[Eriks2009THC]

I like B gear. When I use it to accelerate can't see a big drop in MPG. When I use it to slow down feel like it should help my battery fill level.
When low juice and slowed down from D to B MPG went from 40mpg to 60mpg so that impressed the wife but otherwise she hates it when I use B.

She thinks I will break car. =( Any idea where to get info on how to use and why?

 

[venzoid]

Discussed at length in this thread: Describe How U improve MPG with a Hybrid

 

[Eriks2009THC]

Yes, true. Most people talk about that with a hybrid.

Any idea how to properly use B gear?

 

[Eriks2009THC]

I should be able to accelerate faster with it? Or just tear up my engine faster with it?

 

[Eriks2009THC]

Ok, will go see if I can find that thread.

 

[Eriks2009THC]

Can't find any forum posts about B gears there, sorry. Other ideas?

 

[Ron AKA]

The topic seems controversial. The best information on how to use it and how it works is at this link.

I think the Reader's Digest version is that B mode forces the use of the engine brake. Intake valve timing is advanced to make it act more like a conventional Otto cycle, instead of the more efficient Atkinson cycle. It pumps air, and that is why it slows the car down.

By some accounts it seems to increase regeneration braking too. However, you can do that by putting on the brake to the point where regeneration is not maxed out.

I would suggest the only application for B mode is when you are going down a long hill, and your battery is full, and you need to use the brakes to slow to a safe speed. It would be better to use the B mode than use your disk brakes.

Bottom line is that engine braking pumps air, and the energy is lost forever when you use it. I can't see there is any way using B mode would harm your car, just your wallet.

EDIT: It may be helpful to read up on how engine braking works. Ignore the parts about Jake brakes and diesel brakes. You want the conventional gasoline engine braking part.

 

[Eriks2009THC]

Wow those pages after googling Otto and Atkins engines took a while.
So it doesn't actually seem like using an Otto style in the car would be needed. Why the B option. Just for fun? They clearly can't expect people to watch battery levels or move the gear shift. Hmm.

 

[Ron AKA]

So it doesn't actually seem like using an Otto style in the car would be needed. Why the B option. Just for fun? They clearly can't expect people to watch battery levels or move the gear shift. Hmm.

The Atkinson design is used in the hybrid due to the improved efficiency, and the ability of the electric motors to make up for the poorer power performance of the Atkinson. However, the improved efficiency is achieved at the expense of reducing pumping losses in the engine. It is those pumping losses that make an effective engine brake. So, the B mode it to revert back to the Otto cycle pumping with a change in intake timing.

B mode does have a use though. It saves your brakes on a very long or steep hill. You would only really need it when you might shift down a gear if you had a manual transmission, or down into a lower gear on an automatic.

 

[ukrkoz]

i should be able to accelerate faster with it? Or just tear up my engine faster with it?

1. No
2. No

 

[ukrkoz]

Braking

When you want to slow the car more rapidly than rolling resistance, aerodynamic drag and engine braking, you press on the brake pedal. In a conventional car, this pressure is transmitted by a hydraulic circuit to friction brakes at the wheels. Brake pads rub against metal disks or drums and the energy of motion of the car is converted to heat as the car slows down. The Prius has this exact same braking mechanism, but it has something else as well - regenerative braking. Whereas during coasting MG2 produces some generator drag to simulate engine braking, when the brake pedal is pressed, the electrical power generation of MG2 is stepped up and a much greater generator drag contributes to slowing the car. Unlike friction brakes, which waste the car's kinetic energy as heat, the electrical power produced by regenerative braking is stored in the battery and will be re-used later. The computer calculates how much deceleration will be produced by regenerative braking and reduces the hydraulic force transmitted to the friction brakes by an appropriate amount.
In a conventional car on a steep hill, you might decide to change to a low gear to increase the intensity of engine braking. The engine spins more rapidly and holds the car back more, helping the brakes to slow it. The same option is available in the Prius should you decide to use it. If you move the mode selector lever to the "B" position, the engine will be used for engine braking. Whereas normally the engine is stopped during braking, in this mode the computer and motor/generators arrange for it to turn over without fuel and with an almost closed throttle. The resistance it offers slows the car, reducing brake heating and allowing you to ease up a little on the pedal.


http://www.ecrostech.com/prius/original/PriusFrames.htm

 

[ukrkoz]

Can't find any forum posts about B gears there, sorry. Other ideas?

seriously, why don't you read through this:
Discussed at length in this thread: Describe How U improve MPG with a Hybrid

I lost count of how many c/ps on B drive I posted there, along with personal experiences and recommendations.