Prius C by New York Times

Well, like almost all 'automotive journalists', he gets B mode wrong.

 

I'm getting my C in 2 weeks. I don't know what B mode is- can you explain it to me? I noticed it during my test drive but didn't try it out.

 

it's used to slow down the car during long downhill sections where you know that the traction battery would be fully charged at the bottom..

it DOES NOT give you better regen (which is the common misconception... and also the same mistake the journalist made).. what is does.. is cause the engine to spin faster.. helping you to slow down.. but in doing so... WASTES energy..

you do not get any more regen from B than if you just use your normal brakes.. but B is good for those long downhill grades so you don't cook your friction brakes

 

Thanks for the info! Was just reading your thread on your vinyl bumper mod. Looks fantastic!

 

I wonder how much energy he missed capturing. If he went down his 300 meter hill all the way in B mode, he collected about 40% of the potential regen, or lost up to 1428840 joules if max regen is 60% efficient. That is about 0.4 kwh. I suspect that the battery filled up anyway despite his error, but I do not know the route to be sure. If use of B mode in fact wasted otherwise recoverable energy, the MPG would have been about 3.6% higher.

For those interested, Hobbit explains B-mode

 

If Toyota had been less honest, they would have labeled it L, most folks know when to use L in a conventional Automatic, (almost never) and so would use B mode correctly. But it uses compression braking (like a Jake Brake on a semi) not lower gearing, so Toyota did not call it L. If you use B mode from the top of Long descents, Cajon Pass, as an example, or Hwy 2 to Wrightwood, your brakes stay cooler. Hot Brakes are dangerous. And it is when you should use L in an Automatic, not short hills in SF. Your HV Battery may well fill if there is more than a 600 foot drop.

 

B is kind of like downshifting. CVT style.

 

My salesman didn't understand it either.

This is the lowdown on B: When going down big hills, your battery could become completely full to the point the the regen braking ends and the friction brakes would have to do the entire job of slowing the vehicle. B mode does an "engine braking" action so the friction brakes don't have to do the entire job. I would only shift to B after I had found that my traction battery had filled and regen had ended. I love a full battery...

 

In fact, if the road was right and his speed low, in this case he may not have done better, but his description of why he was in B indicates either he does not understand it or that he is describing it extremely poorly.

We read some reviews and think 'How could they even GET such low MPGs?' but bad practices like B mode, gentle acceleration, and 'last second' braking can explain why some 'automotive journalists' can be such bad drivers.

 

Then it is something he shares with Toyota Service Dept. Manager and techs who gave the hybrid owners presentation. The described it exactly as the article describes it, enhanced use of the cars regenerative charging system to slow it down.

I'm sure Toyota service department of the dealer who provided the car gave him the same explanation.

 

You know, I hate Smilies.

But whenever people start discussing "B" mode? I do this...op2:

I'm convinced nobody knows how "B" mode is suppose to really be used. Or exactly what it is doing...

It's Toyota's little inside joke on Prius Owners....

 

Not quite all the lowdown - after the battery is full the system will go into "Automatic B mode" to reserve the friction brakes even without touching the gear knob.

Under these conditions you will be able to hear the engine speed up.

kevin

 

One can see a difference in the Regen graphic using B vs. D so it is safe to say that it is increasing the regen effort to slow the car down which is what the Toyota service people say at the owners seminars (and to reporters).

Does it engage the engine also, possibly. It sounds like it does on a long descent. I would think it would be possible to just use the electric if the battery is fully charged by energizing the traction motor current in reverse to make it a brake. Not sure which is easier to engineer and easier on the car, electric "brakes" or using the engine as a brake.

 

I assume this would only be when your foot is off the gas, right? Either coasting down a hill or while using the brake pedal with the battery full?