The most misunderstood aspect of the Toyota hybrid synergy drive system

A Toyota Master Diagnostic Technician explains in simple terms how the B mode works in Toyota Hybrids.

 

The hybrid battery may have some reserve capacity to absorb energy beyond an indicated full charge, but at some point the maximum charge can be reached. Probably not in normal driving, but it can happen. If you are coasting down hill, the energy will have to be dissipated. How will you do that with a full battery? You can't keep forcing a charge indefinitely. It's just basic physics.

With any CVT transmission, any "gear" is really just a simulation. The B is intended to stand for braking, so it could be considered a gear, I suppose, but I consider it a mode. Although it may never occur for some drivers, and the B mode may seem superfluous, I have been on some very long downslopes, where you descend thousands of feet in elevation over long distances, like 20 miles. This can be quite dangerous for some vehicles, including 18 wheelers, if the driver is too heavy on the brakes, which can be destroyed by overheating. This is the cause of many runaway rigs, and the reason runaway lanes were built, and why the engine brake was invented.

In most conventional passenger vehicles, this is not a serious problem. Engine drag is usually sufficient to keep speeds in check, as it was when I drove my VW Golf in British Columbia last year. some of the signs warned of a 6% grade or higher. I was able to handle this by downshifting, but occasionally, this was not enough, so I applied friction brakes for short, intense periods. This was intended to slow the car rapidly, and then allow the brakes to cool. Other drivers were not as careful, as evidenced by the smell of burnt brake linings in the air.

With my Prius, it's a different story. There is no downshifting; I only have D, R, P and B on the shifter. It wouldn't matter anyway, because the engine is shut off descending a hill. I can rely on a bit of "simulated engine drag" from regeneration without applying the brakes. This is evident, by seeing the CHG indicator on the instrument panel, as it does whenever a Prius decelerates as you let off the accelerator. Of course it's not real engine braking, because as the guy says, the engine is off.

I can rely on this simulated engine braking as I begin my long descent, but at some point, the hybrid battery could get to full charge. At this point, there's no place for regeneration to put the energy. Friction brakes are the other option, but not a good one, for reasons previously explained. As the expert said, this is where Prius engineers had an ingenious idea, the B mode. I don't know what happens if you use it in normal driving (no reason for this that I can see) it may provide regeneration, but you will get the same braking effect by applying regen braking with the pedal.

Some doubters have denied that a Prius will pick up dangerous speeds on a hill when regen braking is not available. What's stopping this? The engine is stopped, and friction braking will be inadequate. Applying B mode will run the engine in a special mode, where the valve timing is reconfigured to compress air, acting as an engine brake. As the expert in the video said, this feature is not well documented, and it took a deep dive to research it. But common sense can give you the same conclusion.

 

I'm surprised it took so long for the know-it-all to appear. It's a bit puerile to deliberately not read the posting, and then make stupid comments about it.
Everyone is entitled to their own opinions, but not their own facts.

 

As he said, this is one of the most misunderstood aspects about the Prius drivetrain.
As evidenced by all the ignorant comments being made about it.

 

Like I said, all you need is common sense.