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.

 

[TugboatEng]

In my experience, the cost of installing a sleeve is the same as plating a cylinder. It's $90 for the sleeve and $90 to install it. It costs $200 to plate a cylinder (my pricing may be outdated).

I did have a bad experience with one of the USA based platers.

 

[GregLocock]

You may glean some handy numbers from https://www.eng-tips.com/threads/peak-cylinder-pressures.215499/

 

[XR250]

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.

How much actual testing is going to be done before releasing to the end users? Does not sound like a great scenario to me.

 

[BrianPetersen]

Motocross engines are already high-performance engines. The cooling jackets and stress situations within the engine (not limited to just the cylinder) are probably already quite optimised from the factory. There may be some wiggle room for overbore, but I'm betting, not much.

You're going to be stuck with the cylinder head stud/bolt diameters and locations, and the cooling passages in and out of the cylinder head. Those, plus the need to achieve a reasonable "fire ring" in the head gasket clamping area, are going to limit what you can do. Replacing an integral plated-aluminium cylinder with a separate cylinder block and (presumably) cast-iron liner isn't going to be acting in your favour.

All of the cases of really-big overbores that I know of, are situations where the same engine design family allowed for multiple bore sizes, or where the manufacturer allowed for "future expansion" that one could make use of. The motocross engines are pretty much designed "at" 450cc (class limit).

Is there a possibility of simply overboring (and replating) a stock cylinder ... How thin do you dare to make the cylinder walls??

Umatek in Quebec is the main place that does it in Canada. https://umatek.com/en There's another outfit called Mongoose in British Columbia that does it, but their internet presence is iffy. Millennium Technologies is the one that I'm aware of in USA, but I've not crossed paths with them. The technology is available.

 

[G_Alex]

I have looked around at what established companies are doing for the bike in question, Most are just boring out stock cylinders and redoing the coating but . I have adjusted my cylinder wall (one piece aluminum with nikasil coating) to a larger major diameter slightly to make up the wall thickness. And yes it's hard to find any space so it's all a balancing act between case wall / water-jacket area / cylinder wall. The customer we are designing and building these for will be doing all the testing and selling, he is deep in racing of pretty much all types from mx to nhra top fuel so I am sure he will be able to thrash whatever we supply to an extreme extent. I was just given the oem cylinder. I don't have access to the rest of the bike, working from hand measurements and an outsourced structured light scan. I am going to use the same alloy as the oem as it is the proper material, the customer already figured out working suppliers/vendors for material and boring/nikasil from his previous design i was given as reference so we just need to do the machining and the rest will be done elsewhere.

 

[CWB1]

Containing combustion is the easy part. The reason 90%+ of aftermarket liners are bought from a few specialty manufacturers is bc the rest of the aftermarket is famous for durability issues. When you change liner or block geometry, fit/press, or coolant passages you run a real risk of cracking liners bc of stresses and vibration on the liner, and cavitation erosion of the outside of wet liners. If you have FEA, CFD, and know the critical engine modes its an easy analysis, if you dont you're just guessing.

I'd send the customer down the road to preserve your shop's reputation. Any serious hobbyist or parts reseller wanting custom liners would've called a liner manufacturer, spent the $2-300 per, and had quality parts and installation procedure in-hand a week or three later. Folks otherwise are likely shady cheapskates trying to scam hobbyists.