[Too Fast]

Not according to what I was taught in geometry. For the velocity of the piston to become zero would require there be point on the crank journal that had a shape that allowed the piston to become stationary while the crank could continue to rotate.

Or another way of looking at this would be viewing the rod journal as the crank rotated there would have to be point the stopped moving.

Correct. At some point in that rotation, the crank throw stops moving up, or down.

Pretty simple, and obvious, really.

Since the piston is connected via the connecting rod, it also stops so it can reverse rotation. Why are you arguing that it never stops?

I think you are confusing crankshaft rotation with vertical piston movement. The piston must stop moving, the crank never does.

 

[A Guy]

The big thing is we're talking an instant. An instant isn't a measurement of time, but rather a precise moment of time. For an instant the piston stops at both TDC and BDC

A Guy

 

[Rockster]

Correct. At some point in that rotation, the crank throw stops moving up, or down.

Pretty simple, and obvious, really.

Since the piston is connected via the connecting rod, it also stops so it can reverse rotation. Why are you arguing that it never stops?

Be wary of simple and obvious.

Are you saying that the crank which can be rotating at anywhere from around 750 RPMs to 6000 (or more) stops its rotation at TDC and BDC of every stroke?

The big thing is we're talking an instant. An instant isn't a measurement of time, but rather a precise moment of time. For an instant the piston stops at both TDC and BDC

A Guy

There is no point in the flow of time that time stops. Pick the smallest increment of time you can imagine. It can be divided into an infinity of smaller increments of time.

For the piston to stop moving at TDC and BDC the crank would have to stop moving at TDC and BDT of every piston stroke of every piston. Do you believe that this happens?

 

[ChallyTatum]

There is no point in the flow of time that time stops. Pick the smallest increment of time you can imagine. It can be divided into an infinity of smaller increments of time.

For the piston to stop moving at TDC and BDC the crank would have to stop moving at TDC and BDT of every piston stroke of every piston. Do you believe that this happens?

Yes it does, as I pointed out if you take the derivative of the motion of the piston you will see periodic points in its velocity that are zero. To suggest the piston never stops just because it appears to the naked eye that it doesn't is along the same lines of suggesting the earth is flat because it does not appear round when standing on the surface. The crank never stops but the piston does at TDC and BDC, why is this so hard to grasp? Even if the piston stops for 0.001 ms...it still stops despite appearances.

 

[Rockster]

Yes it does, as I pointed out if you take the derivative of the motion of the piston you will see periodic points in its velocity that are zero. To suggest the piston never stops just because it appears to the naked eye that it doesn't is along the same lines of suggesting the earth is flat because it does not appear round when standing on the surface. The crank never stops but the piston does at TDC and BDC, why is this so hard to grasp? Even if the piston stops for 0.001 ms...it still stops despite appearances.

Funny how you believe 0.001ms is a good choice for the stop time... You think that's a pretty reasonable time for the piston to stop I guess... Trouble is for the piston to stop would require time stop and it doesn't.

IOWs, you believe the crank nominally rotating at say anywhere from 750 RPMs up to 6000 RPMs can stop for 0.001ms at TDC and BDC so the piston can stop? And this happens for each piston of a multi-cylinder engine?

 

[ChallyTatum]

Funny how you believe 0.001ms is a good choice for the stop time... You think that's a pretty reasonable time for the piston to stop I guess... Trouble is for the piston to stop would require time stop and it doesn't.

IOWs, you believe the crank nominally rotating at say anywhere from 750 RPMs up to 6000 RPMs can stop for 0.001ms at TDC and BDC so the piston can stop? And this happens for each piston of a multi-cylinder engine?

No what is funny is you believe the piston never stops. I keep telling you to take the derivative of the displacement of the piston and you will see where the piston stops. Just because you can't see it does not mean it does not happen. I will say what never seems to stop is you and your ability to spread false information.

 

[Rockster]

No what is funny is you believe the piston never stops. I keep telling you to take the derivative of the displacement of the piston and you will see where the piston stops. Just because you can't see it does not mean it does not happen. I will say what never seems to stop is you and your ability to spread false information.

Time never stops thus the velocity never stops, does not even pause, even for 0.001ms...

You would have us believe time stops twice per rotation of each crank journal. Who's spreading false information?

 

[ChallyTatum]

Time never stops thus the velocity never stops, does not even pause, even for 0.001ms...

You would have us believe time stops twice per rotation of each crank journal. Who's spreading false information?

OMG I keep telling you how to mathematically show when a piston stops and here you are trying to use anything but math to explain why it appears not to stop. Seriously you should just stop.

 

[CE9311]

Know a great movie once said,

"We can't stop sir! We must slow down first!"

Speaking on physics and math....

Look at spinning objects. The whole thing spins at the same rpm, however the further the outer edge is the faster it is going relative to the inner section. Just because the outside is going faster does not mean it is spinning more rpms though.

However in space, to slow down one goes higher in orbit, and to speed up one lowers the orbit. Ain't that funny?

Just like this whole silly arugment about pistons. Yes, they do stop. There is no physical way an object can change direction in a 180 degree swap without stopping even if for the smallest amount of time.

 

[Rockster]

Know a great movie once said,

"We can't stop sir! We must slow down first!"

Speaking on physics and math....

Look at spinning objects. The whole thing spins at the same rpm, however the further the outer edge is the faster it is going relative to the inner section. Just because the outside is going faster does not mean it is spinning more rpms though.

However in space, to slow down one goes higher in orbit, and to speed up one lowers the orbit. Ain't that funny?

Just like this whole silly arugment about pistons. Yes, they do stop. There is no physical way an object can change direction in a 180 degree swap without stopping even if for the smallest amount of time.

Again the problem is you assume there is a smallest amount of time. There is not.

 

[Too Fast]

Be wary of simple and obvious.

Are you saying that the crank which can be rotating at anywhere from around 750 RPMs to 6000 (or more) stops its rotation at TDC and BDC of every stroke?

I give up.

The piston HAS TO STOP, TO REVERSE ITS ROTATION. You are arguing that the piston does not stop.

The crank keeps spinning, it never stops.

You have never explained how it doesn't stop, lol.

As the piston is stopped at TDC, note it is Top Dead (as in not moving) or Bottom Dead (same) Center, the connecting rod keeps moving. The piston has stopped for the very small amount of time. Not the crank, silly.

 

[Tape]

The piston does not stop at either end of its stroke. A trace of the piston's motion describes a sine wave. There is no flat spot at either the top or bottom of the wave.

Other than that...

His advice to use what the automaker recommends is good advice.

The piston stops
You give out a lot of wrong information throughout this forum not counting this thread.

 

[ChallyTatum]

I think he is getting confused with motion that can be described with a continuous curve and equating it to always moving as he keeps referring to piston motion begin described by a sinusoidal function which has no flat spots. If you use math (calculus) you can easily show piston speed does go to zero but rather than using math he likes to explain it in terms of time/space and interpretations from visual observations.

 

[A Guy]

I just can't...he's hung up on observing the stop and applying a duration of time rather than understanding an instant. He's also hung up on the curve of a sine wave implying the piston itself is moving in a curve rather than linearly. That we're talking about reversing direction, not following a curve

At the simplest you'd think that explaining how the piston would not just continue in the same direction if it didn't stop to reverse direction. There is an instant when velocity (v) equals 0 v=0...it's apparently something he can't wrap his head around and we'll never be able to convince him even if he looked and easily found 1,000 explanations

A Guy

 

[ChallyTatum]

Not to beat a dead horse but the graph below shows piston position (blue), velocity which is first derivative of position (green) and acceleration which is the second derivative of position (magenta). Red circles show the points at which the piston velocity is zero (note the max piston speed is at the halfway point between TDC and BDC).

Fun fact, the third derivative of position (or the first derivative of acceleration) is described as a jerk.

Piston Motion: The Obvious and not-so-Obvious, by EPI, Inc.

.

 

[Too Fast]

It won't matter. Rockster will still argue his point. I went round and round with him about vented gas tanks, he will not give up even when wrong.

 

[EvoRaat]

But wait, theres more: For every action there is an equal and opposite reaction! This is the filtered version.

 

[Rockster]

I think he is getting confused with motion that can be described with a continuous curve and equating it to always moving as he keeps referring to piston motion begin described by a sinusoidal function which has no flat spots. If you use math (calculus) you can easily show piston speed does go to zero but rather than using math he likes to explain it in terms of time/space and interpretations from visual observations.

You say it yet you then go on to try to explain it away.

The geometric wave form described by the path the rod journal has when the crank is moving always has motion. Thus the piston is always moving.

 

[ChallyTatum]

You say it yet you then go on to try to explain it away.

The geometric wave form described by the path the rod journal has when the crank is moving always has motion. Thus the piston is always moving.

Dude, I have shown mathematically that the derivative of the motion of the piston has points of zero velocity...i.e. points where the piston is not in motion. The graph that describes the motion is continuous but that does not mean the velocity is never zero. Jeebus, go learn calculus and report back because I am done trying to teach you. You do know that the slope at the max and min points of the smooth sine wave you love to keep bringing up is zero, has that slope is also velocity. Spend more time looking at the graph I posted earlier and less time trying to argue. You keep saying the piston does not stop, ok then how does it change direction...it must stop (have zero velocity) in order to change direction.

 

[ChallyTatum]

In fairness Rockster is not alone in his belief.

Does the piston ever stop moving in a running engine?

Does the piston ever "stop" moving in a running engine? well guys...does it?

www.physicsforums.com