Camshaft Design- New 1

[EngJW]

I would like to start this thread over again. The last one had drifted too far off topic. My proposal for the topics would be the following:

1. Cam profile design and ramp design
2. Cam and lifter materials
3. Camshaft manufacturing
4. Valve spring design, materials, manufacturing
5. Valve train dynamics

I don't think we should get into applications (what cam should I put in my car?) or valves and ports. That should be the subject of another thread.

Most of the above subject matter is in the hands of a few specialty companies and consultants, but I doubt if a lot of it is proprietary. Engineers involved in engine design and testing could benefit from at least a working knowledge in these areas. If a problem comes up, it would be good to be able to take a first shot at it before turning to consultants, especially if your company does not have deep pockets.

Thanks,
John Woodward

 

[EngJW]

For engine cams, the follower motion is specified typically in the form of a lift table. The lift table is the same for any type of cam but the lobe shape will differ depending on the follower type- flat, roller, etc. The follower motion is important to the engine; the lobe shape is important to the cnc machine or grinder.

The equation I am most familiar with is a 7th order polynomial with exponents 0,1,2,p,q,r,s. You have to know the boundary conditions, the maximum lift, and duration. You then fool with the exponents until you get the best compromise in terms of velocity, acceleration, jerk, and area under the lift curve. I don't know if the specialist cam companies use the same methods, but this one came from one of them.

Some good sources for this information are the SAE series Computer Applications in Valve Gear Design (title is something like that) or the Machine Design articles about Polydyne cams. You would have to search back to the 1950s and some of the names are Englemann, Dudley, and Stoddart.

 

[magnograil]

The cam profile can be described by a single polynimial curve but a 7th order polynomial will only give 6 inflexions. If the curve is drawn on a XY axis (X being theta, Y lift) and the start/end are tangent to the X axis that gives three inflexions for the lift and three for the return. Quieting ramp, acceleration, deceleration, dwell, (-)acceleration, (-)deceleration, quieting ramp. It works but the dwell at full lift has to be a single curve. I expect it also limits the jerk curve to a straight line but have not worked through the equations.
A spline curve would allow smooth transitions through the jerk curve.

 

[EngJW]

This curve would be used on either the opening or closing side. You would have to input the ramp end conditions. I believe the jerk curve is continuous but some people could be using higher order curves to get continuous derivatives beyond jerk.

 

[optimum]

What ever happened to ceramics in the use of valves and seats? Ceramic would be good hi heat applications.

 

[EngJW]

Anyone remember the Polymotor, back in the 80s or so? Most of the engine was plastic with ceramic components. I think some parts were still metal or it used metal inserts. This engine ran for a while in GTP racing.