Are you saying that the XLE coasts "farther" (less resistance)? Or slows down faster (more resistance)?

 

The XLE-V6 slows down much faster than the TCH. On a modest downgrade it's easy for the TCH to exceed 80 mph where the XLE-V6 stays at 65 or less.

 

Interesting. Our TCH is my first Toyota since the early 80s. It's coasting behavior is almost identical to our other car, a 2001 Buick LeSabre.

 

With the ICE fully warmed up, and provided your battery isn't "full," when you take your foot off the accelerator and coast in 'D' at speeds below 64 km/h (~40 miles per hour), the ICE stops turning and the car simulates engine braking by providing a small amount of regenerative braking using MG2. This is done to simulate the small retarding effect that a normal automatic transmission gives under similar conditions. The car slows down slowly as some of its kinetic energy is transformed into electrical energy that gets stored in the battery. Coasting at speeds above 64 km/h in 'D' under similar conditions, the ICE must spin in order to protect MG1 from over-revving. In this "fuel-cut" mode, a similar amount of braking is achieved by using MG2 to retard the car slightly, and sending some of the electricity so generated to the battery and some to MG1. The power sent to MG1 slows MG1 down (and so prevents over-revving), and in the process causes the ICE to be spun (with the fuel cut) at its idle speed of ~1000 rpm. Spinning the ICE without gasoline in this way provides a small amount of true engine braking as well as regeneration to the battery provided it's not "full."

Coasting in 'B' is similar, except that the ECU now spins the ICE much faster (~3000 rpm or more depending on the car's speed), in order to provide much greater true engine braking. If the battery isn't "full," significant regenerative charging will also occur. 'B' mode simulates the greater engine braking one gets from putting a normal automatic transmission into 'L.'

I have found, for example, that descending the Niagara escarpment (~100 m = ~328 ft) using regenerative braking in 'D' will consistently cause my battery to read "full" — almost the only time I have seen it "full" is after a significant descent like this.

If the battery is "full," then all the braking will have to be done "lossily" (i.e., no with no regeneration) by using a combination of engine braking and normal friction braking from the brake discs.

Stan

 

Regenerative braking: by drawing power from MG2 and depositing it into the battery pack, the HSD can simulate normal compression braking while saving the power for future boost. The HSD system has a special transmission setting labelled 'B' (for Brake), that takes the place of a conventional automatic transmission's 'L' setting for engine braking on hills. If the battery is full, the system switches to conventional compression braking, drawing power from MG2 and shunting it to MG1, speeding the engine with throttle closed and so slowing the vehicle. The regenerative brakes in an HSD system absorb a significant amount of the normal braking load, so the conventional brakes on HSD vehicles are undersized compared to brakes on a conventional car of similar mass.

Another quote from http://en.wikipedia.org/wiki/Hybrid_Synergy_Drive and sounds like B isn't quite the only time it'll do engine braking.

 

Finding nothing in the manual, is there a upper limit in terms of speed when using the "B" on the 08 TCH?

 

No upper limit on speed -- everything is computer controlled and there are no gears that shift so there is no damage moving to any selection at any speed.

 

Hello from Prius world,
I would like to tell what we learned on Prius.
(Following cases are below 41 mph and the HV battery is not full.)

The B-mode behavior is changed at 20 mph.

Above 20 mph, the ICE spins with fuel-cut. The kinetic energy is converted to mechanical engine braking and regenerating braking, therefore the regeneration is more in D-mode plus slight foot brake than in B-mode.

Below 20 mph, the ICE does not spin in B-mode. It looks the regeneration is same in D-mode plus foot brake as in B-mode if the speed down rate is same.

On approaching to a stop, Attila found the brake pedal value 17 is the best regeneration.
http://vassfamily.net/ToyotaPrius/CAN/brindex.html

Hope TCH drivers will find the magic speed, 20 mph on Prius, and the magic brake pedal value.

Regards,
Ken@Japan

 

Correct, the XLE is gearing down with the friction and compression of the V6. According to the scan gauge the TCH shows the engine turning about 1050 rpm at .02 gallons per minute. The traction battery will be charging but not enough to slow the cars descent as it gains speed.

 

I've found that 17 mph is the transition speed for the TCH in B mode. Above 17 mph the ICE starts, at 17 mph and below the ICE remains off while coasting in B.

My commute has a downhill section of about 3/4 mile that traffic often limits speeds to around 15 to 20mph. Very often I can coast down this section in B mode without the ICE starting as long as I don't exceed 17 mph.