Intellectual engine questionnnnn...

[lambo]

There is no straight line on a circle, so tit should instantaneously change from up to down, no?

[LJ851]

I'm going edit my original statement.

If there was zero play in the rod bearings, wrist pin, and piston to cylinder clearance AND you had really accurate measuring devices you probably could see continuing motion. The crank throw is a circle so theoretically there is no gap between rising and falling.

I would say it effectively pauses for a very brief period of time but in reality it is just moving VERY little at that time.

[Holland]

There is no straight line on a circle, so tit should instantaneously change from up to down, no?

The crank moves in a circle, but the piston moves in a linear fashion. I don't know why there would even be an argument here though, for an object like a piston, confined within it's own space it has to stop in order to return.

No it doesn't pause but it has to reach a point of zero movement if only for a brief moment.

[slammin.e28]

I can see it doing both, but I always want to come back to it stopping.

With all the clearance for oil (remember, no metal on metal contact should be occurring in an engine), I can see the flexing of the oil film between the components creating an environment for a continuous circle to occur.

I mean, the engine is going so fast. Does it actually have time to stop and reverse direction?

[kronus]

there is no effectively, instantaneously, or anything like that. The piston stops or doesn't stop. If the piston is moving in 1 dimension, there is a time between when it's moving up and when it's moving down at which its speed is 0. When something's speed is 0 it is by definition stopped.

[Holland]

I can see it doing both, but I always want to come back to it stopping.

With all the clearance for oil (remember, no metal on metal contact should be occurring in an engine), I can see the flexing of the oil film between the components creating an environment for a continuous circle to occur.

I mean, the engine is going so fast. Does it actually have time to stop and reverse direction?

Well how else can it reverse direction then? It surely can't turn around.

[Holland]

there is no effectively, instantaneously, or anything like that. The piston stops or doesn't stop. If the piston is moving in 1 dimension, there is a time between when it's moving up and when it's moving down at which its speed is 0.

This is what I was trying to say in a nutshell.

[slammin.e28]

Well how else can it reverse direction then? It surely can't turn around.

It makes a very tight and quick semi circle. Like a hump. Basically, it's closer to the bore on one side, then to the middle, then closer on the other side. It goes around, not straight up and down.

[kronus]

It makes a very tight and quick semi circle. Like a hump. Basically, it's closer to the bore on one side, then to the middle, then closer on the other side. It goes around, not straight up and down.

if that's happening your piston is undersize and will quickly embed bits of itself into the cylinder walls.

[StereoInstaller1]

there is no effectively, instantaneously, or anything like that. The piston stops or doesn't stop. If the piston is moving in 1 dimension, there is a time between when it's moving up and when it's moving down at which its speed is 0. When something's speed is 0 it is by definition stopped.

Even on a "roller based moving belt exercise apparatus"?

[slammin.e28]

These guys just kinda blew my mind.

http://www.physicsforums.com/showthread.php?t=353906

[gstuning]

Lets say that the pistons top surface stops given that there is no rocking motion in the piston at the time of TDC.

The rod stays straight, the cranks motion moves the rod clockwise which pulls the piston back down.

[AndrewBird]

There is no straight line on a circle, so tit should instantaneously change from up to down, no?

I'm going edit my original statement.

If there was zero play in the rod bearings, wrist pin, and piston to cylinder clearance AND you had really accurate measuring devices you probably could see continuing motion. The crank throw is a circle so theoretically there is no gap between rising and falling.

I would say it effectively pauses for a very brief period of time but in reality it is just moving VERY little at that time.

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.

[Thizzelle]

I have heard arguments for it continuing in motion, from engine builders, etc.

I'm like whuut?

they are retarded, it's physics

[lambo]

The crank moves in a circle, but the piston moves in a linear fashion. I don't know why there would even be an argument here though, for an object like a piston, confined within it's own space it has to stop in order to return.

No it doesn't pause but it has to reach a point of zero movement if only for a brief moment.

But the pistons movement is defined by the crank's circular motion. The tangent to a circle is defined by the slope at a single point so the slope changes from positive to negative at an infinitesimally small point at the top and bottom of the crank.