B Mode, Battery Charge and Charge Rate

No such thing as freewheeling in the Prius transaxle. All the mechanical energy going in has to come back out. There are 3 input/outputs: MG1, MG2+wheels, and the engine. If only one is putting torque in, the other two are receiving it. Maybe all goes to one, maybe all to the other, maybe they share evenly. The direction and ratio of sharing is controlled electrically by MG1 & MG2.

If you run the engine, this spins the planet gears. They can drive the ring (MG2+wheels) to move the car, or they can spin the sun gear (MG1) and the car will sit still. If you accelerate the engine, the other two I/Os must deal with this mechanically- either the car accelerates, the sun gear goes even faster, or both.

Now look at the opposite flow: You can spin the road wheels (ring gear) and the transaxle will force that input torque to the engine and MG1 outputs. If you do this at a high enough speed, MG1 will be spinning very fast. In fact dangerously fast. So the engine must spin with wheels+MG2 once you've exceeded a certain road speed. It's completely optional for the engine to inject fuel and make sparks during this time. Logically, when the engine is burning fuel it will be adding power to the system. When it isn't, it becomes a natural drag to brake the system.

It's important to understand that in normal operation, MG1 controls your effective gear ratio, dynamically, and it does so by the application of electrical supply or loading. If the MG1 spins one direction, the transmission works as a reduction set between the engine and the wheels. If MG1 reverses direction, the net effect is an expansion of the effective ratio. The rotational speed of MG1 sets the actual gear ratio in effect between engine and MG2+wheels.

The mechanical path is still there, in fact it can't be broken without disassembling the transmission. N mode decouples the electrical link between MG1 and MG2, so you lose the ability to regulate the gear ratios between the 3 I/Os.The transaxle will still enforce mechanical equilibrium between the 3 inputs and outputs; switching to neutral just surrenders your (and your computer's) ability to influence these ratios electrically.

The net result is very similar to a traditional transmission in neutral, with a critical difference that only applies in the case of a Prius rolling down a hill at high speed in neutral. In that case, the Prius is at risk of uncontrolled overspeeding of individual parts of the drivetrain in addition to the primary risk of runaway speeding of the vehicle itself.

 

@jerlands, go to about 51:38 here where he deals with B mode. You can start sooner to get the details of D mode if you want. TLDNR is that you get extra regeneration in B until the battery is full. At that point, the current generated is used to rotate MG1 to spin the ICE as an air pump.

Good point, @fuzzy1. The energy used to control MG1 is coming from MG2 and not going to the battery in the first place. The more I learn about this system, the more amazed I am at the engineering.

 

While it might be reasonable to believe this based on the phrasing used by the good professor, it is not the Toyota design. The purpose of B is to direct less of that extra power to the battery so it takes longer to fill, expressly to provide more braking effect, to spare the disc brakes from overheating, and to still provide a reservoir (or buffer) to store electrical energy for a longer period of time. Instead of storing the extra energy in the battery, the extra energy is wasted by turning the engine (ICE) over in 'injectors off' mode. However if or when the battery does reach full, all of the excess energy is then directed to the engine and there is a more radical reconfiguration of the engine (using the VVT) to make it a very serious compressor as it needs to replace the MG1/2 regeneration resistance with the equivalent mechanic resistance via the engine. Most people will never experience this last mode in their entire Prius driving experience, as the topography they drive is just too flat. This mode is similar (sound-wise) to the sound of exhaust braking employed by the prime mover of a tractor/trailer unit.

 

I'm pretty sure I have experienced the maximum engine braking in a Gen 3 and a Gen 1 (US 250 east from Rockfish Gap is always enough of a descent to fill my battery, and even with the engine revving at the limit, I still have to snub the car sharply with the friction brakes three or four times on the way down).

I can't say that it ever sounds like jake braking in a diesel rig. It just sounds like gasoline engine revving high, same as it would in a conventional gas-engine car if downshifted.

My understanding is that the jake arrangement is used in diesels because they don't have throttle valves. So a diesel really is compressing serious volumes of air, and the Jacobs arrangement tweaks the exhaust valve timing to release it near TDC, before it re-expands and returns that energy to the truck.

By contrast, when a gasoline-engine car is using engine braking, even a Prius, it is running more as a vacuum pump, pulling against a closed throttle. The Prius engine has adjustable intake valve timing, but the exhaust timing is fixed, so it really couldn't do jake braking if it wanted to. Without being able to do the jake trick, there wouldn't be much point opening the throttle further to compress more air: sure, it takes energy to compress it, but you get the energy right back again on the downstroke.

 

Yup, that's the way I understood it till I saw that part of that video. Could be that the professor was mistaken. Wouldn't be the first time a teacher goofed. Sounds like a good application for Hybrid Assistant or some other app to compare the current in both modes.

 

In neutral, MG1, MG2, and the ICE do not spin. You can be going 100 mph and pop it into neutral. No problem. I've put my various Prii into neutral to coast at speeds up to 80 mph. There is no resistance from either MG or from the ICE. It just coasts. Otherwise, it wouldn't be "neutral."

 

There can be, though only in exceptional circumstances. Neutral in a Prius normally means both MG1 and MG2 are electrically cut off. (In fact, that's the only thing Neutral in a Prius means.)

If the car notices MG1 in danger of overrevving in neutral, the only recourse it has is to temporarily bend the rules and act like it is not in neutral. That is a thing it can do, in three situations (New Car Features manual, 2004, page TH-46):

• If you are in N, and you brake, and a wheel locks, ABS will be used, and the ECU will send some power to MG2 to get the locked wheel spinning again, despite being in N.
• If you shift to N while braking, the ECU will briefly continue electrical regen braking despite being in N, and smoothly taper it over to friction.
• If in N and either MG1 or MG2 is in danger of overrevving, and some electrical control will be applied, despite being in N.

Heh. You have been hoodwinked by the cleverness of the firmware. There is absolutely nothing capable of mechanically disengaging when you shift to Neutral. Everything that was spinning before is still spinning after, and the same nomograph tells you what the possible speeds are for all three players. If you are above the engine-must-spin-to-protect-MG1 speed and pop it into N, even the engine must continue to spin. The car is just programmed to do all of this transparently enough to convince you something else is happening.

 

You're right about MG2. I goofed. MG2 is always connected to the front wheels. MG1, which was your PPM concern in neutral, is not directly connected to the wheels. So, you can still be in neutral at any speed.

 

Take a look at this nomograph. Draw a straight line between any MG2 rpm and any engine RPM, continue it to the MG1 line and it will show you the revs of MG1 for that combination of MG2 and engine speeds, and it can't ever be different, under any circumstances, even neutral. It's a consequence of the mechanical gearing, which never changes, and never disengages.

If you fix the engine RPM at zero, some road speeds / MG2 speeds will be out of range for MG1. The only possible thing the car can do to avoid that is have the engine RPM not be zero.

 

If the ICE isn't moving, then MG1 and MG2 rpms are locked together in a fixed ratio. Neither can spin independently of the other.

And all three shafts still have fixed torque ratios. Either all of them are torqued, or none of them. You can't have torque on just some of them.

The mechanical path is still there, spinning everything in ratios predetermined by the gearsets. But if there is no load, that means no torque on any shaft.

Power = RPM x Torque. When torque is zero, then there is no power flow, no matter how fast anything may be spinning.

Which parts are driving vs not driving, or alternator vs motor, is not determined by relative speeds. It is determined by the polarities of the shaft torque and spin direction.

I.e. the ICE can drive MG1 even if does not exceed the speed of MG2. And MG1 can be an alternator without speed increasing.

You have insufficient information to make those statements. More information about the specific scenario is needed.

 

Nothing, absolutely nothing, other than shredding metal, will ever change the revs of MG1 for a given combination of road speed and engine revs.

Perhaps you mean "will slow the car down", which would end up keeping MG1 revs in range by changing the combination of road speed and engine revs, which is the only way that can happen.

Do keep in mind that, as soon as you allow MG1 to generate, it gets harder to turn, and harder in proportion to how much you let it generate ... and through the gear set, a corresponding torque will be applied at the engine.

If the engine currently isn't spinning, it won't take very much torque to begin spinning it. (You can pretty much turn over a Prius engine by hand; feels like working a big sewing machine).

A Prime has an extra one-way device built into the flywheel, so at least its engine can't be spun backwards.

 

If the ICE doesn't spin, then MG1 critical speed is exceeded.

But making it generate, puts torque on its shaft, which necessary puts torque on the ICE shaft too, which will cause it to spin.

But in neutral, MG1 is de-energized, so it can't generate in the first place (... apart from the protective "bending the rules" statements in post #30, which will have side affects such as moving the ICE).

Remember that one can be in Neutral with the ICE operating and spinning, if it was already operating before entering Neutral. This protects MG1 by reducing its speed. And note also that the Prime has much different limit speeds and RPMs, and an ICE-shaft sprag clutch not installed on any prior Prii.

 

By spinning the engine. Fuel & spark optional, but that sucker's turning.

This assumes that the car is putting some loading on MG1 to hold that element below its critical speed.

The ratio between A & B is altered by the speed and direction of C.

A = engine
B = MG2/wheels
C= MG1

If C is hitting an RPM limit, the computer can* use electrical loading to drag that element. This will naturally force any additional incoming torque from B straight to A.

*when it isn't in neutral. Remember, in neutral MG1 and MG2 are completely de-energized. If more torque comes in from B, it will be applied to both A & C in a relatively undefined, uncontrolled manner.

 

I did not mean to say what you have taken from my relatively throwaway statement. I merely meant to try and describe the sound to someone who had not heard it before, because it can be quite alarming when it does happen for the first time and usually the thought is, "What's happening? Is my engine about to shake itself apart?" I 100% agree that it is engine braking. However, I hear more than just a high revving engine, maybe that is just me.

I definitely did not mean any inference at all about the mechanism by which the braking effect is effected, I don't know the ins and outs of how it does it, I just know that it does it and does it extremely well. I merely said that the sound is something like exhaust braking, agreed nowhere as loud, but that is the sort of sound I hear mixed in with an engine revving at (near) max.