Intellectual engine questionnnnn...

[10Toes]

Well, it doesn't pause if that is what you are asking. It is at the top (or bottom) of the stroke the same amount of time it is in every other position throughout its stroke. While it might seem like it is stopping, it is still technically moving, just to a very small degree.

But the piston speed changes with the rod angle, it moves slower towards the TDC and BDC so this is false.

[TwoJ's]

Above is correct ^

These posts nailed it.

I think the confusion is with the usage of the word stopped. Stopped implies that the object stops moving for a period of time, no matter how small that period of time is. A piston does not do this, its motion is continuous, switching from moving up to moving down instantaneously. There is no gap between movement. It does slow down to 0 when going up, but then instantly start moving down again with no pause between.

Where do you get this definition of stopped.

If V=0, it is stopped. Like others have said, it is for an instant, meaning there is no theoretical time value. Imagine the circle that the crank journal travels. When a tangent line is horizontal, the vertical velocity of the piston is zero (stopped)

The reason that a piston may seem to stop or hang at TDC or BDC is because the slope of the tangent line before and after TDC or BDC is extremely small.

It would be as simple as plotting vertical velocity over time and looking at the graph. The shape of the plot would be sinusoidal... and with tangent slopes of zero twice per period. Nitro... think about this too.... you still think it is in every position for the same amount of time? the velocity (slope of a tangent line) on a sine curve doesn't seem constant to me...

[Thizzelle]

read about it here slam
http://www.camotruck.net/rollins/piston-offset.html

[unevolved]

Yep. The position of the piston is effectively the sine of the angle of the crank multiplied by the stroke, plus the rod length. The derivative (or rate of change) of position is velocity, and it definitely crosses the X axis. Here's a graph showing what I'm talking about:

[travish325]

Why does it matter? Lol

If it slows to zero it is stopped. Even if for a fragment of a second it stops, it has to or how can it change direction.

How can you argue that?

Grab a pencil and draw a straight line up then draw back over the line down. Your pencil had to stop to draw back over the line didn't it?

[mechdonald]

Most of you guys are young, correct? As in most are no more than 5-10 years removed from high school? (That's my guess at average age on here)
Don't you young fellars remember anything from Physics class?

The piston accelerates and decelerates in the cylinder (remember the sine function?). It does not instantaneously go from moving up at a constant speed to moving down at constant speed. Your engine would break in seconds.

You all need to read this guy:

http://www.amazon.ca/Top-Dead-Center-Cameron-Magazine/dp/0760327270

Now, there may be some sort of minute sideways movement as the conrod moves over TDC but I think you are just arguing semantics. The piston, being observed in its path along the bore of the cylinder, does indeed decelerate to zero and then reverses.

Stop this silly talk - you all sound the opposite of intellectual :p - or just drunk.

[Thizzelle]

what, most on here said it does stop, the ones who don't need to stick with their art classes.

[10Toes]

blabla bla what people have already said.

Thanks gramps

[Farwest92]

I think they come to a complete stop.

[10Toes]

I think they come to a complete stop.

The oil geyser

[mrsleeve]

Can you go in reverse in your car while driving foward without stopping first?

YES

Old 2 speed power-glide BIG patch of ICE. going forward at about 60mph get on to Ice patch shift to reverse and STAND ON THE GO FAST PEDAL ON THE RIGHT. You will be driving in reverse while still going forward all at the same time..................

Slammin: As has been noted individual piston speed changes with the rod angle and actual crank position, and yes the Piston DOES stop when changing directions at both TDC and BDC though not for very long

[reelizmpro]

As we know, if you tear apart a very old engine you may see some score marks on the cylinder walls. These are typically in the middle but not on top. This represents the angular force applied by the rod/crank which actually pushes the piston towards the wall in that direction but as the rod angle decreases there is less sideways force until the force is basically linear (TDC). Even though the crank is rotating there is still a peak. At that point (TDC) there is little to no sideways movement which isn't something we want anyway. We can pretty much say that the piston has stopped momentarily before the next stroke. This is evident in an old engine where you can feel the groove where the piston reaches TDC and goes to the next stroke. The movement is not continuous (in other words...stopped) like the rest of the stroke.

[Todd Black 88]

Most of you guys are young, correct? As in most are no more than 5-10 years removed from high school? (That's my guess at average age on here)
Don't you young fellars remember anything from Physics class?

The piston accelerates and decelerates in the cylinder (remember the sine function?). It does not instantaneously go from moving up at a constant speed to moving down at constant speed. Your engine would break in seconds.

You all need to read this guy:

http://www.amazon.ca/Top-Dead-Center-Cameron-Magazine/dp/0760327270

Now, there may be some sort of minute sideways movement as the conrod moves over TDC but I think you are just arguing semantics. The piston, being observed in its path along the bore of the cylinder, does indeed decelerate to zero and then reverses.

Stop this silly talk - you all sound the opposite of intellectual :p - or just drunk.

This here is where you can be tricked, or believe the continuous motion theory. We all assume the piston goes STRAIGHT up and down, whereas, it probably does the slightest variation of a parabola due to the Conrod changing sides on the crank at tdc/bdc, causing a slight curve of a stroke at these points as the piston minutely moves from one side of the bore to the other. .......or I could be talking out of my ass......you decide:D

[travish325]

YES

Old 2 speed power-glide BIG patch of ICE. going forward at about 60mph get on to Ice patch shift to reverse and STAND ON THE GO FAST PEDAL ON THE RIGHT. You will be driving in reverse while still going forward all at the same time..................

Haha I was waiting for that!

Rephrase can you change foward inertia of a car into backwards inertia with out stopping. No you can't.

[Todd Black 88]

Can you go in reverse in your car while driving foward without stopping first?

Rockford. Easy.:D