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nuttmeg44 07-06-2008 12:10 AM

B vs D
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Forgive me if this has been discussed, if it has could someone help me find it?
Our dealer went over it but I still am not sure about what the difference is...
D obviously is for drive :-)
B is for ???
We read the manual but it still seems odd to me. I am really not blonde, promise.
I might be remembering wrong but he said something about city driving and using B. I only drive 5 miles to work and back on streets that are mostly 35 mph.
Just picked up the car today and he went over lots! :-D

Plan on playing with it more tomorrow and doing more research.

ehurtley 07-06-2008 01:40 AM

Re: B vs D
'B' is "Engine Braking". It's the Prius equivalent of 'L' on a conventional automatic.

It's sole purpose is for going down long hills. When going down a hill, you generally don't want to ride the brake pedal, so you downshift. Well, since the Prius can't physically 'downshift', it has "Engine Braking" mode. That tells the car's computer to send some of the power being generated to spinning the gas engine, essentially 'wasting' that energy, rather than building it up in the battery. If you're going down a long enough hill, you'll fill up the battery and it will do that anyway. 'B' mode just forces it to do it before the battery is full, so you don't have to ride the brakes.

You shouldn't drive around in 'B' mode for normal driving; although it won't hurt anything. Your car will just be loud, and you won't get as good gas mileage.

Oh, and when you're accelerating, it doesn't matter at all. The car accelerates exactly the same in 'B' as it does in 'D'. The difference is solely when coasting and braking.

jeromep 07-10-2008 09:25 PM

Re: B vs D
ehurtley's description of B mode is basically correct. There is no low gear or gears in the Prius, so the function of lower gears in a conventional vehicle have to be simulated by some other means.

I won't speak to some of the technical specifics of what occurs in B mode. DanMan knows the most about this as he has gone through the technical manuals. The other problem is that my memory about this issue is a little weak right now (I've been away from the boards for a while because, well..., life goes on after you buy a Prius and live with it for a while).

B mode does a number of things. First, it engages compression engine braking. The vehicle allows the engine to spin with no fuel applied. This creates compression braking. This is akin to a long haul truck with a Jacobs engine brake. The biggest difference is that this is the smallest application I've seen this technology adapted. Make no mistake, this is not a Jacbos brake, as in a truck, but the application is essentially the same. Turn the engine into an air compressor, which creates a load or force that acts against the drive train.

B mode also increases the base amount of power that the vehicle is regenerating in a coast. So the vehicle combines increased electrical resistance with increased engine compression resistance and you end up simulating the behavior of a low gear.

It works very well. Washington state is pretty mountainous and as such if you travel anyplace, especially east to west or west to east you have to cross the Cascades. Descending from one of the mountain passes gives me a chance to use this feature. Descending from a mountain pass really proves the usefulness of B mode.

Oh, and if you have your cruise locked in and activate B, it kicks out your cruise speed lock until you return the mode selector to D and then you can reengage cruise.

hyperion 07-10-2008 11:43 PM

Re: B vs D
Honda uses the same system in the Civic hybrid and eliminated any confusion years ago by labeling the shift selector with an "L."

jeromep 07-11-2008 03:32 PM

Re: B vs D

Originally Posted by hyperion
Honda uses the same system in the Civic hybrid and eliminated any confusion years ago by labeling the shift selector with an "L."

I suppose Toyota could have labeled B as L, but I don't think that would accurately describe the actual function of the engine braking system on the Prius. The L mode on the CVT system that the Hybrid Civic uses is not the same in exact construction as a conventional automatic, but a CVT such as the Civic Hybrid's limits the gear ratios it uses in L mode to lower ratios than the normal full range of ratios that would be used for cruising. The Prius has no such variability in gear ratios which occur in the gearbox. In essence the Prius has one gear ratio, and that occurs at the final drive.

I am admittedly unclear on the specifics of the Hybrid Civic's L mode, however I can only imagine that some quantity of regeneration occurs when the system is placed in L, and that some quantity of engine braking is provided by the engine even if there is no fuel flow being provided. I see the biggest difference between B and L in the major design difference between the Civic and the Prius. First, the only component of the Prius drive train that is connected to the final drive and the wheels is MG2. MG1 and the engine are not connected directly to the final drive, and the behavior of a planetary gear set would allow MG2, directly connected to the final drive to spin with the vehicles movement while MG1 and the ICE could remain totally stationary even though they are always connected to the planetary gear set. What this means is that in a coasting or free rolling situation the only drive train item that can act on or against the force of free forward motion is a motor/generator. The maximum amount of regenerative resistance provided by MG2 has a limit and on a steep enough grade two factors could negate the regenerative abilities of MG2. 1. The upper limit of electrical resistance that MG2 could provide could be overcome by gravity and momentum. 2. Once the battery has reached its upper charge limit, there is no place for the vehicle to store the regenerative energy and MG2 is then no longer able to provide any regenerative resistance.

At this point, this is where the "Jacobs style" engine braking function becomes important. A combination of behavior by the Prius' control systems, starts the ICE spinning without fuel and certain valve timing settings maximize the compression resistance that the engine now combines with the electrical resistance provided by MG2. Should a long descent cause your battery to become completely full and MG2 can no longer provide electrical resistance, the ICE through compression will provide resistance. Also, if you have your foot on the brake the brake pads will come into play to work against the downhill forces as MG2 is unable to provide sufficient enough resistance.

In the Civic Hybrid setup, the input and output ends of the CVT transmission are very conventional in nature. When the Civic Hybrid is in D, S or L, the engine is connected to the wheels through the CVT, at whatever variable gear ratio it is set at, and through the final drive to the wheels. The only way to disconnect the engine from the final drive is to place the transmission in neutral, which then disengages the engine.

When you place a Hybrid civic in L, the CVT chooses a lower gear ratio, allows the flywheel mounted motor/generator to provide electrical resistance and also spins the crank in the engine, which may or may not have fuel flowing to it. Or rather, the Civic Hybrid may behave in a low gear mode more conventionally. It already does by virtue of the CVT actually choosing a lower gear ratio as part of the actions of the Civic's L mode. This is not even an option available to the Prius because of its design.

In the end, B and L may accomplish the same things, but the inherent design differences between these two vehicles means that the behavior of engine braking is executed differently. This gives justification to Toyota's choice to label their low gear substitute (or simulation) as B for engine braking, rather than labeling it L which indicates choosing a lower gear.

Hybridization is changing vehicles, much the same as jet power changed aviation. Labeling B mode as L on the Prius would be no different than labeling the reverse thrust lever on a 747 as reverse pitch so that it could reflect the conventional nature of propeller driven aircraft. Reversing pitch on a propeller driven aircraft has the same braking effects as reversing thrust on a jet aircraft, but the method by which the reversal of thrust is accomplished is different, as such it is labeled differently.

hyperion 07-12-2008 12:21 AM

Re: B vs D
If you can't relate "L" to exactly the same desires in a standard car folks have been driving for years just to keep things standard and easily explainable then there is little room for discussion. The Honda and the prius use "B" and "L" for exactly the same reason. Only thing is, the "Civic" driver has never been confused. The use of the selectors is explained almost identical in both owners manuals.
In a a prop driven aircraft to go into reverse you pull the throttle levers into "reverse" which changes the pitch in the propellers into a reverse pitch setting while pulling the same throttles further back adds more power for stopping.
In a jet aircraft when you pull the throttles back into "reverse thrust" as you continue back you actually add more thrust to the engines while extending diverters to change the direction of the airflow coming out of the back of the engines. (simplified)
Toyota actually promoted the Prius as a "standard" automobile but then added more than a couple of systems for the "technies" that actually refuted this exclamation.
Honda took a standard Civic and put a hybrid system into it but the only evidence of this in the Civic is by a couple medallions affixed to the body. When my son "lends" his Civic to a friend the only thing he has to explain is that it is perfectly normal for the engine to shutdown at every stopsign.

richard schumacher 07-13-2008 12:50 AM

Re: B vs D
Let's keep this simple: "B" is used on long downgrades as an alternative to riding the brakes. Don't use it in normal driving because it will reduce MPGs. (Nuttmeg44, I don't know where you live but there's probably no city in the US where "B" is needed. Driving in the Rockies, yes. In this context "need" = "the brakes would get too hot if you didn't use B".)

Phoenix 07-22-2008 03:43 AM

Re: B vs D
I've used "B" in San Francisco...

Also, the cruise control works in "B" on the 2002 Prius.

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