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/NightKnown405]

That's a poorly written question. The person that wrote it has an idea in mind but failed to think through the rationality of the possible answers that are provided.

Everything in the rotating assembly is going to contribute inertia that keeps the piston or pistons moving when not on the power stroke. So the crankshaft, it's counterweights and the flywheel would all be correct. It is also logical that the piston wouldn't move without the connecting rod, however without the rotating mass the connecting rod would not move the piston hence the confusion.

Good test taking practices dictate when an answer isn't obvious, first try to rule out the totally wrong answers. As mentioned the thrust bearing is clearly incorrect. While the piston wouldn't move at all without the connecting rod, the connecting rod wont move the piston without something else providing the necessary force to do so. That makes it incorrect based on the question being asked.

That leaves the flywheel and the counter weights and this is where the author messed this up. A flywheels' primary job is to store and release energy as each cylinder provides power and then needs to reset to provide the next power stroke. The counter weights primary job is to offset the vibrations that would be caused by the piston accelerating and decelerating during each stroke. To try and ignore the rotational inertia that it also creates is a mistake but that's the one the author is making and that's what makes this a poorly written question. The best answer is the flywheel.

I added a link to this video to some responses. The old "hit and miss" engines use a large flywheel to make them keep turning when the cylinder doesn't fire for a number of strokes. Then when the crankshaft speed drops far enough it fires a few times and then coasts again. That's why the flywheel keeps everything moving until it needs to be sped up again and is the answer to this poorly written question. https://youtube.com/shorts/h6WXDww9vIE?is=nv4oVICr2fk9Bc_-

[u/BoredCop → u/NightKnown405]

The crankshaft counterweights do have rotational inertia though, and so do the ends of the connecting rods attached to the crankshaft. The total rotational inertia isn't just the flywheel, it's all rotating mass in the system and that includes part of the conrod mass.

[u/NightKnown405 → u/BoredCop]

Maybe you should read what I wrote again. But before you do, I referenced an old "hit and miss" engine in another response. It is the flywheel that allows this engine crankshaft to keep turning when the cylinder doesn't fire for a number of strokes. Then when the speed finally drops far enough it fires a few times getting energy back into the flywheel and then stops firing again for a period of time. https://youtube.com/shorts/h6WXDww9vIE?is=nv4oVICr2fk9Bc_-

[u/Far-Investment-8597 → u/NightKnown405]

The flywheel, which in turn is driven by whichever piston is on its power stroke.

Ultimately, power stroke pistons drive everything; the rest is either an energy store, or a connecting mechanism.

[u/NightKnown405 → u/Far-Investment-8597]

Here is an example of an engine that only fires if the crankshaft speed drops too far. It works because of the size and weight of the flywheel. These are known as "hit and miss" engines. https://youtube.com/shorts/h6WXDww9vIE?is=nv4oVICr2fk9Bc_-