How are limitations of dwell over nose estimated?

[Zmechanic]

In the wrong hands, these measurement systems can cause more problems then they solve.
The upgrade on the Audie Cam Pro measures every 1/40th of a degree, with an accuracy of .000004".
But what is is measuring?

A colleague of mine designed an LVDT conditioner with such resolution he jokes that it can measure the wavelength of purple light, very similar to the resolution you are referring to above (I suspect they are also using an LVDT).

The problem was he could never prove it. The closest he got was remotely triggering the measurement from home in the middle of the night and praying the security guard didn't walk by or the HVAC didn't come on.

There's so much ambient electrical noise, vibration, air currents, temperature gradients, etc, it would take a dedicated Lab to get the advertised precision.

[user-23911]

here is a displacement function that is sinusoidal, differentiate to get velocity and again for acceleration which are all sinusoidal



if i use the same curve to make a displacement that is 4th order polynomial then velocity is 3th order and acceleration 2nd order



simple high school mathematics

But it's a grade F for fail.


Here's a correct example, taken from wiki.

https://en.wikipedia.org/wiki/File:Chro ... itesse.svg

Which just goes to show what a sorry state America's education system is in.

Little wonder Buzz has finally lost his marbles and admitted he never went to the moon...just as well, it would have missed the target.

[gruntguru]

Does anything HAVE to come to a completely motionless state to change directions, yes or no ? Mark H.

To "reverse" direction yes. So when you drop a tennis ball vertically there is a point when the ball is completely stationary so velocity = zero. Acceleration however is not zero, the ball is experiencing its maximum acceleration.

On cams. Most profiles accelerate the valve until it reaches maximum velocity (on the flank) then immediately start decelerating it (at whatever rate the springs are capable of at max RPM plus a margin of safety). The deceleration continues right across the nose where lift is max and velocity is zero.

If the cam profile has dwell (usually only done if the lift must be limited) the deceleration stops (goes to zero) at max lift, velocity stays at zero and lift stays at max. The dwell section is just a little circular section inserted into the cam profile at max lift. (There will usually be slight modifications to the profile at the start and finish of the dwell to reduce jerk. Jerk is the derivative of acceleration - sudden changes in acceleration result in high values of jerk.)

[gruntguru]

here is a displacement function that is sinusoidal, differentiate to get velocity and again for acceleration which are all sinusoidal



if i use the same curve to make a displacement that is 4th order polynomial then velocity is 3th order and acceleration 2nd order



simple high school mathematics

But it's a grade F for fail.


Here's a correct example, taken from wiki.

https://en.wikipedia.org/wiki/File:Chro ... itesse.svg

Which just goes to show what a sorry state America's education system is in.

Little wonder Buzz has finally lost his marbles and admitted he never went to the moon...just as well, it would have missed the target.

Nice post Joe.

The Chronogramme is what you might do on the flank of the cam to eliminate jerk as acceleration switches to deceleration. It is not relevant to the nose area of any cam with no dwell (99% of cams). Such a cam has zero jerk at max lift - the acceleration (deceleration) remains constant over the nose.

Back to my previous question. What is the acceleration of a piston at TDC?

[digger]

here is a displacement function that is sinusoidal, differentiate to get velocity and again for acceleration which are all sinusoidal

if i use the same curve to make a displacement that is 4th order polynomial then velocity is 3th order and acceleration 2nd order

simple high school mathematics

But it's a grade F for fail.


Here's a correct example, taken from wiki.

https://en.wikipedia.org/wiki/File:Chro ... itesse.svg

Which just goes to show what a sorry state America's education system is in.

Little wonder Buzz has finally lost his marbles and admitted he never went to the moon...just as well, it would have missed the target.

you don't seem to understand that people know that it IS possible for acceleration to be zero at peak lift, the function i posted is simply to show that it isn't always the case as mathematically there are functions that do not have zero acceleration at peak lift. but you knew that....



for those interested here is some stuff written by blair

http://www.profblairandassociates.com/p ... 20Lift.pdf

[swampbuggy]

Gruntguru......Your question was (is) what is the acceleration of a piston at TDC ( Top Dead Center ). The crankshaft rod journal is EXACTLY centered to the centerline of said bore, the centerline of the con-rod is EXACTLY centered with the bore centerline, and the wristpin is centered in the piston. Lets take a 3.250" stroke with a 1.850" rod journal and a 6.350" con-rod. Lets go from 1 degree BTDC to 1 degree ATDC ( 2 degrees) of crank rotation. It would be hard for me to believe, and i believe hard to prove that there is any movement in those 2 degrees ???? Mark H.

[user-23911]

If you use calculus which is an analogue method of measurement, the answer will be zero.
If you use software which has an insufficient sampling rate, the answer will not be zero. The actual answer will depend on the sampling rate.

So the answer is....it depends on how you measure it.


1769 James Cook recorded the transit of Venus in Tahiti.
They knew where and when thanks to calculus.
No computers back then.

[user-23911]

These are graphs are from the raw Cam DR data with no smoothing of an older Comp Cams SBF Roller.

Stan

ab-comp161-cam-lift.gif

ab-comp161-cam-vel.gif

ab-comp161-cam-acc.gif

Now...going back to this example which is so obviously wrong as far as the acceleration numbers goes.......
Peak lift is where things change.
We've got a negative acceleration number just before peak lift.let's call it .000052
What's actually happening?
We've got a deceleration as velocity reduces towards zero........but the acceleration / deceleration doesn't get to zero.
But just after peak lift the number is the same only now it's accelerating , velocity is increasing in the opposite direction.
If it's accelerating, it's got a positive not a negative number for acceleration.
What's gone wrong?


There's a big challenge for the engineers out there?


Take the number 1
Divide it by 3
What's the answer?
Multiple that by 3
What's the answer?
Is it the same number you started with ?

[digger]

These are graphs are from the raw Cam DR data with no smoothing of an older Comp Cams SBF Roller.

Stan

ab-comp161-cam-lift.gif

ab-comp161-cam-vel.gif

ab-comp161-cam-acc.gif

Now...going back to this example which is so obviously wrong as far as the acceleration numbers goes.......
Peak lift is where things change.
We've got a negative acceleration number just before peak lift.let's call it .000052
What's actually happening?
We've got a deceleration as velocity reduces towards zero........but the acceleration / deceleration doesn't get to zero.
But just after peak lift the number is the same only now it's accelerating , velocity is increasing in the opposite direction.
If it's accelerating, it's got a positive not a negative number for acceleration.
What's gone wrong?

1) as its approaching peak lift the velocity is positive (slope on the displacement graph) its decelerating (slope is getting shallower and shallower) meaning a negative acceleration

2) as its just gone past peak lift the velocity is increasing (slope is getting steeper and steeper) but in the NEGATIVE direction

by definition acceleration = (final velocity - initial velocity) / time

the answer for acceleration still comes out negative because the velocity is negative so the acceleration is actually still negative just like as you approach the peak in 1)

if you just use speed instead of accounting for the velocity which has direction (positive or negative) then you get the wrong answer as you have

thats maths and physics for you...

[Warp Speed]

We can bla bla bla about the math involved, but who can follow that?!?! Lol
When I first read the thread title, I thought, that's a pretty complex question. "How are the limitations of dwell over the nose estimated"
Dwell over the nose typically causes some bad dynamics. How to estimate a given systems limitation to them is pretty tough right?

[user-23911]

That was the answer I was expecting.......so from that.......whereabouts on the acceleration curve does our positive acceleration (acceleration)and negative acceleration(deceleration) change to become.....
Positive acceleration(deceleration) and negative acceleration (acceleration)............and why does it change instantaneously without anybody knowing if I hadn't pointed it out?


BTW as I posted yesterday, there's no such thing as negative velocity, velocity is a vector so it has a magnitude and direction. Negative velocity is used by convention in order to show velocity in the opposite direction.




It's all about if you use erroneous measurement methods, the result is GIGO.
So therefore your conclusions are wrong.


Common sense will always be that if you use a lobe with a flat top it will cause less stress than one with a sharp top, less wear on the nose too due to increased contact area.

[mekilljoydammit]

Good lord. Dunning Kruger effect strikes again, I guess. I'm out, have fun.

[Zmechanic]

Common sense will always be that if you use a lobe with a flat top it will cause less stress than one with a sharp top,

Holy-canoli that's so exactly backwards...

[digger]

That was the answer I was expecting.......so from that.......whereabouts on the acceleration curve does our positive acceleration (acceleration)and negative acceleration(deceleration) change to become.....
Positive acceleration(deceleration) and negative acceleration (acceleration)............and why does it change instantaneously without anybody knowing if I hadn't pointed it out?

BTW as I posted yesterday, there's no such thing as negative velocity, velocity is a vector so it has a magnitude and direction. Negative velocity is used by convention in order to show velocity in the opposite direction.

It's all about if you use erroneous measurement methods, the result is GIGO.
So therefore your conclusions are wrong.

yes its quite simple and actually has nothing to do with measurement its to do with maths, physics and how things are defined and calculated.

the speed (velocity magnitude) slows down approaching the peak lift and speeds up after yet the deceleration is still negative. Stan's first example shows that even with case of dwell the acceleration might reach zero if designed to do so but it didn't go positive.

https://study.com/academy/lesson/contin ... blems.html

whats the slope at x=0 in example 1 in the link?

but your 3.5* degree wheel and dial indicator that's accurate to 0.01 mm is bound to prove everything

[MadBill]

So if the lobe has 360 cam degrees of dwell, can lifter acceleration still be non-zero?