Shifting into neutral

How far does the prius go in N?

so far for me about 2 mi.
I try to figure out how to make it roll 3 mi.
so far no go that far.

gotta pump up tiers and see if I get 5 more ev mi. or mp-ev

 

This depends more on location and geography than anything else.

In my driveway, both my Prius and the household's nonhybrids will go about twenty feet, stopping at the same place where rainwater tends to pool.

From Dante's Viewpoint in Death Valley National Park, my old inefficient gas hog with substantial rolling resistance and air drag should have been able to coast in N nearly all the way to the Furnace Creek Visitor Center, about 25 miles. Because some braking is required to stay on the road on the turns, the Prius could probably extend that only to 26 or 27 miles.

Perhaps you would like to better define your question?

 

Yeah, that is what I was getting at.

25 to 27 miles sounds nice. Have you ever tried donner pass? I would like to drive both Dante's Viewpoint and Donner if I ever get out that way again.

Ok how about this?
How far does your prius go in N?

 

I never experimented with coasting in neutral with my Prius Plug-in, but I don't see any reason to do so with any hybrid. Coasting is giving away free stored kinetic energy that would normally go towards charging the hybrid battery, and dissipates it all to air and rolling friction instead.

In any case, there is no actual "neutral" in the Prius. The engine remains connected to the wheels at all times, never disengaging, even in "neutral." For the car to appear to be free-wheeling with the engine stopped, motor-generator 2 (MG2) must run backwards in exact proportion to the rolling speed of the car.

It's hard to imagine any scenario where this would increase either (EV or gasoline) overall mileage.

 

I like seeing how far the car will go and I like using the shifter like I'm driving a indy car
I'm not trying to convert you Greg

 

I never argue with someone who just wants something to do with their hands. I only discourage N to those who want improved MPG. (I find myself shifting back into D from D at stop lights, just habit from owning a manual)

 

The only reason I can think to shift into Neutral is to roll a tire into contact with a curb when parking on a hill.

 

The Donner Pass descent includes a number of uphills, not quite short enough to coast over the top from a legal or safe speed at the preceding bottom. EV may get you over most (maybe not all without a PiP), but N won't.

The Dante's Viewpoint descent has a stop sign in the middle, inconvenient located on the only flat spot. A true N descent would require blowing through it, but EV mode would easily cover it without burning fuel in the ICE.

 

Not trying to be a pest here, but neutral in a normal car is nothing like "neutral" in the Prius. The car actually has to work to give the appearance of coasting.

 

Correct, the shift knob is the same, but the actual transmission works totally different.

 

I turned on the heater today for the second time, yuck.

 

The Prius only needs to shut down the field excitation of the motor/generators, turning them into freewheeling flywheels. That hardly counts as actual work.

 

A couple of thoughts/comments...

- MG1 and MG2 are permanent magnet motors, there is no field winding that can be de-energized.

- My understanding is that in neutral the MGs are "unloaded" allowing them to freewheel. They may generate voltage, but without a load there is no current flow and consequently no power generated / consumed. I'm not entirely clear how this physically implemented in the electronics.

- Coasting (in neutral or by gliding in D) is inherently more efficient than regen. During acceleration power from the engine and/or motor is used to accelerate the mass of the vehicle to a given speed. Some of that energy is lost to various losses (electrical, energy conversion, friction, etc). The rest is "stored" in the mass of the vehicle as kinetic energy. When maintaining a constant speed, the energy produced by the engine / motor is strictly used to continue overcoming losses, the kinetic energy stored in the vehicle mass is constant for constant velocity. When drive power from the engine / motor goes to zero, the kinetic energy stored in the mass of the vehicle is in essence converted to "drive power" to continue moving the vehicle forward. Due to the losses still present, this kinetic energy store is gradually consumed and the vehicle slows to a stop. Adding regen braking provides an additional loss from a mechanical perspective, depleting the kinetic energy store more quickly. However some of that extra energy "loss" has now been stored into the battery for future use. At best if the regenerative braking, battery storage, and electric drive systems were 100% efficient then the two cases would be equal in overall efficiency. In other words you could either use the kinetic energy stored in the vehicle to carry it forward now, or to accelerate it later with equal overall efficiency. But since the regenerative braking, battery storage and electric drive systems are inherently less than perfectly efficient, the overall efficiency of the regen and use later case will inherently be less than the gliding/coasting to a stop case.

- In theory it would seem like coasting in neutral and gliding in drive (by feather the pedal to stop the ICE and zero out the regen drag) should be equivalent from an efficiency perspective. However I do believe that using neutral has several down sides, compared to gliding in drive:
1) If you switch to neutral before the ICE has stopped, my understanding is it will keep idling unnecessarily as long as you are in N.
2) If you switch to neutral at high speeds, and/or accelerating downhill the ICE is unable to start if needed to keep MG1 rpms in the safe range.
3) If you have to brake while coasting in N, any regen energy that would have been reclaimed is thrown away.
4) As your supplemental electrical loads (AC, lights, etc) pull down the HV battery SOC, the ICE will be unable to start if needed to keep the SOC within the normal range.

In my opinion gliding in D generally makes a lot more sense than trying to use N. It should be just as efficient, and has no strange / potentially harmful operating conditions to worry about.

Rob

 

The rotating part uses perment magnets, but the fixed part uses electromagnets. In N, they are not powered.

 

The permanent magnets are on the rotors. That windings are on the stator. Neutral is implemented by de-energizing / disconnecting the stator windings.

Maybe I should not have said 'field' windings.

 

This brings another thought to mind. The motor-generators are limited to 10,000 rpm, which is why the ICE must start up at 62 mph (100 kph) to protect the MG from over-speed. With the ICE stopped, there is no alternative but for the MG to spin the opposite direction, while the vehicle is in motion.

Which brings me to this question: If you are traveling 63 mph or over and you shift into neutral, what happens?

 

That sounds right. I tend to think of a brushless DC PM motor like a brushed DC traction motor turned inside out. The field is generated by the magnets in the rotor, and the "armature" windings are in the stator, so commutation is provided by the drive signals rather than through brushes contacting the rotor. I'm an EE though, this MechE stuff always makes my head spin

 

At 63, the engine is running, it stays running as long as you are in N, it stays running.

The risk is if you coast from 61 to 63 and up.

 

There's some good stuff in here, which I haven't figured out yet. I think it applies to a pre-Gen III Prius, because the top speed at ICE zero-rpm is only 42 mph.

 

Yes, gen 1 and gen 2 could over rev M/G 1 at about 42 MPH, gen 3 and later add a second planetary gearset to slow M/G1