Mechanical aptitude test

[kowe69]

The question about the engine is crap, The motor sucks the air into it hense vacuum!

The FACT is that it is atmospheric pressure that forces air into the cylinder.

I'll explain: When the intake valve opens and the piston travels downward, a low pressure area is created. There is still pressure present, it is just lower then the pressure outside of the intake. In the Salt Lake valley, air pressure is approx. 12.5 psi. At sea level it is approx 14.5 psi. The atmosphere has mass. And it only has pressure for one reason, gravity. The force of gravity pulling air downward creates pressure. The lower you are, the higher the pressure. This is similar to water pressure in the depths of the ocean. It also explains why engines run better and race cars run faster at sea level.

Another example: hypothetically, say the earth has air but no gravity. There would be no pressure. A naturally aspirated engine would not run because the is no pressure to force air into the cylinder. With a supercharger or turbo to create the pressure it (in theory) would work.

There is no such thing as suction. Only high pressure and low pressure or absolute 0 which would be compaired to outerspace. When you turn on your vacuum and you hear and feel air rushing into the hose, this is only because the 12.5 psi of air pressure forcing air into an area of lower pressure created by the impeller.

A vacuum gauge for example, does not really measure vacuum. It measures the difference between atmospheric pressure and an area of lower pressure.

Another example. Take a Ford 7.3L diesel engine. It is equipped with a manifold absolute pressure (MAP) sensor and a barometric pressure (BARO) sensor. The MAP sensor measures pressure inside the intake manifold and compares it to pressure outside the manifold (atmospheric pressure). With the engine OFF the pressure inside is equal to the pressure outside. It would read as 0 psi. Add boost from the turbo and it may read 20 psi. But that is actually 20 psi above atmospheric pressure. When you combine the two it is more like 32.5 psi above absolute 0 psi. The BARO sensor however, only measures atmospheric pressure in relation to absolute 0 psi. It would read 12.5 psi and will only change as the vehicle changes elevation.

And finally another example. Hold your lips shut tight and attempt to exhale. The pressure created by your lungs is trying to force air out into an area of lower pressure. That being outside your mouth. You are creating an area inside your lungs that has a higher pressure than the 12.5 psi outside your lungs.

My point is that differences in air pressure will always try and find a way to equalize. High pressure will always travel toward low pressure.

Hope this helps explain it to those having trouble understanding.

 

[kowe69]

As far as the test goes...these are pretty standard questions in the automotive industry. I took tests like this as part of Ford's curriculum, on ASE certification tests and the army has a similar but more broad test used to determine job placement for new recruits.

I scored 84% which was surprising to me. I expected to score higher. I know of one question that I checked the wrong answer and clicked submit. It would not allow me to go back and change the answer afterward. I did not however review the test to see which others I missed. I would like to know so I may take it again. But I believe that for the most part the answers that they consider correct, probably are.

 

[78mitsu]

The FACT is that it is atmospheric pressure that forces air into the cylinder.

the FACT is that a rational person would realise that if the piston wasn't moving the engine wouldn't be running so the pressure inside the cylinder would be equal to the pressure outside the cylinder. so it doesn't matter what the atmospheric pressure is.

The reason(cause) the air flows into the piston is that the piston goes down in the hole when it does that the volume of unoccupied cylinder increases since all things seek their lowest energy the air decides to go from the outside to the inside. The atmospheric pressure doesn't force itself down the hole else you could never shut off a motor because the air is forcing it to run. Doesn't matter how you frame it the quesiton is stupid!

 

[4554x4]

An interesting thought came to me. Awell tuned engine can be measured with a vacuum guage. for instance, if you had retarded timing, a weak or broken valve spring, or a burnt valve, your vacuum readings will be low. Also a fuel injected engine will have a MAP sensor. ''manifold absolute sensor'' it is used by the computer as one of the imputs to determine what the engine needs next, more advance or retard the timing etc. In my mind we are both right, just describing it differently. (science vs laymans terms) Kind of like listening to lawyers talk, then asking, what in the hell did that mean? Then he explains it so ordinary people understand.

 

[78mitsu]

It's my turn to be an a$$ today.

 

[kowe69]

.....the air decides to go from the outside to the inside. The atmospheric pressure doesn't force itself down the hole else you could never shut off a motor because the air is forcing it to run. Doesn't matter how you frame it the quesiton is stupid!

Ok....your right if you say so...I guess everything I learned in college was all lies.....

 

[BlackSheep]

From an engineering perspective that test is crap. Too many factors left out to accurately answer the questions and in many of the questions the true answers weren't even an option.

I don't agree. It is a basic aptitude test. All the questions were fair, and when I reviewed the test, the 5 I got wrong, were mostly just due to brain farts or just clicking too quickly on the answer.

Only one I didn't agree with and that was the question about the fans. I might not be seeing it correctly.

 

[BlackSheep]

.....the air decides to go from the outside to the inside.

see, all those physicists in the world don't know a thing. The air just does whatever it decides to do...

p.s. Air won't do anything unless there is a pressure differential. The piston moving down does cause a decrease in pressure, thus the higher pressure (due to the atmosphere!!!) pushes the mass of air into the (lower pressure) area inside the cylinder.

If you don't believe it, go check out any basic physics book.

 

[RockMonkey]

Only one I didn't agree with and that was the question about the fans. I might not be seeing it correctly.

I took a while on that one to make sure I was visualizing it correctly. When I visualized it in my head I found the answer was different than my initial gut reaction. The worm gear one took me a minute to visualize and determine the right answer as well.

 

[kowe69]

see, all those physicists in the world don't know a thing. The air just does whatever it decides to do...

p.s. Air won't do anything unless there is a pressure differential. The piston moving down does cause a decrease in pressure, thus the higher pressure (due to the atmosphere!!!) pushes the mass of air into the (lower pressure) area inside the cylinder.

If you don't believe it, go check out any basic physics book.

I think we are beating a dead horse here blacksheep......

 

[Devel]

94%:greg:
470 points
not bad for bein only 19:greg:,I guess going to college for automotive technology helped some, i missed 3 questions, mainly the pully's and planetry questions are what got me. i must be tired or somthing

 

[waynehartwig]

Ok....I guess everything I learned in college was all lies.....

You don't know how right you could be there...

 

[BlackSheep]

I think we are beating a dead horse here blacksheep......

yeah, hit it again!!!!!

 

[XT Utah]

Here you go....

The Intake Stroke:

On the intake stroke, the intake valve has opened. The piston is moving down, and a mixture of air and vaporized fuel is being pushed by atmospheric pressure into the cylinder through the intake valve port.


http://www.siu.edu/~autoclub/frange.html

 

[kowe69]

Here you go..............http://www.siu.edu/~autoclub/frange.html

Thank you! I was looking for a reference and couldn't find one at the time.

 

[78mitsu]

Here you go....

The Intake Stroke:

On the intake stroke, the intake valve has opened. The piston is moving down, and a mixture of air and vaporized fuel is being pushed by atmospheric pressure into the cylinder through the intake valve port.


http://www.siu.edu/~autoclub/frange.html

just because you say the same thing over and over, doesn't make it true, the sky is green, the sky is green, the sky is green: see.

I'm not debating the reason the air moves from high pressure to low pressure, that pretty simple, what I'm debating is the point that the atmouspherice pressure "PUSHES" the air into the cylinder.

this is a simple cause->effect relationship,

I'm saying that the atmospheric pressure doesn't cause the air to enter the cylinder, IT'S THE PISTON GOING DOWN THE HOLE.

 

[RockMonkey]

just because you say the same thing over and over, doesn't make it true, the sky is green, the sky is green, the sky is green: see.

I'm not debating the reason the air moves from high pressure to low pressure, that pretty simple, what I'm debating is the point that the atmouspherice pressure "PUSHES" the air into the cylinder.

this is a simple cause->effect relationship,

I'm saying that the atmospheric pressure doesn't cause the air to enter the cylinder, IT'S THE PISTON GOING DOWN THE HOLE.

You'd think this horse would be dead by now, but it looks like it's still twitching a bit.

You're skipping a step. The piston goes down the hole, creating a lower pressure zone inside the cylinder, causing the atmospheric pressure to push air into the cylinder to equalize the pressure.

 

[notajeep]

So.... does it suck from the inside or blow from the outside??






















Or does it even freakin matter? :brett:

 

[XT Utah]

just because you say the same thing over and over, doesn't make it true, the sky is green, the sky is green, the sky is green: see.

I'm not debating the reason the air moves from high pressure to low pressure, that pretty simple, what I'm debating is the point that the atmouspherice pressure "PUSHES" the air into the cylinder.

this is a simple cause->effect relationship,

I'm saying that the atmospheric pressure doesn't cause the air to enter the cylinder, IT'S THE PISTON GOING DOWN THE HOLE.

Well, I come from NJ where the sky actually is green .

Here's another fun topic for discussion. A rocket is not actually moved by it's engine's thrust. It moves in the opposite direction of the thrust because of conservation of momementum.

 

[kowe69]

........I'm saying that the atmospheric pressure doesn't cause the air to enter the cylinder, IT'S THE PISTON GOING DOWN THE HOLE.

Your logic makes no sense. If there was NO atmospheric pressure, the piston moving down would have no effect at all. Because if there was not a high pressure outside of the intake, it would be impossible to have a LOWER pressure in the cylinder. You can't have a pressure lower than absolute zero.

Go read a physics book and educate yourself before trying to prove your point when you don't know the facts.