Cylinder design pressure for aftermarket 4 stroke cylinder.

[G_Alex]

Hi there, I have been tasked with designing a big bore cylinder for a 13:1 compression ratio 450cc single-cylinder 4-stroke MX engine making ~65 bhp. I am trying to decide what reasonable static cylinder pressures I should use in Inventor stress analysis. I don't have any other FEA, CFD, or CHT software—or formal education in those areas—so my analysis is limited. From my research, something like about 2 ksi across the ID and full stroke of the cylinder is probably reasonable for verifying normal operation and fatigue life. For my extreme pre-ignition/knocking case, I’ve been applying a 6mm band (carbon band height on OEM) from the top of the cylinder/case interface on the bore at 4.5 ksi, and using 2 ksi for the rest of the wall to be extra conservative.


Even under those unrealistic conditions, my design is mostly alright—the cylinder wall stays below yield, except for some small but high stress concentrations. These happen where the case has interrupted bearing contact with the cylinder barrel at the coolant outlet ports. This is my first time doing anything motorsports-related, so I'm very open to suggestions or critiques.


Also, if you have any design rules or tricks to share on water jacket design and analysis, I’d appreciate it. I’m short on space because I’m trying to keep the cylinder compatible without modifying any of the other parts. Keeping the cylinder wall strong has already forced me to narrow the water jacket flow area, even though I’ve expanded it outward from stock. But it's not a working design if it can't flow properly. Sorry about the mixed units—the OEM part was designed in metric, but my workplace uses inches and psi, so my units are all over the place.

 

[TugboatEng]

It appears you're using a cylinder liner. MX bike manufacturers stopped using liners in the mid-1980's for a reason. Is there a reason you want to walk so far backwards?

 

[G_Alex]

It appears you're using a cylinder liner. MX bike manufacturers stopped using liners in the mid-1980's for a reason. Is there a reason you want to walk so far backwards?

My customer only wants a small batch to start. The shop I work at is setup for machining welding and heat treat only, before I got involved my customer had previously finished a working liner/case cylinder design for a similar bike of a different make, and gave me that package as reference along with the charge of "Make the bore Øxyz, maximise cooling headroom, good luck all the competitor shops doing these went out of business.". I do agree it is not the proper way to design and manufacture the part but it's what I have been asked for. I think I can probably get it working, but it remains to see how much my cnc programmer / operator will be cursing my name when he has time in a few months to actually try make them. And my customer will be selling them to people who aren't very cost sensitive so as long as I can get it working its should be good for a first rev.