You guys should have asked this caller if he was having any trouble with his gas tank running over. He obviously drives one of the new AlGoreMobiles that runs on air.
It sucks air into the (formerly) exhaust pipe, converts it to gasoline, which goes back into the tank, and the energy that is spun off in the process propels the car.
AlGore gets a percentage of the profits when the guy drives up to the de-gassing pump at the local 7-eleven and sells a tank full of newly minted fuel back to the Exxon.
Bottom line: feed in more old shirts and blouses… it helps the AlGoreMobile use less air, which I am sure will soon be a new EPA requirement anyway.
Keep on fixin’ em. I love to listen. John Lockridge
I had that happen once, and discovered I had taped one of those mileage magnets on the gas line, too close to the positive post of the battery. The magnet would occasionally reverse the polarity of the system, and the starter would run backward causing the tailpipe to act like an intake manifold. But only when the trunk lid was open. I suspect this was because the courtesy light in the trunk was somehow entering the equation. As soon as the trunk was closed, the system worked normally. As far as the nylon blouse, it probably melted, as someone said earlier, and the nylon fibers are now acting as a liner in the tailpipe and should reduce friction / backpressure. Is the car getting better mileage now?
When the exhaust valve opens, an event called “blowdown” occurs where a slug of hot gasses and a positive pressure pulse race out the port, manifold (header) and exh pipe to the exit. There the pulse is reflected back as a negative pulse, which, if the light fabric was lying againt the opening could “pull” it in. These reflected negative pulses are at the heart of all “tuned” racing exhaust systems. The negative pulse is timed (by header length) to reach the chamber during “overlap” and help scavange the chamber and start the intake flow during a certain “tuned” RPM range. It seems unlikely that in a production, muffled exhaust that the pulse could be strong or long enough to make the blouse disapear up the pipe - but the kid says so… -Art
For those who doubt the negative blouse pulse effect, I’ve attached a page from a landmark Honda SAE paper from 1970. The grapic shows the negative pulse arriving at the chamber during the valve overlap period. Unfortunately, by this time, the blouse had vaporized. -Art
Mike’s car needs a valve job. It likely has more than one exhaust valve that is burnt and not properly sealing. On the intake stroke it is sucking in air (and shirts) from both the intake and exhaust ports.
As a teenager in the late seventies, my friends would drive big V8’s from the sixties with various issues. We checked for burnt valves by holding one end of a dollar bill at the end of the tail pipe. If the bill was being pushed outward by exhaust, life is good; flapping against the end of the tail pipe indicated minimal valve issue; being sucked into the pipe said it was time for valve rework or sell it to an unsuspecting underclassmen.
The answer is quite obvious to all Hogwarts fans. You have most likely heard of Dobby the House Elf who gains his freedom when his master offers him an article of clothing. Well, you must have run across his cousin, Hitchie the Bumper Elf who is now free due to the generous clothing offered to him as he hid behind the bumper, right over the exhaust pipe. The little girl, either knowingly or by accident must have placed her blouse so that Hitchie was able to assume it was a gift and he immediately snatched it up and gained his freedom. Great blessings upon this lucky car owner as whenever he gets into a jam, speed trap, flat tire or whatever, all he has to do is call upon Hitchie to resolve the issue. May your wand never droop and your spirits never falter.
I just re-listened to the call and another idea came to mind. If the caller, like many people, momentarily revved the engine a bit (say up to ~2000+ RPM) on starting, then, when he released the throttle, there would be a surge in “manifold vacuum” in the intake manifold and port as the blades suddenly closed. Since the overlap period repesents a massive “leak” of gasses from the exhaust side to the intake, thru the chamber (which is why “performance” cams cause a rough idle), then while the engine R’s came down (a couple of seconds), all of the cylinders would be virtually simultaneously drawing exhaust into the intake manifold to quench the high intake manifold vacuum. That rush just might be high enough and long enough to “swallow” a small, filmy kid’s blouse. Once inside, it could easily get hooked on a muffler “burr” and stay put 'til it eventually burned up or vaporized.
You do realize, ajbock, that these curves are for a single cylinder. In a four cylinder or greater engine that momentary vacuum is overwhelmed in the exhaust by another cylinder pushing exhaust out.
And, since the piston in the cylinder represented in the Honda SAE paper won’t have any other cylinder assisting it, unless the inertia of the piston plus the flywheel is great enough it would not pull a blouse into the exhaust pipe. The blouse would plug the hole and cause the piston to stop.
The two theories I proposed are totally independant and unrelated. The first is clearly a “grasping at straws” example, but was presented simply to point-out that negative waves (pulses) do occur in the exhaust system and could pull or jerk something momentarily into the exit pipe. It is true that multiple cylinders sharing the same bank (duals) or a single consolidated system, especially including mufflers, would have so much wave interference that most individual pulses would be nullified or “damped out”. However, common American drag cars, like my bracket racer, have 4 into 1 headers on each bank that do not destroy the pulsation effect and are of a calculated length (~32" in my case, based on a local speed of sound at ~1,300 fps), that utilize the sonic effect of the reflected waves. I’m aware that that sonic effect is independent of the “inertia” effect of the “slug” as it traverses the individual header pipes and combines in the collector so that the cylinders are mutually supportive.
In the second theory, which I think is far more viable, the intake manifold serves as a fairly large “vacuum reservoir” and a common vacuum gauge, that is used for tuning, samples that depressed pressure value available to all cylinders simultaneously. If you bring an engine up to say 2000 RPM, and shut the throttle suddenly, you will see the gauge jump because of the increased manifold vacuum. All of the cylinders “see” that depession simultaneously and that is what causes “reversion” (the passage of exhaust gasses into the intake ports and runners). Evidence of reversion is very common in racing engines even if they are only slightly “overcammed”. It is a grey powder coating the surfaces. I suspect that the mass and “filmyness” of the blouse would be far more relevant than the mass of the flywheel or flexplate and torque converter. Also, in the second theory, the mass or behaviour of the components of any one cylinder would be irrelevant.
But the real question is … did they fess up to the rental company that there may or may not be a shirt up the tailpipe? Because that would be a pretty great conversation…
crossover pipes. In a stock vehicle with two banks the waves interact via the crosspipe. There’s never a negative pulse at the tailpipe. Never will an intake valve opening to a lower pressure intake manifold creat a negative pressure pulse that will make its way to the tailpipe.
racing engines are, as you say, “overcammed”. Lift and/or duration are very different than in stock engines, allowing much more interface between the exhaust port and the intake manifold. Even then, a properly designed header set will time the pulses to prevent two pulses from trying to go down the same pipe at the same time.
I suspect that it would take more vacuum to suck even a thin blouse through an average exhaust pipe that you’re envisioning. Remember that the force will only be the psi times the surface area, and the average tailpipe doesn’t have much area.
All very good points. I think possibly because of my reluctance to accuse the caller of being a prankster or a fraud, or of calling his 9 yo daughter a fool or a liar, I was eager to come up with some rational explanation. I don’t presently have a motor vehicle or I would try to repicate it myself with a standard american auto (my Schwinn just won’t do). I did look for a light piece of silky material I have around here some place but couldn’t find it. I figured I could talk a neighbor into trying it.
Remembering that it happened just as he started the car, and maybe reved the engine a bit before letting off, it should be easy to experiment. Has anyone tried that?
Also, one of the other contributors mentioned, as a teen, they witnessed a dollar bill being drawn into the exhaust pipe exit under cerain conditions, implying a negative pulse. However, since he was just a teen at the time, he was probably either mistaken or a liar too.
A couple of things I should mention: When the intake valve opens (during overlap) with high intake manifold vacuum present, exhaut gasses are suddenly “sucked” into the intake manifold, through the chamber and still open exhaust valve - that is a negative pulse.
When Exhaust Emissions first raised its ugly head in the mid-sixties, one of the first things Detroit engineers did was increase overlap on production cams (generally by reducing lobe centerline separation angles and emulating racing cams) to provide a “natural” exhaust gas recirculation (EGR). The idea, of course, was to “reburn” the dirty residual exhaust gases that the still rising piston was scraping off of the quenching cylinder walls.
I’ve tried to attach an illustration so anyone following can clearly see what we mean by overlap. The little curve included is a typical piston velocity curve to help.
Actually, manufacturers now are returning to designing overlap in specifically in lieu of EGR systems. However, exhaust gasses being pushed into the manifold from the other cylinders will easily excced the drop in pressure from the overlap. A single cylinder will experience the low manifold pressure drawing some exhaust gas back in, but the tailpipe will never feel it. It’ll long since have been dissipated and overwhelmed before it gets there.
I dunno, I guess all kids at some time or other tell a story when caught in a bind in order to cover their mistake. I don’t remember any specifically, but I’m sure I did too. I’m reluctant to use the term “lie” for something like this. This is just a typical kid thing.
I also acknowledge that as implausable as it seems it might have actually happened. I’ve seen enough things in my passing decades that amazed me to realize how little I’ve actually seen.
tsm, This has been a fun exchange! I just wish we could find out what actually happened. I’m going to find a way soon to experiment but today its really windy and rainy here. Its probably not a good idea to do exhaust system experiments inside a closed garage either! Thanks for the dialog - I did enjoy it.
PS: After I try some experiments I’ll report my findings, whatever they are. -Art
Good luck. Like so many anomolies, if it did actually happen you’ll probably have an impossible task duplicating it.
I too have enjoyed the exchange. The entire mystery of physics is always enlightening to discuss. Keeps my neural synapses alive. I have very few people in my daily life with whom I can have these types of debates.
I’m looking forward to the show today (NPR 10 am here) and I’m wondering if the guys will have any follow-up on the missing blouse story. I don’t know how these shows are actually produced so I can’t even guess what the chances are. They said they were going to try out the described scenario and see what happens, so it would be nice to find out the results of their tests before I try out my own.
tsm, Don’t feel like the Lone Ranger - I’m in the same position, which is why I so enjoyed our discussion. -Art
I’m with the dual exhaust camp. Especially if one side of the exhaust was “decorative”. My theory: there is a venturi that creates a Bernoulli effect in the junction where tail pipes split. All of the exhaust gas goes across the junction and out one side, the other “decorative” tail pipe has a reverse exhaust flow. The blouse is still in the pipe. Since the air flows backwards, it may stay relatively cool. The ventury may have been intentionally designed and installed to assist in passing smog tests.
I like the Bernoulli theory but since this was a rental vehicle it seems unlikely that they would have a decorative pipe, unless it came from the factory that way. Still, I must plead ignorance here because I’ve never heard of this being done before. Since I was a kid, back in the '50s, adding duals generally meant headers (on a V8), full length pipes and separate, low restriction mufflers to reduce “back pressure”. Later, as mentioned by tsm, a crossover was added by the collectors because David Vizard said so! =Art