Something that hasn't been discussed widely is the role of engine vacuum in the unintended acceleration issue.
The reason that you only have one chance to brake a car which is experiencing a stuck throttle or unintended acceleration is that at wide-open-throttle, the engine is not generating any manifold vacuum. Without manifold vacuum, on most cars, the hydraulic brake booster will ingest air as the brakes are applied. Pumping the brakes will fill the booster very quickly. Without vacuum, the brake pedal will be very hard, and it will require a huge amount of brake pedal force to stop the car.
7 comments:
http://ifitsgotanengine.com/2004/11/12/unintended-acceleration/
Exactly what I said way back in 2004.
But you can still turn off the engine or shift to neutral, right?No need to drive yourself to death.
Do you remember the engine vacuum demonstration by the "expert" in the old 60 Minutes attack on Audi?
The only problem with their line of attack was that the Audi 5000 used a power-steering-type hydraulic pump for brake boost. Oops. Audi and Citroen were the only ones not using vacuum assisted brakes at the time, but their expert tried to claim runaway engine robbed the brakes of vacuum assist.
Thanks for the clarification. I keep thinking of the CHP officer and his family who died in the crash that spawned the Toyota recall. It's easy to sit on the outside and say, "well, why didn't he do x, y and z?" My understanding is they were on the phone to 911 and that their terrifying high speed drive lasted quite a length of freeway. It was a loaner car, one where you need to push a button to shut it off, so likely he didn't know he had to do that. One wonders if there were any factors preventing him from shifting into neutral other than his non-familiarity with the specific model. I think how horrible it is every time people discuss the acceleration problem or the recall so nonchalantly. People reported seeing sparks and flames coming from the tires, likely from brakes that were doing nothing to slow him down. He was an adept driver and managed to drive miles and miles without hitting or injuring any other person.
Anonymous said:
"The only problem with their line of attack was that the Audi 5000 used a power-steering-type hydraulic pump for brake boost. Oops. Audi and Citroen were the only ones not using vacuum assisted brakes at the time, but their expert tried to claim runaway engine robbed the brakes of vacuum assist."
Only the diesels used the hydraulic assist. They also didn't have unintended acceleration.
"Only the diesels used the hydraulic assist. They also didn't have unintended acceleration."
Wrong on the facts, sir or madam.
I owned a 1987 5000 S quattro, it most certainly did have the hydraulic assisted brakes. I helped during a routine servicing conducted by a very experienced mechanic who was excited to see that design in the Audi.
My family had several Audi 5000s, the ones after model year 2004 all had hydraulic brake assist, with very firm and direct pedal action. It included manual and automatic gasoline engined cars. Dorri732 is not correct about the diesel-only application.
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