Mechanical aptitude test time - Page 2

Pete. · 10-25-07, 06:57 AM

Only 80%

Oh well.........

Dave S · 10-25-07, 07:16 AM

Hey Guys
If you want to check your answers. You can at the end of the test.
In top right corner of the result page put your cursor on the magnifying glass and click on it. It will then go through each question one by one.

Seems like we have a few Stars on the forum.

**candc** · 10-25-07, 07:18 AM

phew i passed only just 88% that got the old brain thinking lots of eyes closed and finger drawing the guys at work thought i was nutts

Dalton · 10-25-07, 07:21 AM

I thought I was pretty hot on mechanical aptitude, and was confident by the end that I had got them right. Oh no, 80%. Bugger.

I agree that some of the questions are ambiguous. For instance, the normally aspirated engine one. The piston cannot suck air in without there being atmospheric pressure outside (it won't suck anything in if it's in space).

Dalton

Lee · 10-25-07, 12:44 PM

92%. Managed to sort out the block and tackles but fluffed a couple by just going too fast - reduction vs overdrive, atmospheric pressure. Got the Bernoulli water pressure question well wrong and 48? - well whatever, it's just for fun but in practise the piston is going down, creating the 'space' which creates the vacuum with pulls in the air which is always pushing in all directions! The engine would not run in space, so talking about so that hypothetical situation is not relevant 'in practise'. Is it aimed at mechanics or Astrophysisists?

Chris_K · 10-25-07, 02:40 PM

90% and the ones I missed were from over-thinking. Like thinking on the wiring diagrams that the switch was in series with the parallel lamps and calling it a series-parallel.

Goes to show what happens when you think too much. I'll try to avoid that in the future. Build still going slowly...

scott · 10-25-07, 03:20 PM

Quote:

Originally Posted by aero

3) The one about air entering the cylinder only because of semantics. I stuck with the engineer's answer which is atmospheric pressure pushing the air in, but the answer above it (being pulled in by suction due to the piston moving down) is arguable also correct because atmospheric pressure wouldn't be able to push air in unless the pressure inside the cylinder drops. This happens, of course, due to the piston moving down and creating a "vacuum" (in gage pressure). I visualize things a lot and in this case it all depends upon which side of atmospheric pressure (above or below) your mind views this scenario from. The other possibility of creating the pressure difference to push air in is to increase the pressure on the outside of the cylinder. This is not what happens in this case, so one can think of it as "sucking" if one pleases.

You are correct as gasses flow from high pressure to low pressure. The reason the air/fuel mixture flows into the cylinder is because of the low pressure area in the cylinder created by the piston going down-----suction!
Yes, the atmospheric pressure is now relatively higher and forces the air/fuel mixture in, but only because the cylinder pressure was lowered by the action of the piston. So, does the mixture flow because of the relatively higher atmospheric pressure? Yes. Does the mixture flow because of the suction created in the cylinder? Yes.

Also, their question 44 was very poorly worded. The FORCE on the top of the piston equals the FORCE on the bottom, but the PRESSURES are different. Force does not equal pressure by definition, and they are being ambiguous in this instance. I did get it right because I guessed what they really meant.

Chris P. · 10-25-07, 03:22 PM

Actually, I don't recall any of the gear ones being ambiguous in regards to CW / CCW because all you need to know is if the gear they are asking about is going the SAME direction or NOT as the first one. I only wish my score on this test could reflect my overall college grade.

Rob · 10-25-07, 08:42 PM

Jim C & JacMac,
I still must oh so humbly disagree about the atmospheric pressure.
Jim, negative pressure is also known as vacuum. So long as you are not speaking in relative terms, in which case I would call it low pressure.
JacMac, I assumed this engine was running. If the engine was hand cycled then I would agree, atmospheric pressure would be responsible for filling the cylinder. However, if the engine is running, it is vacuum. The piston dropping is creating volume faster than atmospheric pressure (1 bar) can fill it. Therefore, vacuum is responsible. Otherwise, you would not show vac. on a gauge at the intake.

just my thoughts on the subject....

Jac Mac · 10-25-07, 09:28 PM

Quote:

Originally Posted by VintageVenom

Jim C & JacMac,
I still must oh so humbly disagree about the atmospheric pressure.
Jim, negative pressure is also known as vacuum. So long as you are not speaking in relative terms, in which case I would call it low pressure.
JacMac, I assumed this engine was running. If the engine was hand cycled then I would agree, atmospheric pressure would be responsible for filling the cylinder. However, if the engine is running, it is vacuum. The piston dropping is creating volume faster than atmospheric pressure (1 bar) can fill it. Therefore, vacuum is responsible. Otherwise, you would not show vac. on a gauge at the intake.

just my thoughts on the subject....

Rob, while I can relate to your reasoning-remember two things:
1. This test was about or from a Diesel engine perspective, so there is no throttle to create a manifold 'vacuum' as such. Only at high speeds will the restriction of the valve or runners cause a 'vacuum' to occur.
2. The term 'Vacuum', is the root cause of this misunderstanding ( Dont worry-after some 30 odd years in the motor trade I decided to learn to Fly in the 1990's- this brought on many discussions and the inevitable differences in tech speak that the two trades have used ). For Aviation purposes they seem to treat everything as Higher or Lower pressure areas or states. WE in the automotive world tend to adopt a ZERO point where everything above that 0 point is Pressure & everything below is a Vacuum, while Scott & others in Aviation simply use high & low pressure areas to differentiate these. The weather is a classic example;If you have a 'High to the south & a Low to the north the flow will be from High to Low to generate a southerly airstream or Wind!
Still confused-I hope not, but can understand if you are!

Jac Mac

BenL · 10-25-07, 10:08 PM

Quote:

Originally Posted by MN12

450 90%

Cool test and yes it will tell you you need to click onthe icon in the upper left for review answers

While taking the test if you click on the button in the lower left (two rectangles), it will show you a list of the question titles, whether you got question right or wrong. You can go back and look at the questions, but you can't change your answers.

Dalton · 10-25-07, 11:43 PM

Lets assume you are at a party where things are going slowly, there are no sheep around, and it needs revving up. Start asking people pulley type questions, such as can you pull yourself up easily in a "bucket" with a rope and a few pulleys. Fairly soon voices will be raised and people nearly hitting each other over the correct answers.

Personally, I'll just be keeping my lack of expertise to myself, over at the bar.

Dalton

Rob · 10-26-07, 06:33 AM

JacMac,
Hmmm good observation and remark about it being a diesel application. And potentially quite valid. Hmmmm

WRT the vacuum definition, funny you bring those particular examples/points up, because they were the exact ones I was thinking about when I typed my response. No joke.. :-) I was wondering how most others perceived the term.

Good stuff....

woodz428 · 10-26-07, 08:35 AM

I got a 98%, as has been said, it doesn't let you know WHAT you got wrong. I think I got one of the pulley questions wrong, I haven't used a pulley system in years and it was pretty distant.

Scott Calabro · 10-26-07, 09:30 AM

Well since we have drifted, what the heck!

"Vacuum" as used in the automotive world, is really slang for "Low pressure".

Try to think in terms of absolute pressure (not gauge pressure) as it may help visualize how the mixture charge goes into the cylinders in a piston engine.

Yes, the expanding area inside the cyl. creates a low pressure due to restrictions in the intake system, hence the higher pressure air from the atmosphere flows into the cyl.

I bet no one would argue that in a supercharged or turbo-supercharged engine that "vacuum" is what draws the charge into the cylinders!

Its the same thing, just at higher pressure differentials.

Cheers,
Scott

Big-Foot · 10-26-07, 09:54 AM

Scott and I are in agreement on the physics involved being a pressure differential.

One thing that's always bothered me though is when people speak of Vacuum. There truly is no such thing as a complete vacuum on the planet Earth - manufactured or otherwise. Only partial vacuums. The only true vacuum is that of outer space well beyond our atmosphere. This was of a primary concern of NASA in the Space Shuttle program during the design and construction process of the tires. Imagine what a blow-out would do to the shuttle in space!