Does the piston ever stop moving in a running engine?

[evans71]

Does the piston ever "stop" moving in a running engine?

well guys...does it?

 

[FredGarvin]

Of course it does. Standard equations of piston motion clearly show that the velocity has zero points at many locations.

http://en.wikipedia.org/wiki/File:Graph_of_Piston_Motion.png

 

[Phrak]

The vertical position of the piston from center is roughly determined by the offset of the rod bearing from the centerline of the crankshaft,

D = R sin θ.

The velocity of the piston is the time rate change in the position, D'.

A little more exactly, if you include the length of the piston rod, L, from the center of the rod bearing to the center of the wrist pin,

D = R sin θ + √(L²-D²sin²θ)

Solve for D'.

Because the wrist pin isn't located through the vertical centerline of the piston, it get's a little more involved.

 

[Lsos]

Velocity is a measurment of motion. If it's 0, then it's not moving. It's kind of hard to argue this fact, despite what you might feel is the correct answer.

This actually creates some problems. For example, most metal parts in a running engine never actually come into direct contact with each other, because they glide over a thin film of oil. This includes bearings, pistons, and piston rings. However, since the piston comes to a full stop at top and bottom dead center, there actually IS metal-to-metal contact, and wear occurs.

 

[Ranger Mike]

where is there metal to metal contact?

 

[Lsos]

Between the piston rings and the cylinder wall. It's not a big deal, but nevertheless, it's something...

 

[Ranger Mike]

it is a fine point..actually the pressurized oil squirts out of the con rod bearing and oils each cylinder..so the piston rides on a thin film of oil ..this is why the bottom piston ring is an oil scraper to control the oil..i would state that the wrist pin to con rod is a near metal t o metal contact but again the pin is oiled by the con rod bearing and oil carries away heat..i would state that the piston ring to piston top and bottom may make contact...cam to lifter is metal contact...

 

[A_Paradox]

No, the piston never stops "moving" in a running engine.

It changes direction and the instantaneous linear velocity equals zero at TDC and BDC but "movement" or motion involves more than simply instantaneous velocity.

Apart from internal sonic vibrations, which are not what the question appears aimed at, it must be the case that the piston "stops" going up, then goes down, until it "stops" going down, after which it starts to go up again. My question is: Why is this an issue?

 

[Lsos]

OP...since you made a poll about what on the surface looks like an extremely simple question, I think everyone (at least I am) is under the impression that you know something nobody else does, and will soon come back with some revolutionary statement about engine dymanics and/or a new theory of gravity. What's the story?

 

[xxChrisxx]

Plot the wave form representing the reciprocating motion of the piston.

Then on the resulting sine wave, show the points at which the piston reaches zero velocity.

What's your point?
TDC, BDC Velocity = 0Please do NOT try to get clever with this to make a futile point that its always 'moving', because it isn't. Not even in the ideal case.

 

[A_Paradox]

A sin wave representing the motion of a reciprocating piston, is a representation of the motion of the piston; it is not the piston itself. It seems to me that Mr Omega has run afoul of the rocky reef of absolutist [or perfectionist] thinking. what I mean is that, like Zeno of long ago, the lure of excessively simplified concepts can lead us into obsessive reliance on mental constructs which potentially exhibit properties intrinsic to the representational system itself.

The real world item however is not constrained by the nature of the system we may use to represent it. Mathematics is only ever indicative of the real world -but fantastically useful none the less.

 

[xxChrisxx]

I still don't know what he was getting at though, because a sinewave has zero points.

 

[FredGarvin]

Plot the wave form representing the reciprocating motion of the piston.

Then on the resulting sine wave, show the points at which the piston reaches zero velocity.

Sure. That was done in the link I provided. However, the velocity is the time derivative of that plot and guess what...there are zeros. They happen to align perfectly with the inflection points of the displacement plot. Aint calculus grand?

It is physically impossible for the piston to NOT come to rest when changing directions.

 

[Ranger Mike]

talk about beating a dead horse...

 

[xxChrisxx]

The OP clearly meant acutal linear piston motion. Not vibrations or anything like that. You may have taken it to mean that, but the complexity of the OP's question was below that.

If you wish to insist that it has to be perfectly motionless, including vibrations, then I agree if an engine is running no part of it ever stops 'moving'. But that's a little bit pedantic for the purposes of this thread.

When referring to velocity at a time, generally people mean at that instant. If I meant average velocity, I would have said average.

 

[tbobtrasman]

It gets into definitions... atoms never stop moving except at absolute zero... but an object can not, on a linear path, take a reciprocal course without first stopping. The piston might rock and/or pivot in the cylinder but the implied relevant motion is along the cylinder axis. It's a bar bet, an ill-defined question designed to evoke a snap answer that can then be argued against.

Math over words every time... there is no kinda 3...

 

[xxChrisxx]

Time factors aren't used directly in the equation determining piston motion as the piston position is a function of crank angle. The crank angle is a function of time (under operating conditions). This being an implicit use of the time variable means its TIV.

 

[xxChrisxx]

State your opinion. I will freely admit I've not had to classify systems for quite a while for maths purposes, and also I've never been the hottest at maths (someone who is good at maths will turn up and gie the correct answer to this sono enough). So I wouldn't be 100% surprised if I was wrong.

However, as far as I can remember. Time variant systems require the time variable to be explicitly used.

 

[xxChrisxx]

Well as a mathematical piece of geometry. No, the crank angle is not a function of time. The pistion displacement (height) depends soley on where the crank is pointing.

However under operating condition, crank angle vaires with time. eg at 1 rpm, the crank moves 6 degrees in 1 second. At 2 rpm the crank moves 12 degrees in 1 second. NOTE! The piston is still only being displaced by the same amount, determined by the crank angle (irrespective as to how quickly crank angle is changing)

Either way the time variable is not explicity used, so it's time invariant.

So (time for me to cock up equations :D)Displacement Of Piston x

x(t) = R sin [theta](t)

T is not an explicit variable.

Were you asking this as a genuine question or just trying to be clever?

 

[xxChrisxx]

Once again the question as stated was, "Does the piston ever stop moving in a running engine."

A "running" engine infers that the crankshaft is constantly rotating.

Is the "movement" or motion of the piston (which is connected to a crankshaft by a lever) time-variant or time-invariant?

Running or not, the equation describing piston motion wrt crankshaft is time invariant.

 

[Archosaur]

Since we're all being devil's advocates:

Hey, OP, there is not enough info to answer your question.
I need a frame of reference!

 

[evans71]

I asked here because this question has been a warzone on one of the other websites I post on.

while most say yes a few still argue no. I've talked to 2 math teachers both of them say no it never stops moving but it slows down so much that you can consider it "motionless" or that since it rotates in a perfect circular motion it cannot stop moving because circles have no flat spots.
also the word "stopped" has been argued over because I guess it means 2 instances in time where the piston is motionless

so I figured I would ask some bright minds here and see what they had to say

 

[xxChrisxx]

The maths teachers you asked are wrong.

You can't get from going very slowly in one direction to going very slowly in the other direction without crossing a point of zero motion in the middle. Even if it stops, for an instant in time, then it does stop.Real pistons acually stop for a relatively long time at TDC and BDC (can be on the order of a degree of crank rotation although its usually much less where there is acutally zero movement), due to the conrod not being infinitely long. The shorter the conrod (for a given stroke) the longer the 'dwell time'. In infiitely long conrod means that the piston, if plotted against time would produce a sine wave, and would only be slopped for an instant.
http://i260.photobucket.com/albums/ii3/cdcracing/crank.png

Image is probably a bit big for a direct link.

We know it must dwell by looking at the acceration curve. If the displacement is sinusodal motion, it must have a sine acceleration curve. A flat spot on the curve means a dwell.
http://upload.wikimedia.org/wikipedia/commons/c/c2/Graph_of_Piston_Motion.png

 

[Archosaur]

Several answers:

1. No. Relative to certain things (the sun, for example) it does not stop.

2. Not possible to tell. Even relative to the shaft, a piston is the construction of subatomic particles. To prove through observation that the center of mass of these particles perfectly reverses direction would violate the Heisenberg uncertainty principle.

3. Yes. If the piston is an ideal piston, drawn on a white board in a math classroom, then, relative to the ideal shaft, the instantaneous velocity at the endpoints of its oscillation is zero, so by definition, it is stopped.

4. No. An actual piston in an actual engine does not go straight up and down. There are vibrations and other factors that reduce the possibility of a perfect 180 degree turn to 0%.


My verdict:
An actual piston does not stop relative to the shaft, BUT as students of physics, we work with ideal forms of things. Ropes, spheres, wires, and yes, even pistons sometimes.
An ideal piston is stopped at crest and trough of its oscillation. Your math teacher missed a golden opportunity to show that velocity can be zero and acceleration can be non-zero simultaneously. Judging by your teachers response to you (that a circle has no flat parts, etc.) I would say that your teacher was talking about an ideal piston, and that he was just good old fashioned wrong.

 

[xxChrisxx]

Nice summary.EDIT: Nevermind I was going to talk about what practical assumptions can be made about a real engine, but it would just confuse matters and isn't terribly relevant.

 

[GSXR1000]

I am pretty sure that it is not move at TDC and BDC and I am pretty sure that that is when acceleration is at a max. This is probably why there is so much confusion about this one. Also, the acceleration is o when velocity is at a max which would be approximately mid stroke. So my answer is YES IT STOPS MOVING as long as we are talking relative to its position in the engine and not its position to relative to its obit around the sun or something silly like that!

 

[Phrak]

Once again the question as stated was, "Does the piston ever stop moving in a running engine."

A "running" engine infers that the crankshaft is constantly rotating.

This question was answered by Fred in post 2, complete with a reference, and reiterated in post 15, and a few others.

For this plot of piston position, velocity and acceleration,

Click here---> http://upload.wikimedia.org/wikipedia/commons/c/c2/Graph_of_Piston_Motion.png"

the zeros of velocity are cleary shown.

 

[MacLaddy]

Is the very bottom of the piston where it connects to the crankshaft considered the piston, or is that considered the push rod?

You'll have to forgive my complete ignorance of physics, as I have not even taken a single class, but it seems to me that the top of the apparatus (the piston)- moving consistently "Up & down," obviously must stop to change directions. (not counting all your quantum effects)

However, as it's on a pivot, the "rod," (if that is considered part of the piston) does not stop moving; ever. It must move continuously as it's connected to a continuously moving crankshaft.

That is, until gas is $8.00 a gallon and you can't afford to drive the Hot Rod El Camino anymore.

 

[FredGarvin]

The piston never stops moving. It instantaneously changes direction.

Instantaneous velocity and motion are not the same thing.

My God you are so full of it. An instantaneous change in direction means an infinite acceleration which also means an infinite force. That is not what is happening at all. Please provide ONE SINGLE SOURCE for your idiotic claims. Enough of this already. You can cloud the issue all you want with piston slap and the like but you darned well know that is NOT what the OP was after.

Post some references now or shut up.

 

[xxChrisxx]

I'm glad you said it Fred, I'm tired of this contant dancing around with wordplay.

To be honest this thread should have been stopped dead after post 2.