"Rev protector"?


CL8
03-13-2010, 12:51 AM
I have been reading tips on what to do if, like some Camrys and Corollas,
your vehicle gas pedal gets stuck to the floor.

Toyota and others say to to press with both feet on the brake, then put the vehicle in neutral, then pull over and turn off the engine.


My boss insists pushing the gear shift to neutral should be the last resort because it will blow the engine with the throttle depressed to the floor.

But I also read modern engines have "rev protectors" that will protect the engine if the gear goes to neutral with the gas pedal down.

What is the truth and what would any of you say?

Is it going to ruin the engine to push it to neutral at 60+ mph, or would turning the engine off first be a better choice?


thanks,
cl8

vgames33
03-13-2010, 02:37 AM
Most modern engines have a rev limiter. You can still damage the engine by bouncing the limiter for an extended period of time, but I would risk it to know that the transmission is in neutral rather than drive in a runaway situation.

RahX
03-13-2010, 08:06 PM
A lot of cars have a park/neutral rev limit WELL below the maximum available when in drive/reverse. I don't know if toyota has it setup like this but there is a very limited chance that you will blow your engine by putting it in neutral. The engine should hold up more than long enough in the time between when you put it in neutral and shutting the key off.

MagicRat
03-13-2010, 09:29 PM
I

Toyota and others say to to press with both feet on the brake, then put the vehicle in neutral, then pull over and turn off the engine.


My boss insists pushing the gear shift to neutral should be the last resort because it will blow the engine with the throttle depressed to the floor.


I think the most important issue here is safety. Flipping it in neutral at speed may be hazardous because you are coasting and have lost the ability to maintain speed or accelerate.
Speed and acceleration are useful to have when trying to pull off on a busy highway, so, imo its safer to leave it in gear and just use the brake until you are on the side of the road and ready to come to a full stop.

Years ago I had a '77 Jeep (with a high-powered 401 V8) which used to stick wide open on occasion (Defective carb throttle body). Leaving it in gear and relying on the brake was preferable to neutral when pulling over, imo.

CL8
03-13-2010, 11:53 PM
Thanks for the info Rahax!

So here, I guess you are meaning, the "rev limit" will kick in if the gear is pushed from drive or reverse to neutral or park while at a high speed, correct?

A lot of cars have a park/neutral rev limit WELL below the maximum available when in drive/reverse.And I agree with what you said MagicRat, however many of the reports say the brake failed to stop at all when pushed in Toyota vehicles with the throttle down.

So given that situation, would it be preferable to turn off the engine or go to neutral?
The issue with shutting off the vehicle is loss of power steering and braking, but if the brakes weren't working anyway, would that matter?

MagicRat
03-14-2010, 05:02 PM
And I agree with what you said MagicRat, however many of the reports say the brake failed to stop at all when pushed in Toyota vehicles with the throttle down.
This is mostly not an issue.
Virtually any modern car has brakes which can easily control an engine at full throttle.
You can easily try it yourself. Drive down the road. Mash the throttle to the floor (left foot) then hit the brake at the same time with your right foot. The car will slow down almost as quickly as it would normally.

I think the problem here is with inexperienced drivers. When the throttle gets stuck, drivers are faced with an unfamiliar situation and do not know what to do. They panic and do nothing or simply do not step on the brake hard enough.

It is remotely possible that the brakes could fail at precisely the same time the gas pedal gets stuck, but this is extremely unlikely.


So given that situation, would it be preferable to turn off the engine or go to neutral?
The issue with shutting off the vehicle is loss of power steering and braking, but if the brakes weren't working anyway, would that matter?
I think shutting off the engine and coasting is the worst choice.
You will lose the power assist for the brakes so they are less effective after a couple of pumps, and usually lose power steering, so the steering is stiffer, as well as forward power. All of this is bad.

If in the very unlikely situation the brakes cannot fully control the car, go to neutral.

Interestingly enough, the Prius, (like some other electronic throttle cars, I think) will drop to idle when you engage neutral, regardless of the throttle position. Look here:
http://www.automotiveforums.com/vbulletin/showthread.php?t=980304

CL8
03-15-2010, 04:17 PM
This is mostly not an issue.
Virtually any modern car has brakes which can easily control an engine at full throttle.
You can easily try it yourself. Drive down the road. Mash the throttle to the floor (left foot) then hit the brake at the same time with your right foot. The car will slow down almost as quickly as it would normally.
I frequently do this maneuver with my students, the brake does stop the car, but much more slowly.
The reports say the drivers were pushing as hard as they could on the brake, but with no braking power. This is I believe why they think it it is a problem with the electrical system, not the gas or brake pedal.
I think the problem here is with inexperienced drivers. When the throttle gets stuck, drivers are faced with an unfamiliar situation and do not know what to do. They panic and do nothing or simply do not step on the brake hard enough.

It is remotely possible that the brakes could fail at precisely the same time the gas pedal gets stuck, but this is extremely unlikely.


I think shutting off the engine and coasting is the worst choice.
You will lose the power assist for the brakes so they are less effective after a couple of pumps, and usually lose power steering, so the steering is stiffer, as well as forward power. All of this is bad.

If in the very unlikely situation the brakes cannot fully control the car, go to neutral.

Interestingly enough, the Prius, (like some other electronic throttle cars, I think) will drop to idle when you engage neutral, regardless of the throttle position. Look here:
http://www.automotiveforums.com/vbulletin/showthread.php?t=980304Very interesting. Those videos convince me neutral is the way to go in the Prius, however many new drivers will have an older vehicle for their first car that would ruin the engine if you pushed it to neutral. Would the Camry and Corolla be the same?

MagicRat
03-15-2010, 11:49 PM
The reports say the drivers were pushing as hard as they could on the brake, but with no braking power. This is I believe why they think it it is a problem with the electrical system, not the gas or brake pedal.?
Brakes systems are entirely non-electrical, by design. So there is no electrical system failure that would affect the brakes.

Remember the Audi 5000? About 25 years ago, it too had problems with "sudden acceleration" where drivers would simultaneously experience uncontrolled acceleration combined with complete brake failure. Some drivers would swear they were pushing on the brake as hard as they could with no effect.
However, these cars, when examined later, would have fully-functioning brakes and throttles.

It turns out those Audis had a slightly different pedal location, (relative to the drivers seat) that virtually all other cars, making it more likely for less-experienced drivers to hit the gas pedal, when they thought they were on the brake. As they thought they were stepping on the brake hard, they were actually stepping on the gas pedal.

I simply do not believe that these Prius drivers were using their brakes properly when they claim they could not stop. Likely there is a defect in the car, but my bet is that it is ergonomic, and not mechanical.

RahX
03-16-2010, 03:07 AM
Found this nifty description of Prius brakes:
When you step on your brake pedal in a conventional system, you're pushing a piston that is pushing fluid out of a cylinder (think bicycle pump), through brake lines, to a hydraulic caliper that squeezes stationary brake pads against a rotor that spins along with the wheel. When you step on the brake in a Prius, your still pushing fluid with a piston, but instead of going to the calipers, the fluid goes to a "dead end" chamber with a piston that pushes on one or two springs. The only purpose of the chamber is to provide a pedal feel for the driver. When the brake is pressed with moderate pressure, the piston is only touches the softer of the 2 springs, providing a little give in the brake pedal. This simulates line and caliper flex normally felt on a conventional system. When brake pedal is pressed harder, the piston comes in contact with the second, stiffer, spring, providing a harder brake pedal, simulating the feel of a conventional brake system once the "slack" has been removed. The real braking work is controlled by a brake control computer, which monitors the brake pedal position with two redundant sensors. The brake computer then uses the electric motor / generator MG2 to slow the car and charge the battery or applies pressure to the calipers using pressure stored in the ABS accumulator. If there were a serious brake control problem, two solenoids open, by-passing the dead end chamber and computer control, allowing the brake system to function just like a conventional system.

I COULD see the ABS on a normal toyota causing the loss of brake trouble but it is very unlikely.

jdmccright
03-16-2010, 09:04 AM
Most modern cars with computer-controlled engines will have an electronic rev limiter, just as most cars also have a speed governor. If the ECM senses rpms going too high, it will either cut the fuel injectors or skip a spark of the plugs, or both, to limit the engine's acceleration. A brief jump into the red may not elicit a limiting action, but a steady acceleration with accelerator pedal senors showing the driver wanting to go higher definitely will cause the ECM to cut power.

As mentioned before, older vehicles may not have the rev limiting software in their ECM and it is difficult to know which ones do and don't.

As far as regaining control of a speeding vehicle, stand on the brakes...DO NOT FEATHER THEM, shift into neutral, and guide the car to the roadside, then shut it off. Braking properly is critical. Half-hearted braking only serves to heat the pads and rotors up, causing brake fade and reducing stopping power.

I'd rather have a dead engine than a dead me. And the brakes are likely toast after this so the car is undriveable anyways.

CL8
03-17-2010, 02:53 AM
anyone know what year rev limiters were first used?

MagicRat
03-17-2010, 01:07 PM
anyone know what year rev limiters were first used?
For decades, rev limiters were not needed because most auto engines would not be damaged by short-term over-revving, simply because they would not rev very high.

Up until the 1950's, most auto engines were side-valve engines and/or long-stroke, small-bore engines. As a by-product of their design, they became inefficient at high revs, so even at full throttle, they would not rev up high enough to damage the engine, at least for short periods of time.

More modern, over-head valve engines could rev higher, but even they encountered two separate rev-limiting features of their design. One was "valve float", where, at high revs, the valves do not close fast enough. In this case, valve function becomes inefficient, and the engine will not rev past a certain point, regardless of how far open the throttle is.

Another factor is the old-style points-type ignition. At high revs, the breaker points may 'bounce' where the points are opening and closing so quickly, they lose the ability to precisely control the spark timing, thus power was reduced at high revs, limiting the maximum revs possible.

Engineering modern features such as higher valve-spring pressures, overhead cam engines and electronic ignitions overcome these rev-limiting problems, and make the use of rev limiters practical.

As far as I know, deliberate rev limiters were first employed in some early aftermarket electronic ignitions, starting in the late 1950's. Some of these limiters were adjustable. Rev limiters were employed in many (but not all, I think) factory electronic ignitions, starting in the late-60's/early 70's.

RahX
03-18-2010, 01:44 AM
Up until the use of exhaust catalyst, spark was the main way to rev limit a vehicle. Beyond that they employed many methods as cutting spark with fuel still flowing will only dramatically shorten the life of the catalyst. Modern engines cut injectors and never spark. Engines using an electronic throttle body (no physical throttle cable) USUALLY keep the throttle from opening past a predetermined angle keeping the engine from revving. Which also works well for cruise control (hold throttle open at the desired angle) and speed governing as well. You can push on that gas pedal all day and you won't go any faster. Valve float is also caused by valves that slam shut and bounce open as well as valve springs that are vibrating out of control due to unwanted spring oscillation. Also, if you use too heavy of an oil, at high RPM it can pump up hydraulic lifters keeping them from letting the valve shut completely.

CL8
03-19-2010, 02:39 AM
Good info on the history of rev limiters MR and RahX!

Now my question is, how much more effective are the rev limiters of today than those of yesteryears?

I mean how safe would it be for me to advise a drive student to get in an empty parking lot or road, push the throttle to the floor, then after 5 seconds with the throttle to the floor, push it in neutral to see how it stops the vehicle?

How harmful would this be on a modern engine, or would it do no to little damage?

Thanks,
cl8

MagicRat
03-19-2010, 11:08 AM
Good info on the history of rev limiters MR and RahX!

Now my question is, how much more effective are the rev limiters of today than those of yesteryears?

I mean how safe would it be for me to advise a drive student to get in an empty parking lot or road, push the throttle to the floor, then after 5 seconds with the throttle to the floor, push it in neutral to see how it stops the vehicle?

How harmful would this be on a modern engine, or would it do no to little damage?

Thanks,
cl8

Simply have your students lift-off the gas a fraction of a second after they flip it into neutral, so the engine does not have time to rev-up much.

Another concern here is the ability of your students to use the shifter properly. They should practise shifting from Drive to Neutral without moving the lever too far and getting Reverse.

If a car throttle is stuck down, an inexperienced driver might get too anxious and move the lever too far, and engage Reverse. On lower-powered cars, the engine would probably stall. On higher-powered and RWD cars, you might just get the drive wheels spinning backwards while you are moving forwards, leading to a loss of control.
And such a move is always very hard on the transmission.

Virtually all automatic transmissions have some kind of interlock device, where you can shift from drive to neutral, but no further, unless you push a button or move the lever in a particular way.
So, have your students practice the shift from Drive to Neutral in the car, with the engine off first. :)

534BC
03-19-2010, 08:32 PM
Hope you don't mind, what exactly is going on with the various "throttle sticking" runaways anyways?

CL8
03-20-2010, 03:23 AM
Simply have your students lift-off the gas a fraction of a second after they flip it into neutral, so the engine does not have time to rev-up much. I wouldn't mind doing this maneuver with my students, however when I have them push to neutral after making a turn when I turn off the vehicle, I stress to NOT push the gas pedal before restarting AFTER putting it in neutral, about a third of them forget and push on gas in neutral anyway. It would be even harder to remember to let off the gas for a fraction of a second.

My boss wouldn't want me or my students pushing to neutral from a high speed (unless she was sure it wouldn't hurt the engine or transmission)

Another concern here is the ability of your students to use the shifter properly. They should practice shifting from Drive to Neutral without moving the lever too far and getting Reverse.

If a car throttle is stuck down, an inexperienced driver might get too anxious and move the lever too far, and engage Reverse. On lower-powered cars, the engine would probably stall. On higher-powered and RWD cars, you might just get the drive wheels spinning backwards while you are moving forwards, leading to a loss of control.
And such a move is always very hard on the transmission.

Virtually all automatic transmissions have some kind of interlock device, where you can shift from drive to neutral, but no further, unless you push a button or move the lever in a particular way.
So, have your students practice the shift from Drive to Neutral in the car, with the engine off first. :)Yes I have them practice that before emergency maneuvers, and as you said, every car I have taught students in has a device preventing going from neutral to reverse, but allowing you to push it from drive to neutral.

CL8
03-20-2010, 03:27 AM
Hope you don't mind, what exactly is going on with the various "throttle sticking" runaways anyways?

Toyota Corollas, Camrys and a few others have reports of this happening.
I just went to Toyota today and had them do what they needed to do to fix
my Corolla at no cast to me. (actually my boss since it's her car)

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