenon applies in the gen 2 Prius if you're driving with the cruise control set-- you have to cancel the cruise control first, before applying the brakes if you want to use any regen braking while slowing down. The alternative there is to briefly tap the brake to cancel the cruise control and let off the pedal completely, and then hit the brake pedal again to start using regen braking. This is because of the throttle/brake interlock that prevents the kind of unintended acceleration that caused such a ruckus a few years back-- applying the brakes with any throttle applied forces the system to use 100% mechanical brakes. It's also electrically impossible to do regen braking while also applying electric motive force to the wheels since regen braking is done with the same MG that's used to drive the wheels-- it can't try to move in both directions at once.

Another thing to keep in mind is that the Prius (at least all but the PIP-- I'm not sure mechanically how the PIP's HSD system differs if at all in terms of maximum MG RPM) can only go up to about 42 MPH without turning the ICE to prevent MG1 from spinning too fast-- it has a maximum safe RPM of 6,000, and at speeds over 42 MPH without the ICE turning, it exceeds 6,000 RPM and may cause premature failure. The PIP can go up to 63 MPH on all electric compared to the non-PIP Prii that can only go up to 42 MPH, but I'm not sure if it does this without electrically spinning the ICE to prevent MG1 from spinning too fast or not. Any gen 2 or newer Prius can go over 42 MPH on all electric provided that the HV battery has a high enough state of charge-- it just has to do so by turning the ICE using battery power through MG1 which draws about an extra 10 amps, or 2KW of power from the HV battery in order to spin the ICE without using any gas. That's about the same amount of power as the A/C will draw when you first turn on the A/C on a hot, sunny, 100 degree F day where the interior is much hotter than the set temperature. The non-PIP Prius will only do this when the HV battery is charged to higher than somewhere around 55% SOC or so, and I don't know anything about the PIP in this regard.

What all of that means in terms of coasting in neutral or learning how to properly pulse & glide in D is that if you shift into neutral at speeds below 42 MPH or so and then go down a hill that speeds you up over 42 MPH, the computer system has no way to start the ICE to prevent damage to MG1 because the electric motors that start the ICE have no electrical connection in neutral. It also means that the ICE can't manage the level of charge on the HV battery as it deems necessary, so if you're not paying attention, you could wind up with an HV battery whose SOC may drop well below the minimum safe level of charge if you're coasting for an extended period of time, particularly if you're doing something else that draws a lot of current like using the A/C on a very hot day. That's not as much of a problem with a PIP if it's been charged and you haven't exhausted your EV power yet, but you want to make sure you never lose track of the HV battery's SOC and make sure you never let it drop below 40% (of HV charge-- again, I'm not sure how this works in the PIP, but this applies to all the non-PIP Prii).

In reality, in a non-PIP Prius, you never want to let it fall below 51.5% SOC unless you're going to have enough regen opportunity to bring it up to a minimum of 53.5% (your SOC will normally drop below 51.5% while driving at 55-75 MPH because as long as you don't do a quick, panic stop, there's more than enough regen opportunity to bring it up over 53.5% when braking from that speed), because when the SOC drops below 51.5%, the computer switches to a rapid-recharge mode which runs the ICE at a much higher RPM than usual in order to quickly bring the HV battery SOC back to a more typical level-- which means you're burning up gas that you wouldn't have if you'd just let the computer manage the system instead of doing it by hand. When this happens to me, it can quickly shed all of the MPG I'd been accumulating on my trips-- I'll often see the trip MPG drop by anywhere from 1 MPG to as much as 4-5 MPG while the computer runs the ICE to bring the SOC back up to 53.5% where it switches back to it's ordinary operating mode.

In my driving, I'll switch between just normal gas pedal operated pulse & glide, and neutral glide, depending on the situation I know I'll be encountering, because I can manually adjust slightly better than the computers can in many cases. I've also learned how to manage the HV battery based on what my needs will be over the upcoming 5-15 minutes so that I maintain at least the normal operating mode and avoid going into the rapid-charge mode and wasting fuel. Since I'm a pizza guy, I get tons and tons of practice, which is why I know when it'll help me to use neutral glides vs. using standard P&G, but people who don't spend a fair bit of time learning and practicing will probably not want to dedicate all that attention to it for the meager gains to be had.

The mileage difference for me can be noticeable in terms of MPG per tank, but in terms of actual money spent per week, I really don't notice the difference most of the time at all. I might squeak out an extra 3-5 MPG by switching between neutral glide and normal P&G, but in terms of fuel consumed or money spent, that barely registers at all. The main reason I still switch to neutral glides personally is because I do a lot of stopping and waiting between deliveries, and I save gas by leaving the car on while I'm waiting for my next delivery if the delay will be less than 10 minutes on a cooler night, or up to 20-25 minutes in the summer time, because I don't have to go through the 50 second warm-up phase when I come back out to the car. The neutral glide technique helps me in this regard because it allows me to arrive back at the store with a significantly higher HV SOC than if the computer manages it, because the computer likes to keep SOC between about 54% and about 59%. It's rare with normal P&G in my usage scenario to reach over 60% on most deliveries, but with neutral glides I can get back to the store almost always with between 59% and 65% SOC, which buys me several minutes of waiting without the SOC dropping below 51.5% where I'll be wasting gas to get the SOC back above 53.5%.

If your driving scenario doesn't involve something like this, then you'll likely not really benefit much if at all from using neutral glides, but if you still want to play around with it, by all means, experiment away, because that's how I've figured out how to best use my Prius to burn the least amount of fuel on the job and save me the most money. Just remember all the caveats to it, because if you ignore those, at worst you could cause damage to your drive train that will cost a couple grand to fix, and at best, you'll not be saving quite as much money as you could if you know how the system actually works and what makes it more efficient vs. less efficient.

If you're driving a PIP, then I suspect more savings to be had by experimenting with switching between EV and HV than by switching between neutral glide and normal pulse & glide. If I ever get a PIP, that'll be the first area that I experiment in.

Oh, another thing I forgot to mention-- if the ICE hasn't shut down when you shift into neutral in a Prius or other HSD-equipped hybrid, the ICE will not turn off once you're in neutral. It will continue to idle while burning gas, which means that under some circumstances, you'll get lower fuel economy by using neutral than you would by leaving it in D where it can either turn the ICE off, or shut off the fuel flow and turn it electrically instead. This is another one of the reasons that most people shouldn't bother with neutral in a Prius, and one of the other complicating variables that you have to account for if you're going to play around with trying to get the most mileage out of your Prius.

 

Wow, that's a mouthful. My summary: neutral coast = bad... Unless you deliver pizza.

 

The post from babybird was one of the most insightful posts I've read on priuschat on the subject of neutral coasting. I thought I knew it all, but the point about the engine not turning off after entering neutral when the engine was already on, was another thing that needed to be added to my Prius Gen III knowledge base.

One other snippet I've learned this past winter is that during warmup below 42 mph when you need to coast by slightly pushing the gas pedal is that you are actually increasing fuel consumption (engine slightly speeds up) whereas putting into neutral allows you to coast with less fuel being used (engine will stay at idle). Once engine is warmed up this of course no longer applies.

I like using neutral because I can keep my eyes on the road instead of the HSI while coasting and I can then more accurately sense the deceleration force due to the regen when putting it back into D and slightly pushing the brakes as I approach a stop or slow down. Of course then I optimize the regen to give a 60 Amp recharge rate. Without a Neutral coast, which sort of zeros out the body's perception of deceleration, one may not properly perceive the actual deceleration force of the regen braking, and misjudge when to initiate optimum regen when one has to come to a stop. I'm sure many of you have felt times when the perception of braking from regen was very little, with the deceleration force inversely related to speed for a given regen rate (i.e. a given regen rate might which produce -0.02 g of deceleration force at 60 mph will produce -0.08 g of deceleration at 30 mph) an inverse squared relationship excluding aero effects.

 

This discussion about using Neutral should be completely moot to my fellow PiP drivers in California, where the motor vehicle code prohibits using Neutral while coasting, at least when coasting downhill on a highway:
V C Section 21710 Coasting Prohibited
How this could ever be enforced is another matter...

(Edit: OOPS.. I just realized I already mentioned this earlier in this thread. Sorry.)

 

I was going to weigh in.....but there's nothing I can add to the excellent posts above.

 

My first half mile is downhill. The ICE comes on unless I use Neutral for that half mile. After that its EV and HV as appropriate, no more Neutral

 

Why would the ICE come on if you are in EV in a plug in?

 

My guess (based on the behavior of my Gen II in the mountains) is that it spins the ICE to help control speed down hill-- similar to shifting into B mode. And since the battery is fully charged, there's no room for it to use the regen to keep the speed reigned in, so it spins the ICE. And judging from what I've read on the PiP forums, I gather that since it's spinning the ICE anyway, it just goes ahead and starts the engine warm-up cycle right then too.

These are all just my own guesses though, I don't have any sources that I can back that up with other than my own experience in my Gen II plus what some PiP owners have said.

 

The ICE comes on because the battery is fully charged after the over night charging and the ICE keeps the battery from over charging. I believe it is effectively using the B mode of operation. However, once the ICE comes on it has to warm up before it will let you go back into EV so it ruins the short trip mileage.

 

ok, I think I get it. What if you were to stop the charge just a little short of full?

 

You have no control over the charge when using the timer, it goes until it is full. It would be very inconvenient to do anything else and its easy to use neutral for that half mile.

 

How downhill is it? Do you have to use your friction brakes a lot?

 

I have to use the friction brakes for part of it. Is it worth it? I don't know. I just added an SOC gauge to my ScanGauge II display. Maybe I can more accurately determine when to let the regeneration start doing the braking.

 

Is there an alternate way that isn't too much longer (distance or time) that doesn't have the big downhill right at the beginning?

 

Clever question Actually there is and I do go that way sometimes but I'm not concerned about modest use of the brakes.