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Thread: B vs D
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post #5 of (permalink) Old 07-11-2008, 03:32 PM
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Default 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.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Millenium Silver Metallic (color code 1C0) over gray, package 5 (AI)
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