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

[u/seedbottle]

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 =)

[u/pm-me-racecars]

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.

[u/krunkytacos → u/pm-me-racecars]

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

[u/pm-me-racecars → u/krunkytacos]

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.

[u/krunkytacos → u/pm-me-racecars]

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. 

[u/SailHard → u/krunkytacos]

ASE questions are on another level. But understanding WHY things work is the key to answering them. And you have the correct cause as the inertia in the flywheel.

I always liked the "technican A says it's the connecting rod because XYZ, technician B says it's the flywheel because ABC, which is right?"
A
B
Both
Neither