A reader Shawn asks :
Hi, My mother just bought a new Camry '07 as well and we both had a question and we cant seem to find an answer anywhere. We were wondering what the differnce is in the gears on the shifter, like what is different about using "D" vs. "B" like what is differnt about them and when should we use them?
The Short Answer : Use the B mode when you are driving downhill for long distances (like down a mountain for more than 30 minutes). The “B“ mode is NOT a lower gear, like you would use for more traction, or towing or something like that. At almost all times you should be in “D“ mode.
The Long Answer :
A normal car has multiple gears, and as you shift (either manually or on automatic) the gears are swapped out, and the gearing ratio changed, to provide efficiency and torque at different speeds.
A CVT (Continually Variable Transmission) has an effective infinite number of gears, and using a complicated system of belts and pulleys will give you any arbitrary gearing ratio.
The Prius and Camry use the “Hybrid Synergy Drive”, which is often considered a CVT, but it really isn't. The Prius and Camry only have one gear. As more power is needed, the gear spins faster. But the gearing ratio never changes. However, Power is split from this gear to either to drive the car, or charge the battery system. The ratio of power to wheels vs battery is dynamic, and so the car acts like a CVT most of the time.
One of the tricks used by the Hybrid drive to get maximum efficiency, is changing the timing on the engine. When you are coasting, The engine is still turning over, even though no (or minimal) gas is being used. The car changes the timing of the air intakes on the pistons. This is done so you aren't pushing around a lot of air, which makes the car more efficient.
The braking system in the hybrid is also different. The hybrids have a smaller, less powerful set of brakes than would be typical for a car of their size and weight. This is because the hybrid uses regenerative braking to charge the battery, which added to the normal brake system provides all the braking you need.
Now that we have the background, the actual answer!
When you are driving down a mountain, you have the brakes on constantly. In the hybrid system, eventually the battery will get full, which means you are using just the normal brake pads. As the brake pads heat up from the friction, eventually you get to a point of brake failure (brake fade), where they cannot absorb any more heat. This can happen in a normal car too, but its easier to do in the hybrid because of the smaller brakes.
So just like the car can be more efficient by changing how the engine works, it can also be LESS efficient. When you go into B mode, it stars opening the air intake valves at the least efficient time. This causes the engine to push around a lot of air, which uses up energy, and helps slow you down, taking work off of the brakes. I believe the system also starts using the electic motor and gas motor at inefficient times, to try and keep a buffer in the battery system to absorb power.
So essentially, you should ignore B mode, unless you are driving down a mountain. It is not used for towing, snow, up hills, or any other time when you would use the low gear in a normal car.
Here is another article that goes into depth on the B mode, and may explain it better than I have http://techno-fandom.org/~hobbit/cars/b-mode.html
More info on how the Hybrid Synergy Drive works is at Wikipedia : http://en.wikipedia.org/wiki/Hybrid_Synergy_Drive