electronic throttle
in [Technical]
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From: Bill Putney on 7 Mar 2010 11:16 dsi1 wrote: > I certainly believe you. You can get a feel for the amount of reserve > vacuum boost on your car by simply repeatedly pressing down on the > brakes without starting your engine. If your brakes are working > properly, you'll feel the pedal getting firmer until you'll only be able > to move the brake pedal a couple of inches of deflection. At that point, > you'll have used up all your vacuum reserve. I figure that you should be > able to get around 3 stabs at the brakes with mostly full boost. This > means you'll only get maybe two chances for full braking after the > initial attempt at braking. That's the breaks I guess. :-) My recollection is that boost is noticeably diminished after the second stab, greatly diminished by the third stab - engine overpowers brakes for most common vehicles. -- Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
From: jim beam on 7 Mar 2010 11:24 On 03/07/2010 08:10 AM, Bill Putney wrote: > jim beam wrote: >> On 03/06/2010 08:26 PM, Bill Putney wrote: > >>> I don't necessarily disagree with the rest of your post, but that part >>> of your post is definitely incorrect. Have you ever played with your >>> power brakes while simultaneously pressing the accelerator? Anything >>> more than one or two initial stabs at the brakes depletes the vacuum >>> stored in the booster, and with even slight power simultaneously being >>> demanded of the engine, the vacuum is not enough to directly power the >>> brakes, much less re-charge the vacuum in the booster. >> >> i have done this. with the engine off, the vacuum remains until the >> pedal is released - thus if you stomp the pedal and keep it there, you >> don't need to keep replenishing the vacuum. and you will stop the car. >> with the engine running, there is no vacuum issue, and the brakes are >> still powerful enough to stop the car. on my honda anyway. > > I have real trouble believing that a large majority of people would, in > a sudden inadvertent acceleration situation, be content to press the > brakes one time and not try to pump them once or twice. why? if the car in front of you suddenly jams on their brakes, do you apply then release your pedal? i think your answer is "no" - unless you're skidding and know what cadence braking is. and if you know what cadence braking is, you should know that to stop a car with the engine on full throttle, you apply the brakes hard and quickly - you don't monkey about with multiple brake applications that can cause excess heating and fade. > After that, the > brakes will be almost totally ineffective because of loss of vacuum. no. fade maybe, but vacuum is always present if the engine is running. even if it's not, you still have vacuum reserve for three applications. > >>> People don't believe that, but try it on your car: On a deserted road at >>> highway speed, stab the brake pedal a couple of times while holding the >>> gas pedal down a little bit to load the engine slightly (this works >>> anywhere from slight to WOT throttle). I guarantee you (unless your >>> brake booster gets its vacuum from something besides the intake vacuum - >>> like a separate electric motor-driven vacuum pump) that after two or >>> more stabs at the brake pedal, the braking power will be extremely low - >>> so low that the engine will have no trouble overpowering the brakes. No >>> vacuum in the booster essentially equals no brakes. >> >> with respect, i think you're confusing vacuum with fade... > > No - I'm not. While you could certainly induce fade with a certain > prolonged script of usage of the brakes, what I'm talking about is true > over what I would say would be the real world typical scenario (before > the fade issue becomes real - which - yes - it would over a longer > period, but not likely if the 2 or 3 stabs had already occurred in the > relatively short period that I would expect). It is a fact that the > vacuum cannot recharge with almost no vacuum in the intake - it doesn't > recharge by magic. I guarantee you that after a third stab of the brakes > on an engine vacuum-driven power brake car, the brakes will loose the > fight with the engine - fade has nothing to do with that over the first > few seconds that we would be talking about (during which the first 2 or > 3 stabs would occur real world). if that is your experience, then i think you must have a vacuum leak. even with wide open throttle, there is sufficient vacuum in the manifold to create significant braking assist. > > i've >> experienced that too, one particular time on a major hill in san >> francisco approaching a busy intersection. yes, it's scary stuff. but >> when i changed the pads on my civic from after-market to oem, all fade >> problems disappeared. even fully loaded, repeatedly decelerating from >> speed. [i learned my "honda oem is best" lesson that way.] >> >> >>> >>> Also, once the booster is depleted of vacuum during that experiment, the >>> vacuum charge in the booster will remain depleted until a second or so >>> after the throttle is released - IOW - deplete it and continue applying >>> the throttle (again - doesn't have to be anywhere near WOT) for several >>> seconds. Every once in a while, while still applying the throttle, try >>> the brakes again. You will not have any effective braking until *after* >>> you release the throttle. >>> >>> I urge anyone who doesn't believe what I claim above to try it before >>> commenting. >> >> i have. my results and comments are as above. > > And both of us could design the script to create either end result we > wished. I submit that in the real world, most people would try pumping > the brakes in desperation if the sudden acceleration scenario actually > happened - the depleting stabs would occur before fade became a factor. > sorry dude, it's incredibly rare for a panicking driver to pump their brakes. that's why abs exists - abs does that for them, and only in the event of traction limit being exceeded.. -- nomina rutrum rutrum
From: Bill Putney on 7 Mar 2010 12:44 jim beam wrote: > On 03/07/2010 08:10 AM, Bill Putney wrote: >> jim beam wrote: >>> i have done this. with the engine off, the vacuum remains until the >>> pedal is released - thus if you stomp the pedal and keep it there, you >>> don't need to keep replenishing the vacuum. and you will stop the car. >>> with the engine running, there is no vacuum issue, and the brakes are >>> still powerful enough to stop the car. on my honda anyway. >> >> I have real trouble believing that a large majority of people would, in >> a sudden inadvertent acceleration situation, be content to press the >> brakes one time and not try to pump them once or twice. > > why? if the car in front of you suddenly jams on their brakes, do you > apply then release your pedal? i think your answer is "no" - unless > you're skidding and know what cadence braking is. and if you know what > cadence braking is, you should know that to stop a car with the engine > on full throttle, you apply the brakes hard and quickly - you don't > monkey about with multiple brake applications that can cause excess > heating and fade. We aren't talking about what the savvy drive does and knows about. What I've said still stands for the scenario we are talking about and for many drivers. We weren't talking about what the best thing would be to do. >> After that, the >> brakes will be almost totally ineffective because of loss of vacuum. > > no. fade maybe, but vacuum is always present if the engine is running... Not enough for any effectiveness of brakes. Do you even know what happens to plenum vacuum at light, moderate, and heavy throttle? If you knew the numbers, then you wouldn't be saying that an engine running (with throttle open) will have enough vacuum to run the brakes - because it simply isn't true. Anyone who has used a vacuum gage to any extent knows that vacuum plummets when the throttle is open. > even if it's not, you still have vacuum reserve for three applications. That part is arguably true. I'd say you're loosing effectiveness after 2, but what's one more jab of the brakes between friends. The result will be that the booster will absolutely *not* charge back to any usable level under acceleration. Plenum vacuum is just too low. Again - you don't seem to know that - if you want to argue that point, then give me some numbers for vacuum for a common engine of your choice at idle, and in gear under light, moderate, and heavy acceleration. If the numbers you come back with are honest, you will prove what I'm saying. >>> with respect, i think you're confusing vacuum with fade... >> >> No - I'm not. While you could certainly induce fade with a certain >> prolonged script of usage of the brakes, what I'm talking about is true >> over what I would say would be the real world typical scenario (before >> the fade issue becomes real - which - yes - it would over a longer >> period, but not likely if the 2 or 3 stabs had already occurred in the >> relatively short period that I would expect). It is a fact that the >> vacuum cannot recharge with almost no vacuum in the intake - it doesn't >> recharge by magic. I guarantee you that after a third stab of the brakes >> on an engine vacuum-driven power brake car, the brakes will loose the >> fight with the engine - fade has nothing to do with that over the first >> few seconds that we would be talking about (during which the first 2 or >> 3 stabs would occur real world). > > if that is your experience, then i think you must have a vacuum leak... That is simply not the case. > even with wide open throttle, there is sufficient vacuum in the manifold > to create significant braking assist. That is profoundly incorrect - period. >> i've >>> experienced that too, one particular time on a major hill in san >>> francisco approaching a busy intersection. yes, it's scary stuff. but >>> when i changed the pads on my civic from after-market to oem, all fade >>> problems disappeared. even fully loaded, repeatedly decelerating from >>> speed. [i learned my "honda oem is best" lesson that way.] >>>> I urge anyone who doesn't believe what I claim above to try it before >>>> commenting. >>> >>> i have. my results and comments are as above. >> And both of us could design the script to create either end result we >> wished. I submit that in the real world, most people would try pumping >> the brakes in desperation if the sudden acceleration scenario actually >> happened - the depleting stabs would occur before fade became a factor. > sorry dude, it's incredibly rare for a panicking driver to pump their > brakes... That's your *theory*, or you have something to back that statement up? Admittedly, my theory that some percentage of drivers would pump the brakes is not provable as far as I know, but I do state it as conjecture, not as fact as you are stating your argument. > that's why abs exists - abs does that for them, and only in the > event of traction limit being exceeded.. You're not talking about the same situation. Trying to stop a vehicle with runaway acceleration is not the same setup as emergency accident avoidance as far as paniced driver psychology. (Yes - there are similarities, but don't turn that similarity into being equal.) -- Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')
From: Grumpy AuContraire on 7 Mar 2010 13:01 bjn wrote: > On Sat, 06 Mar 2010 10:38:19 -0500, Bill Putney <bptn(a)kinez.net> wrote: > >> jim beam wrote: >>> if you buy all this fear-mongering idiocy that electronic throttle is a >>> problem, and that brakes, transmissions and ignition kill switches can >>> all simultaneously fail causing a driver to lose control, it might be >>> worth auto manufacturers of all stripes to adopt a slightly different >>> implementation of electronic throttle [e.t.] - if not for mechanical >>> reasons, but to shut the idiots up... >> The lawyers, politicians, and news media can convince the public of the >> impossible (failure even a totally fail safe system) any time they >> decide to do it depending on political or monetary motivation. IOW - >> the people and companies who do a good job of designing are going to get >> punished anyway (unless they know how to play the game in a corrupt >> system). There are people in our society whose life goal is to make >> sure that that happens. > > > The problem is that now lawyers, politicians and news media are driving (no > pun intended) solution. The way I see them talking, cars will wind up with > a fail-safe throttle that is more fail-safe than the controls of a jumbo > passenger jet. > I'm not sure about this but for sure... The causes you cite certainly contributed in getting to where we're at! Oh, don't forget that little incident when a B-777's engines went to idle about a minute before touch down at Heathrow about a year ago. Aircraft was totaled but there were no major injuries. Cause has been assessed to software/computer glitch. JT
From: Bob Flumere on 7 Mar 2010 13:43
On Sun, 07 Mar 2010 08:35:07 -0800, jim beam <me(a)privacy.net> wrote: >On 03/07/2010 08:16 AM, Bill Putney wrote: >> dsi1 wrote: > >maybe if the engine is stopped and vacuum gone. but if the engine is >running, you still have vacuum. Absolutely not so with any naturally aspirated internal combustion engine. Manifold vacuum depends entirely on the air moving through the engine and being "throttled" by the butterfly in the intake. This vacuum only exists because the engine is pumping out air that cannot be resupplied through the closed or partially closed throttle plate(s). Throttle open (stuck, actuated improperly, or opened wide by driver) always equals ZERO (or certainly very low) manifold vacuum. Low manifold vacuum means zero power assist from a vacuum assisted brake booster. This gives the driver a very hard brake pedal after only couple of strokes and surely a major reduction of the amount of line pressure that the average person could muster with only leg power. ie: the feeling that the brakes have "failed". Unless there is some kind of a vacuum backup assist pump.. (some trucks and buses), not that I'm aware of on Toyotas, or the brakes are power boosted by some other means, hydroboost etc. there is going to be little or no power assistance to help the driver control the "runaway" vehicle if the throttle is in the opened position. Neutraling the runaway engine, allowing the RPM to rise until there is some other limiting factor STILL won't produce much, if any intake manifold vacuum. It would of course help the driver to regain control (if only with no power assist to the brakes). Switch "off" may stop the engine and if the still engaged transmission allows it to stop turning, there still won't be any brake assistance because there still won't be any vacuum as long as the throttle is (stuck?) open. Furthermore, if the engine should stop turning when switched "off", we now have the added element of no power steering asist, (if the steering is assisted by an engine driven pump). Hope this somewhat basic expanation helps to explain what the driver of a "runaway" feels and experiences. |