How are limitations of dwell over nose estimated?

[SchmidtMotorWorks]

But you didn't answer the question. How can you transition from deceleration to acceleration instantaneously without passing through zero?

If I understand your question correctly, a conventional valve lift profile does transition from deceleration to acceleration instantaneously at maximum lift.

The length of time that the velocity is zero is zero.

At any instant before or after, no matter how microscopically small, the position and velocity will have changed.

The rate of acceleration is generally relatively constant near maximum lift.

There are curves that you could make that would have zero acceleration at max lift, but those would not be appropriate for anything operating above maybe 5 RPM, maybe some kind of machinery cam, but even then, it would be a poor design if it was planned to reverse direction immediately afterward.

With regards to a previous discussion, I want to be sure that you understand that curvature of the lift curve indicates acceleration, and that generally the radius of curvature near maximum lift is relatively constant, so acceleration is also relatively constant.

[digger]

If I understand your question correctly, a conventional valve lift profile does transition from deceleration to acceleration instantaneously at maximum lift.

deceleration or acceleration are actually "semantics" for lack of a better word.

According to wiki

"In physics, acceleration is the rate of change of velocity of an object with respect to time"

note the word velocity not speed

mathematically the acceleration remains negative the whole time

if i go from velocity +1 to velocity = 0 in 1 sec the acceleration is (0-1)/1 = -1

if i go from velocity 0 to velocity = -1 in 1 sec the acceleration is (-1-0)/1 = -1

the speed is going from 1 to 0 to 1 i can see how people might think its decelerating then accelerating but strictly it isn't

[SchmidtMotorWorks]

If I understand your question correctly, a conventional valve lift profile does transition from deceleration to acceleration instantaneously at maximum lift.

deceleration or acceleration are actually "semantics" for lack of a better word.

According to wiki

"In physics, acceleration is the rate of change of velocity of an object with respect to time"

note the word velocity not speed

mathematically the acceleration remains negative the whole time

if i go from velocity +1 to velocity = 0 in 1 sec the acceleration is (0-1)/1 = -1

if i go from velocity 0 to velocity = -1 in 1 sec the acceleration is (-1-0)/1 = -1

the speed is going from 1 to 0 to 1 i can see how people might think its decelerating then accelerating but strictly it isn't

I understand your point, but few people will grasp it.

[CamKing]

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?

If the valvetrain is still enough, I think the thing you would focus on is spring frequency, to keep the valvetrain stable with dwell.

I meant to type "If the valvetrain is STIFF enough"

[David Redszus]

Just as a calculation exercise, suppose we were to grind the nose of a cam flat for a period of 10 degs. A flat top cam with a dwell of 10 degs straddling peak lift. This would be a double dwell cam, one dwell period at the base circle and one at the nose.

For those 10 degs of rotation:
What would the velocity be?
What would the acceleration be?
What would the jerk be?

How would peak velocity and acceleration change?

[strokersix]

Zero, zero, zero.

Greater and greater.

Assuming similar total duration.

[strokersix]

Also assuming by "flat" you mean no change in lift.

[David Redszus]

Zero, zero, zero.

Greater and greater.

Assuming similar total duration.

Duration has not changed; we are concerned only with the dynamics at or near peak lift.
And, yes a flat nose means no change in lift.

What does occur is that two new acceleration "spikes" appear at the edges of our flat nose surface.
If we progressively reduce the flat top dwell to 5 deg, 3 deg, 3 deg, 1 deg, .01 deg....what happens to our derivatives?

[Zmechanic]

Zero, zero, zero.

Greater and greater.

Assuming similar total duration.

Duration has not changed; we are concerned only with the dynamics at or near peak lift.
And, yes a flat nose means no change in lift.

What does occur is that two new acceleration "spikes" appear at the edges of our flat nose surface.
If we progressively reduce the flat top dwell to 5 deg, 3 deg, 3 deg, 1 deg, .01 deg....what happens to our derivatives?

Gets better and better (assuming I'm understanding you right and you mean reducing the duration of the dwell spot). Of course, this is what the cam is "asking" for from the valvetrain. If you flatten off the top, the cam is asking for a pretty ugly acceleration out of the valvetrain components for them to suddenly drop velocity and stay in one place. Whether or not they can follow that profile is a whole other story. Kind of makes me think of the old school cams meant to loft the valve.

It's kinda the same thing as a jump for a car or a motorcycle. The ramp just quits, but the car/motorcycle does not. Peak effectiveness of the cam would be that there is zero net normal force by the lobe on the lifter over the nose. I believe some unlimited engines operate past this point though.

[user-23911]

If we progressively reduce the flat top dwell to 5 deg, 3 deg, 3 deg, 1 deg, .01 deg....what happens to our derivatives?

They're always zero.


Even if the flat top is 0.0000001 deg.

[David Redszus]

At the corners of the flat top nose, negative acceleration spikes appear.

As the width of the flat top narrows, the negative acceleration spikes diminish in amplitude and get closer to each other.

Finally, when the flat top nose is a single point, the spike is gone; it becomes another negative acceleration value like its neighbors.

The negative acceleration curve produced by the derivative of the lift across the nose is always very dirty, i.e. irregular in shape.

The next concern is the amplitude of the negative acceleration curve and its duration. How many degrees does the cam produce negative acceleration?

[user-23911]

Finally, when the flat top nose is a single point, the spike is gone; it becomes another negative acceleration value like its neighbors.

But it NEVER disappears.

Even with a flat top of 0.0000000000000000000000000000000000000000000000000000001 deg.

But what happens is that the slope of the curve either side gets steeper.

[gruntguru]

At the corners of the flat top nose, negative acceleration spikes appear.

I think you mean "jerk" spikes appear?
If the original profile had constant acceleration over the nose, the same profile with "dwell" inserted will have the same acceleration before and after the dwell period, and zero acceleration during the dwell. There won't be any acceleration spikes.

[gruntguru]

How do we go instantaneously from deceleration to acceleration without passing through zero?

We go from deceleration (scalar) to acceleration (scalar) by changing the direction of the velocity.

The direction of the acceleration (vector) does not change as we go over the nose. The acceleration of the valve head is always "towards the seat". It NEVER goes to zero in a normal (no dwell) cam.

[user-23911]

I'm beginning to wonder how many so called engineers on here are real ones and how many just make it all up as they go?


Here's a really simple maths lesson which shows exactly what's going on and how it all goes wrong with software.

https://www.youtube.com/watch?v=w3GV9pumczQ

Another one too if you don't know what a "tangent line " is.
https://www.youtube.com/watch?v=O_cwTAfjgAQ
You can't become an engineer if you don't know this?
Not a real engineer.