B mode

Or you use front defrosters, or the outside temp is below (?) 18F, or ...

 

Yes, but you use it more quickly, thus either burning fuel sooner which is more fuel, or needing to charge more. Both cost more.

 

Is this the case with a Prime? Any energy back into the battery is still energy.

 

If you need to slow the car down, capturing the energy in the battery for reuse is more efficient than braking it off as heat.

If you don't need to slow the car down, letting the car just keep the energy it has in kinetic form is more efficient than paying conversion-loss tax on it twice to get it into the battery and back out.

 

Good point.

 

The issue may have had nothing to do with B gear.

In B gear, the car doesn't run the engine, it spins it to get rid of excess energy. With the standard hybrid, the battery is to small to take in all the energy the motor can potentially produce from regen braking. So the engine is allowed to spin and engine brake like other cars. Then if the battery charge into the upper threshold, the car spin the engine with M/G1 to reduce the state of charge. Just sitting in cabin, it is difficult to distinguish between the engine spinning and it running.

The bigger battery in the Prime means more braking energy can be captured, so B is mostly more regen. The battery can still go over the charge threshold, and thus lead to spinning the engine. At that point, the software feels that is necessary to enter the warm up cycle, and leave EV mode.

Or if the battery charge is too high.

Tesla didn't create it. Multiple EV concepts had the same set up through the years. Possibly even BEV conversions did too. My understanding, holding the pedal in the neutral position that allows gliding is generally easier than it is in the Prius.

 

Yes. That's why I was asking about reliability/wear. There has to be a clutch mechanism in the drive train somewhere, and that's were I would expect any degradation to occur. I doubt that excessive use of B mode would cause any significant wear in the m-g itself.

 

I just notice something peculiar in this citation.

Toyota Prius/Driving tricks - Wikibooks, open books for an open world

Under "Using B Shift Position" they state:

"'B' mode will not recharge your batteries more efficiently than just braking and is not recommended for general driving"

Then in the following paragraph "Unnecessary Engine Idling" they state:

"If the gasoline engine is running when you approached the stop it will take the better part of a minute before the computer recognizes the situation and shuts off the engine. There is a trick to getting the engine to shut off promptly. If you approached the stop in B (engine assist braking), you may then when stopped with the brake on, command D (drive) - the engine will stop immediately..."

...which suggests that you can get better efficiency by using B mode at intersections.

 

The last paragraph in "Using B Shift Position" it states:

However, in the case of the Prius "Plug-in" model (which provides an EV drive mode,) the "B" mode can be used to extend the range of the EV battery. If in hybrid mode, it is necessary to switch to EV mode and then engage "B". This is effective when approaching junctions as it is not then necessary to apply the brakes until actually arriving at the junction. Typically the range of the EV mode is extended by .1 - .2 miles each time this technique is used.

 

No clutch. There is a damper that can wear, but nothing at all like a clutch in the Prius.

I don't use "B," and stopping normally in Drive (not using friction till the car is almost stopped and it becomes necessary), I usually add 0.5 to almost a full mile when stopping from 50-60 mph. The brakes do the same thing as "B." You're going to have to use them anyway to come to a stop. The extra step of putting it in "B" gains nothing.

 

Yeah. I keep forgetting this is a Prius Prime forum. In EV mode, what you say makes sense. With any of the other drive modes, I still don't have a clear understanding of how the ice is being used when B is selected, and I have yet to see a satisfactory explanation of it. However, I think I may have found where some of the confusion arises.

When driving in any drive mode other than EV, the energy monitor will often show that the ice is NOT engaged during B mode. That's because, when B Mode is selected, the energy monitor maintains the previous energy flow status in the display, despite what the ice is doing. And even when the energy monitor is showing that the ice is NOT running, that's no guarantee that it isn't. Sometimes the ice runs during B mode, other times it does not. The only time that the energy monitor accurately displays ice status is when the engine was already running before B mode was selected. Otherwise, the only way you can tell for sure is to listen to the engine. In both cases, when you select B, the energy monitor will maintain the previous status (correct or not) until some other event updates the display.

Now that I have explained THAT, are you as confused as I am?

 

I could be wrong, but I'm pretty sure they also electrically reconfigure the mg poles to get more reverse torque, in addition to what is provided by the brake pads.

 

The Prime has a dog clutch on the engine to prevent it from spinning backwards. This allows M/G1 to supply propulsion to the wheels.

I suspect the engine is doing what engines do in traditional cars as they coast in gear; spinning and pumping air.

 

Nope, not in a regular Prius. While the Prime does, it isn't a regular clutch with a wearing friction plate, and is not actually needed to create B mode.

The poles are not changed. I believe they are fixed permanent magnets. There is no electrical need to change the fields either, the motor willswitch to a generator simply by reversing the mechanical torque on the shaft. Though the controllers could change the fields if they have some other reason to do so.

B mode does not apply brake pads, only the driver does that. B mode does send some torque to the engine to create 'engine compression braking'. After regeneration has filled up the big battery, everything goes to the engine.

 

Thanx for clarifying for me fuzzy, jerry already corrected me regarding the clutch and I was speculating on regenerative braking based one what I had already experienced with my gen 4. I included brake pads as one of three sources of deceleration torque (mg, engine inertia/pump and brake pads), typical of B Mode use in an urban environment (i.e. using the friction brakes in addition to B mode as you are approaching a red light). The hybrid system indicator appears to indicate additional recharge when the vehicle still moving fast (35 - 45 mph) before applying brakes, but you are probably right, if permanent magnets are used, you obviously cannot change the stator circuit since you don't have a stator circuit to change. The only other thing I can think of is maybe the two electrical breaking modes (B Mode & pedal brake) use separate mg circuits, in which case you might get additional torque - but the literature doesn't seem to support this.

 

The permanent magnets are in the rotor, not stator. The MG controllers work with the stator.

Everything, propulsion and braking in the various modes, use the same MG circuits. The computers control it all.

 

In particular, the computers control one pair of big transistor switches on each of the three phase connections to each set of stator windings. That's six switches per motor, twelve switches in all.

Which stator poles are pulling and which are pushing and when is all determined by the computers changing the timing of the flips of those switches, as the rotor's permanent magnets pass by.

And there, by gum, you have a transmission.

 

I'm a little surprised none of us has offered you these links that actually show how the system works. The first one is for the Gen 2 transaxle. I put it here because I think it better shows the basic principles involved. The second one is specific to the 4 Gen and Prime transaxles, which are nearly the same as each other but with a couple important differences that make the Prime different than its predecessors. It'll take a while to watch them, but it's worth it. I was sure impressed by the ingenuity of the people who designed this system.

 

Elegant. I knew the eCTV did something like that, but I didn't know exactly how. Thanx.