mechanical aptitude test...
Junior User
Joined: Jul 2007
Posts: 60
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From: Elizabethtown, KY
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?
Postmaster
Joined: Jan 2005
Posts: 4,743
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From: Peoria, IL
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.
Posting Guru
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From: NorCal
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.
BUT it is a lot easier to just say, the piston sucks in air.
Postmaster
Joined: Jan 2005
Posts: 4,743
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From: Peoria, IL
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
http://dictionary.reference.com/browse/suction
Posting Guru
Joined: Apr 2000
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From: NorCal
Originally Posted by Beerstalker
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.
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Junior User
Joined: Jul 2007
Posts: 60
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From: Elizabethtown, KY
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."
So I guess next time I'm in Wally World I'll ask for a "Beer De-Heater."
Originally Posted by cgl
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.
BUT it is a lot easier to just say, the piston sucks in air.
Junior User
Joined: Jul 2007
Posts: 60
Likes: 0
From: Elizabethtown, KY
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.
Got it.
Originally Posted by cgl
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?)
Posting Guru
Joined: Apr 2000
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From: NorCal
Originally Posted by Ironhorse7
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."
So I guess next time I'm in Wally World I'll ask for a "Beer De-Heater."
Touché!
Postmaster
Joined: Jan 2005
Posts: 4,743
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From: Peoria, IL
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.
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.
Posting Guru
Joined: Apr 2000
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From: NorCal
Originally Posted by Beerstalker
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.
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.
All I was doing was trying to help others understand the "suction" theory.
Post Fiend
Joined: Jun 2002
Posts: 13,873
Likes: 3
From: Atlanta GA
> 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?
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?
Posting Guru
Joined: Aug 2005
Posts: 1,473
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From: Coal Country, ND
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!
Try this one on, old Detriot diesels (8v 6v 92 series) that run ONLY a SUPERCHARGER (no turbo) are considered to be N/A!