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

The Prius design has always been to feel rather like conventional cars, which typically supply a small engine braking effect when you are fully off the gas. Prius supplies that "engine braking" effect more efficiently using regeneration (when it can, that is; when it can't because the battery is too charged to accept it, then it supplies the same "engine braking" effect using, well, engine braking, and accepts the efficiency hit).

So to actually coast in a Prius has always taken a bit of driver skill, backing off the pedal but not all the way, finding just that spot where you are neither powering nor retarding the car, which historically has been called 'glide'.

There are 'hypermiler' drivers who have managed to become very good at it. I haven't.

 

In my experience, hypermiling gains you nothing at all in a Gen 5. You can achieve the same result by holding the HV/EV mode button to enter CHG mode which builds up the EV range.

Hypermiling in a Gen 3 can build up your EV miles, but at the cost of fuel economy.

 

For most drivers, coasting without any sort of resistance would be uncomfortable, and probably a bit dangerous. You can achieve that effect by shifting into neutral, but it's actually a simulated condition created by running the electric motor in reverse. There is no actual neutral in a Prius drivetrain.

 

What is a PDA?

 

Neutral a "simulated condition": when a transmission is allowing its input and output shafts to spin (or not) at unrelated speeds and with negligible torque, it's a transmission in neutral. Transmissions are built differently and get neutral in different ways; often something inside the box is freewheeling. Given two transmissions operating in such a condition, neither "condition" is any more "simulated" than the other,

The item that's freewheeling inside a Prius transmission in neutral is MG1, one of the two electric motors inside the box. Its direction of rotation isn't always "in reverse" depending on the speed of the engine (which may or may not be turning) and the speed/direction the car is rolling. If the car is rolling forward and the engine isn't turning, MG1 does spin "in reverse".

That isn't something "created by running the electric motor in reverse". It's spinning that way simply because nothing's stopping it (it's freewheeling) and that's the way it has to spin given how the gears are arranged.

Any time the car stops letting the MGs freewheel, and starts electrically influencing their motion in any way, that's when you're not in neutral anymore.

 

If the electric motor were allowed to freewheel, there would be drag on the drivetrain.
For this reason, MG1 reads road speed (via the computer) and gasoline engine speed, and delivers reverse torque to the drivetrain, to achieve what appears to be no-torque neutral.

 

Just practice a light touch on the go pedal.
You’ll be coasting without even thinking about it before you know it

 

That would be quite something if it were true.

Is there "drag on the drivetrain" when MG1 freewheels and generates no power?

Sure, the same negligible bearing and oil drag that there is in a manual transmission whose countershaft is spinning in reverse because you're coasting in neutral.

No effort is made in a manual transmission to spin the countershaft backward to make neutral appear even less draggy either. It just plain isn't very draggy to begin with.

 

Exactly. If MG1 were allowed to freewheel, the permanent magnets within would create drag to the point of braking. This is true of any high gear ratio, being driven from the low speed end.

 

No. The amount of drag created by MG1's permanent-magnet rotor spinning is determined by the amount of current allowed to flow in the windings.

That is why a strong braking effect is seen when the MG ECU triggers the inverter IGBTs to conduct so that current may flow and power is generated.

Conversely, it's also why there isn't a strong braking effect when the IGBTs are left nonconducting.

I had the advantage of experiencing good demonstrations of the principle as a kid: hand-crank a permanent-magnet generator with different amounts of electrical load connected to it. When there is a significant load to develop power into, you definitely know your arm is producing that power. Conversely, with the output open-circuited, it's an easy spin.

Those kinds of early experiences can be great for cementing an important concept.

 

Is there any Prius gear Paul understands? Two down, only three remaining.

And I've just realised he's got contradictory misconceptions. His whole other thing is about the car dangerously running away downhill without drag when the battery's full.

But this one has him believing the transmission will brake if MG1 isn't driven.

 

Does it make sense to shift to neutral during driving? | PriusChat

 

If you compare the difference with neutral more often you'll find there's actually not as much resistance as you think...

As in, it's common for people new to Prius to notice loss of momentum on hills or after acceleration. Heck, even my little old '86 Toyota truck has more momentum than my Prius when it's in neutral.

But the primary issue you're noticing here is that the Prius is much lighter than a regular car so you have less momentum than a larger heavier vehicle has. This is most notable when going down steep mountain grades.

 

Coasting is a simulated condition created by the interaction between the electric and gasoline engine via the computer. Since both power sources are connected to the drive wheels directly through a planetary differential, and never disconnect, the car must simulate a coasting condition by manipulating each motor.

 

And it works great too. I used it all the time to clean the rust off of the rotors

 

In order to simulate a zero-torque condition, the wheel sensors must tell the computer the vehicle's travel speed in addition to the rotation of the drivetrain. It calculates the exact amount of torque required mainly by controlling MG1. When the gasoline engine is stopped, it simply applies reverse rotation through the planetary differential. When the gasoline engine is running, it applies torque in either direction, as required.

Due to the large amount of gear reduction between the drive wheels and MG1, simply freewheeling the electric motor would produce excessive drag.