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

From Wayne's description of the trip:

• Interstates: 50 to 60 mph <-- This is where I said goodbye to 100 mpg somewhere in Arkansas on the 40 IIRC
• State Highways: 40 to 55 mph
• County and state roads: 35 to 50 mph
• Towns and cities: 25 to 40 mph
• Climbing mountains with truck climbing lanes: 20 to 35 mph
• During overnight hours with no traffic, I dropped down another 10 mph on the county and state roads
• Off-road: 10 to 15 mph

 

[Deleted]

 

You seem to be assuming that the electric MG1 is freewheeling. I don't believe there's any evidence to support that.
When the battery is full and can no longer take a charge, the excess energy must go somewhere. It can either burn the brake pads off or it can be dissipated as air heated by compression.
I didn't write the laws of physics.

 

With a warm soak, I easily get 75+ mpg actual at 55 mph (57 mph and 80+ mpg on display) at the sea level and 42–43-psi tire pressure with my 2021 Prius Prime Limited. At lower speeds and higher altitudes (less aerodynamic drag) and 51-psi tire pressure, 93 mpg if not 100+ mpg should be attainable.

 

Those are your two contradictory misconceptions* next to each other.

As you said about the first misconception, "If MG1 were allowed to freewheel, the permanent magnets within would create drag to the point of braking."

Yet you discount that as a braking mechanism for your second misconception. Why do you need engine braking if MG1 provides sufficient drag?

And where does MG2 come into this? You think MG1 freewheeling produces "drag to the point of braking". Do you think MG2 freewheeling produces "drag to the point of braking" or not?

Pro tip before you make up an answer: since G3 added the MG2 gear reduction, MG2 turns faster than MG1 when the engine is stopped. Only in G1 and G2 was MG2 lower rpm than MG1.

Indeed.

* (nit-picking edit - the second one there isn't a misconception on its own, the sentence above is correct - including that you can't usefully dissipate energy with "MG1 freewheeling braking". But it's the foundation of his misconception that the car won't initiate engine braking in D to produce drag effects if required).

 

Oh, no, not the engine braking (or lack of it) again.

 

What do you believe happens in the neutral-with-power-cut experiment that you proposed in #28 and that I conducted in #30?

 

Magnetic braking is real. Got a fishing reel that makes use of it, and it is the reason that some AWD BEVs have poorer range and efficiency than others. Those BEVs have more powerful magnets in them than a hybrid. The braking is still there in the Prius, as is the friction of the PSD, but the force is a fraction of what the motors apply when generating electricity to mimic an auto transmission coast.

Heard finding that glide spot has become harder in the later generations. Just aim for a little EV power, and you'll see better fuel economy. Seems the hypermilers prefer Hyundai's hybrid system these days.

I do better than the cruise control in Toyotas, Fords, Chvies, and Subarus. CC tends to have a heavy 'foot' when accelerating and braking; it is more on/off operation than variable. It also is unaware of situations where losing speed is okay; going up a hill when there is a down slope on the other side.

 

Ok, what do you believe happens in the neutral-with-power-cut experiment that Paul proposed in #28 and that I conducted in #30?

The MG rotors are built with permanent magnets. The MG stators are built with copper windings.

Do you expect a relationship (via Lenz's law, perhaps, or Biot-Savart) between any "magnetic braking" force and the current that is permitted to flow in the copper windings?

 

I believe the drivetrain drag would be multiplied by the gearing ratio, resulting in sudden braking. That's why the system runs MG1 to compensate, simulating a "neutral" condition.

How do you put the Prius in neutral to push or tow it? | PriusChat
Towing a Prius | Towing on a Flatbed.
https://www.justanswer.com/toyota/mgkd5-put-2008-toyota-prius-neutral-fuse-box.html

Of course you can dispute all these links as you did before, but there really is no "neutral in any Prius, new or old, because they use the same basic transmission. If it were possible to coast in neutral by allowing MG1 to freewheel, all these sources are liars.

 

Got them on ignore so only see a portion of their posts when the thread convo gets too broken up.

But to empathize what I said. Magnetic braking in a MG1 is only enough to be a hinderance in hand cranking it. It in no way will hold the entire in car place. I don't see the difference between it on a Prius in neutral and the sloshing of fluid in an automatic being worth measuring.

It is only a concern in a secondary AWD motor of a BEV because minor inefficiencies have a bigger impact. And then we are comparing the 30 to 40 kW MG1 to a 200+kW motor; much greater magnetic forces involved. To be perfectly clear, all these motors are free spinning when un-energized.

 

Sounds like you think magnetic braking dissipates energy into nothing. It all has to go somewhere, usually in the form of heat. You probably can ignore it in a fishing reel, but not in a 2 ton vehicle barreling down a hill.

 

How do you picture that "runs MG1 to compensate" happening when the system power is cut, as in the experiment that you proposed in #28 and that I conducted in #30?

Excellent, we agree there.

My miter saw has a clever magnetic braking feature. When you release the switch, it (a) disconnects the windings from 120V, and (b) shorts the windings together. The heavy internal current that's allowed to flow because the windings are shorted results in a strong magnetic force stopping the blade posthaste. If the windings are just left open-circuited instead, the blade coasts to a stop.

 

Another point, while we're at it. In your fantasy world, why would the system run MG1 to compensate for drag?

It's MG2 that is connected to the wheels, so any drag compensation would logically need to be done there (just as the drag simulation in D is).

Force applied on MG1 will turn the engine as well as the wheels. You do know that applying torque at MG1 is how the engine is started, right? And the engine is easier to move than the car is, which is why that works. So if you really had to apply sufficient force at MG1 to counter "sudden braking", that engine would spin right up. (Albeit backwards).

Edit: Although I've realised this whole thing is incoherent. For the same reason, MG1 alone couldn't supply enough drag torque to cause sudden braking in the first place. That drag torque would spin up the engine too (forwards) before it managed to slow the car. MG1 resisting backwards motion is how the engine starts when you're moving in EV mode.

You really need to start talking about MG2 magnetic braking if you want your fantasy car to stop without power.

 

Quite the "fantasy" which is backed up by facts and evidence. If your position requires ignoring it, maybe yours is the fantasy. How about some evidence that MG1 can freewheel, creating "neutral" unassisted by power?

 

Funny how those who actually try to tow a Prius, can never get it to coast in neutral unassisted.
Maybe because "neutral" requires power being applied to MG1.

 

[Deleted]

 

I'm sure there was a bit of heat generated in that split-second there. If not, you deserve the Nobel Prize for being the first one to make energy disappear.

 

You proposed an experiment that would show that.

The experiment you proposed was conducted, and the car took the same 3 city blocks to roll out from 15 MPH with power cut as with the car powered in neutral.

What now?