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

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

16 vote(s)
94.1%
2. ### No

1 vote(s)
5.9%
1. Nov 11, 2009

### evans71

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

well guys....does it?

2. Nov 11, 2009

### FredGarvin

3. Nov 11, 2009

### Phrak

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

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 verticle centerline of the piston, it get's a little more involved.

Last edited: Nov 11, 2009
4. Nov 12, 2009

### Lsos

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

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.

5. Nov 12, 2009

### Ranger Mike

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

where is there metal to metal contact?

6. Nov 12, 2009

### Lsos

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

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

7. Nov 12, 2009

### Ranger Mike

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

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...

8. Nov 12, 2009

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

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?

9. Nov 12, 2009

### Lsos

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

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?

10. Nov 12, 2009

### xxChrisxx

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

TDC, BDC Velocity = 0

Please do NOT try to get clever with this to make a futile point that its always 'moving', becuase it isn't. Not even in the ideal case.

11. Nov 12, 2009

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

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.

12. Nov 12, 2009

### xxChrisxx

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

I still dont know what he was getting at though, becuase a sinewave has zero points.

Last edited: Nov 12, 2009
13. Nov 12, 2009

### FredGarvin

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

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.

14. Nov 12, 2009

### Ranger Mike

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

15. Nov 12, 2009

### xxChrisxx

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

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 thats 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.

Last edited: Nov 12, 2009
16. Nov 12, 2009

### tbobtrasman

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

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...

17. Nov 12, 2009

### xxChrisxx

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

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.

18. Nov 12, 2009

### xxChrisxx

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

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.

Last edited: Nov 12, 2009
19. Nov 12, 2009

### xxChrisxx

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

Well as a mathematical peice 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?

Last edited: Nov 12, 2009
20. Nov 12, 2009

### xxChrisxx

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

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