Braking on slippery surface

Yea only regen from the front its not the brakes thats doing the regen its the motor used to help slow the car down with load and this is a front wheel drive so.... but you do have rear drums brakes. Cars always have harder braking from the front and why you see disk on the front. All Prius have abs and some have Vsc to help improve emergency braking (if the vehicle starts to slide out it will apply the brakes to the the other side as needed to help keep control) and mine brakes awesome in the snow, but you wont find a car out there that stops well on ice only better than others. regen is only a small portion of the brake pedal and after you push past that you will get into your hydraulic system (disks and drums) and the prius is like any other abs car.

 

All cars use the front brakes much more heavily compared to the rear ones.

Moreover, I think the braking algorithm is more aware of the situation, and will kick in when the conditions get rough... ABS of course.

 

What does this mean?

Aren't brakes, brakes. I thought the regen effect took over when your foot was off the gas and the car was slowing down. When you apply the brakes, is there a point where your not really applying the brakes, but just calling for more regen?

Help!

 

Nope, "brakes" in the Prius are a request for a certain amount of
stopping torque opposing the present direction of motion. The brake
computer and the hybrid computer closely communicate about this,
with the braking/skid-control ECU asking the hybrid system for a
certain amount of negative torque [i.e. regenerative] and the hybrid
system answering with how much it *can* provide. The braking ECU
supplies the rest by activating hydraulic solenoids to the physical
brakes if needed. This is why leaving it in "D" and braking lightly
and early allows you to stay almost entirely within the braking-
torque range that the hybrid ECU can supply via regeneration, and
avoid burning off your kinetic energy as useless heat in the wheels.
.
_H*

 

Vincent, the Prius is mostly "by wire". When you step on the gas pedal, you don't move a throttle, you move a potentiometer, asking the car for accelleration. The car senses how hard you're asking by the pedal angle and speed of applying presure.

Likewise, with braking, your foot on the pedal sends a request to slow down, as Hobbit explained, and the car balances how hard you want to brake with how much energy can be recovered. There is a hydraulic brake system, but it's usually controlled by a computer. Jam on the brakes hard and it's pure friction stopping those wheels, however.

To make the Prius more like a "normal" car for people who have never driven a hybrid (most of us), Toyota added a simulated automatic transmission drag and creep. Stopped on level ground, take your foot of the brake, and the car creeps forward, just like a "regular" automatic. Driving at speed and take your foot off the gas (stop asking for accelleration) and it feels like an automatic transmission slow down. There's no reason for either of these, except to make you comfortable.

As an aside, the "auto-tranny creep" turns into "hill-hold" if your car is pointed uphill and stopped. The car senses you are in D (and therefore moving forward, even if stopped) and applies enough forward torque to counteract gravity trying to make you roll backwards.

Ask about "by wire" steering sometime...

 

Does it actually increase the forward torque over that usually used for the creeping? I know of at least one hill where it is unsuccessful at preventing rolling backwards; I suspect the hill-hold feature never actually made it in. Of course, it's also possible the hill is steeper than the car is willing to counteract on its own.

 

The car has two electric motors. motor one and motor two. Motor one is a 50k motor used for drive (your creaping) and brake pedal regen (you get more regen the more pedal), and motor two is a 10k motor and used as part of the Transmission planet set to help control gear ratio (awesome design giving unlimited gear ratios by controlling the held gear with motor speed) and two is also used for off pedal regen (letting of the gas going down hill no brake pedal) but only 10k is regened using this motor.

 

Both under control of the computer, not your foot, at least not directly.

Hill-holding works for moderate slopes. It does a good job when climbing out of an underground parking garage in stop-and-go traffic.

 

Hobbit, Bill and Toolbox

I had no idea about the Regen/Brake relationship; thanks for your help.

 

Could someone please amplify the explanation of the the "B" function?

There are different things said about it. One is that it is the most efficient way to regenerate the potential/kinetic energy of a downhill into stored electrical energy. Folks of this opinion recommend using "B" on downhills to charge their battery more rapidly.

My sense is that it just locks the ICE into the drive train so that the pistons are forced into compressing intake air (without fuel) against the exhaust valves, thereby absorbing the energy of motion by exciting the air molecules and slowing the car. There would also be some minimal additional slowing from mechanical friction. However, unless lubrication is inadequate, this should be truly minimal!

Even if utilizing "B" locked the larger MG into the drivetrain as well as the ICE, thereby creating drag electromagnetically, how would this be more effective than simply activating the MG by applying the brakes?

Please advise.

Thanks

storm petrol

PS Those compressed, excited air molecules would heat up. This waste heat would serve to warm the engine (and converter!) somewhat. I suppose that this could have a mild salutory effect on efficiency, especially in cold weather.

 

Bill,

Please inform us about the "drive by wire " steering!

Thanks

storm petrol

 

sorry to jump in Bill

drive by wire: means no direct connection between the steering wheel and the rack. A normal car has a power steering pump, rack, shaft and all are reliant on the engine with use of belt power. you turning the steering wheel directs power to the steering shaft which turns the rack assisted by engine power through fluid. The prius does not have a direct connection between the two and has a electric steering rack which has no fluid and also does not rob power from the engine. disadvantage to drive by wire is you loose feeling of the road as there is no direct connection you wont feel a bump in the road in the steering shaft and right now cost of the racks is about $3000 and fluid racks are $400-800 depending on the car.

another way to look at is the new b2 bomber is fly by wire. meaning the pilot can not fly the plane without use of computers as it is so unstable.

 

Hey Toolbox

Is this really true? What happens with a power failure; do we lose steering control? Where do you all get this steering, braking, accelerator "drive by wire" stuff - I had no idea how computer reliant my new Prius will be. Please direct me to a source of "drive by wire" technical information - I have a lot of reading to do.

Thanks,

 

Jeez... all those responses and nobody botherd to answer the question!

>>>So, when it gets a little icy out, or in black ice conditions, does the
>>>prius get squirrly where another car would be more sure-footed?

The answer is, "No, the Prius does not get squirrely."

Take it from someone with a LOT of 'Prius on Ice/Snow' experience.

In fact, our "BC" model with VSC is all but impossible to make it go out of control or even do a decent donut.

Dang... Prius... Squirrel... now I'm hungry.

 

What are you talking about? Of course the Prius has a direct mechanical connection between the steering wheel and the front wheels through the steering rack, just as any other car does. Just sit in the car when the power is off and turn the steering wheel. There will be resistance, but the front wheels will turn.

The only difference between a Prius and most cars is that the boost, or power assist, is done by an electric motor instead of by a pump driven hydraulic power.

 

IsrAmeriPrius and Tideland,

Thank you for those answers.

Tideland, I am assuming that the twenty mph threshold is relevant for a 2005, yes?

storm petrol

 

I couldn't have said it better myself...

I do want to chime in on B mode. IMO, B mode is like downshifting in a conentional car to provide engine braking. The ONLY time it's really useful is going down a very long hill.

When I drive over the Coast Range mountains, going down hill I use B. Sure, regenreative braking is useful, but it stops when the HV Battery is charged to it's limit and is all green. At that point, it's pure friction braking, just like a conventional car. You don't want to ride your brakes and overheat them. As some have noted, the Prius' brakes are not oversized.

Switch to B mode, however, and as storm petrol surmized, the ICE is engaged without fuel or spark and compression braking slows the car, saving the friction brakes.

I am not sure B mode ever increases regenerative braking.

I've never experienced any "squirrelly" steering, but I haven't driven on ice yet.