4 stroke spring selection

[cbooth]

Hi all,

Anyone have any thoughts on the best way to select valve springs for 4 stroke gas engine. Its my own design 100cc and I have the valve lift determined, but not sure how to assess the required spring compression strength required.
On my previous smaller engines I have made by trial and error using spring steel, but wanted to see if there was a more scientific method?

thanks
Criag

 

[tornitore45]

The spring pre-compression lenght is usually much larger than the travel so the force is nearly constant.
Just to get you in the ball park
Try adding the mass of valve, spring, rocker, pushrod and poppet.
Force should be 2 to 4 times the MASS x Acceleration You should know the acceleration from the cam design.

If you are not to proud to copy other designs look for a lawn mower engine.

 

[cbooth]

Thanks Mauro, that should be fairly easy to calculate. I'll try that.

 

[WOB]

The book "Aircraft Engine Design" by Joseph Liston (1942) has an entire chapter on valve gear. In it, there is a rigorous calculation procedure for determining valve spring specs. All components of the valve train are weighed and the radius of gyration of the rocker arm is measured. That along with the cam profile and the max RPM are plugged into the calculation procedure. It spits out the required valve spring force to prevent valve float. I went thru the procedure for a normal design 1" bore engine with approx. 0.4" dia. valves and a max RPM of about 10,000. The required spring force turned out to be about 2# compression with the valve closed. This was easily implemented and worked perfectly in my engine with a max RPM of 7200 with the propeller I picked.

The book can be found online on the used book sites. It covers every part of engine design in full mathematical detail at the state of the art in 1942 which was near the end of IC engine evolution. It would be invaluable for a scratch engine designer and pretty interesting for the casual builder.

WOB