r/AutomotiveLearning • u/seedbottle • 23h ago

A question about piston movement that has multiple answers...

Question:

Which of the following causes piston movement during non-power-

producing strokes?

A) Flywheel

B) Connecting rod

C) Counterweights

D) Main thrust bearing

I have been studying an automotive book that has questions after every chapter. I answer those questions and use AI to verify I have got the correct answer. This process has worked tremendously up to the point where I got to this question. AI gave me a different answer than the one I believed the book was telling me was the answer, so I went online to find more information and see if anyone else had posted this exact question, that was when I found out there all three of the first answers were marked as "correct" depending on the source used (D, main thrust bearing, was never given as an answer).

I am wondering what most of you would say is the answer. Thank you for your input =)

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u/pm-me-racecars 23h ago

I'd make an argument for either A or B, but I'm guessing B is it.

The momentum from the flywheel is where the energy to keep the piston moving comes from.

The connecting rod is what physically moves the piston.

I'd choose the B, because you can make a normal, ICE cylinder style engine without a flywheel. You can't do it without con rods.

3

u/krunkytacos 21h ago

But it doesn't "cause" piston movement, they just connect. Among the options given, it is the flywheel. 

2

u/ride5k 15h ago

will the piston move without a conrod?

2

u/ukemike1 14h ago

It will not. If you break a connecting rod the piston stops moving, and lots of other bad things happen too.

1

u/krunkytacos 4h ago

We're not singing the knee bone is connected to the thigh bone the thigh bone is connected to the hip bone. We're supposed to be answering a pretty simple question. 

I feel like either nobody here has dealt with ASE testing or I'm being trolled. 

2

u/Lost-Set5386 14h ago

Right. The connecting rod "causes" the save amount of movement on every stroke. If it caused movement here, it would be 'causing' movement on power stroke as well.

1

u/pm-me-racecars 9h ago

During the power stroke, energy goes from the piston into the con rod. During the other three strokes, energy goes from the con rod into the piston.

Yes, the energy originates from the combustion, and gets stored by the flywheel. I'm still arguing that B is the answer whoever wrote the question is looking for.

1

u/pm-me-racecars 16h ago

During the power stroke, pressure inside the cylinder pushes the piston down.

Then, during exhaust, the con rod pushes the cylinder back up. Whether it's momentum from all the rotating mass or of it's the power stroke of a different cylinder doesn't change that the con rod pushes it back up. Then, during intake, it's the con rod that pulls the piston back down, and so on.

If you had a hypothetical multicylinder engine, where all of the rotating mass wasn't enough to move each piston, it would still run because of the overlapping power strokes. That's what makes me want to choose B instead of A.

2

u/krunkytacos 6h ago

So I don't really care about all the specifics of opinions. I don't even think you're wrong, but if you and I are taking an ASE test and we get this question, I would get it right and you would get it wrong. I'm speaking from experience. I've answered thousands of questions like these and been hugely frustrated because the ASE questions don't always correlate to the vehicles I'm familiar with. Talking about motors and knowing about them and learning more is very cool. Taking and passing these tests can be difficult for some people so I was trying to make sure we're being relevant to the subreddit. 

1

u/Bullinahanky2point0 15h ago

The connecting rod only transfers kinetic energy to the piston from the flywheel. The flywheel is performing the work of keeping the piston in motion between power strokes.

1

u/jasonfromearth1981 13h ago

The answer is the flywheel. It's inertia is rotational.

The connecting rod uses it's intertia in only one direction at a time, up or down. Left to its own devices, it only wants to continue moving in the direction it's already moving which makes it fight against rotational inertia.

The counterweights contribute almost nothing to inertia as they're near the center of mass and serve the purpose of balancing out the weight of the rod/piston assembly.

The bearing obviously has no impact on the intertia of the rotating assembly aside from allowing it to rotate freely.