How are limitations of dwell over nose estimated?

[digger]

Are you claiming that on a non-dwell cam, that acceleration is zero at max lift?
If so, you are confused.

both you and Joe are wrong technically

1)
here is a 2nd order polynomial defining one such lift function over the nose. i've done the derivatives using mathematical theory. I've not done it by a numerical approximation so the numbers are EXACT. the exact function is plotted in the title.


as you can see to left of peak lift the velocity is positive then goes to zero then continues to negative.
due to being a 2nd order polynomial the acceleration comes out constant negative
it is also plain to see that velocity is zero at peak lift but acceleration is not

this proves Joe wrong.

2)
here is a 4th order polynomial defining another such lift function over the nose. i've done the derivatives using mathematical theory. I've not done it by a numerical approximation so the numbers are EXACT. the exact function is plotted in the title.


as you can see being a 4th order polynomial the velocity comes out to 3rd order polynomial and acceleration a 2nd order meaning both velocity and acceleration ARE zero at peak lift

Obviously this is a VERY simplified polynomial example as is the first as the lift curve function, but it proves it IS possible to have zero acceleration at peak lift mathematically without "any" dwell. if you were to define something more realistic there would be combinations of different orders with a general function maybe similar to

e.g -Ax^4 + Bx^3 - cx^2 + Dx - E

giving a different result

[SchmidtMotorWorks]

You are correct

In the case above, the curvature is zero at max lift.

If one were interested in minimizing harmonics and getting close to dwell, that would be a pretty good shape to do it.

In the context of a complete curve, that would be a difficult shape to squeeze in so smoothly.

Here is an image where the acceleration curve takes that shape.
It is very hard on the shape of the curves adjacent to it.

It more than doubled the negative acceleration to achieve the same lift.

zero_accel.jpg

[digger]

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?

here is a 2nd order polynomial followed by straight line and then another 2nd order polynomial. I've put the 3 separate equations to show this is real mathematics its not a numerical approximation


so how the acceleration is not definable due to going from constant negative to zero within a "time" of zero. it would need some smoothing otherwise the parts will mechanically be smoothed it for you .

if i numerically approximate work out the derivatives i get the following.



it shows a spike in the jerk but the magnitude will depend on how far apart the samples are (increment size). you need the increment size --> 0 to define and you will find that you cant actually define it.


here is a 4th order polynomial followed by straight line and then another 4th order polynomial


now you can see that jounce/snap would be not definable

[midnightbluS10]

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.

Why would you generalize about the entire group when this topic only involves a few people? Adding to that, how many of those people that "claim to be engineers" are actually involved in this discussion? Not nearly enough to generalize that most people here are fake engineers. Are you an engineer? So are you, then, making it up as YOU go along? If you're speaking about someone specific, then be specific. Call them by name instead of generalizing about the entire group when the entire group doesn't apply. If you're gonna take cheap shots, at least aim for the right person.

[David Redszus]

In the design world of camshafts, a dwell is a flat spot on the lift curve. A cam can have multiple dwells (used in automated machinery) but virtually ALL automotive camshafts are single dwell. That dwell period is represented by the camshaft base circle.

A single dwell cam starts lift from zero (on the base circle), rises to peak lift, then falls back down to the base circle. There is no dwell at maximum lift; the cam nose is rounded, never flat.

When we consider an entire camshaft revolution, we encounter a slight acceleration as the follower moves from the base circle onto the opening ramp. This is followed by a steep and strong positive acceleration as the cam opens the valve.

About half way up the lift curve, across the nose and half way down the lift curve, the acceleration becomes negative.
And as the valve closes the acceleration again becomes positive.

So we should see two positive acceleration spikes and one prolonged negative value acceleration curve.
While acceleration values must pass through zero as they change from positive to negative and back to positive, a single dwell cam will never indicate zero acceleration across the nose.

Only Jon's curves indicate the real acceleration experienced by a valve.

[digger]

Calculus says that the slope of the curve is the slope of the lane that's drawn as a tangent to the curve.
At peak lift the slope is zero.

Yes at peak lift the slope is zero. This means velocity is zero at peak lift but everyone knows this and it isn't being disputed

[David Redszus]

Calculus says that the slope of the curve is the slope of the lane that's drawn as a tangent to the curve.
At peak lift the slope is zero.

Yes at peak lift the slope is zero. This means velocity is zero at peak lift but everyone knows this and it isn't being disputed

Every word I have written is true and is well documented. For your information, I write the programs that perform the calculations and do not use generic software.
Your graphs are incorrectly labeled; you should check them for correctness before posting. And calculus doesn't say anything. It is merely a tool that is used to obtain an insight into the movement of objects.

At peak lift, the velocity is indeed zero, but the acceleration is not, at least not for a single dwell cam. Over the nose the acceleration is always negative except for double dwell cam lobes. When was the last time we saw a double dwell cam used in an automotive application?

You tend to blather away about high school math and never really address the OP question.

Just what is the dwell over the nose? Can you explain the purpose and application of camshaft derivatives? Why do we bother with them? How should they be used?

[Zmechanic]

Calculus says that the slope of the curve is the slope of the lane that's drawn as a tangent to the curve.
At peak lift the slope is zero.

For once we agree! It is. Its slope is the value produced by the first derivative. Which is velocity. The second derivative is acceleration. Throughout this entire thread it's feeling more and more like you are confusing the two.

[naukkis79]

Basic physics : Acceleration is force/mass. So if you have 100kg force against 0,1kg valve train mass you have a 1000g acceleration potential, even when valve velocity is zero, velocity isn't needed for calculations.

Valve train events are limited with mechanical forces from acceleration and deceleration, and from valve spring ability to reverse motion speed. So in ideal world cam will accelerate and decelerate rapidly at near zero lifts and spring force controls motion after that in spring-rated acceleration so theoretical ideal cam profile is symmetrical. In practical things are more complicated and as pictures there from actual cam profiles targets near constant acceleration for motion reversing to maintain valve train harmonics.

[gruntguru]

Joe. Here is an example you can try. Reverse your car down the street at say 10 mph. While still travelling backwards engage a forward gear and smoke the tyres. Although travelling backwards (forwards velocity equals negative 10 mph), you will be experiencing a constant forward acceleration of say +0.5g. Keep the tyres smoking and the +0.5g acceleration will continue - even at the instant when the forward velocity is equal to zero - and continuing when the forward velocity becomes positive. You can verify the direction and magnitude of this acceleration by feeling your back being pressed into the seat.

[David Redszus]

So what is dwell over the nose?

[CamKing]

So what is dwell over the nose?

I sometimes dwell velocity over the nose.

[SchmidtMotorWorks]

Every word I have written is true and is well documented. For your information, I write the programs that perform the calculations and do not use generic software.
Your graphs are incorrectly labeled; you should check them for correctness before posting. And calculus doesn't say anything. It is merely a tool that is used to obtain an insight into the movement of objects.

At peak lift, the velocity is indeed zero, but the acceleration is not, at least not for a single dwell cam. Over the nose the acceleration is always negative except for double dwell cam lobes. When was the last time we saw a double dwell cam used in an automotive application?

You tend to blather away about high school math and never really address the OP question.

Just what is the dwell over the nose? Can you explain the purpose and application of camshaft derivatives? Why do we bother with them? How should they be used?

There is a case as Digger points out where velocity and acceleration can be zero at maximum lift without any dwell.

The lift curve goes to zero curvature at the point of max lift.
So for that instant, geometrically, it does not have velocity or acceleration without any dwell.

[SchmidtMotorWorks]

So what is dwell over the nose?

I sometimes dwell velocity over the nose.

0

[naukkis79]

I sometimes dwell velocity over the nose.

That was the original question, why would you do that? (At max lift dwell velocity means zero velocity which also means that there has to be zero acceleration too.)

After seeing videos for actual valve springs operation at high speed I might guess that sometimes there can be momentarily zero force from valve spring at max lift from spring harmonics and dwelling valve could cure it.