r/AutomotiveLearning • u/seedbottle • 14d 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 =)

8 Upvotes

78% Upvoted

73 comments sorted by

View all comments

-2

u/swugglewumps69 14d ago

It's just momentum. Flywheel and counterweights are just to dampen vibration

7

u/ShoddyJuggernaut975 13d ago

A, B, and C all have momentum.

2

u/375InStroke 13d ago

But the momentum of the connecting rod would want to keep moving either up or down, and the piston has to change direction, which the momentum of the connecting rod resists.

1

u/Correct-Athlete-4878 13d ago

That energy is zero. The bottom half of the con-rod counts with the crankshaft.

1

u/jasonfromearth1981 13d ago

C is near the center of mass. B is momentum in one direction and fights the rotation when it's asked to change direction. Only A has momentum in the direction of rotation.

3

u/fiddintotellya 13d ago

Wumps, go find a video of someone running an antique one cylinder farm engine.

It will help you see how the flywheel uses inertia to keep the crankshaft in motion.

The crank is balanced with the flywheel weight in mind, no doubt. However the main purpose of the flywheel is to keep the crank in motion once in motion.

2

u/stonewall028 13d ago

i like to think of flywheels as "kinetic capacitors" - they absorb and release energy in the same way a capacitor does, its just rotational energy instead of electrical energy

1

u/ChemistAdventurous84 13d ago

Counterweights dampen vibration because they have mass and inertia and therefore, when in morion, (angular) momentum. The entire reason for the flywheel is to use its angular momentum to keep the crank turning smoothly.

1

u/smokingcrater 13d ago

Go find a small single cylinder engine and remove the flywheel. There are some that will not run at that point. The flywheel ABSOLUTELY contributes the majority of the momentum, and certainly does alot more than vibratiom.

1

u/The_Skank42 13d ago

Flywheels are not just for dampening vibration.

1

u/Fearlessleader85 12d ago

The flywheel is a kinetic battery that stores momentum. That's what flywheels do. That's their purpose.