# Since we are all PhD.'s is BSology...

Raw760
Posers and Puzzles 07 Oct '05 08:02
1. 07 Oct '05 08:02
I ask you this...

Does a piston in an internal combustion engine ever stop as the engine runs?
Remember, it is directly connected to a crank moving in an endless circle!

If the answer is no; then why would we argue that anything doing a similar 180 change of direction does stop before doing so?
2. 07 Oct '05 18:442 edits
it passes through a point in time where its velocity is zero. It never stops, in the sense that there is always an acceleration or jerk on it.

also, keep in mind that the crank does not move at the same speed at all times, it moves faster right after the explosion in the chamber.
3. AThousandYoung
All My Soldiers...
07 Oct '05 20:01
Even though it's attached to a moving crank, that crank at two points in time moves perpendicular to the line of motion of the piston. At those two points in time the piston stops.
4. Suzianne
Misfit Queen
07 Oct '05 21:09
Originally posted by AThousandYoung
Even though it's attached to a moving crank, that crank at two points in time moves perpendicular to the line of motion of the piston. At those two points in time the piston stops.
Quite correct. The connecting rod between piston and crankshaft is attached so that it allows the connecting rod to sway back and forth. It's *not* rigidly attached to the piston. It's more correct to say that the piston is attached to the connecting rod, which is then attached to the crankshaft, rather than saying that the pistion is directly attached to the crankshaft. This allows the piston to stop at two points in the movement of the crankshaft, just as you said.
5. 08 Oct '05 01:051 edit
Originally posted by logicalhippo
also, keep in mind that the crank does not move at the same speed at all times, it moves faster right after the explosion in the chamber.
That may be true if the engine only has one cylinder.

What about a any other combination, 4-6cylinders, V5-12cyliners, modern engines which can electronically control the number of operating cylinders.

The piston is only at its highest velocity at the center of its stroke. Piston velocity actually slows at the top and bottom of the stroke(becasue of how its conected), but not the crank RPM's.

Of course the crank does not move at the same speed at all times.
But the rise or fall of RPM's is not because of a single combustion pushing a single piston..

The rpm of the crank, or "engine speed" keeps all pistons moving in unison, or the engine would fail. All pistons are contionuosly pushing or being pulled by the crank, So I would imagine its RPM's rise and fall very smoothly.

Any mechanics like to add their 2cents?
6. 08 Oct '05 01:082 edits
Originally posted by AThousandYoung
Even though it's attached to a moving crank, that crank at two points in time moves perpendicular to the line of motion of the piston. At those two points in time the piston stops.
Do you mean TDC and BDC of the piston stroke? Please explain?

How can the forece of crank move perpendicular to the piston's line?
I think that may cause a failure.
There are only a two single points opposite of each other in the revolution, where the push or pull is not parallel to the piston's line of motion.

A line of motion or any line requires two points of reference, que no?
7. 08 Oct '05 01:32
Originally posted by Suzianne
the piston is attached to the connecting rod, which is then attached to the crankshaft.
This I am aware of.
8. 08 Oct '05 02:42
It was laziness on my part to not word the mechanics more clearly.
I *never* said ridged anything. That would not make sense.

The question was fairly clear I thought.
9. 08 Oct '05 04:347 edits
Originally posted by Raw760
There are only a two [b]single points opposite of each other in the revolution, where the push or pull is not parallel to the piston's line of motion.

A line of motion or any line requires two points of reference, que no?[/b]
Let me add to or rephrase this. Referring to an in-line engine for simplicity.

The horizontal(perpendicular) position of the connecting point of the connection rod to the crank shaft (as it turns) *in relation* to the piston's vertical line of motion, has no affect of stopping vertical travel of the piston.

Only the vertical(parallel) position of this point (as well as the length of the connection rod) affects the pistons length of vertical travel and velocity(wait, sorry velocity depends on relation the two and TDC or BDC).
TDC and BDC are the extents of this motion.

As the connecting point revolves with the crank, there is no travel where the rod's push or pull is perpendicular to the piston's line of motion.. Not even TDC or BDC, these are single points.

A line of motion or any line still requires two points of reference, right?
And velocity is a measure of distance over time.

So the all pistons would not cease to have a measurable velocity until the engine stopped.

Anyone?...Anyone?...Bueller?
10. Suzianne
Misfit Queen
08 Oct '05 05:11
AThousandYoung already got this right. Can you read?
11. 08 Oct '05 05:131 edit
Yes I can. Do you ever have anything new to say?
12. Suzianne
Misfit Queen
08 Oct '05 05:16
Originally posted by Raw760
Yes I can. Do you ever have anything new to say?
Not on this topic, since AThousand Young is correct, not much more needs to be said.
13. 08 Oct '05 05:17
Does she ever say anything new?
14. 08 Oct '05 05:19
You must be Princess-Know-Everything-others-do.
15. 08 Oct '05 05:393 edits
Originally posted by AThousandYoung
Even though it's attached to a moving crank, that crank at two points in time moves perpendicular to the line of motion of the piston. At those two points in time the piston stops.
The point of connection is on a circular path. It will never run perpendicular or parallel to anything.
BECAUSE ITS DIRECTION IS CONSTANTLY CHANGING.

So long as the engine runs the piston will be pushed and be pulled in relation to this point. So it will always have a measurable velocity.
Even from TDC and BDC.

CAN YOU READ THIS PRINCESS???