How does a blouse get sucked up a car's tailpipe? Very good question!
But, that's apparently what happened to Mike's daughter. And, this week on Car Talk, Tom and Ray heard all about it. (You can hear the call, right here.)
Could it be vacuum from "valve overlap," as Ray asserts -- despite the gales of laughter from his dubious brother?
It sounds crazy, but it did actually happen. After all, we have a witness: Mike's nine-year-old daughter. Now, we're looking for a plausible explanation.
What do you think? Answers from physicists, muffler technicians and everyone else, gratefully accepted!
The explanation I find most plausible would be the engine kicked back and actually ran backwards briefly. When an engine turns backwards, the airflow is reversed, into the exhaust and out of the intake.
As the engine is cranked the cylinders which open after a unsuccessful compression cycle will have manifold vacuum in them when the exhaust valve opens. Thus there would be a sucession of slight exhales with sharp inhales as the cranking progressed.
During the start, the first cylinder to initate a combustion will cause one or more cylinders that did not fire to open their exhaust valves with manifold vacuum plus a full expansion stroke. Thus there would be an initial draw back through the exhaust system followed by the exhale of the first firing exhaust stroke.
That firing exhaust stroke would force the cold air out of the exhaust pipe. When that hot gas hit the cold exhaust manifold, converter, interpipe, and muffler, there would be another short draw back as the hot gases decreased in volume as it cooled.
If the car was already warmed up and the shirt was made of nylon, then is it possible that the little girls little shirt shrank into the exhaust pipe?! Of course it would have to have been really windy not to smell burning nylon! That’s it… it was windy and the shirt really just blew away…lol
There was a little dog under the car. Said little dog grabbed shirt and ran off with it in the opposite direction. Or maybe it had shiney buttons and a sqirrel grabbed it and did the same. The shirt never came back out of exhaust, where was the smoke?
There was nothing unusual about this incident. All the valves opened and closed at their correct times, and the exhaust system was in perfect working condition. The engine rotated in the correct direction. This incident can happen with any ordinary gasoline piston engine. It will not work with diesel engines and jet engines.
When the engine initially starts, the throttle is closed. This allows the pistons to create a vacuum on the intake stroke and retain that vacuum within the cylinders until the beginning of the exhaust stroke. As the exhaust valve opens, air from within the exhaust system fills the cylinder; hence, air flows “backwards” through the exhaust system.
Because the engine was likely of an overlapping configuration (i.e. the piston and valves overlap the same space in their movements), the exhaust valve must close before the piston reaches Top Dead Center of the exhaust stroke. This prevents some of the air that was sucked into the cylinder at the beginning of the exhaust stroke from being re-exhaled at the end of the exhaust stroke (as the piston rises and pushes the air back out). Therefore, over several cycles of the starting phase, the exhaust system experiences overall vacuum. The air that disappears from the exhaust system ultimately ends up in the intake system where there was initially a lot of vacuum.
Of course, once the gasoline starts igniting on the power strokes, air flows in the correct direction through all systems (intake, cylinder, & exhaust).
See how simple that was to figure out? I’m not even a mechanic or engineer (yet).
Small children are known to have challenges with reality similar to those of aging alcoholics. I suppose it’s possible that an amount of suction may have pulled the blouse from her hand, I doubt it actually went into the exhaust system or some evidence would have presented itself. More likely, the child related her perception of reality, and the blouse went under the vehicle or someplace where it went unobserved.
I’m thinking that a venturi effect is the most likely solution. Only possible of there was a hole somewhere in the exhaust or if the car has more than one tail pipe.
When a pressure wave reaches an open ended pipe part of its energy gets reflected back down as a rarefaction (suction) wave.
Similar to a musical wind instrument.
So gas can puff out followed immediately by a puff of suction.
It doesn’t have to start out as suction in the cylinder or around the exhaust valve.
I pondered this long and hard after hearing the story on the show - and I’m thinking the idea that a blouse would go UP an exhaust pipe is so unlikely, I started looking for corroborating evidence.
Wouldn’t the blouse burn in the exhaust system - and wouldn’t that create smoke and/or a smell? Would the blouse go COMPLETELY up the exhaust system with nothing hanging out?
None of these things were mentioned so I don’t believe the story. So how about some alternatives?
How about a gust of wind at the exact time the engine started - and the blouse went under the car? How about the exhaust gas forced the blouse to go the OTHER direction - and it was under the car beside the car in question?
I tend to agree with CapriRacer - more non-corroborating evidence is that after the engine started, it didn’t blow back out. A little confusing about whether it was cold or hot. Said they went to the airport car rental place to pick up a car, yet she was by the trunk changing her clothes.
smith, maybe I wasn’t clear. If the EGR valve was open then maybe a partial vacuum was pulled on the exhaust side when the car was being cranked. Does that make more sense?
There was a hole in the exhaust larger than the tailpipe diameter. Most likely ahead of the muffler on the outside radius of a bend in the exhaust. The initial starting blast created a larger than normal vacuum on the tailpipe, and once the blouse hit the muffler it was stuck. He said that he revved the engine to try and blow it out, but in fact it was just further implanting the shirt in the muffler.
I like all the ideas about the valve lap and EGR, but the engine is a compressor. 99% of the air goes one way. And the volume of exhaust gas out to air in is about 4:1. Even the recirculating exhaust produces a net flow downstream.
-Kevin
(psychology major, mechanical engineering minor) And I stand by the perception error over the exhaust malfunction.
Edit- I just listened to it again and it was a rental car. Not too many rental cars with gaping holes in the exhaust. On the other hand, rental cars do get thrashed pretty hard, and a renter might not know the normal sound of the exhaust. OR, the previous renter cut a hole in the catalytic converter, removed the platinum/palladium and left a hole.
Maybe the rental car was from rent a wreck. I think the only probable time there might be a negative pressure from the tailpipe is while cranking the engine, not after the engine is running. Most cars do not crank too long before starting. If only we had a modej and year to try and replicate the experience.
My thoughts. Why is there a blouse running loose in the trunk. I could guess the blouse was tossed due to some damage the kid did not want to fess up to, and Dad, you’ll never guess what happened!
Listening to the podcast of this show when we got to this question I was actually shouting “Bogus” at my iphone, until I had a moment of insight and realized I could explain something else troubling that happened recently. Ray’s answer achieved such an enormous level of bogosity that a Superbogon explosion occured, and when the resulting wavefront from the blizzard of bogons emitted from this incident crossed the Atlantic something very unfortunate happened.
A previously unknown Quantum Enbogusment effect interfered with the positron experiment at CERN, momentarily altering the laws of physics and changing the speed of light, giving the unfortunate CERN particle physicists a bogus experimental result.
All we need now is for Tom and Ray to broadcast their show from the center of CERN so that the physicists can capture and observe that semi-mythical elementary particle, the Higgs’ Bogon. Once found we will be able to synthesize a Grand Unified Theory that seamlessly unites Car Repair and particle physics.
as a bend and a hole at the bend somewhere before the opening at the back. that could create a suction when the air blew past the hole from the engine starting. That would require that the caller check his exhaust system as a hole could be dangerous.