Does GIII have brake logic override?

Discussion in 'Gen 3 Prius Main Forum' started by maomao, Mar 16, 2010.

  1. David Beale

    David Beale Senior Member

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    The Prius steering doesn't get "harder" than it would be without power steering if the power assist fails. You don't have to fight hydraulic fluid, it's electric, and if it fails it is just a bit more mass in the steering.

    The brakes are pretty much the same. The "brake assist" is actually two things. First, there is hydraulic boost. Then there is the true "brake assist" which senses when you are in a panic (rapid press of the brake pedal, I think also with removal of foot from accelerator, though with cc on perhaps not). In this case FULL BRAKING is engaged, subject to antilock, of course. If you test this, be careful everyone in the car is belted properly and there are no loose items to fly about!
    The above posters clearly did not engage brake assist. It is VIOLENT! As in "stop as quickly as possible to save your life!" Most people don't realize how quickly a car can stop. Even a Prius. ;)

    Note that if the electronics fails the brakes are STILL -normal- hydraulic brakes, just like we've had since about 1930. The pedal just moves a little further. Oh, and they work better than those older cars, as you have disks in the front at least, perhaps all the way round! The valves in the brake master cylinder are controlled by physical pedal movement, not by the computers.

    Now, I used to own a GIII RX-7. One failure mode on that car was a braking problem. Because it had twin turbo-chargers on the intake, and the brakes used vacuum boost, there was a one way valve between the intake and the brake booster (the engine could only suck on the brake booster). That valve could, and did fail. When it did you could have about 10 PSI of -pressure- on the booster. If the throttle stuck open the driver couldn't overcome that with both feet on the brake pedal. NO BRAKES! And 250 HP pushing you in a car that could reach 280 km/hr! Quickly! We learned to use the vehicle key, and place the o n - o f f switch in the o f f position!
     
  2. Susan4ET

    Susan4ET Member

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    If the Prius design was such that its unassisted hydraulic brakes can alone overcome and stop an accelerating at high speed (80, 100, 120 mph?) Prius before brake fade then you can predict the Prius would stop. I don't know, I don't assume, I don't predict that the car was or had to be designed this way--it would just feel good to know.
     
  3. qbee42

    qbee42 My other car is a boat

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    The Prius can't go 120 mph, unless you drop it from somewhere high. As for any lower speed, yes, the brakes can bring it to a stop.

    What do you need to make yourself comfortable about this issue?

    Tom
     
  4. Susan4ET

    Susan4ET Member

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    What hasn't left me feeling comfortable has been the numerous videos demonstrating how the car works perfectly as designed to stop the vehicle. That's not news and isn't very helpful to me. The underlying problem of a really stuck accelerator isn't being demonstrated by anyone. I want to see Toyota with Brake Override disabled stopping the car from 70 mph while the ICE is running full speed ahead. Everyone seems to think it can and that that was the design--stuck accelerator with no brake override.

    So far it seems that all these unscheduled accelerations involve drivers that have trouble stopping the car. Do they not understand what the brake pedal does? Are the incidents all driver error or incidents that relate to the floor mat?
     
  5. justlurkin

    justlurkin Señor Member

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    The HSD computer in the Prius is programmed to automatically decrease the gasoline engine power whenever the brake pedal is depressed more than 50%, to protect MG1 from overload/burnout that would destroy the transaxle.

    So to demonstrate stopping the Prius from 70mph with the ICE running full speed with brake override disabled (i.e. with the ICE running full tilt all the time while it's coming to a stop), Toyota would have to do some really fancy software surgery on the HSD computer for that to happen, and the end result would be a destroyed transaxle.

    You can take a look at the Prius Power Split Device simulator and see what happens if the brake override is theoretically disabled-- Hold the MG2/outer ring gear/drive wheels at 0 RPM (full hydraulic braking) while revving the ICE to max RPMs (almost 4500 RPMs), and see what happens to the RPMs of the central sun gear/MG1. MG1 would rev up to 16,000 RPMs-- WAY past the 10,000 safety limit set by the HSD computer to prevent it from burning out. Toyota Prius - Power Split Device

    Once MG1 burns out, it ceases to provide countertorque to the ICE, which means the ICE running wild would just freewheel without transmitting any torque to the outer ring gear/drive wheels, which means the transaxle is destroyed and the car will come to a stop.

    This transaxle self-destruction capability adds an extra measure of safety against a runaway ICE in the Prius, even if somehow brake override (auto ICE cutoff when both gas and brake pedals are stood on) is disabled.
     
  6. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Re: Does GIII have brakes priority function?

    What you seem to want is a Prius without any 'Nanny' software, something even Toyota can not seem to lay their hands on. This article is about Acceleration alright, but not Unintended Acceleration. Notice how many times they mention the computers on their Prius on both pages 1 and 4.

    Got Hybrid? - Sport - Auto Reviews - Car and Driver

    If these guys can't get a Prius without the 'Nanny' software, no one can.
     
  7. Susan4ET

    Susan4ET Member

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    I know I have read this before. I'm sorry you've had to repeat it. But maybe some good comes of it or maybe not. If one fits this into the instances of stuck acceleration where Brake Override failed the investigators would have found destroyed transaxles--and they haven't have they?

    On top of that I suppose Toyota at some point did demonstrate this and did destroy a transaxle?

    Feeling more comfortable... Seems like Toyota and car rental agencies and dealers are going to have to do a better job training drivers on their cars then.
     
  8. Susan4ET

    Susan4ET Member

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    Re: Does GIII have brakes priority function?

    But that isn't what I'm asking at all. I was asking for the proof that the Prius design would safely brake a car going 70 mph with the throttle stuck open (another way of putting it this time I suppose). To do that they would have to disble Brake Override. And it sounds like the unexpected answer is that it is physically impossible to create the situation since the transaxle would be destroyed and the car would come to a stop under hydraulic brakes alone with the ICE still going full tilt.

    That's all good enough for me--maybe.:)

    Playing devils advocate it is still easy to wonder about what has been overlooked and if we will ever know. I wonder what happens to the "victims". Do they still sue and does Toyota just pay them off? Hopefully when Toyota gets into the courtroom this time they will be more willing now to talk about driver training and driver error. "So your floor mat stuck the accelerator and you panicked and never applied the brakes properly or in time." "How do you figure we now owe you $1M?"
     
  9. justlurkin

    justlurkin Señor Member

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    I'm sure Toyota did... Back in 1995 when they were developing the Prius.

    The Power Split Device transaxle was invented by the American technology company TRW back in the 1970s, but at that time it wasn't practical because nobody could make a computer system to control it back then. We are talking about computers like those used on the Apollo moon mission that had the computational capacity of a 4-function calculator.

    The challenge was to develop a "drive-by-wire" control system that will vary the operation the three components of the transaxle (MG2 traction motor, ICE and MG1 countertorque motor) in a coordinated manner AND do it all on the fly to accomodate road conditions (remember the "lurch" braking over rough road surfaces?) and demands from the driver (through the throttle and brake pedals).

    So Toyota had to do a lot of experimentation with computer software to see what works with the PSD and what doesn't. No doubt they burnt out a lot of MG1 and MG2 motors doing it. :D
     
  10. qbee42

    qbee42 My other car is a boat

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    That is the key question. Looking at the available data, and considering past SUA events with other cars, my bet is on driver error. Almost all of these events turn out to be someone pressing on the wrong pedal.

    However, previous events do not prove that the same is happening now.

    Tom
     
  11. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Re: Does GIII have brakes priority function?

    I think we are saying the same thing.
     
  12. lolder

    lolder New Member

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    I do not think the transaxle/MG1 will destruct in any these tests. I think the ICE will be run by the computer at an RPM that will not overspeed MGI while supplying no torque to the drive wheels. It will be turning the planetary carriage ring of the HSD just fast enough to not overspeed MGI. For the ICE to run away, there have to be multple simultaneous failures, any one of which will be coded before an additional one can create a runaway. For instance, for the ICE to accelerate, the throttle plate has to be opened and the fuel injection has to be increased; both separate functions controlled by the computer. The HSD also has to let the ICE accelerate. With old carburetor cars, if you opened the throttle plate, you automatically got more fuel. This is ALL pedal error or scams!
     
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