I second that...so if the NA question wasn't piston suction...then what was it?

 

atmospheric pressure...

 

Got a 98. Don't know which one I got wrong but I think it was the one with the six pulleys because none of the available answers were what I thought it should be.

 

I scored a 96. Missed the NA Engine and the water pressure. I have a hard time understanding how natural ambient air pressure (about 17 psi at sea level) is higher than the draw of, say, a 3.74 inch bore traveling 4.13 inches every .0005 seconds (assuming a 6.0l Ford at 2000 rpm). If that were true, why do I measure the intake performance with a vacuum gage instead of a pressure gage?

 

I got a 94. Didn't pay close enough attention on the first pulley question, or the two fans question. I disagree with the answer they give on the naturally aspirated engine question, it should be from the suction of the piston. If it wasn't for the piston moving down the pressure in the cylinder would be just as high as atmospheric pressure if not higher, and therefore air wouldn't enter the engine. Because the piston does move down, it lowers the pressure inside the cylinder to below atmospheric pressure, which then causes the air to enter.

 

I got an 82... i missed the ones i thought I knew and just flew through. As for the N/A question... think about it like this... As the piston travels down it does not "suck" per se. What it is actually doing is creating a "low pressure" zone which the higher atmospheric pressure will force it's way in to fill. Much like your lungs do not "suck" air. Your diaphragm pulls down, creating a low pressure zone in your lungs. The higher outside atmospheric pressure then forces it's way in. So technically the test is correct.

BUT it is a lot easier to just say, the piston sucks in air.

 

That's exactly what suction is though, creating a low pressure situation that causes a fluid to move from a higher pressure area to a lower pressure area.

http://dictionary.reference.com/browse/suction

 

Right. Which requires atmospheric pressure. Another reason we cannot breathe without an atmosphere. That is why you cannot have suction inside a vacuum. (Ironic isn't it?)

 

Yeah, that's like saying that ice doesn't "cool" the beer, it absorbs the "heat" out of it...

So I guess next time I'm in Wally World I'll ask for a "Beer De-Heater."

 

Now THAT I can understand. A suction cup (for example) won't work in space because there's no ambient pressure to push it against what ever surface your trying to stick it to. There's no differential...

Got it.

 

Touché!

 

On the exhaust stroke the piston compresses all of the exhaust gas which causes it all to flow out the exhuast valve in order to equalize with the atmospheric pressure. So after the exhuast valve closes the pressure inside the cylinder is equal to that of the atmosphere. Without the intake stroke creating a low pressure situation in the cylinder, no air would flow in when the intake valve is opened.

If this was a turbocharged or supercharged engine, where the intake manifold pressure is greater than atmospheric pressure it could force some air into the cylinder without the piston moving since the pressure would want to equalize, but this was supposed to be a question about a naturally aspirated engine.

 

Correct. In a turbo or super charged scenario; The increase in MAP will be ABOVE atmospheric pressure, which will in turn cause the intake to achieve maximum equalization at a higher rate. Still the same basic effect tho.

All I was doing was trying to help others understand the "suction" theory.

 

> why do I measure the intake performance with a vacuum gage instead of a pressure gage

Because it is the carb.'s venturi creating the vacuum and that is why it drops at WOT? Why you don't have much vacuum in a diesel?

 

I got a 78 (damn pulleys and weights!). But I got the N/A question right. I remembered some schooling on that one. I remember our instructor saying that it is atmospheric pressure alone that causes air to enter the cylinders, there was an uproar that day in class.


Try this one on, old Detriot diesels (8v 6v 92 series) that run ONLY a SUPERCHARGER (no turbo) are considered to be N/A!