Pulse and Neutral?

Putting the car in "N" is effectively the same as pushing in the clutch at any speed. It does not hurt the engine, traction battery or anything else. As long as the engine is running, say you are going 42 mph or faster when you do it, no problem. But there is one exception and there are local driving laws that ban shifting into neutral when "on down grades."

If you are traveling under 42 mph and the engine is off and then roll down a grade in "N", you can cause MG1 to exceed the 'design limits.' Now those of us who have 'tickled the sleeping dragon' know that nothing happens instantly ... there is some head room. But so far, no one has reported the 'forbidden experiment,' an actual failure from doing this. There are two hypothesis.

The first is that mechanical forces could cause MG1s rotor to disintegrate or fly apart. My NHW11 (2003 Prius) has been to just over 50 mph in "N" with the engine off and the MG1 rotor still seems to be working. My understanding is the NHW20 model could go to ~65 mph with the engine off before reaching the 10,000 rpm limit ... Hobbit would have more details.

The second hypothesis is the inverter circuitry might not be able to manage the high voltages generated by a too rapidly spinning MG1. This could lead to just burning up the inverter and MG1 stators or possibly going into an uncontrolled traction battery charge mode.

We have competing hypothesis of the failure mode; several experimental 'tickling of the dragon' and no field reports of the failure mechanism. So generally speaking, I don't risk my car anymore.

One last thing, I don't do pulse and glide except around or under 25 mph in my neighborhood while the engine is still warming up, the first 5 minutes. Otherwise, I use cruise control or if hilly, pedal to 'drive a constant load.' I will shift into "N" if approaching a distant stop but it is because I know I will be coming to a stop.

Bob Wilson

 

I think it has something to do with the way the fuel injectors work in neutral vs. gliding or at high speeds using very little battery power like the long coast when approaching and taking an exit. There is a post about this somewhere but it's been so long since I read it that I don't remember where to look.

In a nutshell I believe it was better to perform a false glide (act like you're going to glide but because your speed is too high you'll engage the hybrid battery but the drain is so low that it is negligible) than it is to switch to neutral. I could be wrong though.

 

You're in an area that I try to avoid so I would recommend posing the same question over at CleanMPG.com. There is a recent SAE paper on the subject, Jeongwoo Lee, Douglas J. Nelson and Henning Loshe-Busch in their paper "Vehicle Inertia Impact on Fuel Consumption of Conventional and Hybrid Electric Vehicles Using Acceleration and Coast Driving Strategy" (SAE 2009-01-1322) and they had this to say about it:

So please don't take my understanding as any sort of endorsement. That I use it on a neighborhood street at speeds under 25 mph while the engine is warming up is not an endorsement. It has to do with engine warm-up fuel consumption.

Bob Wilson

 

Here is how neutral works.

If you select neutral with the engine running, it stays running and uses gas to do it.
If you select neutral with the engine not running it stays not running.

That seems simple.

So if you are above 42 MPH and select neutral, then the engine is burning gas to stay running. If you use warp stealth, then the engine is spinning, but using no gas. This is usually the more efficient technique.

There is a mode refered to as Warp Neutral. That is where you get the ICE (gas engine) to turn off in a glide below 41 MPH (usually confirmed with SG) select neutral and let your downhill speed increase to above 42 MPH. Since you selected neutral with the ICE off, the ICE will not come on above 42 MPH unless D is selected.

I use this technique all the time and I make sure to stay below 62 MPH because that is the speed at which MG1 will overspeed with the ICE off.

Warning 62 MPH is where MG1 will overspeed and use this technique at your own risk.

 

Bestmapman,

Is there written documentation about the 62mph and MG1 overspinning...what is your source? I'd like to read more about it if it's available.

I know that above 40 mph in neutral the engine is protected as it keeps spinning (confirmed with my SGII) but have never heard of 62 mph neutral presenting an over-spinning issue with MG1. Would love to read more about it if you know where.

 

Because coasting with the engine running uses little gas but more than if the engine was off. So with the MFD 99mpg is as high as it will go. Using SG the iMPG might show 300mpg or even 2000mpg instead of the 9999 it would show if you were using zero fuel. I have not plugged my SG in for some time though so I may be wrong. lol

 

If your gf laughs at you for getting a SGII see if she's laughing when the money you save each month in gas costs buys her a new dress or takes her out to dinner!

 

On the hills that I can start below 42 MPG, it saves the small drain on the SOC. On the highway usually above 42 MPH I use warp stealth. I live in a hilly area and there is one freeway onramp that I enter that is downhill and continues for 3 miles downhill. I enter the onramp in EV mode and select N about 32 MPH and get all the way up to 60 before I have to tap the brakes to stay below 62 MPH at the bottom of the hill. I then select D and it immediately starts the engine and power is available immediately.

 

It is explained well in this thread.

 

There is plenty of documentation, in particular at Graham's Toyota Prius, Graham Davies paper.
In the Prius trans-axle, the fixed gearing means that the rotational speeds of the ICE, MG1 and MG2 are in constant relationship to each other, according to the following equation:

MG1 = 3.6 x ICE - 2.6 x MG2.

In addition, MG2, the main traction motor is related to the road speed of the car by the following equation:

MG2 = 59.1 x V (mph) ... where V is the road speed in MPH. (It varies slightly as the rolling radius varies with tire pressure)

So when the ICE is off (=0), MG1 = -2.6 x MG2.

Thus at 62 mph, MG2 = 59.1 x 62 = 3664.2 rpm.
So MG1 = -2.6 x 3664.2 = - 9527 rpm (rounding the fraction), which is close to its' published limit speed of 10,000 rpm.

Note too that it is also rotating backwards and acting as a generator.