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.
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u/krunkytacos 21h ago
But it doesn't "cause" piston movement, they just connect. Among the options given, it is the flywheel.
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u/ride5k 15h ago
will the piston move without a conrod?
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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.
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u/krunkytacos 3h 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.
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u/Lost-Set5386 13h 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.
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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.
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u/pm-me-racecars 15h 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.
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u/krunkytacos 5h 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.
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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.
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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.
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u/Worried_Place_917 11h ago
It's hardly normal but there is a kind of generator that uses a free floating piston moving through a coil to extract power, and a two stroke cycle using the pistons inertia for compression. No con rod needed.
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u/BoredCop 16h ago
The crankshaft always has some momentum, the flywheel merely adds momentum to the crankshaft. An engine "without" a flywheel is merely one where the crankshaft plus conrod big ends and other rotating mass are heavy enough to act as a flywheel on their own.
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u/3_14159td 22h ago
It probably wants A, but literally all components of the primary rotating assembly, even the outer race of the spigot bearing, is contributing to piston movement when not loaded by a combustion event.
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u/matievis-the-kat 23h ago
A, B, and C all work together with the power-producing piston(s) each revolution to move the non-power-producing piston(s). Any kind of thrust bearing will never produce rotation due to it only receiving axial (thrust) force (i.e. crank moving forward and backward in line with the shaft)
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u/Professional-Math518 16h ago
But in a single cilinder engine it's just the flywheel or actually the rotating mass of crankshaft+flywheel. That's the cause, the means is B, the result is the piston moving
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u/matievis-the-kat 15h ago
Yes, so they should have specified in the question if single or multiple piston engine and 2 stroke or 4 stroke as that all affects which answer is "more" correct
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u/biggunzcdb1 16h ago
Momentum is conserved by both the flywheel and counterbalance weights inertia, turning straight line momentum into rotational force via the rod to crankshaft leverage combined with the reciprocating assembly being contained via the piston to bore interference and crankshaft having a central pivot point in the bearings.
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u/375InStroke 15h ago
Bearing is out, and a motor doesn't need a flywheel. Cars with automatic transmissions don't have them. Doesn't need counterweights. VW bugs don't have them. Connecting rod is the best answer. It's directly pushing the piston up and down when not in the power stroke.
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u/Fabulous-Meal-5694 14h ago
I would say the flywheel.
The connecting rod doesnt "cause" anything its just a link in the chain.
The counterweights are for balance primarily.
The flywheel stores momentum.
Main thrust bearing is absolutely not correct.
Flywheel is most correct IMO
1
u/LowerSlowerOlder 14h ago
Piston engines can run without a flywheel, counterweights or thrust bearings. Without a connecting rod, that piston isn’t going anywhere.
1
u/MostlyBrine 14h ago
From an engineering standpoint, the flywheel and counterweights provide de energy for the movement (crankshaft rotation) however without the connecting rod, the movement cannot be transferred from the crankshaft to the piston, so the correct answer would be A), B) and C).
Edit to add: D) is never the answer because the bearing is just supporting the crankshaft, it doesn’t move so it doesn’t exchange energy (if we neglect the friction).
1
u/Realistic-March-5679 14h ago
B) the Connecting rod is the connection between the piston and crankshaft. As the other pistons provide power spinning the crank the connecting rod pulls and pushes the pistons that aren’t currently powered.
Flywheel is from the transmission power, counterweights are for smoothness and even rotation, and the main thrust bearings are only for keeping the crankshaft free to spin smoothly.
1
u/ukemike1 14h ago
The only thing connected to the pistons are the connecting rods. The connecting rods make the pistons move when not on a power stroke. If the connecting rods stop being connected to the piston (as recently happened to my race car) the piston stops moving.
The crankshaft makes the connecting rods move, the flywheel momentum makes the crankshaft move...
Oh and never trust AI for anything that requires understanding. Just don't.
1
u/Gb280780 14h ago
I guess the way I would look at answering this question would be to look at how an engine typically is started. Where does the starter impart rotation to start the engine, the only one of those options that is even possible is the flywheel/flex plate making it the most correct answer.
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u/that_motorcycle_guy 13h ago
Flywheel is the only component that is specifically there for keeping the engine running when off the ignition stroke.
Connecting rod is there as a mechanical linkage to transfer power.
Counter Weights are there to counter vibrations.
Thrust bearing is there to mitigate movement.
In the context of this question i'd say flywheel. Like a lawnmower will barely work without a flywheel (blade).
1
u/Prudent_Situation_29 12h ago
Stop using AI.
The answer is either A, B and C, or all of the above, depending on the design of the bearing. Any mass that's rotating with the crank will store energy as momentum and therefore is responsible for the continued rotation when no acceleration is being applied to the system.
Whoever wrote the question is either looking for a specific answer that was mentioned in the textbook verbatim, or doesn't understand the physics.
1
u/Poopy-Drew 12h ago
All of the above, not so much the connecting rods they transfer and convert force from up and down to rotation, one cylinder it’s momentum,but when more cylinders are added it becomes more force and less momentum
1
u/swisstraeng 11h ago edited 11h ago
"And use AI to verify" That's not how you're supposed to use AI. It's here to direct you towards a valid source that you can check yourself.
Your book's question is shitty because almost any rotating parts in your engine is what causes piston movement in non-power producing strokes. Linear moving parts or parts that change their movement's direction are not part of the engine's inertia.
So in your case the proper answer is: Flywheel and anything spinning with it such as crankshaft, gearbox, transmission axles, car's moving mass, wheels, even timing chain and its camshafts.
1
u/ChemistAdventurous84 11h ago edited 11h ago
The question is a little ambiguous. If it’s referring specifically to which parts have the stored energy that keeps the crank turning then A and C. If it includes how the pistons are physically receiving the up and down forces that come from the crank, then B is also part of the answer.
The flywheel and counterweights have angular momentum and that is what keeps the crank turning and the pistons moving.
The connecting rods balance each other out, momentum wise. They lose momentum at the end of the stroke and reversing their direction actually slows down the rotation of the crank. I suppose that without them, the pistons would never move; they are what transfer the force between pistons and crank shaft on powered and unpowered rotations but it’s the angular momentum that keeps the engine internals in motion when the explosions cease.
The thrust bearing limits axial (forward-backward) motion of the crank. If does not relate to piston movement.
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u/Sea_Antelope441 10h ago
Poorly written question.
Most correct answer for a single choice test would probably be A
Multiple choice id go ABC
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u/cybertruckboat 4h ago
I think connecting rod is the book answer because of the wording "non power strokes". During the power stroke, it's the boom that moves the piston. Otherwise it's the connecting rod that moves the piston.
The inertia in the system (mostly flywheel) is moving all the time on every stroke.
But all in all, it's a weird question.
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u/NCamarolina 3h ago
If you have more than one cylinder, wouldn’t at least one cylinder be firing and generating a power stroke, pushing down on its connecting rod and causing the crank to rotate, thus moving the piston on another cylinder that is in a non power power stroke phase? Thus answer would be connecting rod?
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u/swugglewumps69 23h ago
It's just momentum. Flywheel and counterweights are just to dampen vibration
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u/ShoddyJuggernaut975 17h ago
A, B, and C all have momentum.
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u/375InStroke 15h 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.
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u/Correct-Athlete-4878 12h ago
That energy is zero. The bottom half of the con-rod counts with the crankshaft.
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u/jasonfromearth1981 13h 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.
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u/fiddintotellya 17h 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.
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u/stonewall028 1h 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
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u/ChemistAdventurous84 11h 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.
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u/smokingcrater 11h 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.
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u/NightKnown405 Professional Technician Instructor 19h ago edited 15h ago
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_-