Stuck accelerator and antilock brakes?

[cgallery]

Will a stuck accelerator confound anti-lock brakes?

I've read reports in the past that braking systems would be able to compensate for a stuck accelerator (basically overcoming the power of the engine).

But I wonder what happens when you floor the accelerator and the brakes simultaneously?

Do anti-lock braking systems take accelerator position into account?

Would having a floored accelerator make the anti-lock brakes think the car is in a slide, and therefor cause the brakes to pulse, multiplying the problem?

Just curious. No personal experience in this area.

[jackellis]

I'm not an auto expert but I think the brakes operate independently from the accelerator. I base this SWAG on the widely reported fact that modern autos have literally dozens of microprocessors (as does just about everything else these days).

It would make sense for the many systems in a car to operate independently of one another. Otherwise, the software gets pretty complicated in a hurry. Especially since the microprocessors in question cost next to nothing.

[mpc]

Very interesting question.

With the "old" independent setups, the "dumb hydraulic brakes" would do exactly what your left foot told them to do... and typically would lock up enough to stall the engine. With ABS, the wheel will stop for an instant and then the ABS will say "we're skidding" and relax the hydraulic pressure until the wheel is turning again... then the pressure is allowed to increase until the sensors say the wheel is locked again. Will that be enough to stall the engine? Don't know but I'd bet probably not thanks to auto tranny slippage.

Many engine computers today monitor the brake lights: if you're foot is on the pedal, the engine torque is limited (via ignition timing, or more commonly by cutting off fuel injector pulses & ignition/spark activation for half the cylinders) to prevent damage to the auto tranny and especially the torque converter. All that torque going into the tranny... and no output shaft RPMs due to the brakes means the engine's output energy is getting converted to heat inside the tranny. Bad. So reduce the engine's output. This reduction is to "safe" levels though, not to idle power. Typically 30 to 50% of max power. Folks used to "brake torque" auto tranny cars (pressing both brake and throttle pedals while stopped) to get the maximum acceleration after "side stepping" the brake pedal for drag racing, stop-light racing, etc.

It would be a simple matter to monitor the brake pedal (via the brake light switch/circuit again) and cut off engine power especially with the latest "drive by wire" throttle setups. On those cars, the pedal turns a rotary sensor (like the twist knob volume controls of a radio/old TV) and the engine computer then operates an electric motor to open/close the throttle plates - no cable directly linking the pedal to the throttle. Such a system could easily be programmed with "brakes are ON so reduce engine to IDLE power regardless of throttle pedal position." No more brake torque launches for racers. I'll have to see what my Genesis does... it's electric throttle "drive by wire."

I remember reading Toyota was going to implement a change to their cars to specifically limit engine output if/when the brake pedal was depressed so that tells me those cars probably would NOT stall like the older (simpler) cars, at least not with nominal brake pedal force.

mpc

[Pappy]

In the case of the recent problem with Toyotas and some Pontiacs, the advice i heard on the news was to apply maximum force to the brakes, shift to neutral, and shut the engine down after the car is stopped. The biggest concern is to not panic or to shut the engine down with the car still moving, losing the power assist to both the brakes and steering.

This is going to allow the engine to rev unloaded, unless there is a rev limiter circuit built in, and risk damage to the engine. That would still be preferable to the possibility of brake failure with the accelerator stuck open and the transmission engaged.

[Daryl]

If you have an automatic transmission, you won't stall the engine no matter how hard you hit the brakes, if manual you can use the clutch and let the brakes do their job.

[herb fellows]

In the case of the recent problem with Toyotas and some Pontiacs, the advice i heard on the news was to apply maximum force to the brakes, shift to neutral, and shut the engine down after the car is stopped. The biggest concern is to not panic or to shut the engine down with the car still moving, losing the power assist to both the brakes and steering.

This is going to allow the engine to rev unloaded, unless there is a rev limiter circuit built in, and risk damage to the engine. That would still be preferable to the possibility of brake failure with the accelerator stuck open and the transmission engaged.

I heard the same thing. I was wondering why they didn't throw downshifting first into the mix. Even with an automatic trans, it seems this would begin to slow down the car and make the brakes' work easier and more effective. This is assuming you are doing more than maybe 30-40 mph, because the lowest gear would create a braking effect at that speed., no?

[JimD]

If you google "unintended acceleration" you should be able to find some tests that were done on the "stomp on the brakes and the accelerator" question. The car stops. If you look at the time from 0-60 versus the time from 60-0 (distance may be more readily available and would provide the same answer) you understand why. All cars, even the most powerful you can think of, have brakes capable of larger deacceleration than the acceleration they can get from the engine. The brakes are more powerful than the engine, in other words. Maximum throttle can do no more than increase the stopping distance.

The best theory I've heard on the trooper in California that tragically crashed a Lexus, I think it was, was that he tried gentler application of the brakes to the point of dramatically overheating them (witnesses reported fire) so that they failed to provide the necessary stopping force when he recognized the car was out of control. Maybe that is possible, I have seen no controlled tests. But if you jump on the cool brakes with a car stuck at full throttle, the car stops. Period. No matter what the lawyers say. It has been proven.

This is mainly a legal liability issue. Most unintended acceleration cases are simply instances of the driver pushed the wrong pedal. It's hard for people to accept and I can understand that but I think that is clearly the fact. Toyota has tested their pedal asemblies and I think the number of pedals that did not move in the desired direction was less than 1 in a million or something. There were a few more that moved to the intended rest position more slowly than they should. Toyota can do a better job in both avoiding the mat wedging the gas open (a much more feasible issue) stopped by hinging the pedal (as in my BMW) or providing more secure placement of the mat (velcro in my BMW) and in having a pedal assembly that returns to zero in all cases.

It seems possible in the era of DSCs to disconnect the throttle signal separate fromt the pedal position if the car is in rapid braking. Most of us want the car to do what we tell it, however, so I would want this given very careful thought. I can't envision where this would be a problem but you wouldn't want to trade one issue for another issue.

I have had a mat wedge the throttle open. Fortunately I was able to reach down and slide it back where it belonged. Some people have not been as fortunate, I suspect, and others panicked and did the wrong thing (pushing harder because they thought their foot was on the brake).

Audi had huge problems with this previously.

Jim

[Alex Franke]

My gas pedal got stuck floored once while passing (in a V8) -- after the initial "WTF?!?" moment when the car just kept growling/accelerating, I hit the brakes to slow it back down to "just speeding". I didn't notice any of the automatic "pumping" that you get when antilock brakes are active. The truck ahead of me was coming up quickly, so I threw it into neutral, and braked again, while reaching down (figuring it must be the floor mat) to yank the mat back.

When it went into neutral, I *think* it stopped revving up before it hit redline, and before I was able to pull the mat back. (I wasn't focusing on that at the time, but I seem to remember that.) I guessed that this was a safety feature of the car, but I'm not sure. (Lincoln LS V8, by the way.)

It was over in a few seconds, but it was a pretty frightening few seconds! The other drivers on the road must have thought I was nuts!

To be clear, it was not the stock mat that caused this (not Lincoln's fault) -- it was another mat I added to trap water and grime. The stock mat actually hooks to the floor so it doesn't slide.

[jackellis]

I have a 2008 Tacoma. Mats hook to the floor, so I have not removed them. No problems so far.

My airplane weighs 3100 pounds fully loaded and has two disc brakes. While standing on the brakes, I can apply full throttle (235 HP) and not worry about having the airplane move. Airliners can do the same thing. The brakes have plenty of stopping and holding power.

For light airplane pilots, an engine problem or retractable landing gear that's stuck in the wrong position immediately becomes the insurance company's problem. The airplane's job is to protect you, not the other way around. Same thing applies here. Engines are cheap compared with the cost of even a short hospital stay, and I can always buy another truck.

[LCHIEN]

antilock brakes are an independent system.

They will not pulsate when slowing the car down, until the wheels are detected to have stop turning (e.g. they are locked up) at which point the circuit momentarily releases the braking force until the wheel starts moving again and repeats until the brakes slow the car to a stop at which point it takes very little force to keep the car from rolling. Braking hard that does not lock the brakes will not cause pulsation

The point of ABS is that once the wheel starts skidding the friction and stopping effectiveness is greatly reduced. A rolling wheel in firm, non-skidding contact with pavement, being braked is the optimal stopping method. Just keeping the wheel from skidding allows the most braking force without losing traction.

A car with ABS should be able to stop a car with a stuck accelerator.

[cgallery]

I guess I'm confused because I don't exactly understand how anti-lock brakes work.

On my Honda (w/ anti-lock), if I get going to a considerable (30-MPH) speed on a snow covered road, then slam on the brakes, I can feel the brakes pulsing.

I'm putting all my considerable weight on the brake pedal, so I imagine I've locked-up all four wheels.

I thought anti-lock brakes worked by detecting rotational speed variances among the wheels. So if one wheel isn't spinning, and three are, I thought the system was supposed to pulse the brakes on that one wheel.

Or are the systems more sophisticated and know (for example) that my last speed was 30-MPH, and it takes X seconds to stop from 30-MPH even on dry pavement, so therefor if all four wheels lock up in less than X, it is time to pulse all the brakes?

Or is there some way the system knows my speed independent of the rotation of the wheels? Like a sensor that knows the front-end of the car is plowing?

[JimD]

I found an article describing a test that Car and Driver did on at least 3 vehicles. One was a Camry with features very similar to the Luxus 350 the trooper in CA was driving. With feet firmly on both brake and accelerator, it stopped. No surprise except Consumer Reports said they couldn't stop. I never liked Consumer Reports for automobile advice. They also tested an Infinity. It has a circuit to discontinue opening the throttle if you step on the brake. So it stopped normally. The other car was a souped up Mustang with 540hp or something like that. It took 900 feet to stop. But it stopped.

I posted on a BMW forum to see whether my bimmer has a circuit like the Infinity and was told it does. It also has velcro to keep the mat in position and an accelerator pedal hinged off the floor (so nothing can slide under it). My guess is Toyota would have saved a lot of lawsuit money if they had systems like BMW.

Jim

[cgallery]

I posted on a BMW forum to see whether my bimmer has a circuit like the Infinity and was told it does.
Jim

I tested my Honda (2009 Odyssey) myself and it doesn't seem to have such a circuit.

My test wasn't scientific, I basically drove like an old person (foot on accelerator AND brake) and the engine didn't return to idle like I thought it would if I had a system like your BMW.

But I'm still confused (above) over exactly how anti-lock brakes work.

[Alex Franke]

FWIW, I found this over at http://auto.howstuffworks.com/auto-p...ock-brake1.htm

The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed.

[cgallery]

FWIW, I found this over at http://auto.howstuffworks.com/auto-p...ock-brake1.htm

Thanks, that helps a lot!

I was spending on my time googling "sudden acceleration anti-lock brakes" to see how they would interact.