Prius Doesn't Coast Very Far When You Let Off On The Gas

It definitely starts up in Neutral. I was in a car wash that required the car to be in neutral, as it was pushed through the car wash. It was easy to hear the gas engine start up. It had no effect on the rolling motion at all, The only way this could be accomplished is by running the MG1 in reverse to prevent the torque from the gas engine going to the wheels.

Time to be shed of the fantasy that MG1 has nothing to do with creating zero torque on the wheels to simulate "neutral." Both power sources are directly connected to the drive wheels, and can not disengage, ever.

 

Considering how MG1 controls the torque to the wheels, including zero torque while in neutral, It's nonsensical to think that MG1 is not used to roll the car in Neutral.

 

From the Gen 4 Prius Prime new-car features manual.

 

That's not wrong, there is very little torque being transferred from MG1 to the drive wheels when in Neutral, but it doesn't say it's freewheeling, especially when the gas engine kicks in. Whatever the state of either power source, in Neutral, the job of the computer is to provide zero torque to the drive wheels, by controlling the rotation of MG1. MG2 sets the drive ratio of the planetary differential, acting as a CVT, and never produces direct torque to the drive wheels. It also allows MG1 to run in reverse as necessary.

The document describes the net effect of using Neutral, not how it works. If you would have us believe that the computer is not actively monitoring the torque to the drive wheels while in Neutral, that goes against the evidence.

You can call me a liar, but there are several conditions that can cause the gasoline engine to start up when the car is in neutral.
1. Battery needs to be charged.
2. Heater needs heat from the gas engine.
3. CHG mode is activated.
I have seen all of these things happen while in Neutral. To keep the car from moving, MG1 must counter the rotation of the gas engine.
If MG1 is freewheeling, as you seem to think, why would the computer not be doing its job while the gas engine is off? Are we to believe that for some reason, the computer is disengaged while the car is in Neutral? What evidence is there for this?

When the car is in Park, it's also in Neutral, but with the parking pawl engaged. The only difference from neutral is that the car won't roll. In that case, MG1 has little to do but monitor the torque on the drivetrain. The gas engine may be doing all the work of driving MG1 backwards, which it must do, because the drivetrain can not disengage. It makes more sense for the computer to be controlling MG1 instead of strangely letting it freewheel.

 

This is an official manual written by Toyota. It is clearly saying that the electrical power to both MG1 and MG2 is entirely cut off in the neutral gear. If what you were saying (the MG1 and MG2 power are tuned for zero torque in the neutral gear) were the case, it would be explained in glory detail by the new-car features manual, as that’s what the new-car features manual does. However, on the contrary, what you are claiming contradicts the designers and builders of the system at Toyota; therefore, it is simply wrong.

 

Has Paul got MG1 and MG2 the wrong way around all this time?

I guess that answers my question about why the obsession with MG1 and ignoring the more relevant MG2.

 

I may have named them incorrectly, but that doesn't invalidate what I said.

 

I think you have ignored a great deal of evidence to focus on your own interpretations. Please answer my question about what happens to the electric drive motor when the ICE kicks in. You claim it won't, but mine definitely kicks in all the time, for reasons previously explained.

More importantly, why would the electric motor completely disengage when in neutral, as you claim.

 

I'm going to withdraw from this impasse for now, to educate myself and understand exactly how it works before commenting further.

 

Okay, I guess we can go back and try to dig through some of Paul's earlier work, having swapped in MG2. Even if we can't get through to Paul, there is some interesting detail here.

First, on the things Paul claims to have experienced - I strongly discount everything he claims to have seen his car do as quite likely made up, given his track record. But I'd like anyone else to chiming in on their experiences of Neutral engine start conditions. I haven't experienced any of those, but then I've hardly spent any time in N, unlike Paul who is apparently in N lots. I will have a go at reproducing.

But those points are an irrelevant tangent to the main point which is that MG1+MG2 don't need to push to get a neutral effect - which is of course why Paul is so interested in them. If the drive system is getting more relaxed than it used to be in previous generations about when it's prepared to use MG1+MG2 for engine starts, that doesn't change the mechanics of the power split device, which has not changed since G1.

But we can look at what happens when the engine does start. The engine is started by MG1 pushing forwards. That produces a forward torque on the engine and a backwards torque on the wheels. To ensure the engine moves, something needs to resist the torque at the wheels harder than the engine resists. In the earliest generation of Priuses, I'm not sure there was any MG2 drive during the start, rather than just relying on pushing against the mass of the car (and being held in position by the parking pawl and/or foot brake).

In a G2 you would feel the car rock slightly as the engine started in P if your foot was off the brake. In current generations, I believe MG2 does push forwards to balance out - making it a bit smoother so you don't notice this.

The reason there's something to balance is that you've got two other inputs applying significant torque. (But still not that significant - engine spin-up torque is still significantly less than car-moving torque).

But that's only worth doing (even then only as a refinement, not a necessity) during the startup when MG1 is pushing. Once the engine has started, MG1 can freewheel again - that's exactly how one decouples the engine from the motion of the wheels.

The PSD functions like any standard open differential - if one connection can spin without resistance, the other two are effectively disconnected, as any input power goes to the spinning connection.

And sure, if the hybrid system is on while in Neutral, it will be monitoring the system - checking for faults, and ready to start the engine if any engine start condition arises. But it doesn't need to tell either MG1 or MG2 to push, as just having a loose MG1 and a heavy car is sufficient to let the car move freely with the engine having no significant effect.

Anything it did choose to do would be fine-tuning at most. As ChapmanF measured, there's no discernible difference between the neutral coast in hybrid system READY versus non-READY. That's the evidence.

A test I'm wondering about is engine-running neutral, with the wheels off the ground. At that point you've now got two loose connections to the PSD - the wheel resistance could be low enough that MG1 is no longer effectively zero drag by comparison, and the PSD could start to split torque meaningfully again, so the running engine might be able to start the wheels spinning in the air. Does the system do anything to stop the wheels turning? If it didn't, the torque would be small - you'd be able to just hold the wheels in place by hand, redirecting the motion to MG1.

Note you'd have to hold both wheels as they have a differential too. If any car was in D and pushing forwards with both wheels off the ground, you'd be able to hold one wheel still so the other one spins. Exactly the same as you holding both wheels so a freewheeling MG1 spins in N.

 

Give it a rest. Let's just withdraw and reconsider our arguments.

 

Traction control will step in on that test if it hasn't been disabled.

 

The eco driving mode only changes the throttle mapping and A/C power—nothing else. However, since you have more range/precision on the gas pedal at low throttle in the eco mode, you will have more control over regenerative braking.

 

Neutral means neutral: No power of any sort is applied to the drivetrain. This is the definition of neutral. There is no other definition for any vehicle. There is no such thing as tuning the motive power by software for constant-speed cruising at neutral. That would be cruise control—not neutral gear. This is explained in the new-car features manual I posted above. You can also easily verify on the power gauge on the display that there is no negative or positive power at neutral.

In the unlikely scenario of the engine kicking in in neutral to recharge the battery through the MG1 with the car moving, it is somewhat complicated. This is what the new-car features manual says about when the engine recharges the battery with the vehicle stopped:

“HINT: When the vehicle is stopped, negative torque is applied to the motor (MG2) to prevent the tires from rotating.”

So, with the vehicle moving, they would still have to apply a negative torque to the MG2 to keep the engine from sending a net power to the wheels.

If the battery is fully charged, the engine would simply turn off, and the six bipolar transistors to all three phases of both the MG1 and MG2 would be switched off to entirely cut off the electric power to them, and there would be no power applied to the wheels, as it should be in the neutral gear by definition and as confirmed by the new-car features manual.

 

What? Is this a feature of the Prime? I was not aware that one could hold in the HV/EV button to enter "charge" mode. How does this work? Is it true for Gen 4 Primes as well?

I assume this is Prime-only as my 2024 Prius does not have an HV/EV button.

 

It is Prime-only, and yes, the Gen 4 Prius Prime has it.

 

I don't know about the Gen 4, but I didn't have it on my Gen 3. Instead of that, some people actually thought they were saving fuel by hypermiling (accelerating and coasting to build up battery charge). Actually, I think the opposite is true.