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

That's quite a lot of ersatz technical detail to handwave into existence behind a method of operation you have made up.

The thread you linked in post #16 is a fine old thread from 17 years ago, but nothing in it supports your idea that neutral requires the ECU doing any electrical drive of MG1.

I can see how, if you squint, you might read that into what Tom wrote in that old post of his:

But Tom had a deeper understanding of the system, and he was (correctly) saying that differences in "how the control system runs MG1 and MG2" account for all of the transmission's different operating modes.

It just happens that, for the operating mode called N, "how the control system runs MG1 and MG2" is, it leaves them both open-circuited and electrically dead. That's what makes N different from all other modes, where at least one of MG1/MG2 will always be under electrical control.

When MG1 is open-circuited, freewheeling it does not produce a large drag.


The frequent PriusChat refrain "there's no neutral, the gear configuration never changes!" could just as easily be "it's got nothing but neutral, the gear configuration never changes!" Seriously, if you showed this transmission to a savvy mechanic who could only see its mechanical layout, the mechanic would say "you have shown me a joke transmission. It is always neutral. There is no way the engine ever turns the wheels or vice versa. This free rotor in the middle just ends up spinning."

To understand how it ever does anything else, you have to understand the interactions of MG1 and MG2 under electrical control. To understand neutral, you don't.

 

You do have to have a feel for the forces involved.

Like ChapmanF, I had the experience of powering a motor-generator via a hand-crank connected to different loads, and that is something that makes you go "oh!".

We're used to electrical power spinning motors, so we sort of have an intuitive sense that motors provide force under electrical power, and they don't push without electrical power.

But we're not so used to interacting with generators, so we don't have an intuitive sense that turning of generators requires the driveshaft to supply the electrical power.

You connect a light-bulb to a generator, and you can feel it. An open-circuit motor-generator has a little spinning resistance, but it's nothing compared to a generator attached to a load. If you're attached to a 60W light-bulb, you've got to provide 60W of mechanical power. And that is a lot. I think far too few people have attempted to hand-power electrical loads to understand how big those loads are. While the mechanical resistance of an electrically-disconnected motor-generator is almost nothing.

The resistance from spinning an open-circuit MG1 (and MG2!) is similar to that caused by spinning the output of a normal transmission in neutral or clutch-disengaged, and far less than that caused by spinning an engine (which is why MG1 turns while the engine stays put), and less than other things slowing the car like tyre rolling resistance. It is totally comparable in feel to a conventional "neutral".

(Another thing to not forget is that you're spinning both MG1 and MG2 - whatever you think the resistance of an open-circuit motor-generator is will be coming from both.)

Sure, you could apply a bit of power to MG2 to cancel out drivetrain losses, but that would just make it even more glidey than a normal car. There was no point in Toyota doing that for N because the Prius N was intended to act as a genuine neutral - no electrical torque applied - as well as feeling like a conventional neutral, not some sort of "speed holding" mode. They achieved both aims.

On the contrary, it's D that has to apply use electrical torque to get a drag effect by capturing power (from MG2). If it was true that N required supplying power, then D could have been achieved by simply cutting that power. And the Prius would have been spectacularly inefficient, due to all the power being consumed to overcome the drag of MG1 and MG2...

 

Not exactly what you asked, but FWIW, it is not too difficult to balance the throttle pedal input so little or no regeneration is happening if you have the car in ECO.

ECO mode allows a lot of initial pedal travel with modest result, so fine adjustments can be made. I don't know that it makes the car any more efficient, just as I've no confidence that sport mode makes it any faster. Sport mode does cram a high percentage of the throttle pedal range into the initial travel, something many manufacturers have done for decades to make the car seem peppy on a test drive.

I understand the desire to coast as a recreational hypermiling game. One advantage of the mild regenerative braking that occurs when you lift off the pedal is that it may encourage you to keep up a constant speed until closer to a stop, a more ordinary pattern. Coasting to a red light too slowly can frustrating for other drivers.

 

Except that I was right.

 

At some time in your life about something, I have no doubt. This thread keeps bringing down your average though.

Nine words into that post, you bring in another bogus detail already, and the most interesting part is that you didn't need it for anything. If you were sure that the ECU controlling the MGs needs to get travel speed and rotation info from somewhere (which is true), but you didn't really know from where, you didn't have to say from where. If you had known that it gets that info from the MG1 and MG2 resolvers, and not from the wheel sensors, you could have said that, and been right. Otherwise, you could just have left the from where part unstated, and still not been wrong. Instead, you chose to make something up, when nothing was forcing you to do so.

The MG ECU needs very precise rotation info for both MG1 and MG2 because it has to switch inverter IGBTs on and off in real time as the rotor magnetic poles pass the windings and the rotors could be doing several thousand RPM.



It gets that information from the resolvers mounted right on MG1 and MG2, which are able to resolve the rotors' absolute positions to small fractions of a degree in real time, and are wired directly to the MG ECU.





The wheel sensors, though, are wired to the skid control ECU. They are used for skid control and ABS. That ECU also sums up their readings into a basic "MPH" reading available over the CAN bus to anything in the car that wants to know more or less how fast the car is going.

But there is no way the MG ECU could be relying on those. It needs way more precise rotation info. And it's not even on the CAN bus. It has a private serial link to the power management control ECU. For it to use the wheel sensor data would mean too-imprecise information arriving two network hops too late for real-time use.

Which is why your choice to pad out your post with the 'detail' that the wheel sensors supply this information creates not just another error but a pretty ludicrous one, the kind that shows you're really not even making an effort to understand how the system works. It's as if you think that just dropping extra technical-sounding factoids into a post will make you seem knowledgeable—but it doesn't really work that way if you can't be bothered to take the time to get them right.

 

Some might believe you, purely on your assertiveness,
but I have facts and the laws of physics on my side.

 

Fortunately, the ones who aren't awed by my assertiveness are able to confirm the details I use—which is the approach I'd recommend more anyway—and see that I haven't just made them up.

The laws of physics may not really take sides; they just do what they do, but the people who haven't been making details up sometimes are less surprised by what happens.

 

If you could turn off the entire system without engaging the parking brake, you will be sure to find that the car would not be coasting in neutral, merely by virtue of the electric motor freewheeling. The combined effects of drivetrain fiction and the high gearing ratio would bring the car to a quick stop.

 

If you insist on not using the gas pedal because your foot is getting tired, why not use the DRCC at all times then? You can set it to your desired coasting speed, whether it is 25 mph, 35 mph, etc. Note that you can easily exceed the set speed by using the gas pedal.