Cam With Dwell At Max Velocity Graph

[DaveMcLain]

Yesterday we were taking apart an engine for a customer it is an Onan 2 cylinder flathead engine from a Case garden tractor. Anyway I decided that it would be fun to plot the cam profile. It is interesting because it has a lot of dwell at maximum velocity along with very long lash ramps. When is this technique used or is it used at all on modern high performance cam lobe designs?

The engine has .750 diameter lifters and it uses .695 of that diameter. It measures 231 and 226 duration intake and exhaust on a 110 lobe sep. .305 lift on the intake .301 on the exhaust.

Here's a graph of the intake lobe: the exhaust looks just about the same..



The dark blue line is the lift, the green line is the tappet velocity and the light blue line is the acceleration. I left off jerk for clarity. See how the cam has a big acceleration , goes to zero then goes negative as it moves toward the top of the lobe. The exact opposite happens on the way back down. The constant velocity ramp ends at about .020 lift which is also probably the lash point on the cam.

When looking at the lobe it is interesting and it has a shape sort of like a Gothic arch or a tall Gambrel style barn roof.

[Stan Weiss]

Dave,
What are the valve spring open and seat pressures?

Stan

[SchmidtMotorWorks]

It looks like the design technique may have been to draw the velocity profile with lines and parabola, then integrate to get lift.

The peak in the acceleration curve at max lift is probably a byproduct of the manufacturing process such as incomplete clean-up when grinding or improper polishing or a machine that does not reverse the motion of the follower well due to insufficient pressure or bearing issues.

[DaveMcLain]

Dave,
What are the valve spring open and seat pressures?

Stan

Figuring a 1.375 installed ht the spring gives 37lbs on the seat and 80lbs open.

[CamKing]

Very old school, but I was just looking at some current Mercedes AMG cams, and they're still designed that way.

I bet you're using some amount of smoothing on the graph, and that the acceleration curve is steeper.

[Stan Weiss]

Dave,
I think that deal is more about minimum wear and long parts life than getting the most HP.

Stan

[ingram]

There are a couple of cam design techniques being used here. The amount of lift for the duration and the relatively small tappet face requires a dwelled maximum velocity technique in order to control the maximum velocity. If that technique was not used, the maximum velocity would be too high for the lifter diameter. Another technique is used to control the nose radius so it is not too small. That creates the profile shape you described.

[volodkovich]

Very old school, but I was just looking at some current Mercedes AMG cams, and they're still designed that way.

I bet you're using some amount of smoothing on the graph, and that the acceleration curve is steeper.

What exactly are you commenting on that is 'old-school'? Dwelling max velocity? Or the constant velocity ramp?

Interesting they compromised the valve lift profile instead of putting a bigger lifter in. That amount of v-dwell is going to have some nice Fourier content!

[CamKing]

What exactly are you commenting on that is 'old-school'?

The whole design. That's a stupid amount of acceleration for a cam that size.
It would be very easy to get more area and/or lift at the same duration, without over-riding the kifter, and with less peak acceleration.

[Walter R. Malik]

What exactly are you commenting on that is 'old-school'?

The whole design. That's a stupid amount of acceleration for a cam that size.
It would be very easy to get more area and/or lift at the same duration, without over-riding the kifter, and with less peak acceleration.

Not to mention the fact it is symmetrical...

[Stan Weiss]

What exactly are you commenting on that is 'old-school'?

The whole design. That's a stupid amount of acceleration for a cam that size.
It would be very easy to get more area and/or lift at the same duration, without over-riding the kifter, and with less peak acceleration.

Mike,
Isn't the point of max acceleration where the constant velocity lash ramp meets the lift curve?

Stan

[SchmidtMotorWorks]

Dwell at Max Velocity_zpszk6u8wns.jpg

This is one way that the profile could be divided into segments.

[DaveMcLain]

Here's some more information on that lobe:

Durations

.003 399
.004 392
.006 376
.015 304
.020 266
.040 237
.050 230
.100 197
.150 164
.200 131
.250 94
.300 29

What's interesting to me is how is has a super long slow lash ramp but then suddenly accelerates very quickly. I wonder how much of this has to do with the engine being a flathead vs something with a rocker arm?

[CamKing]

Here's some more information on that lobe:

Durations

.003 399
.004 392
.006 376
.015 304
.020 266
.040 237
.050 230
.100 197
.150 164
.200 131
.250 94
.300 29

What's interesting to me is how is has a super long slow lash ramp but then suddenly accelerates very quickly. I wonder how much of this has to do with the engine being a flathead vs something with a rocker arm?

Just screwing around, this is what I came up with with no dwell, .305" lobe lift, and a max acceleration of .00008472
.020 268
.040 241
.050 231
.100 196
.150 164
.200 132
.250 95
.300 29

[SchmidtMotorWorks]

What's interesting to me is how is has a super long slow lash ramp but then suddenly accelerates very quickly. I wonder how much of this has to do with the engine being a flathead vs something with a rocker arm?

A long constant velocity lash-ramp makes it possible to assemble the engine with parts made to looser tolerances without clearance adjustment.