Camshaft lobe design

[David Redszus]

This is a topic which I feel needs expanded discussion.

Given a specific lobe lift and duration points, how many curves can be drawn through these three points?

How does the shape of each curve affect the performance of that lobe? How would this be determined?

[F-BIRD'88]

Good questions.

[CamKing]

Question 1: An infinite number of curves could be designed to fit a given duration, and lift.

Question 2: How the shape of that curve effects the engine performance, is reliant on two different factors. First, is how the lobe curve dynamically controls the valvetrain. Second, is how the curve controls the airflow, in and out of the cylinder.
On the Intake side, you want the lift at each degree to be high enough to not choke the flow, but not so high, that the pressure above the valve becomes as low as the pressure below the valve.

[PRH]

On the Intake side, you want the lift at each degree to be high enough to not choke the flow, but not so high, that the pressure above the valve becomes as low as the pressure below the valve.

That’s a mouthful right there!!

[Erland Cox]

But if the flow is not choked at one rpm it will probably bee choked at a higher rpm.
So if the intake valve opens to fast, will the engine lose power or give more power at a higher rpm?
If it is not choked by something else.

Erland

[David Redszus]

But if the flow is not choked at one rpm it will probably bee choked at a higher rpm.
So if the intake valve opens to fast, will the engine lose power or give more power at a higher rpm?
If it is not choked by something else.

Erland

Hi Erland
Where is the choke point? At the valve? At the port?
How does the valve velocity affect the choke point?

What should the valve velocity be? How do we know?
How does valve (cam) velocity relate to PAD?

[CamKing]

But if the flow is not choked at one rpm it will probably bee choked at a higher rpm.
So if the intake valve opens to fast, will the engine lose power or give more power at a higher rpm?
If it is not choked by something else.

I design it for max HP RPM.
If you open it too fast, you'll lose power below peak HP.

[naukkis79]

I design it for max HP RPM.
If you open it too fast, you'll lose power below peak HP.

I always believed that it's just mechanical limits of how fast you can open and close valves. For desired max Hp RPM it should alway be better to use faster valve motion and less open time for same flow window, for whole power range, or at least how I believe.

[Erland Cox]

But if the flow is not choked at one rpm it will probably bee choked at a higher rpm.
So if the intake valve opens to fast, will the engine lose power or give more power at a higher rpm?
If it is not choked by something else.

Erland

Hi Erland
Where is the choke point? At the valve? At the port?
How does the valve velocity affect the choke point?

What should the valve velocity be? How do we know?
How does valve (cam) velocity relate to PAD?

In the scenario not opening to fast at the valve.
The valve velocity or lift versus piston motion and RPM controls pressure in the cylinder and the pressure controlls the port velocity and the reflections.
If you look at a port velocity diagram the engine always loses power if the intake port loses velocity after the exhaust valve hs closed.
I have only the possibility to simulate it but there are always RPM:s out of phase when you lose intake velocity and power.

Erland

[naukkis79]

In the scenario not opening to fast at the valve.
The valve velocity or lift versus piston motion and RPM controls pressure in the cylinder and the pressure controlls the port velocity and the reflections.
If you look at a port velocity diagram the engine always loses power if the intake port loses velocity after the exhaust valve hs closed.
I have only the possibility to simulate it but there are always RPM:s out of phase when you lose intake velocity and power.

Erland

If intake flow fills vacuum in cylinder flow stops and usually it can't start to flow again so VE drops badly. But it's not about valve motion but valve open timings.

And that's easily curable, you tune intake tract longer which adds dead time to valve open to where intake flowing actually starts, nothing starts to flow into intake tube before valve open suction pulse travels from valve to end to intake tube end - before that only air entering into cylinder can be only from pressure reduction from air that already is in intake tube. ( in high rpm-tune air can be entering into tube from reflection of valve closing affected pressure rising effectively making engine to see intake tube shorter)

With tuned intake and fast opening/long duration cam there's always pretty sharp increase in engine torque where piston demand starts to be strong enough to keep cylinder not filling up too soon - with extremely high flowing head, long duration cam/long intake tract combo that point might be doubling engine torque within 100rpm.

[Erland Cox]

I will try that in a sim and see what happens.

Erland

[CamKing]

I design it for max HP RPM.
If you open it too fast, you'll lose power below peak HP.

I always believed that it's just mechanical limits of how fast you can open and close valves. For desired max Hp RPM it should alway be better to use faster valve motion and less open time for same flow window, for whole power range, or at least how I believe.

Right now, we are talking about cams with the same duration and lift, and just changing the curves between those points.
Shortening the duration, and increasing the valve motion to get back the area, is a different subject.

[David Redszus]

Right now, we are talking about cams with the same duration and lift, and just changing the curves between those points.
Shortening the duration, and increasing the valve motion to get back the area, is a different subject.

Exactly right, Mike.

Just finished CamDr measurement of several cam lobes and found substantial differences in velocity curves.

When we refer to fast opening valves, do we mean the slope of the valve velocity curve, the peak valve velocity, or do we mean the crank angle at which peak valve velocity occurs?

I have inlet valve data that shows assymetric velocity curve slopes prior to and after peak valve velocity. Why?
Is there a benefit to a steep velocity opening and a shallow closing velocity curve? Or vice versa?

Again, as Mike said, we are dealing with cams with identical lift and duration, but with different curves.

[CamKing]

Is there a benefit to a steep velocity opening and a shallow closing velocity curve? Or vice versa?

yes. Look at piston velocity per degree, from TDC to BDC, and you will see why.

[Racer57]

Is there a benefit to a steep velocity opening and a shallow closing velocity curve? Or vice versa?

Wouldn't the steep opening velocity be needed to lift the valve as much as possible in the 75 degree range when the piston is at its max speed??? Not quite sure what to think on the shallow velocity curve maybe Camking can elaborate more on this?