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Originally Posted by Deltona Couple
Sorry, but for the record I will just have to disagree with you on this one. I see the point you are trying to make, but in a real world situation, there is "14psi" of normal atmospheric pressure at all times at sea level. so a moving piston would create a vacuum based on the principal of air flow through a restriction (i.e. the open valve) If you were to actually connect a vacuum/pressure gauge to the cylinder you would see the fluctuations of vacuum (and pressure when on compression stroke). We did this experiment many years ago in our physics class when I was getting my degree in automotive technology. The question we had was based on a naturally aspirated engine, turning at normal cranking RPM of a standard V-8 engine. The solution was that since the movement of air at atmospheric pressure was too slow, a vacuum HAD to be introduced by the piston, otherwise the cylinder could not fill up fast enough to produce proper combustion. (based on volume of air that can be moved at atmospheric pressure, v/s volume of air that can be moved under vacuum)
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Well, for the record, are you trying to say that the test and I are both wrong?
I'm confused (but curious) by your description of your experiment...any more details on exactly what you were trying to prove?
There are two measurements of pressure--gauge pressure, which compares something to atmospheric, and absolute pressure, which is the actual scientific pressure present. If you had some way of measuring absolute pressure on a cycling cylinder, it would have just read less than 14psi on the down stroke, higher (atmospheric x the compression ratio, usually about 9:1, so 126psi-ish) on the upstroke, and extremely high during combustion.
I'm not sure what you mean exactly by "The solution was that since the movement of air at atmospheric pressure was too slow", as air that is at atmospheric pressure won't move at all! It won't move unless there is a pressure difference, such as the one set up by the low pressure volume created by a dropping piston.
In the real world, when you are talking about moving gasses, it isn't a matter of opinion--there is no such thing as suction or vacuum. These are both laymans terms to describe the observed effects of a pressure differential. When speaking scientifically, forces from pressure differences are always a push, never a pull. (and yes, dirty jokes about the fact that therefore nothing can ever be sucked abound...luckily they can still be blown
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Our two (real) choices were:
In a NA engine, air enters the cylinder...
From suction caused by the piston going down
From atmospheric pressure pushing it in
Since suction doesn't exist, then the answer becomes obvious. The expanded volume of the cylinder created by the descending piston temporarily creates a low pressure volume inside the cylinder, and atmospheric pressure pushes new air in. If you still don't agree, think about how the situation would change if you changed atmospheric pressure. Bring it to the top of a mountain where there's less air pressure? Guess what, less air gets pushed into the motor, and you have less power.
I have a feeling that you and I are just using different words to describe what's happening, however in this specific case, the question was one of definition, so which definition is more accurate is what it has to come down to. My gas dynamics prof would be horrified if I ever put "suction caused by the piston pulls air through the valve" on any of my tests.