In 1 (ONE.... O-N-E) 2008 model....

 

Jet 1 you have the target locked on.

 

Does the hybrid brake function when the car is off?
Generally when engine is off with all vehicle, it takes more foot power to stop the vehicle when applying the brakes due to the hydraulic.
I am wondering how does the hybrid brake work when engine is off since it is electrically assisted by the 12V battery?
any feedback on that from users and owners??

 

Guss, the hydraulic brakes are still hydraulic just like a conventional car. The difference is what supplies the power boost. Instead of a vacuum off of a running gas engine supplying pressure to provide power brakes, you have an electric pump supplying that pressure. They designed it to work when the engine is on or off since the engine will not be running all the time in a hybrid. I think the question you are asking is if the key is off will the brakes have power assist and the answer is no. I can feel the pedal height change when I start the truck but the gas engine may not actually be running at that time. I put my foot on the brake before I start the truck and when I release the key to run, I can feel the pedal move toward the floor as the electric pump booster comes on line.

You also have the regenerative braking that the motor/generators (M/G's) supply when coasting or braking above a certain speed. If more force is required than the M/G's can supply, the power hydraulic brakes suppliment the M/G's. I don't know how GM programmed them to work together. Maybe someone else does. Since this is a very heavy vehicle, it depends more on the hydraulic brakes than some other hybrids do. For example, my Prius has very small hydraulic brakes and they only work when the car is traveling less than 8 mph. This trucks brake system is programmed differently I think.

You still have the mechanical parking brake pedal on the left side of the floor that you can set to mechanically park the rear brakes. This is not done by a hydraulic pressure, it is done by a cable that goes all the way back to the rear brakes and actuates the rear brake calipers. At least I believe it still works that way, I saw the cable climbing around under there but did not trace it all the way back.

One of these days, the requirement for a mechanical and hydraulic brake will probably disappear. They may not be needed and are there now as redundant systems and probably required by some law that was written due to liability for some accident. Believe me, the electric motors could lock down the wheels almost instantly if designed to...maybe not the 60kW motors we have but certain setups could.

If I have any of this wrong, somebody jump in and correct it or add to it if you know more particulars. Hope this helps Guss and hope I answered your question.

 

Mel,
Thanks for your detailed and informative reply.
Thats the exact answer I was looking for, much appreciated
so if the 12V battery suddenly dies for some odd reason, the hydrolic brake would work, but you would need to apply more pressure to the padel, if I understand the system correctly?
Hillbilly_Hybrid any feedback from your end is highly appreciated on teh above

Thanks again,

Guss

 

I have no experience w/GM hybrids but the Prius' hydraulic brakes WILL kick in if you're too hard on the brakes, esp. if you slam on them. It's not limited to the last 8 mph. They'll also be the ONLY brakes to function if you brake while in neutral. There's no regen in neutral.

Since there's discussion about creep (to simulate the behavior of automatic non-hybrid vehicles), I found this article http://edocket.access.gpo.gov/2003/03-12051.htm via http://www.techno-fandom.org/~hobbit...-linkfarm.html recently. I haven't had a chance to read the whole thing but the synopsis from the latter was NHTSA/DOT starter interlock and transmission safety standards NPRM for hybrids, that explains the whole rationale about "creep" force and directional cueing for a driver.

FWIW, I can't speak to how GM designed the braking system on their two-modes but the ones on the Prius seem quite complicated. See http://www.techno-fandom.org/~hobbit...s03.html#sld16, http://www.autoshop101.com/forms/Hybrid05.pdf and http://www.autoshop101.com/forms/Hybrid16.pdf. There's also a bit of an explanation about its stroke simulator at http://priuschat.com/forums/gen-iii-...tml#post949068. You can't bleed the brakes on a Prius w/a proper scantool either. Procedure at http://www.techno-fandom.org/~hobbit...qc/f040001.pdf.

 

The thing that makes them complicated is the antilock function, stability control, and the control interface between the regen braking and the hydraulic brakes...all mathmatical magic.

I was active on priuschat 3 years ago and could swear that the function of the hydraulic brakes on that car was only below 8mph. Yes, I have had to panic stop in that car before and I am suprised the seats didn't come unbolted from the floor! I read the info you supplied and I stand corrected if the info is correct. I did not go back and look at all my Prius stuff. I won't make that statement without further research. Could it be that in a hard stop, wheel rotation is confused by antilock and the interface between the systems, but they are not intended to be used for braking above 8mph??? That number is stuck in my head on the Prius brakes.

 

I was up late last night and posted my above response then. I realized I may have simplified a very complex system, as cwerda mentioned about the Prius brake system. They are complex because of the way that the Regenerative system interfaces with the hydraulic system plus the added complexities of the stability control and antilock systems. And thanks for reminding me about the regen lockout when in neutral (on the Prius) as I had forgotten about that. Just finding neutral in a prius is not like a conventional transmission since the lever is shift by wire! Matter of fact, the great thing about the transmission is it isn't really a transmission as we knew them in the past. I like the way you just hit the power button to turn it off and that too puts the transmission in park.

All of these hybrids are a technological marvel and are blazing a trail into the future. Many people that have resisted them will wake up one day and everything will be different on their automobiles. Sometimes having an understanding of the evolution helps you greatly in your ability to use and understand your car, at least it does for me. There is so much more information available on the Prius and way more user input. I for one would love to see three things...First, we need more active participation within forums on the GM 2-Mode Hybrids, we need more printed information about design with technical information we can understand, and lastly we need past and active engineers and service tech's participation on our forums. That will not only answer questions but allow feedback from the field to the teams that develope what will come.

Thank you all for your participation here on this forum. I am not completely pleased with this forum because I can't seem to get a reply from any admin or moderator with simple answers or suggestions...but I am pleased that we have a growing and active group of hybrid owners/drivers/enthusiats that contribute. Thanks.

 

Mel,
Thanks for your insight.

 

One? Not according to Chevy. There has been at least 4...again, that's from Chevy, not me.

My objective was not to have the vehicle replaced, it was to have the issue resolved. Obviously after several attempts this is not possible.