NHTSA Tracking Braking Loss on Prius Hybrids

i being a veteran PC'er and one of the first to defend Priuses vehemently, but one thing that i should not have to make perfectly clear.

i would not, in anyway, under any circumstances,take lightly or risk the health and safety of your family any more than i would mine.

of its a question of my car or my son, the car loses by a long long way. and i trust my car to carry my son safely. in that i have no doubt i am driving a safe car

 

If there is someone who is experiencing the "brake problem" and they are close to Cincinnati, Ohio, I will be willing to come visit and drive with you so I can see what you and your vehicle are experiencing.

It is time to get some of the I have a problem people together with the I don't have a problem together to see what is really happening on both sides.

 

Are we really going to debate the semantics of "glitch" vs "failure" here? This is not a ghost, a phantom, or an imagined problem. This is

1. Real
2. 100% replicable
3. Severe - braking power is 100% lost for about 1/2 second. In 1/2 second at 30 mph, this equates to 20 feet of unintended travel. That's plenty of time and distance to hit a pedestrian or another vehicle.

You can debate whether it's a true and complete "failure" of the brakes or whether it's just a miscalibration, but whatever it is, it unavoidably leads to the results above. Don't get me wrong, I absolutely love my new Prius, but I don't know how anyone can argue that that's not a severe problem.

 

I would question the two points (in blue) based on the following information provided by autobloggreen/automotive engineer. It does not appear to be "100% replicable" and it does not appear to be "severe" (as per your defintition) - whether "severe", or not...anytime stopping sdistance is increased, that's an important issue to be addressed. :

"The problem, which has been reported to National Highway Traffic Safety Administration (NHTSA) by several dozen drivers, relates to a loss of deceleration when transitioning from regenerative braking to friction braking. At this time, NHTSA has not opened a full investigation but it is monitoring the situation.

This is actually quite a complicated issue and it highlights one of the reasons why it takes automakers so long to bring cars to market. Read on after the jump for some discussion on what might be happening.

The problem is that hybrids (and upcoming electric vehicles) actually have two different braking mechanisms. They have the traditional hydraulic friction braking system that has been used for most of the past century. However, they also use regenerative braking to convert kinetic energy back to electrical energy to charge the battery. The problem arises from the fact that there is only one control mechanism, the brake pedal. So electronics and complex hydraulics are used to blend the amount of braking from the two systems to provide a net deceleration in proportion to the amount of pressure the driver applies to the pedal.

This would all be fine if the behavior of both systems was completely predictable. If that were the case, both could be modeled mathematically and the estimated torque from each could be calculated. Unfortunately, friction brakes can exhibit significant variability caused by wear, humidity, temperature and numerous other factors. The controls and calibration engineers spend tens of thousands of man hours testing and developing the systems to attempt to ensure that they behave predictably and consistently in over the life of the car and under different operating conditions.

The problem reportedly being experienced on the new Prius relates to an apparent loss of deceleration when the brakes transition from regen to friction braking. Having worked on the controls for similar brake systems in a previous life as an engineer, this is not an uncommon problem. Typically, it occurs when the friction brakes are producing less torque than the model predicts.

A number of things could cause this. The most basic is the brake linings, which could have less friction than expected. In this case, the control algorithm should be able to learn and compensate for this. The problem might also be related to the pressure sensors in the hydraulic unit that are part of the closed loop control. Sensors can have erroneous signals or drift over time and temperature. Again, there are mechanisms to correct for this, but they are not perfect. Not being familiar with the details of the Prius system, we can only speculate about has changed in the new version.

Based on the information in the reports, there is one more possibility. Apparently, at least some of the owners experienced the perceived loss of deceleration after hitting a bump or pot-hole. When we reviewed the Prius, we noted that its newly revised suspension setup seemed to handle larger road imperfections well, but it was a bit over-damped on small sharp inputs. This could cause the tire to lose adhesion with the road in this case and thus start to exhibit slip. The brake system could be detecting this sudden increase in wheel deceleration (as opposed to vehicle deceleration) and proactively reducing brake torque to try and prevent impending wheel lock and the need for full-blown ABS control. If (and we stress IF) this is being done too aggressively, it could be what drivers are feeling. We never experienced any such behavior when we drove the Prius, but we wouldn't rule it out under the right conditions. If you own a third-generation Prius and have experienced anything like this, let us – and more importantly NHTSA and your Toyota dealer – know. It may not be a real problem, but if it is Toyota, will need as many data points as possible in order to diagnose and fix it."

 

Again your attemp to rachet up the decibel level in this discussion and inflame emotions with just 6 posts leaves your intentions open to interpretation.

1. It is real
2. It may or may not be replicable. Some have tried and done it; some have tried and failed to do it. So it's not 100% replicable
3. Your statement here is simply a WAG. Nobody has put a stopwatch to this 'feeling'. It might be 1/2 sec or as Blind Guy noted previously it might not exist at all except in the driver's perception.

From my own experience ( in some distant foggy past ) when I first encountered it, there was a strange sensation of loss of deceleration as if I had hit a one foot patch of ice but the vehicle 'caught' and stopped as suddenly as I had expected. At no time was it 'out of control'. Since then I've filed this issue somewhere very deep on things I have to watch out for on a daily basis, I think it's right next to rabid dogs.

A coworker mentioned early on that she did feel this sensation soon after she got her 2010. She was moving very slowly as she turned from a city road onto a local street. As she slowed, braked and turned she drove over a concrete drainage channel when she felt it. 'It felt like someone hit me from behind' ...but the vehicle didn't go anywhere. No loss of anything. Since then ... nothing.

 

"Severe - braking power is 100% lost for about 1/2 second. In 1/2 second at 30 mph, this equates to 20 feet of unintended travel. That's plenty of time and distance to hit a pedestrian or another vehicle. "

Utter BS.




....Troll

 

If you are able to reproduce it 100% and actually own a Prius, can you make a video?

It is not that we do not believe you -- an extraordinary claim requires some type of proof. This should be an easy one for you since you are able to reproduce it every time.

 

Unless you provide some kind of video or proof of what you are saying, you will be regarded as a troll. For a low post count person to come here and post this is just not credible. In order for you to be believed, you need to back up your claim with proof. Just stating these outrageous claims is meaningless.

We will see if you are a troll or not. Post the proof.

 

That true of any claim, not just something like this.

We see MPG postings routinely that fall apart after the detail is revealed.
.

 

I share the frustration as this matter continues, reinvigorated at times
by low post count new members. The idea that there might be an
orchestrated campaign involved in this is particularly enraging.

That said, it seems like a faint hope at best that the typical
owner/driver can provide anything even close to proof that the
condition alleged significantly affects slow speed stopping distances.

You can't prove a "feeling" or a "sensation." You can attest to having
one of them, but you can't prove its validity. It's that "perception"
thing.

As to proving that the alleged condition causes a meaningful change in
braking time or distance, back in Post #51 I said:

In reading this thread, one question comes repeatedly to mind;
Just how much difference in stopping distance does the matter cause?

It seems like getting the answer would not be simple:
* First, there would be the matter of the road surface irregularity; its
height and its horizontal extent.
* Then, the nature and timing of the driver's response; ride it out or
apply more brake. How much more? How is it measured?
* Finally, the matter of accurate, reproducible measurements may
require continuous measurements of deceleration, as well as
distances.

But, it sure would be helpful to know, whatever the number/s is/are.

It will take a full blown SAE/NHTSA investigation to generate
meaningful data and in its turn an unbiased evaluation and report. But
before that can happen they have to devise a valid testing strategy
and methodology.

It will probably take a year or more to get a report.
Until then, what... The End of The World as We Know It?

Everyone is just going to have to wait...
and then wait some more...
With any luck, or better yet, mature evaluation of the situation, the
ugly exchanges will subside.

"Patience is a virtue, have it if you can.
Seldom in a woman, never in a man."
My late Grandmother.

FWIW: Post #51

 

If you have an iPhone, you can get DashCommand app. I believe that will tell you the G without having connected to the OBD-II port. It uses accelerometer in the iPhone.

That data will be very useful!

 

You guys are making a mountain out of a mole hill. I have been driving a Gen II Prius since Oct of 2004. The Gen II's produce a similar scenario to the one you are describing. When braking and hitting a pothole, manhole cover or just a slick spot on the road simultaneously, the car feels like it loses braking, it's not. All you have to do is keep applying the same amount of pressure to the brake pedal the car will slow and stop quite normally. What you are doing is what I did when this situation happened, you let pressure off the brake pedal, wrong thing to do. You have to keep applying the same or slightly increase the pressure. As far as your 1/2 second your perception of time is impaired, it is a much shorter interval.

 

yesterday I was able to get a partial slip. Yes I was trying, because I want to know what others are feeling.

I was going approximately 25mph, with the charge about half way on the HSI screen, over railroad tracks. I felt a momentary loss of braking, I pushed harder and braking re-engaged. I don't think I had the full experience that others have had.

For this to happen, it may require both driver behavior (proper speed range and brake pedal position) and road conditions. That leads me to believe it is

1) real
2) repeatable for some just not me
3) a problem that should be fixed. If it is 0.5 seconds, then at 30mph the vehicle would move 22 feet. The problem as described does not happen under full braking, so lets say 11 extra feet in stopping distance. This won't be dangerous in the great majority cases, but one extra accident is one too many.

 

I have lived with this supposed condition for 5+ years and once you get used to it you compensate automatically and then it should pose no problems. If the driver panics and releases the brakes then yes it might cause a low speed accident. The problem most people do not know their cars, they buy them and then just get in and drive, never testing how the car reacts in possible emergency situations. I have always taken a new car to an empty parking lot on a dry clear day and done a number of tests to see how the car reacts to unusual input. I have also done it on snow a cover empty parking lot to see the cars reaction under adverse conditions. Knowledge is power and knowing how your car reacts in adverse situations is critical.

 

You are quite right that perhaps this is just a normal aspect of the hybrid braking system. But to some people, the "reality" or the "sensation" is very, very disturbing. Plus given the redesign of the hybrid powertrain coupled with a stronger engine, it's reasonable to conclude that perhaps the symptoms of this problem are more acute with the new 2010 than with the Generation 2. So your molehill might not be the same molehill as a 2010 Prius, even if they are similar.

I haven't read every single post, or every single thread but this seems to be happening most when you have organic or natural deceleration to below 30-35 mph that suddenly gets coupled with a rapid brake on a unstable road surface. Could it be that the more powerful engine and more torque available of the new 2010 just makes these situations more common?

One of Toyota's goals was to try to make the 2010 act and behave more like a mainstream "normal" car. Given that it is still a hybrid, with a Hybrid Braking system did Toyota suceed too well? My point being a smaller engine, means slower acceleration, and maybe more leisurely deceleration, which could mean that the parameters for this braking "glitch" aren't as extreme or met as often in a generation 2 as opposed to a generation 3.

Just speculation, I don't really know.

 

You might want to read a few things about the possible source of the issue, before BLINDLY speculating as to the cause. As noted earlier by an automotive engineer that use to work and now reviews hybrid cars - he said that (perhaps) the key change with the 2010 from prior Prius models) is the much stiffer suspension in the G3, and how the breaking system would compensate (or not) to the loss of tire contact after hitting a larger bump. Softer suspensions tend to absorb the bumps from pot holes, etc without the loss of tire contact with the road.

Auto Racing: Suspension, wrc cars, stiff suspension

"hard suspension gives you best stability given that the road surface is smooth... hard suspension will decrease body roll to a bare minimum and car will be very low thus able to take corners without fear of flipping

problem is with bumpy surfaces were hard suspension will tend to cause the wheels to slightly fly off the surface of the road and accordingly grip is lost and the car will tend to move around and bounce and the control is not so good

you can see that F1 cars got very stiff suspension 'cause they run on smooth circuits but on WRC cars they run on normal roads, cut corners and some times even jump so that kind of "F1 hardness" is not good for WRC cars"




The dynamics of the suspension, the specifics of the road conditions, and the unique aspects (for hybrids) of blending traditional frictional breaking with regenerative breaking, make this a complex and often irreproducable problem... which is proabbly why there have only been 33 reported cases - to the NHTSA - out of 100'000's of G3 drivers.

All this said, I often wonder how many of these complaints (and NOT specific to this breaking issue) are made by those individuals who really haven't expereinced the problem first hand, but simply wish the company, or product ill will?? Case in point - when I read some of the consumer comment on Amazon.com (for example) about some electronic gizzmo, you can find just about every possible opinion. I often wonder how many of the positive (or negative) comments are made by employees of the company, or by employees of a competing company? Since there's no validation process with the internet, it's easy for anyone to post something negative/positive. I'm not saying that some of the breaking complaints aren't real...I'm just saying that I take a LOT of what I read here with a big grain of salt.

 

I'm not an automotive engineer, but I am an engineer (rockets and aircraft, but cars only as a hobby). I believe I have a much simpler explanation for this phenomenon than I've seen posted anywhere thus far. [since I haven't seen it suggested before makes me suspect I may be wrong, but here goes...]

First, forget about ABS, Traction Control, or some sort of smart switching between friction and regen braking. We don't need those systems to explain the behavior.

I agree with the quote above that points out the Prius has two separate braking systems; they could be refered to as friction and regen, as above. Alternatively, we could think of them as direct wheel braking (friction acting independently on each wheel) or drivetrain braking (energy from the drive wheels...in this case the two front wheels...driving torque into the motor/engine where the energy gets dissipated or stored.

Note that non-hybrid cars have both forms of braking, too. Wheel/friction braking is virtually the same in both hybrid and non-hybrid. Drivetrain braking in a non-hybrid occurs when downshifting causes deceleration/braking by transmitting the energy from the car's forward motion, through the drive wheels, through the drivetrain, and finally into the engine, where it is dissipated by compressing air in the ICE cylinders (i.e., converted to heat). In hybrids, drivetrain braking transfers energy through the drivetrain into the motor where it is used to create battery charge.

The drivetrain braking is the key to what I'll call the hiccup in braking many have experienced on the Prius (me too, BTW). Recall that the drive wheels connect to the power source (engine/motor) through a differential gear (or equivalent) that allows the two wheels to turn at different speeds, which is needed to allow the car to turn without chewing up tires; both hybrid and non-hybrid cars have a differential gear system of some kind. Also recall that if you're stuck in snow or mud and one wheel slips in a two-wheel drive car, the other wheel receives no torque precisely because of this differential gear; the engine provides torque to the drivetrain, and the torque serves only to spin the free wheel. Another perhaps overly simplified way to look at it is that the differential mechanism ensures both wheels receive equal torque but don't need to spin at the same speed, and since a free spinning wheel has nothing to push against, the torque is nearly zero despite the high speed. [neglect the fancy traction control systems for a moment]

The differential gear mechanism works both ways, too; both when applying power to accelerate as well as when dissipating energy to decelerate. When decelerating/braking with the drivetrain (downshifting in a non-hybrid or regen braking in a hybrid), the differential mechanism will cause the torque (in the case of deceleration, "negative" torque) to roughly equalize between the two drive wheels. If a bump in the road causes one of the drive wheels to slip or lose contact with the road, the torque in the other wheel will drop to equal the torque of the slipping wheel; that is, near zero. So, for the brief instant that the bump causes one of the drive wheels to slip or lose contact with the ground, the drivetrain braking will drop to near zero. As soon as the slipping wheel grabs the ground again, both wheels will again provide the negative torque required to decelerate/brake.

Of course, if we're using wheel/friction braking and one wheel comes off the ground due to a bump, the other three wheel continue to provide the same braking. Instead of losing up to 100% of braking on a bump when using drivetrain braking, friction braking will only lose 25%; a much less noticable difference.

Presuming this theory correct, then the actual increase in stopping distance can be estimated. Assuming a bump causes one drive wheel to leave the ground completely, and that when it's on the ground it provides normal braking, the increase in braking distance would be the distance the free wheel traveled from the bump to where it landed back on the ground; I'd estimate this in inches unless you're in a car chase on "The Streets of San Francisco" (now I'm showing my age).

As for Toyota fixing this "feature", based on my theory the only thing they could do would scale back or eliminate the regen braking system, which I certainly don't want.

BTW, my theory could be tested using a non-hybrid car. Drive it over the same spot where the Prius has the braking hiccup, but instead of using the brakes on the standard car, downshift to decelerate without using the brakes. See if you get a similar loss of braking/deceleration in a non-hybrid using drivetrain braking. I suspect a front wheel drive car with similar weight would be a good comparison to the Prius. I would do it with my wife's car, but I don't have a repeatable location to try it. But, I have driven standard transmission cars all my life (until the Prius), a lot of the time in the mountains, where I use downshifting frequently. I'm almost certain I've experienced the same sensation in my previous non-hybrid cars under these circumstances.

I welcome comments.

Bill

P.S. I'm convinced this is 1) real, 2) happens for waaay less than 0.5 seconds (e.g., inches of travel), and 3) is not a safety issue I'm worried about.

 

That's the best summary yet. That is exactly what happens as well as why it is not a concern. If stopping distance is important in a given scenario, you are hitting the brakes harder and using the friction brakes, NOT the regenerative brakes. Hence, the loss of stopping distance is EXACTLY what you would find in a non-nybrid car (25% less until the wheel hits again). If you are only using the regenerative brakes, then the distance does not matter, as you aren't into a critical stopping situation.


PLEASE, do NOT "fix" this Toyota. Nothing's broken!

 

If you have only driven a Gen II please do not act like a total fan boy and claim a) you have driven with this situation in a gen III and know exactly what is going on, b) its a great conspiracy against toyota since you don't have safety issues in a GenII so no one could in a different car with different motors in regen braking and most likely different abs, vsc, trac and suspension.

I'm not saying, I'm just saying.

As for the theory of momentary lack of traction re-engaging in a split second, my experience differs. The brake release lasted longer than this. The sensation I felt was consistent with the explanation that the regen brakes (not just the one for the slipping wheel) were released then brakes re-engaged.

 

Regen braking is only provided by the front wheels because they are driven by MG2. So, only two wheel braking in regen.