KB_S1 wrote |
In drive-by-wire cars you cannot activate the throttle and brake simultaneously.
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I don't think that this is
strictly true. -Of the three cars which I own only one is DBW, and it is my only 'automatic' (twin-clutch, technically). On it, you can definitely activate both, and doing so is part of the
'launch control'. The brake is a mechanical linkage still, and still operates uninhibited... I'd bet money that this is true in all cars. -My wife's car is also DBW, but she doesn't let me play with that car... she's seen the effect on our bank balance when the 'urge to improve' inspires me!
One problem however, is that the servo in power-assisted braking is vacuum-driven, and (in most engines) is generated by the engine when the throttle is CLOSED. SOME vehicles (such as the newer Infiniti G37) don't even HAVE a throttle... they determine how much air goes into the cylinders by varying the time that the valves are open (no intake camshaft) and/or the distance to which they open. In these cars (and diesel vehicles, which also have no throttle) there is therefore NO throttle-generated vacuum, and so there's usually a mechanical vacuum pump run off one of the main motor shafts, and an electric pump for when the engine is not running.
In a 'traditional' system, the vacuum is generated when the throttle is partially open or essentially 'closed'. The downward 'pulling' of the pistons pulls air past the open intake valves, and draws a significant vacuum (typically 20+ inches of mercury at warm idle, even more at running speed with throttle-lift) behind the (closed or partially-closed) throttle plate, usually in a plenum which is connected to the vacuum chamber in the brake servo.
When the 'gas pedal' is 'mashed to the mat' the throttling-plate opens and this vacuum disappears, so the vacuum chamber is fed from a line with a "check-valve" in it (which is analogous to a 'diode' in an electrical circuit and the servo chamber is like a 'capacitor') -the check valve lets air flow one way (AWAY from the servo) but not the other, and the servo 'stores' the vacuum in the vacuum chamber.
You can witness an example of this in a traditional servo-brake system, by running your car at idle while parked, and testing what it feels like to press the brakes firmly. -Now stop the engine and press the pedal firmly again. -IT should feel about the same. Now try a third time... then a fourth -Keep going.
The vacuum chamber usually has enough for about three full application/release cycles, and then you should probably notice that the brake pedal suddenly feels VERY 'hard'. -At this point the vacuum is exhausted, and you now have no more servo-assistance. Your braking power is now about one-tenth to one-fifth of what it was before, and at 70MPH (or in fact any other speed) your stopping distance will be extended accordingly.
For a traditional throttle-body-equipped-intake vehicle, the intake generates no vacuum when the gas pedal is "in the mat", and so the brakes would act as if the car had the engine stopped... after a few cycles, the driver would have a 'hard-pedal', the required braking effort would be enormous, and it's pretty much certain that a human driver would not have enough strength even then to overcome a 'screaming' motor.
MDM, wrote |
If the engine goes nuts, what would turning the ignition off do?
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Yes, absolutely. -However, in the awful case of the Caifornian highway crash which was recorded in a 9-1-1 call, the driver (who was a 40-ish year old off-duty police officer IIRC) was driving a 'loaner' vehicle which had been provided to him by the Lexus dealership while his personal vehicle was in for service, and -as in the case of most disasters- it was not a SINGLE problem which made this so calamitous, but the 'perfect storm' convergence of several smaller problems...
These 'service loaner' vehicles at 'prestige-brand' dealerships are sometimes cleverly used as 'lures' to put existing brand owners (who have cars of a few years old and may be at the stage where they're considering their NEXT car purchase) in the mindset that the newer versions are familiar, yet "cooler" in some way. -For example, they sometimes have navigation systems in them. Or a more comprehensive entertainment package... All the little things which make a buyer think "well, I wasn't
necessarily looking to replace mine, but this new gadget is so neat to have... I wonder how much it would be to get the newer model?"
In the Lexus crash, the car had a pushbutton ignition engine stop/start, with an electronic "key", the presence of which is detected by RF. Basically you leave it in your pocket and just just pull the door handle to open the car. -No fumbling for keys, no scratching paint trying to find the lock in a dark parking lot. And to start the engine there's no fumbling down the side of the steering column... you just press "engine start" while the automatic shift lever is in 'park' or 'neutral'.
STOPPING the engine also requires the engine to be in 'park' or 'neutral', so that the driver doesn't accidentally shut off the engine while in the fast lane due to an accidental 'fumble' while trying to turn the radio down when a phone call comes in, for example...
So the "engine start/engine stop" button DOESN'T stop the engine... unless you VERY deliberately hold the button down for an extended period, like three to five seconds.
The people in the car didn't appear to know this... it wasn't their car, and while there's a strong possibility that their personal vehicle had a traditional metal-keyed ignition switch, I suspect that MANY pushbutton-start-engine vehicle owners don't know that trick... I mean, how many people actually READ the owner's manual? (okay... apart from car-geeks like me!!!)
MDM, wrote |
would it be possible to put the car in neutral?
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Yes. Absolutely. The engine would scream the song of near-self-destruction of course, but who cares? -This is -tragically- life or death stuff.
Sadly though, if you listen to the final phone call of the front seat passenger in the California Lexus crash, you get the impression -or at least
I do- that after a few seconds (during which the phone signal is -frustratingly- poor, and some precious seconds are lost as a result), he's not LISTENING to the dispatcher who answers... he's called to talk, and seems to be certain that they've already TRIED everything. At that point they're at 130MPH reaching surface streets and heading towards a required stop at a busy intersection...
Possibly the fact that the driver represented an 'authority figure' (police officers of my acquaintance seem to have "the presence") to the passengers in the car had the effect that when HE said there was nothing that he could do to stop the accident, the passengers just accepted that nothing could be done. -Certainly I feel that the dispatcher said the right things, but the passenger really wasn't 'listening'. (Perhaps understandably... he was -correctly- fairly certain that he was about to die...)
The full 50-second recording is
here but -BE WARNED- it's quite disturbing.
Keith