[DavidW]

Can anybody explain the technical reasons behind Toyota's driving instruction advising users not to stay in engine braking mode for long? Is it to reduce mechanical wear and tear on the hybrid drive or to avoid loss of fuel efficiency, or both? In this mode, is the engine compression in fact being used mechanically to brake the vehicle, or is engine braking being simulated by engaging the regenerative brakes progressively when you take your foot off the gas pedal? It certainly feels like regular engine braking, especially on downgrades.

 

[mrv]

In B, not only do you have the regular regenerative braking (which happens in most instances when your foot is not on the accelerator), but you also have engine (compression) braking. The gasoline engine spins (although often without fuel) to help slow you down. For normal driving, B mode will give you lower fuel economy. The only time you should use B is when going down a steep/long hill (to avoid burning out the brake pads), and there are a few instances in snowy/icy driving when it may help.

 

[G. McCurdy]

I've been wondering about this too. I had been using the B a lot, but not so much now.

When one steps on the brake peddle, does the electric motor do some of the braking while in D? Perhaps a percentage applied to the brake system hydraulics while another being the electric regenerative braking?

 

[RSnyder]

You are advised not to use B too much because under most circumstances it results in lower mileage. It will also put slightly more wear and tear on the engine, while reducing wear and tear on the brakes (wear and tear on the hybrid drive is not significantly affected, though the electronics do heat up slightly more). Note that engines cost more than brakes.

Even in D the electric motor (the bigger one) does some of the braking. In fact, it is applied first, before the mechanical brakes, so at light braking levels, it's the only thing doing the braking.

 

[DanMan32]

Regenerative braking occurs in D as well as B. With the recent Prius version, no friction brakes are used much of the time, until you drop below the speed where regenerative braking is sufficient (about 7MPH) or panic stops. Regenerative braking is also used to simulate engine braking that an automatic transmission would normally exhibit.

In B, regenerative braking is a bit more agressive. Also actual engine braking is used where the engine is spun by the wheels, with minimal fuel (I believe some is used just to keep the engine from stopping). If you are using the engine to help stop, you're wasting kinetic energy to heat energy going nowhere useful. Also, I have found that once you stop in B, the engine tends to keep running which is another waste of energy.

You might ask, if regenerative braking is so good, (it can for the most part stop the car by itself) why use B to have the engine help decelerate?
Because the regenerative braking requires a place to store the recovered energy, which would be the battery. The battery has a fininte storage capacity. Once its capacity is reached, you can't have any more regenerative braking, so you start using your friction brakes. B would reduce the use of regeneration so you don't run out of capacity too soon, and uses the engine rather than the friction brakes to supplement the regeneration.

Use B when decending long steep hills, so you can use the engine rather than your pads/shoes to supplement regeneration braking.

 

[mikepaul]

Use B when decending long steep hills, so you can use the engine rather than your pads/shoes to supplement regeneration braking.

Did we decide that 8 or so miles down a steep hill at 70+ (D, not B) will hurt the battery, once it goes green? Entering NC from VA on I-77, I usually let people pass me for a while, just rolling along at 60 or so until gravity kicks in harder and I pass 70. I love those nearly 10 minutes of 99.9MPG...

 

[DanMan32]

Nah, won't hurt the battery. Once it is maxed out of its usable limits, the friction brakes will be used, and the engine will be started and stopped several times unil the battery is drawn down a bit. When you start cruising or accelerate, mostly MG2 will be used yielding fantastic, though short lived, mileage.

 

[redwein]

Use B when decending long steep hills, so you can use the engine rather than your pads/shoes to supplement regeneration braking.

Did we decide that 8 or so miles down a steep hill at 70+ (D, not B) will hurt the battery, once it goes green? Entering NC from VA on I-77, I usually let people pass me for a while, just rolling along at 60 or so until gravity kicks in harder and I pass 70. I love those nearly 10 minutes of 99.9MPG...

I just went back and forth across the country and over the Rockies twice. I never used "B" and rarely had to use the brake pedal at all going down the hills. When the car was in cruise control, it always stayed within 5 MPH of the setting. I did get to full green bars but I never lost that "feel" of the car being slowed down by regenerative braking. If it isn't necessary in this situation, I'm not sure it is really necessary anywhere.

 

[Phoenix]

Regenerative Braking & "B"

Not sure what route you drove over the Rocky Mountains, but there are lots of places in Colorado where regenerative braking AND "B" are insufficient to hold speed down on long steep grades. These roads need regular friction brakes, too, to keep the classic Prius at safe speeds.

I found that on these grades, our Prius needed cautious driving to avoid overheating the "regular" brakes. Fortunately, those roads are infrequent, and the really steep parts are only a few miles long. Most drivers would be OK, but the inexperienced mountain driver could experience brake fade and an exciting trip down the mountain.

 

[hyperion]

Most of these "steep" descent highways have warning signs for "eighteen wheelers" to shift into lower gears. This would be the only time I would even remember that my Prius had a "B" selection. That's how I read the description of "B" mode in my owners manual.

 

[redwein]

Re: Regenerative Braking & "B"

Not sure what route you drove over the Rocky Mountains, but there are lots of places in Colorado where regenerative braking AND "B" are insufficient to hold speed down on long steep grades. These roads need regular friction brakes, too, to keep the classic Prius at safe speeds.

I found that on these grades, our Prius needed cautious driving to avoid overheating the "regular" brakes. Fortunately, those roads are infrequent, and the really steep parts are only a few miles long. Most drivers would be OK, but the inexperienced mountain driver could experience brake fade and an exciting trip down the mountain.

The steepest was I-70 through Denver and into Utah. It topped out at over 11,000 feet. The speed limit was 70 or 75 much of the way down and, without using engine braking or much of the pedal at all, I was easily able to keep it in the 80 to 85 range. Granted that wasn't the speed limit but it was pretty much in keeping with the flow of traffic. I did notice that with the cruise control set, the car tended to attempt to keep itself slowed down. When I turned off the cruise control, for an experiment, the car did speed up more and I had to use the pedal. Maybe that is the difference.