2004 Toyota Prius Long-Term Road Test


2004 Toyota Prius: "B" Is For Bad

October 19, 2010

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A lot of people wonder what the deal is with the "B" position on the shifter of the 2004 Toyota Prius (and most other Toyota Hybrids). Some people figure that B increases the level of regenerative braking and plows more juice into the battery.

Not so fast. Turns out the opposite is true.

Compared to coasting in plain-old D, engine speed goes up in B to provide more genuine engine braking. RPMs increase in this mode, just as they would if you had downshifted a regular transmission, with the drag coming from the engine pumping air through itself while the fuel injectors sit dormant. But the Prius planetary transmission's gear ratio regulating element, electric motor-generator MG1, has to expend some battery energy to command the engine's crankshaft speed up to create this extra drag.

It gets worse. Engine braking represents a lost opportunity for the electrical regenerative braking system, the system by which all hybrids collect most (and in some cases all) of the electricity that makes them hybrids in the first place. The mechanical engine pumping losses that are engine braking could and should be avoided so they can go through the regenerative path to the battery instead. But the use of B robs the charging system of this chance and, ultimately, robs your wallet in the form of lost mpg.

In a regular car it would be madness to tell someone they should use the brakes to slow the car when going downhill, but here in the Prius, a so-called "strong" hybrid with sizable electric motors, that's oftentimes better for efficiency and no serious threat to the integrity of your regular friction brakes.

That's because the regenerative braking system in a strong hybrid can develop a significant amount of slowing by using the electric motor as a generator. In the 2010 and 2011 Gen 3 Toyota Prius, for example, the regen system can produce decelerations of up to 0.6g. Our somewhat less-efficient Gen 2 Prius (rated at 46 mpg combined instead of 50 mpg for the Gen 3) doesn't do quite as well in this regard, but the regen braking effect it can produce is still significant.

Need perspective? Most normal stops are made at 0.2 to 0.3g. Impatient folk stop at 0.4g or so. You might touch 0.6g if you get surprised by a yellow light. Unless your license has several points on it, over 90% of your braking occurs below 0.6g.

Unless the grade is very steep or very long, the foot braking mode you'll be in on a downgrade will be an electronic one, with your battery as benefactor. Your brake pads and rotors can't overheat if they're not being asked to do much.

Fun fact: our 2004 Prius has over 85,000 miles on the clock. The original brake pads still have some meat left and the rotors still look good and run true. That's regenerative braking for you.

But you can overdo it. The battery in a Prius can only hold so much, and a long grade can top it fully, at which time the computer begins to rely more heavily on the friction brakes. Keep one eye on your battery monitor and shift into B if there's still grade remaining when the gauge reads full.

Of course fully electric cars have no possibility of engine braking, so this B mode discussion is a moot point. But their batteries and electric motors are so large that they can soak up just about everything the longest grade can throw at them.

Dan Edmunds, Director of Vehicle Testing @ 85,354 miles

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