Joel5.0

Whatever that stuff is.... you are not sharing.............. anyway....... you are mixing and mismatching metrics, reference points and data. Velocity = scalar + direction, not a point in space, in order for that vector to change direction to the opposite side, it needs to contract to the value of 0 first, not "pass through it", the object stays at rest (as in motionless) for a very short period of time, therefore, it dwells.

Didn't find an example with a bullet..... will a cannon ball, piano or pumpkin do?.......... run this simulation, fire the object at a 90° angle, it will help you visualize the moment the object stops in midair (V = 0 = dwell period) + it might help you understand the basics of physics a little better.

golden_eye

+1 this is probably the easiest way to put it. You need to read up on basic physics. To keep it simple, a piston only moves up and down in one dimension. In order for it to change direction from up to down it needs to come to a stop first, even if only for an instant.

Li0nHart

the physics is about grade 10 or 11....the projectile or piston at TDC only reaches "0" velocity at it's maximum verticle height. We can calculate the point at which this occurs with the projectile exactly by using the following forumula....

H = (v x t) + (0.5 x g x t2)

in order for the projectile to dwell of a small amount of time, you would need to calculate another point in time in which the clock moves ahead but the height stays the same. This would alter the curve (flatten it) to the point that it would no longer be parabolic...that does not happen.

in order to approach the point at which V=0 you have to take "baby steps"...you can use Limits in calculus to approach V=0 but you will never find another point in time that is the same as V=0

you can visualize it by thinking of the following paradox.. if you walk towards a line and take a step that is 1/2 as small as the previous one you will never reach the line...no matter how small a step you take, you will always be able to cut the remaining distance in half....alternatively, you can just take one big step and cross right over the line without stopping at all.

..interestingly...if you were to graph the time vs distance of your steps...the curve formed would be similar to that of the projectile and piston.

it is also interesting that if you calculate the point at which verticle velocity = 0, maximum height, the point at which the projectile stops going up, and the point at which the projectile starts going back down..they are all the same point...everything occurs at the same instant.

thanks everyone for keeping the discussion civil...

it was asked what engine building i have done...i have built two engines...one 302 and one 350. Yes I measured dwell time with a dial indicator over the open bore...yes dwell time did exist...but this is due to the slop in our manufacturing processes and the accuracy of the dial indicator.

Li0nHart

been a while since I have gotten back to this...

you basically answered it yourself...."V = 0 = dwell period".....the dwell period is zero therefore indicating no actual dwell....in theory that is.

if it did indeed stop (even for the briefest moment) where would this pause be made up in the constant ration of the crank?

Joel5.0

JesusChrist!.... allow me to decipher that relationship you quoted for you.......

When V (as in velocity) = 0 (as in no movement whatsoever) = dwell (as static, no movement or when V=0) period (as in time lapse, interval). You do have a lot of homework to do.

jkrum10238

haha heated argument, this is easy throw a damn baseball straight up in there air and tell me if it stops (ovb not for a long period of time) before it reverses its direction and comes back down

Bubba 95SN

Stop with the graph, it is confusing you. Take a head off and rotate engine while watching the piston. Will be clear as a puddle. Bubba

dreamer1uk

Really intellegent people for some reason have the hardest time in understanding something so simple and i have no clue why, lol.

Albert Einstein 'supposedly' had a difficult time understanding extremely basic math....

Li0nHart

We can help you...


I'll throw it back at you (pun intended)....tell us how long it pauses for? ...yes the answer is easy

You don't need to throw it straight up though. The same formula applies if you throw it at a 45 degree angle or any angle for that matter.

One really easy way to wrap your head around it is to consider only half of its motion. The point at which you say it stops before reversing direction. Climb up a tree, hold out your arm with the ball and drop it. Would you say it pauses in the air like "Wile E Cyote" before it starts to fall?


Hehe...no need to get snooty...yes I have done my share of undergrad physics homework...mind you that was 10 years ago. V=0 is not an interval or lapse in time it is an exact instantaneous point in time. V=0 at the top of a constantly curving curve does not indicate any dwell. Dwell, by the nature of the word, occurs over a period of time. V=0 occurs at a single point on a parabolic graph...which indicates and exact point in time, but not a moment or period in time as there is only 1 point in time at which upward/downward velocity = 0 occurs.

It's a bit confusing but if you look at the graph a curve such as a parabola, you will see that there is no place on the curve that is perfectly straight (perfectly horizontal if plotting for time). In order for a pause (full stop) to occur, a portion of the top of that graph needs to be flat in the horizontal plane. This simply is not the case. There simply is no formula that produces v=0 in two adjacent distinct points at the top of a parabolic graph of non linear motion. A theoretical piston follows a parabolic curve...it is not a linear as most people assume.

Now of course those of us who have built engines have indeed measured dwell with a dial indicator across a cylinder bore. We can see this dwell on our dial indicators and we can adjust this with rod length etc. What causes the perceived dwell we measure is the fact that we can not create a totally tight and frictionless engine. There is slop in the wrist pins allowing movement/rotation, there is slop in the journal bearings and between the sides of the cylinder wall and piston. Even the best dial indicator can only measure to thousands or hundreds of thousands of an inch or degree. Not nearly precise enough to totally eliminate what we are able measure as dwell.

If the piston and connecting rod truly did pause for any length of time...say even 10,000ths of a second. Then after 10,000 revolutions of the crank, the piston would be lagging behind the crank by 1 second...which obviously doesn't make sense or is even possible.

Joel5.0

... as I said, you have a lot of homework to do. What type of home work?..... simple, you are assuming the crankshaft + rod creates a perfect symmetrical motion of the piston in the cylinder........ and it does not. Or do you think the transition between the upper and lower arcs of the circular stroke motion from the crankshaft causes an instantaneous switch in direction at the piston? .....quick answer = nope (as in negative).

A crankshaft-rod-piston slider mechanism is not a perfect sinusoidal oscillator as you continue to wrongly assume. Why does piston movement is >55% in 50% of the crankshaft rotation downward (as in 90° ATDC)? Doesn't that leave <45% of the piston stroke to be handled by the other 50% crankshaft rotation downward (as in from 90° ATDC to BDC)?

IOW.... when you look at a 360° revolution from the crankshaft in a 3.4" stroke engine, you have 180° accountable for >3.74" of total piston movement and 180° accountable for <3.06" of total piston movement in a full revolution. And yes, there is a dwell period (as in an interval when V=0 or movement in the cylinder's axis = 0) at TDC and BDC.

As I previously said....

Do you feel like doing some homework?.... check the attachment, use the values for a 3.4" stroke, 5.4" rod and solve for a 0° - 360° rotation in 1° increments and check Δx.