Mechanical aptitude test

[4554x4]

found this on Wikepedia under ''manifold vacuum''

When the throttle is opened (in a car, the accelerator pedal is depressed), ambient air is free to fill the intake manifold, increasing the pressure (filling the vacuum). A carburetor or fuel injection system adds fuel to the airflow in the correct proportion, providing energy to the engine. When the throttle is opened all the way, the engine's air induction system is exposed to full atmospheric pressure, and maximum airflow through the engine is achieved. In a "naturally-aspirated" engine, total engine output is thus determined by the ambient barometric pressure. Superchargers and turbochargers can "boost" manifold pressure to above atmospheric pressure, usually by a maximum of 0.7 bar. (wikepidea seems to be using both terms)

 

[Lifelong Jeeper]

I got 420 pts at 84%.

 

[UNSTUCK]

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.

Sorry, can I just add my .02 as well. After combustion is finished, all the air is used up, creating a vaccume by it's self. So now in the cylinder there is vaccum. Next is the exhaust stroke. The exhaust valve opens and the piston then has to push the bad gas out, fighting back pressure. Just before the pistion reaches TDC the intake valve opens, Valve overlap occurs. The whole reason for valve overlap is to allow new air to come in to help push out the bad gas. This new air can only come in due to the vaccum created by the combustion process. The piston is still on it's way up so it couldn't possibly suck the new air in. Now the exhaust valve closes and the process starts again. Two different vaccum sources are used throughout the four stroke cycle.
But yeah, I got that question wrong too.

 

[kowe69]

......The whole reason for valve overlap is to allow new air to come in to help push out the bad gas.......

You bring up a good point, BUT valve overlap has a slightly different effect then what you described. During combustion the fuel and air is ignited causing massive amounts of heat and expanding gasses forcing the piston down. How could this possibly be a vacuum? Even after combustion is complete, the burned gasses are still expanding on there way out the exhaust port. The purpose of valve overlap is to use the inertia of the exhaust flowing out of the cylinder (just at the end of the exhaust stroke near TDC creating another low pressure area) to help bring a new air/fuel charge into the cylinder. This is called the scavenging effect (reference http://www.enjoythedrive.com/content/?id=8178). This is why performance camshafts have more valve overlap and produce more horsepower at high rpm. To build power, it is more beneficial to try and get more air and fuel in the cylinder first. Getting the exhaust out comes second.

 

[Jeremy]

ok. lets say you had a jet plane on a giant treadmill...................:sick:

 

[Cody]

76% :sad:

 

[BlackSheep]

How come we don't have a popcorn smiley....

Sorry, can I just add my .02 as well. After combustion is finished, all the air is used up, creating a vaccume by it's self. So now in the cylinder there is vaccum. Next is the exhaust stroke. The exhaust valve opens and the piston then has to push the bad gas out, fighting back pressure. Just before the pistion reaches TDC the intake valve opens, Valve overlap occurs. The whole reason for valve overlap is to allow new air to come in to help push out the bad gas. This new air can only come in due to the vaccum created by the combustion process. The piston is still on it's way up so it couldn't possibly suck the new air in. Now the exhaust valve closes and the process starts again. Two different vaccum sources are used throughout the four stroke cycle.
But yeah, I got that question wrong too.

you guys really need to read a little further into whatever resources you are using. or maybe you're reading "4 stroke combustion engines for dummies".....

 

[Maverick]

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.

Agreed...well, not crap, but there were some holes.

Brett (98%, missed the stupid venturi one and haven't bothered to look in my books to validate)

 

[timpanogos]

92% - in reviewing I understand all the ones I missed except the "drive ratio Type"

There was a small to large and a small to small to large. Since small to small to large is a reduction, but leaves the final gear turning in the same direction as the first gear ... I put this as "reduction". The simple small to large is also a reduction, but indeed reverses direction. I put the small to large as reverse

What am I missing on this?

 

[Maverick]

Perspective. That one gave me pause too...but with reference to the other gearsets, that was the one that ended up with a different final rotation direction (compared to the others).

Brett

 

[timpanogos]

Perspective. That one gave me pause too...but with reference to the other gearsets, that was the one that ended up with a different final rotation direction (compared to the others).

Brett

Ahhh, i see

 

[Lifelong Jeeper]

Perspective. That one gave me pause too...but with reference to the other gearsets, that was the one that ended up with a different final rotation direction (compared to the others).

Brett

That was how I reasoned it too. It was the only one of the 4 that produced the same direction as the initial gear.

 

[Zombie]

So I think I get it, and I'm a dumbass...

The piston moving down the hole creates a low pressure situation inside the hole (lower pressure than atmospheric pressure...)
So when the intake valve opens, the air that is still at atmospheric pressure pushes into the hole (because there was less pressure inside the hole...)

Make sense now?

Or just read what rockmonkey wrote. It is better worded an less dumb.

 

[RockMonkey]

So I think I get it, and I'm a dumbass...

The piston moving down the hole creates a low pressure situation inside the hole (lower pressure than atmospheric pressure...)
So when the intake valve opens, the air that is still at atmospheric pressure pushes into the hole (because there was less pressure inside the hole...)

Make sense now?

Or just read what rockmonkey wrote. It is better worded an less dumb.

Ooh, I feel a new signature coming on...

 

[Brett]

Ooh, I feel a new signature coming on...

lol....nice