How are limitations of dwell over nose estimated?

[CamKing]

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.)

No. I'm not dwelling lift, I'm dwelling velocity, and that velocity is not zero.
This is for limited lift classes.
Here's an example of the dwell in velocity(0 is max lift)

-3 .39988427
-2.75 .39993493
-2.5 .39997108
-2.25 .39999276
-2 .40000724
-1.75 .40001448
-1.5 .40002172
-1.25 .40002896
-1 .40003620
-.75 .40004344
-.5 .40005068
-.25 .40005792
0 .40006516
.25 .40005792
.5 .40005068
.75 .40004344
1 .40003620
1.25 .40002896
1.5 .40002172
1.75 .40001448
2 .40000724
2.25 .39999276
2.5 .39997108
2.75 .39993493
3 .39988427

[Stan Weiss]

Code: Select all

C:\Program Files (x86)\Microsoft Visual Studio\VB98\carfor\mjcv.cmm
Smooth = NO

       I N T A K E
         Cam Lift        Velocity      Acceleration       Jerk
  1   0.00000000000
  2   0.39988427000   1.59953708000   6.39814832000  25.59259328000
  3   0.39993493000   0.00020264000  -6.39733776000 -51.18194432000
  4   0.39997108000   0.00014460000  -0.00023216000  25.58842240000
  5   0.39999276000   0.00008672000  -0.00023152000   0.00000256000
  6   0.40000724000   0.00005792000  -0.00011520000   0.00046528000
  7   0.40001448000   0.00002896000  -0.00011584000  -0.00000256000
  8   0.40002172000   0.00002896000   0.00000000000   0.00046336000
  9   0.40002896000   0.00002896000   0.00000000000   0.00000000000
 10   0.40003620000   0.00002896000   0.00000000000   0.00000000000
 11   0.40004344000   0.00002896000   0.00000000000   0.00000000000
 12   0.40005068000   0.00002896000   0.00000000000   0.00000000000
 13   0.40005792000   0.00002896000   0.00000000000   0.00000000000
 14   0.40006516000   0.00002896000   0.00000000000   0.00000000000
 15   0.40005792000  -0.00002896000  -0.00023168000  -0.00092672000
 16   0.40005068000  -0.00002896000   0.00000000000   0.00092672000
 17   0.40004344000  -0.00002896000   0.00000000000   0.00000000000
 18   0.40003620000  -0.00002896000   0.00000000000   0.00000000000
 19   0.40002896000  -0.00002896000   0.00000000000   0.00000000000
 20   0.40002172000  -0.00002896000   0.00000000000   0.00000000000
 21   0.40001448000  -0.00002896000   0.00000000000   0.00000000000
 22   0.40000724000  -0.00002896000   0.00000000000   0.00000000000
 23   0.39999276000  -0.00005792000  -0.00011584000  -0.00046336000
 24   0.39997108000  -0.00008672000  -0.00011520000   0.00000256000
 25   0.39993493000  -0.00014460000  -0.00023152000  -0.00046528000
 26   0.39988427000  -0.00020264000  -0.00023216000  -0.00000256000

Stan

[Zmechanic]

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.

Yes there can be zero net normal force on the lobe over the nose. Anything beyond that is into valve loft where the valve motion is no longer following the lobe.

[modok]

just ignore joe. He must be pretending not to understand. A cam is just like throwing something up in the air, it stops at the top but the acceleration doesn't.

Reading the title of the thread did get me wondering

..... how much dwell can you get away with? Certainly the flexibility of the parts would accommodate some.
I know some cam grinders you can stack masters and stretch a few more degrees out of it, creating a small dwell.

[volodkovich]

How much dwell? Depends on how much excitation the valve spring can take without losing control.

Normally we do a Fourier Power series decomposition of the lift curve (or a Fourier series decomposition of the acceleration curve) and compare it to the valvespring natural frequency. But it is all theory until you have some real world testing to determine what magnitude of each excitation frequency corresponds to a dangerous valve spring surge. Once you have that data you can predict fairly well how a new profile will perform with a given spring/valvetrain - up until the valve starts to loft.

[modok]

4 degrees?

[CamKing]

..... how much dwell can you get away with? Certainly the flexibility of the parts would accommodate some.
I know some cam grinders you can stack masters and stretch a few more degrees out of it, creating a small dwell.

We have some very successful limited lift circle track cams that have 32-48 degrees of dwell over the nose. These engines only have to turn 6,800rpm.

[speedtalk]

IMG_1194.jpg

.
I'm not sure if this helps the discussion, but here is Spin-Tron valve motion at 2000 and 8800

[Zmechanic]

I'm not sure if this helps the discussion, but here is Spin-Tron valve motion at 2000 and 8800

Haha, I thought for sure you were coming in to lock this thing up.

But in any case, kinda looks like that one is getting some decent pushrod flex. Both on the opening flank, and then when it lofts and catches back up with the lobe. Pretty cool...

[CamKing]

IMG_1194.jpg
.
I'm not sure if this helps the discussion, but here is Spin-Tron valve motion at 2000 and 8800

If you plotted this against the acceleration curve of the lobe, you would see that the pushrod is flexing as the valve is accelerating toward max accel, then, as the acceleration rate is reducing, the pushrod is trying to straighten back out, and that increases the valve velocity beyond what the spring can control, and the valve lofts. When the valve comes back down on the back side, the sudden increase is valve acceleration causes the pushrod to flex again, until the valve reaches the seat, and bounces back up.

[The Badger]

Camking, how are the limitations of the amount of dwell on your cams figured? Is it the limits of the profile, limits of valve spring paired with valve weight? Or something completely different?

[CamKing]

Camking, how are the limitations of the amount of dwell on your cams figured? Is it the limits of the profile, limits of valve spring paired with valve weight? Or something completely different?

Spring is the biggest issue for the actual dwell, but the more dwell, the faster the acceleration rate to get to the dwell, so that relates to the same limitations for any aggressive lobe design.

[williamsmotowerx]

Dwelling over the nose is the most stressful thing you can do to a valvetrain.

You see it all the time in cam grinders who try to manipulate a copied cam.

[David Redszus]

Camking, how are the limitations of the amount of dwell on your cams figured? Is it the limits of the profile, limits of valve spring paired with valve weight? Or something completely different?

The maximum valve acceleration does not occur over the nose of the cam. It occurs much earlier when the valve spring force has not reached its peak. Not even close.
Which is why the valve is launched free of the cam while the valve is still on the opening slope.
If we increase valve spring seat force to control valve float, the spring force over the nose becomes excessive.

Since (once again) F = M * A, the spring Force required is determined by the Mass of the valve train (valve assembly, push rod, lifter, etc) and the Acceleration determined by the cam lobe and rpm.

The optimum solution requires very light components with high strength; its called valve train stiffness.

The cam plays an important role since it determines the valve acceleration. An improperly designed cam lobe, even with necessary lift and duration, can produce problems of valve train instability at certain engine speeds and even self destruction.
This is due to resulting vibrations which can produce amplified harmonics; wear, float, bounce. and piston dome impact.

[williamsmotowerx]

What happened to page 10? There was a bunch of posts deleted?